An efficient receiver scheme for downlink ZP-CDMA

A signal transmitted through a wireless channel may be severely distorted by intersymbol interference (ISI) and multiple access interference (MAI). In this paper, we propose an efficient CDMA receiver based on frequency domain equalization (FDE) with a regularized zero forcing (RZF) equalizer and parallel interference cancellation with a unit clipper decision function (CPIC) to combat both the ISI and the MAI. We call this receiver the FDE-RZF-CPIC receiver. This receiver is suitable for downlink zero padding CDMA cellular systems. The effects of the decision function, the channel estimation, the number of cancelled users, and the user loading on the performance of the proposed receiver are discussed in the paper. The bit error rate (BER) of the data received by the proposed receiver is evaluated by computer simulations. The experimental results show that the proposed receiver provides a good performance, even with a large number of interfering users. At a BER of 10^?3, the performance gain of the proposed receiver is about 2 dB over the RAKE receiver with a clipper decision function and PIC in the half-loaded case (eight users) and is much larger in the full-loaded case (16 users).     

Interference Cancellation with Space Diversity for Downlink MC-CDMA Systems

Modern wireless communications require an efficient spectrum usage and high channel capacity and throughput. Multiple-input and multiple-output (MIMO), Linear equalizers, multi-user detection and multicarrier code-division multiple access (MC-CDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate multiple access interference (MAI), eliminate Inter symbol interference (ISI) and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Multiple-output (SIMO) downlink MC-CDMA systems. It employs an interference cancellation scheme to suppress the interference caused by the multipath fading channel. Also, the proposed scheme is developed for MIMO MC-CDMA system. The performance analysis of Downlink MIMO MC-CDMA systems with V-BLAST over frequency selective fading channel is investigated under various number of transmit and receive antennas. The simulation results show proposed SIMO equalization with parallel interference cancellation scheme is effective in reducing the ISI and the MAI. It improves the performance significantly and the simulation results show that MIMO MC-CDMA with V-BLAST multi-user detection provides high data rate and the BER significant improvement.     

Performance Analysis of the Downlink CP-WCDMA System with Turbo Code in Frequency Domain

Modern wireless communications require an efficient spectrum usage and high channel capacity and high throughput. Turbo code, linear equalizers, multi-user detection and wideband code-division multiple access (WCDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate MAI, eliminate ISI and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Single-output downlink cyclic prefix CP-WCDMA systems. It employs frequency domain interference cancellation schemes to mitigate the interference caused by the multipath fading channel. Also, the proposed scheme is developed for the downlink Turbo code CP-WCDMA system to maximize the throughput of the proposed system.     

基于自适应步长ICA的CO-OFDM系统的偏振效应均衡研究

提出一种基于自适应步长独立成分分析(ICA)法的 信道均衡算法,用于相干光正交频分复用 (CO-OFDM)系统偏振效应的补偿。基于50Gbit/s 16QAM CO-OFDM系统,偏振效应严重影响其系统传输性 能。接收端在完成系统公共相位误差(CPE)补偿后,用本文所提 算法对系统偏振效应进行 盲均衡,根据每次迭代中信道频响分离矩阵的变化量自适应调整迭代步长,然后进行 盲相互干扰(ICI)相位噪声 补偿。仿真结果表明,本文算法的系统性能补偿效果接近于差分群延迟(DGD) 为零时 的系统性能,从而证明本文算法可以有效补偿偏振效应对高速光CO-OFDM系统 性能的 劣化。与固定步长ICA算法相比,本文算法能够极大地提高了迭代算法的收敛速度,极大 地降低了算法复杂度。     

Adaptive chip-rate equalization of downlink multirate wideband CDMA

We consider a downlink direct sequence-code division multiple access (DS-CDMA) system in which multirate user signals are transmitted via synchronous orthogonal short codes overlaid with a common scrambling sequence. The transmitted signal is subjected to significant time- and frequency-selective multipath fading. In response to this scenario, a novel two-mode receiver is proposed that accomplishes chip-rate adaptive equalization aided by filtering and/or cancellation of multiaccess interference (MAI). In the acquisition mode, a code-multiplexed pilot is used to adapt the equalizer from cold start or loss-of-lock. The use of MAI filtering results in a third-order least mean squares (LMS) algorithm, which has significant advantages over standard (i.e., first-order) LMS in nonstationary environments. In the tracking mode, decision-direction facilitates MAI-cancellation in the equalizer update, which enhances performance. The receiver monitors pilot decision quality as a means of switching between the two modes. The performance of the adaptive receiver is studied through analysis and simulation.     

