- 學位論文
An Adaptive Receiver Design for OFDM-Based Cooperative Relay Systems Using Conjugate Transmission
使用共軛傳輸之OFDM合作式中繼系統的適應性接收機設計
摘要
Cooperative relay systems have attracted much attention in wireless communications in recent years due to its various potentials in the enhancement of diversity, achievable rates and coverage range. By using spatially distributed relays capable of forwarding data through statistically independent links, cooperative relaying can fulfill conventional multiple-input multiple-output (MIMO) transmission in a more feasible way. It is more appealing to wireless high rate multimedia application to employ orthogonal frequency division multiplexing (OFDM) in cooperative relay systems, since OFDM has desirable properties for wireless transmission such as high bandwidth efficiency and resistance to the multipath delay spread. However, the performance of OFDM-based cooperative relay systems is still sensitive to the intercarrier interference (ICI) induced by the frequency offset. The ICI problem becomes more complicated since the signals forwarded by relays would suffer from statistically independent channel fadings and frequency offsets, which has rarely been dealt with in conventional ICI cancellation schemes. Therefore, the thesis first focuses on some existing ICI cancellation schemes based on the two-path conjugate transmission, including conjugate cancellation (CC), phase rotated conjugate cancellation (PRCC), and the adaptive receivers, which provide remarkable performances for OFDM systems not using relays. We point out the performance deficiency of the receiver designs in the above schemes when conjugate transmission is carried out in the two-relay cooperation scenario. Then, we develop an adaptive receiver that is suitable for OFDM-based cooperative relay systems based on conjugate transmission. In the proposed scheme, not only the phase rotations are applied, but also the amplitudes are adjusted on the two receiving paths. We provide theoretical derivation for the optimal values of both the phase rotations and the amplitude scale using criteria of maximizing the carrier-to-interference ratio and minimizing the ICI power, respectively. We also develop an adaptive process for updating the phase rotations and the amplitude scale using the normalized block least mean-squared (BLMS) algorithm to track channel and frequency offset variation under time-varying environments. Simulation results show that the proposed adaptive receiver is better than other related works for OFDM-based cooperative relaying systems. It is also demonstrated that the proposed scheme is robust against limited channel estimation errors
並列摘要
合作式中繼系統近年來受到了廣泛的研究與探討，其主要利用空間分集的概念，使傳送端資料藉由多個中繼點通過各自獨立的通道傳送，以實現一個虛擬的多輸入多輸出系統，藉此提升系統效能。而使用正交分頻多工(OFDM, orthogonal frequency division multiplexing)技術之合作式中繼系統由於兼具高速資料傳輸率、高頻譜效率及有效抵抗選擇性衰減通道的優點，因此，對於未來需要高速傳輸率之無線通訊應用服務，此系統有著格外顯著之重要性。 然而，使用OFDM傳輸技術之合作式中繼系統仍存在系統效能容易受頻率偏移所導致的載波間干擾影響的問題。尤其在信號透過空間上不同的中繼點轉送至接收端時，將遭遇獨立的通道衰減和頻率偏移效應，相較於傳統未使用中繼傳輸之OFDM系統，其載波間干擾的問題將變得更為複雜。因此，本篇論文將首先針對目前幾種OFDM系統中基於共軛傳輸之載波間干擾消除方法進行探討，包含共軛消除(CC)、相位旋轉共軛消除(PRCC)和兩種適應性接收機設計。這些方法固然在OFDM系統中能提供不錯的效能表現，然而卻不足以直接應用在OFDM合作式中繼系統。基於此，我們提出一個使用共軛傳輸，並可適用於OFDM合作式中繼系統的適應性接收機設計。此方法除了在兩條共軛傳輸路徑上進行不同之相位旋轉，更進一步調整其振幅以達到更好的載波間干擾消除效果。我們分別利用最大化載波對干擾比以及最小化干擾能量準則對相位旋轉和振幅調整進行最佳化。此外，為了能應用在時變通道環境，我們亦使用了正規化BLMS演算法，發展了相位旋轉和振幅調整之適應性方法，以追蹤通道和頻率偏移的變化。電腦模擬結果顯示，在OFDM合作式中繼系統中，本篇論文所提出之基於共軛傳輸的適應性接收機可達到比其他方法更好的系統效能。