Error probability performance for W-CDMA systems with multiple transmit and receive antennas in correlated Nakagami fading channels

The bit error rate (BER) performance of a two-dimensional (2-D) RAKE receiver, in combination with transmit diversity on the downlink of a wide-band CDMA (W-CDMA) system, is presented. The analyses assume correlated fading between receive antenna array elements, and an arbitrary number of independent but nonidentical resolvable multipaths combined by the RAKE receiver in the general Nakagami-m (1960) fading channel framework. The impact of the array configuration (e.g., the number of transmit antennas and receive antennas, the antenna element separation) and the operating environment parameters (such as the fading severity, angular spread and path delay profile) on the overall space-path diversity gain can be directly evaluated. In addition, the exact pairwise error probability of a convolutional coded system is obtained, and the coding gain of a space-path diversity receiver is quantified.     

Multiuser detection and diversity reception for asynchronous CDMA mobile communication

In CDMA mobile communication systems, multiple access interference can be canceled by multiuser detection technique. The Degradation by channel fading can be reduced by diversity reception. This paper investigates a family of multiuser receivers that combined decor-relating detection, antenna diversity and RAKE multipath diversity. The performance of the multiuser receivers is analyzed. The results demonstrate a significant increase in the performance of the receivers by using multiuser detection and diversity reception.     

Combined multiuser detection and diversity reception for wirelessCDMA systems

Code division multiple-access (CDMA) techniques using interference cancellation are being explored for the capacity increase in third-generation universal mobile telecommunications systems. However, multipath fading is a major constraint on the performance of wireless CDMA systems, with multipath propagation worsening the effects of multiple-access interference, and fading on propagation paths leading to the near far problem. Multiuser detection, exploiting the knowledge of other users to cancel multiple-access interference, has the capability of eliminating the near far problem and providing a significant capacity increase in CDMA systems. On the other hand, diversity techniques effectively combat the fading effects of the channel. This paper investigates multiuser receivers that combine explicit antenna diversity, RAKE multipath diversity, and multipath decorrelating detection. Both coherent reception with maximal-ratio combining and differentially coherent reception with equal-gain combining are analyzed. The results demonstrate a significant increase in up-link capacity over the conventional RAKE receiver, at the expense of complexity. In the case of limited receiver complexity, where the number of correlators is less than the number of resolvable paths at the RAKE front-end, antenna diversity is shown to be effective in reducing residual multiple-access interference     

Joint application of STBC and RAKE receivers in multipath Rayleigh fading channels

This letter investigates the issues on joint application of STBC systems and RAKE receivers in a CDMA system. We show the advantages of using STBC-RAKE receivers in multipath Rayleigh fading channels via error probability analysis and extensive computer simulations. Specifically, this study leads to a better understanding of the interplay between spatial diversity by STBC and multipath diversity via RAKE receivers.     

Performance analysis of cellular CDMA networks overfrequency-selective fading channel

An effect of multipath fading on the performance of a cellular code-division multiple-access (CDMA) system is analyzed in this paper. A wide-sense stationary uncorrelated scattering (WSSUS) channel model and the coherent binary phase-shift keying (BPSK) with asynchronous direct-sequence (DS) spreading signal are assumed in the analysis. The average error probability for both the forward link and reverse link of a cellular CDMA system over a frequency-selective fading channel using a conventional correlation-type receiver and RAKE receiver are derived. The impact of imperfect power control and channel capacity of a cellular CDMA system is also investigated. The closed forms of average error probability derived in the paper can save a lot of computation time to analyze the performance and channel capacity of a cellular CDMA system. The analytical results show that the performance and maximum transmission rate of cellular CDMA systems degrade with an increase in the number of simultaneous users and the number of interfering cells. The signal-to-interface ratio (SIR) for the reverse link derived in this paper can directly describe the interrelationships among a number of paths, number of users, number of interfering cells, fading factors, and maximum variation of a received unfaded signal     

Theoretical analysis of reverse link capacity for an SIR-basedpower-controlled cellular CDMA system in a multipath fading environment

