非步式跳頻多址進接系統在衰褪通道中的多用戶檢測

Abstract

展頻多重近接系統之使用者容量主要受限於同頻道干擾(cochannel interference,CCI)及頻道間的干擾(interchannel interference, ICI) 。本篇論文針對跳頻多址進接(frequency-hopped multiple access)結合 複頻鍵移(MFSK)信號之系統， 探討多用戶檢測相關之各類課題。在同步 跳頻多址進接/複頻鍵移系統中，兩個來自不同向度或不同載波頻率之波 形是相互正交的。而在非同步的作業環境中，正交性並不存在，頻道間的 干擾再也無法忽略。因此我們研究各種方法來減輕同頻道干擾及頻道間的 干擾，其中又以下列三項為主(1)改善向度組合結構，(2)使用最佳跳頻型 式(optimal hopping pattern)，(3)利用跳頻型式的代數結構。我們分析 了在無頻率選擇性的萊思或瑞雷分佈之通訊環境中，利用上述各種方法所 造成的影響。對於(1)我們針對非同步跳頻多址進接/複頻鍵移信號推導出 一最大可能性向度組合結構，並提出一最大可能性接收機及分析其性能， 對於(2)我們針對以隨機跳頻與最佳跳頻形式的兩種系統比較其性能。對 於(3)我們提供一個二階段式的解碼流程並評估其性能。從分析與模擬所 得的數據顯示，在非同步系統中，即使採用二階段式解碼器及最佳跳頻形 式，性能的退化也很顯著。因此我們提出另外兩種方法來進一步減低頻道 間的干擾:導引信號的廣播及保護時段的插入。經由數值分析後我們發現 ，同時利用上述兩種方法，的確可以改善不少系統的容量及頻譜效率。 This thesis studies various aspects of multi-user detection for frequency-hopped multiple access (FHMA) systems that employs multilevel FSK (MFSK) signaling. For a synchronous FHMA/MFSK system two chip waveforms are orthogonal if they come from different hops or different tones (carrier frequencies). However, in an asynchronous operating environment where systems user are not coordinated in time, orthogonality will no longer exist and interchannel interference (ICI) cannot be neglected any more. The capacity of a spread spectrum multiple access (SSMA) system is primarily limited by cochannel interference (CCI). Hence, the foremost design concern of a multi- userdetector is to separate the desired signal from other undesired cochannel signals. SinceICI in asynchronous FHMA systems can be too significant to be ignored we have to payextra attention to mitigate ICI. We examine several ways to minimize both CCI and ICI: (i) improved diversity (hop) combining rules to suppress the CCI/ICI, (ii) the use of the optimal hopping pattern that ensure minimum mutual interference in synchronous mode, (iii) using the algebraic properties of the optimal hopping pattern to cancel the CCI/ICI. The impacts of all these methods in a communication environment that is characterized by frequency-nonselective Rician or Rayleigh fading are analyzed. For (i) we derive a maximum likelihood (ML) diversity combiner for asynchronous FHMA/MFSK signals. A close approximation of the ML receiver is proposed and its performance is analyzed. For (ii) we compare performance of systems using random hoppingpatterns and that of those using a set of optimal hopping patterns. As for (iii) we proposed a new two-stage decoding algorithm and evaluate the resulting performance.Since performance degradation of the asynchronous systems are significant even with theuse of the two-stage decoder with the optimal hopping pattern, we present two morealternatives to further reduce ICI: the deployment of a broadcasting pilot signaland the insertion of a guard time. Numerical results indicate that the implementationof both schemes do bring substantial improvement.