A Comparative Analysis of Linear Modulation Schemes in the Presence of FLI in Wireless Optical Systems

Abstract

In this chapter three optical modulation techniques such as on off keying (OOK), binary phase shift keying (BPSK), differential phase shift keying (DPSK) are evaluated in terms of Q-factor and bit error rate (BER) in order to assess their suitability in typical indoor optical wireless communication. The modulated signals are subjected to additive white Gaussian noise (AWGN) and florescent light interference (FLI) representative of practical scenarios. The received signals are demodulated with and without using matched filter (MF). The BER in both the cases are evaluated through simulation studies using MATLAB and compared for each of the three modulated signals. It has been observed that in each of the demodulation process the rate of reduction of BER is significant when the demodulation is done by using MF.

1. Introduction

The day to day increasing number of wireless devices has drastically increased the crowd in radio frequency channels. Starting from 1G to 4G the objective of development has been to increase the data rate and channel reusability. This is due to the fact that the Radio Frequency (RF) band assigned for communication is quite limited while the applications and user number are increasing day by day. The inter-channel interference and electromagnetic interference (EMI) are two impairments of the RF channel. Hence an alternative needs to be investigated in order to meet the demand of the growing number of users. Question arises as to the suitability of channel that can be a replacement of such a highly reliable and high data rate supporting channel (?). Optical communication is a promising technique to provide a reliable and high speed data transmission. Again fiber optic cables are free of EMI. The major issue associated with optical fiber cable is pulse dispersion which limits the data rate of the signal to be transmitted through it. Also this dispersion increases with increasing distance. In rural areas, areas with rough, difficult to access terrains, the installation and maintenance of optical fiber cables is not commercially viable. For this reason wireless radio channels are becoming important as an alternative. With increasing applications of wireless devices, the demand for radio channels is also increasing, but the channel band width is limited. Different frequency reuse techniques are being developed as a potential solution. Another solution has been to use optical wireless channel as an alternate option of radio channel, so that the crowd in radio frequency can be reduced. It may be considered to be a hybrid of optical and wireless technology.

Wireless optical communication is a technique to transmit and receive optically modulated signals in free space or air [1]. So the frequency band that the signal uses is the optical band. But ambient light interference is a major problem while optical signal is transmitted through wireless medium [2]. This necessitates solutions to have to be found out to make optical wireless communication a practical potential candidate.

Optical communication techniques use different modulation techniques where some techniques are power efficient and some others are spectrally efficient. The spectral efficiency of a modulated signal depends upon the modulation scheme as well as the line-coding technique. In [3] a comparative study has been carried out between the performance of On-Off Keying (OOK), Pulse Position Modulation (PPM) and Digital Pulse Interval Modulation (DPIM) systems in presence of florescent light interference (FLI) by considering normalized optical power required (NOPR) and optical power penalty (OPP) as the performance metrics. A 64 channel inter-satellite wireless optical channel (IS-OWC) based system has been described in [4], in which three most preferable modulation techniques such as Alternate Mark Inversion (AMI), Differential Phase Shift Keying (DPSK) and Chirp-RZ have been compared on the basis of their Q-factors, BER and eye diagram as achieved at different data rates. Their spectra were compared based on their bandwidth. The results reveal DPSK as the best among all. By using space and polarization diversity techniques in [5], a high speed IS-OWC system has been designed and simulated. The analysis has been made on the metrics like BER, Q-factor, eye opening etc. The aim is to find out the practicability of OWC for establishing an inter satellite communication. In [6], advanced modulations, multiplexing, coding schemes, detection schemes, switching and controlling schemes as applicable in typical optical communication have been discussed with possible directions for making further research. In [7] an experimental analysis has been done for OOK-return to zero (RZ) and non return to zero (NRZ) signal in a Light Fidelity (Li fi) network and it has been found that OOK-NRZ has a wider coverage area as compared to that of OOK-RZ. In [8] performance of RZ modulation has been found to be better than NRZ modulation in Gigabit Passive Optical Network (GPON) system. The OOK-RZ and NRZ signals have been analyzed over Free Space Optical (FSO) channel in different weather conditions in [9] wherein the analysis is carried out by taking different wireless media. [10, 11, 12, 13] gives analytical observations on some advanced modulation techniques such as FSK, DPSK, QDPSK based on RZ and NRZ pulses.

