Time-varying narrow-band interference rejection in asynchronousmultiuser DS/CDMA systems over frequency-selective fading channels

In this paper, we handle the problem of joint suppression of multiple-access interference (MAI) and narrowband interference (NBI) in fading, dispersive channels. The detectors we consider are linear, one-shot structures, which allow for possible window enlargement and signal-space oversampling to improve performance. We focus on both zero-forcing and minimum-mean square-error design strategies, showing that the presence of NBI generally requires a time-varying processing of the observables, no matter what the optimization criterion. A thorough performance assessment of the proposed detectors is also presented, either through analytical formulas or through computer simulations. We finally deal with the problem of blind suppression of both MAI and NBI, introducing batch-estimation procedures to be implemented offline, which require very little and sometimes no prior knowledge as to the interference structure     

Constrained MMSE receivers for CDMA systems in frequency-selective fading channels

A constrained minimum mean square error (CMMSE)-RAKE receiver for multipath fading channels is developed by extending the CMMSE receiver for flat fading channels. Based on the observation that interpath interference causes a bias of the channel estimator in , a receiver that can remove such a bias is proposed, plus a closed-form expression of the bit-error rate of the receiver is derived. Computer simulation is used to demonstrate that the proposed receiver can outperform existing RAKE receivers.     

Symbol-by-symbol based adaptive interference canceller forasynchronous DS/CDMA systems in multipath fading channels

The authors propose a new interference cancellation scheme called the symbol-by-symbol based adaptive interference canceller (SAIC), which adaptively estimates and removes both multiple access interference (MAI) and intersymbol interference (ISI) at the output of the Rake receiver. The SAIC is considerably simpler to implement than existing techniques; computer simulation results demonstrate that it can perform much better than conventional Rake receivers     

一种频率选择性衰落信道下的盲自适应去相关Rake接收机

多用户检测是DS－CDMA系统中的一项关键技术,而Rake接收是解决多径效应的一种有效方法,本文将基于Kalman滤波的多用户检测器与Rake接收相结合,提出了一种频率选择性衰落信道下的盲自适应去相关Rake接收机,研究结果表明,这种接收机具有较强的抑制多址干扰和克服“远－近”效应能力,并且能快速收敛。     

A constrained MMSE receiver for DS/CDMA systems in fading channels

The convergence problem of minimum mean square-error (MMSE) receivers is discussed, and to overcome the problem, a constrained MMSE receiver is proposed. In addition, we propose the orthogonal decomposition-based least mean square algorithm to implement the constrained MMSE receiver adaptively. Through computer simulations, it is shown that the proposed receiver provides significant performance improvement in the bit-error rate over the conventional matched filter receiver and currently available MMSE receivers.     

A new blind adaptive interference suppression scheme foracquisition and MMSE demodulation of DS/CDMA signals

We present an efficient blind algorithm for estimating the code timing of a desired user in an asynchronous direct-sequence code-division multiple-access (DS/CDMA) system over frequency-nonselective-fading channels. The proposed algorithm acquires the code timing explicitly and results in a near-far resistant minimum mean-squared error (MMSE) demodulation without requiring the knowledge of the timing information, amplitudes, and transmitted symbols of all transmissions. The only required knowledge is the information of the signature sequence used by the desired transmission. Several computer simulations are done for additive white Gaussian channels, Rayleigh fading channels, and two-ray Rayleigh fading multipath channels, respectively. Numerical results show that the new algorithm is near-far resistant to the multiple-access interference (MAI) in the DS/CDMA system     

An accurate method for approximating the interference statistics of DS/CDMA cellular systems with power control over frequency-selective fading channels

This letter proposes an approximation method by characteristic function (AM-CF) method to approximate the distribution of interference in DS/CDMA cellular systems. This method considers the effects of frequency-selective multipath fading; it also assumes perfect power control and a rectangular/sine chip waveform. The AM-CF method can yield results that fit the Monte Carlo simulation results more accurately than the conventional standard Gaussian approximation method.     

