OFDM系统中HARQ方案与性能仿真

研究了正交频分复用(OFDM)系统中采用的混合自动重传请求(HARQ)技术,对其性能和实现方案进行了分析;在采用高阶调制时,为均衡同一映射符号内比特可靠性差异采用星座重排方案,并对星座重排技术进行了研究;在典型城区多径信道环境下,对不采用HARQ、采用Type I HARQ、采用Type Ⅲ chase合并的HARQ和采用带有星座重排的Chase合并的HARQ4种方案的误码率和吞吐量性能分别进行了仿真;仿真结果表明,星座重排技术结合Chase合并的HARQ机制是无线通信系统中提高传输质量和吞吐量的一种高效可行的方案.     

TD-LTE系统中上下行HARQ研究及DSP实现北大核心

HARQ(混合自动重传请求)技术能够很好地补偿无线移动信道时变和多径衰落对信号传输的影响,已成为LTE(长期演进)系统中不可缺少的一项关键技术。针对TD-LTE(时分双工的LTE)系统中的上下行HARQ过程,提出了一种在DSP(数字信号处理器)中的实现方案,通过在DSP中运行,验证了该方案的有效性和可行性。     

3G长期演进系统中的混合自动重传请求技术

3G长期演进系统(LTE)的标准化工作已经全面展开,并得到了大家的广泛关注。各种先进的无线传输技术即将在该系统中得到应用,包括混合自动重传请求(HARQ)技术。根据重传发生时刻的不同,HARQ可以分为同步和异步两类。同步HARQ由于接收端预先已知传输的发生时刻,HARQ进程的序号可以从子帧号获得;异步HARQ进程的传输可以发生在任何时刻,HARQ进程的处理序号需要连同数据一起发送。由于HARQ技术能够很好地补偿无线移动信道时变和多径衰落对信号传输的影响,已成为LTE中的关键技术之一。该技术将会随着3G长期演进系统的发展不断完善。     

基于判决反馈的TurboDPSK

本文针对衰落信道中DPSK(differential phase-shift keying)信号的特点,提出了一种基于判决反馈的Turbo DPSK解调/译码算法,它利用判决反馈进行衰落信道的估计并计算信息比特的似然比.与基于BCJR(Bahl-Cocke-Jelinek-Raviv)的Turbo DPSK解调/译码算法相比,它可以大大降低运算复杂度,减少迭代译码过程中所需的存储空间.仿真结果表明,采用这种新的迭代解调/译码算法可以获得和基于BCJR的迭代解调/译码算法几乎一样的性能.     

基于迭代原理的低复杂度均衡算法

研究了均衡和信道译码的联合迭代译码,在此基础上提出了一种基于迭代原理的低复杂度均衡算法。算法通过将指数运算近似为乘累加运算,然后在不同时机分别利用最小均方误差符号估计值或信道译码器反馈的先验信息来计算迭代均衡中的软信息,避免了在信号空间中的穷尽搜索计算,从而降低了复杂度。仿真和分析表明,在多径信道下,该算法以些微性能损失为代价有效的降低了均衡计算复杂度,尤其适用于高阶调制方式。当采用8PSK调制方式时,本文算法对于均衡过程中复杂度较高的ρ_n,i的计算次数仅为原来的1/4。      

一种CQI辅助的HARQ改进方案

下一代移动通信系统在高速移动的条件下,需要更高的吞吐量和更大的带宽。这就需要采用更好的信道编码（turbo码等）、HARQ策略和信道均衡技术。但是,turbo译码是一个复杂的过程,在进行信号处理时会耗费用户大量的时间和功率。因此与turbo译码相关的HARQ策略会对系统性能产生很大影响。提出了一种CQI辅助的HARQ策略。这种策略主要利用了已经测量得到的CQI来跳过HARQ在首次传输和重传过程中不必要的解调和译码过程。通过采用HARQ策略,当误帧率高于门限时可以节省UE端大约55%的功率。     

基于校正子的第一类混合ARQ及其性能分析

基于校正子的计算提出了一种第一类混合ARQ (HARQ) 方案,该HARQ采用Reed-Solomon (RS)码作为纠错子系统,将检错功能巧妙融于译码过程中。对系统误比特率和吞吐量的分析与仿真表明,短码宜采用前向校正子结构,长码宜采用后向校正子结构,而系统的吞吐量与可靠性之间的最佳折衷,可以通过选择恰当的最大重传次数实现。     

