The arrangement of 16
It is well known that cellular mobile phone systems have evolved from 1G and 2G that use frequency and time division multiple access (FDMA and TDMA) systems respectively, to code division multiple access (CDMA) of third generation (3G) systems Chen et al., 2006. Furthermore, the exploitation of spatial diversity from the emergence of advance antenna technologies such as smart antenna and space time signal processing have given rise to induce another multiple access scheme called space division multiple access (SDMA) systems Fang, 2002. Among these schemes, the system capacity and spectrum efficiency are the key factors to compare the performances of various mobile communication systems. Since radio frequency (RF) spectrum is a limited resource, these techniques have approached their fundamental limitations. Flexible utilization of such resources in space, time and code has led to great improvement in system capacity. For a given bandwidth, the system capacity for narrowband radio systems such as FDMA and TDMA is dimension or bandwidth limited. In contrast, the system capacity of CDMA and SDMA systems is interference limited. Any reduction in interference in CDMA and SDMA systems converts directly and linearly into increased capacity Yu et al., 2004, Chen et al., 2008.
Multiple access schemes such as FDMA and TDMA increase their system capacity and spectrum efficiency by dividing the different network planning phases more clearly into individual parts to allow different frequencies to be used at different time moments Castaeda & Lara, 2008. In CDMA systems, the same frequency is used simultaneously in adjacent cells and the interference level should be taken into account in the coverage-planning phase Niemela & Lempiainen, 2003. Furthermore, cell splitting and sectorisation to form SDMA systems with use of directional antenna could also result in increase of system capacity and spectrum efficiency over the omnidirectional antenna system Godara, 1997. Although these approaches do significantly increase the system capacity and spectrum efficiency, each scheme basically is attempting a more efficient use of the same resource.
It is well known that CDMA system is characterized as being interference limited. Independent simultaneous transmissions by mobile users at different locations in a cell give rise to the near-far phenomenon. To combat the near-far problem, power control is used to ensure equal signal levels are received from all mobile users at different location Hashem & Sousa, 1997. Therefore, power control is considered the most important system requirement for CDMA systems to increase the system capacity on the reverse link by overcoming the near-far problem Cameron & Woerner, 1996, Uthansakul, 2002. Since all the cells can operate with the same channel in CDMA cellular network, a significant source of interference apart from traffic in its own cell is the traffic from neighbouring cells. Thus, the system capacity of CDMA systems is determined by the amount of co-channel interference that it can tolerate, which is comprised of intra-cell interference and inter-cell interference Wu et al., 1998. If the traffic load in neighbouring cells is reduced, more traffic can be accepted in the observed cell Chatovich & Jabbari, 1999. However, because of power control from observed cell base station (BS), transmitting a high power level in reverse link may result in high interference to neighbouring cell BS Hashem & Sousa, 1997. Therefore, in CDMA systems, if the capacity of a single cell increases it creates higher interference to its neighbouring cells and thus impacts their capacity.
Other approach that shows a promise for substantial capacity enhancement is the use of spatial filtering with exploitation of smart antenna at cell site BS Zheng et al., 1996. Hence, the deployment of SDMA system has been recognised as one of the most promising techniques for controlling co-channel interference in cellular systems, leading to the required system capacity improvement Liberti & Rappaport, 1998. The beamforming ability of smart antenna technology has been adapted to increase the gain of the desired signal while null interference sources resulting in the improvement of the system capacity Huang et al., 2001. The narrow beams from smart antenna are steered toward desired users in order to filter out interference caused by co-channel users located in the same cell and from adjacent cells Galvan-Tejada & Gardiner, 2001.
