Performance and Capacity Issues for 3G WCDMA in Micro and Macro Cellular Environment

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1 Performance and Capacity Issues for 3G WCDMA in Micro and Macro Cellular Environment R Moyo and P J Chitamu Centre for Telecommunications Access and Services School of Electrical And Information Engineering University of the Witwatersrand, Johannesburg {r.moyo, p.chitamu}@ee.wits.ac.za Abstract: The Wideband Code Division Multiple Access (WCDMA) has been identified as one of the technologies used to meet the high capacity and high bandwidth requirements of the third generation mobile system. The main challenge of this technology is multiple access interference and various multiuser detection (MUD) methods proposed to alleviate this problem. In the search for suitable multiuser detection solutions most emphasis has been placed on the performance and complexity under certain ideal conditions. The receiver operates in radio environment with characteristics that may widely vary. The question of robustness of techniques under various conditions has not been fully explored. In this paper, we first present a survey of the detection techniques that have been proposed for the third generation (3G) cellular system and then propose and investigate suitable receiver structure that can cope well in a wide range of operating environments and meet the 3G prescribed data rates. Key words: CDMA, multiple access, interference multiuser detection I. INTRODUCTION The evolution of the mobile cellular systems from the second generation to the third generation (3G) systems, are mainly driven by the popularity in the mobile voice services, demand for other services such as data, video and multimedia applications and international roaming. The primary requirements of 3G systems can be summarized as: Voice services requiring a quality of service which is better or the same as that of the fixed networs Full coverage and mobility for 144 Kbps, but preferably 384 Kbps Up to 2 Mbps for limited coverage and mobility High spectrum efficiency compared to the existing system Bacward compatibility with pre-existing networs and flexibility to introduction of new services. These requirements and the lac of spectrum availability lead to an increased demand for bandwidth efficient multiple access schemes. At the moment, the Wideband Code Division Multiple Access (WCDMA) is largely preferred as the multiple access technology for 3G systems. Unlie the Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) schemes which rely on orthogonal user portioning in the time-frequency plane, wasting spectrum in between time/frequency channels, in CDMA the whole spectrum is available to each user and the separation is in the code domain, resulting in more bandwidth. The main attraction of CDMA is potential capacity, coverage and efficiency improvements. However, despite its advantages, there are challenges in the design of WCDMA receivers due to complexities associated with the detection and degradation of performance due to the Multiple Access Interference (MAI), which limits the capacity of the system. Various multiple user detection (MUD) techniques have been proposed to mitigate MAI, but in practice, performance is not only degraded due to MAI, it is also affected by the channel fading and self-interference due to multipath propagation especially when operating in the mobile environment. The interest in this wor is due to the fact that there is a broad range of possible operating environments, in which the performance and achievable capacities of the various MUD techniques have not been fully investigated. Section II of the paper gives a brief overview of the WCDMA technology, section III briefly discusses the radio channel characteristics, section IV gives an overview of multiuser detection, and section V loos at the implementation issues and the wor proposed for this research. II. WIDEBAND CODE DIVISION MULTIPLE ACCESS WCDMA is an air interface that provides the opportunity to design a system with the properties of fulfilling 3G requirements. Important concepts in WCDMA are the introduction of the pilot channel associated with each data channel that maes coherent detection possible on the reverse lin. The minimum 5MHz bandwidth maes the data rates of 144 and 384 Kbps achievable, and is large enough to resolve more multipath than in the narrow band version, IS-95. The ey technical parameters are shown Table 1[1].

