How fading affects CDMA: an asymptotic analysis with linearreceivers

Using asymptotic analysis, we study the effect of frequency-flat fading on code division multiple access (CDMA) systems with linear receivers and random spreading sequences. Specifically, we let the number of users grow without bound, while the ratio of number of users to spreading sequence length is kept fixed to a value α. We treat separately the cases of slow fading (nonergodic channel) and of fast fading (ergodic channel). For the former channel, we derive the outage probability, while for the latter we compute the channel capacity. In both cases, multiple classes of users with different qualities of service are dealt with. As α→∞, the system throughput tends to the same limit of 1.44 bit/symbol as for the nonfading channel with both single-user matched filter (SUMF) and linear minimum mean-square-error (MMSE) receivers. The outage probability exhibits a floor for all α with the SUMF receivers, while with MMSE receiver the floor is present only for α>1. We also address the tradeoffs involved in the allocation of available bandwidth between spreading and coding     

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     

Quasi-maximum-likelihood multiple-symbol differential detection for time-varying Rayleigh fading channel

The maximum-likelihood multiple-symbol differential detector (ML-MSDD) has better bit-error-rate performance than many other detectors for differential modulation. Unfortunately, the computational complexity of ML-MSDD quickly becomes prohibitive as the observation window size grows. While low-complexity MSDD algorithms for the time-invariant Rayleigh fading channel have been considered before, there is a need for low-complexity MSDD algorithms for general time-varying Rayleigh fading channels. A polynomial-time complexity approach called semi-definite relaxation (SDR) is employed to achieve differential detection with near maximum-likelihood (ML) performance. The proposed SDR quasi-maximum-likelihood (QML) multiple-symbol differential detection (SDR-QML-MSDD) is efficient in that its complexity is polynomial in the observation window size, even in the worst case, while it exhibits almost the same performance as ML-MSDD does.     

Performance comparisons of channel estimation techniques in multipath fading CDMA

The problem of pilot-symbol-aided estimation of multipath fading channels in up-link code-division multiple-access (CDMA) systems is considered. The transmitted symbol streams of each user are divided into time-slots; and each time-slot contains a number of pilot-symbols followed by information data symbols. Channel estimation is based on interpolation of the channel values corresponding to the pilot symbols in adjacent time-slots. Existing channel estimation techniques, including the weighted multislot average method and the wavelet expansion method, are studied. Two new channel estimation methods, namely, the robust channel interpolator, and the polynomial channel interpolator, are developed and are compared with these techniques. It is seen that the two new channel estimation methods significantly outperform the existing methods in multipath fading CDMA systems, for a wide range of Doppler values, and under various receiver schemes (with single or multiple receive antennas), such as the RAKE receiver, the interference cancellation receiver, and a receiver which performs iterative channel estimation and interference cancellation.     

一种快速时变衰落信道估计方法研究

由于现代无线通信系统对快速移动通信服务的要求越来越高,传统的信道估计方法在快速时变信道环境下的估计性能下降幅度很大。文章提出了一种快速时变信道环境下的信道估计方法,在发射端发射2个调频率相反的LFM信号,在接收端在峰值搜索的过程中结合谱校正技术搜索信号幅度谱的峰值和对应点的坐标,实现谱峰值的超分辨率估计,利用分数阶傅里叶变换的时移和频移性质,根据发射端信号的峰值点与收端信号的峰值点之间的差值,估计出信道的时延和频移,并给出信道的衰落幅度和初始相位的估计方法。仿真结果表明,该方法较传统信道估计方法的估计精度有较大的提升且实时性高。     

Adaptive joint multiuser detection and channel estimation in multipath fading CDMA channels

The problem of joint multiuser detection and channel estimation in frequency-selective Rayleigh fading CDMA channels is considered. First the optimal multiuser detector for such channels is derived, which is seen to have a computational complexity exponential in the product of the number of users and the length of the transmitted data sequence. Two suboptimal detectors are then developed and analyzed, both of which employ decorrelating filters at the front-ends to eliminate the multiple-access interference and the multipath interference. The symbol-by-symbol detector uses a Kalman filter and decision feedback to track the fading channel for diversity combining. The per-survivor sequence detector is in the form of the Viterbi algorithm with the trellis updates being computed by a bank of Kalman filters in the per-survivor fashion. Both suboptimal detectors require the knowledge of all waveforms of all users in the channel and the channel fading model parameters. Adaptive versions of these suboptimal detectors that require only the knowledge of the waveform of the user of interest are then developed. The adaptive receivers employ recursive-least-squares (RLS) minimum-mean-square-error (MMSE) filters at the front-end to mitigate the interference, and use a bank of linear predictors to track the fading channels. It is shown that the front-end RLS-MMSE filters can be implemented using systolic arrays to exploit massively parallel signal processing computation, and to achieve energy efficiency. Finally, the performance of the suboptimal detectors and their adaptive versions are assessed by simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Performance of CDMA/PRMA protocol for Nakagami-m frequencyselective fading channel

