數位行動與個人通訊系統之同步與通道補償技術研究

Abstract

因應未來無線行動與個人通訊之服務需求，高速率傳輸已成為最明顯的趨勢。為了使多階差分調變傳輸技術適用於叢發式高速率無線行動與個人通訊系統，本篇論文中提出多項新型同步與通道補償技術，包含符號時序回復、頻率漂移補償、通道補償及框同步等。由於接收端的設計與傳送端採用的調變方法及無線傳輸通道特性有關，所以本論文分成四個部分進行研究，並且加以討論。 本論文之第一部分，針對M階差分相移鍵(MDPSK)調變系統，提出一種結合差分檢測的同步架構，其中包含符號時序回復和頻率漂移補償技術。經由理論分析與電腦模擬驗證，經過差分相位檢測器後，Gardner的時序誤差檢測器，仍可維持S型之輸出特性曲線，並且該輸出特性曲線不僅與差分相移鍵的階數M無關，而且與解調時所殘餘的相位誤差和頻率漂移也無關。此外，頻率漂移會造成差分檢測器之輸出訊號產生一個固定的相位旋轉量。因此，在差分檢測器之後，加上一個頻率漂移補償器，用以消除此一相位旋轉量對系統效能所造成的損害，所以可使訊號檢測時對頻率漂移的容忍程度增加。電腦模擬結果顯示，此一同步架構，在高斯白色雜訊及無頻率選擇性的衰落通道中，即使有殘餘的相位誤差和頻率漂移，仍具有相當不錯的表現。 本論文之第二部分，提出一種新型之前置訊號輔助式通道估計與補償技術，利用接收到的前置訊號，來估計因多路徑衰落與頻率漂移所造成的通道失真。此外，使用檢測後訊號，可增加通道估計的準確度。同時，通道失真在訊號檢測前即被修正。與傳統的作法相比，此一通道估計與補償架構，估計過程中的所需之儲存延遲時間，僅需若干個符號時間長。電腦模擬結果顯示，在具前置訊號之16階差分振幅相位移鍵(16DAPSK)調變系統中，不論經由無頻率選擇性的衰落通道或是具頻率選擇性的衰落通道，此一通道估計與補償架構均可有效地改善系統效能。 本論文之第三部分，提出一種具有適應性內插濾波器的符號時序回復架構。在本地振盪器產生的取樣時脈中，所包含未知的取樣時序誤差量，可利用時序誤差估計技術，從接收到的取樣訊號中估計出。先將取樣訊號經由一個二次方非線性運算，再從其傅立葉轉換之第一係數的相位值，萃取出時序誤差估計值。當要做訊號檢測時，將先前接收到的取樣訊號，經由一個受控於時序誤差估計值之可程式延遲器，再利用適應性內插濾波器，同時達到消除時序誤差及多路徑衰落造成之符際干擾，其中適應性內插濾波器的係數係由時序誤差估計值及錯誤訊號所更新。電腦模擬結果顯示，在具有多路徑衰落與頻率漂移和相位誤差的無線傳輸通道中，此一同步與通道補償架構，與傳統架構相比，可得較好的效能表現。 由於前面三部分所提之同步與通道補償技術均需利用前置訊號輔助，因此系統均需有一個框同步架構與其互相搭配結合使用。本論文第四部分，提出一種利用分數間隔式序列匹配關聯器組成之新型框同步架構，它並不需要具備精確的符號同步電路。同時，也進一步提出一種利用位元序列匹配關聯器之改良式框同步架構，用以降低硬體複雜度。電腦模擬結果顯示，在以前置訊號為同步序列之16階差分振幅相位移鍵(16DAPSK)調變系統中，此一架構利用適當的臨界參數可有效地控制搜尋同步序列時之誤警率與漏失率，用以達到框同步之目的。

For future mobile and personal communication systems, high data-rate transmission is the most important requirement. In order to employ the multilevel differential modulation transmission in the burst mode high data-rate digital mobile and personal communication systems, several new techniques, including symbol timing synchronization, frequency offset compensation, channel compensation and frame synchronization, are proposed and investigated in this thesis. Since the design of the receiver depends on the modulation method in the transmitter and channel characteristics in the wireless environments, this thesis is separated to four parts. In the first part, we present a synchronization scheme, combining symbol timing recovery and frequency offset compensation, associated with the differential detection for MDPSK signals. The characteristic (average output versus timing error) of Gardner's timing error detector after differential phase detector has been proved having an S-curve shape. It is not only independent of the modulation level of DPSK signals, but also the residual phase error and frequency offset in demodulated signals. The frequency offset compensation can increase the tolerance of frequency offset in data detection, and reduce the degradation of BER performance caused by frequency offset. Computer simulation results show that the proposed synchronization scheme has good performance for MDPSK signals in the AWGN channel and frequency-nonselective fading channel even with frequency offset and residual phase error. In the second part, we propose a novel channel estimation and compensation technique for the preamble-assisted DAPSK transmission. The received preamble signals are used to estimate the combined distortions due to multipath fading and frequency offset. The detected data symbols as well as preamble symbols are used to enhance the accuracy of estimation process. The combined distortions can be corrected in the data symbol at the same time. The storage-delay time of the estimation process is only several symbols long. A series of computer simulations have been carried out to investigate the BER performances of the preamble-assisted 16DAPSK signals with uncertain frequency offset in the frequency-nonselective and frequency-selective fading channels. In the third part, we present a symbol timing recovery scheme employing adaptive interpolation filter. The unknown timing error of the sampling clock generated by fixed a free-running local oscillator can be estimated from the received samples by the timing error estimation. The received samples are first processed by a square-law nonlinear operation. Then, the timing error can be extracted directly by the phase of the first Fourier coefficient at symbol rate. For symbol detection, the received samples are first delayed by a programmable delay block, which is controlled by the timing error estimate. And then, the adaptive interpolation filter is adopted to mitigate both the unknown residual timing error and intersymbol interference due to multipath fading simultaneously. The tap-weights of the adaptive interpolation filter are updated both by the timing error estimate and the error signal. Computer simulations show that the proposed adaptive interpolation filter scheme outperforms the other conventional schemes in the wireless environments with multipath fading, frequency offset, and phase error. Since the aforementioned synchronization and channel compensation techniques require some preamble signals, a frame synchronization scheme is necessary at the receiver. In the fourth part, we present a simple frame synchronization scheme using fractionally-spaced sequence-matched correlators without exact symbol synchronization. At the same time, a low-complexity frame synchronization scheme using a single bit-sequence-matched correlator is also presented to reduce the hardware complexity. A series of computer simulations have been carried out to evaluate the performance of the proposed frame synchronization scheme with a suitable threshold parameter for the preamble-assisted DAPSK transmission in the wireless environments.