A novel multiple access scheme for mobile communications systems

Indian Journal of Radio & Space Physics Vol. 36, October 2007, pp. 430-435

A novel multiple access scheme for mobile communications systems Poonam Singh, R V Raja Kumar & T S Lamba Department of Electronics & Electrical Communication Engineering, Indian Institute of Technology, Kharagpur 721 302 (WB), India {psingh, rkumar, tsl} @ece.iitkgp.ernet.in Received 19 June 2007; accepted 30 August 2007 This paper presents a novel multiple access scheme for future mobile communications using Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). The proposed system can support different classes of users each with different data rates and can provide very high spectral and system efficiency for the uplink and downlink. It is able to meet the demands on flexibility in data rate and provides scalability with respect to bandwidth using variable time slots, spreading factors and number of subcarriers. It exploits the advantages given by the combination of the spread spectrum technique with multicarrier modulation as well as TDMA. Keywords: Code division multiple access (CDMA), Orthogonal frequency division multiplexing (OFDM), Multi carrier CDMA, Multi-carrier direct sequence code division multiple access (MC-DS-CDMA) PACS No: 84.40 Ua

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Introduction The primary goal of next generation wireless system will be the convergence of multimedia services such as speech, audio, video, image and data. This implies that a future wireless terminal will be able to connect to different networks in order to support various services by guaranteeing high speed data. The rapid increase in the number of wireless mobile terminal subscribers and users of wireless local area networks (WLAN) and wireless local loops (WLL) highlights the importance of wireless communication. The most important objective of the global 4th generation (4G) wireless systems is to offer cellular users broadband multimedia services everywhere. The 4G cellular systems will support much higher data rates (100 Mbps-1 Gbps) than 3G cellular. When selecting a multiple access scheme, perhaps the most important question is the number of admissible users per cell for a given available total bandwidth, for given radio propagation conditions and for a required transmission quality. All the existing multiple access schemes, orthogonal frequency division multiple access (OFDMA), time division multiple access (TDMA) and code division multiple access (CDMA), taken together provide plenty of resources for providing subscribers with a wide variety of services and applications and accommodating new applications yet to be imagined.

Frequency Division Multiple Access (FDMA) is the multiple access technique employed in the first generation of cellular communication systems, e.g. the Analog Mobile Phone Service (AMPS) system, where the system bandwidth is divided into several channels and each user is assigned a distinct channel. The commonly used multiple access schemes for second and third generation wireless mobile communication systems are based on either TDMA, CDMA or the combined access schemes with FDMA. In a TDMA system all users can use the entire channel bandwidth and are distinguished by allocating short and distinct time slots to each user. In CDMA, all users are allowed to use the entire system bandwidth all the time. The signals of users are distinguished by assigning different spreading codes. The FDMA and TDMA techniques can accommodate N users on a channel whose bandwidth is N times the bandwidth of individual user signals without any mutual interference, but not a single additional user can be supported beyond this limiting number. The CDMA does not have a hard limit on the number of users that can be accommodated, but is subject to multi-access interference (MAI), which increases linearly with the number of users1. Orthogonal Frequency Division Multiplexing (OFDM) is a special case of multi-carrier modulation2, where a single data stream is transmitted over

SINGH et al.: MULTIPLE ACCESS SCHEME FOR MOBILE COMMUNICATIONS SYSTEMS

a number of lower rate subcarriers. By using a large number of subcarriers, a high immunity against multipath dispersion can be provided since the useful symbol duration Ts on each sub-stream will be much larger than the channel time dispersion. Hence the effects of inter-symbol interference (ISI) will be minimized. To eliminate ISI almost completely, a guard time is introduced for each symbol. The guard time is chosen larger than the expected delay spread. The OFDM can be easily realized by using discrete Fourier transform (DFT) or more computationally efficient Fast Fourier Transform (FFT)3. Today, progress in digital technology has enabled the realization of a FFT also for large number of subcarriers, through which OFDM has gained much importance4,5. In OFDMA (Orthogonal Frequency Division Multiple Access), the channel bandwidth is divided into a number of subchannels. The subchannel is a subset of carriers out of the total set of available carriers. In order to mitigate the frequency selective fading, the carriers of one subchannel are spread along the channel spectrum. A CDMA scheme is a potential candidate for a third generation system. However, it has to cope with the presence of MAI. The present third generation (3G) systems which use CDMA can provide a maximum data rate of 2 Mbps for indoor environment, which is quite less than that needed for recent multimedia applications that require very high bandwidth with mobility. The most important objectives of 4G wireless systems are to take care of severe ISI, which results from the high data rates and to provide spectral efficiency in the available limited bandwidth. Multi-carrier modulation with spread spectrum technique known as Multi Carrier Code Division Multiple Access (MC-CDMA) is a promising technique for future wireless multimedia communications. A lot of research has been devoted to hybrid schemes such as MC-CDMA, MC-DS-CDMA (Multi-Carrier Direct Sequence Code Division Multiple Access), where multi-carrier and spread spectrum system have been combined8,9. The MCCDMA system avoids MAI due to an FDMA scheme at subcarrier level and also exploits the diversity gain offered by spread spectrum technique. It allows one to benefit from several advantages of both multi-carrier modulation and spread spectrum system by offering high flexibility, high spectral efficiency, simple and