The performance of the code-division multiple-access/packet-reservation multiple-access (CDMA/PRMA) protocol for a Nakagami-m frequency selective fading channel is obtained by simulation for mixed voice/data traffic     

Performance of asynchronous orthogonal multicarrier CDMA system infrequency selective fading channel

An asynchronous multicarrier (MC) direct-sequence (DS) code-division multiple-access (CDMA) scheme for the uplink of the mobile communication system operating in a frequency selective fading channel is analyzed. The bit error rate performance of the system with either equal gain combining or maximum-ratio combining is obtained. Numerical results indicate that the system performs better than that of the conventional DS-CDMA system and another MC-DS-CDMA system     

室内多载波CDMA系统在深度衰落环境下的能量受控均衡策略

多载波CDMA(MC-CDMA)是将多载波调制OFDM与DS-CDMA结合起来的多址访问技术。如何从接收到的信号恢复出发送信号，涉及到频率分集接收和均衡。本文提出的能量受控均衡技术，在多用户的环境下能较好地恢复扩频码的正交性且避免信道深度衰落的影响，而且，实验表明该方法对于不同的信道状况均能很好适应。     

Performance of multicarrier CDMA with successive interference cancellation in a multipath fading channel

A high capacity, low complexity, and robust system design for a successive interference cancellation (SIC) system is developed and analyzed. Multicarrier code-division multiple access (MC-CDMA) is used to suppress multipath and to overcome the multipath channel estimation problem in single-carrier SIC systems. In addition, an optimal power control algorithm for MC-CDMA with SIC is derived, allowing analytical bit-error rate expressions to be found for an uncoded system. Low-rate forward error-correcting codes are then added to the system to achieve robustness. It is found that the capacity of the coded system approaches the additive white Gaussian noise capacity for SIC, even in a fading multipath channel with channel estimation error. This indicates that MC-CDMA is very attractive for systems employing SIC.     

Multiuser decision-feedback receivers for asynchronous CDMA systemsover a mismatched flat Rayleigh fading channel

In this paper, the design of decision-feedback multiuser (DFM) receivers for asynchronous code-division multiple access (CDMA) systems over a mismatched flat Rayleigh fading channel is studied, and tight upper and lower bounds on the pair-wise symbol-error probability, which has been averaged over the channel fading, time delays, and phase angles, are derived for some of the DFM detection schemes. The performance bounds, which are computed by efficient analytic techniques, can serve as reliable references for CDMA system design. It is shown that the proposed system, which takes into account the effect of mismatched fading on the system design, has some gain over the system where the estimation of fading is assumed to be exact     

Hybrid diversity combining techniques for DS‐CDMA over a multipath fading channel

The combined effect of antenna diversity and spread spectrum (SS) diversity for a direct sequence code division multiple access (DS‐CDMA) system using differential binary phase‐shift keying (DBPSK) modulation is evaluated. Both maximal ratio combining (MRC) and selection diversity (SD) are considered and the bit error rate (BER) performance for four following diversity schemes are compared: (1) SS diversity with MRC and antenna diversity with SD; (2) SS diversity with SD and antenna diversity with MRC; (3) MRC of both SS diversity and antenna diversity; (4) SD of both SS diversity and antenna diversity. It is shown that antenna diversity with MRC and SS diversity with selection has a performance very close to that of antenna diversity and SS diversity both with MRC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Consistent estimation of Rayleigh fading channel second-orderstatistics in the context of the wideband CDMA mode of the UMTS

We address the problem of estimating the second-order statistics of a frequency-selective Rayleigh channel in the context of the wideband CDMA mode of the UMTS. The data to be transmitted are sent over slots on which the channel is assumed to remain constant. Each slot contains a pilot symbol sequence from which it is possible to estimate the current value of the channel. The covariance matrix of the channel is usually estimated by a denoised version of the empirical covariance matrix of the trained channel estimate. However, in the UMTS, this estimate is not consistent in the sense that if the number of slots used to estimate it tends to infinity, it does not converge to the true covariance matrix of the channel. We propose a new consistent estimate of the covariance matrix and evaluate the performances of two Wiener-like channel estimation schemes based on the proposed estimate. The performances of the new approach are evaluated by means of the bit error rate provided by a RAKE receiver based on the proposed channel estimates. It is shown that our estimate of the covariance matrix allows significant improvement in the performance of the RAKE receiver     

Efficient BER evaluation of linear multiuser detectors with imperfect channel estimation for CDMA fading channels

In this paper, we present a unified mathematical framework to analyze the bit-error rate (BER) performance of general linear coherent multiuser receivers with diversity reception and imperfect channel estimation for doubly selective Rician-fading asynchronous code-division multiple-access channels. BERs of linear receivers with channel state information and data-aided channel estimation are analyzed, and both exact and low-complexity approximate BER evaluation formulas are presented. Furthermore, by using a Markov chain steady-state analysis, a tight BER approximation for receivers with decision-directed channel estimation is proposed. Numerical and simulation results verify the accuracy of the proposed BER evaluation methods.     