In this chapter OOK, BPSK and DPSK optical transreceiver systems are designed with an objective of supporting high data rate (as needed for 5G communication), low energy consumption, high received power and an affordable design complexity. The simulation of the above three systems are first done by using Optisys16.0 by taking both RZ and NRZ line coding for a certain set of parameters to find which type of line coding will give a better result. Then the performance of each system has been investigated in presence of FLI and AWGN while considering the BER as the performance parameter. Simulation studies are presented to show the superior BER achieved through the use of a matched filter (MF) as compared to no MF at the receiver.

2. System components

Here the intention is to transmit the signal in free space. Again the signal which will be received at the other end that will be an optical signal and hence the antenna must be an optical antenna. The receiver must be capable to convert the optical signal into the detectable form. So there are some special system components used to perform all these operations.

2.1 Wireless optical channels

In any communication process, channel plays an important role. The channel performance characteristics somehow depend on the design of the transmitting and receiving antenna. Wireless optical channel includes the frequency range of infra-red, visible light and ultraviolet [1]. The optically modulated signal is radiated to freespace or air at the transmitter side. At the receiver side, the optical signal is accumulated by using telescope. In this analysis Optical Wireless Channel (OWC) has been used, however another technique named Free Space Optics (FSO) is also being used in some fields. The equation of wireless optical channel is same as RF channel [4].

Wireless optical links are of two types—Optical Wireless Channel (OWC) and Free Space Optical Channel (FSO) [1]. The differentiation has been established between the two channels by considering the difference between the transmitting and receiving antenna used for the two techniques. OWC can be said to be as an improved version of FSO [5]. FSO needs LOS path and it works effectively for short range [1, 14]. For long distance communication OWC is used.

Figure 1 represents an FSO based system where the transmitting antenna is simply a light radiator and the receiving antenna is a photo detector which has the ability to convert the photonic power into voltage signal. The idea of FSO is based on the transmission of collimated light beam from one location to another by using low power IR laser [1].

Figure 1.

Block schematic of a simple FSO based system.

Figure 2 represents a simple block diagram of OWC system. The concept of OWC involves the use of single mode fibers directly as light launchers and light collectors.

Figure 2.

Block schematic of a simple OWC based System.

2.2 Continuous wave (CW) laser

CW laser is being used as the optical source. Continuous wave means that it is continuously pumped and emits light continuously, not in form of pulses [1].

2.3 Mach-Zehnder (MZ)-modulator

A Mach-Zehnder modulator is simply an interferometer [15, 16]. It is otherwise known as Mach-Zehnder Interferometer (MZIM). It is a combination of two electro-optic phase modulators (EOPM) in parallel structure as shown in Figure 3. It splits the input optical signal into two fields by the two arms of the MZIM and at the output end these two fields are again recombined by means of interference.

Figure 3.

MZ interferometer.

MZIM is again of two types—single drive and dual drive. In single drive MZIM one arm is driven by an input voltage $\mathit{V}\left(\mathit{t}\right)$ while no voltage is applied to the other arm. Therefore the output optical field is given by [16]

$$E_0\left(t\right)=\frac{E_i\left(t\right)}{2}\left[1+e^{\mathit{j\pi }\frac{\left(V\left(t\right)+V_{\mathit{\text{bias}}}\right)}{V_{\pi }}}\right]=F_i\left(t\right)cos\left[\frac{\pi }{2}\frac{\left(V\left(t\right)+V_{\mathit{\text{bias}}}\right)}{V_{\pi }}\right]e^{-j\left[\frac{\pi }{2}\frac{\left(V\left(t\right)+V_{\mathit{\text{bias}}}\right)}{V_{\pi }}\right]}$$E1

where

${\mathit{E}}_{\mathit{i}}\left(\mathit{t}\right)\mathit{=}$ Input optical field.

${\mathit{V}}_{\mathit{\text{bias}}}\mathit{=}$ DC voltage.