基于IRA码的HARQ的性能分析

混合自动重传请求(Hybrid Automatic Repeat Request,HARQ)技术是保障数据可靠传输最有效的差错控制技术。在本文中,采用不规则重复码(Irregular Repeat Accumulate,IRA)码作为前向纠错码,得出了在AWGN信道下的吞吐率。仿真结果表明,IRA码在HARQ系统中的吞吐率接近turbo码,但译码复杂度远远低于Turbo码。     

HSDPA技术中HARQ的一种新组合译码算法

文中介绍了HSDPA中组合译码的组合方案,并在传统Chase组合译码算法的基础上提出了一种改进的新组合译码算法。最后通过计算机仿真,结果表明采用新算法的HARQ在有效减少重传次数方面明显优于采用无组合译码算法的HARQ。     

MIMO-OFDM系统的低复杂度递推信道跟踪算法

该文针对时变多径信道下的MIMO-OFDM系统,基于变分贝叶斯原理,提出了一种新的联合信号检测和信道跟踪的低复杂度半盲贝叶斯迭代接收机。针对该接收机,基于递推变分期望最大化(RVBEM)算法,提出了一种RVBEM信道跟踪算法。由于RVBEM算法需要进行矩阵求逆,因此以该算法为基础推导得到了一种时频域联合递推的低复杂度信道跟踪(TF-LCRVBEM)算法。TF-LCRVBEM算法不仅完全避免了矩阵求逆运算,还通过合理的近似使得算法只具有线性复杂度。分析和仿真表明,在时变多径信道下,所提迭代接收机具有远优于传统接收机和接近理想接收机的性能。     

分组相关快衰落信道下自适应Turbo码译码算法研究

分析了分组相关快衰落信道的特性，推导出该信道下Turbo码译码算法；研究了迭代次数对Turbo编码系统的影响，在小信噪比弥散度条件下，提出基于平均信噪比的最佳迭代译码次数自适应选择方案，可以兼顾译码性能和译码速度，得到较低的平均误比特率和较高的平均译码速度。仿真结果说明，本文提出的Turbo码译码算法，降低了对信道估计精度要求的同时，得到精确信道估计时的性能；对于目标误比特率为10^-4时，采用自适应Turbo译码算法，与固定迭代4次相比，平均误比特率降低了40％，提高了系统性能；而与固定迭代8次相比，迭代次数降低了约1／4，提高了译码速度。     

时变信道中自适应VR-HARQ研究

为了更好地解决时变信道中可靠性与吞吐率这一对矛盾,本文提出一种基于线性预测的自适应冗余可变混合ARQ(VR-HARQ)方案.在该方案中,由于时变信道的慢衰落特性,时变信道被等效为有限状态的Markov过程,每个Markov状态对应信道的不同信噪比,同时,文章详细地描述了信道吞吐率与误码率之间的数学关系,推导了每个Markov状态的最佳编码方案,然后系统采用自适应线性预测算法,根据当前个时段的信道状态,估计下一个时段信道的信噪比以及其所对应的Markov状态,最后根据吞吐率最大原则选择合适的纠错编码方案.仿真结果表明:自适应线性预测VR-HARQ方案的性能明显优于传统VR-HARQ方案.     

结构半随机LDPC码的递增冗余HARQ方案

提出了结构半随机LDPC码,分析了其在不同码率时,编、译码具有兼容性的特性。介绍了基于结构半随机LDPC码的递增冗余HARQ方案,给出了这种方案的吞吐量性能封闭解。在AWGN信道和Rayleigh衰落信道下,通过仿真将新方案的性能和随机LDPC码的性能进行了比较,结果显示,结构半随机LDPC码的递增冗余HARQ方案性能接近随机LDPC码,但编、译码更简单,参数选择范围更广。     

Multi-user receiver scheme and uplink performance of space–time-coded CDMA system in Rician fading channels

The uplink performance of multi-user space–time-coded code-division multiple access (STC-CDMA) system in Rician fading channel is presented. A simple and effective multi-user receiver scheme is developed for STC-CDMA system. The scheme has linear decoding complexity when compared to the existing scheme with exponential decoding complexity, and thus implements low-complexity decoding. Based on the bit error rate (BER) analysis and moment generation function, theoretical BER expressions are derived for STC-CDMA with orthogonal and quasi-orthogonal spreading code, respectively. It is shown that these expressions have more accuracy. Using these expressions and the approximation of error function, closed-form approximate BER expressions are obtained, which can simplify the calculation of the derived theoretical BER. Simulation results show that the developed low-complexity decoding scheme can achieve almost the same performance as the existing scheme. The theoretical BER are in good agreement with the corresponding simulated values. Moreover, the presented approximate expressions are also close to the simulated values due to the better approximation. Under the same system throughput and concatenation of channel code, the presented full-rate STC-CDMA system has lower BER than the corresponding full-diversity STC-CDMA systems.     