However, in order to achieve an ideal SDMA system, smart antenna must carefully form its radiation patterns to capture the desired user and to nullify sufficiently interfering users. Therefore, the smart antenna requires high accuracy in propagation channel response estimation Cho et al., 2002. If there are
The wireless channel usually characterized by the path loss, shadowing and fading Feuerstein et al., 1994. In urban areas, multipath propagation is common, whereby the receiver observes a number of copies of the transmitted signal, each with a different time delay Adachi et al., 2005. This provides a form of multipath fading. In a digital communication system, the delay-spread of multipath propagation could also cause inter-symbol interference (ISI) Lien & Cherniakov, 1998. The characteristics of the spreading sequences in CDMA system provide a crucial effect on the performance of the whole communication systems. This signature sequences in general determine how much interference is received at a receiver from other mobile users and influence the extraction capability of the desired signal from noise-like spectrum Xie & Rahardja, 2005. On the other hand, since the reverse link of a CDMA system is usually asynchronous, in the sense that the arrival times for each mobile user signal are different Thompson et al., 1996, Choi et al., 2007. Therefore, the spreading sequences of CDMA systems are characterized with ISI as well as multiple access interference (MAI) Peterson et al., 1995, Guo & Wang, 2008. In multipath propagation environment, multiple copies of transmitted signal arrive at receiver with different time delay will cause ISI. A MAI occurs if the orthogonality among spreading sequences is lost Ishida et al., 2000. The MAI is caused by asynchronous in a CDMA system where each mobile user will observe interference from all other mobile users in the system, since the transmitted signal will not be orthogonal in delay-spread environment Thompson et al., 1996. Traditional CDMA spreading sequences such as m-sequence Golomb, 1992, Gold codes Gold, 1967, and Kasami codes Kasami, 1966, exhibit non-zero cross-correlation which results in high MAI in asynchronous reverse link transmission. Another family of orthogonal codes is constituted by Walsh codes Harmuth, 1970 and orthogonal Gold codes Popovic, 1997, do retain their orthogonality in the case of perfect synchronization, but also exhibit non-zero cross-correlation in asynchronous transmission Wei et al., 2005. Recently, an attractive family of large area synchronized (LAS) CDMA spreading sequences is introduced in Li, 2003 has exhibited zero correlation zone (ZCZ) or interference free window (IFW) near zero delay time offset, resulting in zero ISI and MAI within the IFW. The LAS spreading sequence is constituted by the combination of Large Area (LA) code Li, 1999 and Loosely Synchronous (LS) code Staňczak et al., 2001. More specifically, the interference-free in CDMA system only become possible when the maximum channel-induced delay-spread is within the designed IFW duration. However, in the system design especially using omnidirectional antenna, not all multipath signal components arrive within IFW time offset. Since the total duration of IFW expressed in terms of the number of chip intervals depend on the minimum zero padding implanted between non-zero pulses interval, thus the number of minimum zero padding must be increased to maximum delay-spread of the channel in LAS sequence in order to accommodate all multipath signal components. This implies that the duty ratio of LAS spreading sequences is low when the number of minimum zero padding is increased. Therefore, a specific drawback of LAS-CDMA is that its relatively efficient orthogonal codes demanded in wireless systems are limited, and hence reduce its spectrum efficiency. Besides that, the implementation of LAS sequences is very complex that additional components are necessary.
There have been many multiple access systems for the cellular system designed to improve its system performance. Several works have been carried out to show the improvement in the system capacity using the joint multiple access system. A careful selection of joints multiple access from two or more individual systems can determine the fitness of the joint system. Interference-limited systems such as CDMA and SDMA are susceptible to time of arrival (TOA) and angle of arrival (AOA) of individual user signals. Thus, a non-uniform traffic can severely degrade the performance of CDMA and SDMA systems. In this chapter, a joint multiple access of CDMA and SDMA system is proposed. The performance of this joint multiple access system is also vulnerable to the non-uniform traffic. Although the performance of this joint multiple access system has been previously studied in several papers Liberti & Rappaport, 1994, Naquib et al., 1994, Buracchini et al., 1996 and Ng & Sousa, 1998, none of them considers to evaluate the most realistic of system performance in this joint multiple access.
In this chapter, a new approach called dynamic space-code multiple access (DSCMA) system arising from the combination of CDMA and SDMA systems is designed, and its system performances are then investigated. An innovative approach to eliminate the existing interferences in DSCMA system is introduced. The spreading sequences of Large Area Synchronous Even Ternary (LAS-ET), which exhibited an interference free window (IFW) in their correlation, are exploited here. The spatial signature from smart antenna narrower beam is exploited to drive all the multipath propagation signals to arrive within the IFW in reverse link transmission. The size of IFW is adaptable with the size of smart antenna beamwidth through dynamic space-code (DSC) algorithm. Therefore, the result of combined dominant signature from DSCMA system will yield a perfect interference cancellation so that the system capacity increases dramatically.
