數位通訊系統同步技術之研究

Abstract

本論文主要探討高速率電話線傳輸和數位無線電通訊系統的同步問題, 同 步的層次則包括時序回復與載波相位回復等相關技術o高速率電話線傳輸 由於嚴重的符際干擾､近端串音與殘留回音增加資料還原和時序回復上的 困難o 為增加時序回復的可靠度, 我們提出一包率取樣的時序回復技術o 該法乃利用適應性通道估測技術自估測得到的通道響應中拮取時序資訊o 進一步利用該通道估測器將通道等化､時序回復與微調自動增益等功能結 合以簡化硬體複雜度o無線電通道會因城市密集的建築物或其他地形障礙 而造成多重路徑反射的干擾, 使得接收到的信號發生嚴重衰落(Fading)o 首先, 我們評估一類非線性時序回復技術在衰落通道中的效能並比較之o 由於衰落效應導致同步相位解調困難, 而差分解調雖然毋需載波相位的回 復, 但是頻率漂移則會影響系統效能o 在文中, 我們提出一快速頻移估測 器並分析頻移量和系統效能間的數學關係, 更進一步評估該快速頻移估測 器在衰落通道中的效能o 落效應與符際干擾兩因素對一般LOS(Line-of- Sight)無線電通訊系統其載波相位回復的影響, 並分析I-Q 載波相位回復 迴路在該系統中的效能o In the dissertation, we study the clock and the carrier synchronization for the high-rate digital subscriber lines( HDSL) and the digital wireless communication systems. In the HDSL transmission, the severe intersymbol interfere- nce(ISI), the residual echo and the near-end crosstalk(NEXT) from the adjacent cables will make the timing recovery and data ion become more difficult. In the dissertation, we propose a rate sampling timing recovery method based on an adaptive estimator. The timing information is extracted from the estimated channel response rather than from the received signal itself. Furthermore, the timing recovery, the channel equalization and fine-tuned automatic gain control (AGC) can be unified simply by an adaptive channel estimator such that the overall hardware lexity can be reduced. In the radio propagation, the received signal suffers from ipath fading due to the reflections of buildings or terrain. we evaluate a class of nonlinear timing recovery techniques forπ/4-DQPSK signal in the Rayleigh fading channels. Since detection does not require carrier phase recovery, it is more able than coherent detection in the fading channels. However, ncy offset will degrade the performance of the differential Thus, we propose a fast frequency offset estimator for the signal. The relation between the frequency offset and the symbol rate performance is developed analytically. Its performance in Rayleigh fading channel is also analyzed. Finally, we study the performance of I-Q carrier recovery the line-of-sight(LOS) digital radios subject to the impairments multipath fading and intersymbol interference(ISI).