${\mathit{V}}_{\mathit{\pi }}\mathit{=}$ Driving voltage required to create a $\mathit{\pi }$ phase shift on the light wave carrier (usually 3–6 V).

The dual drive MZIM takes two driving voltages, ${\mathit{V}}_{\mathbf{1}}\left(\mathit{t}\right)$ and ${\mathit{V}}_{\mathbf{2}}\left(\mathit{t}\right)$ such that ${\mathit{V}}_{\mathbf{2}}\left(\mathit{t}\right)\mathit{=}\mathit{-}{\mathit{V}}_{\mathbf{1}}\left(\mathit{t}\right)$ . Therefore, considering ${\mathit{V}}_{\mathbf{1}}\left(\mathit{t}\right)\mathit{=}\mathit{V}\left(\mathit{t}\right)$and ${\mathit{V}}_{\mathbf{2}}\left(\mathit{t}\right)\mathit{=}\mathit{-}\mathit{V}\left(\mathit{t}\right)$, the output optical field is given by [16]

$${\mathit{E}}_{\mathbf{0}}\left(\mathit{t}\right)\mathit{=}\frac{{\mathit{E}}_{\mathit{i}}\left(\mathit{t}\right)}{\mathbf{2}}\left[{\mathit{e}}^{\mathit{j\pi }\frac{\left(\mathit{V}\left(\mathit{t}\right)\mathit{+}{\mathit{V}}_{\mathit{\text{bias}}}\right)}{{\mathit{V}}_{\mathit{\pi }}}}\mathit{+}{\mathit{e}}^{\mathit{j\pi }\frac{\mathit{-}\left(\mathit{V}\left(\mathit{t}\right)\mathit{+}{\mathit{V}}_{\mathit{\text{bias}}}\right)}{{\mathit{V}}_{\mathit{\pi }}}}\right]$$

$$\mathit{=}{\mathit{E}}_{\mathit{i}}\left(\mathit{t}\right)\mathbf{cos}\left[\frac{\mathit{\pi }}{\mathbf{2}}\frac{\left(\mathit{V}\left(\mathit{t}\right)\mathit{+}{\mathit{V}}_{\mathit{\text{bias}}}\right)}{{\mathit{V}}_{\mathit{\pi }}}\right]$$E2

In all these analysis Dual-drive MZ-modulator has been used however single-drive MZIM can also be used with the same arrangement.

2.4 Avalanche photodiode (APD)

APD is a highly sensitive photodiode [15, 16] that is used to convert the photonic energy into electrical energy. As compared to PIN diode it can detect lower level light energy. Hence these diodes are basically used at the receiver side in long distance transmission. APD provides inherent current gain through repeated electron ionization process. Hence a multiplication factor $\mathbf{M}$is associated with the expression of responsivity of an APD which is given by

$$\mathrm{M}={\mathrm{I}}_{\mathrm{T}}/{\mathrm{I}}_{\mathrm{P}}$$E3

where, ${\mathbf{I}}_{\mathbf{T}}$ = total output current; ${\mathbf{I}}_{\mathbf{P}}$ = primary photocurrent.

3. Optical OOK, BPSK and DPSK modulation systems

The signal wave forms and the logical statements of the above said optical systems are quite similar to that of the electrical systems but the functional blocks of the optical systems are completely different from the electrical systems. It is obvious that the system components used here must support the optical signals. So here the system block diagrams are explained well with their corresponding Optisys16.0 simulator set up that has been used here to analyze the performance of the systems.

3.1 Optical ON-OFF keying (OOK)

It is the simplest modulation technique used in optical modulation schemes. The binary data is first converted into NRZ pulse then given to the MZIM as the driving voltage. The optical signal from CW laser is given to the input of the modulator. In the output the ON-OFF keying optical signal is generated. Figure 4 shows the block schematic of a typical OOK modulator. Figure 5 represents a simple optical receiver in which a single Avalanche photo diode (APD) has been used to convert the optical signal into electrical signal. Then the signal is passed through a LPF to get the desired frequency. The data regenerator will generate the binary signal from the electrical signal. Figure 6 shows the entire setup in Optisys16.0 to simulate the OOK system with OWC channel.

Figure 4.