基于低密度校验码的OFDM编码调制译码算法

低密度校验码(LDPC)具有编码增益高、译码速度快、性能接近Shannon限的优点。LDPC码应用于OFDM，能有效地提高多径环境下OFDM的BER性能。本文首先简单介绍LDPC码及其概率域上的译码算法，在此基础上对译码算法作融合，阐述概率似然比的译码算法。为了把LDPC应用于OFDM系统上，提出了多电平调制下的LDPC译码的算法。仿真结果表明，在AWGN和Rayleigh信道下，此算法正确有效。     

Reed-Solomon error control coding for Rayleigh fading channels withfeedback

The use of nonbinary block error control codes over Rayleigh fading channels with feedback is examined. It is assumed that the fading is slow with respect to the rate of symbol transmission. Expressions are derived for the probabilities of channel symbol error and erasure, which are in turn used to develop expressions for code symbol error and erasure. Two erasure generation mechanisms are considered, one based on the existence of channel amplitude side information, the other not. This analytical framework is used to evaluate the performance of the Reed-Solomon/hybrid-ARQ protocol (RS/HARQ) over fading channels with feedback. The RS/HARQ system uses erasure decoding in a hybrid-ARQ protocol to provide excellent reliability performance at the expense of a reduction in throughput. The RS/HARQ protocol allows for the variation of the erasure threshold and the effective diameter of the decoding operation     

GF(2p)LDPC编码的相位调制OFDM系统在瑞利衰落信道的性能

OFDM是一项能有效对抗高速无线通信中多径衰落的关键技术，为了进一步提高OFDM系统的误码性能，许多信道编码技术已被应用于OFDM系统中，二元域LDPC码以其近香农限的误码性能和较低的译码复杂度成为研究的热点。在AWGN信道下，多元域LDPC码比等效码长的二元域LDPC码有更好的纠错性能。本文提出了一种将多元域LDPC码经过MPSK调制后用于OFDM系统的新方法。仿真结果表明，在多径衰落信道下，通过合理选择多元LDPC码域的阶数和调制的方法，多元域LDPC编码的高阶调制OFDM系统比等效码长的二元域LDPC编码OFDM系统具有更好的性能，并且由于采用了多元域LDPC的快速BP译码，译码复杂度只是稍有增加。     

Performance analysis of amplify-and-forward relaying with hybrid-ARQ in Rayleigh fading channels

Hybrid automatic repeat request (HARQ) is a well-known technique for improving system throughput and link performance of wireless communication systems, including cooperative communication systems. The amplify-and-forward (AF) relaying method is one of the most attractive cooperative diversity schemes because of its low complexity. In this article, the end-to-end performance in terms of block error rate (BLER) and normalized throughput of AF relaying with HARQ transmission under the Rayleigh fading channel is analyzed. Numerical results validate the proposed analysis and demonstrate the gain of HARQ schemes in AF relaying systems. This analytical method can be extended to the systems with other HARQ protocols and other cooperative relaying schemes.     

An adaptive modulation scheme for simultaneous voice and datatransmission over fading channels

We propose a new adaptive modulation technique for simultaneous voice and data transmission over fading channels and study its performance. The proposed scheme takes advantage of the time-varying nature of fading to dynamically allocate the transmitted power between the inphase (I) and quadrature (Q) channels. It uses fixed-rate binary phase shift keying (BPSK) modulation on the Q channel for voice, and variable-rate M-ary amplitude modulation (M-AM) on the I channel for data. For favorable channel conditions, most of the power is allocated to high rate data transmission on the I channel. The remaining power is used to support the variable-power voice transmission on the Q channel. As the channel degrades, the modulation gradually reduces its data throughput and reallocates most of its available power to ensure a continuous and satisfactory voice transmission. The scheme is intended to provide a high average spectral efficiency for data communications while meeting the stringent delay requirements imposed by voice. We present closed-form expressions as well as numerical and simulation results for the outage probability, average allocated power, achievable spectral efficiency, and average bit error rate (BER) for both voice and data transmission over Nakagami-m fading channels. We also discuss the features and advantages of the proposed scheme. For example, in Rayleigh fading with an average signal-to-noise ratio (SNR) of 20 dB, our scheme is able to transmit about 2 bits/s/Hz of data at an average BER of 10 ^-5 while sending about 1 bit/s/Hz of voice at an average BER of 10^-2