Industries where low latency services are very important.
\r\n\t* Technologies that allow control and reduction of air pollutants are encouraged to be discussed. In particular, technologies employed to treat greenhouse gases and precursors of acid rain.
\r\n\t* Poor agricultural practices, improper waste management and extractive activities that contaminate soil with special attention focused on emerging techniques permitting to diminish these pollutants.
\r\n\t* In the treatment of freshwater and marine and coastal waters, technologies that should be taken into account are those focused to eliminate chemicals and pathogens from mining and industrial effluents.
\r\n\t* Renewable energy technologies should also be discussed, special interest in those having the lower environmental impact. In this case a watchful life-cycle analysis has to support the proposal.
\r\n\t* New technologies and materials allowing the energy storage in a competitive mode should be also taken into account for the reason that they have a direct impact in the decrease of pollutants.
\r\n\t* As a final point, technological innovations applied to conserve and study endangered species will be also considered.
Internet traffic is growing very rapidly. Based on Cisco Visual Networking Index (VNI) forecast and methodology, global IP traffic has increased more than 4 times in the past 5 years, and will increase 3 times over the next 5 years. Moreover, it will reach 1 zettabyte (ZB) per year or 83.8 exabytes (EB) per month in year 2015 and exceed 1.4 ZB per year or 120.6 EB per month threshold by the end of year 2017. Video-on-demand traffic will triple by year 2016. For these reasons there will be need for optical access networks which are capable to handle with large data amounts, consume less energy and are cost effective .
PONs has been considered to be one of the most promising solutions for access networks due to its broad bandwidth and low-cost infrastructure . Especially, spectrum sliced wavelength division multiplexed passive optical network can be an attractive and cost effective solution to satisfy the growing worldwide demand for transmission capacity in the next generation fiber optical access networks [1, 2]. In principle there are two major factors that will influence the telecommunication networks of the future. The first one is the need to support high bandwidths and low latency and the second one is to use an architecture that is both cost and energy efficient . Recent studies have uncovered that a large amount of electricity is consumed by telecom equipment’s in broadband enable countries. Therefore it is important to put research effort for minimizing energy consumption in fiber optical access networks .
Traditional WDM systems have multiple transmitter lasers operating at different wavelengths, which need to be wavelength selected for each individual channel operated at a specific wavelength [1, 2]. It increases complexity of network architecture, cost and wavelength management . The strength of spectrum sliced WDM-PON technology is use of one common broadband seed light source and its ability to place electronics and optical elements in one central office (CO), in that way simplifying the architecture of fiber optical network . By using only one light source instead of one for every user we can make optical access system more energy efficient or in other words “green”. It is reported that the total energy consumed by the infrastructures of communication networks including Internet take up more than 3% of the current worldwide electric energy consumption. In specific, the access networks contribute to a larger portion of the overall energy consumption when compared to the core and transport networks . Spectrum sliced WDM PON systems benefit from the same advantages as traditional WDM, while employing low cost incoherent light sources like amplified spontaneous emission (ASE) source or light-emitting diode (LED) [7, 9].
The optical bandwidth per channel of SS-WDM PON system is large compared to the bit rate. Therefore, dispersion significantly degrades the performance of this system more than it is observed in conventional laser-based systems . The influence of dispersion needs to be studied in order to understand the characteristics of a spectrum sliced WDM PON system employing standard single mode optical fiber (SMF).
It is very important to build a new type optical system based on widely used frequency grid, recommended by international standards because of such a system potentially is much more compatible with other already existing WDM-PON optical systems. The main benefit includes the reduction of network architecture complexity as well as cost per one user. It is possible by replacing the classic WDM-PON system (where one laser source is used for each user) with our proposed spectrum sliced dense WDM PON system with CD compensation (where one seed broadband ASE source is spectrally sliced and used for multiple users) [11, 12].
The more information we are transmitting the more we need to think about parameters like available bandwidth and latency. Bandwidth is usually understood by end-users as the important indicator and measure of network performance. It is surely a reliable figure of merit, but it mainly depends on the characteristics of the equipment. Unlike bandwidth, latency and jitter depend on the specific context of transmission network topology and traffic conditions. As network latency we understand delay from the time of packet transmission at the sender to the end of packet reception at the receiver . If latency is too high it spreads data packets over the time and can create an impression that an optical network is not operating at data transmission speed which was expected. Data packets are still being transported at the same bit rate but due to latency they are delayed and affect the overall transmission system performance .
It should be pointed out, that there is need for low latency optical networks in almost all industries where any data transmission is realized. It is becoming a critical requirement for a wide set of applications like financial transactions, videoconferencing, gaming, telemedicine and cloud services which requires transmission line with almost no delay performance. These industries are summarized in references [15, 16] and shown below (see Table 1).
|\n\t\t\t\tIndustry \n\t\t\t\n\t\t\t||\n\t\t\t\tApplications and services\n\t\t\t\n\t\t|
|\n\t\t\t\tEducation\n\t\t\t\n\t\t\t||• video conferencing\n\t\t|
• rich learning content
• dynamic e-learning platforms
• presentation applications
• dynamic administration tools
• cloud-based applications
|\n\t\t\t\tHealthcare\n\t\t\t\n\t\t\t||• Picture Archiving Communications Systems (PACS)\n\t\t|
• telemedicine, telehealth applications
• diagnostic imaging
• Electronic Medical Records (EMR)
• patient portals
• mobile healthcare applications and equipment
|• live-streaming breaking news\n\t\t|
• television shows
• movies over Internet
• transfer large files, images, and videos from the field to studios around the world
• real-time gaming
|\n\t\t\t\tGovernment\n\t\t\t\n\t\t\t||• interaction between communities and their governments\n\t\t|
• transportation management,
• emergency response and general commerce
• circulation of documents
• self-service portals
|\n\t\t\t\tLegal\n\t\t\t\n\t\t\t||• sharing large, bandwidth-intensive files quickly and securely\n\t\t|
• secure and high speed access to critical files "24 hours a day, 7 days a week"
|\n\t\t\t\tFinance\n\t\t\t\n\t\t\t||• High-Frequency Trading (HFT) and high speed information exchange\n\t\t|
• financial transactions
• connections to brokers, dealers, exchanges, hedge funds and information feeds
In fiber optical networks latency consists of three main components which adds extra time delay: the optical fiber itself, optical components, and opto-electrical components. Therefore, for the service provider it is extremely important to choose best network components and think on efficient low latency transport strategy [17, 18].
There are many researches about improvement of transmission speed in fiber optical networks while impact and sources of latency is not investigated sufficiently. As mentioned before, latency is a critical requirement for a wide set of applications. Even latency of 250 ns can make the difference between winning and losing a trade . Surely “latency reduction” is among the hot key-words for vendors, end-users and telecommunication service providers. According to a recent market analysis, the request for ultra-low latency services is increasing dramatically and opening many opportunities especially in the field of end-to-end high-speed optical services . Latency reduction is very important in financial sector, for example, in the stock exchange market where 10 ms of latency could potentially result in a 10% drop in revenues for a company . No matter how fast you can execute a trade command, if your market data is delayed relative to competing traders, you will not achieve the expected fill rates and your revenue will drop . Low latency trading has moved from executing a transaction within several seconds to milliseconds, to microseconds, and even nanoseconds. Nowadays, a millisecond improvement in network speeds offers competitive advantage for financial institutions .
It is important to look at latency as consisting of different components. Latency is a time delay experienced in transmission system and it describes how long it takes for data to get from transmission side to receiver side. In a fiber optical communication systems it is essentially the length of optical fiber divided by the speed of light in fiber core, supplemented with delay induced by optical and electro optical elements plus any extra processing time required by system also called overhead . Signal processing delay can be reduced by using parallel processing based on large scale integration CMOS technologies .
Added to the latency due to propagation in the fiber, there are other path building blocks which affect the total data transport time [17, 24]. These elements include opto-electrical conversation, switching and routing, signal regeneration, amplification, chromatic dispersion compensation, compensation of polarization mode dispersion (PMD), data packing, digital signal processing (DSP), protocols and addition forward error correction (FEC) . Data transmission speed over optical metro network must be carefully chosen. If we upgrade 2.5 Gbit/s link to 10 Gbit/s link then CD compensation or amplification may be necessary, but it also will increase overall system latency. For optical lines with transmission speed more than 10 Gbit/s (e.g. 40 Gbit/s) a need for coherent detection arises. In coherent detection systems CD can be electrically compensated using DSP which also adds latency. Therefore some companies avoid using coherent detection for their low-latency network solutions .
From the standpoint of personal communications, effective dialogue requires latency < 200 ms, an echo needs > 80 ms to be distinguished from its source, remote music lessons require latency < 20 ms, and remote performance < 5 ms. It has been reported that in virtual environments, human beings can detect latencies as low as 10 to 20 ms. In trading industry or in tele-health every microsecond matters. But in all cases, the lower latency we can get the better system performance we can achieve [25, 27].
In standard single-mode fiber (SMF) major part of the light signal travels in the core while a small amount of light travels in the cladding. Optical fiber with lower group index of refraction provides an advantage in low latency applications .
Therefore it is useful to use a parameter “effective group index of refraction (neff) instead of “index of refraction (n)” which only defines the refractive index of core or cladding of single mode fiber. The neff parameter is a weighted average of all the indices of refraction encountered by light as it travels within the fiber, and therefore it represents the actual behavior of light within a given fiber . Figure 1 illustrates the impact of profile shape on neff by comparing its values for several Corning single mode fiber products with different refractive index profiles.
It is known that speed of light in vacuum is 299792.458 km/s. Assuming ideal propagation at the speed of light in vacuum, an unavoidable latency value can be calculated as following in Equation (1):
However, due to the fiber’s refractive index light travels more slowly in optical fiber than in vacuum. In standard single mode fiber defined by ITU-T G.652 recommendation the effective group index of refraction (neff), for example, can be equal to 1.4676 for transmission on 1310 nm and 1.4682 for transmission on 1550 nm wavelength . By knowing neff we can express the speed of light in selected optical fiber at 1310 and 1550 nm wavelengths, see Equations (2) and (3):
By knowing speed of light in optical fiber at different wavelengths (see Equation (2) and (3)) optical delay which is caused by 1 km long optical fiber can be calculated as following:
As one can see from Equations (4) and (5), propagation delay value of optical signal is affected not only by the fiber type with certain neff, but also with the wavelength which is used for data transmission over fiber optical network. It is seen that optical signal delay values in single mode optical fiber is about 4.9 μs. This value is the practical lower limit of latency achievable for 1 km of fiber in length if it were possible to remove all other sources of latency caused by other elements and data processing overhead .
