Evaluation of a DS/CDMA multiuser receiver employing a hybrid formof interference cancellation in Rayleigh-fading channels

This paper assesses the performance of a multiuser detection DS/CDMA receiver based on a hybrid scheme of successive interference cancellation (SIC) and parallel interference cancellation (PIC). Two configurations of the proposed hybrid IC are presented and compared with existing SIC and PIC schemes. The performance criteria used for comparison are complexity, delay, power control requirements, and average bit-error rate (BER) performance obtained by simulation in Rayleigh-fading channels with additive white Gaussian noise (AWGN). The suggested hybrid IC combines good average BER performance, short delay, acceptable complexity, and also operates under slow power control     

时分导频位CDMA系统上行链路阵列接收到部分多址干扰抑制技术

针对时分导频位CDMA系统上行链路,本文提出了导频位辅助LMS－DRMTA算法加上部分多址干扰对消的空时联合干扰抑制接收方案,由于阵列的干扰抑制作用,其输出信号中只剩下少量强的多址干扰信号,部分干扰对消法只对阵列输出中的强多址干扰信号进行干扰估计并抑制,这样一方面大大降低了系统处理的运算量及缓存空间,另一方面又消除了因弱多址干扰估计的不准确而民的系统性能下降,并且相对于自适应全多址干扰对消有更好的系统稳定性,仿真实验表明该方案比全干扰用户对消法具有更好的干扰抑制效果。     

Performance of Multiuser Detection in Multirate CDMA Systems

Multiuser detectors for asynchronous multirate code-division multiple-access (CDMA) systems are considered. A multirate CDMA system model able to fit several system concepts in the same framework is presented. The performance of the multi-code (MC) system with one processing gain and the multiple processing gain (MPG) system is compared. Upper bounds for the expected value of the asymptotic multiuser efficiency of the decorrelating detector for a synchronous multirate CDMA system with three effective users are derived for a system with one and two processing gains. The numerical results show that the performance of the decorrelating detector for MC and MPG systems do not differ significantly. The bit error rates (BER) of the decorrelating, parallel interference canceler (PIC) and groupwise serial interference canceler (GSIC) with either PIC or decorrelator within the group are compared. The results show that the decorrelating and the GSIC detectors yield the best performance. For all interference cancellation schemes the BER saturates at high SNRs due to decision errors degrading the multiple-access interference (MAI) estimates.     

Performance and optimization of multistage partial parallel interference cancellation for wideband CDMA systems in multipath fading channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA.     

Impact of Imperfect Power Control and Channel Estimation on the Performance of Multiuser Detectors in Synchronous DS-CDMA System

Simulation performance comparison of various linear multiuser and parallel interference cancellation (PIC) detectors in the presence of imperfect power control and channel estimation is presented. Results show that imperfect power control degrades even the performance of a single-user detector. Therefore, tight power control is highly indispensable for suboptimal detectors to maintain a good performance. When power control is not perfect, interference cancellation detectors can outperform linear multiuser detectors. Among cancellation detectors, the conventional [1] and partial PIC [2] detectors are fairly sensitive to channel estimation error, while the LMS PIC [3] is quite robust in this regard.     

多码CDMA系统中联合并行干扰抵消与迭代信道估计方法

本文针对多码CDMA系统提出了一种联合并行干扰抵消与迭代信道估计方法。该方法首先通过导频符号对信道进行估计，然后使用RAKE接收机后的软信息对信道参数进行修正，最后通过并行干扰抵消(PIC)去除多码干扰(MCI)。仿真结果表明，经过多次迭代后，该方法可显著地降低多码CDMA系统的误比特率平台。本文同时还给出了信道估计质量、码道数和PIC次数对多码CDMA系统性能的影响。     

一种比较决策多用户检测器的研究

以多速率码分多址(CDMA)传输为出发点，以满足不同用户信号的信干比(SIR)检测要求为目标，给出了在具有决策反馈的并行干扰抵消(PIC)检测框架下，通过实施基于概率数据关联(PDA)预估计软判决和并行干扰抵消硬判决的两种决策比较，进而选择最佳判决的方法，有效地抑制了由于信号估计不准确而造成的错误干扰抵消及相应的判决错误。文中给出了仿真结果，并分别就单一的软、硬判决及比较决策的检测性能进行了分析和比较。     

Parallel Interference Cancellation in Multiuser CDMA Channel Estimation

Parallel interference cancellation (PIC) based channel parameter estimators for frequency selective fading channels are proposed for the uplink in code-division multiple-access (CDMA) mobile communication systems. The performance of PIC based algorithms depends heavily on the quality of the multiple-access interference estimates, which can be improved by using adaptive channel estimation filters. The performance of two adaptive complex channel coefficient estimation filters has been verified in a fading channel by computer simulations. According to the results, the PIC based adaptive channel estimators outperform clearly conventional, successive interference cancellation, and decorrelation based adaptive channel estimators. The PIC method is also used in delay tracking. By using the principles of sample-correlate-choose-largest (SCCL) delay trackers, a robust algorithm for multiuser delay tracking in fading channels is obtained.     

