行動衛星通訊之功率控制和調變╱編碼設計

Abstract

以往衛星通訊大都使用Ｃ頻帶或Ｋｕ頻帶，但近年來這些頻帶已相當擁擠 幾近飽和狀態，因此使用Ｋａ頻帶或更高頻帶已是各國新一代通訊衛星的 趨勢。Ｋａ頻帶的信號波長與雨珠大小相當，因而雨水所造成的信號衰減 情形顯得非常嚴重。當遇到下雨時，衛星通訊可能因信號品質惡化而中斷 ，所以許多補償雨量衰減的方法便應運而生。其中最主要方法為從衛星傳 下單頻波束以供地面接收站測量信號衰減程度，來作為上傳時電功率大小 控制的指標。其它諸如使用地域多重接收、增加錯誤編碼、降低資料傳輸 速率等也都不失為解決之道。過去鮮有人討論下傳單頻波束強度的估計問 題，在本論文的第一部份，我們提出一種利用高敏感度輻射計來測量下傳 單頻波束強度的方法，並詳細分析其統計特性。行動衛星通道大都有複路 徑衰褪的現象和遮蔽效應。由於地面行動接收機的天線不具方向性，使得 上述的影響更為明顯，不僅增加了信號正確接收的困難度，更使得同相解 調接收之可行性減少許多。本論文的第二部份，對於非同相解調的多頻鍵 移與差分相位鍵移信號在各種不同衛星行動通訊之傳輸通道中的性能作一 探討研究。其中除考慮多重接收與二元或多元的校正碼兩種方法外，同時 亦分析在實際設計上的考量如：測距轉換、量化以及解隔結合的先後次序 （前解隔結合法與後解隔結合法）等之影響。 One of the major design concerns in designing a Ka-band Satellite communication system is the deep rain fades often encountered at frequencies above 20 GHz. In order to maintain reliable transmissions, certain rain compensation method must be employed. Solutions for alleviating the rain fade include adaptive forward error control, resource sharing, uplink power control, and site diversity. Oftentimes one may have to use more than one of these techniques to compensate for a large rain attenuation. The first part of this thesis dwells on the issue of uplink power control. Attenuation level estimated from a downlink beacon is used for uplink power control. Using a highly sensitive switched radiometer with a narrowband pre- filter, we propose algorithms to estimate the downlink power based on the sampled radiometer outputs. Multipath fading and shadowing effect usually exist in a mobile satellite channel. These effects become more noticeable when an omnidirectional antenna is used by a ground receiver. In this case, noncoherent signalings like MFSK and DPSK are more practical and robust than other coherent signals. The second part of this thesis analyzes cutoff rate performance of multichannel MFSK and DPSK signals in various mobile satellite environments. System design issues like the diversity combining, the choice of binary or M- ary codes, the metric conversion and quantization losses, and the deinterleaving order are all investigated.