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robust detection techniques and narrow band interference rejection capability10,11. The performance analysis of MC-CDMA and MC-DS-CDMA in multipath fading channels are given in12-14 and their performances have been compared in15,16. In this paper, a new hybrid multiple access scheme is proposed to get higher capacity and more flexibility. This scheme uses TDMA, CDMA and OFDMA to accommodate data rates from 8 kbps to 100 Mbps in a bandwidth of 20 MHz. The data rates can be increased further by using higher modulation schemes, e.g. 16-QAM or 64-QAM under good channel conditions. The number of time slots per frame, spreading factor and number of subcarriers are variable and provide flexibility to support variable data rates. This paper is organized as follows. Section 2 discusses briefly the existing multiple access schemes, Section 3 discusses the proposed multiple access scheme, Section 4 gives the system model for implementation of this scheme and the performance analysis for the proposed MC-CDMA based system is given in Section 5. Simulation results are discussed in Section 6 to show the effectiveness of the proposed method. Some concluding remarks are made in Section 7. 2

Proposed scheme The proposed multiple access scheme uses a combination of TDMA, CDMA and OFDMA and thus increases the number of users that can be accommodated in a channel. Each user is assigned a time slot or a number of time slots depending on its data rate. The spreading factor is variable for different service classes and hence the number of subcarriers is also different. The spreading factor is chosen according to data rates for each application. Then the data are spread using the user specific spreading code of length L and the spread data are transmitted on different subcarriers using OFDM. The chips of spread data symbol are transmitted in frequency direction over several parallel subchannels (MCCDMA) or in time direction over several multicarrier symbols (MC-DS-CDMA). The MC-CDMA based system has been proposed for the downlink wireless access of the 4G mobile radio system, because the orthogonality among the channels is maintained by using orthogonal codes when synchronous transmission is used from a base station. The channel estimation accuracy is also maintained by using a common pilot channel with

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high transmission power. However, if MC-CDMA based approach is used in the uplink, the orthogonality among signals of different users is destroyed increasing the MAI, because the transmitted signal of each user is affected by different channel variations1216 . The MC-DS-CDMA system has been proposed for uplink as it transmits the same spread data symbol in parallel over a number of subcarriers, so the signals of different users can be distinguished in the receiver. But spectral efficiency of the system decreases and more complex receivers are needed to handle large number of users. The MC-CDMA is proposed for the downlink and MC-DS-CDMA for uplink in order to optimize both the spectral efficiency and mobile power consumption. In both cases, variable time slots, spreading factors and variable number of subcarriers are used. The maximum achievable data rate depends on the available channel bandwidth, number of time slots and the spreading factor. 3

System model

3.1 Transmitter and receiver

Figures 1 and 2 show respectively, the structures of proposed transmitter and receiver. The proposed system supports M different service classes, each with different data rates Rm. The users are assigned one or more time slots per frame depending on their data

