同步與非同步OFDM-CDMA信號在衰褪環境下的偵測

Abstract

(Orthogonal Frequency Division Multiplexing, OFDM) 是一種可以有效抵抗選擇性衰褪的方法。另一方面，分碼多重存取(CDMA)則提供較傳統多工更多的容量。因此，結合OFDM與CDMA應可提供更好的抗多重路徑與抗衰褪的多重存取。 在本論文中，我們首先利用模型式最小平方法逼近 (Model-based Least-square Fitting, MB-LSF) 通道估測器來估測同步系統中的通道，並探討通道估測器的設定如何影響估測結果，以及影響單一使用者信號的偵測。另一個重點則是單一使用者非同步OFDM-CDMA信號的偵測。我們建立了非同步OFDM-CDMA信號的數學式，以及信號在系統各模組下的數學表示法。同時利用最大可能性 (Maximum-likelihood, ML) 演算法建立最佳與次佳的單一使用者信號偵測器，以及利用最大可能性序列偵測器(Maximum-likelihood Sequence Detector, MLSD)來偵測單一使用者信號。最後並提供不同信號偵測器的效能比較。 另外，我們提出兩種不同的方式來建立信號偵測器，分別是後解擴式決定 (post-depsreading decision) 以及先解擴式決定 (pre-depsreading decision)。同時比較這兩種方式的差異與對信號偵測的影響。

Orthogonal Frequency Division Multiplexing (OFDM) is an e?ective multi-carrier waveform for combating frequency-selective fading. On the other hand, the code division multiple access (CDMA) technique has attracted a great deal of interest for its potential to o?er system capacity larger than those achieved by other conventional multiple access techniques. Therefore, it is conceived that OFDM combined with CDMA (OFDMCDMA) can provide enhanced anti-multipath fading capability while accommodating a large number of active system users. This thesis studies the fundamental issue of detecting synchronous and asynchronous OFDM-CDMA signals in frequency-selective fading environments. We derive the optimal single user detectors for synchronous systems based on a novel model-based leastsquare tting (MB-LSF) channel estimation scheme. For asynchronous OFDM-CDMA systems the situation is a bit more complicated. We consider both pre-despreading and post-despreading single-user detection scenarios. Optimum and suboptimum single user detectors based on maximum-likelihood symbol by symbol detection and maximumlikelihood block-by-block detection are obtained. Numerical examples are provided to compare performance of various detector structures and to study the impacts of system and channel parameters so that optimized performance can be obtained.