2. The Properties of Orthogonal CDMA Sequences
Traditional ways of separating multiple access signals in time or frequency such as TDMA and FDMA are relatively simple by making sure that the signals are orthogonal and non-interfering. However, in CDMA different mobile users occupy the same bandwidth at the same time. They are separated from each other through the use of a set of orthogonal sequences. Two waveforms
In discrete time, the two sequences
As an example, the following two sequences or codes,
Hence, their cross-correlation is zero.
In order for the set of codes to be used in a multiple access scheme, an additional property is needed. In addition to the zero cross-correlation property, each code in the set of orthogonal codes must have an equal number of +1s and –1s Faruque, 1996. This second property gives that particular code the pseudorandom nature. A direct sequence CDMA (DS-CDMA) system spread the baseband data by directly multiplying the baseband data pulses with a peudorandom or PN sequence that is produced by a PN code generator. A single pulse or symbol of the PN waveform is called a chip, where the chip rate is much higher than the data bit rate Lee, 1991.
2.1. Welch Bound in CDMA Systems
The CDMA system is a multiple access scheme in which several independent users access a common communication channel by modulating their data symbols with preassigned spreading sequences. The receiver observes the sum of the transmitted signals in additive white Gaussian noise (AWGN) channel. The decoder for a given mobile user treats the sum of the interfering signals from other mobile users as noise. The spreading sequences are chosen to create good single user channels for the individual coding systems. In fact, however, the channel created by the spreading sequences is susceptible to MAI Rupf & Massey, 1994. In 1974, Welch in Welch, 1974 had shown that the lower bound for the acceptable sidelobes of auto-correlation and cross-correlation functions are set around
2.2. LAS-ET Sequences
The original LAS codes proposed in Li, 1999 are synthesized by seeding LS codes in LA codes to improve it spectrum efficiency. An
In order to exploit the characteristics of LA sequences proposed in Li, 1999 without altering the size of its IFW, a modified version of the sequence such LAS-ET sequences Ng et al., 2009 is employed in DSCMA instead of LAS-CDMA sequences proposed in Li, 2003 which exhibit a small IFW. Figure 1 shows the correlation properties of the
3. Reverse Link Capacity of SDMA System
The conventional SDMA systems increase its capacity by spatial filtering the interferences. The system continuously adapts its narrower beam from smart antenna system to steer each mobile user with the main lobe while isolating interferences with nulls. Hence, SDMA is allowed to reuse the limited radio resources (frequency, time and code) within a cell. From Equation (1) in Ng et al., 2008, the nulls’ AOA,
Figures 2aa and 2b show the typical SDMA system for
The interfering users are only allowed to be located at null AOAs, otherwise co-channel interferences between mobile users will occur. Any additional mobile user into this system after the limited nulls are fully occupied will also cause co-channel interference to other mobile users.
It has been reported that smart antenna can synthesize a high directive beam toward the desired user while nulling the interfering users to increase capacity. However, fully nulling the interfering users in SDMA system do not take place because there are two major interference sources, which are side-lobes and co-channel interferences. The interfering users will not always locate at the nulls of the desired user radiation pattern especially in randomly distributed traffic environment as shown in Figure 3.
4. Dynamic Space Code Multiple Access (DSCMA) System
Non-uniformly distributed traffic usually degrades the performance of CDMA and SDMA systems severely in the reverse link. The imperfect correlation properties of the traditional CDMA spreading sequences result in ISI and MAI at non-zero delay spread. The random positions of mobile users will cause MAI among them in the SDMA system, where positions at nulls of the desired user radiation pattern are rarely achieved. Therefore, the non-uniform traffic causes loss of orthogonality to distinguish each mobile user in the conventional interference limited systems.
Here, a promising solution to deploy the BS with smart antenna system to perform the joint multiple access of CDMA and SDMA systems is proposed. The CDMA and SDMA systems are adapted to each other dynamically to form DSCMA system. This proposed multiple access scheme is a novel interference cancellation scheme that employ the spreading sequences of CDMA system into spatial signatures of SDMA system through DSC algorithm. In DSC algorithm, the size of dedicated IFW from LAS-ET spread sequence is adapted dynamically to the size of half power synthesized beamwidth from smart antenna beamforming system as shown in Figure 4. In this joint multiple access scheme, each user is assigned an LAS-ET sequence within a high directivity beam. Hence, the integration of these two signature schemes, spatial filtering and spreading sequence, creates a dominant signature scheme called DSC signature. Therefore, by using this dominant signature scheme, the inherent interferences in CDMA and SDMA systems environment can be eliminated.
