輪碼於複徑衰褪通道下之性能表現

Abstract

摘要 平行聯接迴旋碼 (parallel concatenated convolutional code)，或稱 為渦輪碼 (turbo code)，是一種簡單且功能強大之通道編碼。資料於 編碼器輸入端經交錯器 (interleaver) 產生另一組排序不同之資料， 再分別經兩個遞迴式系統迴旋編碼器 (recursive systematic convolutional code) 以產生兩個同位檢查位元 (parity check bit) 。而於接收端，將收到的三種資料 (即原始資料與兩組同位檢查位元) 利用對數可能性法則 (log-likelihood principle) 與最大後置機率判定 法 (max- imum a posteriori probability rule) 進行遞迴式解碼。 電腦模擬的結果顯示其 更正隨機錯誤 (random error) 的能力相當驚 人-於高斯通道下，可使位元錯誤率 (bit error rate) 相當接近謝農 式極限 (Shannon limit)。但於實際的複徑衰褪通 道 (multipath fading channel)，由於叢發錯誤 (burst error) 急劇增加， 錯誤 更 正能力相對地大幅降低。因此必須針對通道特性改良編碼系統結構， 以 對抗衰褪 通道的影響。 在本文中，除介紹、比較與討論各種形式之渦 輪碼外，並就其在同調 與非同調模式操作下， 針對調變方式，通道種 類與特性，通道狀態之估計方法，分 集 (diversity) 與解碼方法， 做 一完整之模擬與討論 其中之通道狀態估計乃採用 數種已有的引導信號 (pilot signal) 技術， 以降低信號經衰褪通道後產生之叢發 錯誤， 並產生可用於遞迴解碼計量值 (metric) 計算之通道狀態估計。 AbstractA class of powerful error-correcting codes called parallel concatenated codes, or turbo codes, has been proposed recently. Such a code consists of two or more parallel recursive systematic convolutional codes, each "subcode" is used to encode the same but independently interleaved data sequence. Together with a maximum a posteriori probability (MAP) iterative decoding rule, turbo codes have been shown to achieve bit error rate performance close to the Shannon's AWGN limit. The performance and behavior of turbo codes in practical multipath fading channels, however, remains to be determined. The fading effect usually incurs burst errors that greatly hinder a channel code's error correcting capability. Modification of the decoder structure is needed to account for the influence of multipath fading. The purpose of this thesis is to investigate the impacts of channel conditions and various system design alternatives on the overall system performance. Channel models used are: independent and correlated Rayleigh fading channels and those based on Jakes' model. Given the channel model we then study how the channel state information is derived and such information can be used to help decoding. Computer simulation is employed to quantify various system behavior and to provide numerical examples that give us system solutions.