階層式分碼多工系統在多媒體服務下導向性功率之設計與分析

Abstract

由於良好的抗干擾與有效使用頻譜等特性，分碼多工存取系統（CDMA）已成為第三代無線通訊裡最有潛力的技術。本論文之目的在藉由推導理想的導向性功率（Pilot Power）數學模式以求得其最佳的導向性功率比例，以便能夠增大系統之容量。從涵蓋範圍的觀點來看，較大的導向性功率可以有較大的涵蓋範圍。然而，若從功率分配的觀點來看，越大的導向性功率會使得負載通道的信號分配到比較小的功率。因此，在導向性功率與傳輸功率中間存在一個最佳的比例，能讓系統的容量達到最大。 在下一代的無線通訊需求中，隨著數據使用量的提升，包含上網瀏覽WWW、接收信件E-mail、影像傳輸等多樣化的服務需求將加入原本只有單純語音服務的行列。在這樣多媒體服務的環境下，為滿足不同服務的品質需求 (QoS)，將使得資源管理變得更為複雜。也由於多媒體服務的產生，系統中存在有某個小區域擁有較高負載的這種情形將常常發生。在此種異質性網路下，系統功率調配的技術便須要重新考量。

Heterogeneous cellular architectures have posed a new challenge on the design for the CDMA system. This paper presents an analytical framework to determine the optimal pilot power ratio with the goal of maximizing the CDMA system capacity with different cell sizes and structures. From the coverage perspective, larger pilot power leads to a larger coverage area. However, from a power budget standpoint, the more the pilot power for the control channel, the less the signal power for the traffic channels. Thus, there exists an optimal ratio between the pilot power and the traffic signal power from the capacity maximization perspective even in a single cell environment. Furthermore, in the environment with multiple cells, a large pilot power may indirectly cause stronger interference to other neighboring macrocells or the embedded microcell due to higher transmission power from the user terminals. The mobile terminals need to increase transmission power to maintain the uplink signal quality when they are located at the cell boundary of a larger cell due to a larger pilot power. Consequently, it is important to incorporate the inter-cell interference into the design of pilot power for the CDMA system. In this paper, we derive analytical expressions for the optimal pilot power ratio in both the homogeneous cellular systems and the heterogeneous microcell/macrocell overlaying systems. Our proposed analytical framework takes account of the impact of co-channel interference, non-uniform user density, and cell sizes. Compared to the traditional fixed pilot power ratio method in a hierarchical cellular system, our results show that the proposed analytical pilot design approach can increase the capacity by 23.1\% with non-uniform user density and by 16.9\% with uniform user density, respectively.