結合多載波分碼多重進接及渦輪多用戶偵測之上鏈基頻接收機的設計與模擬

Abstract

本論文提出一結合多載波分碼多重進接系統和渦輪碼的上鏈基頻接收機。在上鏈傳輸系統中，使用者間互相的干擾是影響系統表現的主要原因，因此在我們的接收機中使用了多層級平行干擾消除，在每一層級中將資料決策後的結果重建以估計每位使用者對其它使用者的干擾訊號，然後於下一層級中將干擾訊號消除掉，由於本論文中重建干擾訊號的地方在渦輪解碼器後，故將之稱為渦輪多用戶偵測。由電腦模擬結果可以看出，在理想通道估計下，系統的表現可以趨近於單一使用者的理論值；在通道估計下，系統的表現則與通道估計的準確度有關。 在作干擾訊號的重建時，使用軟式決策會比硬式決策好，但渦輪解碼器只有系統位元的軟式決策值，因此我們利用資訊傳遞演算法去推導出同位位元的軟式決策值，並由電腦模擬看出，在干擾嚴重時，同位位元使用軟式決策去重建干擾訊號的表現比用硬氏決策好。此外，我們也利用訊息傳遞演算法去推導出和渦輪解碼器相同的式子，並利用此演算法來解釋渦輪解碼。

We propose an uplink baseband receiver for a turbo coded multicarrier CDMA system. Because other users’ interferences are the main cause of the degradation of the system’s performance in the uplink transmission, we adopt a multi-stage parallel interference cancellation (PIC) multiuser detection to improve the performance. The interference is estimated and regenerated from the result of data decision in each stage, and it is canceled from the received signal in the next stage. In our proposal, the interference is regenerated from the soft decision of the turbo decoder, so we name this architecture turbo multiuser detection. From the result of the computer simulation, we can see that the system’s performance can be improved to approach the theoretical single user bound on the condition of ideal channel estimation. The simulation result also shows that the performance improvement severely depends on the accuracy of channel estimation when turbo multiuser detection is applied. Besides, interference regeneration using soft decision performs better than one using hard decision. However, the turbo decoder offers only the soft decision of systematic bit, so we propose a method to derive the soft decision of parity bits of turbo code by using the concept of message passing algorithm. The simulation result shows that it performs better when soft decision of parity bits is used and the improvement is apparent especially when interference is serious. In the thesis, we also use the message passing algorithm to derive the same decoding equation of turbo decoder and explain the turbo decoder by using the message passing algorithm.