Performance Analysis of Downlink W-CDMA Systems in Weibull and Lognormal Fading Channels Using Chaotic Codes

In this paper, a subspace based blind channel estimation scheme for downlink W-CDMA systems using chaotic codes under Weibull and Lognormal fading channel conditions is proposed and compared with W-CDMA system using PN codes. The algorithm provides estimates of multiuser channels by exploiting the structural information of the data output. The subspace of the (data + noise) matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms and signal constellations. The proposed channel estimation algorithm is also implemented for multiuser—orthogonal frequency division multiplexing (OFDM) system. Performance measures like bit error rate (BER) and root mean square error (RMSE) are plotted for Weibull and Lognormal fading channels. Signal constellations under Weibull and Lognormal channels are also plotted. Analytical and Simulation results for BER and RMSE are compared for W-CDMA system using PN codes and chaotic codes. Simulation results show that, chaos-based W-CDMA outperforms the PN-based W-CDMA in terms BER and RMSE. Simulation results of multiuser-OFDM system shows that performance is further improved when compared to the W-CDMA system.     

Blind RLS-Based Detection Using GSC with DF for STBC MC-CDMA Systems Under Mismatched Channel Estimation

Under Rayleigh fading channels, a common problem with blind multiuser space-time block coded (STBC) multicarrier code division multiple access (MC-CDMA) detectors is that their performance is very sensitive to the signature waveform mismatch (SWM) induced by channel estimation distortion. This paper presents a robust detector using the generalized sidelobe canceller with a double-feedback filter that effectively mitigates the channel estimation distortion for the STBC MC-CDMA system during multipath fading. Numerical simulations show that the proposed detector substantially provides robustness against small-to-medium SWM scenarios for a downlink STBC MC-CDMA system.     

Blind multiuser channel estimation in asynchronous CDMA systems

In asynchronous code division multiple access (A-CDMA) systems transmitting over multipath channels, both intersymbol interference (ISI) as a result of interchip interference (ICI) and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Recently, Liu and Xu (see Proc. 29th Asilomar Conf. Signals, Systems and Computers, 1996) introduced a blind estimation algorithm for synchronous CDMA (S-CDMA) systems to estimate the multiuser channels. However, this algorithm cannot be directly applied to an asynchronous CDMA (A-CDMA) system. In this paper, we study a similar blind estimation scheme that provides estimates of the multiuser channels by exploiting the structure information of the data output and the users' delays. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms. By utilizing antenna arrays, we extended our approach to overloaded systems, where the number of users may exceed the spreading factor     

Adaptive feedforward/feedback architectures for multiuser detectionin high data rate wireless CDMA networks

We consider the design and performance of nonlinear minimum mean-square-error multiuser detectors for direct sequence code-division multiple-access (CDMA) networks. With multiple users transmitting asynchronously at high data rates over multipath fading channels, the detectors contend with both multiple-access interference (MAI) and intersymbol interference (ISI). The cyclostationarity of the MAI and ISI is exploited through a feedforward filter (FFF), which processes samples at the output of parallel chip-matched filters, and a feedback filter (FBF), which processes detected symbols. By altering the connectivity of the FFF and FBF, we define four architectures based on fully connected (FC) and nonconnected (NC) filters. Increased connectivity of the FFF gives each user access to more samples of the received signal, while increased connectivity of the FBF provides each user access to previous decisions of other users. We consider three methods for specifying the FFF sampling and propose a nonuniform FFF sampling scheme based on multipath ray tracking that can offer improved performance relative to uniform FFF sampling. For the FC architecture, we capitalize on the sharing of filter contents among users by deriving a multiuser recursive least squares (RLS) algorithm and direct matrix inversion approach, which determine the coefficients more efficiently than single-user algorithms. We estimate the uncoded bit-error rate (BER) of the feedforward/feedback detectors for CDMA systems with varying levels of power control and timing control for multipath channels with quasi-static Rayleigh fading. Simulations of packet-based QPSK transmission validate the theoretical BER analysis and demonstrate that the multiuser RLS adapted detectors train in several hundred symbols and avoid severe error propagation during data transmission mode     

