提高MIMO系统容量的检测算法研究

为了满足通信容量的需求，在发射端和接收端设置多元素天线阵列构成多输入多输出系统，可以显著提高频谱的利用率，基于贝尔实验室提出的非线性迫零检测算法，提出了反向迫零检测算法．理论和仿真证明，在相同信噪比条件下，反向迫零检测算法获得的系统容量高于非线性迫零检测以及线性迫零检测算法．与奇异值分解算法相比，该算法虽然得到的系统容量有所减小，但是由于所需的运算量相对较小，因而易于实现．     

一种频分MIMO声呐的波达方向估计算法

为了使声呐阵列在有限的载体空间内获得较高的角度分辨率,设计了基于频率分集的多输入多输出（MultipleInput Multiple-Output,MIMO）声呐。该MIMO声呐采用N发M收的布阵方式,接收阵为M元均匀线阵,发射阵由N个阵元组成,且各发射阵元发射中心频率不同、包络相同的窄带信号。建立密集式频分MIMO声呐的回波模型,并以此模型为基础提出了波达方向估计算法方向-相位域多重信号分类（Direction and Phase Domain-Multiple Signal Classification,DPD-MUSIC）算法。仿真实验中以双频MIMO声呐为例,将频分MIMO声呐的DPD-MUSIC算法的估计性能与单输入多输出（Single-Input Multiple-Output,SIMO）声呐MUSIC算法的估计性能进行了对比。仿真结果表明,频分MIMO声呐利用DPD-MUSIC算法可以获得优于等接收阵元数SIMO声呐的角度分辨率和角度估计精度。     

RZF预编码与MMSE-SIC检测联合的大规模MIMO系统预编码设计算法

研究了大规模多输入多输出(MIMO)系统的预编码设计算法。针对大规模MIMO系统通常利用增加用户天线数来提高系统频谱效率的方法会导致用户间干扰增大,从而对系统性能产生负面影响的问题,提出了一种将正则化迫零(RZF)预编码与最小均方误差-串行干扰消除(MMSE-SIC)检测相结合的改进算法。该算法通过在基站端采用RZF预编码对信号进行预处理以平衡用户间干扰和噪声干扰的影响,继而在接收端运用检测性能优异的MMSE-SIC算法来进一步减轻信号中的干扰,从而达到提升系统容量的目的。实验结果表明,这种将RZF预编码与MMSE-SIC检测相结合的改进算法,在用户间干扰较大时具有较好的适用性,且在完全已知和未完全已知信道状态信息情况下的频谱效率均优于传统RZF算法。     

差分协作系统中的软输入软输出多符号差分球形译码

在差分协作系统中,为了解决硬判决多符号差分检测(multi-symbol differential detection,MSDD)译码过程中,部分信息丢失导致性能损失以及该算法中最大似然检测计算复杂度高的问题,提出了一种软输入软输出的多符号差分球形译码(SISO-MSDSD)算法.该算法利用球形译码思想,在一定搜索范围内计算编码比特的对数似然比(LLR),去除先验信息可获得编码比特的外信息,经内部译码器判决得到源信息.仿真分析表明,比较硬判决MAP算法,SISO-MSDSD算法性能明显提高,且在信道频偏较大时,随着分组长度增加能有效提高检测性能.此外,在低SNR区域,SISO-MSDSD算法对复杂度的改善也十分明显.因此,SISO-MS-DSD可作为一种较好的软检测算法应用于差分协作系统中.     

密集式MIMO声呐高分辨成像实验研究

为验证密集式多输入多输出(Multi-Input Multi-Output,MIMO)声呐高分辨成像算法的有效性,进行了水池实验并获得了期望的高分辨成像结果。首先,将成像算法分为角度维高分辨成像和距离维高分辨成像两种,分别建立了各自的成像处理流程。角度维高分辨成像基于虚拟阵列波束形成算法,距离维高分辨成像基于大带宽信号合成算法。然后,根据两种处理流程选择合适的发射信号和阵型,并据此搭建密集式MIMO声呐高分辨成像平台进行水池实验。最后,通过与传统单输入多输出(Single-Input Multi-Output,SIMO)声呐的成像结果进行对比,证明了MIMO声呐可结合发射信号、阵型和相应的处理流程获得更高的角度分辨率和距离分辨率。     

