分碼多重存取通訊之基於粒子群最佳化的強健多使用者偵測

Robust Multiuser Detection Based on Particle Swarm Optimization for CDMA Communications

孟昭立(Jhao-Li Meng)

指導教授 ： 張安成

共同指導教授 ： 洪瑞鍾(Jui-Chung Hung)

嶺東科技大學/資訊學院/資訊科技應用研究所/碩士(2011年)

https://doi.org/10.6823/LTU.2011.00001

摘要

以展頻為基礎的分碼多重存取(CDMA)技術於現今的蜂巢式和個人通訊系統中因具備下述特性上的潛力而於眾多的多重存取方式中受到注意並且扮演重要的角色，如：抗多重路徑能力、軟性系統容量和軟式交握等；多重存取允許多個使用者分享有限的資源，例如頻寬和時間，而直接序列分碼多重存取(DS-CDMA)為最普遍使用的技術，然而其中有兩個限制系統效能的主要因素即多重存取干擾(MAI)與遠近效應；最大相似度偵測可有效地對抗MAI的問題，然而其計算複雜度以使用者的數量成指數增加，線性多使用者偵測器具有較低的計算複雜度但是卻無法提供足夠的效能，因此本篇論文主要在探討利用粒子群最佳化(particle swarm optimization, PSO)演算法以低於ML偵測的計算複雜度進行最大相似度結果的搜尋並提高多使用者偵測效能的策略。標準的PSO演算法在粒子移動速度的更新上所有粒子皆使用相同之線性遞減的慣性因素因而導致快速收斂的特性，然而最佳解搜尋的快速收斂無法保證其結果為全域最佳解，因此於本論文中提出利用牛頓法更新粒子移動速度的PSO演算法，以提高最佳解搜尋的精確度並利用適應性的慣性權重以加快收斂速度。
實際的行動通訊系統中，天線陣列因其具有指向性的輻射場型可提高訊號接收的效能與容量而被廣泛的使用，但由於訊號到達角度估測實際上的困難，使得天線陣列的波束構成存在指向誤差的問題，雖然基於特徵空間(eigenspace-based, ESB)的波束構成器於存在指向誤差的情況下具有其強健性，但是在較大的指向誤差情形中ESB波束構成器同樣會出現所欲訊號消除的現象，為了減輕由於指向誤差與有限取樣所導致的效能退化，本論文亦同時針對基於決策導向(decision directed, DD)以提高強健性之ESB波束構成器進行討論，然而DD-ESB波束構成器的輸出SINR經證明亦將在指向誤差相對較大時發生效能退化的情形；事實上經由均勻線性天線陣列參考點所獲得的接收訊號僅為時間域的資料，因此所提出之結合基於PSO碼片估測程序與決策導向的方法可用以形成在沒有任何特定訓練序列下的強健性DD-ESB波束構成器，模擬結果證明所提出的DD-ESB波束構成器提供了較現存ESB、RESB與DD-ESB波束構成器具有較佳的強健性。

關鍵字

PSO演算法 ； 以牛頓法更新粒子移動速度的PSO演算法 ； 分碼多重存取 ； 最大相似度 ； 天線陣列 ； 訊號到達角度估測 ； ESB波束構成器 ； 基於決策導向的ESB波束構成器

並列摘要

Spread spectrum-based code division multiple access (CDMA), has taken on a significant role in cellular and personal communications because of such characteristics as potential capacity increases over anti-multipath capabilities, soft capacity, and soft handoff. Multiple-access allows multiple users to share limited resources such as frequency (bandwidth) and time. Direct-sequence CDMA (DS-CDMA) is the most popular of CDMA techniques. However, there are two key limits to present DS-CDMA systems: multiple-access interference (MAI) and near-far effect. In DS-CDMA systems, the multiuser detection (MUD) has been used to combat MAI and near-far effect. The maximum-likelihood (ML) detection can effectively combat the MAI problem; however, its complexity increases exponentially with the number of users. Linear multiuser detection has lower computational complexity, but it cannot provide sufficient performance. In this thesis, we investigate the MUD strategy using particle swarm optimization (PSO) algorithm, which has lower computation complexity, to search the maximum likelihood results.
Recently, PSO algorithm is a population based heuristic global optimization technology has been proposed. In PSO algorithm, the individual is called particle which using identical inertia weight in velocity updating and has a superior convergence speed compared with other evolutionary algorithm. However, it cannot be guaranteed that all the particles converge to a global best position due to high convergence speed. Therefore, this thesis presents a modified PSO algorithm with velocity updating by using Newton method to improve the accuracy of optimal solution searching and convergence speed for MUD.
Antenna array has been widely implemented in practical mobile communication system because of its directional radiation pattern can improve the performance of signal receiving. Due to the estimation of direction of arrival (DOA) of signal has difficulties in nature, adaptive array beamforming always exists the problem of pointing error. To mitigate the performance degradation due to pointing error and finite sample effects, this thesis also deals with eigenspace-based (ESB) beamforming based on the decision-directed (DD) correction with robust capability. However, it is shown that the output SINR of the ESB beamformer is degraded when the pointing error is relatively larger. As a matter of fact, the received signal obtained from the reference element of the uniform linear array is only time-domain data. In conjugation with DD and PSO-based chip estimation procedure, the proposed approach can be used to form a robust DD-ESB beamformer without any specific training sequence. Simulation results show that the proposed DD-ESB beamformer provides more robust capabilities than the existing ESB, RESB, and DD-ESB beamformers, respectively.

並列關鍵字

PSO algorithm ； modified PSO algorithm with velocity updating by using Newton method ； code division multiple access ； maximum-likelihood ； antenna array ； estimation of direction of arrival ； ESB beamformer ； DD-ESB beamformer

參考文獻

參考文獻

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