空时编码联合波束形成技术研究现状分析

通过采取适当策略把空时编码和波束形成技术结合起来,可同时获得波束形成增益和分集增益,进一步改善系统性能、提高系统容量.文中分别从不同算法和天线配置结构上,分析介绍了两技术相结合的原理及特点,讨论了几个典型的空时分组码与波束形成的结合技术,分析了它们的内在机理及其优缺点,最后对该结合技术的发展趋势进行了展望.     

结合波束形成与发射分集的闭环下行发射方案

多天线技术是提高下行发射性能的一种有效方法。波束形成(BF)和发射分集(TD)均通过使用多个天线来有效地改善系统性能并提高信道容量。该文提出了一种将BF和TD结合用于闭环通信系统的自适应下行发射方案, 它包括适合于慢变信道的分集阵列方法和适合于快变信道的波束形成阵列方法。基站可以根据实际的信道状况自适应地调整发射方法。文中详细地介绍了自适应方案的实现算法,并对算法的复杂度和适应环境进行了分析。仿真结果证明新方案可以同时获得BF增益和TD增益,在使系统的误比特率(BER)性能大幅提高的同时,增强了系统对信道变化的适应能力。     

常见分集合并系统的性能分析

在移动通信中,分集技术是一种最有效的抗衰落技术。本文对3种常见的线性合并分集技术进行简要分析,给出它们的基带表示和合并器输出信噪比的概率密度函数(pdf),由此给出它们的合并增益。针对系统采用MPSK调制的情况,对瑞利衰落信道的3种合并分集系统的比特误码率(BER)性能进行理论研究,分别给出选择性合并(SC)和最大比率合并(MRC)系统的理论比特误码率表达式;对于等增益合并(EGC)分集,给出了一种近似的EGC系统的输出信噪比的pdf,由此导出EGC的一种近似的BER表达式,由蒙特卡罗仿真结果可以看出此近似的BER数值结果是准确的。数值结果显示:MRC性能最好,EGC性能稍差,而SC性能较差。文中给出的分析方法对于实际分集系统的理论研究具有普遍的指导意义。     

基于协方差反馈的多天线系统优化发送研究

为了克服多天线信道相关性的影响，提出一种新的自适应发送方案。应用空时分组码特征波束成型技术和格形编码调制（TCM）来获得分集增益和编码增益。针对采用和不采用交织器两种情况，基于成对差错概率（PEP）准则。分析了系统的统计性能，分别得到了使系统编码增益和分集增益最大化的TCM设计准则。根据注水法则和Lagrange乘子法求得波束间功率分配算法最优解。此外，码距作为优化功率加载算法中的权重因子，有效降低了获取波束成形分集的信噪比门限。分析和实验结果表明此方案复杂度低。能有效克服相关衰落。     

一种改进的FDA–MIMO雷达波束形成算法

频率分集阵列多输入多输出(FDA-MIMO)雷达由于其波束具有距离依赖特性而受到广泛关注。建立了FDA-MIMO雷达的回波模型,将传统的特征空间(ESB)波束形成算法扩展到该体制雷达。针对该算法在低信噪比(SNR)性能较差的问题,提出了一种改进的ESB波束形成算法。仿真结果表明该体制雷达能够抑制来自角度主瓣的距离依赖干扰,并证明了新算法的有效性。     

多波束机会波束成形系统性能比较

研究了在多输入单输出（MISO）系统中通过加大、加快信道的波动来提高多用户分集增益的机会波束成形技术,并在此基础上研究了在每个时隙中采用多个加权矢量的多波束机会波束成形技术,着重对它们的性能作了仿真比较。结果表明,多波束机会波束成形技术可以提高系统吞吐量。     

一种适用于SF-OFDM系统的波束形成技术

在正交频分复用（OFDM）系统中使用空频传输分集是未来宽带无线通信中的重要技术。由于在各个终端采用多天线进行收发，导致使用空频-OFDM（SF-OFDM）传输分集的系统受到增强的共信道干扰（CCI）。本文提出了一种适用于SF-OFDM系统的波束形成方案，该方案能有效的抑制共信道干扰。在该方案中，基于盲转向的波束形成被应用于SF-OFDM系统中。分析和仿真结果表明，该方案具有良好的共信道干扰抑制能力。此外，该方案能够利用不同波达角（DOA）信号间的相对时延来增强系统性能。     