基于空时块编码和正交脉冲的MIMO系统性能研究

为了有效利用脉冲无线电(IR)的短距离、高数据速率以及多输入多输出(MIMO)通信系统的更大容量,提出了一种基于空时块编码(STBC)和正交脉冲的MIMO超宽带(UWB)通信系统。具体实现是通过对标准单输入单输出(SISO)脉冲无线电系统的直接序列超宽带作为多址(MA)技术的IR-MA信号模型以及信道模型、收发器结构和检测方法的分析,提出了一种把空时块编码和正交脉冲相结合的新方法,并采用FS-rake相干接收(CR)和非相干接收(NCR)技术及IEEE UWB信道模型对其获得的空间分集和编码性能进行了研究,从而得到可以相对较好地描述高信噪比时的性能变化趋势的理论上限值。仿真结果表明,这种STBC-IR方案相比于传统的单链路-脉冲无线电(SL-IR)方案,可获得更好的空间分集和编码增益,可用于增加传输距离和减少rake接收机的复杂性,而且在UWB通信中,STBC-IR方案还能很好地消除定时抖动的影响。     

分布式MIMO声纳中运动目标参数快速估计

分布式MIMO声纳由于自身的优点而成为当前研究的热点,但常规的目标参数估计方法均存在着一定的缺陷,无法满足分布式MIMO声纳的要求.针对这种情况,本文提出一种适用于分布式多输入多输出(MIMO)声纳中使用单接收阵元的运动目标参数快速估计方法,给出了单接收阵元情况下的信号模型,提出了快速估计方法并给出了流程图,并通过数值...     

比特排序的低复杂度K-best检测算法

为了降低多输入多输出(multiple-input multiple-output,MIMO)系统中K-best检测算法的计算复杂度,提出一种基于硬件操作中比特计数思想的比特排序(bit-sort,BS)K-best算法和一种进一步简化BS K-best算法的动态比特排序(dynamic bit-sort,DBS)K-best算法.这两种算法都是通过依次查找和计数每条路径的累积权重值的对应比特位,从而快速地找出所需的K条最短路径.此外,DBS K-best算法根据路径累积权重的大小只找出不大于K条的最短路径,因此,它比需要找出固定K条最短路径的BS K-best算法更加简便.仿真结果表明,BS K-best算法和DBS K-best算法均可以在逼近次优的球形译码(SD)算法的误码率性能的同时又大大降低了K-best检测算法的计算复杂度,且DBS K-best算法又进一步简化了BS K-best算法的计算.     

MIMO信道树形矢量量化编码反馈方法

为了进一步降低有限反馈系统的反馈量,提出了一种多输入多输出(MIMO)信道树形矢量量化编码反馈方法.对于具有时间相关性的信道,首先统计出在已知当前码字的条件下下一个码字出现的条件概率,然后根据条件概率进行条件熵编码,编码结果反馈到输入端进行波束成形.该方法利用树形码本提高最优码字的搜索效率,减少计算量;利用信道的时间相关性提高编码效率,降低反馈量.仿真结果表明,在具有时间相关性的信道中,采用所提方法能以较少的计算量和较低的反馈量获取与传统方案相近的系统性能.     

基于波束训练的大规模MIMO多用户度调度算法

在毫米波多用户大规模天线阵列多输入多输出(MIMO)系统中,由于使用混合波束形成结构,使得用户调度问题面临新的挑战,调度问题转变为在同一时频资源选择调度用户子集和用户与基站之间通信波束对的问题,需要从用户和波束两个维度来进行考虑。大规模MIMO由于在基站端和用户端使用大量的天线阵列,如果使用穷举搜索算法运算量极大,在实际应用中是不现实的。本文提出了一种基于波束训练的多用户调度方案,该方案分为2步。第1步,由于波束间的干扰大多数来自于相邻波束,因此对相邻波束进行分组,同时根据用户反馈的信干燥比(SINR)最大值的基站波束索引号把相关的用户映射到对应波束组。第2步,为了减小干扰,每个波束组的最小波束索引根据一定的原则来选取调度用户,然后通过基站和用户之间的波束训练选取最接近于SINR门限值的用户波束来进行通信。仿真结果表明,该方案具有逼近最优穷举方案的性能并且具有较低的计算复杂度。     

差分酉空时协作系统的多符号裁减自动球形译码

鉴于差分协作系统中多符号差分检测(multiple-symbol differential detection,MSDD)算法计算复杂度高的缺点,引入裁减自动球形译码(pruning automation sphere detection,PASD)算法来降低差分酉空时协作系统的检测复杂度.PASD算法是在球形译码的基础上,结合自动球形译码和裁减球形译码的思想而提出的.通过对扩展节点所消耗的复杂度和系统的误码率两方面来分析译码算法的性能.仿真分析表明,在差分酉空时协作系统中,PASD算法在误码性能几乎不变的情况下,复杂度曲线比球形译码(sphere detection,SD)算法收敛迅速.其中在分组长度N=5、信噪比SNR=15dB的情况下,PASD、SD算法的扩展点数分别为14.138 6、78.950 5,大大降低了系统的复杂度;同时,当SNR>16dB时,协作节点的增加有利于提高系统的误码性能,系统的复杂度性能并没有很大的损失.     

