城市建筑对移动通信信号多用户检测的影响

分析城市建筑的结构、高度和密度对移动通信信号多用户检测的影响。在蜂窝移动通信系统中,当城市环境传播模型变化致使信号处于深衰落时,最小均方误差(MMSE)检测器经常失去对信号的相位跟踪。本文提出了一种次优化子空间盲MMSE检测算法,运用目标用户特征波形和辅助向量,对信号相位进行去耦处理,使盲MMSE检测器具有次优化低复杂性的结构和算法,并且适合于城市衰落信道。     

SC-FDE系统中归一化FBLMS算法的研究

单载波频域均衡（SC-FDE）系统传统上采用最小均方误差（MMSE）算法和快速块最小均方（FBLMS）算法进行频域均衡,文中对单载波频域均衡系统中归一化FBLMS算法进行了仿真研究,并与MMSE算法和FBLMS算法进行比较。测试和分析了系统的均方误差和误码率。仿真结果表明：该算法克服了收敛速度和失调量之间的矛盾,提高了算法的收敛速度,降低了误码率,该结果对SC-FDE系统的进一步研究具有参考价值。     

一种空时分组码预编码的均衡算法研究

为了提高正交空时分组码的MIMO系统性能,提出了采用预编码的空时分组码信号检测方案,该方案在发射端将正交分组编码和预编码矩阵相结合,在接收端采用MMSE均衡译码算法。分析了最小均方误差(MMSE)均衡译码算法和最大似然(ML)译码算法,将此两种算法的性能与最大比值合并(MRC)的性能进行了仿真和比较。仿真结果表明,MMSE算法与ML算法性能相比在误比特率为10-5时有约0.5 dB的增益。     

SC-FDE在无人机遥测下行链路中的应用研究

该文针对无人机高速率下行数据链通信中存在的码间干扰和多径衰落,在对无人机信道进行建模的基础上,采用了迫零均衡(Zero Forcing,ZF)和最小均方误差均衡(Minimum Mean Square Error,MMSE)两种算法的单载波频域均衡(Single Carrier Frequency Domain Equalization,SC-FDE)技术对此码间干扰和多径衰落进行了抑制;通过仿真得到采用SC-FDE技术使得误码率明显下降,从而验证了在无人机高速率下行数据链通信中,采用SC-FDE技术能有效抑制码间干扰和多径衰落。     

一种联合均衡和多用户检测的SDCMA盲算法

该文提出了用于MC DS-CDMA系统中的两种SDCMA(最陡下降常模算法)盲检测方法,一种是SDCMA盲均衡算法,在此基础上,提出了另一种联合均衡和多用户检测的SDCMA盲算法。同时还将这两种算法与基于子空间的MMSE(最小均方误差)盲检测算法进行比较。仿真结果表明,这两种SDCMA算法的BER(误比特率)性能均优于子空间法,而且联合均衡和多用户检测的SDCMA盲算法利用同时对均衡权值向量和多用户检测器向量进行自适应更新,能较大程度地改善系统性能,是这3种盲算法中性能最佳的一个。     

同步CDMA系统中的MMSE检测器研究

分析了基于实域估计的最小均方误差(MMSE)检测器及其在同步CDMA系统的应用。首先对同步CDMA系统进行了描述,并在此基础上对这种新的MMSE检测器在同步CDMA系统中的设计和近似实现方法进行了探讨。     

MIMO系统中基于ZF/MMSE检测的 自适应功率分配方案

 本文研究了MIMO系统中的功率分配技术.针对基于ZF/MMSE检测的MIMO系统,提出了一种简单的自适应功率分配方案.该方案在不使用预编码技术对MIMO信道进行对角化的前提下,遵循最小化系统BER的准则,为发送数据流分配发射功率.与采用SVD预编码的功率分配方案相比,本文提出的方法减少了系统需要的反馈信息量.仿真表明该方法能够有效地提高系统的BER性能,而且基于MMSE检测的系统性能接近于采用SVD预编码的功率分配性能.     

V-BLAST系统中的迭代软判决干扰抵消检测算法

分层空时结构(BLAST)是一种通过使用多根发射天线和接收天线,以及在接收端使用先进的信号处理技术来获得高频谱效率的技术。主要研究了基于MMSE的迭代软判决干扰抵消(ISDIC)算法在V BLAST系统中的应用。仿真结果表明,相对于传统的检测算法,其性能至少有1 dB的增益。     

