就長碼的直接序列分碼多重存取(long-code DS-CDMA)系統而言，限制系統所能容納最大用戶數目的主要原因，是因為用戶間所使用的展頻碼彼此非正交的關係，這會造成用戶訊號間相互的干擾，也就是所謂的多重存取干擾(MAI)，為了克服多重存取干擾的問題，有許多方法被提出，而多用戶檢測(MUD)是其中最重要的技術之一，多用戶檢測技術最重要的優點就是它可以降低多重存取干擾以及有效的提高系統效能，然而，這同時會改變每個用戶訊號決策統計的統計性質，事實上，系統輸出訊號的決策統計其平均值與變異數也不再是保持非時變的，因此傳統架構在AWGN模型下的單一用戶Viterbi解碼器將不再適用，必須考慮存在於解碼器中可以被改善的空間，才能進一步提升整體系統效能。
對於長碼分碼多重存取系統下被用於連結在多項式接收器以及線性連續干擾消除接收器輸出的解碼器，本篇論文將提出其改善後的演算法，演算法的改進是藉由解釋檢測後訊號的決策統計為漸進的高斯分佈(asymptotic Gaussian)，並透過矩陣代數運算的方式算精確地求出其時變的平均值與變異數，此外還將介紹一種近似平均值與變異數的方法，即利用圖形理論的方式可以得到各階動差的近似值，進而求得平均值與變異數的近似值，再將近似的平均值與變異數用於改善後的Viterbi解碼演算法中，而最後得到的效能增益其模擬結果將被顯示在論文裡。

The most important advantage of multiuser detectors is that they can degrade multiple access interference and increase system performance efficiently. However, they also change mean and variance of each user’s decision statistic in the same time. In fact, decision statistics of system outputs are not longer time-invariant either. Therefore, the conventional single-user Viterbi decoder may be further improved.
This paper will propose modified algorithms for the decoders used in conjunction with polynomial receiver and linear successive interference cancellation (LSIC) receiver for DS- CDMA systems with long spreading sequence. The modifications are based on explaining the detected signal as asymptotic Gaussian and calculating time-dependent mean and variance accurately by matrix algebra. In addition, an approach method of the mean and variance is also presented and put to use in the modified decoding algorithm. The performance gains of the modified Viterbi decoders will be shown in the paper.

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