一种利用信源冗余的Turbo均衡新算法

为克服无线衰落信道中严重的符号间干扰(ISI)的影响,提出了一种新的利用信源冗余的Turbo均衡算法。该算法将联合信源信道译码技术与Turbo均衡技术结合起来,在均衡、译码、信源之间建立起软信息交互的环路,有效提高了整体接收的性能。外信息传递(EXIT)图分析与计算机仿真均表明,尽管信源冗余给译码器带来的性能提升较为有限,但是将这部分信息反馈回均衡器后,在严重ISI信道,信源冗余度为70%时,整体接收的性能改善约为9.5 dB,基本达到了理想加性高斯白噪声（AWGN）信道下的误码性能。     

一种利用信源冗余的Turbo均衡新算法

为克服无线衰落信道中严重的符号间干扰( ISI)的影响,提出了一种新的利用信源冗余的Turbo均衡算法。该算法将联合信源信道译码技术与Turbo均衡技术结合起来,在均衡、译码、信源之间建立起软信息交互的环路,有效提高了整体接收的性能。外信息传递( EXIT)图分析与计算机仿真均表明,尽管信源冗余给译码器带来的性能提升较为有限,但是将这部分信息反馈回均衡器后,在严重ISI信道,信源冗余度为70%时,整体接收的性能改善约为9.5 dB,基本达到了理想加性高斯白噪声( AWGN)信道下的误码性能。     

IDMA水声多用户下行通信链路迭代接收算法

水声通信网络的快速发展对多用户水声物理层通信技术提出了迫切的需求。本文主要针对水声多用户下行通信链路中存在严重码间干扰和多址干扰的问题,提出了一种交织多址（IDMA）水声多用户下行通信链路迭代接收算法,能够根据译码器输出的软信息对估计的信道进行迭代更新,同时根据信道估计结果重建各用户对当前用户的多址干扰（MAI）,并使用干扰抵消器予以抵消,通过在均衡器、译码器和干扰抵消器之间的迭代逐次提高当前用户信息的输出信噪比,最终实现正确解调。其中,信道估计算法采用LMMSE初始化的分簇OMP稀疏信道估计算法,多普勒估计采用时频二维搜索结合自相关相位估计的方法,计算机仿真结果表明该接收机具有与已知信道时接近或相当的性能。进行了多用户水声通信海上试验,在4.5km、6km和7km三个距离实现了对2个用户、3个用户和4个用户的同时通信,每个用户的通信净速率约为500bps,证明了本文所提出的方法能够有效应用在多用户水声通信中,为水声通信网络的发展提供了一种重要的物理层通信技术手段。      

Error Rate Analysis of SIMO-CDMA with Complementary Codes Under Multipath Fading Channels

Broadband communication systems of the current generation are likely to offer higher bit rates for delivering high-speed multimedia services to the end users. The achievable capacity and data rate of wireless communication systems are limited to fading channels varying with time, leading to multiple access interference (MAI) and multipath interference (MPI). In this paper, we investigate downlink single-input multiple output transmission for complementary coded code-division multiple access (CC-CDMA) systems working in channels with multipath fading. Here, parallel interference cancellation is employed for analysis of CC-CDMA with different frequency domain equalization schemes to eliminate MPI and MAI over multipath fading channels. Error rate analysis for CC-CDMA employing receiver diversity is assessed using simulations under varying channel parameters. Further, we compare different equalization schemes to show the superiority of regularized zero forcing in reducing the error rate of CC-CDMA systems.     

SAGE based iterative receiver for joint synchronization and channel estimation in uplink MIMO-OFDMA systems

A new Turbo iterative receiver structure is proposed for the uplink multiple-input multiple-output orthogonal frequency division multiple access （MIMO-OFDMA） systems. The space-alternating generalized expectation-maximization （SAGE） algorithm is naturally embedded in the framework of iterative receiver to perform synchronization and detection usin~ the Turbo detector outputs. In each iteration, the expectation step intends to remove the multiple access interference （MAI） caused by other asynchronous users, and the maximization step is utilized to estimate the required parameters （i.e., timing offset, carrier frequency offset, channel state information, etc.） sequentially for each user. Simulation results show that the proposed algorithm can approach the performance of ideal receiver closely, while the processing complexity is rather lower than the conventional detectors.     