2 Table 1 Technical parameters of the WCDMA Channel bandwidth 5,10, 20 MHz Chip rate 4.096/8.192/16.384Mcp s Multiple Access DS CDMA Variablerate speech Frame length 10 or 20ms Pacet Data ms Balanced QPSK (downlin) Spreading QPSK (Uplin) Modulation Complex spreading circuit Data QPSK (downlin) BPSK (Uplin) Coherent detection Pilot Symbol assisted Channel Coding Convolutional and turbo coding Scrambling code 10ms Interleaving 10/20/40/80 ms Multirate Variable spreading and multicode Spreading factor Variable length orthogonal sequences Downlin for channel separation. Gold sequence2 18 for Spreading user and cell separation Variable length orthogonal sequences Uplin for channel separation. Gold sequence 2 41 for user separation Soft handover Handover Inter-frequency handover Power control Open and fast closed loop (1600Hz) Power control step size dB Inter base station timing Asynchronous WCDMA is based on wideband direct spread spectrum technology (DSCDMA) in which a high rate pseudo-random noise sequence (unique code sequence/spreading code) modulates the data for each user. Due to the high frequency of the sequence, the modulated signal bandwidth occupies a much broader bandwidth than the data signal. At the receiving end, with the code sequences of the user nown, received signal is decoded to recover original data. This is made possible by the fact that cross-correlations between the code of the desired user and the codes of the other users can be made to be small. The receiver auto-correlates the received signal with the synchronously generated replica of the spreading code to recover data from the informationbearing signal. Any cross-correlations result in part of the interfering signal being taen into the information bandwidth. In most cases users signals do not use completely orthogonal spreading codes, then there is always residual interference nown as multiple-access interference (MAI). The interference affects users in the same cell (intra cell interference) and other cell (inter cell interference) because of universal frequency reuse. Multiple-access interference can present a significant problem if the power level of the desired signal is significantly lower than the power level of the interfering user. Propagation mechanism causes the signal received by the base station from a user terminal close to the base station to appear stronger than the signal received from another terminal located at the cell boundary, such a phenomenon is called the near-far effect. To solve this problem, a transmitted power control (PC) signal in the reverse lin (mobile-to-base path) is used to achieve a constant received mean power from each user at the base station. The W-CDMA standard controls the power level of each mobile and base station with each new slot (using the TPC bits), thereby yielding a control rate of 1.6 Hz. The problem of MAI has received attention from various researchers adopting different approaches, mainly in the areas of code waveform design, investigating spreading codes with good cross correlation properties, power control, forward error correction codes and sectored /adaptive antennas. One aspect that has drawn most attention is Multiuser detection (MUD). III. RADIO CHANNEL MODELLING An important requirement for assessing a wireless technology such as 3G WCDMA is to have an accurate description of the wireless channel in the 3G service area environments. It is nown that channel models are heavily dependent upon the operating environment and the architecture of the radio system. The wireless channel will normally be characterized by path loss, multipath delay spread, fading characteristics, Doppler spread and interference, both co-channel and adjacent channel interference. For the case of path-loss, the most widely used path-loss model for signal strength prediction and simulation in macro cellular environment is the Hata-Oumura model [2]. Using path-loss models, it is possible to predict the average received signal strength and hence the coverage footprint for base stations. In general, the path-loss at a distance d is generally given as: d P L L d o 10 + σ d o ( d) P ( ) + n log X (1) = where n is the path-loss exponent, P L (d o ) is the path loss at a reference distance of 100m and X σ represents the shadowing effect that follows a log-normal distribution. The typical value of the standard deviation for X σ is between 4 and 16dB depending on the terrain [2].

