DS/CDMA 蜂巢式系統截斷式迴路傳輸速率調整之效能分析

Abstract

在直接序列-分碼多工擷取(DS-CDMA)無線通訊系統中，所有使用者的訊號都在同一頻帶內不分時地傳送。在上傳(uplink)時，由於非同步(asynchronous)的特性使得使用者間的訊號失去正交性(orthogonality)，導致使用者間的訊號互相干擾。並且由於通道(channel)的衰減(fading)，訊號遭嚴重衰減時，將受到其他使用者較強訊號的干擾，而導致近遠效應(near-far effect)，影響通訊品質，我們可以藉由調整手機發射功率或傳輸速率來減低通道衰減的效應。 在本篇論文中，我們分析分碼多工擷取蜂巢式(cellular)系統中，使用截斷式迴路傳輸速率調整(truncated closed-loop rate adaptation)的效能。截斷式迴路傳輸速率調整是指手機當通道衰減在一定門檻(threshold)以內時，調整傳輸速率以抵抗通道的衰減；當通道衰減超過一定門檻時，手機即停止傳輸。我們在本篇論文中首先以數學模型分析在截斷式迴路傳輸速率調整中截斷門檻(truncated threshold)的設定對系統容量(system capacity)、平均傳輸速率(average transmission rate)、平均整體傳輸速率(average total transmission rate)、每位元平均傳輸能量(average transmission energy per bit)和暫停延遲(suspension delay)等效能的影響，我們也觀察截斷門檻對中止延遲(suspension delay)的影響。從數值分析和模擬的結果我們發現在某些截斷門檻下，截斷式迴路傳輸速率調整比截斷式迴路功率控制具有較好的效能。

In this thesis, we study a truncated closed-loop rate adaptation scheme for reverse links in a Direct Sequence-Code Division Multiple Access (DS/CDMA) cellular system. In this scheme, a mobile station (MS) suspends its transmission when the channel condition is bad; otherwise, an MS transmits with a rate adapted to compensate for the short-term fading so that the received signal power level is constant. We analyze this rate adaptation scheme in a DS/CDMA cellular system and derive closed-form formulas of performance measures such as system capacity, average transmission rate, average total transmission rate, and MS’s average transmission energy per bit, and suspension delay. We also obtain the maximum achievable total transmission rate of a DS/CDMA cellular system. The truncated closed-loop rate adaptation can achieve this optimal performance under an infinite cutoff threshold. Numerical and simulation results show that, with proper selection of the cutoff threshold, the truncated closed-loop rate adaptation scheme can substantially improve system capacity, average transmission rate, total transmission rate, and reduce power consumption than the truncated closed-loop power control scheme.