Group-blind intersymbol multiuser detection for downlink CDMA with multipath

Group-blind multiuser detectors for uplink code-division multiple-access (CDMA) were recently developed by Wang and Host-Madsen. These detectors make use of the spreading sequences of known users to construct a group constraint to suppress the intracell interference. However, such techniques demand the estimation of the multipath channels and the delays of the known users. In this paper, several improved blind linear detectors are developed for CDMA in fading multipath channels. The proposed detectors utilize the correlation information between consecutively received signals to generate the corresponding group constraint. It is shown that by incorporating this group constraint, the proposed detectors can provide different performance gains in both uplink and downlink environments. Compared with the previously reported group-blind detectors, our new methods only need to estimate the multipath channel of the desired user and do not require the channel estimation of other users. Simulation results demonstrate that the proposed detectors outperform the conventional blind linear multiuser detectors.     

Blind equalization and multiuser detection in dispersive CDMAchannels

The problem of blind demodulation of multiuser information symbols in a high-rate code-division multiple-access (CDMA) network in the presence of both multiple-access interference (MAI) and intersymbol interference (ISI) is considered. The dispersive CDMA channel is first cast into a multiple-input multiple-output (MIMO) signal model framework. By applying the theory of blind MIMO channel identification and equalization, it is then shown that under certain conditions the multiuser information symbols can be recovered without any prior knowledge of the channel or the users' signature waveforms (including the desired user's signature waveform), although the algorithmic complexity of such an approach is prohibitively high. However, in practice, the signature waveform of the user of interest is always available at the receiver. It is shown that by incorporating this knowledge, the impulse response of each user's dispersive channel can be identified using a subspace method. It is further shown that based on the identified signal subspace parameters and the channel response, two linear detectors that are capable of suppressing both MAI and ISI, i.e., a zero-forcing detector and a minimum-mean-square-error (MMSE) detector, can be constructed in closed form, at almost no extra computational cost. Data detection can then be furnished by applying these linear detectors (obtained blindly) to the received signal. The major contribution of this paper is the development of these subspace-based blind techniques for joint suppression of MAI and ISI in the dispersive CDMA channels     

Blind adaptive reduced-rank detection for DS-CDMA signals inmultipath channels

Blind adaptive multiuser detection for direct sequence code division multiple access (DS-CDMA) signals over static and time-varying intersymbol interference (ISI) limited channels is considered. Blind adaptive detectors must be robustified for ISI channels, when there is significant mismatch between the received signature vector and the transmitted code (assumed known at the receiver). A new low-complexity detector is presented that improves on some previously proposed methods without explicit estimation of the ISI channel. The key innovation is a reduced-rank detector architecture combined with an efficient subspace tracker that yields direct accurate estimation of the desired user's received signature. Several representative simulation examples of detector output signal-to-noise-and-interference ratio (SINR) for fading channels are provided in support of our claims of improved efficacy of the method     

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.     

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.     

On CDMA with space-time codes over multipath fading channels

We explore code-division multiple-access systems with multiple transmitter and receiver antennas combined with algebraic constellations over a quasi-static multipath fading channel. We first propose a technique to obtain transmit diversity for a single user over quasi-static fading channels by combining algebraic constellations with full spatial diversity and spreading sequences with good cross-correlation properties. The proposed scheme is then generalized to a multiuser system using the same algebraic constellation and different spreading sequences. We also propose a linear multiuser detector based on the combination of linear decorrelation with respect to all users, and the application of the sphere decoder to decode each user separately. Finally, we consider the generalization to multipath fading channels where the additional diversity advantage due to multipath is exploited by the sphere decoder, and a method of blind channel estimation based on subspace decomposition is examined.     