Photonic crystal fibers (PCFs) can have very low effective refractive index, and can propagate light much faster than in SMFs . For example, hollow core fiber (HCF) may provide up to 31% reduced latency relative to traditional fiber optics [30, 31]. But there is a problem that attenuation in HCF fibers is much higher compared to already implemented standard single mode fibers (for SMF α=0.2 dB/km but for HCF α=3.3 dB/km at 1550 nm) . It is reported even 1.2 dB/km attenuation obtained in hollow-core photonic crystal fiber .
Chromatic dispersion (CD) occurs because different wavelengths of light travel at different speeds in optical fiber. CD can be compensated by dispersion compensation module (DCM) where dispersion compensating fiber (DCF) or fiber Bragg grating (FBG) is employed .
A typical fiber access optical network will require DCF approximately 15 to 25% of the overall fiber length. It means that use of DCF fiber adds about 15 to 25% to the latency of the fiber [17, 18]. For example, 100 km long optical metro network where standard single mode fiber (SMF) is used, can accumulate chromatic dispersion in value about 1800 ps/nm at 1550 nm wavelength . For full CD compensation is needed about 22.5 km long DCF fiber spool with large negative dispersion value (typical value is-80 ps/nm/km) . If we assume that light propagation speed in DCF fiber is close to speed as in SMF then total 100 km long optical network’s latency with CD compensation using DCF DCM is about 0.6 ms.
Solution how to avoid need for chromatic dispersion compensation or reduce the length of necessary DCF fiber is to use optical fiber with lower CD coefficient value. For example, non-zero dispersion shifted fibers (NZ-DSFs) were developed to simplify CD compensation while making a wide band of channels available. NZ-DSF fiber parameters are defined in ITU-T G.655 recommendation . Today NZ-DSF fibers are optimized for regional and metropolitan high speed optical networks operating in the C-and L-optical bands. For C band it is defined that wavelength range is from 1530 to 1565 nm, but for L band it is from 1565 to 1625 nm .
For commercially available NZ-DSF fiber chromatic dispersion coefficient can be from 2.6 to 6.0 ps/nm/km in C-band and from 4.0 to 8.9 ps/nm/km in L-band. At 1550 nm region typical CD coefficient is about 4 ps/nm/km for this type of fiber. It can be seen that for G.655 NZ-DSF fiber CD coefficient is about four times lower than for standard G.652 SMF fiber [27, 38]. Since these fibers have lower dispersion than conventional single mode, simpler modules are used that add only up to 5% to the transmission time for NZ-DSF . This enables a lower latency than using SMF fiber for transmission. Another solution how to minimize need for extra CD compensation or reduce it to the necessary minimum is dispersion shifted fiber (DSF) which is specified in ITU-T G.653 recommendation. This fiber is optimized for use in the 1550 nm region and has no chromatic dispersion at 1550 nm wavelength. Although, it is limited to single-wavelength operation due to non-linear four wave mixing (FWM), which causes optical signal distortions .
If CD is unavoidable another technology for compensation of accumulated CD is a deployment of fiber Bragg gratings (FBG). DCM with FBG can compensate several hundred kilometers of CD without any significant latency penalty and effectively remove all the additional latency that DCF-based networks add . In other words, a lot of valuable microseconds can be gained by migrating from DCF DCM to FBG DCM technology in optical metro network . Typical fiber length in an FBG used for dispersion compensation is about 10 meters. Therefore, normally FBG based DCM can introduce from 5 to 50 ns delay in fiber optical transmission line .
One of solutions how to avoid implementation of DCF DCM which introduces addition delay is coherent detection where new transmission formats like quadrature phase-shift keying (QPSK) can be used. However, it must be mentioned that it can be a poor choice from a latency perspective due to the added digital signal processing (DSP) time they require. This additional introduced delay can be up to 1 μs [24, 31].
Another key optical component which adds additional time delay in optical transmission line is optical amplifier. Erbium doped fiber amplifiers (EDFA) is widely used in fiber optical networks. EDFA can amplify signals over a band of almost 30 to 35 nm extending from 1530 to1565 nm, which is known as the C-band fiber amplifier, and from 1565 to 1605 nm, which is known as the L-band EDFA. The great advantage of EDFAs is that they are capable of simultaneously amplifying many WDM channels and there is no need to amplify each individual channel separately. EDFAs also remove the requirement for optical-electrical-optical (OEO) conversion, which is highly beneficial from a low-latency perspective. However it must be taken into account that EDFA contains few meters erbium-doped optical fiber (Er3+) which adds extra latency, although this latency amount is small compared with other latency contributors. Typical EDFA amplifier contains up to 30 m long erbium doped fiber. These 30 m of additional fiber add 147 ns (about 0.15 μs) time delay .
Solution how to avoid extra latency if amplification is necessary is use of Raman amplifier instead of EDFA or together (in tandem) with EDFA. This combination provides maximal signal amplification with minimal latency. Raman amplifiers use a different optical characteristic to amplify the optical signal [9, 27]. Raman amplification is realized by using stimulated Raman scattering. The Raman gain spectrum is rather broad, and the peak of the gain is centered about 13 THz (100 nm in wavelength) below the frequency of the pump signal used. Pumping a fiber using a high-power pump laser, we can provide gain to other signals, with a peak gain obtained 13 THz below the pump frequency. For example, using pumps around 1460–1480 nm wavelength provides Raman gain in the 1550–1600 nm window, which partly cover C and L bands. Accordingly, we can use the Raman’s effect to provide gain at any wavelength we want to amplify. The main benefit regarding to latency is that Raman amplifier pump the optical signal without adding fiber to the signal path, therefore we can assume that Raman amplifier adds no latency .
Any transmission line components which are performing opto-electrical conversation increase total latency. One of key elements used in opto-electrical conversation are transponders and muxponders. Transponders convert incoming signal from the client to a signal suitable for transmission over the WDM link and an incoming signal from the WDM link to a suitable signal toward the client. Muxponder basically do the same as transponder except that it has additional option to multiplex lower rate signals into a higher rate carrier (e.g. 10 Gbit/s services up to 40 Gbit/s transport) within the system in such a way saving valuable wavelengths in the optical metro network .
The latency of both transponders and muxponders varies depending on design, functionality, and other parameters. Muxponders typically operate in the 5 to 10 μs range per unit. The more complex transponders include additional functionality such as in-band management channels. This complexity forces the unit design and latency to be very similar to a muxponder, in the 5 to 10 μs range. If additional FEC is used in these elements then latency value can be higher . Several telecommunications equipment vendors offer simpler, and lower-cost, transponders that do not have FEC or in-band management channels or these options are improved in a way to lower device delay. These modules can operate at much lower latencies from 4 ns to 30 ns. Some vendors also claim that their transponders operate with 2 ns latency which is equivalent to adding about a half meter of SMF to a fiber optical path .
For low latency optical metro networks it is very important to avoid any regeneration and focus on keeping the signal in the optical domain once it is entered the fiber. An optical-electronic-optical (OEO) conversion takes about 100 μs, depending on how much processing is required in the electrical domain . Ideally a carrier would like to avoid use of FEC or full 3R (signal power, shape and length) regeneration. 3R regeneration needs OEO conversion which adds unnecessary time delay. Need for optical signal regeneration is determined by transmission data rate involved, whether dispersion compensation or amplification is required, and how many nodes the signal must pass through along the fiber optical path .
It is necessary to minimize the amount of electrical processing at both ends of fiber optical connection. FEC, if used (for example, in transponders) will increase the latency due to the extra processing time . This approximate latency value can be from 15 to 150 μs based on the algorithm used, the amount of overhead, coding gain, processing time and other parameters .
Digital signal processing (DSP) can be used to deal with chromatic dispersion (CD), polarization mode dispersion (PMD) and remove critical optical impairments. But it must be taken into account that DSP adds extra latency to the path. It has been mentioned before that this additional introduced delay can be up to 1 μs .
In this section we present optimized model of 160 km long optical metro access network operating at 10 Gbit/s bitrate. The aim of this section to show how much optical network can be optimized in terms of latency by replacing conventional latency inducing optical and electrical components to low-latency components. In Figure below we propose two optical transmission networks – conventional and latency optimized, see Fig. 2(a) and\n\t\t\t\t\tFig. 2(b).
First optical metro network (see Fig. 2(a)) consists of two transponders, DCF DCM for CD pre-compensation, two EDFA amplifiers to boost optical signal, and two 80 km long ITU-T G.652 SMF fiber spans.
Pre-compensation configuration of accumulated chromatic dispersion was chosen because our researches proved it more effective than post-compensation method . Fiber optical line is chosen 160 km in length because typically fiber optical metro networks are up to 200 km long .
Second network scheme is latency optimized (see Fig. 2(b)), where we replaced conventional transponder by ultra-low latency transponders, DCF DCM replaced by FBG DCM and EDFA amplifiers are replaced by Raman amplifiers.
For total latency calculations of both optical systems we assume that 64 byte packet which adds 51.2 ns network interface delay is transmitted. Here we used typical latency parameters we described previously in this work and compared results; see from obtained data (see Table 2).
