於無線多輸入多輸出與多重存取通道下高分集與多工權衡且具低複雜度收發機設計

Abstract

在通訊系統中，一般而言大部分偵測或解碼的複雜度及其效能有某種權衡關係。在我們的研究中，我們提出數種不同傳送訊號的設計來使得接收機除了具有較低偵測複雜度，也證明我們的設計具較佳的分集與多工增益曲線，並模擬其中斷機率及位元錯誤率來驗證我們所提出的設計。在單一使用者多輸入多輸出通道下，由其他學者的證明得知，V-BLAST使用低複雜度線性接收機的分集與多工增益會比最大概似接收機來得差，但是最大概似接收機具有相當高的偵測與解碼複雜度。對此議題，我們首先證明我們提出的傳輸模式的分集與多工增益曲線，接著提出在單一使用者多輸入多輸出通道下可達到此分集與多工增益曲線的晶格時空碼，換句話說，如果使用我們提出來的晶格時空碼，一方面可以到達我們所證明出來的分集與多工增益曲線，另一方面我們可以使用低複雜度的線性接收機即可以達到最大概似接收機的最大分集增益。在多用戶多輸入多輸出多重存取通道下，由於傳送天線數量遠大於接收天線的數量，因此基地台端有極高的偵測與解碼複雜度。在我們的研究中，我們使用少量的通道回饋，證明我們提出的兩種傳輸模式的分集與多工增益曲線可以改善傳統沒有通道回饋的分集與多工增益權衡曲線，並且使得基地台具有傳統球狀解碼器的偵測與解碼複雜度。

In wireless communication, many researchers and engineers chase low detection complexity solutions. However, many such solutions trade performance for possible complexity reduction. In this dissertation, high DMT performance and low detection complexity transmission designs in both single user and multi-user, multiple-input-multiple-output (MIMO) communications are proposed. Firstly, in single-user MIMO system, linear receivers are often of more practical interest than maximum likelihood (ML) receivers, but at a cost of much worse diversity gain performance. Such statement on performance loss is due to the assumption of using an independent and identically distributed (i.i.d.) complex Gaussian vector as channel input. By removing this assumption, we find that the diversity performance of MIMO linear receivers can be significantly improved. In an extreme case, the maximum diversity gain can be the same as that of ML receivers. Specifically, in this dissertation we investigate the diversity multiplexing tradeoff (DMT) performance of MIMO linear receivers with colored and possibly degenerate Gaussian channel inputs. By varying the rank of the covariance matrix of the channel input vector and by allowing temporal coding across multiple channel uses, we show that the MIMO linear receiver can achieve a much better DMT performance than the currently known. Explicit optimal code constructions are also provided, along with simulation results to justify the above findings. For the case of (2 x 2) and (3 x 3) MIMO linear receivers, simulation results show that the newly proposed lattice space-time codes provide significant gains of 26.5 dB and 29.6 dB in E_b/N_0 at bit error rate 10^{-6}, compared to the conventional schemes, respectively. Secondly, in MIMO-multiple-access channel (MAC), the base station has less number of receive antennas than the total number of transmit antennas of all users, sphere (lattice) decoding for the existing MIMO-MAC codes requires an exhaustive search of exponentially large size before it can process the root node in sphere-decoding tree. In this dissertation, two coding schemes are proposed and are shown to have a constant sphere-decoding complexity, independent of the number of users as well as the number of transmit antennas. The schemes require a channel feedback, but at an extremely low rate. The first scheme is based on user selection, and the second scheme selects jointly the users and transmit antennas, using a fast antenna selection algorithm proposed by Jiang and Varanasi. It also requires an additional design of rate-assignments that maximizes the overall DMT performance. It is shown that both schemes yield DMT performances far superior to the optimal MIMO-MAC DMT without channel feedback. In the case of ten-user MIMO MAC, each with single antenna and two antennas at base station, simulation result confirms that the first proposed scheme and the second proposed scheme can provide an astonishing SNR gain of 2.8 dB and 14.5 dB at outage probability 10^{-6} compared to the optimal coding schemes without feedback, respectively.