Capacity estimation in a code-division multiple-access system is closely related to power control schemes, which complicates the analysis due to the interaction between the signal power and the interference from other users and from other paths. For a signal-to-interference ratio (SIR)-based power control scheme, most previous work has been restricted to a single-cell system or to a multiple-cell system neglecting the effect of multipath fading. This paper is to give a theoretical foundation to the possible reverse link capacity of a multiple-cell system with perfect SIR-based power control, assuming two different multipath Rayleigh fading channel models: uniform and exponential power delay profiles. The effects of the numbers of resolvable propagation paths and RAKE fingers, and other system parameters such as the required E_b/I₀, the processing gain, and the maximum allowable transmit power of a mobile station, are investigated. The results are compared between single- and multiple-cell systems. When the number of resolvable paths is one or the number of Rake fingers is one, the link capacity becomes zero in a multiple-cell environment. This can be avoided by the use of antenna diversity. Antenna diversity reception is found to linearly increase the link capacity as the number of antennas increases     

Multiuser detection in asynchronous CDMA frequency-selective fading channels

Multipath fading severely limits the performances of conventional code division multiple-access (CDMA) systems. Since every signal passes through an independent frequency-selective fading channel, even modest cross-correlations among signature sequences may induce severe near-far effects in a central multiuser receiver. This paper presents a systematic approach to the detection problem in CDMA frequency-selective fading channels and proposes a low complexity linear multiuser receiver, which eliminates fading induced near-far problem.We initially analyze an optimal multiuser detector, consisting of a bank of RAKE filters followed by a dynamic programming algorithm and evaluate its performance through error probability bounds. The concepts of error sequence decomposition and asymptotic multiuser efficiency, used to characterize the optimal receiver performance, are extended to multipath fading channels.The complexity of the optimal detector motivates the work on a near-far resistant, low complexity decorrelating multiuser detector, which exploits multipath diversity by using a multipath decorrelating filter followed by maximal-ratio combining. Analytic expressions for error probability and asymptotic multiuser efficiency of the suboptimal receiver are derived that include the effects of multipath fading, multiple-access interference and signature sequences correlation on the receiver's performance.The results indicate that multiuser detectors not only alleviate the near-far problem but approach single-user RAKE performance, while preserving the multipath diversity gain. In interference-limited scenarios multiuser receivers significantly outperform the RAKE receiver.This paper was presented in part at the Twenty-Sixth Annual Conference on Information Sciences and Systems, Princeton, NJ, March 1992 and MILCOM'92, San Diego, CA, October 1992. This work was performed while author was with the Department of Electrical and Computer Engineering, Northeastern University, Boston, USA.     

异步CDMA移动通信系统中采用分集接收的多用户检测器

在CDMA移动通信系统中,多址接入干扰可以通过多用户检测技术来消除,而分集接收可以减小信道衰落的影响。本文给出了一类应用解相关检测、天线分集、RAKE多径分集技术的接收机,并对这一类接收机的性能做了分析。结果表明,在上行链路中,应用多用户检测和分集接收能够极大地提高接收机的性能。     

Performance analysis of an improved multi‐carrier CDMA system under frequency‐selective Rayleigh fading channels

In this article, an analytical method is proposed to study an improved orthogonal multi‐carrier DS/CDMA system, which uses QPSK for both spreading and carrier modulations without redundant sub‐carriers and time interleavers in each sub‐channel. The article concerns in particular such a system and its performance under frequency‐selective Rayleigh fading channels with various delay‐power profiles. The bit error probabilities under varying multipath‐diversity orders (using maximal ratio combing RAKE reception) and different numbers of resolvable multipaths are derived. It has been shown that the system, despite of its structural simplicity, performs well in mitigating frequency‐selective fading. The performance is also evaluated taking into account various other parameter sets, such as numbers of users and sub‐carriers, lengths of PN codes and the number of fingers in an RAKE receiver, etc. Copyright © 2003 John Wiley & Sons, Ltd.     

Blind equalization in antenna array CDMA systems

Multipath induced interchip-interference (ICI) alters waveforms of transmitted signals and presents a major obstacle to direct-sequence (DS) code-division-multiple-access (CDMA) communications. For systems with aperiodic pseudorandom (PN) spreading sequences, the primary way to counter fading is through employing RAKE receivers that enhance the signal-to-interference ratio (SIR) by combining multipath signals from the desired user. In this paper, we formulate a discrete-time model for antenna array CDMA systems and study the 2-D RAKE receiver problem by casting it into an optimum vector FIR equalizer design and estimation framework. A novel aspect of the present work is the full exploitation of the potential of 2-D RAKE receivers without requiring any detailed knowledge of the multipath channels     