As shown in previous section, the co-channel interference between two mobile users in SDMA system occurs when both of them are located close to each other. For example, assuming that the desired user,
Therefore, it is necessary to prefer spreading sequences that exhibit zero correlation between each other to drive all the asynchronous signal components to drop within the smart antenna’s narrow beam maximum propagation delay spread. Hence, a spreading sequence that exhibits large size of IFW is required to accommodate large beamwidth of smart antenna radiation pattern. Considering Figure 3 again, there is group of beams with theirs AOA respectively to accommodate 17 randomly distributed mobile users. Each beam is assigned to different mobile user with an LAS sequence order, C1, C2, C3, C4, C5,…, Cm, where m is the maximum number of total available sequence. These sequences are assigned to mobile users in chronological order upon their arrival and can be reused dynamically whenever needed.
To illustrate how directive beam can improve the reverse link in a single cell of DSCMA system, consider the case in which each mobile user has an omnidirectional antenna, and the BS tracks each mobile user in the cell using a directive beam. Assume that the beam pattern,
5. Reverse Link Interferences in DSCMA System
In DSCMA system, a BS equipped with smart antenna transmits signal to each mobile user in forward link transmission using a synthesized narrow beam and a dedicated spreading sequence. The signal is perfectly synchronized at transmission so that it arrives at mobile receiver in synchronism with zero delay spread. Consequently, due to the orthogonalities of both spatial signature and spreading sequence in zero delay spread among the
However, this synchronism in forward link transmission cannot be maintained in reverse link transmission where all the signals from
5.1. Intra-cell Interference
Suppose that each cell has
In DSCMA system, the
The arbitrarily interferences level,
5.2. Inter-cell Interference
In the multi-cell of DSCMA system, the interference analysis in reverse link becomes complicated. This is because the mobile users are power controlled by their own cell BS. The membership of the user is determined by the maximum pilot signal power among the cells and not the minimum distance from a cell BS. The mobile users are connected to a BS that offers the lowest signal attenuation rather than the closest BS Gilhousen et al., 1991. Because of power control, the interference level received from mobile users in neighbouring cells depends on two factors: attenuation in the path to the desired user's cell BS, and attenuation in the path to the mobile user's cell BS. Thus, in the fourth power law of distance, the user’s transmitted power
Hence, from (14) and (15), the interference to signal ratio,
where the first term is due to the attenuation caused by distance and blockage to the given BS, while the third term is the effect of power control to compensate for the corresponding attenuation to its BS. Since
For all values of the parameters in (16),
6. DSCMA System Signalling
To simplify the derivation, only the baseband signal of transmitted signal is being considered. Hence, in DSCMA system, the transmitted signal from the ith user,
Assuming that the desired user is user 0 and all the other K – 1 users are interfering users. The received signal,
7. Probability of Error Evaluation in DSCMA System over AWGN Channel
A bit error rate (BER) expression for DSCMA is derived over MAI from the other K – 1 users in an AWGN channel. The derivation is performed at the baseband level, which will simplify the analysis. From the previous section, the first term of (22), S is the transmitted signal of user 0, where
Therefore, the variance of (26) becomes
The term I in (22) is the MAI component of the K – 1 interferers, which is given by
The signal to interference plus noise ratio (SINR) is then given as
Therefore, from (24), (28) and (31), the SINR for the DSCMA system can be expressed as
Since the amplitude of each mobile user’s signal is
Assuming that the combining noise and interference components have a Gaussian distribution, then the BER is given as
where Q(x) is the Gaussian Q-function.
For the interference limited of DSCMA system where thermal noise is not a factor due to
where the resultant of
which is a expression of BER performance in DSCMA over AWGN channel.
8. BER Performance and System Capacity in DSCMA System
Two different types of BS antenna, omnidirectional antenna and smart antenna are exploited for BER performance comparison through simulation. The system simulation will evaluate the BER performance of DSC algorithm in DSCMA by considering interference from both intra-cell and inter-cell interferences. The BER expressions over these interferences have been derived in the previous section. From these BER expressions the system capacity in DSCMA can be estimated by looking at the number of mobile users that the system can support at 0.001 BER (BER < 10-3). Since the cell is split into three sectors, the inter-cell interference sources are only considered from two neighbouring cells for each sector. The system chip rate is 1.2288 Mcps resulting at a data rate of 4.8 Kbps for sequence length of about 256 chips per bit. This data rate is used to transmit the multimedia type data. The data transmission will go through a wireless channel with fourth power of distance loss and 8 dB shadowing. The voice activity factor is not taken into consideration in this simulation. The system parameters for the system simulation are summarised in Table 2.
