運用拍擊干擾補償技術增進簡易60GHz光載微波無線訊號系統之有效傳輸距離

Abstract

近年來，智慧型手機與平板電腦的普及，結合了高解析度數位頻道(High Definition TV)的需求，現今無線通訊系統所提供的有限頻寬服務沒有辦法滿足此類移動式裝置高傳輸速率的需要。因此，拓展更廣的頻寬來達到更高的傳輸速率是目前無線通訊系統所面對的課題。利用60GHz無線頻帶系統來達成高速無線通訊的目標，是必然的產業趨勢。然而，一般同軸電纜(Co-axial cable)無法有效傳遞如此高頻的電訊號，造成了實際應用上的困難，因此我們提出簡易光載無線(Radio-over-fiber)傳輸訊號的技術，結合了光電轉換器的特性，可以將如此高頻的訊號以光為介質傳輸，而在用戶端還原為60GHz的微波無線訊號。藉由光傳輸上低功率消耗的優勢，可以將此高頻訊號傳輸距離有效地延長。現階段簡易光載無線技術的有效傳輸距離在約略三公里。簡易光載微波技術的有效傳輸距離受到限制主要來自於光纖射頻凋零(RF fading)，此存在於簡易光載微波技術的問題造成了有效傳輸距離的限制，有效地減低此存在於光載微波技術的影響，能夠增加此技術的有效傳輸距離自三公里至十公里以上。在此篇論文中，使用正交多頻分工技術傳輸訊號，透過等化器及訓練符元的設計，解決I/Q不平衡對OFDM訊號以及系統拍擊噪聲之影響。此論文成功的傳輸60GHz頻段上7GHz免授權頻寬的OFDM訊號，其QPSK訊號格式之有效傳輸距離達到12公里及3公尺無線傳輸。並成功傳輸8QAM訊號，其傳輸速度最高達到20.8125Gbps。

With the continuously growing demand for wireless video-based interactive and multimedia data services, 60-GHz millimeter-wave wireless systems have been viewed as a promising candidate for multi-gigabit-per-second wireless access due to 7-GHz license-free band (i.e. 57-64 GHz). However, 60-GHz wireless signal suffer from high path loss. Hence, the cell size of 60-GHz pico-cells is generally limited to 10 m, which is ideal for in-building applications. Consequently, wireless access networks at 60-GHz require numerous base stations (BSs) to connect the access points. Therefore, the radio-over-fiber (RoF) system approach, which distributes RF signals from a central station (CS) to multiple BS over an optical fiber, is a promising means of reducing the overall cost of 60-GHz wireless access networks owing to its nearly unlimited bandwidth and extremely low propagation loss [1-6]. For certain applications such as in-building, it is imperative that the employed RoF links are as simple as possible to reduce cost, while providing the needed performance. Recently, we proposed simple 60-GHz RoF systems with one single-electrode Mach-Zehnder modulator (MZM), and their analysis have been presented in [7]. Compared with conventional optical double sideband RoF systems using 60-GHz MZM or electro absorption modulator [8], the proposed RoF system can support longer fiber transmission distance (4km vs. 500m) and has less bandwidth requirement of optical OFDM transmitter (35.5 GHz vs. 64 GHz). In this paper, we further improve the fiber transmission distance from 4km to 12km. However, the generated 60GHz OFDM signal will have signal-to-signal beat interference (SSBI) after square-law photo-detection. Therefore, we will employ signal-to-signal beat interference mitigation algorithm to improve signal-to-noise ratio (SNR). 60-GHz 13.875-Gbps QPSK and 20.8125-Gbps 8-QAM OFDM signal with 3-m wireless transmission are experimentally demonstrated and transmitted over 12-km and 6-km single-mode fiber, respectively.