Space-time block-coded multiple access through frequency-selectivefading channels

Mitigation of multipath fading effects and suppression of multiuser interference (MUI) constitute major challenges in the design of wide-band third-generation wireless mobile systems. Space-time (ST) coding offers an effective transmit-antenna diversity technique to combat fading, but most existing ST coding schemes assume flat fading channels that may not be valid for wide-band communications. Single-user ST coded orthogonal frequency-division multiplexing transmissions over frequency-selective channels suffer from finite-impulse response channel nulls (fades). Especially multiuser ST block-coded transmissions through (perhaps unknown) multipath present unique challenges in suppressing not only MUI but also intersymbol/chip interference. In this paper, we design ST multiuser transceivers suitable for coping with frequency-selective multipath channels (downlink or uplink). Relying on symbol blocking and a single-receive antenna, ST block codes are derived and MUI is eliminated without destroying the orthogonality of ST block codes. The system is shown capable of providing transmit diversity while guaranteeing symbol recovery in multiuser environments, regardless of unknown multipath. Unlike existing approaches, the mobile does not need to know the channel of other users. In addition to decoding simplicity, analytic evaluation and corroborating simulations reveal its flexibility and performance merits     

Low-complexity blind multiuser detection of DS-CDMA signal in dispersive channels

The problem of blind multiuser detection in dispersive channels of Code-Division Multiple-Access (CDMA) in the presence of both Multiple-Access Interference (MAI) and In-terSymbol Interference (ISI) is considered. In practice, it is showed that by incorporating the desired user's signature waveform and the auxiliary vector, the information of the user can be identified using the suboptimal subspace method. The major contribution of this paper is to propose a minimum-mean-square-error detector with the suboptimal subspace-based blind technique for joint suppression of MAI and ISI in the dispersive CDMA channels.     

Soft Decision Error Assisted Layered Multiuser Detectors for MIMO 2D Spread MC DS-CDMAs

In this paper, we present two layered multiuser detectors (MUDs) for MIMO frequency-time-domain (FT-domain) multi-carrier (MC) direct sequence code division multiple access (DS-CDMA) systems with an antenna array at the base station. We assume that multiple users are active and individually utilize multiple transmit antennas in the MC DS-CDMA system. The users are organized into groups, and each user is assigned a unique Time-domain (T-domain) signature code. Moreover, users in the same group share a unique F-domain signature code. As a result, they can exploit the T-domain and F-domain signature codes to spread their multiple symbols in parallel, and then transmit the FT-domain spread signals from the corresponding multiple antennas over the fading channels to the base station. However, because of the non-ideal channel effect and/or the use of non-orthogonal signature codes, the FT-domain spread MC DS-CDMA system is affected by multiple access interference (MAI) in the same way as CDMA-like systems. To mitigate the effects of MAI and improve the system’s performance, we propose two layered MUDs that exploit the layered soft decision errors. Specifically, in a trade-off between the performance and the computational complexity, only the soft decision errors of one user/one group are used in the proposed layered MUDs. The results of simulations and a complexity analysis demonstrate that the proposed layered MUDs outperform existing approaches and the computational complexity is modest.     

Error Rate Analysis of SIMO-CDMA with Complementary Codes Under Multipath Fading Channels

Broadband communication systems of the current generation are likely to offer higher bit rates for delivering high-speed multimedia services to the end users. The achievable capacity and data rate of wireless communication systems are limited to fading channels varying with time, leading to multiple access interference (MAI) and multipath interference (MPI). In this paper, we investigate downlink single-input multiple output transmission for complementary coded code-division multiple access (CC-CDMA) systems working in channels with multipath fading. Here, parallel interference cancellation is employed for analysis of CC-CDMA with different frequency domain equalization schemes to eliminate MPI and MAI over multipath fading channels. Error rate analysis for CC-CDMA employing receiver diversity is assessed using simulations under varying channel parameters. Further, we compare different equalization schemes to show the superiority of regularized zero forcing in reducing the error rate of CC-CDMA systems.     