|Cell radius, R||Unity|
|Number of sectors per cell||3|
|Number of interfering neighbouring cells per sector||2|
|Type of data||Multimedia|
|Spreading factor, SF||About 256|
|System chip rate, W||1.2288 Mcps|
|Data rate, R||4.8 Kbps|
|Path loss exponent¸||4.0|
|Standard deviation of shadowing,||8 dB|
The radiation pattern of a smart antenna represents gains of different AOA along a 120o azimuth span sector. It is assumed that K separate narrow beams and K different spreading sequences can be generated from BS and directed to each of K users within a sector of interest. Assume that a sector antenna beamwidth is 120o in the three sectors per cell configuration. This beamwidth size attributes to a maximum excess delay,
|Number of elements,||Beamwidth, BW (o)||Maximum angular spread, (o)||Maximum excess delay,|
This table showed that the narrower beam of a smart antenna will reduce the TOA of a transmitted signal. This implies that the maximum excess delay of a channel can be reduced when more elements in a smart antenna system is exploited. To analyse the system performance in DSCMA, all the simulations are executed by considering smart antenna systems with
For comparison purposes, the BER of conventional CDMA and SDMA systems will be the first to be evaluated. In the CDMA system, the omnidirectional antenna gain,
|Sequences||Capacity, (BER < 10-3)|
On the other hand, in the SDMA system, the correlation property,
|Number of elements,||Capacity, (BER < 10-3)|
In the AWGN channel, it is necessary for DSCMA to perform in perfect synchronous manner to obtain the orthogonality among the mobile users in zero delay spread. However, perfect synchronisms rarely exist because each mobile user signal arrives at BS receiver with different delay. Therefore, the orthogonality between spreading sequences is no longer held in non-zero delay spread.
The BER performance of the DSCMA system in AWGN channel for various spreading sequences, vis m-sequence, Gold, Walsh-Hadamard and LAS-ET sequences are shown in Figures 9a - 9d respectively. In each figure, the BER performance is evaluated by exploiting the smart antenna with different number of elements,
|Number of elements,||Capacity, (BER < 10-3)|
Table 6a. DSCMA system capacity of BER < 10-3 for m-sequence with different number of antenna elements,
|Number of elements,||Capacity, (BER < 10-3)|
|Number of elements,||Capacity, (BER < 10-3)|
|8||6.85 x 103|
|16||2.75 x 104|
|32||8 x 104|
|64||2.4 x 105|
DSCMA system capacity of BER < 10-3 for Walsh-Hadamard sequence with different number of antenna elements,
|Number of elements,||Capacity, (BER < 10-3)|
|4||2.4 x 1033|
|8||5.67 x 1033|
|16||1.27 x 1034|
|32||2.04 x 1034|
|64||2.9 x 1034|
DSCMA system capacity of BER < 10-3 for LAS-ET sequence with different number of antenna elements,
All the evaluated system capacities in DSCMA system here are based on the interference level that the system can tolerant. Hence, these attained results are not the ideal system capacity performance. There are some other factors need to be considered in conforming to this issue such as the totally bandwidth available, the number of spreading sequences that can be synthesized, and the limitation of system signal processing. Therefore, all the attained results are only suited for the comparison purpose and are not representing the real scenario.
In general, it shows that the DSCMA system performance is improved with spreading sequences of m-sequence, Gold, Walsh-Hadamard and LAS-ET in ascending order as in conventional CDMA system. This is because the mean square correlation property,
All the energies from higher number of antenna elements,
It can be concluded that non-uniform traffics can severely degrade the performance of CDMA and SDMA cellular systems. The DSCMA system described in this chapter is a double signatures system that can distinguish more users by cancelling the existing interference in multipath environment. This multiple access system uses LAS-ET sequences to create IFW near zero delay spread in its cross-correlation function. In order to ensure all the signal components drop within the IFW, a narrow beam with higher directivity smart antenna system is exploited. The size of IFW is adapted to the smart antenna half-power beamwidth using DSC algorithm. Therefore, all the interferences induced in non-uniform traffics can be dramatically reduced in DSCMA system and thus resulting in higher system capacity.
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