On the Design of Comb Spectrum Code for Multiple Access Scheme

A new methodology that applies frequency orthogonality to the spreading code design has been presented. The main purpose is to reduce the receiver complexity of the cyclic prefix code division multiple access (CP-CDMA) over the downlink channel and to enable parallel transmission over the uplink channel. The receiver complexity has been found for requiring full-band FFT and IFFT calculations in the channel equalizer. To handle this problem, we developed the comb spectrum codes organized in frequency orthogonal groups, in each of which the codes have time correlation orthogonality. For the downlink channel, the reduction of complexity in signal processing of a CSCDMA receiver is due to that the separation of groups is simple and each separated group is in the form of a narrower bandwidth CP-CDMA. Thus, the full-band FFT and IFFT can be replaced by the partial FFT and partial IFFT as shown in this proposed approach. For the uplink channel, we assign the code groups to different users to form a group division multiple access (GDMA) for enabling an independent channel equalization of each user at base station. The theoretical analysis and the simulation results agree well with each other and both confirm the effectiveness of this approach.     

A novel prefiltering technique for downlink transmissions in TDD MC-CDMA systems

We discuss a prefiltering technique for interference mitigation in the downlink of a time division duplex (TDD) multicarrier code-division multiple access (MC-CDMA) system. The base station (BS) is equipped with multiple transmit antennas, and channel state information (CSI) is obtained at the transmitter side by exploiting the channel reciprocity between uplink and downlink transmissions. The prefiltering coefficients are designed so as to minimize a proper cost function that depends on the signal-to-interference-plus-noise ratios (SINRs) at the mobile terminals (MTs). The resulting scheme allows using a simple despreading receiver, thereby eliminating the need for channel estimation and equalization. Numerical results show the advantages of the proposed scheme over some existing solutions.     

通用滤波多载波通信系统中干扰抑制均衡算法

作为5G多载波技术强有力的候选对象,通用滤波多载波利用子带滤波技术抑制带外功率泄露,进而降低同步要求和获得更高的频谱效率。本文首先针对通用滤波多载波在慢时变多径信道下的性能进行了分析和研究;其次为消除多径信道所带来的干扰,提出了适用于该多载波系统的信道估计方案,该方案设计了具有重复样式的导频结构进行信道估计,复杂度低;最后针对通用滤波多载波在多径信道下容易遭受符号间干扰的问题,提出了基于干扰消除的Zero-Forcing均衡算法和基于迭代干扰消除的均衡算法,两种算法均能够在消除ISI的基础上进一步地消除ICI和IBI。仿真结果表明,本文提出的信道估计和均衡算法能有效消除通用滤波多载波技术在多径信道下所经受的ISI、ICI和IBI。      

CDMA系统下行链路的干扰抵消

CDMA蜂窝移动通信系统是一种采用多址技术的通信系统，即采用不同的地址码来区分用户、基站和信道。然而由于其在多径衰落信道中的自相关和互相关特性的不理想造成了多址干扰。在扩频通信系统的下行链路中，有效地抵消多址干扰是进一步改善系统性能和提高系统容量的重要途径。本文提出一种应用于CDMA下行链路的干扰抵消算法，分析了下行链路接收机的实现原理及过程，并对其中的关键算法进行了详细地分析，仿真结果表明，所选取的下行接收算法能改善系统的性能。     

Low-Cost Approximate LMMSE Equalizer Based on Krylov Subspace Methods for HSDPA

The throughput of the high speed downlink packet access (HSDPA) sub-system of UMTS suffers significantly from multiple access interference in the wireless channel. A linear minimum mean square error (LMMSE) equalizer at the receiver achieves higher throughput than a conventional RAKE receiver, at the cost of higher complexity. We introduce an iterative algorithm based on Krylov subspace projections, approximating the LMMSE equalizer with negligible loss of performance for the receiver. Slow variations of the channel can be exploited to allow further acceleration of the algorithm. Computational complexity as well as storage requirements are strongly reduced     

Compensation of non-reciprocal interference in adaptive MIMO-OFDM cellular systems