A statistical approach to the analysis of DS/CDMA cellular systemsemploying RAKE receivers and sectorized antennas

RAKE receivers and sectorized antennas are used in direct-sequence/code-division multiple-access (DS/CDMA) cellular systems to improve the system performance. This paper presents a statistical method for analyzing the performance of DS/CDMA cellular radio systems employing RAKE receivers and sectorized antennas. Average bit error rates in the system are estimated considering the multipath fading effects of the environment. (The fast fading is assumed to be Rayleigh distributed, and the distance-dependent means of the multipath components have an exponential power delay profile.) The analysis of RAKE receivers quantifies the performance improvement that could be achieved by increasing the number of RAKE fingers. Sectorized antennas improve the system performance by reducing the interference at the receiver. In a perfectly sectorized system, assuming three sectors per cell, the capacity of the system can be improved by a factor of three. However, due to the imperfection in practical antennas, it is not possible to achieve this improvement. The performance of systems employing practical sectorized antennas (with finite front-to-back ratios and overlapping sectors) is compared with the performance of perfectly sectorized systems. The analysis shows that the incremental performance improvement diminishes with each incremental increase in the number of RAKE fingers. Performance degradation due to finite front-to-back ratio is shown to be insignificant for practical values of the front-to-back ratio of sectorized antennas. However, the reliability of mobile reception can be degraded significantly in areas where adjacent sectors overlap     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

Performance analysis of linear multisensor multiuser receivers forCDMA in fading channels

Bit-error probability (BEP) analysis for linear multiuser receivers with multiple sensors in frequency selective Rayleigh fading channels is presented. The analysis is applied to evaluate the BEP in antenna diversity reception and in a cellular CDMA system. Diversity and multiuser receivers are compared based on the examples. It is observed that adding new diversity antenna elements improves performance even if the correlation between the antenna elements is relatively large (up to 0.7). However, the large correlation values pose a significant reduction in the diversity gain in comparison to the zero correlation. It is also seen that the macroscopic diversity improves the performance of receivers significantly in cellular CDMA networks. When comparing diversity and multiuser receivers it is concluded that multiuser receivers are necessary to provide low BEPs. It is also highly beneficial to have at least two diversity antennas available, in particular, if there is no multipath diversity provided by the channel. The results also show that the reduction of intercell multiple-access interference yields a significant performance advantage in cellular networks. It is also demonstrated that the combination of spatial diversity and a multiuser receiver provides a significant receiver performance or system capacity gain in comparison to implementing only one of them     

Performance of the decorrelator receiver for DS-CDMA mobile radio system employing RAKE and diversity through Nakagami fading channel

Multiple-access interference (MAI) and multipath fading are two of the most significant factors limiting the capacity and performance of direct-sequence code-division multiple-access (DS-CDMA) systems. In this paper, synchronous multiuser receivers that combine antenna diversity, RAKE reception, and a multipath decorrelator for MAI cancellation are analyzed in a Nakagami faded environment using a maximal ratio combiner or a selection combiner. A coherent binary phase-shift keying employing DS-CDMA is considered. Arbitrary branch correlation is also considered for any diversity order in the case of identical severity fading on the branches.     

Performance of antenna diversity multiuser receivers in CDMA channels with imperfect fading estimation

The performance of antenna diversity coherent and differentially coherent linear multiuser receivers is analyzed in frequency-nonselective Rayleigh fading CDMA channels with memory. The estimates of the complex fading processes are utilized for maximal-ratio combining and carrier recovery of the coherent multiuser receiver. To analyze the impact of channel estimation errors on the receiver performance, error probability is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimation as well as the fading-induced error probability floor. The impact of fading dynamics on the differentially coherent decorrelating receiver with equal-gain combining is quantified. While performance of multiuser receivers at lower SNR is determined by both the fading dynamics and the number of active CDMA users, performance at higher SNR is given by an error probability floor which is due to fading only and has the same value as in a single-user case. The comparison of the two receiver structures indicates that the coherent decorrelating receiver with diversity reception may be preferable to the differentially coherent one in nonselective fading CDMA channels with memory.     

