Transmit Antenna Selection Algorithms In MIMO Systems

Antenna Selection is low-cost and low-complexity alternative to capture many advantages of MIMO systems. It becomes very important for wireless communications because of its significance in theory analysis and applications. Based on basic theories in antenna selection, we focus our work on transmit antenna selection algorithms and its optimization in both spatial multiplexing systems and closed-loop transmit diversity systems. The main achievements of the dissertation are listed as follows:(1) Transmit antenna selection for spatial multiplexing systems in lower complexity receivers such as the linear receiver is investigated. The minimum post-detection SNR of multiplexed substreams is a dominant parameter in the system. Although the linear receiver is the simplest spatial multiplexing receiver, the minimum post-detection SNR of substreams is probably too little. To heighten the minimum post-detection SNR, the thesis adopts BLAST receiver which has significant improvement over the linear receiver by an operation called interfere cancellation. Transmit antenna selection algorithm in a BLAST receiver is proposed, which opts for the antenna subset that has largest minimum post-detection SNR of substreams. Simulation shows that the proposed algorithm is superior to the algorithms in the linear receiver in both error rate and channel capacity.(2) Transmit antenna selection for closed-loop spatial multiplexing systems is studied. In the systems, the equal power transmission scheme in channel capacity is optimal if the transmitter has no information about the channel or total transmit power is high enough. The extended MRT scheme combined with waterfilling power allocation can obtain the maximum capacity in low total transmit power, however, it requires a large amount of feedback. A transmit antenna selection algorithm based on the partial MRT scheme and its power allocation algorithm are presented to reduce the amount of feedback. In the systems with two RF chains, we demonstrated that the proposed algorithm can achieve the maximal channel capacity with half amount of information required in the extended MRT scheme. Simulation shows that the algorithm can reduce the amount of feedback in low total transmit power, and enhance the system capacity in high total transmit power, comparing to the extended MRT scheme. (3) Hybrid transmit antenna selection and maximal-ratio transmission (TAS-MRT) systems are investigated, In order to reduce the system cost and computation complexity, the algorithm of determining the minimum number of transmit antennas is put forward. It takes average SNR as the performance metrics. Besides, the algorithm of determining the minimum number of transmit antennas based on average BER is presented. These algorithms will add a minimum number of transmit antennas to compensate the performance loss induced by the reduction in the number of RF chains. In order to reduce the amount of feedback, the quantization of transmit beamforming vector is studied, and two algorithms are given: Lloyd vector quantization and Grassmannian subspace packing (GSP). Simulation demonstrates that the performance degradation of TAS-MRT systems induced by the quantization of transmit beamforming vector is less than that of MRT systems. TAS-MRT systems with Lloyd algorithm in average BER are superior to those with GSP algorithm.(4) Hybrid transmit antenna selection and equal-gain transmission (TAS-EGT) systems are investigated. The algorithm of calculating the minimum number of transmit antennas based on average SNR is obtained. The algorithm will add a minimum number of transmit antennas to pay the performance drop induced by the reduced number of RF chains. Since the system performance is not entirely proportional to the number of RF chains, the algorithm of determining optimal number of RF chains based on average SNR is brought forward. The algorithm can search the number of RF chains which gain maximal performance. Besides, the algorithm of determining the minimum number of transmit antennas based on average BER is presented for the systems with two RF chains. The algorithm of quantizing transmit beamforming vector is explored. The quantization algorithm based on sector phase constraining is presented. The algorithm has low complexity because of no requirement for exhaustive search of all possible combinations. Simulation shows that the algorithm with certain number of quantization bits can approach the minimal BER achieved by no phase quantization (namely, ideal equal-gain transmission).