In a time-division-duplex communication system, the channel knowledge can be obtained at the transmitter side due to channel reciprocity and it can be used to increase the spectral efficiency of a multiple-input multiple-output (MIMO) communications. However, the interference structure between transmission directions does not necessarily correlate. The obtained quality of service at the receiver may differ significantly from the desired one if the transmission parameters are assigned based on the reverse link measurements only. In this paper, the performance of an orthogonal frequency division multiplexing (OFDM) cellular system with adaptive MIMO transmission is studied in the presence of non-reciprocal inter-cell interference when the downlink interference structure is known at the receiver and only limited feedback information about the interference is available at the transmitter. The results are compared to those with perfectly known interference structure per each sub-carrier. The system level impact of realistic interference non-reciprocity scenarios is studied via network simulations. Linear minimum mean squared error (MMSE) filter is applied at the receiver to suppress the impact of structured inter-cell interference together with a simple and bandwidth efficient closed-loop compensation algorithm. Both link and system level simulation results show that the proposed compensation algorithm with a simple scalar power offset feedback combined with interference suppression at the receiver results in nearly the same performance as the ideal case     

Spreading code allocation strategy for downlink multicarrier code division multiple access transmission in a correlated Rayleigh fading channel

In this paper, we address the problem of multiple access interference (MAI) in a downlink multicarrier code division multiple access system. Because in the realistic case of correlated faded subcarriers, MAI greatly depends on the codes assigned to users, one way of improving performance without increasing receiver complexity consists in an appropriate code selection. We propose a code allocation strategy with the following properties: possible use with any code sequence and equalizer, low complexity, and efficient management of load variations. The allocation problem is formulated as a minimization problem of a cost function related to MAI. First, we provide analytical expressions for the channel frequency correlation function after maximum ratio combining, equal gain combining, and minimum mean square error equalization and for the MAI power. Then, by approximating these expressions, we define a simple cost function and build an iterative algorithm on the basis of the minimum maximum criterion to select the spreading codes. Finally, a complete analysis of the allocation efficiency is provided versus key parameters, in particular, the degree of correlation between the faded subcarriers, the system load, and the equalization techniques. Copyright © 2012 John Wiley & Sons, Ltd.     

Iterative decoding and channel estimation for space-time BICM over MIMO block fading multipath AWGN channel

In this paper, we consider a generic model of space-time bit-interleaved coded modulation (ST-BICM) on a multiple-input multiple-output (MIMO) Rayleigh fading multipath channel. A practical low-complexity receiver structure performing iteratively MIMO data detection, channel decoding and channel estimation, is presented. The MIMO data detection, employing a reduced-state list-type soft output Viterbi algorithm enables to cope with severe channel intersymbol interference (ISI) without MIMO prefiltering. Among other results, simulations show that our approach can dramatically improve the downlink performance of time-division multiple access (TDMA) systems with high order modulation, keeping a reasonable complexity at the receiver side.     

Modified Frequency-Domain Equalization for Channel Shortening in Reduced-CP OFDMA Systems

In orthogonal frequency-division multiple access (OFDMA) systems, the cyclic prefix (CP) length needs to be no less than the longest delay spread of the channels of many users, reducing bandwidth efficiency more significantly than in single-user orthogonal frequency division multiplexing systems. In this paper, we address OFDMA downlink data transmission when a short CP is used to mitigate the inefficient bandwidth usage. Previous time-domain equalizers (TEQs) can be used to shorten the channel; however, they tend to increase noise by introducing spectral nulls. A previous work on per-tone equalization (PTEQ) structure involves a multitap frequency-domain equalizer (FEQ) for each tone, and shows better performance than TEQ-based receiver structure. We propose a novel receiver structure with only one-tap FEQ for OFDMA systems with a reduced CP, exploiting the unused subchannels for a user. We formulate an optimization problem to set the FEQ coefficients at the unused subchannels such that the channel is shortened (approximation is involved) and noise is not enhanced at the used subchannels. With the aid of computer simulations, it is demonstrated that the proposed equalization method is superior to the conventional TEQ-based receivers, and is comparable to the previous PTEQ-based receiver in terms of the achievable SNR, error performance, and bandwidth efficiency. Although the throughput curve versus synchronization delay of the PTEQ is smoother than that of the proposed receiver, the proposed method shows proper throughput over a wider range of the delay values than PTEQ receiver when the system parameters are set so that the complexities are comparable.     

基于PSP均衡技术的CPM检测算法

针对时变频率选择性衰落信道,研究了连续相位调制（CPM）信号的逐幸存检测算法。该算法在未知信道状态的条件下,利用训练序列对信道参数进行初始估计。在对CPM信号进行Viterbi解调过程中,采用PSP技术实现信道的无延时跟踪。基于频域均衡的CPM检测算法虽然可以有效抗多径干扰且计算复杂度较低,但不能对时变信道进行跟踪。仿真结果表明,在时变多径信道下,基于PSP均衡的CPM检测算法能有效地进行信道参数估计,比频域均衡算法具有更好的误码性能。