Two-stage nondata-aided adaptive linear receivers for DS/CDMA

Closed-form steady-state performance analysis of the signal-to-interference plus noise ratio (SINR) at the output of well-known adaptive implementations of the linear minimum mean-square error (MSE) receiver for direct-sequence code-division multiple access show that nondata-aided (NDA) schemes may suffer from a considerable performance degradation with respect to their data-aided counterparts. Motivated by this fact, we propose a new two-stage NDA scheme where symbol-by-symbol predecisions at the output of a first adaptive stage are used to train a second stage. We derive closed-form steady-state performance analysis for both the two-stage and classical decision-directed schemes, taking into account detection errors in decision-directed adaptation. Our analysis shows that the SINR of the two-stage algorithm is close to optimal over a large range of values, while the SINR of the decision-directed scheme is far from optimal when the optimal SINR is small. Finally, we consider the case of time-varying fading channels. We derive modified recursive least square and least mean square adaptation schemes by considering SINR maximization rather than MSE minimization (that is useless under the assumption of zero-mean random channels). The resulting two-stage receiver shows good tracking properties in heavy near-far conditions (at least for moderate normalized Doppler bandwidth), while the decision-directed receiver may easily loose tracking after deep fades.     

Performance analysis of adaptive receivers for DS/CDMA communication systems

Both intersymbol interference (ISI) and multiple-access interference (MAI) are the important issues in code division multiple access (CDMA) multiuser communication systems. The step size of the traditional least-mean-square (LMS) adaptive receivers must be substantially adjusted to overcome the effect due to different channel responses or different numbers of active users. The normalized-LMS (NLMS) algorithm can automatically manipulate the adjustment to avoid the problem just discussed. This paper proposes the symbol-based interference rejection filter with NLMS algorithm and derives both finite impulse response (FIR)- and infinite impulse response (IIR)-type algorithms. We also derive the optimal step sizes and minimum mean-square errors (MSEs) for both the FIR and IIR symbol-based receivers. The complexity of our proposed FIR receiver is lower than that of the conventional chip-based receivers. In addition, it is shown that the bit error rate performance of our proposed symbol-based receivers is superior to conventional one in the simulations. Simulations also show the correctness of our theoretical analysis of minimum MSE. Copyright © 2008 John Wiley & Sons, Ltd.     

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.     

Adaptive modulation in ad hoc DS/CDMA packet radio networks

This paper investigates the benefit of adaptive modulation based on channel state information (CSI) in direct-sequence/ code-division multiple-access (DS/CDMA) multihop packet radio networks. By exploiting varying channel conditions, adaptive modulation can be used in ad hoc networks to provide upper layers with higher capacity links over which to relay traffic. Using the /spl alpha/-stable interference model, the distribution of the signal-to-interference ratio (SIR) is obtained for a slotted system of randomly, uniformly distributed nodes using multilevel coherent modulation schemes. Performance is evaluated in terms of the information efficiency, which is a new progress-related measure for multihop networks. Three types of adaptivity are analyzed, differing in the level of CSI available: 1) full knowledge of the SIR at the receiver; 2) knowledge of only the signal attenuation due to fading; and 3) knowledge of only the slow fading component of the signal attenuation. The effect of imperfect channel information is also investigated. Sample results are given for interference-limited networks experiencing fourth-power path loss with distance, Ricean fading, and lognormal shadowing.     

Space-time interference suppression in DS/CDMA networks

Low-complexity adaptive receivers based on smart antennas with post- and pre-despreading structures are presented. Comparisons between three space-time (ST) schemes employing minimum-mean-square-error, a constant modulus algorithm, and a decision-directed algorithm for constant modulus signals are investigated in detail     

Blind adaptive channel estimation for dual-rate DS/CDMA signals

This letter proposes the subspace-based blind adaptive channel estimation algorithm for dual-rate quasi-synchronous DS/CDMA systems, which can operate at the low-rate (LR) or high-rate (HR) mode. Simulation results show that the proposed blind adaptive algorithm at the LR mode has a better performance than that at the HR mode, with the cost of an increasing computational complexity     