Iterative vector channel estimation/MAP/PIC for CDMA systems in time-selective correlated multipath fading channels

Recently, Gao et al. proposed the expectation-maximization (EM) vector channel estimation for single-user direct-sequence code-division multiple-access (DS-CDMA) systems to estimate the correlated fading channel coefficients of the paths of a user. They also used one D-lag decision-feedback maximum a posteriori (MAP) detector to detect the information bits. A straightforward extension (noniterative) to multiuser synchronous CDMA cases, i.e., adding the parallel interference cancellation (PIC) multiuser detector into their scheme, results in poor performance. Therefore, we propose an iterative scheme which combines vector channel estimation, decision-feedback MAP detection, and PIC. The partial PIC is applied in the sequence hypothesis stage of MAP in each iteration before the final MAP decision. The proposed scheme has twice the decision delay of the previous scheme for single-user cases. The performance of the proposed system in the time-selective, correlated multipath fading channel environment is investigated. Simulation results show that the proposed scheme significantly outperforms the noniterative scheme. Simulation results also show that the proposed scheme performs better than its scalar channel-estimation version.     

Interference and Complexity Reduction in Multi-stage Multi-user Detection in DS-CDMA

Multi-stage parallel interference cancellation (PIC) technique gives good performance compared to successive interference cancellation (SIC) method, but biased decision statistic and complexity problems are raised due to imperfect estimation of multiple access interference (MAI) as number of stages increases. partial parallel interference cancellation (PPIC) technique is proposed to cancel the interference partially stage by stage to overcome biased problem. The complexity reduction for PIC detection is based on the convergence nature of interference cancellation which is called the difference PIC (D-PIC) detection technique. In this paper we combine (PPIC and DPIC) these two techniques and propose a multi-stage multi-user (PD-PIC) detector for performance improvement and complexity reduction compared to conventional PIC detector. The performance is degraded as the number of users increases in each technique.     

Low-complexity turbo equalization and multiuser decoding for TD-CDMA

The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) decoder and turbo decision feedback equalizer for code division multiple access (CDMA). In their scheme, an estimate of the interference signal (both multiple-access interference and intersymbol interference) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, they optimize the feedback weights at each iteration. Then, they consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. The authors show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and they propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of third-generation systems, contemplating a combination of time-division multiple access and CDMA and including frequency-selective fading channels, user asynchronism, and power control. The main conclusion of this work is that, for such application, soft-input soft-output decoders (e.g., implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.     

PIC detector for distributed space-time block coding under imperfect synchronisation

A parallel interference cancellation (PIC) detection scheme is proposed to suppress the impact of imperfect synchronisation. By treating as interference the extra components in the received signal caused by timing misalignment, the PIC detector not only offers much improved performance but also retains a low structural and computational complexity     

Trickle-based interference cancellation schemes for CDMA systems

In this paper, we introduce a novel approach to interference cancellation (IC) for code division multiple access (CDMA) uplink transmission. Several models combining principles of serial (SIC) and parallel (PIC) interference cancellation are discussed. The proposed scheme is derived from the analysis of these hybrid models and applies a user configuration algorithm (termed “trickle”) in order to provide improved bit-error-rate (BER) performance. The algorithm utilizes an adaptive matrix to compute the required configuration to be used for the subsequent interference cancellation stage. We demonstrate that significant performance improvements can be achieved over various hybrid schemes. A reduced-complexity version of the trickle algorithm is also introduced where the processing delay is greatly reduced while maintaining similar performance. We present several numerical examples through which we demonstrate the efficacy of the proposed algorithms relative to existing interference cancellation algorithms.     

Adaptive detection in asynchronous code-division multiple-accesssystem in multipath fading channels

In code-division multiple-access systems transmitting data over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. In this paper, we address interference suppression, multipath diversity and processing gain protection for multiuser detection with less noise enhancement by using a parallel cancelling scheme. The proposed detector consists of a RAKE filter, forward filter, and feedback filter with different functions for each filter. The RAKE filter increases the signal-to-noise ratio by taking the advantage of multipath and code diversities. The forward filter is proposed, in combination with the feedback filter, to remove the effects of MAI and ISI by parallel cancellation. In order to avoid performance deterioration due to unreliable initial estimation in the parallel cancellation, a cost function with proper weighting is introduced to improve the performance of the proposed detector. In the proposed design method, a recursive least square algorithm is employed to update the tap-coefficients of all filters for MAI and ISI cancellation. Finally, the performance of the proposed detector is analyzed and compared with other detectors     

Performance Analysis of Parallel Interference Cancellation for Multi-carrier DS-CDMA