Modified sum-product algorithms for decoding low-density parity-check codes

The authors deal with the sum-product algorithm (SPA) based on the hyperbolic tangent (tanh) rule when it is applied for decoding low-density parity-check (LDPC) codes. Motivated by the finding that, because of the large number of multiplications required by the algorithm, an overflow in the decoder may occur, two novel modifications of the tanh function (and its inverse) are proposed. By means of computer simulations, both methods are evaluated using random-based LDPC codes with binary phase shift keying (BPSK) signals transmitted over the additive white Gaussian noise (AWGN) channel. It is shown that the proposed modifications improve the bit error rate (BER) performance up to 1 dB with respect to the conventional SPA. These results have also shown that the error floor is removed at BER lower than 10^-6. Furthermore, two novel approximations are presented to reduce the computational complexity of the tanh function (and its inverse), based on either a piecewise linear function or a quantisation table. It is shown that the proposed approximations can slightly improve the BER performance (up to 0.13 dB) in the former case, whereas small BER performance degradation is observed (<0.25 dB) in the latter case. In both cases, however, the decoding complexity is reduced significantly     

克隆选择算法在多用户信号检测中的应用

为了减少计算复杂度,改善CDMA(Code Division Multiple Access)系统性能．将具有解决复杂组合优化问题能力的克隆选择算法(CLONALG)应用于码分多址系统多用户信号检测中．确定每一个用户传输的符号位．仿真结果表明,基于克隆选择算法的多用户检测器不仅具有克服多址干扰,抗“远-近效应”的能力,而且通过调整算法参数可以在检测时间和系统平均误码性能间进行均衡,与最佳多用户检测器相比明显地降低了计算复杂度．     

Pilot overhead reduction in turbo coded OFDM systems employing an iterative channel estimation under low signal-to-noise ratio environments

The authors evaluate the improved energy and spectral efficiency by pilot overhead reduction of turbo coded orthogonal frequency division multiplexing (OFDM) systems employing an iterative phase estimation algorithm. Developed from the recently proposed iterative phase estimation schemes, the phase estimation and compensation process is embedded into the basic iterative turbo decoding process for the application to OFDM systems with just a slight complexity overhead. At each decoding iteration, sub-carrier phase rotations are estimated from the extrinsic information arranged in each sub-carrier and are compensated for the next decoding iteration. This enables the iterative phase estimation algorithm to successfully work under very low signal-to-noise ratios even without pilot symbols. The pilot symbols are just very rarely inserted only for breaking the erroneous phase estimation propagation frame to frame in case of large residual phase offset beyond reliable decoding range. Simulation results show that the iterative phase estimation algorithm drastically reduces the pilot insertion overhead and thus, it achieves improved spectral efficiency as well as bit error rate (BER) performance by saving pilot energy compared to the conventional method.     

SISO algorithms based on Max-Log-MAP and Log-MAP turbo decoding

Novel soft-input soft-output (SISO) decoding algorithms suitable for turbo codes are proposed with good compromise between complexity and bit error rate (BER) performance. The algorithms are based on the application of the max/max* (i.e. Jacobian logarithm) operations at different levels when computing the decoder soft-output value. It is observed that some decoding schemes from the authors' previously published work fall into the family of methods described here. The effect is to provide a range of possibilities allowing system designers to make their own choices for turbo code BER performance against complexity     

A hybrid multiuser detector for DS-UWB systems with matched filter and error-bit recognizer

In this article, we propose a hybrid algorithm of multiuser detection (MUD) for direct sequence ultra-wideband (UWB) communication systems, based on matched filters (MF) and error-bit recognizers (ER). The ER embraces three steps: bit mapping, K-means clustering, and sign judgment. In this detector, we construct a mapping function and map the output base-band bits from MFs into a one-dimensional feature space to get the amplitude information. After that, we set an initial value and classify the bits into right bits and wrong ones by K-means clustering. Finally, we make the sign judgment to prevent some right bits being picked out as wrong bits and correct the wrong bits to reduce the bit error rate (BER) of the system. Compared to a conventional detector based on MFs and minimum mean square error detector, to a large extent, our detector can improve the BER performance of a system because of high recognition probability of the wrong bits in K-means clustering. What is more, the simulation results show that the proposed hybrid MUD is not sensitive to the number of users. It can achieve a good BER performance and low computational complexity when there are 20 UWB transmitters in the channel.     