As one can see from obtained data (see Table 2), total latency amount is reduced by 21.6% or 216.24 μs. In this case the main benefit comes from optimization of chromatic dispersion compensation scheme from DCF DCM to FBG DCM and replacement of conventional transponders to low latency transponders.
|\n\t\t\t\tLine component\n\t\t\t\n\t\t\t||\n\t\t\t\tConventional network\n\t\t\t\n\t\t\t||\n\t\t\t\tLatency optimized network\n\t\t\t\n\t\t|
|2 x 80 km ITU-T G.652 SMF fiber spans\n\t\t\t||4.9 μs x 160 km = 784 μs\n\t\t\t||4.9 μs x 160 = 784 μs\n\t\t|
|1 CD compensation module (DCM)\n\t\t\t||DCF DCM: 25% of 784 μs = 196 μs\n\t\t\t||FBG DCM: 50 ns\n\t\t|
|2 Optical amplifiers\n\t\t\t||EDFA: 2 x 0.15 μs = 0.3 μs\n\t\t\t||RAMAN adds no delay\n\t\t|
|2 Transponders\n\t\t\t||2 x 10 μs = 20 μs\n\t\t\t||2 x 4 ns = 8 ns\n\t\t|
|2 Network interface delays\n\t\t\t||2 x 51.2 ns = 102.4 ns\n\t\t\t||2 x 51.2 ns = 102.4 ns\n\t\t|
|\n\t\t\t\tTotal latency value\n\t\t\t\n\t\t\t||\n\t\t\t\t1 ms\n\t\t\t\n\t\t\t||\n\t\t\t\t0.784 ms\n\t\t\t\n\t\t|
Our research is based on powerful and accepted mathematical simulation software OptSim. It solves complex differential nonlinear Schrödinger equation (NLSE) using split-step Fourier method (SSFM). This equation describes optical signal propagation over the fiber and can be written as Equation (6) :
where A(t, z) is complex optical field; z is fiber length, [km]; is linear attenuation coefficient of an optical fiber, [km-1]; is the second order parameter of chromatic dispersion, [ps2/nm]; is the third order parameter of chromatic dispersion, [ps3/nm]; is nonlinear coefficient, [W-1.km-1]; t is time, [s]. NLSE takes into the account linear and nonlinear affects and they influence to optical signal distortions. The principle of split-step method is better illustrated by (6), which can be written as follows :
In general split-step method is based on assumption that linear and nonlinear effects affect optical signals independently. This statement can be considered as true if we assume that all fiber length z is being divided into sufficiently small spans Δz, and only then these linear and nonlinear effects by turns are taken into account for each segment.
There are two basic algorithms for realization of SSFM: time domain split step (TDSS) and frequency domain split step (FDSS). These two algorithms differ only with an approach that is being used for calculation of linear operator. While in both cases nonlinear operator is being calculated in time domain .
Operator is being fully characterized by its impulse response h(t) and it is mathematically correct to calculate its influence to A(t, z) optical field using products of mathematical convolution. In TDSS case it can be written as follows :
This algorithm calculates this convolution in time domain and precisely obtains time delay values between signals with different wavelength. In OptSim software this TDSS algorithm is realized using finite impulse reaction (FIR) filters. This sophisticated technique provides complete control of an overall mistake that may occur during all process of calculating. By contrast FDSS calculates in frequency domain but firstly for this algorithm is necessary to calculate fast Fourier transformation (FFT) from A[n] signal samples and from h(t) impulse reaction. Then it is necessary to use invers FFT (FFT-1) to convert obtained data array to time sample domain. FDSS algorithm can be mathematically described using following equation :
As one can see, then in this case circular convolution is used for obtaining signal sample array. This array may contain fewer samples than it is necessary to obtain actual convolution products—sample array. Hence this algorithm is easier to implement than TDSS and it requires less computation time and resources but serious errors may occur during calculation .
Equation (7) can be solved assuming that and operators are independent and fiber span Δz length is small enough (5-100 meters, depending on the simulation accuracy requirements). Then optical signal after propagation over Δz span can be described in the following manner :
For the evaluation of system performance will be used such parameter as Q-factor and BER value. Q=7.03 (16.94 dB) corresponds to the commonly used reference for BER of 10-12. Q factor and BER confidence interval magnitude depends on the total number of simulated bits Ntotal :
whereμ1,0 and σ1,0 are the mean and the standard deviation of the received signal, when a logical “1” and “0” is transmitted, and π≈ 3.14 .
Q-factor uncertainty range for 1024 simulated bits that is used in our schemes is equal to 0.77 dB. For 1024 of simulated bits these intervals are:
Spectrum slicing technique is one of basic techniques available in WDM PON systems in order to reduce the cost of components and simplify the passive network architecture. There incoherent broadband light source (BLS) is sliced and equally spaced multi-wavelength channels is generated . The aim of spectrum slicing is to employ a single BLS for transmission on a large number of wavelength channels (see Fig. 3).
At first, BLS is sliced with arrayed-waveguide grating (AWG). Afterwards, optical slices are modulated, multiplexed by second AWG and transmitted over standard single mode optical fiber line. Channels are demultiplexed by third AWG located after SMF and received by direct detection optical receiver. This receiver can be PIN photodiode or avalanche photodiode (APD). Broadband light sources like LED, SLED (superluminescent diode) or ASE can be used in spectrum sliced systems for data transmission. Transmission power available for each separate optical channel depends on the slice width. It should be considered that a larger slice will increase not only the total channel power but also increase the influence of dispersion and therefore the number of available WDM channels . In our research we choose ASE as BLS for spectral slicing because it has the highest optical output power compared to other above mentioned broadband light sources.
Erbium doped fiber amplifier (EDFA) emits high power amplified spontaneous emission (ASE) noise in C band (wavelength from 1530 to 1565 nm) and L band (wavelength from 1565 to 1625 nm) if there is no signal to be amplified. This effect is used to design broadband ASE light source in this research. ASE noise generation and gain occurs along all EDFA fiber length and it depends on erbium ion (Er3+) emission and absorption spectrum . Broadband ASE light source realization can be done in different ways: by using one EDFA or connecting several amplifiers in cascade mode (one after another). The latter method allows achieving almost flat ASE output spectrum and higher output power because of better Er3+ions usage along several amplifiers.
In our research the broadband ASE light source is constructed from two EDFAs combined in cascaded mode. The smoothest ASE output spectrum can be achieved if 400 mW output power for all pump lasers is used, where first EDFA amplifier is pumped in co-propagating direction on 1480 nm wavelength, and second EDFA is pumped in co-propagating direction on 1480 nm as well as in counter-propagating direction on 980 nm (see Fig. 4).
Erbium doped fiber span with length of 9 m is used for first EDFA and 12 m long erbium doped span is used for second EDFA. In this manner a broadband ASE source with almost flat spectrum and total output power on the output of cascaded EDFA system about+23 dBm (200 mW) is being constructed. The output spectrum of realized broadband ASE noise-like light source which will be spectrally sliced using AWG unit is shown in Figure below (see Fig. 5).
The highlighted area in figure shows the 1545.322 nm to 1558.983 nm wavelength range (192.3 THz to 194.0 THz frequency, centered on 1552.524 nm wavelength or 193.1 THz in frequency) which will be used for our 8-channel and 16-channel spectrally sliced WDM-PON systems.
As one can see (see Fig. 5), fluctuations of optical power level are minimal and spectrum in this employed region is almost flat.
This section describes simulation scheme of SS-WDM PON system with up to 16 channels, 2.5 Gbit/s transmission speed per channel (total system’s capacity is up to 40 Gbit/s) and chromatic dispersion compensation module (DCM).
The performance of simulated scheme was evaluated by the obtained bit error ratio (BER) value of each channel in the end of the fiber optical link. Basis on ITU recommendation G.984.2 it is taken into account that BER value for fiber optical transmission systems with data rate 2.5 Gbit/s per channel must be below 10-10.
As one can see in Figure 6, SS-WDM PON simulation scheme we built consists of up to 16 channels. Frequency grid is anchored to 193.1 THz and channel spacing is chosen equal to 100 GHz frequency interval. This frequency grid and interval is defined in ITU-T recommendation G.694.1. Broadband ASE light source is spectrally sliced using 8-channel or 16-channel flattop AWG filter (AWG1) with channel spacing equal to 100 GHz (0.8 nm in wavelength). Using this AWG unit we can obtain equally arranged optical channels (slices) with dense channel interval 100 GHz. Insertion losses of AWG units are simulated using additional attenuation blocks (attenuators). It is taken into account that simulated high-performance AWG multiplexers and demultiplexers are absolutely passive optical components with insertion loss up to 3 dB each. After spectrum slicing operation implemented by AWG1, optical slices are transmitted to optical line terminals (OLTs). OLTs are located at central office (CO). Each OLT consists of data source, non-return-to-zero (NRZ) driver, and external Mach-Zehnder modulator (MZM). Each MZM has 5 dB insertion losses, 20 dB extinction ratio, modulation voltage Vπ of 5 Volts and maximum transmissivity offset voltage 2.5 Volts.
Generated bit sequence from data source is sent to NRZ driver where electrical NRZ pulses are formed. Afterwards formed electrical NRZ pulses are sent to MZM modulator. MZM modulates optical slices received from AWG1 and forms optical signal according to electrical drive signal. These formed optical pulses from all OLTs are coupled by AWG multiplexer (AWG2) and sent into standard optical single mode fiber (SMF) defined in ITU-T recommendation G.652 .
Information from OLT is transmitted to an optical network terminal (ONT) or user over the fiber optical transmission link called optical distribution network (ODN). ODN consists of AWG multiplexer, two optical attenuators, dispersion compensation module (DCM), SMF with length up to 20 km and two AWG demultiplexers. SMF fiber used in simulation setup has large effective core area Aeff=80 μm2, typical attenuation α=0.2 dB/km, dispersion coefficient D=16 ps/(nm km) and dispersion slope Dsl=0.07 ps/(nm2 km) at the reference wavelength λ=1550 nm.
As one can see in middle of Figure 6, two simulated CD compensation methods implemented in DCM module are FBG and DCF. In our research we use DCF fiber with Aeff=20 μm2, attenuation coefficient α=0.55 dB/km, dispersion coefficient D=-80 ps/(nm km) and dispersion slope Dsl=0.19 ps/(nm2 km) at the reference wavelength λ=1550 nm. For full compensation of chromatic dispersion accumulated by 10 km of SMF fiber we need about 2 km of DCF fiber. Simulated FBG is tunable in terms of both reference frequency and total dispersion value which can be compensated. Additional attenuator is used for simulation of FBG’s 3 dB insertion loss. For the most accurate results we used real parameters of standard DCF fiber and tunable FBG in our simulation setup. In the end of fiber optical link 8 or 16 optical channels are separated using AWG demultiplexer (AWG3) which is located in remote terminal (RT). Receiver section includes ONT units. Each ONT consists of sensitivity receiver with PIN photodiode (sensitivity S=-25 dBm at sensitivity reference error probability BER=10-10), Bessel electrical lowpass filter (3-dB electrical bandwidth BE=1.6 GHz), optical power meter and electrical probe to evaluate the quality of received optical data signal (capture eye diagram or spectrum). Optical signal is converted to electrical signal using PIN photodiode and filtered with Bessel electrical filter for noise reduction.