ON-OFF keying modulator.

Figure 5.

ON-OFF keying demodulator.

Figure 6.

Simulation layout of OOK Transreceiver using OWC in Optisys16.0.

3.2 Optical binary phase shift keying

Here the binary data is first sent to BPSK pulse generator and then given to the MZIM as the driving voltage. The optical signal from CW laser is given to the input of the modulator. In the output the BPSK optical signal is generated. Figure 7 shows the block schematic of a typical BPSK modulator. Figure 8 represents a simple optical receiver in which a single PIN diode has been used to convert the optical signal into electrical signal. Then the signal is passed through a LPF to get the desired frequency. The data regenerator will generate the binary signal from the electrical signal. Figure 9 shows the entire setup in Optisys16.0 to simulate the BPSK system with OWC channel.

Figure 7.

BPSK modulator.

Figure 8.

BPSK demodulator.

Figure 9.

Simulation layout of BPSK Transreceiver using OWC in Optisys16.0.

3.3 Optical differential phase shift keying (ODPSK)

Figure 10 represents an DPSK modulator. The binary signal is first converted into a Duobinary code by using a duo binary pre-coder. The internal circuitry of a Duobinary precoder has been given in Figure 11 which is simply a two input XOR gate having one input connected to the output and the other input is the binary data to be encoded. The initial value of D is set to 1, then from the next step the output is fed back to the input. An example of te conversion is done in Table 1. The signal conversion from binary to duobinary has been shown in Figures 12 and 13. Thus, the duo-binary coded data is generated which is then fed to the pulse generator. Then the pulse is given to the first modulator which converts the electrical signal into optical signal. The second modulator take the driving voltage from a sinusoidal function generator that finally gives a differerentially phase shifted optical signal or optical DPSK signal.

Figure 10.

DPSK modulator.

Figure 11.

Duobinary Precoder.

dk	D	bk
1	1	0
0	0	0
0	0	0
1	0	1
1	1	0
0	0	0
1	0	1
1	1	0
1	0	1
0	1	1

Table 1.

Duobinary coding.

Figure 12.

Input signal to Duobinary Precoder.

Figure 13.

Output of Dubinary Precoder.

Figure 14 represents the block schematic of optical DPSK demodulator where the optical receiver block consists of an APD and a DPSK decoder. The signal is then passed through an LPF and then the binary data is generated from the electrical signal by the data regenerator.

Figure 14.

DPSK-demodulator.

Figure 14 represents the block schematic of optical DPSK demodulator where the optical receiver block consists of an APD and a DPSK decoder. The signal is then passed through an LPF and then the binary data is generated from the electrical signal by the data regenerator (Figure 15).

Figure 15.

Simulation layout of optical DPSK transreceiver using OWC in Optisys16.0.

4. Effect of ambient light interference

Sun light causes the maximum amount of background noise and it is the major source of shot noise at the receiver. The ambient light has an operating wavelength range of 850 nm to 1550 nm. It can be modeled as a white Gaussian noise with PSD given by [1].

$$N_0=2q{I}_b$$E4

where $q\mathit{=}$ charge of electron; $I_b=$ amount of current.

Incandescent lamps have a maximum Power Spectral Density (PSD) around 1 μm. It can produce an interference of near perfect sinusoid with a frequency of 100 Hz [1]. So the first few harmonics carry a significant amount of energy which can be avoided by using an High Pass Filter (HPF).

Fluorescent light has a switching frequency range in between 20 and 40 kHZ [17, 18]. The detected electrical signal may contain harmonics of the switching frequency. These harmonics may extent to the range of Megahertz frequency range and can create a much more serious impairment to the receiver in case of wireless optical system [19, 20]. When a signal is received along with channel noise and also FLI, the signal cannot be detected only by thresholding. The received signal has an inherent delay in reaching the receiver and is also subject to amplitude distortion occurs due to FLI besides the omnipresent additive noise. All these render an unacceptable signal to noise ratio (SNR) at the receiver input. So it becomes difficult to sample the signal at an accurate instant. Hence it becomes impossible to detect the signal with acceptable accuracy. In this case a special kind of matched filter is to be used. The formulation of this filter is described in the next section.