3 The issue of multipath delay spread is even more complicated especially in a mobile environment as it compounds the fading characteristics by induced fast deep fades on the received signal. Because of its wide bandwidth, WCDMA always has frequency selective fading characteristics. These vary depending on the area. Propagation measurements results reported in [3] produced delay spreads values ranging from 2µs to about 70µs with the mean delay spread values of between 3µs and 20µs [3]. Such areas could have completely different performance and capacity results from typical urban areas with average of 15µs delay spreads. Of interesting in this study is to consider practical channel characteristics in analysing MUD schemes. Wideband channel are usually modelled as shown in equation (1) where each ray fades independent of the other at the chipsampling rate. In 3G WCDMA however, there is provision for different spreading factor of between 4 and 256. This enables the system to support variable rate services at the same time enable different delay spread profiles to be accommodated by varying the effective symbol period. L (, ) ( ) = ( jφ h t τ β δ t τ ) e l l (2) l = 1 l where β l (), τ l (), Φ l (), are the l th path gain, delay and phase for the th user and L is the deterministic number of resolvable paths. IV. MULTIUSER DETECTION TECHNIQUES FOR WCDMA The single user detector for DSCDMA, in its simplest form, is a matched filter of the desired signals, which considers other users' signal as noise. In multiuser detection (MUD), a joint detection and interference cancellation (IC) is performed. A. MUD problem The basic problem of multi-user detection is that of inferring the data contained in one or more signals embedded in a nonorthogonal multiplex, the entire multiplex of which is received in ambient noise and intercell interference as described by the following equation r K B s + = 1 i= B () t A b () i ( t τ it ) + n() t I() t = where K, A,b i, s, τ,,t and n are the number of active users, received amplitude, i th bit, normalized signal, delay, symbol interval, noise process of user and I represents intercell interference and direct sequence DSCDMA signalling waveform is described as s () t = 2a () t sin( ω c + ϕ ), t [ 0, T ] t [ 0, T ] (3) (4) where ω c is the carrier frequency, ϕ is the phase of the th user relative to some reference and the spreading wave forms a are in the form N 1 j= 0 () t = a ( t jt ) a ψ, j In this case, a,0, a,1, a,2. a,n_! is a signature sequence of +1 or 1 assigned to the th user and is a normalized chip waveform of nominal duration T c =T/N (N is spread by a factor). The signature spreading codes and chip waveform are typically chosen such that the auto-correlation and cross-correlation properties reduce the effect of MAI even in multipath propagation environment. In this context, multiuser detection refers to the problem of detecting all or parts of symbol matrix B (whose entries are given by, b i, = b (t)) from the multiplex. In the demodulation of any given user in such a system, it is necessary to process the received signal in such a way as to minimize the two types of detrimental effects: the MAI caused by the remaining K-1 users in the channel and the ambient channel noise (AGWN). The conventional receiver uses a ban of optimum single-user detectors operating independently. Since in general the input to every threshold has an additive component of MAI, the conventional receiver is not optimum in terms of errorprobability. For direct sequence spread spectrum systems (DSCDMA) the signal constellation has large bandwidth, therefore the cross-correlations between the signals can be ept to a low level for some relative delays and acceptable performance can be achieved. Nevertheless, if data demodulation is restricted to single-user detection systems, then the cross-correlation properties of the signal constellation carry the entire burden of complexity required to achieve a given performance level, and when the power of some of the interfering users is dominant, performance degradation can become. User detection of asynchronous signals is inherently a problem of sequence detection, that is, observation of the whole received waveform is required to produce a sufficient statistic for any symbol decision. Hence one-shot approaches (where the demodulation of each symbol taes into account the received signal only in the interval corresponding to that symbol) are sub-optimal. The observation of the complete intervals of the overlapping symbols of the other users gives additional information about the received signal in the bit interval under consideration. Thus the general structure of optimal systems for determining the data symbols from the received waveform consists of an analog front end that extracts the matched filter outputs, followed by a decision algorithm that infers optimal decisions from the collection of these outputs. The matrix of matched filter outputs Y whose entries are given by c (5) ( t it ) r( t) (6) yi, = s τ dt forms a sufficient statistic for the matrix B of data symbols whose entries are given by b i, = b (t)), that is all information