MIMO-OFDM系统中一种新的信道估计方法

提出了多输入多输出-正交频分复用(MIMO-OFDM)系统中一种新的信道估计方法.根据最小均方误差的准则,推导出了求解信道频率响应的方程,方程求解的运算量主要集中在一个矩阵的求逆上.对每一个符号,此矩阵是固定的,所以矩阵的求逆仅需计算一次,整个求解过程的运算量较低.计算机仿真给出了这种方法的归一化均方误差和误码率性能.仿真结果表明,本文方法在多普勒频移为60Hz和360Hz下,其归一化的均方误差值相当,表明本文方法可以在不同的移动环境中良好工作.在不同的数据调制方式和不同的多普勒频移下,本文方法的误码率性能优于最小平方算法,略差于线性最小均方误差算法,但复杂度大大降低.     

Joint channel tracking and intercarrier-interference equalisation for vertical Bell Labs layered space time-orthogonal frequency division multiplexing in vehicle ad hoc network

Vertical Bell Labs layered space time-orthogonal frequency division multiplexing (VBLAST-OFDM) systems can achieve high spectral efficiency in quasi-stationary links and with channel state information (CSI) matrix knowledge. Owing to the high speeds of nodes in vehicle ad hoc network (VANET), the channel is fast fading thus raising the need for channel tracking. Furthermore, inter-carrier interference (ICI) causes an error floor at high signal-to-noise ratio (SNR) even with perfect CSI knowledge. In this paper we investigate channel tracking and ICI mitigation for VBLAST-OFDM. The analysis of ICI shows that it increases with speed, number of subcarriers and/or number of transmit antennas. The authors then introduce a simple channel tracking algorithm for VBLAST-OFDM. Simulation results show that our algorithm reduces the bit error rate (BER) of a 2 x 4 VBLAST system by 102² at 40 dB SNR and 100 km/h speed compared to obtaining a channel estimate from a training sequence only. The change in the channel response is estimated using the channel tracking algorithm and then passed to an ICI equaliser to enhance performance and reduce the error floor caused by ICI at high SNR. Equalising five pairs of subcarriers gives 4 dB improvement for 2 x 4 VBLAST at 180 km/h relative speed. The performance is enhanced as more subcarriers are included in the ICI equaliser at the expense of increased receiver complexity.     

一种降低BICM-ID系统误比特率的高阶APSK星座映射方法

本文提出一种基于蝙蝠算法的星座映射方法,旨在解决采用高阶振幅移相键控APSK调制的BICM-ID系统中误比特率高的问题.所提出的星座映射方法包括以下步骤:建立系统模型并初始化参数;随机产生多组解;计算每一组解的误比特率,取误比特率最小值对应的一组解为最优解;开始迭代,产生新解并在最优解周围产生局部解;更新当前全局最优解,当达到最大迭代次数时,输出最优解.所提方法不仅星座映射的效率高,而且误比特率性能优于现有映射方法,实验结果表明,对32APSK而言,当系统的误比特率为10-5时,采用本文所提方法优化后需要的信噪比为8.73 dB,相比于DVB-S2标准中方法和预编码方法所获得的信噪比增益分别是0.09 dB和1.88 dB.     

Channel estimation for time-hopping pulse position modulation ultra-wideband communication systems

Ultra-wideband (UWB) communication systems are used in indoor environments with dense multi-path characteristics. Therefore channel estimation has an important role in the receiver of these systems. A new approach for data-aided (DA) and non-data-aided (NDA) channel estimation is proposed, which is called the pulse compression (PC) method. This method is useful for UWB systems employing time-hopping pulse position modulation. The PC method requires only some basic operations such as sampling, overlap-add and finite impulse response filtering. The PC method, in both DA and NDA scenarios, in spite of its low complexity, outperforms the maximum-likelihood (ML) method in channel parameters estimation. The bit error rate (BER) of the DA method, in single-user scenario, performs as well as the ML method, and in multi-user scenario, in the worst case, there is only 0.5 dB loss compared with the ML method. In the case of NDA scenario, the proposed method outperforms the NDA-ML method, that is, in the single-user scenario about 4 dB gain at the BER of 1023 is observed. In multi-user scenario, it outperforms significantly the NDA-ML method, and its performance loss in comparison with the perfect channel knowledge scenario is about 3 dB at the BER of 1023.