卫星移动通信系统低信噪比抗干扰数字接收机设计

针对阵列天线卫星移动通信抗干扰能力差、传播损耗大等特性,设计了一种数字波束形成技术( DBF)和扩频技术相结合的数字接收机。其中,DBF算法采用基于递归最小均方算法( RLS)的解扩重扩盲自适应波束形成算法,使用VxWorks实现权值计算,FPGA实现波束形成;扩频方式采用直接序列扩频,在FPGA中实现。仿真分析与样机测试显示,通过两项技术联合使用,在信噪比低至-45 dB条件下仍可以实现可靠通信,同时有效加强了系统抗干扰能力。     

基于斜投影的波束形成算法

采样协方差矩阵求逆(SMI)的波束形成方法在少快拍数、高信噪比和相干信源情况下波束形成性能下降。对角线加载技术能改善方向图畸变,但是加载量的确定一直是一个比较困难的问题。该文提出基于斜投影的波束形成算法,它对接收信号斜投影,可有效消除干扰,进而提高波束形成的鲁棒性。仿真结果表明:该算法在高、中、低信噪比下都具有较好的波束形成性能;且在少快拍数和相干信源情况下仍具有较好的波束形成性能。该算法只需要期望信号的方向矢量和接收信号,是一种性能优越且鲁棒的波束形成算法。     

高速率移动通信的正交波束空间分集—OFDM方法

为进一步提高OFDM的传输速率，提出了正交波束空间分集—0FDM方法。利用智能天线的数字波束形成（DBF）技术，首先在时域构造出多个正交空分信道，然后对每个空分信道进行OFDM解调，最后根据选择性分集算法获得频域序列。理论分析和仿真结果都表明，至少有一个空分信道的频响特性优于全向信道的频响特性，并且各正交空分信道频响特性间的相关性很低，因而分集合成信道的频响特性明显优于全向信道、每个窄分信道的频响特性。与单天线OFDM系统相比，正交波束空间分集—0FDM方法能够实现更高的传输速率。     

Transmitter diversity for OFDM systems and its impact on high-ratedata wireless networks

Transmitter diversity and down-link beamforming can be used in high-rate data wireless networks with orthogonal frequency division multiplexing (OFDM) for capacity improvement. We compare the performance of delay, permutation and space-time coding transmitter diversity for high-rate packet data wireless networks using OFDM modulation. For these systems, relatively high block error rates, such as 10%, are acceptable assuming the use of effective automatic retransmission request (ARQ). As an alternative, we also consider using the same number of transmitter antennas for down-link beamforming as we consider for transmitter diversity. The investigation indicates that delay transmitter diversity with quaternary phase-shift keying (QPSK) modulation and adaptive antenna arrays provides a good quality of service (QoS) with low retransmission probability, while space-time coding transmitter diversity provides high peak data rates. Down-link beamforming together with adaptive antenna arrays, however, provides a higher capacity than transmitter diversity for typical mobile environments     

Adaptive antennas for space-time codes in outdoor channels

Space-time codes have been introduced to improve mobile system performance in a multipath fading environment. We consider a multiple-input multiple-output (MIMO) system with m mobile antennas and n base station antennas, in which there are L multipaths at the base station at distinct angles of arrival. We show that when the channel has no intersymbol interference (ISI), then adaptive antennas in the form of beamforming, can be combined with space-time coding, to achieve a diversity gain of mL and a large signal-to-noise ratio (SNR) gain whenever n/spl ges/L. When the channel has ISI, beamforming can be used by the MIMO systems to achieve an SNR gain over a single-input multiple-output system, although both systems have the same diversity gain.     

空时块码与波束形成结合方案及性能分析

Alamouti空时块码与波束形成相结合的方案只使用了两个功率均分的波束,在很多情况下并非最优。该文提出了新的空时块码与波束形成相结合的方案,新方案能够根据信道互相关矩阵的特征值,展开不超过发射端天线数量的波束,在满足空时块码发射天线数量等于波束数量的条件下,与任意码率空时块码结合。该文还利用Gauss-Chebyshev积分对线性调制下新方案的误符号率进行了性能分析,得出了简单的数值计算表达式。新方案由于同时获得了分集增益和波束形成增益,显著提高了系统的性能。     

Adaptive space-time transmission with side information

In this letter, a simple adaptive transmit diversity technique with side information is presented. The proposed scheme has a number of attractive characteristics. First, it is simple to implement. Second, it is efficient in terms of bandwidth requirements for side information feedback. Finally, it adapts to the quality of the side information to accordingly configure the space-time signal set to be transmitted so that additional gain in signal-to-noise ratio can be achieved at the receiver. In effect, it can be viewed as a flexible combination of suboptimal beamforming and space-time block coding.     

