Performance Analyses for Power-Controlled Wideband CDMA Systems

• Main Authors: Jeng-Shin Sheu, 許正欣
• Other Authors: Jyh-Horng Wen
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/73434179566884567377

博士 === 國立中正大學 === 電機工程研究所 === 90 === In a code-division multiple access (CDMA) cellular system, all frequency and time resources are allocated to all subscribers simultaneously. Thus, the performance and capacity of such cellular system are mainly limited by cochannel interference. Transmitter power control (TPC) is very important for cellular CDMA systems. It is used to determine the lowest possible power level for maintaining a required signal quality in order to minimize the interference in the system. The objective of this dissertation is to analyze the performance of power-controlled multimedia CDMA systems. The term multimedia, here, is defined as multiple SIR (signal to interference ratio) targets because the concept of SIR is widely used for the performance measure of power control. The mobile cellular system is primarily constituted by physical and MAC layers within the framework of OSI layers. Thus, we would analyze both physical-level and MAC-level performance for power-controlled multimedia CDMA systems in this dissertation. In the first place, the performance bound for multimedia TPC is investigated. This performance bound is attained from an optimum SIR-balanced TPC together with a removal algorithm. Next, we reformulate the downlink portion of this optimum SIR-balanced TPC in an alternative method. Computer simulations show that the alternative downlink TPC has a performance identical to that of this optimum SIR-balanced TPC. However, the alternative downlink TPC has extremely low complexity as compared to that of the original optimum TPC. Exploiting this alternative TPC, we proposed a hierarchical structure which is feasible for carrying out the eigen-decomposition necessitated by the original optimum SIR-balanced TPC. Moreover, the linear prediction method and adaptive on-off strategy were proposed to tackle both unbalanced SIR and global outage, respectively. Differential coding is proposed to reduce the number of bits required for the transmission of power-controlled commands. In the sequel, the attention is turned to the derivation of the physical-layer performance for power-controlled CDMA systems. 3GPP and cdma2000, are analyzed by a unified approach. Analytical results prove that RAKE receiver and power control strategy are really necessary for improving the performance of CDMA systems. The RAKE receiver collects multipath signals to achieve a better performance. The contribution of the multipath reception is significant especially in the case of small path-strength decay factors. Even for the worst case of power control, our analyses indicate that the CDMA system with power control still outperforms that without employing power control scheme. Finally, we evaluate the capacity for spectrally overlaid power-controlled CDMA systems. The spectrally-overlaid performance over coexistent CDMA systems involving cdma2000 (wideband) and IS-95 (narrowband) is investigated. It was shown that the overlaid performance is not only dependent on the shape of transmission pulse, but on the distribution of IS-95 (narrow-band CDMA, NCDMA) subscribers over the whole available frequency band. This observation suggests that we should locate the NCDMA users over the whole frequency band carefully in order attain a better coexistence performance.