非理想定时跟踪下CDMA并行干扰消除检测器的性能

以异步码分多址(DS-CDMA)系统硬判并行干扰消除检测器为模型,在定时同步错误情况下.由于并行干扰消除(PIC)分析的复杂性,以前的工作主要是依靠仿真方法.文献[5]分析了软判决PIC在定时跟踪错误下的性能,给出了封闭形式的误码率表达式.本文则分析了硬判决PIC残余多址干扰的结构及统计特性,导出了定时跟踪错误下封闭形式的误码率表达式.通过仿真与数值计算,分析了定时跟踪错误对硬判决并行干扰消除算法的影响.     

软判决并行部分干扰抵消检测器权重因子的确定

在本文中，我们给出线性软判决并行部分干扰抵消（SD－PPIC）检测器的另一种解释。根据这种解释，我们可以得到一种确定SD－PPIC检测器部分抵消权重因子的简便方法，这种方法得到的权重因子可以保证SD－PPIC检测器以最快的速度收敛到它的极限性能，而这一极限性能我们证明就是经典解相关检测。     

基于并行干扰抵消的OFDM/OQAM系统中的信号检测方法

在多径衰落信道中,基于交错正交幅度调制的正交频分复用(OFDM with Offset QAM,OFDM/OQAM)系统使用迫零均衡器进行信号检测时,不能完全消除信道复数特性和滤波器实数正交特性引入的时域符号间干扰和频域子载波间干扰,及信道估计误差导致的误码率性能损失。该文利用对数据初始判决并重构相邻载波及符号间干扰的思想,通过分析采用迫零均衡信号检测时的残余干扰与信道估计误差干扰,提出了一种基于并行干扰抵消和迫零均衡器结合的OFDM/OQAM信号检测方法,并在IEEE 802.22技术标准的两种典型多径衰落信道中进行了计算机仿真与比较研究。仿真结果表明,与基于迫零均衡的检测方法相比,基于并行干扰抵消的迭代信号检测方法在误码率为1%时,可获得1 dB至2 dB的性能提升。     

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。     

频率选择性MIMO衰落信道中基于导频的软均衡算法

MIMO无线传输技术的有效应用有赖于高效的空时干扰抑制策略.针对频率选择性衰落MIMO信道特点,本文提出一种联合判决反馈干扰抵消和粒子滤波的软均衡方案.首先利用导频,确定了各发送信号最强径与接收天线之间的对应关系以及基于信干噪比的判决反馈干扰抵消顺序;其次,依据后验估计准则,充分利用径强度及接收信号结构信息,构造了接收信号的重要性分布函数,采用粒子滤波算法对感兴趣用户信号实施最优判决并进行干扰抵消.仿真结果表明:相比于其他均衡方法,本算法可以有效抑制均衡误差的传播,同时具有较低的计算复杂度.     

时变信道下OFDM系统中基于载频间干扰抑制的迭代检测算法

为了有效地抑制OFDM系统载频间干扰（ICI），提出一种并行干扰消除的迭代检测算法。基于检测信号的信干噪比最大化的准则，通过对接收信号进行判决、重构、干扰抵消处理，建立了ICI抑制的迭代检测算法，给出了信号检测流程。通过对时变信道的分析，研究了时变信道中归一化频偏对系统陛能的影响，给出了性能仿真结果。仿真结果表明所建议的算法可有效地利用时变信道的时间分集特性来提高系统性能，计算复杂度从O（N^3）降低到O（N）。     

自适应部分干扰抵消多用户接收机

在DS/CDMA系统中,采用多用户检测技术可以有效地克服多址干扰(MAI,MultipleaccessInterference)及远近效应。本文提出了一种自适应部分干扰抵消(APPIC,AdaptivePartialParallelInterferencecancellation)算法,根据匹配滤波器输出的判决统计对多址干扰进行选通抵消,以提高系统性能。同时本文给出了自适应部分干扰抵消接收机的非相干实现方案,并给出了仿真结果。     

水声通信中基于软判决的块迭代判决反馈均衡器

在单载波频域均衡水声通信系统中,混合结构的时-频域判决反馈均衡器(H-DFE)计算复杂度高,不利于实时实现;而基于硬判决的块迭代判决反馈均衡器(HD-IBDFE)存在错误符号判决造成系统性能下降问题,同时需要估计判决符号和发射数据之间的互相关函数。该文对水声通信中基于软判决的块迭代判决反馈均衡(SD-IBDFE)接收机算法进行了研究,通过对均衡器输出信号进行软判决,并将符号软信息进行反馈,提高了系统性能,同时采用迭代信道估计方法来适应水声信道的时变性。通过仿真比较得出,该方法在水声信道条件下明显优于HD- IBDFE。对湖上试验数据处理结果表明,在浅水1.8 km通信距离下,单通道无编码QPSK调制可实现10-3的误码率并达到3000 bps的有效数据率。     

基于LDPC编码的MIM0系统迭代接收机研究

研究了基于LDPC编码无线MIMO通信系统中的软输出最小均方误差干扰抵消迭代(MMSE PIC)检测算法.针对初次迭代检测时PIC输出的后验比特对数似然比(LLR)不可靠的问题,利用MMSE滤波器输出的高斯近似表示,给出了基于后验概率估计的迭代干扰抵消检测算法,以提高检测器输出的编码比特对数似然比的可靠性.仿真结果表明...     