Fig. 1—Proposed transmitter

Fig. 2—Proposed receiver

rates. The spreading factor and the number of subcarriers are also variable and are chosen according to data rates for each application. The modulation scheme employed is Quadrature Phase Shift Keying (QPSK), 16-QAM (Quadrature Amplitude Modulation) or 64-QAM depending on the data rate of users and channel conditions. The complex valued data symbol bk is multiplied with the user specific spreading code of length L. The complex valued data sequence obtained after spreading is then modulated onto a number of subcarriers using OFDM. In MC-CDMA based systems, the number of subcarriers are equal to number of chips after spreading. In MC-DS-CDMA based systems, the number of subcarriers are chosen according to the bandwidth requirement, which is decided by the data rate and modulation scheme used. Thus each data symbol is spread over many subcarriers. A guard time of Tg is usually inserted. The OFDM symbol duration including a guard interval is Ts = T + Tg, where T is the actual symbol duration. Here, one data symbol per user is transmitted in one OFDM symbol. In the synchronous downlink channel, the modulated signals of K active users are added before transmission. In receiver, after removing the guard interval and taking inverse OFDM the received sequence is equalized to get the transmitted data. After channel estimation, the output of each subcarrier is equalized and then coherently combined over the parallel subcarrier components, i.e. despreading of signals is performed. Finally, the regenerated symbol sequences are parallel to serial converted to recover the transmitted binary data. The proposed scheme can transmit data at rates varying from 8 kbps to 10 Mbps. The system bandwidth is assumed to be 20 MHz and the final chip rate is 40 Mcps. The number of subcarriers varies from 8 to 4096 with a subcarrier spacing varying from 4.88 kHz to 2.5 MHz. Duration of one frame is 10 ms, consisting of 160 time slots, each slot duration being 62.5 µs. It consists of 2560 chips. Each user is assigned 1 to 160 time slots per frame depending on its data rate. The spreading factor varies from 4 to 32. The data symbols are time-multiplexed with the pilot symbols and the resultant symbol sequence is converted from serial to N parallel sequences. Each data-modulated symbol sequence is duplicated into M parallel copies and each duplicated symbol is multiplied by a chip from the spreading code. An orthogonal multi-carrier signal is generated using the IFFT


and a guard interval is inserted every generated symbol. Table 1 summarizes the simulation parameters assumed in this paper. 3.2 Channel model

A mobile radio propagation channel is characterized by frequency selective multipath channel consisting of many propagation paths with different time delays. The simulation is first carried out under ideal channel conditions, where the noise is considered only due to AWGN (Additive White Gaussian Noise). Then the ITU-R defined Vehicular A propagation channel model having six Rayleigh faded discrete paths is considered. As per this model, a channel is modeled as an FIR filter, whose impulse response can be expressed as P −1

hi (n) = ∑ ai (n) δ(n − τi )

… (1)

i =0

with a i (n) and τ i being the complex path gain and time delay of the ith propagation path in a P path model. The channel parameters are summarized in Table 2. 3.3 Channel estimation

When coherent detection is used in receivers, information about the channel state is required and has to be estimated by the receiver. The basic Table 1—Simulation parameters Parameters

Values

Bandwidth Chip rate Data rate Spreading Codes Spreading factor Modulation Number of sub-carriers

20 MHz 40 Mcps 8 kbps-100 Mbps Walsh Codes 4-32 QPSK, 16-QAM, 64-QAM 32-4096

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principle of pilot symbol aided channel estimation is to multiplex reference symbols, known as pilot symbols, into the data stream. The receiver estimates the channel state information based on the received, known pilot symbols. The pilot symbols can be scattered in time and/or frequency direction in OFDM frames. Here frequency domain equalizer has been used, so pilot symbols are multiplexed with data symbols in frequency direction. At the receiver, the guard intervals of the received signals are removed and the resultant symbol sequence is converted to a modulated signal of each subcarrier using FFT. For channel estimation, the channel impulse response at each subcarrier is found by averaging the impulse response measured for each of the dedicated pilot symbols. Using the obtained channel impulse response, the output of each subcarrier is equalized and then coherently combined over the N parallel subcarrier components, i.e. de-spreading the signals. Finally, the regenerated symbol sequences are parallel to serial converted to recover the transmitted binary data. 4

Performance analysis It is found that MC-CDMA systems suffer from multi-access interference when the channel is frequency selective fading. In fact, MAI is a major factor that limits the performance of CDMA based systems. The MAI can be reduced by using orthogonal codes. But the orthogonality of these codes could be destroyed in a multi-path environment. For high data rates, the delay spread may be longer than the duration of several chips and the induced MAI will limit the system performance. The transmitted signal of kth user is given by

s k (t ) = bk (t )c k (t )

… (2)

where bk(t) is the data and ck(t) is code for kth user. Due to multi-path fading, the received signal will be given by K

Table 2—ITU-R defined Vehicular A propagation channel model Tap

Relative delay, ns

Average gain, dB

0 1 2 3 4 5

0 710 1110 2090 3730 4510

0 –1 –9 – 10 – 15 – 20

r (t ) = ∑ s k (t − τ ) ⊗ h(t ) + n(t )

… (3)

k =1

where ⊗ represents convolution, τ the propagation delay, h(t) the channel impulse response and n(t) the AWGN, with a double-sided power spectral density of No/2. Also K

r(t)= ∑ bk (t − τ)c(t − τ) ⊗ h(t ) + n(t ) k =1

… (4)