5. Matched filter

The matched filter is an optimal linear filter for maximizing the SNR in presence of additive noise. It is used to detect highly noisy signal [21].

Let us consider the general equation of a received signal,

$$R_k=T_k+n_k$$E5

where

${\mathit{R}}_{\mathit{k}}\mathit{=}$ received signal.

${\mathit{n}}_{\mathit{k}}\mathit{=}$ additive noise.

${\mathit{T}}_{\mathit{k}}\mathit{=}$ scaled and shifted version of a known signal

$$T_k={\mu }_0{S}_{k-j_0}$$E6

${\mathbf{\mu }}_{\mathbf{0}}\mathbf{=}$ unknown scaling.

${\mathbf{j}}_{\mathbf{0}}\mathbf{=}$ unknown shift.

To detect the original signal the optimal estimation of ${\mathit{j}}_{\mathbf{0}}$ and ${\mathit{\mu }}_{\mathbf{0}}$ is needed.

Let the optimal estimation of ${\mathit{j}}_{\mathbf{0}}$ and ${\mathit{\mu }}_{\mathbf{0}}$ be ${\mathit{j}}^{\mathit{\ast }}$ and ${\mathit{\mu }}^{\mathit{\ast }}$ respectively which are obtained by minimizing the least square residual between the observed sequence ${\mathit{R}}_{\mathit{k}}$ and probing sequence${\mathit{h}}_{\mathit{j}\mathit{-}\mathit{k}}$. This ${\mathit{h}}_{\mathit{j}\mathit{-}\mathit{k}}$ is otherwise the impulse response of the matched filter.

$$j^{\ast },{\mu }^{\ast }=arg{min}_{j,\mu }{\sum }_k{\left(R_k-\mu .h_{j-k}\right)}^2$$E7

By solving the equations the values of ${\mathit{\mu }}^{\mathit{\ast }}$,${\mathit{j}}^{\mathit{\ast }}$ and ${\mathit{h}}_{\mathit{j}\mathit{-}\mathit{k}}$ are found to be

$$\therefore {\mu }^{\ast }=\frac{{\sum }_k{T}_k{h}_{j-k}}{{\sum }_k{h}_{j-k}^2}$$E8

$$j^{\ast }=\mathit{arg}{max}_j\frac{{\left({\sum }_k{T}_k{h}_{j-k}\right)}^2}{{\sum }_k{h}_{j-k}^2}$$E9

$$h_{j-k}=\nu .T_k=\kappa .S_{k-j_0}$$E10

Hence, the probing signal $h_{j-k}$ is proportional to $S_{k-j_0}$.

To detect the correct signal the observed signal must be convolved with the matched filter impulse response.

If the signal received by the receiver including FLI is given by ${\mathit{R}}_{\mathit{k}}\left(\mathit{t}\right)$, then output of the matched filter is given by

$$m_0\left(t\right)=R_k\left(t\right)⨂h_{j-k}\left(t\right)$$E11

i.e. the convolution of the received signal and the impulse response of the matched filter.

For the system here under observation ${\mathit{T}}_{\mathit{k}}$ is the signal transmitted in the form of OOK or BPSK or DPSK where the additive noise includes both AWGN as well as FLI.

The FLI interference noise power added to the transmitted signal is given by [20]

$$n_{\mathit{FLI}}=R_{a}q{\int }_{-\infty }^{\infty }p\left(\tau \right)h_I^2\left(t-\tau \right)\mathit{d\tau }$$E12

${\mathit{R}}_{\mathit{a}}\mathit{=}$ responsivity of the receiver

$\mathit{q}\mathit{=}$ electronic charge

${\mathit{h}}_{\mathit{I}}\left(\mathit{t}\right)\mathit{=}$ receiver impulse response

$$p\left(t\right)=H\left(t\right)A_r$$

$\mathit{H}\left(\mathit{t}\right)\mathit{=}$ irradiance produced by the background light at the receiver side

${\mathit{A}}_{\mathit{r}}\mathit{=}$ photodetector response area

For the sampled version of transmitted signal the interference also occurs at every sample.