基于多用户反馈的判决反馈均衡器的研究

本文提出的一种新颖的基于多用户反馈的判决反馈均衡器,解决了在CDMA多用户检测中传统自适应判决反馈均衡器误码率高、系统容量小的缺点.它由具有误差反馈滤波器的判决反馈均衡器(Decision Feedback Equalizer with Error Feedback Filter,DFE-EFF)构成,并在判决后反馈多用户数据抵消多址干扰(多用户反馈干扰抵消).文中给出其结构图,分析各种判决反馈均衡算法.理论证明,具有误差反馈滤波器的多用户反馈干扰抵消判决反馈均衡器(多用户反馈干扰抵消DFE-EFF)较各种判决反馈均衡器为最优,它能同时有效处理ISI,MAI和噪声的干扰.仿真结果表明,在误码率性能和系统容量两方面,多用户反馈干扰抵消DFE-EFF比DFE、DFE-EFF均有较大改善.     

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.     

基于软判决的MMSE-OSIC接收机

串行干扰相消接收机是广泛应用于多输入多输出系统中的一种接收端信号处理与检测技术。在串行干扰相消接收机中,先检测的层的判决准确性会对后续层的检测产生影响。检测过程中产生的判决误差,将对后续层的检测引入残留干扰,进而影响后续层检测的正确性,造成差错传播。本文提出一种基于软判决的最小均方误差检测带排序串行干扰相消接收机,可以有效减轻基于硬判决方法的带排序串行干扰相消接收机的差错传播问题。在重构干扰信号时,该接收机使用根据软判决信息得到的符号期望,替代硬判决方法,能够有效降低干扰相消时各层之间由判决误差引起的残留干扰;该接收机还可以精确估计残留干扰的功率以及各层数据流的后处理信干噪比,改善了基于硬判决方法的串行干扰相消接收机存在的各层后处理信干噪比估计值偏大的问题。可靠性更高的软判决结果有效降低了残留干扰,更精确的各层后处理信干噪比抑制了残留干扰对后续数据流检测的影响,提高了判决结果和排序过程的准确性。仿真结果证明,基于软判决的最小均方误差检测带排序串行干扰相消接收机可以有效避免差错传播的产生,因而获得较大的性能提升。      

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     

DS-OCDMA Receivers Based on Parallel Interference Cancellation and Hard Limiters

In an incoherent direct-sequence optical code-division multiple-access (DS-OCDMA) system, multiple-access interference (MAI) is one of the principal limitations. To mitigate MAI, the parallel interference cancellation (PIC) technique can be used to remove nondesired users' contribution. In this paper, we study four DS-OCDMA receivers based on the PIC technique with hard limiters placed before the nondesired users or before the desired user receiver, or both. We develop, for the ideal synchronous case, the theoretical upper bound of the error probability for the four receivers. Significant performance improvement is obtained by comparison with conventional receivers in the case of optical orthogonal codes. The paper highlights that the number of active users with null error probability is doubled, compared with conventional receivers. Finally, we show that, thanks to their good performances, the PIC structures permit considerably reducing the minimal code length required to have 30 users with bit-error rate$≪10^-9$. So, the hardware constraints are relaxed for realistic application.     

Low Complexity Iterative ICI Cancellation and Equalization for OFDM Systems Over Doubly Selective Channels

In this paper, a low complexity iterative intercarrier interference (ICI) cancellation and equalization technique is proposed for use in OFDM systems over doubly selective channels. In the iterative parallel interference cancellation/minimum mean square error (PIC/MMSE) detector has a high complexity and a restriction on the structure which can not remove the ICI in the initial stage. Therefore, an error propagation occurs due to the ICI regenerated by the incorrect output of soft-input soft-output (SISO) decoder. In order to reduce the error propagation, an MMSE detector based on the successive interference cancellation (SIC) is used in the initial stage. The low complexity MMSE detector is also derived to minimize the error propagation by quantifying the decision error before SISO decoding. In the first iteration, simulation results show that the proposed scheme outperforms the conventional PIC/MMSE scheme by about 3 dB at bit error rate $({rm BER})=1times 10^{-3}$ while maintaining the equivalent computational complexity. In the subsequent iteration, it is possible to cancel the ICI out in the received signals by the aid of soft log-likelihood ratio (LLR) fed from the SISO decoder. Converting the LLR to the decision error probability, the error covariance matrix is obtained more accurately. As a result, the error propagation can be effectively reduced by dealing with only the dominant components, when considering decision errors. Finally, simulation results show that the proposed scheme outperforms the conventional PIC/MMSE scheme.      

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.     

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.     

Data-aided channel estimation for turbo-PIC MIMO detectors

We consider data-aided channel estimation for multiple input multiple output (MIMO) systems when iterative parallel interference cancellation (PIC) is performed for signal detection. We compare some data-aided channel estimation methods based on expectation maximization (EM) algorithm or on hard estimated transmit symbols. In particular, we propose a modified EM-based approach and show that when few iterations are to be performed, it provides considerable performance improvement.     

Analytical performance analysis of the linear multistage partial PIC receiver for DS-CDMA systems

In this letter, we derive a recursive expression for the decision statistic of a linear multistage partial parallel interference cancellation (PIC) receiver for direct-sequence code-division multiple-access systems that allow us to present two closed-form expressions (exact and approximate) for the bit-error probability of this receiver. The bit-error rate is computed in terms of the number of PIC stages, set of cancellation parameters, crosscorrelation between the users' codes, amplitude of the users' signals, and noise power.