在半正交空時區塊碼系統下之最小錯誤率功率分配法

Abstract

在新一代無線通訊中，傳送多樣（Transmit Diversity）為一廣受矚目的技術，其中空時區塊碼尤其常被廣泛使用與討論。有別於常見的正交空時區塊碼 (Orthogonal Space-Time Block Code, OSTBC)，在本論文中，吾人集中於探討一種特殊的半正交空時區塊碼 (Quasi-Orthogonal Space-Time Block Code, QOSTBC)，俗稱ABBA碼。半正交空時區塊碼的概念是藉由犧牲部分空時碼之間的正交性以換取較高的資料傳輸率。利用ABBA碼特有的通道代數架構，吾人在接收端利用QR分解來偵測訊號；而在傳送端，則提出一種最小錯誤率的功率分配法來改善平均的傳送錯誤率。一般而言，如果通道可事先得知，即可針對通道做功率分配；有別於此，吾人設計的方法是針對整體通道的平均錯誤率做功率的分配。在不考慮錯誤傳遞的情況下，吾人可推導出一個平均錯誤率的上界；此上界在針對通道特性做平均後，得到一個上界的平均錯誤率公式，進而對此上界的公式做最小錯誤率的功率分配。由模擬結果可以看出，吾人的方法在中高訊號與雜訊比的條件下，可以提供接近於聯合最大可能解碼的表現。

It is well known that transmit diversity is a popular technique in modern wireless communication. In this thesis, we focus on one of quasi-orthogonal space-time block codes with full rate (the so-called ABBA code). By exploiting a distinctive channel matrix structure induced by the ABBA code, we derive an explicit formula of the associated QR-decomposition. We propose a minimal BER power allocation scheme for the ABBA code over i.i.d. Rayleigh fading channels under the QR-based successive detection framework. Under a fixed channel realization, we propose optimal power allocation schemes depending on whether or not inter-layer error propagation is taken into account first. Instead of relying on BER under a fixed channel realization, the design criterion adopted by us is the overall mean BER averaged with respect to the channel distribution. Without inter-layer error propagation, we derive an upper bound of the average BER. The closed-form formula is obtained by averaging the upper bound of mean BER with respect to the channel distribution. We then minimize the closed-form formula and an optimal power allocation scheme is obtained. Numerical simulation shows that the resultant performance is almost identical to that of the joint maximum-likelihood decoding in the medium-high SNR region.