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In the receiver for first user, the received signal is multiplied by the corresponding code in the receiver and the detected signal after equalization is given by T

d1 (n) = ∫ r (t )c1 (t ) dt 0 K

T

k =1

0

=∑

∫ [b (t − τ)c (t − τ) + n(t )]c (t ) dt k

k

1

… (5)

Equation (5) can be written as

T d 1 ( n) =

∫ bk (t − τ

)c1 (t − τ )c1 (t )dt

0  T K T  + ∑ ∫ bk (t − τ )ck (t − τ )c1 (t )dt  + ∫ n(t )c1 (t )dt   k =2 0  0 … (6) The first term in the above equation represents the desired signal at the output of first user's receiver, second term represents the interference from other (K–1) users, known as MAI and the third term corresponds to the output due to AWGN. In the first term, the delayed version of the received signal is being multiplied by the code of first user, so some amount of self-interference is introduced. In the second term, the delayed signal is multiplied by codes of other users, so even if the codes are chosen to be perfectly orthogonal, the cross-correlation will not be zero and a significant amount of MAI will be present. In general, the MAI for kth user is given by

T  MAI = ∑  ∫ bl (t − τ)cl (t − τ)c k (t )dt    l =1l ≠ k  0  K = ∑ bl (t − τ)ρ kl … (7) l =1l ≠ k

be longer than the duration of several chips and the induced MAI will limit the system performance. In MC-DS-CDMA systems all the chips are transmitted on the same subcarrier, which experiences correlated fading, so MAI is less as compared to MC-CDMA system. It is difficult to analyze MAI exactly, because it is the sum of many interferences, which may not be independent in general. Since the QAM data symbols bk’s are independent and identically distributed (i.i.d.) with zero mean and variance Es (the symbol energy) and the spreading codes ck's are also sequence of i.i.d. random variables with equal probabilities. One can conclude that the interference is at least uncorrelated with zero mean. So applying central limit theorem, MAI can be approximated by a Gaussian process with zero mean and variance N Nc σ 2 N ( K − 1) c 1 σI 2 = n ∑ ∑ 2 2 8π n =1 l =1, l ≠ n (n − l )

where σ n 2 is the noise variance, K the number of users, Nc the number of subcarriers and N the spreading factor. The probability of error for each user is given by Pe (k ) = Q SINR = Q

where

ρ kl = ∫ cl (t − τ)c k (t )dt 0

is

the

cross-

correlation between different users’ spreading sequences. The MAI will be more if this crosscorrelation is large, which increases with the delay spread σ τ . For high data rates, the delay spread may

Eb σ n 2 + σ 2 MAI

… (9)

In a multi-user system, the average bit error rate for K users is given by BER =

K

T

… (8)

1 K −1 ∑ Pe (k ) K k =0

where K is the number of users. 5

Simulation results Figure 3 shows the symbol error rate (SER) performance of the proposed multiple access system in multipath fading channel for different number of users for a data rate of 8 kbps. The modulation used is QPSK and spreading factor is 32. The required bandwidth for one symbol is 128 kHz and number of subcarriers is 32 with a subcarrier spacing of 4 kHz. The RF bandwidth is 20 MHz and so the total number of subcarriers can be 5000, but the number of subcarriers is taken in powers of 2 to reduce computational complexity, so one can take 4096


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References

Fig. 3—The SER plot for the proposed system

subcarriers with a subcarrier spacing of 4.88 kHz. Thus, using CDMA, TDMA and OFDMA, the maximum number of users can be 5000 at a data rate of 8 kbps, giving a maximum spectral efficiency of 2. It is observed that in multi-path fading channel the performance degrades as the number of active users increase due to MAI. Even when Walsh codes are used, which are perfectly orthogonal codes, the MAI is not zero, since each chip of the PN sequence experiences independent fading, which tends to destroy the orthogonality between spreading sequences. This increases the MAI and degrades the SER performance. 6

Conclusion A new multiple access scheme has been proposed, which uses FDMA, TDMA and CDMA and can be used for transmission of different classes of data, e.g. audio, video, internet, ISDN, multimedia, etc. having different data rates and modulation schemes. The parameters, e.g. spreading factor, number of time slots, number of subcarriers, etc., are optimized to get suitable bandwidth scalability. These parameters are chosen according to the required data rate, the available bandwidth, the number of subscribers, etc. The performance of proposed system has also been studied for mobile channels.

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