直接序列/分碼多工接取蜂巢式系統之功率控制機制效能分析

Abstract

對直接序列/分碼多工接取(DS/CDMA)蜂巢式系統的運作而言，功率控制是非常重要的一環。若沒有功率控制，系統容量將受到遠近效應的影響而變得很低，若有適當的功率控制，系統資源才得以公平地互享而展現系統應有的容量。在本論文中，我們將深入分析DS/CDMA蜂巢式系統在功率控制機制的運行下，其系統容量的表現。 首先，我們分析了一種暫停式閉迴路功率控制(Truncated Closed-loop Power Control, TCPC)機制下的系統上鏈容量。該TCPC 功率控制機制屬強度型(Strength-based)功率控制機制的一種。在TCPC機制下，當短程衰耗小於一預設遮斷準位(cutoff threshold)時，行動台暫停其傳輸作業；否則，控制行動台的發射功率以補償短程衰耗，使得在基地台處的接收功率盡量維持在一預設準位。我們成功地導出了下列各效能指標的公式：系統容量、平均系統傳輸量、行動台平均傳輸量、行動台消耗功率、及行動台傳輸延遲。數值結果顯示，所導出的公式具有相當的準確度，此外亦顯示這種TCPC功率控制機制可以達到比傳統的強度型功率控制機制更好的系統效能。另一方面，我們更進一步分析TCPC功率控制機制在考量有功率控制誤差的情形下的效能表現。所導出的公式可以相當準確地算出其系統容量。 接著，我們提出一種AM-CF(Approximation Method by Characteristic Function)估算方法，用以估算DS/CDMA蜂巢式系統中干擾功率的機率分佈。干擾功率是分析DS/CDMA蜂巢式系統效能的重要因子。雖然一般所常用的高斯近似法可以很容易地套用到複雜的系統，但是高斯近似法被發現並不是很準確，造成所據以分析出來的系統容量都是過於樂觀的值。所提出的AM-CF估算方法可適用於考量了多路徑衰耗和具有功率控制的複雜環境，片元波形可以是方波形式或sinc形式。與模擬結果為參考標的，AM-CF估算方法比傳統之高斯近似法有更好的估算準確度。 最後，我們提出一種分析方法，可成功地分析非理想之訊擾比型(SIR-based)功率控制機制的系統容量。透過對研究觀察所得的平均接收訊擾比獨特的性質，我們發現功率控制機制下的系統行為可以用一組線性聯立方程式表示之，並可據以解出每條上鏈的接收功率值。系統容量由接收功率值合理性的機率與平均位元錯誤率所定義並據以求得。只需該聯立方程式係數矩陣各子元素的一階級二階統計特性，並套用中央極限定理，我們成功地導出了系統容量的公式。結果顯示，分析結果與模擬結果分常接近，這顯示所提出的分析方式可以很準確地分析非理想之訊擾比型(SIR-based)功率控制機制的系統容量。

Power control is an important system requirement for DS/CDMA (direct-sequence/code division multiple access) cellular systems. In the absence of power control the effect of near-far phenomena is dominant, and the system capacity is very low. On the other hand, when the power control exists, it allows users to share resources of the system equally between themselves. In this dissertation, the system capacity of a power controlled DS/CDMA cellular system is investigated. Firstly, we investigate the system performance of a truncated closed-loop power control (TCPC) scheme for uplinks in DS/CDMA cellular systems over frequency-selective fading channels. The TCPC scheme adopts strength-based power control. In this scheme, a mobile station (MS) suspends its transmission when the short-term fading is less than a preset cutoff threshold; and otherwise, the MS transmits with power adapted to compensate for the short-term fading so that the received signal power level remains constant. Closed-form formulae are successfully derived for performance measures, such as system capacity, average system transmission rate, MS average transmission rate, MS power consumption, and MS suspension delay. Numerical results show that the analysis provides reasonable accuracy; and the TCPC scheme can substantially improve the system capacity, the average system transmission rate, and power saving over conventional closed-loop power control schemes. Moreover, the TCPC scheme under realistic considerations of power control error due to power control step size, power control period, power control delay, and MS velocity is further investigated. A closed-form formula is obtained to effectively approximate the system capacity of the realistic TCPC scheme. A closed-form formula is obtained to effectively approximate the system capacity of the realistic TCPC scheme. Next, an approximation method by characteristic function (AM-CF) method to approximate the distribution of interference in DS/CDMA cellular systems is proposed. The interference statistics is an important factor for analyzing the performance of a DS/CDMA cellular system. Although the most widely used method, the SGA (standard Gaussian approximation) method, is easy to use and applicable to a complicated circumstance, it is known that the SGA is not very accurate and leads to an optimistic analytical result of the system capacity. This method considers the effects of frequency-selective multipath fading; it also considers perfect power control and a rectangular/sinc chip waveform. The AM-CF method can yield results that fit the Monte Carlo simulation results more accurately than the conventional standard Gaussian approximation method. Finally, a novel analytical method for analyzing the system capacity of an imperfect signal-to-interference ratio (SIR)-based power control scheme is proposed. Based on investigated properties of average received SIR, a set of linear equations is derived to obtain the average received power on each uplink. The system capacity is then obtained according to the feasibility of the average received powers and the corresponding average bit error rates. A closed-form solution for system capacity is successfully derived by employing the first and second order statistics of each element in the coefficient matrix and applying central limit theorem. Results show that the analytical results are consistent with the simulation results; this implies the novel analytical method is quite accurate.