Spectral efficiency of a cellular MC/DS‐CDMA system in Rayleigh fading

The spectral efficiency of a multicarrier direct‐sequence code‐division multiple‐access (MC/DS‐CDMA) cellular system, operating in a mobile radio environment with Rayleigh fading, is investigated and evaluated in terms of channel capacity (in the Shannon sense) per user, estimated in an average sense. The analysis is based on the combination of path‐diversity reception, achieved by a conventional coherent maximal‐ratio combining (MRC) RAKE receiver and the physical frequency diversity potential provided by orthogonal frequency division multiplexing (OFDM). As shown, a closed‐form expression for the achieved spectral efficiency of the considered system's operation is derived following a recently presented general method. Copyright © 2005 John Wiley & Sons, Ltd.     

On spectral efficiency of low-complexity adaptive MIMO systems in Rayleigh fading channel

Adaptive MQAM modulation is used to maximize spectral efficiency of Multiple-Input Multiple-Output (MIMO) systems while keeping bit error rate (BER) under a target level. Closed-form expressions of the average spectral efficiency, coined as discrete-rate spectral efficiency (DRSE), are derived for adaptive modulation MIMO systems using different algorithms. To further enhance the spectral efficiency, a low complexity adaptation scheme is suggested to switch across different algorithms based on the DRSE. In the current letter, we investigate the adaptation scheme that switches between Orthogonal Space-Time Block Codes (OSTBC) and spatial multiplexing with zero-forcing (ZF) detection for MIMO systems with two transmit antennas. Two types of operating environment are considered: flat Rayleigh fading channel without spatial correlation and spatially correlated Rayleigh fading channel with transmit correlation.     

Capacity losses in wireless CDMA networks using imperfect decorrelating space-time Rake receiver in fading multipath channel

In this paper we analyze the impact of system imperfections on the overall cellular code-division multiple access (CDMA) radio network performance. The theory is general and some examples of practical sets of channel and system parameters are used as illustration. A flexible signal model, based on the complex signal format, is used enabling us to model all wideband CDMA standards. For such a signal, we first derive a complex decorrelator structure. In the next step imperfections in the operation of a decorrelating space-time Rake combiner are modeled and analyzed. Relative capacity losses and the network sensitivity function are used as performance measures. Simulations are also performed to confirm some of the key assumptions made in the analysis. Numerical results show that the user capacity varies significantly depending on the multipath profile, diversity order, fading rate, and code crosscorrelations. It is shown that up to 97 % of the system capacity can be lost due to the system imperfections. More, advanced and robust parameter estimators and/or multiuser detectors are needed to alleviate these degradations at the cost of increased complexity.     

Space-frequency-Doppler coded OFDM over the time-varying Doppler fading channels

In this paper, space-frequency-Doppler coded OFDM (SFDO-OFDM) scheme over the time-varying Doppler fading channels via the time-frequency duality is proposed. Based on the basis expansion model (BEM) and the time-frequency duality, through the circulant matrix diagonalized processing, the nonlinear time-varying Doppler fading channel is dually converted to the virtual frequency-selective linear channels. With OFDM module, subgrouping the subcarriers in OFDM through the block matrix method and fatherly general complex orthogonal coding (GCOD) on each corresponding block subcarriers, SFDO-OFDM codes for the general multiple input multiple output (MIMO) is thus constructed. And concatenating it with the signal constellation precoding, full maximum diversity gains including the inherent Doppler fading are achieved. Theoretical analysis and corroborating simulation results demonstrate that, comparing with existing Doppler coding alternatives, the proposed scheme can effectively and robustly combat the Doppler fading with high bandwidth efficiency and even lower bit error ratio (BER).     

On the asymptotic capacity of stationary Gaussian fading channels

We consider a peak-power-limited single-antenna flat complex-Gaussian fading channel where the receiver and transmitter, while fully cognizant of the distribution of the fading process, have no knowledge of its realization. Upper and lower bounds on channel capacity are derived, with special emphasis on tightness in the high signal-to-noise ratio (SNR) regime. Necessary and sufficient conditions (in terms of the autocorrelation of the fading process) are derived for capacity to grow double-logarithmically in the SNR. For cases in which capacity increases logarithmically in the SNR, we provide an expression for the "pre-log", i.e., for the asymptotic ratio between channel capacity and the logarithm of the SNR. This ratio is given by the Lebesgue measure of the set of harmonics where the spectral density of the fading process is zero. We finally demonstrate that the asymptotic dependence of channel capacity on the SNR need not be limited to logarithmic or double-logarithmic behaviors. We exhibit power spectra for which capacity grows as a fractional power of the logarithm of the SNR     

Proposal of a combined CFR estimation method for practical bidirectional ANC-OFDM system in higher time-varying fading channel

Telecommunication Systems - In bidirectional analogue network coding based on orthogonal frequency division multiplexing (ANC-OFDM) system, two users can transfer their data to each other via the...