4 in the received waveform that is relevant to maing inferences about B is contained in Y. Therefore the main job of optimal multiuser detection is to map the Y matrix of observables to a matrix of B symbol decisions. The nature of the decision algorithm in this process depends on the optimality criterion applied to the decision. If a maximum-lielihood criterion or a minimum-error-probability criterion is used, then assuming the signalling s waveforms satisfies s (t)= 0 for t [0,T], this optimal decision algorithm can be implemented as a sequence detector having O( A K ) time complexity per binary decision [4], where A is the size of the symbol alphabet. B. Performance comparison Verdu's wor shows that optimum (maximum-lielihood sequence) detectors can completely eliminate MAI, thus greatly increase CDMA system capacity. However, the complexity of the optimum detector is exponential to the number of users, which is too complicated for practical implementation. There have been great interests in finding sub optimum detectors with acceptable complexity and marginal performance degradation compared with the optimum detector. Various sub optimum detectors have been proposed, most of which can be classified into two categories, namely linear multiuser detectors and subtractive interference cancellers. In linear multiuser detectors, a linear transformation is performed to the soft outputs of the conventional detector to produce a new set of decision variables with MAI greatly (or completely in decorrelating detector) decoupled. Two of the most cited linear multiuser detectors are the decorrelating detector which chooses the linear filter to have zero output MAI [4]; and the Minimum Mean Squared Error detector (MMSE) which chooses the linear filter to have minimum output energy within the constraints that the response of the filter is fixed [7,9]. In subtractive interference cancellation, estimates of the interference are generated and subtracted out before detection. The cancellation can be carried out either successivelysuccessive interference canceller (SIC)[6,7] or in a parallel manner-parallel interference canceller (PIC)[6,8]. There are many desirable futures for decorrelating detector; it yields the optimal value of near-far resistance performance metric, it does not need to estimate the amplitude of the received signal and it corresponds to maximum lielihood sequence detector when the energies of all users are not nown at the receiver. However, there is a need to compute the inverse of a cross-correlation matrix, which brings in a computational complexity and as a result it is unacceptable for practical implementation. On the other hand, subtractive interference cancellers are much easier to implement compared with linear multiuser detectors, but the performance gap between them is quite obvious. Another disadvantage of subtractive interference cancellation is that they usually need to estimate the amplitude and carrier phase of all active users. Direct performance comparison between the linear detector and interference cancellation detectors was done in [12,13], where the performance of the SIC, partial PIC, decorrelating and NMSE are compared in various conditions such as perfect power control, single path Rayleigh fading and two path Rayleigh frequency selective fading, with a processing gain of 31. The results showed that, under perfect power control, the partial PIC has the best performance, with MMSE and decorrelating detectors having similar performance. In single path Rayleigh fading and 2-path frequency selective Rayleigh fading channels, linear detector shows performance superiority. An investigation of the equal transmission power and near far problem appears [12], where the comparison of the PIC and the decorrelating detectors indicates that for low to moderate signal to noise ratio (SNR) PIC out performs the decorrelating receiver. The decorrelating receiver is only superior under severe near far problems at high SNR. It is therefore necessary to either reduce the complexity of linear multiuser detectors or improve the performance of subtractive interference cancellers. In the interference cancellers, SIC has less computational complexity hence is less hardware intensive, since canceling is done serially the delay bits are added as the number of users increases which could results in large delays. On the other hand PIC causes less delay, but is more hardware intensive to process users in parallel. Comparisons of these were done in [14,15] and with perfect power control, PIC performs better than the SIC. Without power control the reverse is true according to results from [14]. In single path Rayleigh fading and 2-path frequency selective Rayleigh fading [12], the PIC and SIC performances were close. In [15] comparisons of the systems were done under different data rates the lowest BER obtained was from the SIC. V. PROPOSED WORK Many authors indicate that the main issue on implementation is the complexity versus performance and the robustness of the system for different receivers. From the comparisons done it shows that there are changes in the performance with the change in environments. The detector must be able to cope with the uncertainty in timing caused by the channel, both with respect to the nominal propagation delay and multipath propagation. One of the primary challenges to multiuser detection offered by the mobile wireless environment is that essentially all user parameters such as received pulse shapes, amplitude, relative carrier phase, timing and whether or not a particular user is active, are time and space varying.