This section describes simulation scheme of more advanced, bidirectional 8-channel SS-WDM PON system with symmetrical 10 Gbit/s transmission speed per channel, gain saturated SOAs before optical modulators and chromatic dispersion compensation module (DCM) in downstream direction. For cost saving purposes DCM is not implemented in upstream direction because the influence of CD on 20 km long optical transmission line, where conventional laser based system (e.g. distributed feedback laser) is used may be considered as negligible . Total system’s capacity in downstream as well as in upstream direction is up to 80 Gbit/s.
Simulation scheme consists of 8 channels in downstream direction (see Fig. 7(a)) and 8 channels in upstream direction (see Fig. 7(b)). Channel spacing for both directions is chosen equal to 100 GHz frequency or 0.8 nm wavelength intervals. As one can see in Figure 7(a), broadband ASE light source, is spectrally sliced using 8-channel flattop AWG filter (AWG1) with channel spacing equal to 100 GHz. Using this AWG unit we can obtain equally arranged optical slices with dense channel interval. Insertion losses of AWG units are simulated using additional attenuation blocks. It is taken into account that simulated AWG multiplexers and demultiplexers are absolutely passive optical components with insertion loss up to 3 dB per unit. After spectrum slicing operation realized by first AWG1, optical slices are transmitted to optical line terminals (OLTs). OLTs are located at central office (CO).
Each transmitter (Tx) of OLT consists of data source, non-return-to-zero (NRZ) driver, semiconductor optical amplifier (SOA) and polarization-insensitive electro-absorption modulator (EAM). Compared with polarization-sensitive Mach–Zehnder modulator (MZM), the use of EAM gives a 3-dB gain in optical power and signal to noise ratio (SNR) . Each EAM has 3 dB insertion loss and maximum applied voltage is 5 Volts. SOA is gain saturated and amplifying incoming carrier spectral slice and used for suppression of excess intensity noise (EIN) which comes from the spontaneous–spontaneous beating between different wavelength components of the spectrally sliced incoherent broadband ASE light source .
Generated bit sequence from data source is sent to NRZ driver where electrical NRZ pulses are formed and sent to EAM to drive it. Each EAM modulate amplified incoming spectral slices according to electrical drive signal and forms optical pulses. These formed optical pulses from all OLT transmitters (OLT_TX1 to OLT_TX8) are coupled by AWG multiplexer (AWG2) and sent into standard 20 km long SMF fiber.
We also used additional gain saturated semiconductor optical amplifier (SOA) before modulator to suppress excess intensity noise (EIN) which originates from the ASE source .
In general SOAs, due to their structure can be characterized as a semiconductor lasers without feedback, as the same mechanism is used in order to amplify incident light as in laser diodes. Semiconductor optical amplifiers are the type of amplifiers in which electrical energy is applied as a pump to achieve population inversion in the active region, and amplification is achieved via the process of stimulated recombination luminescence. When the intensity of the input optical signal increases, at a certain point the whole carrier population of the upper energy level is occupied by the process of amplification. Therefore, for a specific SOA at the certain intensity level of input optical signal the amplifier gain starts to decrease. Such behavior of semiconductor amplifiers is known as gain saturation. When gain saturation occurs, characteristics of the gain become nonlinear (device gain becomes inversely proportional to the input intensity). Therefore, intensity noise suppression of input optical signal takes place when operating in such a nonlinear mode.
The main idea of intensity noise suppression is following: when intensity of input optical signal is lower, saturated SOA will provide higher optical gain coefficient than during the moment when input optical signal reaches its maximal value. Therefore, on the output of SOA amplitude difference between intensity drops and peaks will not be so large when signal is observed in time domain, and obtained optical radiation will be more similar to the output of continuous wavelength lasers, which are normally used as carriers in cases when external optical signal intensity modulation is used.
Information from OLT transmitter part is sent to receiver part of optical network terminals (ONT_RX1 to ONT_RX8) over the fiber optical transmission link referred as optical distribution network (ODN). In our case for both transmission directions ODN consists of AWG multiplexers/demultiplexers, optical attenuators, and 20 km long SMF spans. In downstream direction FBG DCM is used for chromatic dispersion compensation. It is tunable in terms of both reference frequency and total dispersion value which can be compensated. Additional attenuator is used for simulation of FBG’s insertion loss which is set 3 dB. In the end of downstream fiber optical link 8 optical channels are separated by using AWG demultiplexer (AWG3) which is located in remote terminal (RT).
Each ONT receiver (ONT_RX1 to ONT_RX8) consists of sensitivity receiver with PIN photodiode (sensitivity is-19 dBm at BER=10-12), Bessel electrical lowpass filter (3-dB bandwidth is 7.5 GHz), optical power meter and electrical scope to evaluate the quality of received signal (e.g. show eye pattern). Optical signal is converted to electrical signal using PIN photodiode and filtered with Bessel electrical filter for noise reduction.
As one can see in Fig. 7(b), which represents upstream optical transmission, conventional laser based system (where each user has its own individual light source) is realized. Each user’s transmitter (ONT_TX1 to ONT_TX8) has its own continuous wavelength light source modulated by Mach-Zehnder modulator. Upstream transmission from user side to OLT receiver block (OLT_RX1 to OLT_RX2) is realized over separate 20 km long optical fiber span. OLT receiver structure is the same as for ONT receiver. As it mentioned before, dispersion compensation is not used in upstream transmission for cost reduction and network architecture’s complexity reduction purposes.
In the simulation model it was decided to set all geometrical and material parameters of the active layer of the SOA as it was used previously in . There these parameters were set as in a standard InGaAlAs travelling wave SOA with negligible residual facet reflectivity.
The only parameters which values were changed were the length of active layer and input bias current. In the proposed solution it was required to obtain gain saturation of the amplifier for input optical power of at least 5.5 dBm. Therefore, it was required to reduce the maximal optical gain that can be provided by the SOA. This was achieved by shortening the length of active layer of the SOA from 750 µm to 100 µm, in such a way significantly reducing the population of available carriers in the active layer, hence also lowering the value of amplifier’s maximal gain. It was required to set appropriate value of input bias current to obtain such level of population inversion in active layer, which would provide BER values below the 10-12 threshold for detected signals in all ONTs.
Best results were obtained if pumping current of 370 mA was used for our SOAs located in OLTs transmitters. As mentioned in introduction part, when intensity of input optical signal is lower, saturated SOA will provide higher optical gain coefficient than during the moment when input optical signal reaches its maximal value. Instantaneous optical power at input and output of the SOA is represented in Figures 8(a) and 8(b) respectively.
It can be clearly seen that after processing through the SOA intensity fluctuations of the spectrally sliced optical flow have been reduced significantly. Unfortunately not all of the intensity noise was suppressed. The provided solution of intensity noise suppression applies only to the intensity noise which frequency is low in respect to the spontaneous carrier lifetime. The SOA carrier changes are rate limited by their lifetimes, and carrier changes cannot keep up with the high frequency intensity changes. Suppression of intensity noise, which frequency is high in respect to the carrier lifetime, requires more detailed investigation and a different approach for the SOA configuration .
Spectrum at the input and output of the SOA amplifier can be observed in Figure 9. By comparing both curves, it can be seen that spectrum of spectral slice (i.e. spectrum of spectral sliced carriers) at the output of the SOA is clearly broadened. Also, upper part (peak) of the spectral slice obtained a visible slope with the intensity peak shifted towards to lower frequency.
The spectral broadening of spectral slices, each containing multiple carriers, is caused by occurrence of two non-linear effects which became well seen as the SOA began to operate in the gain saturated mode: intra-channel four wave mixing (FWM) and self-phase modulation (SPM) of the carriers. Appearance of slope at the peak of spectral-sliced carriers is related to SPM, as phase modulation also has the effect of shifting the peak power toward longer wavelengths (lower frequencies) as the light travels through the SOA .
In Figure 10 it is shown spectrum of downstream optical signals on the output of all OLT transmitters (before multiplexing by AWG2 unit) and spectrum in the input of ONTs (after demultiplexing by AWG3).
We found that optimal 3-dB bandwidth value of flat-top type AWG units for maximal quality of received signal and acceptably low crosstalk between channels must be about 90 GHz for spectrum sliced downstream and 50 GHz for laser based upstream system . In case of ASE spectral slicing flat-top type AWG units shows good filtering performance because of its amplitude transfer function which provides good WDM channel separation at the same time passing sufficiently high optical power. The larger is width of spectral slice the better performance we can obtain. However, arising crosstalk between channels must be taken into account in this case . It means that there is a tradeoff between optical filter bandwidth and crosstalk between spectrum sliced channels which can result in system’s performance limitations.
In Figure 11 is shown spectrum of optical signals on the output of laser based ONT transmitters and in the input of OLT receivers after demultiplexing by AWG4 unit. As one can see all 8 upstream channel are well separated and any major spectral distortions is not observed. It must be mentioned, that for CD compensation and performance as well as network reach improvement of proposed 8-channel bidirectional SS-WDM PON system fiber Bragg grating dispersion compensation module (FBG DCM) was used. It is seen in Figure below (see Fig. 12(a)) that without CD compensation system performance is poor and successful data transmission is not possible.
Theoretical value of accumulated CD for 20 km SMF fiber span is about 320 ps/nm. We found that optimal CD amount which must be compensated by FBG DCM for downstream 20 km SMF span is 310 ps/nm. Usage of gain saturated SOA for noise suppression was another key element to build spectrum sliced WDM-PON system operating below desired BER threshold. As it is shown in Figure 12(b), without gain saturated SOA downstream signal performance already after transmitter (back-to-back (B2B) measurement) is seriously affected by noise and BER is well above defined threshold (BER<10-12).
When SOA amplifier and FBG DCM were used for performance improvement of downstream direction, quality of obtained signal in receiver was sufficient and BER was below minimal threshold (see Fig. 13(a)). It is clearly seen that noise at logical “1” level is suppressed and eye opening became noticeably larger. It is due to the fact that gain saturated SOA is amplifying low intensity signal more than comparatively high signal and compress intensity fluctuations existing in input light signal, which in our case is spectrally sliced ASE. In Figure 13(b) we see, that eye diagram of upstream signal is quite good and eye is wide open. Some signal distortion due to CD is also seen, but for transmission distance of 20 km it can be considered as negligible.