6. Proposed system model

Here three different types of modulation techniques are investigated on basis of their BER in presence of Gaussian noise as well as FLI. The modulation schemes chosen here are OOK, BPSK and DPSK. Table 1 represents the simulation parameters considered during the simulation of the system model.

Figure 16 is a simple block schematic representation of the proposed system. The modulator is OOK, BPSK and DPSK. Here the noise has been added in two different stages as shown in this figure. The received signal is passed through a matched filter before entering the demodulator.

Figure 16.

Block diagram of the proposed system.

7. Experimental outputs of the systems in Optisys16.0 simulator for Q-Factor comparison of OOK, BPSK and DPSK Transreceivers at Different Data Rates

The simulation has been done in Optisys16.0 by taking the parameters as given in Table 2. Then by changing the data rates from 10 Gbps to 20 Gbps and 40 Gbps the corresponding BER and Q-factor has been noted down in Table 3.

SL. no.	Parameters	Values
1	Channels/bits	64
2	Bit rates	10, 20, 40 Gbps
3	Input power	30 dBm
4	EDFA gain	20 dBm
5	Length of sequence	128
6	Range	2500 km
7	Samples per bit	64

Table 2.

Parameters to simulate Q-factor and BER at different data rates.

Modulation technique	Data rate (in Gbps)	Q-factor	BER
OOK	10	6.67	0.004998
	20	5.472	0.00886
	40	2.24	0.06324
BPSK	10	19.21842	0.00005321
	20	5.94129	0.00002932
	40	4.03944	0.007106
DPSK	10	24.4962	0.00006759
	20	9.1693	0.0003917
	40	5.6349	0.000712

Table 3.

Q-factor and BER at different data rates.

8. Distance versus received power for signal transmitted in RZ and NRZ linecoding

Table 4 represents observed readings of the power meter at receiver side. It shows the received signal power at different distances for all the three modulated signals taking both RZ and NRZ pulses. In every case the received power for NRZ signal is more than that of RZ signals. Here in every modulation schemes, the received power for NRZ modulation is more as compared to RZ modulation. The simulation is done by taking the parameters as given in Table 5.

Sl no.	Distance in KM	Received power (in dBm)
		OOK-NRZ	OOK-RZ	BPSK-NRZ	BPSK-RZ	DPSK-NRZ	DPSK-RZ
1	100	22.981	20.882	22.321	21.148	24.846	23.115
2	200	22.976	20.863	22.312	20.940	24.842	23.106
3	300	22.832	20.849	22.315	20.911	24.837	23.080
4	400	22.800	20.821	22.296	20.888	24.824	23.066
5	500	22.723	20.699	22.284	20.769	24.818	23.054
6	600	22.673	20.560	22.277	20.660	24.811	23.031
7	700	22.566	20.342	22.273	20.557	24.778	23.007
8	800	22.392	20.318	22.269	20.497	24.759	22.991
9	900	22.188	20.298	22.251	20.311	24.742	22.766
10	1000	21.889	20.008	21.211	19.967	24.728	22.611

Table 4.

Received powers for varying link length.

Sl no.	Parameters	Value
1	Data rate	10 Gbps
2	Input power	30 dBm
3	Center frequency	193.1THz
4	Data length	1024 bits
5	Link length	100 km to 1000 km

Table 5.

Parameters for simulating at varying distance.

9. Experimental outputs of the system with FLI

The FLI effect is analyzed by taking the system parameters as given in Table 6. To visualize the system performance clearly an 8-bit signal is taken and the test results for OOK based system are shown below.

Sl. no	Parameter	Values
1	Length of data	1024
2	Normalized data rate	1, 10 and 20 Gbps
3	SNR	1–40 dB
4	Responsivity of receiver	1

Table 6.

Simulation parameter values for BER analysis.