Performance analysis of space-time transmitter diversity techniques for WCDMA using long range prediction

Transmitter diversity in the downlink of code-division multiple-access (CDMA) systems achieves similar performance gains to the mobile-station receiver diversity without the complexity of a mobile-station receiver antenna array. Pre-RAKE precoding at the transmitter can be employed to achieve the multipath diversity without the need of the RAKE receiver at the mobile station. We examine feasibility of several transmitter diversity techniques and precoding for the third-generation wideband CDMA (WCDMA) systems. In particular, selective transmit diversity, transmit adaptive array and space-time pre-RAKE (STPR) techniques are compared. It is demonstrated that the STPR method is the optimal method to combine antenna diversity and temporal precoding. This method achieves the gain of maximum ratio combining of all space and frequency diversity branches when perfect channel state information is available at the transmitter. We employ the long range fading prediction algorithm to enable transmitter diversity techniques for rapidly time varying multipath fading channels.     

Linear multipath-decorrelating receivers for CDMAfrequency-selective fading channels

Analytical expressions for the error probability of linear multipath-decorrelating receivers with coherent and differentially coherent reception are derived. Both multi-user receivers have superior performance compared to the RAKE receiver and eliminate error probability floor caused by multiple-access interference on a code-division multiple access (CDMA) reverse link. Results also emphasize benefits of using coherent multi-user reception with maximal-ratio combining in frequency-selective fading channels     

Blind multiuser detection in uplink CDMA with multipath fading: a sequential EM approach

We consider joint channel estimation and data detection in uplink asynchronous code-division multiple-access systems employing aperiodic (long) spreading sequences in the presence of unknown multipath fading. Since maximum-likelihood (ML) sequence estimation is too complex to perform, multiuser receivers are proposed based on the sequential expectation-maximization (EM) algorithm. With the prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel, the receivers sequentially estimate the channel using the sequential EM algorithm. Moreover, the snapshot estimates of each path are tracked by linear minimum mean-squared error filters. The user data are detected by a ML sequence detector, given the channel estimates. The proposed receivers that use the exact expressions have a computational complexity O(2/sup K/) per bit, where K is the number of users. Using the EM algorithm, we derive low-complexity approximations which have a computational complexity of O(K/sup 2/) per bit. Simulation results demonstrate that the proposed receivers offer substantial performance gains over conventional pilot-symbol-assisted techniques and achieve a performance close to the known channel bounds. Furthermore, the proposed receivers even outperform the single-user RAKE receiver with Nyquist pilot-insertion rate in a single-user environment.     

Cellular channel modeling and the performance of DS-CDMA systemswith antenna arrays

In this paper, a new channel modeling approach incorporating nonuniform propagation environments is introduced, and the bit error rate (BER) of a direct sequence code division multiple access cellular system incorporating antenna arrays for spatial filtering is derived analytically. Specifically, this paper introduces a channel model for, and analyzes the performance of, a system in an environment where the multipath signals on each of the diversity branches of a RAKE receiver have varying fading characteristics. This scenario would typically describe urban environments where a large number of multipath echoes are present, each with different fading statistics resulting from the nonhomogeneous propagation paths seen by each multipath echo. It is shown that nonuniform fading parameters for multipath signals can severely influence the system performance, especially at high E_b /N₀ levels. Furthermore, it is shown that the conventional assumption of identical fading statistics for all RAKE receiver branches provide a lower bound on the system performance     

A generalized RAKE receiver for interference suppression

Currently, a global third-generation cellular system based on code-division multiple-access (CDMA) is being developed with a wider bandwidth than existing second-generation systems. The wider bandwidth provides increased multipath resolution in a time-dispersive channel, leading to higher frequency-selectivity. A generalized RAKE receiver for interference suppression and multipath mitigation is proposed. The receiver exploits the fact that time dispersion significantly distorts the interference spectrum from each base station in the downlink of a wideband CDMA system. Compared to the conventional RAKE receiver, this generalized RAKE receiver may have more fingers and different combining weights. The weights are derived from a maximum likelihood formulation, modeling the intracell interference as colored Gaussian noise. This low-complexity detector is especially useful for systems with orthogonal downlink spreading codes, as orthogonality between own cell signals cannot be maintained in a frequency-selective channel. The performance of the proposed receiver is quantified via analysis and simulation for different dispersive channels, including Rayleigh fading channels. Gains on the order of 1-3.5 dB are achieved, depending on the dispersiveness of the channel, with only a modest increase in the number of fingers. For a wideband CDMA (WCDMA) system and a realistic mobile radio channel, this translates to capacity gains of the order of 100%