For evaluation of downstream spectrum sliced system performance BER curve was measured (see Fig. 14). It can be observed, that power penalty to receive optical signal for 8-channel spectrum sliced downstream with desired BER<10-12 after 20 km transmission is 3.7 dB. This penalty is introduced by the cross talk effects, dispersion and due to the noise-like nature of broadband ASE light source.
In upstream transmission direction (see Fig. 15) minimal received power to obtain BER<10-12 must be more than-18.9 dBm for B2B configuration and-17.6 dBm for 20 km SMF transmission line.
As one can see in Figure 15, power penalty is about 1.3 dB, which can be explained by the influence of CD which broadens optical pulses as they travel along the optical fiber span.
There are compared two different CD compensation methods for improvement of maximal reach and performance of 8-channel and 16-channel AWG filtered SS-WDM PON system with flattened ASE broadband light source. In Figures 16 and 17 it is shown optical spectra on the output of ASE source and spectra after each flat-top AWG unit. We found that optimal 3-dB bandwidth value of flat-top type AWG unit for maximal system performance must be about 90 GHz for both SS-WDM PON systems. In case of ASE spectral slicing flat-top type AWG unit shows good filtering performance and high OSNR because of its amplitude transfer function that provides good WDM channel separation at the same time passing sufficient high optical power .
First, we studied both SS-WDM PON systems in back to back (B2B) configuration (without DCM and SMF fiber span in ODN) which was the reference to which we compared all other measurements.
In Figures 18(a) and 19(a) we show the BER versus optical received power for 2.5 Gbit/s 8-channel and 16-channel SS-WDM PON downlink transmission in B2B configuration as well as after 10 km 20 km transmission without chromatic dispersion compensation. In Figures 18(b) and 18(c), as well as 19(b) and 19(c) we show the BER versus optical received power for 2.5 Gbit/s 8-channel and 16-channel SS-WDM PON downlink transmission after 10 km and 20 km transmission realizing CD compensation by implementing DCF and FBG respectively.
As one can see in Figure 18(a), power penalty to receive optical signal for 8-channel system with BER<10-10 after 10 km transmission is 2.7 dB. The minimal BER value of received signal after 20 km transmission is above defined BER threshold and it means that qualitative data transmission is not possible in this case.
In Figure 20 are shown eye diagrams of received signal in B2B configuration (BER<10-10) and after 20 km long SMF span (here BER>10-10) without CD compensation.
Theoretical value of accumulated CD for 10 km SMF fiber span is about 160 ps/nm and 320 ps/nm for 20 km span. We found that optimal CD compensation amount that must be compensated by FBG for 10 km span is 125 ps/nm for both 8-channel and 16-channel systems. For 20 km SMF span we must compensate 290 ps/nm in 8-channel system and 280 ps/nm in 16-channel system. In order to make optimal CD compensation in 8-channel system we used 2.5 km DCF fiber before 10 km long SMF fiber span, and 4.7 km DCF fiber before 20 km span. For 16-channel system it was used 2 km long DCF before 10 km span and 4.5 km DCF before 20 km SMF span. In case of FBG it was used incomplete CD compensation, but in case of DCF the optical line was overcompensated.
Usage of DCF for accumulated CD compensation provides extension of network reach from 10 km to 20 km as well as improvement of network performance for both line lengths, (see Fig. 18(b)). In this figure it is seen that, for a BER of 10-10 using DCF for CD compensation, the power penalty to pass from 10 km transmission to 20 km transmission is 1.3 dB. This penalty is introduced by the cross talk effects, dispersion and due to the noise-like nature of broadband ASE light source. The power penalty for BER of 10-10 to pass from 10 km to 20 km is 1.1 dB in case when FBG is used for compensation of accumulated CD for reach extension and performance improvement of proposed 8-channel SS-WDM PON system, (see Fig. 18(c)). The comparison between DCF and FBG in terms of performance improvement shows that FBG provides higher performance improvement.
After investigation of 8-channel SS-WDM PON system the simulation model of 16-channel system was realized. Results show that the overall performance of this system is slightly lower than 8-channel system using the same ASE source. As one can see in Figure 19(a), power penalty to receive optical signal after 10 km with defined BER is 2.8 dB. Performance of transmission system is too low to provide data transmission over 20 km SMF fiber span with BER<10-10 (see Fig. 21).
It is shown (see Fig. 19(b)) that usage of DCF fiber for CD compensation improve access system’s performance and it is capable to operate over 20 km SMF span with BER<10-10. In case when DCF is used for CD compensation the power penalty for BER of 10-10 to pass from 10 km to 20 km is 1.5 dB. If FBG is used (see Fig. 19(c)) power penalty is 1 dB. When FBG is used for CD compensation in 16-channel system BER of 10-10 is reached at-17 dBm received optical power after 10 km fiber span and at-16 dBm power after 20 km long span.
When DCF fiber is used for CD compensation minimal received power must be-16.7 dBm for 10 km SMF fiber span and-15.2 dBm for 20 km fiber span or transmission line. As it is seen from obtained results performance between FBG and DCF for CD compensation and system efficiency improvement of 8-channel and 16-channel SS-WDM PON systems are close. However, FBG provides better CD compensation performance. Instead of DCF fiber it can be used at higher optical powers without inducing nonlinear optical effects and additional signal distortion which can occur due to small effective core area of a DCF fiber.
We report on causes of latency in fiber optical networks and solutions how to minimize it. Latency is a critical parameter in a wide set of applications like financial transactions, telemedicine, cloud services and other real-time applications which requires optical transmission line with the smallest available time delay. It was shown that latency can be greatly reduced by replacing conventional network components with low-latency components and by optimizing optical network path trying to keep it short as possible. Future telecommunication networks must be cost and energy efficient, in the same time supporting high bandwidths and low latency. We showed the realization of cost effective up to 16-channel dense WDM-PON system where spectrum sliced spectrally uniform ASE source is used as a light source. It is shown the design of this broadband spectrally-uniform ASE source with+23 dBm output power in system’s operating wavelength range by using two EDFA amplifiers connected in cascade mode. We also demonstrated excess intensity noise (EIN) suppression by using additional SOA in transmitter section as well as experimentally compared two different chromatic compensation methods, namely dispersion compensation fiber (DCF) and fiber Bragg grating (FBG) for implementation of signal performance improvement in high-speed spectrum sliced optical access systems.
Nowadays, the global navigation satellite systems (GNSSs) are widely used in many human activities, particularly for geodetic positioning and navigation, as well for atmospheric monitoring in scientific research. The GNSS systems have been used to obtain the ionospheric total electron content (TEC) often in near real-time and in global scale because the networks ground-based receivers cover large geographic areas. In the last decades, TEC information has given a great advance in the understanding of the ionospheric structuring and improving our forecasting capacity, which has revolutionized the ionospheric studies. Particularly, the behavior of the ionosphere during geomagnetic disturbed periods has been extensively investigated, showing that F-region response to geomagnetic storms is very complex in space and time, but a general morphology and physical processes have been defined (e.g., [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]; and references therein).
The ionospheric knowledge has been used to improve the GNSS positional accuracy, which can be strongly degraded under severe atmospheric disturbances because they can suddenly change the satellite geometry that is essential for geodetic position, in particular for kinematic precise point position (PPP), and are a limiting factor to achieve centimeter accuracy (e.g., [14, 15, 16]). The main atmospheric disturbances that affect the GNSS quality signals are the ionospheric steep density gradients, signatures of irregularities of the electron density distribution. Special attention has been given to the ionospheric irregularities investigation, which can vary on a wide range of scale sizes, from centimeters to hundreds of kilometers. The formation and the temporal/spatial evolution of these irregularities affect the propagation of radio signals, causing cycle slips and loss of lock on GNSS receivers and degrading the performance of radio communication and navigation systems [17, 18]. At L-band, amplitude scintillations are due to irregularities with a scale size from hundreds of meters down to tens of meters (according to Fresnel’s filtering mechanism), while phase scintillations are caused by structures from a few hundred meters to several kilometers (see, e.g., ). In addition, in your way down to the ground, the radio signals could interfere with itself due small changes in their way along the scattered ray paths, resulting in a sort of “space multipath” . The overall of these atmospheric influences can produce rapid fluctuations in the amplitude and phase of GNSS signals, which are known as ionospheric scintillations.
The investigation of scintillations has shown that their activity is stronger at latitudes within the equatorial ionization anomaly (EIA), particularly during post-sunset hours when plasma bubbles are formed in the equatorial F-region driven by the Rayleigh-Taylor instability [20, 21]. Recently, the formation of ionospheric irregularities and plasma bubbles at equatorial region also have shown that they can be driven by gravity waves [22, 23]. The scintillations have been extensively studied at different longitudinal sectors and latitudes, showing a strong dependence on magnetic local time, season, magnetic activity, solar cycle, and geographic location (e.g., [6, 8, 9, 10, 11, 12, 13, 19, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]; and references therein).
This chapter provides the characterization of ionospheric scintillations observed with GNSS networks in the South American sector (e.g., [34, 35, 37, 38, 39, 40]). The chapter is organized as follows: Section 2 presents the ionospheric variability, Section 3 presents the ionospheric scintillation indices and climatology in South American sector, Section 4 presents the impact of scintillations in a new method for high accurate single frequency precise point positioning (PPP), and finally a discussion section.
The solar heating throughout the atmosphere causes large-scale variations associated with diurnal and semidiurnal tides in the thermosphere , while the extreme ultraviolet radiation (EUV) forms the ionosphere. The physical processes in the thermosphere are primarily driven by solar and magnetic disturbances , which influence the ionospheric production and recombination, as well transport and frictional heating. The ionospheric conditions have been studied for decades, and particularly there is a good understanding of F-region conditions associated with seasonal, solar cycle, and level of magnetic activity variations. Despite that the predictive capability of its condition is still very poor because the variations appear over a wide range of timescales going from minutes to several days and also depend on the magnetic local time and geographic location. Even during quiet times, that is, under undisturbed geomagnetic conditions, significant ionospheric variability is observed. The low latitude ionosphere is controlled by electrodynamic plasma (E × B) drifts driven by thermospheric neutral winds (e.g., [43, 44, 45]). The zonal electric fields drive strong daytime E-region eastward currents in the equatorial region, which form two narrow latitudinal bands centered at the dip equator that are called equatorial electrojet (e.g., ). The zonal electric fields drive equatorial E and F region vertical plasma drifts (e.g., ) that lift the ionospheric plasma at the dip equator, which goes down following the magnetic field lines leading density enhancements located at ~10–20° from the magnetic equator. The overall process, the plasma lifting followed by their diffusion along the geomagnetic field lines and the formation of the two density maxima away from the equator, is called “fountain effect,” and the denser regions are called the crests of the equatorial ionospheric (or ionization) anomaly (EIA) . The thermospheric winds are highly variable because they are driven by changes in the global tidal forcing, and effects of irregular winds, planetary and gravity waves. The planetary waves and tides have been identified as relevant factors affecting the electrodynamics of the lower thermosphere (e.g., [49, 50, 51, 52, 53, 54, 55]).