一种改进的SC-FDE块迭代判决反馈均衡器

单载波频域均衡(SC-FDE)系统能有效地对抗频域选择性衰落信道。块迭代判决反馈均衡(IBDFE)是单载波频域均衡系统中一种有效的非线性均衡算法,但是其计算复杂度较高,且复杂度与迭代次数成正比。鉴于此,提出一种改进算法,在改进算法中,将接收信号经过MMSE均衡得到发送信号估计值作为传统算法反馈支路输出,前馈支路结构保持不变。对比传统IBDFE算法,迭代被取消,故降低了计算复杂度。对传统IBDFE算法和改进IBDFE算法性能进行比较,实验结果证明:改进IBDFE算法性能在与传统IBDFE算法性能相似的条件下降低了计算复杂度。     

频域均衡联合基于能量排序的部分并行干扰删除检测算法

针对频率选择性信道下的DS-CDMA系统,该文提出了一种联合MMSE(最小均方误差)频域均衡与基于能量排序的部分并行干扰删除(MMSE-EOPPIC)的多用户检测算法。该算法首先采用基于重叠剪切法的MMSE频域均衡代替Rake接收机对各用户的数据信息进行初始估计,然后在干扰删除的每个迭代级根据用户能量由强到弱的顺序依次对用户进行部分干扰删除;为了提高每个迭代级检测的可靠性,该文将当前级已经检测出的能量较强用户的硬判决值用于重构当前级能量较弱用户的多址干扰(MAI)。仿真结果表明MMSE-EOPPIC检测算法可以有效提高系统的比特误码率性能。     

Packet access using DS-CDMA with frequency-domain equalization

The next-generation mobile communications system is anticipated to support very high-speed data rates exceeding several tens megabits per second. In this paper, we consider high-speed downlink packet access for direct-sequence code-division multiple access (DS-CDMA) as in third-generation wideband code-division multiple-access systems. Adaptive modulation and coding (AMC), multicode operation and hybrid automatic repeat request (HARQ) will be the enabling technologies. With such high-speed data transmissions, however, multicode operation severely suffers from the loss of orthogonality among the spreading codes since the wireless channel becomes severely frequency-selective. In this paper, we apply frequency-domain equalization (FDE) based on minimum mean-square error (MMSE) criterion instead of conventional rake combining for receiving the packet. A new MMSE-FDE weight is derived for packet combining. The throughput in a frequency-selective Rayleigh-fading channel is evaluated by computer simulation for Chase combining and incremental redundancy (IR) packet combining. It is shown that the use of MMSE-FDE for the reception of multicode DS-CDMA packet gives an improved throughput irrespective of the channel's frequency-selectivity.     

Joint frequency-domain differential detection and equalization for DS-CDMA signal transmissions in a frequency-selective fading channel

It has been revealed that direct sequence code-division multiple access (DS-CDMA) can achieve a good bit-error rate (BER) performance, comparable to multicarrier CDMA (MC-CDMA), by using coherent frequency-domain equalization (FDE) instead of coherent Rake combining. However, coherent FDE requires accurate channel estimation. Pilot-assisted channel estimation is a practical solution, but its accuracy is sensitive to the Doppler spread. In this paper, a frequency-domain differential encoding and detection scheme is proposed for a DS-CDMA mobile radio. Joint frequency-domain differential detection and equalization (FDDDE) based on minimum mean-square error (MMSE) criterion is presented, where a simple decision feedback filter is used to provide a reliable reference signal for MMSE-FDDDE. Also presented is an approximate BER analysis. It is confirmed by both approximate BER analysis and computer simulation that MMSE-FDDDE provides good BER performance close to the coherent MMSE-FDE and shows high robustness against the Doppler spread; it outperforms coherent MMSE-FDE for large Doppler spreads. The proposed MMSE-FDDDE can also be applied to MC-CDMA. A performance comparison between uncoded DS- and MC-CDMA shows that DS-CDMA with MMSE-FDDDE achieves better BER performance than MC-CDMA with MMSE-FDDDE for small spreading factors.     

基于参差重复导言的频率同步算法

具有重复信号的导言是突发通信系统中常用的同步方法。本文设计了一种新颖的参差重复导言,并给出最小均方误差的频率同步算法。与传统算法相比,该算法不仅提高了频率捕获精度,捕获范围也有效扩大且倍数可调。虽然本文仅研究了该算法在单载波频域均衡系统中的应用,但其基本原理同样适用于多载波系统。     

Signal Processing by Generalized Receiver in?DS-CDMA Wireless Communication Systems with?Frequency-Selective Channels