Code-aided blind adaptive new user detection in DS/CDMA systems with fading time-dispersive channels

We consider the problem of detecting the entrance of a new-user in a direct-sequence code division multiple access (DS/CDMA) system operating over a fading dispersive channel. This problem has received considerable attention in the recent past in that it arises in a number of different contexts, such as decentralized user acquisition and data detection, handoff algorithms, soft handover procedures, and cell-search in wideband-CDMA systems. The detection algorithms that we propose in this paper are based on the application of the generalized-likelihood-ratio-test (GLRT) and can be implemented based on the knowledge of the spreading code of the user to be detected. In particular, the proposed procedures do not require any prior knowledge of either the propagation channel impulse response or the timing offset of the user to be detected. We consider both the cases of user detection in the reverse link of a cellular system, wherein it is assumed that the subspace spanned by the signals of the existing previous users is known to the receiver and that of user detection in the forward link, wherein the mobile receiver has no prior knowledge on the multiaccess interference. With regard to the latter situation, we develop a detection algorithm that ensures a constant false-alarm rate with respect to the second-order statistics of the overall disturbance. The performance of the proposed detection structures is finally assessed through closed-form formulas and through some sample plots, showing, for a given probability of false alarm, the probability that the new user is detected.     

Acquisition for DS/CDMA systems with multiple antennas in frequency-selective fading channels

A generalized code acquisition scheme for direct-sequence code-division multiple-access systems with multiple antennas is proposed over frequency-selective fading channels. The proposed scheme is developed on the framework of a generalized configuration of multiple antennas and correlators. The nonconsecutive search method is generalized and extended to multiple antenna systems to exploit multipath signals in improving acquisition performance over frequency-selective fading channels. The proposed scheme also adopts a hybrid decision strategy to make effective decisions on acquisition. The mean acquisition time performance of the proposed acquisition scheme is analyzed and evaluated in frequency-selective Rayleigh-fading channels with general multipath delay profiles and spatial-fading correlations. The effects of nonconsecutive search on mean acquisition time are investigated for various channel environments, and the optimal choice of decision strategy is discussed. Furthermore, effects of various configurations of multiple antennas and correlators, decision thresholds, and correlation interval on the performance are also investigated.     

Symbol-level Rake MMSE receiver for asynchronous DS/CDMA systems inmultipath fading channels

A new nonlinear adaptive minimum mean squared error (MMSE) receiver performing a successful cancellation of multiple access interference in multipath fading channels is proposed. It is observed that the proposed receiver could achieve a significant performance gain over any currently used adaptive MMSE receivers, at the cost of a relatively small increase in complexity and modification of the conventional DS/CDMA system     

Multistage interference cancellation with diversity reception forasynchronous QPSK DS/CDMA systems over multipath fading channels

This paper introduces a multistage interference cancellation (MIC) technique with diversity reception for quadrature phase shift keying (QPSK) asynchronous direct-sequence code division multiple access (DS/CDMA) systems over frequency-selective multipath Rayleigh fading channels. Unlike the previous MIC, which tries to remove the lump sum of the multiple-access interference (MAI) and self-interference (SI), this introduced MIC attempts to cancel only the MAI and part of the SI due to the intersymbol interference, while treating the remaining SI created by the current symbol as useful information for symbol decision. In this technique, the RAKE combining is used to collect signal replicas over multiple fading paths. Upper and lower bounds on the bit error probability are derived using a Gaussian approximation and the characteristic function method. Furthermore, effects of channel estimation error on the performance are studied. Analytical and simulation results show that the introduced MIC can provide a performance extremely close to that in an ideal single-user environment and outperforms the previous MIC even in the presence of channel estimation error     

A multiuser receiver for trellis-coded DS/CDMA systems inasynchronous channels

We propose and analyze a multiuser receiver using a decorrelating filter and Viterbi decoders for trellis-coded direct-sequence code-division multiple-access (DS/CDMA) systems with biorthogonal signal constellation in asynchronous channels. The biorthogonality is implemented by user signature waveforms and the decorrelating filter. The performance of the proposed system is investigated with emphasis on the asymptotic cases: it is shown that the proposed system provides us with some coding gain and near-far resistance. It is confirmed that we can enhance the performance of the proposed system by using base-station antenna arrays     

Performance of various multistage interference cancellation schemesfor asynchronous QPSK/DS/CDMA over multipath Rayleigh fading channels

