數位式光載微波技術應用於正交多頻分工訊號調變系統

Abstract

儘管長久以來光載微波訊號系統一直被用來傳輸訊號，類比式光載微波訊號仍存在許多缺點，包括對非線性系統的容忍度差、易受雜訊影響、傳輸光纖距離越常訊號品質越差等。近年來，數位式光載微波訊號被視為解決這些問題的方法。在數位式光載微波訊號系統中，數位訊號的振幅只會有兩個強度，故有較大的雜訊容忍度及對非線性失真免役。然而，數位式光載微波訊號系統的位元傳輸率由類比數位轉換器的取樣率及解析度所決定，在這篇論文中，我們應用帶通取樣理論來降低系統所需要的位元傳輸率以及系統元件所需要的頻寬。為了更了解數位式光載微波訊號系統，這篇論文將展示一些模擬和實驗結果。模擬結果顯示，系統需要一個適當的帶通濾波器以用來避免雜訊疊加效應，數位類比轉換器的解析度和訊號的峰均功率比、訊號進轉換器時的強度有關。除此之外，我們也提出一個演算法，用來計算對多個訊號同時做取樣時，不要造成訊號之間互相相疊的最小取樣率。實驗顯示，將正交多頻分工訊號傳輸50公里的光纖距離，數位式光載微波訊號系統仍較類比式光載微波訊號系統展示較佳的訊號品質。

The Radio-over-Fiber system has been employed to transmit signals for a long time, but there are still many disadvantages with analog radio-over-fiber (ARoF) systems, such as tolerance to non-linearity effects, noise and dynamic range of the signal with increase fiber length. Recently, digital radio-over-fiber (DRoF) has been proposed as better candidate in order to solve these problems. In DRoF system, only two-level amplitudes for digital signal are used to send an RF signal, hence fundamentally, DRoF is robust to noise and any non-linear distortions caused by microwave and optical components. However, the data rate of DRoF systems depend on the sampling frequency and resolution bit of an analog-to-digital (ADC). In this thesis, we employed a band pass sampling frequency in order to reduce on the total bit-rate of the system and relax the bandwidth requirement of the components used in DRoF systems. In order to understand the fundamental benefit of DRoF system, some simulations and experiments has been carried out in our work. Simulation results show that a pre-aliasing band-pass filter (BPF) is needed for avoiding noise aliasing after sampling the radio frequency (RF) signal. In addition, it has been observed that the required bit-resolution in the ADC depends on the peak to average power ratio (PAPR) and signal amplitude input to the ADC. For multi-services or multi-band RF signals, an efficient and low complexity algorithm has been employed for finding a minimum sampling rate that avoid overlapping different RF signals after sampling. Experimental results show DRoF for OFDM modulated systems show a better performance compared to ARoF system over 50-km fiber transmission.