The parallel interference cancellation for multi-carrier DS-CDMA （which is termed FDC-PIC） is proposed by integrating frequency diversity combination. The simulations are made over FDC-PIC with respect to different decision ways hard, soft and linear decisions, respectively, and we conclude that FDC-PIC acquires superior performance improvement over correlation reception of multi-carrier DS CDMA. With an increase in interference cancellation stages, the system performance is improved further. The initial 2 stages bring about the most dominant performance improvement, but up to the 3rd stage the system performance is improved little. It is also shown by the simulation results that FDC-PIC with soft decision would exhibit the best performance with a high implementation complexity, while FDC-PIC with linear decision acquires performance comparable to that of FDC-PIC with soft decision with a reduced-complexity if the number of the interference cancellation stages is the same, which indicates that FDC-PIC with linear decision has optimal performance/complexity tradeoff and therefore will be suitable for practical application in future.     

小扩频系数下的多级非线性并行干扰删除器的衰减系数

并行干扰删除器由于较佳的性能和较低的复杂度已开始逐步实用化.在日本,并行干扰删除器已用于3G的演示系统当中.但由于3G系统存在较小的扩频系数,并行干扰删除器的干扰再生的方法和结构必须加以调整.本文针对CDMA系统在频率选择性瑞利衰落信道中,小扩频系数下多级非线性并行干扰删除器的干扰再生的问题,明确了衰减系数的概念、产生原因,提出了一种新型应用衰减系数的多级非线性并行干扰删除器(PIC)方案.此外,还详细分析了小扩频系数下衰减系数下多级非线性并行干扰删除器性能的提高.最后,本文给出了衰减系数的仿真实现,仿真结果也表明了应用衰减系数的新型多级非线性并行干扰删除器性能改善的程度.     

Performance of an adaptive successive serial-parallel CDMAcancellation scheme in flat Rayleigh fading channels

The performance of a successive concatenated cancellation scheme for code-division multiple-access (CDMA) uplink transmission in cellular mobile radio is presented. Both serial and parallel cancellation stages are employed. The serial cancellation stage is first used to obtain initial data estimates followed by the parallel cancellation stages to enhance the accuracy of the estimates. The performance of this scheme is evaluated via analysis and simulation. In our analysis, we develop a model to consider the impact of wrongly estimating the phase and amplitude of the channel impulse response on the successive concatenated cancellation scheme. Analysis and simulation results in flat Rayleigh fading asynchronous channels with both perfect and nonperfect channel estimation and with perfect ranking confirms the accuracy of our analytical model as well as the significant improvement in performance compared to the conventional single-user matched filter (MF) detection and the stand-alone parallel cancellation schemes. Analytical results also show that the concatenated scheme has the potential to reach the single-user performance bound for a wide range of user base size, up to K=120 users, with a processing gain of 127 using one serial and three parallel cancellation stages. Finally, we propose a method whereby the receiver adapts with the number of users in order to retain the bit error ratio (BER) performance while decreasing the processing delay     

Iterative detectors for trellis-code multiple-access

Trellis-code multiple-access (TCMA) is a narrow-band multiple-access scheme based on trellis-coded modulation. There is no bandwidth expansion, so K users occupy the same bandwidth as one single user. The load of the system, in number of bits per channel use, is therefore much higher than the load in, for example, conventional code-division multiple-access systems. Interleavers are introduced as a new feature to separate the users. This implies that the maximum-likelihood sequence detector (MLSD) is now too complex to implement. Iterative detectors are therefore suggested as an alternative to the joint MLSD. The conventional interference cancellation (IC), detector has lower complexity than the MLSD, but its performance is shown to be far from acceptable. Even after a novel improvement of the IC detector, the performance is unsatisfactory. Instead of using IC, another iterative detector is suggested. This detector updates the branch metric for every iteration, and avoids the standard Gaussian approximation. Simulations show that the performance of this detector can be close to single-user performance, even when the interleaver and the phase offset are the only user-specific features in the TCMA system.     

Multiuser detection with neural network and PIC in CDMA systems for AWGN and Rayleigh fading asynchronous channels

In this paper, multiuser detection in code division multiple access (CDMA) was performed by using neural network (NN) and parallel interference cancellation (PIC). Neural network is used as a front-end stage of one stage PIC circuit. PIC is a classical technique in multi user detection process and its bit error rate (BER) performance is not good in one stage for most of the applications. For improving its BER performance, generally multi stage PIC which has high computational complexity is used. In this study, we have got better BER performance than the three stage PIC receiver in AWGN channel, almost same BER performance with the three stages PIC receiver in the Rayleigh fading channel.     

MC-CDMA系统PIC多用户检测策略

主要研究了MC-CDMA系统下行链路上各种均衡策略，采用并行干扰消除技术进行多用户检测。计算机性能模拟结果表明：基于MMSE的PIC两级检测是兼顾系统性能和运算复杂度的一种较好的多用户检测方案。