The generalized receiver (GR) based on a generalized approach to signal processing (GASP) in noise is investigated in a direct-sequence code-division multiple access (DS-CDMA) wireless communication system with frequency-selective channels. We consider four avenues: linear equalization with finite impulse response (FIR) beamforming filters; channel estimation and spatially correlation; optimal combining; and partial cancellation. We investigate the GR with simple linear equalization and FIR beamforming filters. Numerical results and simulation show that the GR with FIR beamforming filters surpasses in performance the optimum infinite impulse response beamforming filters with conventional receivers, and can closely approach the performance of GR with infinite impulse response beamforming filters. Channel estimation errors are taken into consideration so that DS-CDMA wireless communication system performance will not be degraded under practical channel estimation. GR takes an estimation error of a maximum likelihood (ML) multiple-input multiple-output (MIMO) channel estimation and GR spatially correlation into account in computation of minimum mean square error (MMSE) and log-likelihood ratio (LLR) of each coded bit. The symbol error rate (SER) performance of DS-CDMA employing GR with a quadrature sub-branch hybrid selection/maximal-ratio combining (HS/MRC) scheme for 1-D modulations in Rayleigh fading is obtained and compared with that of conventional HS/MRC receivers. Procedure of selecting a partial cancelation factor (PCF) for the first stage of a hard-decision partial parallel interference cancellation (PPIC) of the GR employed in DS-CDMA wireless communication system is proposed. A range of optimal PCFs is derived based on the Price’s theorem. Computer simulation results show superiority in bit error rate (BER) performance that is very close to that potentially achieved and surpasses the BER performance of the real PCF for DS-CDMA systems discussed in literature.     

A robust channel estimator for DS-CDMA systems under multipath fading channels

This paper addresses the problem of channel estimation for direct-sequence code-division multiple-access (DS-CDMA) systems with time-varying multipath fading channels. The multipath fading channels are modeled as autoregressive (AR) models. A method is first proposed to convert the time-varying regression model due to the time-varying nature of users' information symbols into a time-invariant one. Then, a polynomial approach is proposed to obtain the minimum mean square error (MMSE) estimator. The uncertainty of the channel model and decision errors of the DS-CDMA detector are taken into consideration in the design of the MMSE estimator. Compared with the Kalman estimator, the computational complexity of the proposed algorithm is much lower. The simulation results show that the proposed estimator provides a comparable estimation performance with the Kalman estimator and is robust for fast-fading channels.     

Performance evaluation of OFDM and single‐carrier systems using frequency domain equalization and phase modulation

In this paper, we study the performance of the continuous phase modulation (CPM)‐based orthogonal frequency division multiplexing (CPM‐OFDM) system. Also, we propose a CPM‐based single‐carrier frequency domain equalization (CPM‐SC‐FDE) structure for broadband wireless communication systems. The proposed structure combines the advantages of the low complexity of SC‐FDE, in addition to exploiting the channel frequency diversity and the power efficiency of CPM. Both the CPM‐OFDM system and the proposed system are implemented with FDE to avoid the complexity of the equalization. Two types of frequency domain equalizers are considered and compared for performance evaluation of both systems; the zero forcing (ZF) equalizer and the minimum mean square error (MMSE) equalizer. Simulation experiments are performed for a variety of multipath fading channels. Simulation results show that the performance of the CPM‐based systems with multipath fading is better than their performance with single path fading. The performance over a multipath channel is at least 5 and 12 dB better than the performance over a single path channel, for the CPM‐OFDM system and the proposed CPM‐SC‐FDE system, respectively. The results also show that, when CPM is utilized in SC‐FDE systems, they can outperform CPM‐OFDM systems by about 5 dB. Copyright © 2010 John Wiley & Sons, Ltd.     

A comparison of long versus short spreading sequences in codedasynchronous DS-CDMA systems

The performance of turbo-coded asynchronous direct sequence code division multiple access (DS-CDMA) using long and short spreading sequences is compared by both analysis and simulation. For coded systems with a conventional matched filter (MF) receiver, three analytical methods with different complexity are compared: the standard Gaussian approximation, the improved Gaussian approximation (IGA), and the density function approach. It is shown that while the standard Gaussian approximation is fairly accurate for the long sequences, it is too optimistic for the short sequences. For the short-sequence systems, the IGA gives an accurate estimate for the performance with much less complexity than the density function approach. The analysis shows that for either the additive white Gaussian noise (AWGN) channel or the flat Rayleigh fading channel and a MF receiver, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations are not time-varying. However, the short-sequence systems are amenable to the use of an interference suppression technique designed to minimize the mean square error. Such a minimum mean square error (MMSE) receiver in the turbo-coded system is shown to outperform the long-sequence system with the MF receiver, especially when there is a near-far problem, as previously observed in a convolutionally-coded system. Finally, similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selective Rayleigh fading channel     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

On water-filling precoding for coded single-carrier systems

We study the spectrum shaping gain in single carrier systems through water-filling precoding. We show that water-filling precoding outperforms other existing alternatives when iterative linear minimum mean square error (LMMSE) detection is involved, which is confirmed by both evolution analysis and simulation results. This technique does not incur any extra receiver cost when it is used together with frequency domain equalization (FDE).