小細胞行動網路中合作式傳輸與重傳機制之跨層設計與效能分析

Abstract

此論文研究在小細胞行動網路中合作式傳輸與重傳之跨層設計與效能分析。首先，我們設計了三種搭配投機分散式空時編碼之合作式中繼重傳機制，此三種機制具有不同的系統複雜度、可提供不同的時間與空間多樣性。利用失效(outage)分析的手法，探討此論文提出之重傳機制可提供的多樣性與吞吐率；中繼站的使用效率則是從每使用多一個中繼站可提升的訊號品質來衡量。儘管在小細胞網路內會有大量潛在可能提供幫助的中繼站，分析的結果顯示僅需要投機式地選擇兩個中繼站使用分散式空時編碼即可在至多兩次的重傳次數之內顯著地提升系統吞吐率。 承襲此結果，我們從跨層設計的角度持續探討合作式重傳機制之優點，為此，我們在TDD LTE-A系統的訊框架構設計實現前言提及之合作式重傳機制的方式。在此我們使用一個簡化的分析模型討論移動性對下行(downlink)吞吐率造成的影響，在不同的移動條件、考量中繼站之暫存器要能穩定與傳輸功率的限制之下，挑選基地台以及中繼站的傳輸速率來最大化下行吞吐率。當使用者在室內、或是用步行速度在移動時，分析與模擬的結果顯示讓所有中繼站偷聽(overhear)來自其餘中繼站的傳輸訊號、並在成功解調之後加入可被投機式挑選並幫忙重傳的候選中繼站集合，此機制可大幅增進吞吐率。當使用者以每小時大約30公里的中等速度移動時，結果顯示只要在原來的候選中繼站集合裡每次重選中繼站幫忙重傳訊號即可提供良好效能。在高速移動(超過每小時80公里)時，較實際且有效的方式是讓候選中繼站集合裡的所有中繼站一起使用分散式空時編碼幫忙重傳。

In this dissertation, the cross-layer design on the coordinated transmission and retransmission protocol and the performance analysis are provided in mobile small cell networks (SCNs). Firstly we design three opportunistic multipoint relaying mechanisms with using opportunistic distributed space-time coding (ODSTC) for coordinated Automatic-Repeat-reQuest (ARQ) according to the protocol complexities for temporal and spatial diversity exploration in retransmissions. Based on an analytic approach of outage analysis, the effectiveness of the proposed ARQ protocols is studied from the perspectives of diversity and throughput performance, and the relay efficiency is investigated from the viewpoint of signal quality improvement per active relay. In spite of a great amount of potential helpers (relays) in SCNs, the numerical results show that opportunistically choosing 2 active relays for ODSTC in at most two rounds of retransmissions is good enough to yield a significant throughput improvement. Based on this inspiring result, we continue to exploit and study the advantage of the coordinated ARQ protocols from a cross-layer perspective. The proposed ARQ protocols are implemented under the frame structure of the long term evolution advanced (LTE-A) time division duplex (TDD) system, and a simplified analytical model is provided to study the mobility impact on downlink (DL) throughput. Subject to the relays’ queue stability and energy constraint, the data rate of the base station and the active relay(s) are selected to maximize the DL throughput under different mobility conditions. The numerical and simulation results show that enabling relay overhearing and opportunistically reselecting relays at every round of retransmission can significantly improve the throughput when the users are in the indoor environment or moving at a walking speed. If the users are traveling at a medium vehicular speed (~30km/h), reselection only is good enough to provide a throughput gain. To the users who travel at a high speed (>80km/h), a more realistic and efficient method is to use all the candidate relays to forward signals with distributed space-time coding (DSTC) together.