Figure 17 represents the original OOK signal which has been transmitted from the transmitter. This is passed through the OWC channel to reach the receiver side. Figure 18 represents the received signal without adding the FLI. Here the signal is noisy due to the addition of AWGN. However, as here the aim is to investigate the effect of fluorescent light interference in addition to AWGN, the FLI has also been added. The signal becomes more distorted as shown in Figure 19. The noisy received signal in the presence of AWGN only can be easily detected by an integrate and dump filter; low pass filter (LPF) and subsequent thresholding. But the signal shown in Figure 18 when down sampled, filtered and thresholded, yields the signal as shown in Figure 20. As we can observe, the signal so recovered does not properly match with the transmitted signal. However, when the signal is passed through the MF, where it is also down sampled, the output looks as given in Figure 21. By thresholding the output of MF, the signal so recovered is shown in Figure 22, which agrees with the transmitted signal.

Figure 17.

Transmitted signal.

Figure 18.

Received signal without FLI.

Figure 19.

Received signal with FLI.

Figure 20.

Detected signal without MF.

Figure 21.

Signal at the output of MF.

Figure 22.

Detected signal with MF.

10. BER variation with respect to SNR

Figures 23–31 represent the BER ∼ SNR curves of OOK, BPSK and DPSK systems at data rate of 1 Gbps, 10 Gbps and 20 Gbps respectively as obtained through MATLAB simulations. Each of these contains a pair of curves, one of which indicates the system performance with MF and the other without the MF. The purpose of taking three different data bits is to verify whether the MF is able to yield an acceptable signal quality in terms of SNR. By observing these figures it can be clearly noticed that for each of the systems the BER reduces at a higher rate and falls down to a more lower value when the demodulation is done with matched filter as compared to that in case of demodulation done without matched filter. A BER comparison for all three modulation schemes at the three different data rates is shown in Tables 6 and 7.

Figure 23.

BER curve for OOK at 1 Gbps.

Figure 24.

BER curve for BPSK at 1 Gbps.

Figure 25.

BER curve for DPSK at 1 Gbps.

Figure 26.

BER curve for OOK at 10 Gbps.

Figure 27.

BER curve for BPSK at 10 Gbps.

Figure 28.

BER curve for DPSK at 10 Gbps.

Figure 29.

BER curve for OOK at 20 Gbps.

Figure 30.

BER curve for BPSK at 20 Gbps.

Figure 31.

BER curve for DPSK at 20 Gbps.

Modulation scheme	Bit rate (GBPS)	Demodulation without MF		Demodulation with MF
		Minimum BER	Value of SNR(dB) required	Minimum BER	Value of SNR(dB) required
OOK	1	0.02	40	0.002	28
	10	0.004	34	0.001	18
	20	0.001	34	0.002	16
BPSK	1	0.003	36	0.003	26
	10	0.004	28	0.005	18
	20	0.002	26	0.003	15
DPSK	1	0.03	40	0.001	28
	10	0.001	35	0.002	18
	20	0.001	32	0.001	18

Table 7.

Table of comparison for the system with and without MF.

The results obtained from Table 6 reveal that in each case, between the minimum BER achieved through the use of MF and without using the MF, there is observed a minimum of 10 dB difference of SNR. With increasing data rate the there is not a significant variation in the minimum achievable BER.

11. Conclusion

Here the results reveal that with an increasing data rate the Q-factor goes on decreasing while BER increases. So the data rate may be a limitation for the transmitters. Making a comparison of the three modulation techniques, DPSK is found to have the best Q-factor in all the data rates taken in analysis. As noted in Table 3, the Q-factor in all cases is falling down with increasing data rate but the falling rate is lower in case of DPSK system as compared to the others. The Q-factor value at the worst scenario, that is at 40GBPS data rate falls to 2.24 and 4.03 in case OOK and BPSK system, while it is 5.63 in case of DPSK system. The detected output with using matched filter has been compared with the detected signal without using matched filter for three modulation schemes at three different data rates to assess their suitability over an OWC. The signal plots give information regarding the mismatch between the transmitted signal and detected signal without using MF whereas the MF insertion gives an acceptable signal quality. Each of the BER versus SNR plots also include two curves which show that the signal detected without MF needs a higher value of SNR to achieve a certain level of BER that can be achieved at a 10 dB (approximately) lesser value of SNR by a system having MF. The MF is not required in case if signal impairment is due to AWGN only. But practically, in OWC, the ambient light interference is a serious issue to be taken into consideration, because this effect cannot be avoided. This necessitates signal recovery through an MF.