The performance of multistage interference cancellation (MIC) and three combining techniques, i.e., multipath decorrelating (MIC-DECO), optimum combining (MIC-OPTM), and RAKE combining (MIC-RAKE) for asynchronous quadrature phase-shift keying/direct-sequence code-division multiple access over frequency-selective multipath Rayleigh fading channels is studied. The analytical bit-error probabilities of the MIC-DECO and MIC-OPTM are derived and shown to be in a good agreement with simulation results. Both analytical and simulation results show that the MIC-DECO, MIC-OPTM, and MIC-RAKE in a multiuser environment provide a good performance close to the ideal performance in a single-user system even in the presence of channel estimation error     

Design and analysis of concatenated coded DS/CDMA systems inasynchronous channels

We propose and analyze concatenated coding schemes for direct-sequence code-division multiple-access (DS/CDMA) systems in asynchronous channels. In the concatenated coding, bandwidth-efficient 2 ^2L-2-state L/(L+1)-rate 2-MTCM with biorthogonal signal constellation is used for the inner code, and (2^L-1,[(2^L -1)/L/2]) RS code is used for the outer code. It is shown that we can get considerable performance gain over the uncoded system without sacrificing the data transmission rate. The proposed system can be used as a coding scheme for reliable and high-speed integrated information services of mobile communication systems     

一种频选衰落信道下的Turbo多用户检测算法

联合MAP多用户检测与信道解码的迭代多用户检测(MUD)技术可显著提高宽带移动CDMA系统的容量和性能.在多径时变衰落的编码信道下,提出一种迭代实现干扰抑制、符号估计、信道解码的Turbo多用户检测算法.在每次迭代中,MUD自适应地实现干扰抑制并输出符号估计的软信息,软输入软输出的信道解码器使用LOG MAP方法实现信道解码并反馈符号估计的软信息作为下一次TurboMUD迭代的先验信息.仿真结果证实了该算法在频选衰落信道下经两次迭代就能逼近单用户编码CDMA系统的接收性能.     

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.     

Analysis of quasi-ML multiuser detection of DS/CDMA systems inasynchronous channels

We consider the quasi-maximum-likelihood (quasi-ML) detectors of the reverse link system that uses antenna arrays in asynchronous channels when the channel vector is time-invariant and time-varying. A channel vector estimation method based on eigendecomposition for time-invariant channels and its adaptive version suitable for the time-varying channels are also proposed. It is shown that the proposed quasi-ML detector can be regarded as a beamformer followed by a decorrelating filter and that the proposed system performs better than the conventional decorrelator scheme. It is also observed that the performance gain of the proposed scheme over the conventional decorrelator system increases as the numbers of active users and antenna arrays increase     

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     

An adaptive CMMSE receiver for space-time block-coded CDMA systems in frequency-selective fading channels

A technique that can suppress multiple-access interference (MAI) in space-time block-coded (STBC) multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) systems is developed. The proposed scheme, called a constrained minimum mean square error (CMMSE) receiver, is an extension of the CMMSE receiver for a single-input-single-output system to MIMO systems. It is shown that the complexity of the proposed CMMSE receiver is almost independent of the number of transmitter antennas. The advantage of the proposed receiver over the existing receivers for STBC CDMA systems is demonstrated by comparing the closed-form expressions of the signal-to-interference plus noise ratio and simulated bit error rates. The results indicate that the proposed CMMSE receiver can provide a significant performance improvement over the conventional receivers and that the gain achieved by suppressing the MAI can be larger than that from increasing the transmitter or receiver diversity.     

Space-time-frequency coded OFDM over frequency-selective fading channels

This paper proposes novel space-time-frequency (STF) coding for multi-antenna orthogonal frequency-division multiplexing (OFDM) transmissions over frequency-selective Rayleigh fading channels. Incorporating subchannel grouping and choosing appropriate system parameters, we first convert our system into a set of group STF (GSTF) systems. This enables simplification of STF coding within each GSTF system. We derive design criteria for STF coding and exploit existing ST coding techniques to construct both STF block and trellis codes. The resulting codes are shown to be capable of achieving maximum diversity and coding gains, while affording low-complexity decoding. The performance merits of our design are confirmed by corroborating simulations and compared with existing alternatives.