The ionospheric F-region often becomes turbulent and develops electron density irregularities during post-sunset hours . During the day, the electrical field is eastward and it reverses to the west after sunset, but during sunset an enhanced eastward electric field develops, the so-called pre-reversal enhancement (PRE) [44, 57, 58]. The PRE drives an upward vertical plasma drift at magnetic equatorial region, and the bottom of the F-region becomes unstable to the Rayleigh-Taylor instability , developing large-scale plasma depletions (plasma bubbles). These bubbles ascend upwards over the magnetic equator, causing a redistribution of ionization similar to the fountain effect. The latitudinal extension of the plasma bubbles is defined by the upper height limit they reach in their rise up above the magnetic equator  and can intersect the crests of the EIA, where the steeper density gradients on the edges of the plasma bubbles favor the generation of smaller scale irregularities [56, 61, 62]. The plasma bubbles practically disappear after local midnight and are most intense during the equinoctial months and during the solar maximum years [63, 64]. These conditions favor the scintillations to occur most frequent and severe around the EIA crests, in particular after sunset due to the generation of irregularities caused by the intersection of the plasma bubbles with the regions of larger background electron density.
The ionosphere can be strongly disturbed during geomagnetic storm-time periods. These storms are terrestrial magnetospheric perturbations caused by the impact of interplanetary coronal mass ejections (ICMEs), and corotating interaction regions associated with high-speed streams (HSS), which are the most geo-effective solar wind phenomena. The coupling process involving the solar wind, the interplanetary magnetic field (IMF) and the magnetosphere strongly affects the high latitude ionospheric electrodynamics. The magnetosphere compression during geomagnetic storms induces intense electric fields and increases its convection. During these processes, the interplanetary electric field (IEF) is mapped along the magnetic field lines to the high latitude ionosphere, but can also propagate across them and promptly appears in the low latitude ionosphere, when it is called of prompt penetration electric field (PPEF). The effect of PPEF at the equatorial ionosphere has been observed during the first hours of the main phase of geomagnetic storms, suggesting a long-duration penetration of interplanetary electric field to the low-latitude ionosphere without shielding (e.g., [5, 65, 66, 67, 68]). The equatorial ionosphere under the effect of the PPEFs is convected upward in the dayside and downward in the night side . The PPEFs are stronger than the fields associated with the normal fountain effect resulting in a higher elevation of the equatorial plasma , and consequently the crests of the EIA departs more from magnetic equator and can reach middle latitudes (~20–30°). At high latitudes, the precipitation of energetic particles enhances ionospheric conductivities and generates intense electrical currents in the auroral zone . The dissipation of these currents by the Joule effect heats the local plasma that expands, changing the lower thermospheric composition and driving large-scale neutral winds [3, 4, 42]. Thus, major geomagnetic storms results in a large-scale ionospheric thermal plasma redistribution involving all latitudes from the equatorial through the polar region. Particularly F2-region shows very complicated spatial and temporal behavior (e.g., ), but a general morphology and physical processes have been established (e.g., [1, 2, 3, 4, 7, 9]). F2-region variations during geomagnetic disturbed periods are called ionospheric storms and are detected as an increase (positive) or depletion (negative) of electron density associated with electrodynamics processes or neutral composition changes (e.g., ), respectively. Their morphology is a function of the amount of energy inputted in the high latitude during the main phase of geomagnetic storm , as well of the station latitude and longitude, local time of storm onset, storm time and season.
Scintillations are rapid fluctuations in the amplitude and phase of GNSS signals produced by the ionospheric irregularities, which are responsible by the refraction and diffraction of trans-ionospheric signals [20, 70].
The ionospheric scintillations are evaluated from 60 s amplitude (S4) and phase scintillation (Phi60) indices, which give information about intermediate (around hundreds of meters) and large (above hundreds of meters)-scale size irregularities, respectively. The scintillation indices are obtained by using GNSS dual-frequency receivers recording data at a high data sampling rate of 50 Hz, which also compute the total electron content values.
The S4 index can be interpreted as the standard deviation of the received power (C/NO) normalized by its mean value and the Phi60 index is the standard deviation (in radians) of the carrier phase computed over 60 s time interval. The indices are obtained as Eqs. (1) and (2) .
where <> denotes 60 s average, I is the signal intensity and ∅ the signal phase at GPS L1 frequency (1.575 GHz) sampled at 20 ms (50 Hz).
The S4 and Phi60 indices are used to classify ionospheric scintillation severity, which can be separated in three categories: strong, moderate and weak scintillations, for example, as shown in Table 1.
|Weak||0.1 < S4 < 0.25 or 0.1 < Phi60 < 0.25|
|Moderate||0.25 < S4 < 0.7 or 0.25 < Phi60 < 0.7|
|Strong||S4 > 0.7 or Phi60 > 0.7|
The morphology of the Earth magnetic field favors the occurrence of strong scintillations at high latitudes, and intense at equatorial region between about 20°N and 20°S magnetic latitudes [20, 72]. Particularly in the South American sector, the F-region irregularities present peculiarities due to the large longitudinal variation in the magnetic declination angle [38, 73, 74], as well by the influence of the South American Magnetic Anomaly (SAMA). The SAMA is the region on the Earth where the magnetic field has the lowest intensity values, allowing enhancement of energetic particle precipitation into the atmosphere . The enhanced ionization in the SAMA region produced by the particle precipitation is a regular feature, which modifies the quiet ionospheric physical conditions increasing the F-layer vertical drift over the eastern sector as compared to the western sector of South America, which can be drastically modified during magnetospheric disturbances (e.g., ; and references therein).
The investigation in the South America sector has been improved in the last decades after using GNSS receivers networks dedicated to monitor ionospheric scintillations. Today, there are three GNSS networks, the GPS Low-Latitude Ionospheric Sensor Network (LISN) that is operating since November 2011 with an array of 45 receivers , 15 of them over the Brazilian territory, where they are complimented with one array of 12 GPS Ionospheric Scintillation Monitoring Receivers (ISMR) of the GPS Scintillation Monitors network (SCINTMON)  and other with 10 ISMR of the Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America and Countering GNSS high Accuracy applications Limitations due to Ionospheric disturbances in Brazil (CIGALA/CALIBRA) project . At high latitude, these networks are complimented with GPS ionospheric scintillation and total electron content (TEC) monitor receivers (GISTM) that covers a large area from sub-equatorial Latin America to the South Pole .
Ionospheric irregularities commonly appear in the regions of enhanced or depleted electron density. These regions are associated with the crests of the EIA anomaly located at latitudes ~15–20° from the magnetic equator, where strong scintillations have been observed particularly during sunset hours. Before GNSS era, Aarons  using ionosondes and VHF systems showed the scintillation intensities, produced by smaller scale ionospheric irregularities, were stronger in the EIA crests after sunset hours under the influence of plasma bubbles. A comprehensive study of the occurrence of irregularities over the south EIA crest in Brazil during two decades was reported by Sobral et al. , which shows the plasma bubbles occurrence has a broad maximum around summer months (from September to April), with a significant increase from low to high solar activity levels during the equinoctial months of March-April and September-October. De Paula et al. , using GPS L1-band receivers in the Brazilian territory, reported the characteristics of small-scale irregularities that produced strong scintillations in the post-sunset equatorial ionosphere from 1997 to 2002, the ascending phase of the solar cycle 23. They reported that the ionospheric irregularities are stronger in the southern crest of the EIA, and present a seasonal variation occurring predominantly from September to March during magnetic quiet periods, being more intense during December (local summer). They also show a large longitudinal variation in the South American sector showing that the irregularities are most intense in the EIA crest in Brazilian sector than over Argentinean sector, which is attributed to the large longitudinal variation of magnetic declination in this sector. During quiet magnetic periods, the irregularities occur in the sunset-midnight local time sector while during magnetic storms their occurrence can extend to the midnight-sunrise sector. Akala et al.  using a chain of GPS receivers along the western longitude sector of South America, during different levels of solar activity, also shows the scintillations occur predominantly at post-sunset hours and decay before or around local midnight, with stronger activity and longer durations in the months of March and January, which means in the March Equinox and December solstice; and in particular the station near northern crest of the EIA recorded the highest occurrences of scintillation especially during periods of high solar activity.
Spogli et al. , using GISTM receivers located between South America, South Atlantic Ocean and Antarctica, defined crucial areas in the ionosphere where the probability of scintillation occurrences is higher. These areas were called ionospheric scintillation “hot-spots” and were defined using a climatological representation given by the Ground Based Scintillation Climatology (GBSC) technique . They showed that there are two main hot-spots over South America, first one associated with the post-sunset (POST) hours at low latitudes located in the magnetic latitude (MLAT) range 15–25°S and magnetic local time (MLT) between 20 and 24 h, and another one associated with particle precipitation region (SAMA) nearby SAMA region located in the MLAT range 22–24°S and MLT between 0 and 24 h. At high latitudes, they identified three scintillation hot spots, one associated with particle precipitation region in the polar cusp (CUSP, MLAT: 74–82°S and MLT: 10–14 h), other associated with place where the irregularities are induced by reconnection from the magnetotail (MLAT: 68–82°S, MLT: 20–4 h), and other one associated with polar cap patches (PATC, MLAT: 82–90°S, MLT: 0–24 h). The POST hot spot shows scintillation intensifications in March and November, in agreement with the intensification of pre-reversal at equinoxes (e.g., ). The hot-spot intensification is stronger in November probably due to the superposition effects associated with the spring equinox and the local summer. The SAMA hot spot also shows one stronger enhancement in November similarly the observed at POST hot spot. They also confirm that in the Brazilian longitudes, the irregularities are more intense in the September (spring) equinox and summer months . At high latitude the two main hot spots identified as CUSP and PATC show enhancements at equinoxes that are attributed to direct particle precipitation.
