分散式籬柵空時碼接收機設計

Abstract

無線通訊中，在慢速衰弱通道環境下，多天線系統(MIMO)[1]加上籬柵空時碼(STTC)是有效改善傳輸錯誤率的方案之一。籬柵空時碼會根據輸入資訊與編碼器狀態，乘上衍生係數，產生發射天線訊號。手持設備因成本與尺寸考量問題，往往僅有單一天線；傳送者欲使用多重天線的特性來幫助傳輸品質，可藉由合作式通訊達到目的。典型的DF合作式通訊，傳送端先將訊號傳給中繼端群，中繼端群將接收訊號解碼過後，再重新編碼傳至接收端。當傳送端與某一中繼端間通道的雜訊干擾甚為嚴重時，中繼端解出的訊號易發生錯誤，此時中繼端假如對此錯誤訊號加以重新編碼，再傳至接收端，會導致接收端解碼錯誤；中繼端群對於不同的訊號作編碼，會導致中繼端群編碼器狀態不一致。本篇論文提出混合式籬柵解碼，來改善此狀態不一致的問題，使得接收端不至於完全喪失解碼能力。當編碼器狀態不一致時，我們稱之為”虛擬狀態”。接收端應用混合式籬柵碼，結合虛擬狀態，改變原有籬柵空時碼結構，以系統觀點來重新定義籬柵的輸出輸入關係；與傳統籬柵空時碼比較，混和籬柵解碼犧牲了錯誤率上的分集程度，來解決中繼端群狀態不一致所引發的解碼問題。此外，吾人利用空時碼概念，討論混合式籬柵碼的分集性能、編碼增益與選擇增益，並在搜尋籬柵空時碼時，提出有效率的演算法。

MIMO with STTC is one of the powerful methods to improve error rate performance in slow fading channel. The signal transmitted by STTC is based on the convolution of the input bit plus memory bit with connection coefficient. Due to the size limitation, most mobile and handheld equipment usually has only one antenna. If MIMO is to be applied, cooperative communication would be a good choice. For DF cooperative communication, source terminal transmits signal to relay in phase 1. After relay decoding the signal, it will re-encode the signal and transmit it to destination in phase 2. However, possibility exists that the relay may decode the signal erroneously. In this case, a series of re-encoded error message will be sent to the destination. In this condition, the conventional trellis decoder will not be able to find the correct path and retrieve the correct information bits. One way to solve this problem is to examine the state status of the encoder. When error bit appears, the states in each relay will not be consistent. In this thesis, we expand the conventional trellis state to incorporate all these inconsistent states. We call this as the hybrid STTC decoding. Another simple approach is to estimate the transmitted signal of each relay separately, and each signal is trellis decoded separately. In this way, the diversity advantage of the MIMO is lost. The hybrid STTC which combines virtual state and correct state to consider the inconsistent problem can still have the advantage of the MIMO diversity. But the diversity decreases as more states are incorporated. Compared with the traditional STTC, hybrid STTC trades the diversity for error detection.