多发射天线的差分酉空时调制信号星座图

在无法获得信道状态信息的瑞利衰落信道里,分集积是高信噪比场景下酉空时信号具有良好性能的首要指标。采用全旋转矩阵的方法获得了极大分集积非群空时码,仿真结果表明,在高信噪比下,该非群空时码性能优于传统的酉空时循环码。     

Opportunistic Beamforming over Rayleigh Channels with Partial Side Information

In recent years, diversity techniques have evolved into highly attractive technology for wireless communications in different forms. For instance, the channel fluctuations of the users in a network are exploited as multiuser diversity by scheduling the user with the best signal-to-noise ratio (SNR). When fading is slow, beamforming at a multiple antenna transmitter is used to induce artificial channel fluctuations to ensure multiuser diversity in the network. Such a beamforming scheme is called opportunistic beamforming since the transmitter uses random beamforming to artificially induce opportunism in the network [1]. Opportunism requires a large number of users in the system in order to reach the performance of the true beamforming that uses perfect channel state information (CSI). In this paper we investigate the benefit of having partial CSI at an opportunistic transmitter. In the investigation, we focus on the maximum normalized SNR scheduling where user?s feedback consists of SNR relative to its channel gain. We show that opportunism can be beneficially used to increase the average throughput of the system. Simulations support the analytical average throughput results obtained as the amount of CSI and the number of users vary.     

The impact of multiuser diversity on space-time block coding

In this letter, analytic performance results are derived for space-time block coding paired with multiuser diversity. We consider a scenario in which K active data users, each of which is potentially equipped with multiple antenna elements, are served by a multi-antenna element base station (BS). We focus on the downlink channel, where a space-time block coding scheme is employed and assume that channel quality information is reported to the BS by all users on a per frame basis. Using a scoring function at the BS, time resources are allocated to the user with the best instantaneous effective signal-to-noise ratio (SNR), facilitating a multiuser diversity mechanism. Using order statistics, we compute histograms and cumulative distribution functions of the effective SNR at the space-time combiner output and assess the interaction between multiuser diversity obtained via scheduling and spatial diversity obtained via the space-time code.     

Error Analysis of Grouped Multilevel Space-Time Trellis Coding with the Combined Application of Massive MIMO and Cognitive Radio

In today’s scenario, demand for error-correcting codes with minimal error constraints for wireless communications. Multilevel coding scheme with trellis codes as component codes provides flexible data transmission rates, coding gain, diversity gain with improved spectral efficiency and low decoding complexity. This paper investigates the potential improvements by using the Multilevel coding scheme with massive Multiple-Input Multiple-Output in Cognitive Radio Networks with trellis codes as component codes. This paper discussed space-time coding with beamforming and antenna grouping according to the channel state information. Multilevel Space-time coding is based on multi-level Quadrature Amplitude Modulation signaling and beamforming to mitigate the effect of primary users for the enactment of secondary users in Cognitive Radio. The primary users provide channels dynamically to the secondary user for an unknown duration. Our transmission use Quadrature Amplitude Modulation based signals, with an adaptive grouping of antenna which weight according to the optimization, which inherently depends upon the resource allocation of the secondary user. The results show that the proposed coded system achieves Bit error rate/Symbol error rate/Frame error rate and Signal to noise ratio varies according to sources sensing time.

     

Evolution, insights and challenges of the PHY layer for the emerging ieee 802.11n amendment

IEEE 802.11n is a newly emerged WLAN standard capable of providing dramatically increased throughput, as well as improved range, reduced signal fading, over the existing IEEE 802.11a/g WLAN standards. These benefits are achieved through use of MIMO (Multiple-Input,Multiple-Output). The latest draft for IEEE 802.11n describes rates up to 600Mbps, exceeding the maximum rate with the 11a/g standards by more than ten times. In addition, techniques such as space-time block coding and beamforming provide the potential of increasing signal strength at the receiver with optimal efficiency, based on the diversity order used. In this paper, a comparative analysis of the physical (PHY) layers in the original main proposals for the 11n amendment (the TGn Sync, WWiSE and TGn Joint proposals) is presented. The key architectural differences governing the performance of these proposals are outlined. In addition, insights are provided into the choices leading to the TGn Joint proposal, which reflects the PHY architecture described in the 11n standard. The insights and challenges described relate to the choices made in the TGn Joint proposal regarding the areas of channel estimation (considering the use of beamforming, channel smoothing), bit interleaving techniques (for maximizing coding gain under channels with high frequency diversity), space-time block coding (STBC) options (designed in an effort to achieve a good balance between achieving high diversity gain and low receiver design complexity), and pilot tone selection (for a reasonable tradeoff of robustness and link-level performance). Performance curves (based on simulation models developed in MATLAB/ SIMULINK) are used to verify the analysis presented. This paper also includes a discussion of some of the future challenges for the 11n amendment.