Blind adaptive algorithms for minimum variance CDMA receivers

Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity. The receiver's output variance is minimized subject to appropriate constraints which depend on the multipath structure of the signal of interest. When multipath is present, the constraint equations can be written in parametric form, and the constraint parameters jointly optimized with the linear receiver's parameters. We develop adaptive solutions for this joint, constrained optimization problem. Both stochastic gradient and recursive least-square-type algorithms are developed. The performance of the proposed methods is compared with other blind and trained methods and turns out to be close to the trained minimum mean-square-error receiver     

Differentially coherent PN code acquisition with full-periodcorrelation in chip-synchronous DS/SS receivers

Differentially coherent detection techniques for PN code acquisition in direct-sequence spread-spectrum (DS/SS) receivers are designed and analyzed in this letter. The full-period cross correlation (FPC) can be modeled as deterministic for certain types of PN sequences whose autocorrelation functions have ideal shapes the same as those of maximum length shift registration (MLSR) sequences. This model is exploited here to simplify the design and performance evaluation of the proposed detection techniques. Such techniques can be confidently employed as alternatives to noncoherent detectors because of their attractive improvements in the signal-to-noise ratio (SNR) and, thus, are very suitable for commercial code-division multiple-access (CDMA) systems, which often operate in severely noisy environments. The results of statistical analyses show that the proposed techniques can provide rapid acquisition and improved performance in SNR     

利用自适应滤波器实现扩频码的捕获

使用自适应滤波器对扩频信号进行扩频码捕获,不仅理论上是可行的,而且此方法还能够实现快速捕获,同时还可以对输入信号内的多种干扰信号起到抑制的作用。自适应滤波器法结构简单,易于实现;同时跟踪采用与捕获同一个支路,不需要额外硬件实现,从而节省硬件资源。自适应滤波器法扩频码捕获时,只需期望扩频码的周期与长度,并不期望码型与干扰信号的任何信息。在稳态时,此方法还具有对多径信号分集接收的潜力。     

Blind adaptive multiuser detection for DS/CDMA over multipathfading channels

Using a hard null scheme, multipath fading and multiple access interference suppression can be realised for a multiple constrained minimum variance (MCMV) detector at the same time. A modified version of the MCMV detector is also presented, which utilises the eigenstructure of the correlation matrix to enhance the performance of the MCMV detector. Numerical results demonstrate the effectiveness of the proposed detectors     

DS/CDMA closed-loop power control with adaptive algorithm

Closed-loop power control in the reverse link of a DS/CDMA system is analysed. The transmitter/receiver based on IS-95 and the radio propagation channel under mobile communication environments are modelled and a new power control algorithm is proposed. This algorithm utilises the burst error characteristics of the channel and can be directly applied to the current IS-95 system. The new power control algorithm, with respect to the SIR (signal to interference ratio), increases the service quality, and finally, enhances the system capacity     

Partially blind adaptive MMSE interference rejection inasynchronous DS/CDMA networks over frequency-selective fading channels

In this work, the problem of joint suppression of multiple-access and narrow-band interference (NBI) for an asynchronous direct-sequence code-division multiple-access (CDMA) system operating on a frequency-selective fading channel is addressed. The receiver structure we consider can be deemed as a two-stage one: the first stage consists of a bank of minimum mean-square-error (MMSE) filters, each keyed to a given replica of the useful signal, and aimed at suppressing the overall interference; the second stage, assuming knowledge of the fading channel coefficients realizations, combines the MMSE filters outputs according to a maximal-ratio combining rule. Due to the presence of the NBI, the resulting structure is in general time-varying, and becomes periodically time-varying if the NBI bit-rate has a rational ratio to that of the CDMA system. Moreover, enlarging the observation window beyond the signaling interval and oversampling the signal space may yield a noticeable performance improvement. For the relevant case that the said ratio is rational, a new cyclic blind recursive least squares (RLS)-based algorithm is introduced, capable of tracking the periodically time-varying receiver structure, and allowing adaptive interference cancellation with a moderate complexity increase. We also come up with a closed-form expression for the conditional bit-error rate (BER), which is useful both to evaluate semi-analytical methods to assess the unconditional BER and to derive bounds on the system near-far resistance. The results indicate that the receiver achieves very satisfactory performance in comparison to previously known structures. Computer simulations also demonstrate that the cyclic blind RLS algorithm exhibits quite fast convergence dynamics     

Steady-state analysis of the adaptive successive interferencecanceler for DS/CDMA signals

