【作者】 王勇；

【导师】 廖桂生；

【作者基本信息】 西安电子科技大学 ， 信号与信息处理， 2009， 博士

【摘要】 MIMO(Multiple-Input Multiple-Output)技术充分开发空间资源,利用多根天线实现多发多收,在不需要增加频谱资源和天线发送功率的情况下,可以成倍地提高信道容量,满足未来无线通信中高速数据传输的需求。以智能天线和空时编码为代表的多天线技术是目前公认的实现上述目标的最佳选择。波束形成技术源于智能天线,增强期望用户信号功率、抑制干扰用户信号是其典型的技术优势,也是SDMA(Space Division Multiple Access)能够得以实现的基础。而STBC(SpaceTime Block Code,空时分组码)则是实现MIMO分集的重要手段,能有效对抗信道的多径衰落。从发展趋势看,两者的有效联合将可以充分发挥多天线的优势,进而实现系统性能的大幅提升。也应看到,天线选择技术是最近发展起来的一种非常有效的低成本、低复杂度的多天线分集技术,具有重要的理论和实际价值,在无线通信领域正发挥着越来越重要的作用。本论文主要针对MIMO系统的发射和接收天线阵元,研究阵元优化调度方法,结合阵列信号处理,提出在不增加硬件投入情况下,经过合理天线阵元调度而提高系统性能的优化算法,并且通过理论分析与仿真实验进行验证。主要包括以下几个方面内容:1.提出一种适用于任意发射天线数的QOSTBC子阵分组发射分集方案,该方案克服传统方法中性能随波达角度(DOA)和角度扩展(AS)变化而剧烈波动的不足。其基本思想是:将发射天线平均分组,空时编码信号经各子阵的波束形成加权后定向发射;接收机利用空时编码矩阵特点进行选择接收,将接收信号分成两组,利用分组内信号的正交性进行组间干扰抑制,由于每组都是Alamouti编码,因此每组都可以获得满速率和满分集增益,将两组合并后仍可以获得满分集增益。从理论分析和计算机仿真的结果都可以看出:该方法在保持QOSTBC满速率优点的前提下,可以同时获得满空间分集增益;而且编码和译码的过程都是基于线性处理的,计算简单。2.传统MIMO系统传输信号需要有与发射(或接收)天线数量相同的射频链路,这使得MIMO系统的成本和复杂度非常高的问题。提出了发射天线和接收天线选择算法及优化设计方法。利用凸优化技术解决基于系统容量最大和译码性能最优两个准则条件下的最优解问题,并对凸优化方法进行了改进,提出了近似逼近的方法,在保持性能的同时显著降低计算复杂度。仿真结果给出了特定条件下最优天线数目;最后,针对传统天线选择算法性能损失较大,将射频链路不再分配给一个天线元素,而是分配给一个天线元素子集的线性组合,通过在接收端的天线子阵变换,使系统性能明显提升。3.研究使用相同频率、时隙和码道的同一小区多用户通信时对码间干扰和信道干扰抑制的两种算法。智能天线阵列上,通过复合加权,同时实现对系统内其他用户的干扰抑制和对期望用户的满发射分集。本文从理论上证明了复合加权等效为两个子权向量的卷积形式,而这两个权向量可以分别用来实现用户间的干扰抑制和构建STBC所需要的不相关的传输信道。第二种方法是将发射端和接收端的天线阵元进行分组,在发送端依据容量最大准则,在接收端依据信干噪比最大准则分别求出发送波束形成权和接收波束形成权,利用Alamouti编码的正交性使得多用户等效传输矩阵也保持正交性,在不需要预先知道共信道干扰的信道状态信息情况下,抑制码间干扰,同时在接收端获得更大信干噪比。理论分析和计算机仿真结果证明了这两种方法的有效性。更多还原

