合作式通訊下以正交分頻多工之接收端使用白化處理對抗多重頻率偏移

Abstract

在正交分頻多功(OFDM)通訊系統下，載波頻率飄移導致載波間干擾(ICI)和傳輸效能的衰減，載波間干擾讓通道矩陣不再只有對角線方向有值，這種情形使得在正交分頻多功信號接收變的困難，在合作式通訊下，同步是一個很重要的議題，不像傳統的多輸入多輸出(MIMO)系統，合作式的天線是分屬於不同的傳送端，因此，對目的端而言，很難同時補償多重頻率偏移(MCFOs)，另外，傳統的訊號偵測方法在這邊可能不是很適用。在這篇論文裡，我們考慮從傳送端到目的端的通道是頻率選擇性衰減通道，提出的方法是結合兩個已分別做完同步的訊號來消除載波間干擾，白化處理被進一步用來獲得良好的位元錯誤率以及對於多重頻率偏移有更好的容忍度。 除了傳輸分集(transmit diversity)之外，這篇論文也探討了空間多功(spatial multiplexing)的情況，我們使用兩個已分別做完同步的訊號，經過符號的重新排列，白化處理被進一步用來獲得良好的位元錯誤率以及對於多重頻率偏移有更好的容忍度。更進一步，在這篇論文使用了線性最小均方誤差(LMMSE)來降低複雜度，並且實做在數位信號處理板(DSP board)上，相對於最大可能序列估計，這個方法在良好的偵測性能和低複雜度間提供另一折衷選擇。

Synchronization is an important issue in cooperative communications. Unlike in conventional MIMO systems, the cooperative antennas belong to different source nodes. Therefore, it is hard for the destination node to compensate multiple carrier frequency offsets (MCFOs) simultaneously and conventional signal detection strategies may not be capable enough to deal with the situation. In this paper, we consider cooperative transmission with MCFOs when the channels from source nodes to the destination node are frequency selective fading. The proposed strategy combines two sets of separately synchronized signal to mitigate inter-carrier interference (ICI) caused by MCFOs. Block-wise whitening is deployed to further enhance the tolerance of MCFOs and improve BER performance. In addition to transmit diversity, in this paper we also consider the case of spatial multiplexing. Techniques such as using two sets of separately synchronized signal, index reassignment, and block-wise whitening are utilized to mitigate ICI. To go a step further, in this paper we use linear minimum mean square error detection to reduce complexity and implement the algorithm on DSP boards. The method is shown to provide good performance complexity trade-off compared to other ICI countermeasures.