適用於衛星通信之高頻譜效益正交分頻多工系統架構研究

Abstract

近年來為因應國軍戰場數位化的目標，軍用通訊系統的傳輸率必須有效地提高，如 此才能與戰場上的士兵進行多媒體聯繫，藉以進行戰場現況即時監控，包括戰場影像傳 輸與情資蒐集。有鑒於衛星通訊無遠弗屆的通訊能力與正交分頻多工(Orthogonal Frequency Division Multiplexing) 的高資料量傳輸特性，結合此兩種技術將是未來軍用通 訊系統的主流。現存正交分頻多工技術需在每一傳送幅元前加入額外的循環字首 (Cyclic Prefix)，方能在面對多路徑通道環境時確保子載波間之正交性並改善通道所造成 的符元間干擾，但此舉卻會導致整體系統吞吐量 (Throughput) 降低。然而，縮短循環 字首雖能有效地提升正交分頻多工系統之頻寬效益 (Bandwidth Efficiency)，但亦會造成 該系統對通道效應更加敏感，因而大幅限制其整體系統表現。因此，如何縮短(甚或去除) 循環字首以改善現存正交分頻多工調變技術之頻譜效益，便成為極具挑戰性的議題。 在本計劃中，吾人將針對縮短循環字首後系統所產生的不完美特性進行理論分析， 並結合空時編碼、盲蔽式通道估測與多重使用者偵測等技術於正交分頻多工系統中，以 提出一種適用於衛星通訊之高頻譜效益正交分頻多工傳輸架構。同時亦將針對此架構發 展包括通道等化及補償都普勒效應之高效能、低複雜度收發機演算法，並進行全系統在 不同通訊環境下之效能評估及整體運算複雜度之計算。預期所得新一代高頻譜效益傳輸 架構將能在不影響整體系統效能的前提下，大幅提昇系統吞吐量，進而滿足未來國軍數 位戰士之需求。

Owing to the demand for multimedia service on the battlefields, e.g., the real-time monitoring and information gathering, the trend of military communication toward high data rate transmission is inevitable. On account of the wide coverage of satellite communication and the high transmission rate of orthogonal frequency division multiplexing (OFDM), researches on combining both of the techniques have become the mainstream for future military communication systems. However, OFDM necessitates extra cyclic prefix (CP) to avoid the inter-carrier interference and to mitigate the inter-symbol interference due to multi-path channels, which results in the undesired decrement of system throughput. A direct reduction of CP can improve the bandwidth efficiency but makes OFDM systems more sensitive to the channel impairments, hence limiting the overall system performance. In this project, we will combine OFDM with space-time coding, blind channel estimation, and multi-user detection to construct a CP-reduced (or even CP-free) OFDM transmission structure with high bandwidth efficiency. A low-complexity transceiver with algorithms for channel equalization and compensation of the Doppler effect in satellite communications are also investigated for performance optimization.