Muella et al.  investigated the scintillation occurrence from 2002 to 2006, during the descending phase of the 23rd solar cycle, at two sites located in the inner regions of the northern and southern crests of the equatorial ionization anomaly in the Brazilian sector. They showed that the scintillation occurrences during sunset hours present a north-south asymmetry, being ~10% higher over the southern EIA crest than over northern one during solar maximum. This asymmetry was considered to be a possible influence of the SAMA on the scintillation activity. The scintillation occurrence also showed a broad minimum in June and maximum in December over both crests. On a recent investigation of scintillation occurrence at Cachoeira Paulista near the south EIA crest, covering almost the two last solar cycles (1998–2014), Muella et al.  showed that the maximum occurrence of scintillations observed during the peak of 23rd solar cycle was 20% higher than that one observed for the 24th, which was the weakest cycle in the last century. This behavior can be attributed to the scintillation intensity dependence on the electron density gradients and the thickness of irregularity layer, which are driven by the intensity of the solar extreme ultraviolet radiation (EUV). In addition, the fewer occurrences of scintillation in the maximum of the 24th solar cycle at Cachoeira Paulista could also be associated with the secular variation in the dip latitude, which changed ~3° in south direction from 1997 to 2014.These results on the long-term trend analysis and climatology of scintillations at the EIA region are shown in Figure 1. The colored contours in the upper panel of Figure 1 show the nocturnal occurrence statistics of scintillation from 1998 to 2014 for S4 > 0.2 as function of universal time (UT = LT + 3 h) and the mean F10.7 cm solar flux index. For the type of the GPS receiver used to measure scintillations, the threshold of S4> 0.2 can be considered above the level of multipath and noise effects, which may produce very weak scintillations (S4< 0.2). The middle panel shows the occurrence for the strongest levels of scintillations (threshold of S4> 0.5), whereas the color scale bar indicates the percentage of occurrence used in the plots of scintillation statistics. The monthly mean variation of the F10.7 cm solar flux is shown in the lower panel and depicts its changes from the ascending phase of solar cycle 23rd to the maximum of solar cycle 24th. Figure 1 reveals that the patches of larger occurrence of scintillations are observed from 23:00 to 04:00 UT between the months of September and March and mainly around the solar maximum years.
The climatology of the onset time of ionospheric scintillations near the southern crest of EIA over Brazilian territory, covering a period between solar cycles 23 and 24, showed that their start time is about 40 min earlier in the months of November and December when compared to January and February, suggesting an association with the ionospheric pre-reversal vertical drift (PRVD) magnitude and time .
An investigation of the equatorial scintillations over São Luis (2.33°S, 44.21°W, dip latitude 1.3°S) was done by Muella et al.  during different solar activity levels of the 23rd solar cycle (1999–2006). The study showed the scintillations occurred more frequently during the years of high solar activity, but strong scintillation variability was also observed during the descending phase of the solar cycle. The scintillations occurred predominantly during pre-midnight hours with a broad maximum in the summer. They observed a weak level of scintillations all over the year, however, during the winter months near the years of solar maximum, some stronger levels of scintillations were observed at comparable rate with the weak scintillations.
The ionospheric scintillations are driven by zonal drift of the irregularities, so the investigation of the spatial and temporal variations of the irregularities have been done in the last decades. The study of the zonal drift driven scintillations in the South American sector has shown that: a latitudinal gradient in the irregularity zonal velocities is associated with the vertical shear of the zonal drift in the topside equatorial ionosphere ; at two magnetic conjugate sites over Brazilian territory, the magnitude of the zonal velocities in the site inside the SAMA region was ~12% larger than in the conjugate one ; during nighttime, there is a strong correlation between neutral winds and scintillation drifts near magnetic equator ; the irregularity of the zonal drifts shows a negative gradient with increasing geomagnetic latitude ; and that the magnitude of the zonal velocities might be reduced at the inner regions of the EIA due to the latitudinal variation in the ion drag force . The nighttime zonal drift velocities of the ionospheric irregularities increase in association with increasing EUV solar radiation [37, 84, 85], which can be produced by the fact that in years of higher solar activity, the thermospheric zonal wind velocities are higher enhancing the solar thermal tide .
The ionospheric electrodynamics conditions depend on magnetosphere-ionosphere-thermosphere system, which can be strongly disturbed during geomagnetic storms. During geomagnetic storms, the magnetosphere is compressed inducing intense electric fields and increasing the magnetospheric convection. The strongest geomagnetic storms are mainly produced by the impact of solar wind disturbances associated with geoeffective solar coronal mass ejections (CMEs) and coronal hole solar high speed streams (HSS) on the magnetosphere, which results in a highly inhomogeneous ionosphere, producing steep electron density gradients and irregularities. The ionospheric electron density distribution changes as a function of the solar wind input energy in the high latitude upper atmosphere, which occurs during the main phase of the geomagnetic storms .
The spatial and temporal variations of F2-region during geomagnetic storms are called ionospheric storms, and their morphology depends on the site location, local time of geomagnetic storm onset, storm time and season. Despite the very complex ionospheric behavior during disturbed periods, there is a general understanding about its morphology and physical processes (e.g., [1, 2, 3, 4, 7, 9]). The negative phase of ionospheric storm is attributed to changes in the thermosphere at middle and high latitudes due to the heating in the auroral zone, mainly by Joule dissipation , occurring at all seasons but in winter. In contrast, the positive phase occurs at middle and low latitudes mainly in winter season and involves more complex physical processes associated with uplifting due vertical drift, plasma fluxes from the plasma sphere, and downwelling produced by storm-induced thermosphere circulation at low latitudes (e.g., [3, 7, 9]). The vertical drift during daytime is controlled by equatorward winds at middle latitudes during the winter, and at equatorial region is driven by the EIA anomaly in association with prompt penetration electric field effect (PPEF) , resulting in an enhancement of the electron concentration because the photoionization is still operating (e.g., [3, 88]).
At middle latitudes, TEC enhancements are observed during the main phase of geomagnetic storms in the dusk sector, which are called storm-enhanced density (SED), and have been associated with large-scale redistribution of ionospheric plasma, covering a large extension from equatorial to polar region . This phenomenon has been observed during the first hours of the main phase storm when the fountain effect at the equatorial region is reinforced by the PPEF, moving EIA crests from low to middle latitudes. The middle latitude EIA crest at dusk sector, under electrodynamic processes, has its plasma redistributed in longitude and latitude generating plumes of SED , which can be transported into dayside cusp where enter the polar cap and form the called tongue of ionization (TOI; [8, 10, 11, 12, 13, 24, 89]; and references there in) at polar region. The ionospheric plasma redistribution during geomagnetic storms is a function of their intensity, magnetic local time, storm time, latitude and season.
The ionospheric regions affected by impact of geomagnetic storms show a strong intensification of scintillation occurrence. At high and middle latitudes, these regions are the night side auroral oval due to the particle precipitation events [12, 91, 92], the cusp on the dayside in association with SED, and the polar cap in association with TOI, while at equatorial latitudes the scintillations are associated with regions under the influence of the EIA anomaly. Similarly, the ionospheric storms, the occurrence of scintillations during geomagnetic disturbed periods also are function of magnetic local time, season, magnetic activity, solar cycle, and geographic location [12, 13, 19, 25, 26]. The ionospheric irregularities can be inhibited during magnetic storms with main phase occurring during daylight hours, but can be intensified during any season when main phase storm coincides with the hours of the pre-reversal electric field is maximum (e.g., ). De Paula et al.  from an investigation of small scale irregularities (~400 m) at equatorial and low latitudes over Brazilian territory obtained that during geomagnetic storms they can occur at any epoch of the year, present largest intensities in the south EIA crest, could extend from sunset-midnight to midnight-sunrise sector during some storms, are enhanced during PPEF occurring during post sunset hours, and can be suppressed during daytime main phase storm under the disturbance dynamo effect. In association with geomagnetic storms, the large-scale irregularities (few km) have shown a seasonal variability . On the other hand, an investigation of the F-region under the impact of the geomagnetic storm occurred on June 2013, from equatorial to middle latitudes in both hemispheres over American sector, showed that the ionospheric irregularities were observed confined in the equatorial region before and during the storm, which shows this storm did not affect the generation or suppression of irregularities .
An investigation of the 26–27 September 2011 moderately intense geomagnetic storm impact in the ionosphere at middle and high latitudes in the South American sector showed that during its dayside main phase two SEDs were observed at middle latitudes . These SEDs were attributed to a combination of processes, including the PPEF effect from low latitudes during a couple of hours just after the storm onset, and dominated by the disturbance dynamo effect from high latitudes during its evolution. The plumes of these SEDs were located near the dayside cusp and result in TOI formations observed in nightside polar cap region. In association with the middle latitude SEDs and polar cap TOIs were observed strong ionospheric scintillations.
New analysis about the GNSS positioning was performed by Prol et al.  in order to evaluate the ionospheric delay retrieved by a new method for TEC calibration when this method was applied to correct the single-frequency PPP. The results revealed the possibility of performing the single-frequency PPP corrected by a TEC calibration and obtaining a similar accuracy to the double-frequency PPP. It suggested that almost all of the first-order ionospheric effect was eliminated by the TEC calibration method. Additionally, the single-frequency PPP corrected by the new method was very sensitive to the impact of the ionospheric scintillations. In fact, the proposed single-frequency PPP appeared to be even more sensitive to ionospheric scintillation in comparison with the single-frequency PPP corrected by traditional ionospheric models and the double-frequency PPP. In order to present the impact of the ionospheric scintillations in the proposed single-frequency PPP, this section describes the developed method for TEC calibration and some experiments and results.
TEC can be expressed as the integral of the electron density along the path between the GNSS satellite () and the receiving antenna (), in a column whose cross sectional is equivalent to 1 m2. It can be written as:
being the ionospheric delay given by the following relation with TEC:
where represents the ionospheric delay and the signal frequency.
The ionospheric delay is related to the GNSS observations by the following equation for the code:
and the following equation for the carrier phase:
where the frequency-dependent terms are referred by , is the geometric distance, is the speed of the light in vacuum, and are the receiver and satellite clock errors, refers to the tropospheric delay, is the multipath, is an ambiguity term, is the wavelength and represents the noise in code () and phase ().