5 One of our focuses is to investigate MUD algorithms, which can cope with the non-stationary, non-ideal condition in a mobile radio environment; in particular, cases where typical delay spreads of over 50µs can be encountered. In implementation, one has to consider a real world situation in which parameter estimation is non-optimal. Also modeling uncertainties have to be considered. Therefore, it is important to investigate the above effect on the efficiency of existing multiuser detection algorithms and possibly propose new receiver structures. VI. CONCLUSION In this paper, we presented a survey of the four most common detection techniques, namely the decorrelating, MMSE, SIC, and PIC. The comparison was based on the performance and complexity since there is very much limited literature on the robustness of these structures. Despite complexity linear detectors performance is not superior to the less complex subtractive detectors in near to practical situations (low SNR, moderate far near problem) with the multistage PIC out performing the decorrelator receiver. The decorrelating is more sensitive to timing errors than the subtractive interference cancellers, however subtractive interference cancellers are more vulnerable to amplitude estimation error than the decorrelating. Since 3G WCDMA have matured and are being deployed in practical situations, there is a need to explore further the detection techniques that are robust enough to cope with a broad range of propagation and service environments where 3G WCDMA is going to be used.. REFERENCES [1] Tero. Ojanperä and Ramjee Prasad, "Wideband CDMA for Third generation Mobile Communications", Artech House Publishers 1998, [2] Theodore S. Rappaport, "Wireless communications Principles and Practice", Prentice Hall 1996, [3] P.J. Chitamu and H. R. Mgombelo,"Propagation Measurement Results for Scintillating Channels from Selected Areas of Rural Tanzania", Procs. of International Conf. on Advances in Systems, Signals, Control, and Computers (SSCC'98), September 1998, Durban, South Africa, pp [4] S. Verdu, "Muliti-User Detection", in Advances in Statistical Signal Processing-Vol. 2 Signal Detection, H. V. Poor and J. B. Thomas, eds. JAI Press: Greenwich, Conn., [5] R. Lupas and S. Verdu, "Linear Multiuser Detectors for Synchronous Code Division Multiple Access Channel", IEEE Trans Inform. Theory, Vol. IT-35, No. 1, pp ,1989. [6] Stefan Parvall and Miael Höö An overview of CDMA Multi-User Detection techniques [7] P. Patel and J.Holtzman. " Analysis of simple successive interference cancellation DS/CDMA", IEEE J. selected Areas ic Communication. Vol. 12, pp June [8] P. Due-Hallen, J. Holtzman and Z. Zvonar. " Multiuser Detection for CDMA ", IEEE Tran. Communication. Vol 38, pp [9] L. M. Honig, U. Madhow and S. Verdu, "Adaptive Blind Multiuser Detection", IEEETrans. Inform. Theory, Vol. IT- 41, No. 4, pp , [10] M. K. Varanasi and B. Aazhang, "Multistage Detection in Asynchronous Code Division Multiple Access Communnications", IEEE Trans. Comm. Vol. COM-38, No. 4, pp , [11] Duel-Hallen, "A Family of Multiuser Decision-Feedbac Multiuser Detectors for Asynchronous Code Division Multiple Access Channels", IEEE Trans. Comm [12] N S C R Michel Buehrer and B D Woerner. " A comparison of multiuser receiver for cellular CDMA", In Proc.IEEE Globecom 1996, pp Nov [13] M.J. Juntti, M Latva-aho and M. Hieila. "Perfomance comparison of PIC and decorrelating receiver in fading channels ", In Proc.IEEE Globecom 1997, pp [14] P. Patel and J. Holtzman. " Performance comparison of a DSCDMA system using a successive interference cancellation IC scheme and parallel interference cancellation IC scheme under fading channels ", IEEE Proc. ICC 1994, pp [15] "Future Improvements to Wide band CDMA" ww.s2.chalmers.sezsz~portenzszpaperszszrv99.pdf/falahati 99future.pdf Biography: R. Moyo received a B Eng Hons Degree from the National University of Science and Technology in 2000, Bulawayo, Zimbabwe. She is currently a full time MSc. student at the University of the Witwatersrand. P. Chitamu received BSc(Eng.)(Hons.) in Electrical Engineering from the University of Dar es Salaam and the MSc(Telematics) and Ph.D. from the University of Surrey and Southampton respectively. He is presently Senior Lecturer in the School of Electrical and Information Engineering at Wits University.

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