The adaptive successive interference canceler (ASIC) is a multistage receiver that sequentially detects and removes cochannel users from the received signal impinging on a single antenna element. Each stage of the ASIC consists of a conventional matched filter (MF) detector and an adaptive interference canceler (AIC) that employs the least-mean-square (LMS) algorithm to recursively estimate the received amplitude of the detected signal. In this paper, we investigate the performance of the ASIC using a Wiener model of convergence for the LMS algorithm, deriving expressions for the asymptotic mean and variance of the amplitude estimate and the steady-state bit error rate (BER). The analyses and computer simulations demonstrate that the performance of the ASIC exceeds that of the conventional SIC (CSIC), which utilizes the MF output as the received amplitude estimate     

Blind adaptive decision feedback CDMA receivers for dispersive channels

Blind adaptive decision feedback (DF) receivers for direct sequence code division multiple access (DS-CDMA) systems in dispersive channels are proposed. Blind adaptive stochastic gradient algorithms are developed for use with the constrained minimum variance and constrained constant modulus receivers along with successive and parallel DF structure.     

随机序列直扩Rake系统在Nakagami信道下的性能

本文分析了用不同波形的二进制随机序列作扩频码的直扩系统在Nakagami下的性能，通过高斯近似，得到了系统的误比频率，最后给出了数值结果。     

On call admission control in DS/CDMA cellular networks

Analytical models are proposed for various direct sequence code-division multiple-access (DS/CDMA) call admission control schemes. Many mathematical call admission models for DS/CDMA cellular networks have been proposed. However, they have shortcomings. First, by ignoring the stochastic traffic load variation or call blocking effect, they failed to sufficiently characterize the second moment of other-cell interference. This leads to inaccurate analysis of a real network. Second, the optimal control parameters were often obtained through an exhaustive search which was very time consuming. Finally, the estimation of system capacity in previous models was obtained by using a simple one-slope path-loss propagation model. However, it is well known that a two-slope path loss propagation model is needed in a line-of-sight (LOS) microcell propagation environment. We propose an analytical model for call admission to overcome these drawbacks. In addition, we combine a modified linear programming technique with the built analytical model to find better call admission control schemes for a DS/CDMA cellular network     

Integrated hybrid adaptive space-time equaliser for interferencerejection in multipath DS/CDMA systems

An integrated hybrid space-time equaliser is proposed for interference suppression in DS/CDMA systems. It outperforms the existing time-domain-only counterpart, and previous work on the integrated space-time receiver in terms of MSE and near-far resistance. A comparison between simulation and analytical results is also presented     

Averaged stochastic gradient algorithms for adaptive blindmultiuser detection in DS/CDMA systems

In this paper, we present a blind adaptive gradient (BAG) algorithm for code-aided suppression of multiple-access interference (MAI) and narrow-band interference (NBI) in direct-sequence/code-division multiple-access (DS/CDMA) systems. This BAG algorithm is based on the concept of accelerating the convergence of a stochastic gradient algorithm by averaging. This ingenious concept of averaging was invented by Polyak and Juditsky (1992)-this paper examines its application to blind multiuser detection and NBI suppression in DS/CDMA systems. We prove that BAG has identical convergence and tracking properties to recursive least squares (LMS) but has a computational cost similar to the least mean squares (LMS) algorithm-i.e., an order of magnitude lower computational cost than RLS. Simulations are used to compare our averaged gradient algorithm with the blind LMS and LMS schemes     

频率复用DS/CDMA系统中自适应盲多用户检测的一种方法

该文给出一种工作于强窄带干扰(NBI)多径衰落信道中DS/CDMA系统盲多用户检测接收机模型。它具有RAKE接收机的基本结构,在每一条并行支路中利用MMSE准则跟踪各路径信号。因此具有抗多径衰落,同时抑制NBI和多址接入干扰(MAI)的能力。模拟结果证明,它的性能优于未考虑NBI抑制的多用户检测接收机。     

直接序列扩频信号的盲解扩方法

本文首先介绍了一种通过解方程组、计算反馈移位寄存器系数,继而估计直扩信号扩频码序列的方法.在此基础上,本文提出了一种新的直扩信号的盲解扩方法,该方法不需要进行扩频码同步,而且其抗误码性能相对于一些传统方法有所提高.最后通过计算机仿真实验验证了盲解扩方法的正确性.