The method to estimate TEC (Eq. (3)) using GNSS observations (Eqs. (5) and (6)), is performed in three steps. In the first step, a phase leveling estimation based on the code information provides the ambiguity terms. In this regard, the difference between ambiguities () of two GPS frequencies (L1 and L2) in a unique arc of data is calculated through:
where only arcs with a minimum of 5 min of continuous data are used. Once the term is obtained for all arcs of a specific day, the receiver DCB (Differential Code Bias - ) is obtained by the daily weighted mean of the phase difference (), the initial TEC, the leveling ambiguity and the satellite DCB (). The receiver DCB is derived by the following weighted mean:
being the standard deviation of the initial TEC, which is derived from the Global Ionospheric Maps (GIMs) of the International GNSS Service (IGS) and their root-mean-square (RMS) maps. In addition, the satellite DCB is obtained from GIMs. Therefore, as the satellite DCB and the initial TEC are obtained from GIMs, it is expected that the estimated receiver DCB is related to the DCB referential frame defined by IGS.
Once the ambiguity leveling (first step) and the receiver DCB estimation (second step) are done, the third step consists in the TEC estimation. TEC is directly calculated along the path of the GNSS signal with the following equation:
where the TEC is derived for each GNSS observation, that is, TEC is estimated with the same time resolution as the phase and code collection rate.
Two close GNSS stations located at Rio de Janeiro in Brazil have been selected to make the experiments. The TEC estimation procedure was performed in the station RIOD (lat. Mag. 36.47° S), and the ionospheric delay correction was applied in the station ONRJ, which is located 12 km away. Using the configuration of short distances apart, it is possible to mitigate the problems of spatial gradients of the ionosphere. In addition, the receivers and antennas are from different brands in order to avoid possible correlations between clock and ambiguities of the stations. In this way, it has been as much as possible to isolate the degradation of the ionospheric scintillations in the PPP results.
Prol et al.  evaluated the analysis of the performance of the new TEC calibration procedure when applied to PPP for 120 days with six distinct configurations of base and rover stations, and the TEC performance is assessed by applying the estimated TEC from the base station to correct the ionospheric delay in a nearby rover receiver. Just to show the potentiality of the new procedure, here are shown the results of the analysis for two specific days with and without ionospheric scintillations.
Figure 2 shows an example of the calibrated TEC in RIOD during epochs with and without evidences of ionospheric scintillations. Each colored line represents the slant TEC calculated for a distinct satellite, that is, one colored line refers to the slant TEC of one GPS satellite. The Day Of Year (DOY) 005 of 2013 is shown in the top panel, which refers to the summer solstice in the south crest of EIA. On the other hand, DOY 191 of 2013 is referred to the winter. As it can be seen, the maximum of TEC in the summer solstice reaches around 150 TECU in the slant direction during the daytime. The magnitude of the TEC in daytime is reduced in the winter for up to 100 TECU, mainly due to the reduced intensity of the solar irradiation. However, TEC variability in the daytime is similar. In contrast, the nighttime between 22 and 04 LT presents a significant difference in terms of the TEC variability. The maximum TEC in the nighttime is reduced from 60 TECU in the solstice up to 30 TECU in the winter. Additionally, the high TEC variability evidenced between 22 and 04 LT is associated to the plasma depletions propagating trough the Brazilian region. In fact, the high magnitude of the TEC observations in comparison with the TEC variability makes the impact of the ionospheric scintillation not so clearing TEC. However, the impact of the ionospheric scintillations is much more easily seen when looking to the single-frequency PPP performance.
RTKLIB is adapted for the use of the calibrated TEC during the single-frequency PPP. Among the PPP configurations, it is used the kinematic mode, a combined solution obtained by forward and backward filters, a cut-off angle of 10°, Earth tides corrections, the estimation of tropospheric delay during PPP, IGS precise ephemerides, satellite clock corrections with a 30 s rate (clk_30s), global positioning system constellation, correction of the phase center variation of the antenna, phase wind up corrections, no strategy for ambiguity solution and corrections of the differential instrumental bias between the civil and precise codes (C1-P1) when P1 was not available. In general, three modes of PPP are analyzed: (1) using the ion-free observation (PPP/if); (2) using L1 and the ionospheric delay from GIMs from UQRG (UPC Quarter an hour Rapid GIM), identified as PPP/uqrg; and (3) using L1 and the calibrated TEC through the proposed method (PPP/tec). Indeed, the PPP/if solutions are obtained using the ion-free observation of the carrier phase, which means that a linear combination is carried out with the L1 and L2 frequencies to eliminate the first-order effect in the ionospheric delay. In the case of PPP/uqrg, the observation used in the PPP Kalman Filter is related to the L1 frequency but corrected from the ionospheric delay derived from UQRG GIMs. The PPP/tec is similar to PPP/uqrg, but the ionospheric delay is derived directly from Eq. (10). The reference coordinates of ONRJ has been obtained from the Sistema de Referencia Geocéntrico para las Américas (SIRGAS) final solutions at epoch 2013, where a time update was performed to make the coordinates consistent with the PPP solutions.
Figure 3 shows the performance of PPP/if, PPP/tec and PPP/uqrg in terms of the three-dimensional (3D) error of the estimated coordinates. Each point represents the error of the PPP solution calculated in each processing epoch in kinematic mode. Since it was used a combined filter of forward and backward solution, there is no need for time convergence in the solution. Consequently, the remaining errors are related to the terms not efficiently mitigated and, as can be seen, the PPP/tec (blue points) was obtained with a high accuracy in many hours, except to the hours when is typically observed ionospheric scintillations.
In general, the proposed method allowed having the single-frequency PPP with a similar accuracy than the double-frequency PPP. The root mean square error (RMSE) obtained for PPP/if in DOY 191 was 0.04 m with a standard of 0.04 m, the RMSE of PPP/tec was 0.09 m with a standard deviation of 0.04 m and the RMSE of PPP/uqrg was 0.47 m with a standard deviation of 0.65 m. In the case of DOY 005, the RMSE of PPP/if was 0.04 m with a standard of 0.04 m, the RMSE of PPP/tec was 0.19 m with a standard deviation of 0.05 m and the RMSE of PPP/uqrg was 0.70 m with a standard deviation of 0.66 m. By itself, this is an outstanding result, since many researchers have already used single-frequency receivers and ionospheric corrections to obtain, in general, an absolute accuracy of 0.5 m in the horizontal and 1 m in vertical for the kinematic PPP [96, 97, 98]. Now, we are showing the possibility of obtaining the single-frequency PPP accuracy similar to that from dual-frequency PPP. The horizontal accuracy in the experiment for PPP/tec was 0.12 m in DOY 005 and 0.05 m in DOY 191 and the vertical accuracy was 0.15 m in DOY 005 and 0.07 m in DOY 191. These accuracies are compatible to the double-frequency solutions. Therefore, the unique consideration for a high accuracy in single-frequency PPP is that TEC needs to be sufficient precise, which was not realistic in some epochs of DOY 005 due to the ionospheric scintillations.
A noticeable degradation of the PPP solution occurs between 00 and 04 hours LT in the summer solstice due to the high TEC variability. This PPP degradation is related to the ionospheric irregularities that impact GPS observations more effectively. At such instances, the uplifted ionosphere due to the pre-reversal drift produces high vertical gradients. It is believed that these gradients set the preconditions for plasma instability, controlling the generation of ionospheric irregularities. Therefore, the change of the GPS signal phase and amplitude imposed by ionospheric irregularities degrades the PPP solution. It is interesting to note that the PPP/if is sensitive, but not too much, to the ionospheric scintillations. In case of PPP/uqrg, it is hard to see the impact because of the high standard deviation in the PPP solution at other epochs. However, the impact of the scintillations is very evident in PPP/tec. This mainly happens because the estimated TEC was set to be very precise during the PPP, so that TEC was included with small values for standard deviation in the PPP Kalman filter. At epochs where only the first-order ionospheric delay is supposed to affect the GPS observations, the PPP/tec solution is very accurate. At epochs where the high-orders terms of the ionosphere delay impact the GPS signal, the PPP degradation becomes evident.
The ionospheric conditions are affected by electrodynamic processes that are driven by solar phenomena. During quiet geomagnetic periods, the effects are clearly associated with the seasonal variation of the solar illumination and with the 11-year solar radiation variation. These conditions can be strongly disturbed under the impact of CMEs (coronal mass ejections) and HSS (high-speed streams) in the magnetosphere producing the geomagnetic storms, which result in steeper electron density gradients and stronger irregularities. These irregularities are responsible for fluctuations in the amplitude and phase of GNSS signals, which can degrade the accuracy of the measurements. Therefore, the climatology of these irregularities is very important to define its spatial distribution and time of occurrence.
The investigation of ionospheric scintillations over South America has shown that they can occur at all longitudinal sectors during quiet geomagnetic periods, but they are stronger at post-sunset hours in the crest regions of the EIA (equatorial ionospheric anomaly), and seems to be more intense in the southern EIA crest, which is under the effect of the SAMA (South American Magnetic Anomaly) [24, 37, 81]. The scintillation occurrences strongly enhance with the increase of the solar activity [20, 21, 35, 37, 99], and during geomagnetic disturbed periods [13, 38, 75, 93, 94].
It was presented the potentiality of a new procedure for TEC calibration, showing a useful tool to correct the first-order ionospheric delay that improves the traditional single-frequency PPP solutions. Additionally, the results show a high-sensitive solution of PPP/tec to the ionospheric scintillations, even more sensitive in comparison with the impact of the ionospheric scintillation in the TEC level, which indicates that this kind of procedures are emerging as having potential for a wide range of applications for those measuring and predicting the ionospheric scintillations.
EC thanks the National Council for Research and Development (CNPq) for individual research support (processes nos. 556872/2009-6, 406690/2013-8, 303299/2016-9) and the National Institute for Space Research (INPE/MCTI). MTAHM thanks the support from CNPq through grant no. 429885/2016-4. FSP and POC are grateful to CNPq (grant no. 309924/2013-8), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP grant no. 2015/15027-7) and to Faculdade de Ciências e Tecnologia of UNESP (FCT/UNESP).
The authors declare that they have no conflict of interest.