Performance of multiuser detection with adaptive channel estimation

An adaptive multipath decorrelating multiuser receiver is considered for application in Rayleigh fading multipath channels with significant Doppler spread. Coherent diversity combining is performed using adaptively obtained channel estimates in a manner that minimizes the impact of estimation errors on data detection. The bit-error rate of the receiver is evaluated analytically, showing dependence on the fading rate of the channel and tracking capabilities of adaptive least mean square and recursive least square algorithms, in addition to the order of multipath diversity and the number of active code-division multiple-access users     

空时编码多载波码分多址MIMO系统下行频率选择性信道盲估计

基于子空间方法的无线信道盲估计由于其算法的固有特性,使得估计结果与实际信道之间存在一个不确定复系数,无法得到无线信道的精确估计。在利用子空间分解方法对空时编码多输入多输出MC-CDMA系统下行频率选择性信道盲估计的基础上,利用发射符号的有限码集特性,将单载波系统下的模糊复系数盲辨识方法推广到多载波多输入多输出系统,从而得到信道的精确估计。Monte-Carlo仿真表明,在信噪比较低的情况下,本方法的信道估计误差仍然较小。     

Batch processing algorithms for blind equalization using higher-order statistics

Statistical signal processing has been one of the key technologies in the development of wireless communication systems, especially for broadband multiuser communication systems which severely suffer from intersymbol interference (ISI) and multiple access interference (MAI). This article reviews batch processing algorithms for blind equalization using higher-order statistics for mitigation of the ISI induced by single-input, single-output channels as well as of both the ISI and MAI induced by multiple-input, multiple-output channels. In particular, this article reviews the typical inverse filter criteria (IFC) based algorithm, super-exponential algorithm, and constant modulus algorithm along with their relations, performance, and improvements. Several advanced applications of these algorithms are illustrated, including blind channel estimation, simultaneous estimation of multiple time delays, signal-to-noise ratio (SNR) boost by blind maximum ratio combining, blind beamforming for source separation in multipath, and multiuser detection for direct sequence/code division multiple access (DS/CDMA) systems in multipath.     

综合业务的CDMA系统的业务容量分析

个人通信与移动计算的发展促进了将综合业务引入移动通信中。ＣＤＭＡ系统是新一代传输综合业务的系统。本文采用一种业务接入控制算法，分析了综合业务的ＣＤＭＡ系统的业务容量。显见，在计算综合业务的ＣＤＭＡ系统容量时，所需的Ｅｂ／Ｉ０、速率Ｒ及激活因子大的业务对系统容量的影响大。同时在上行信道中，非理想功率控制对Ｅｂ／Ｉ０、Ｒ及激活因子大的业务的容量影响也较大。为此，可以对不同的业务置以不同的功率调整步长来减小这种影响。     

Performance of multiple-dwell pseudo-noise code acquisition with I-Q detector on frequency-nonselective multipath fading channels

Multiple-dwell pseudo-noise code acquisition with a noncoherent I-Q detector is analyzed for Rayleigh and Rician fading channels that takes into account the detection correlations resulting from multipath fading. Minimum mean acquisition times with optimized dwell times and thresholds are obtained, and the effects of multipath fading and frequency offsets are evaluated. In addition, a detailed comparison between I-Q and square-law detectors is conducted under various channel conditions.     

PERFORMANCE OF MULTIUSER DETECTION FOR CDMA SYSTEM IN A FLAT RAYLEIGH FADING ENVIRONMENT

In a CDMA communication network using conventional signal detection, system capacity is limited and its performance is degraded by the multi-access interference (MAI). Multiuser detection, which makes use of cross-correlation information between spread spectrum codes, can reduce or eliminate the MAI in a Gaussian channel so as to mitigate the near-far effects and increase the system capacity. This paper extends the multiuser detector to a flat Rayleigh fading CDMA environment, and discusses the bit error rate of typical multiuser detection algorithms in such a environment by combining theoretical analysis and computer simulation. It is shown that multiuser detection is superior to conventional detections in the flat Rayleigh fading channels.