【Abstract】 MIMO (Multiple-Input Multiple-Output) technique utilizing multiple antennas to realize multiple transmissions and multiple receiving can exploit space resource adequately and can improve channel capacity without any loss in bandwidth and transmitting power. So it can meet the need of high data transmission in future wireless communications. Multi-antenna systems, such as smart antenna and space-time code, are well-known as the best choice to meet above targets. Beamforming comes from smart antenna with strengthening desired signal and suppressing inter-users interference as its typical technical superiority. It is also the base to realize SDMA (Space Division Multiple Access). STBC (Space Time Block Code) is the main method to realize MIMO diversity, and it is good at mitigating the influence of wireless multipath fading. In view of development trend, the combination of beamforming and STBC will adequately exert the advantage of multi-antenna systems and result in enhancing the system performance significantly. Antenna selection is low-cost and low-complexity alternative to capture many advantages of MIMO systems. It becomes very import for wireless communications because of its significance in theory analysis and applications.Aiming at MIMO system transmit/receive antenna elements, several scheduling elements optimal algorithms are designed and studied, and logical select antennas algorithms is proposed for low-cost and low-complexity based on array signal processing theory in this dissertation. Then these methods are verified by theoretical analysis and computer simulations. The primary contributions included in this dissertation can be summarized as follow:1. A novel selective receiver switching scheme combined with beamforming is proposed for making arbitrary complex quasi orthogonal STBC matrix achieve full transmission as well as full diversity. An efficient selective receiver switching scheme is proposed for STBC with full code and non-orthogonal design. In the proposed scheme with the aid of beamforming, the scheme divides the received signals into two groups according to the encoded matrix. By this way, it can eliminate the interference from the neighboring signals almost by half. The simulation results with the example of matrix demonstrate that the proposed scheme can provide the improved performance and more transmit diversity. On the other hand, because of the reduced interference, not only the processing complexity at the decoder can be reduced but also it have a stable performance independent of DOA and angular spread. Finally, based on Maximal ratio combining (MRC) techniques, dissertation use the probability density function (PDF) of the square of the maximum singular value of the channel matrix to obtain exact closed-form expressions for BER of the coherent reception of M-PAM and M-QAM. It has the conclusion from analysis result that how to scheduling antenna makes the performance better.2. The MIMO system has radio frequency (RF) the same as antennas. Electronic elements of RF include power amplifier, analog to digital conversion, digital to analog conversion and transducer are very expensive. Transmit antenna selection and equal-gain transmission systems are investigated. The algorithm of calculating the minimum number of transmit antennas based on average SNR is obtained. Since the system is not entirely proportional to the number of RF chains, the algorithm of determining optimal number of RF chains is brought forward. A low complexity approach to receive antenna selection for capacity maximization and error rate minimization, based on the theory of convex optimization. Then, we describe a new scheme for approximately solving this problem. It gives a subset selection as well as a bound on the best performance that can be achieved by any selection of antennas. Finally, antenna subarray weight groups are allocated to a linear combination of the responses of a subset of the available all antenna elements, which maximizes the effective channel capacity.3. Two multiuser schemes which have the same frequency, time slot and channel are proposed for MIMO systems. The impulse response of the channel between the transmit array and receive array must be independent in order to make space time code get full order diversity gain. The transmitter performs eigenvalue-decomposition on the spatial covariance matrix and utilizes the obtained eigenvectors as weighting vectors to form orthogonal eigenbeams to carry space time code. A composite weighting based joint multiuser STBC and beamforming method is proposed. The key feature of the method is that interference canceling for other users and full order transmit diversity for desired user is realized simultaneity. The composite weight vector can be equivalent to a convolution operation of two sub-weight vector, one to realize inter-users interference suppressing and the order to construct uncorrelated channel for STBC, respectively. Then, to combine the cochannel interferences (CCI) suppression ability of beamforming techniques with the Alamouti transmission and achieve the receiver computational simplicity, the MS (Mobile Subscriber) beamforming process maximizes the channel capacity; the BS (Base Station) beamforming process maximizes the output SINR to suppress CCI. Since, the technique preserves algebraic structure of the Alamouti STBC and sustain the orthogonally of the virtual MIMO channel in the presence of beamforming, the Maximum-likelihood (ML) decoding process is achieved simple linear processing.更多还原