LTE下多媒體廣播多播服務單頻網NACK-based重傳機制

Abstract

行動多媒體因為有了PSS串流而有了更多的發展，而因為不斷成長的行動多媒體用戶，串流的使用會造成下傳頻寬被大量佔用。為了讓下傳頻寬更有效的被使用，因而提出了多媒體廣播多播服務的概念。在LTE下多媒體廣播多播服務甚至可以更進一步地利用單頻網技術有效加強訊號的涵蓋範圍以及強度。這樣子的多媒體廣播多播服務將會以遠小於串流技術的系統頻寬來提供用戶多樣化多媒體服務。 然而在這樣子的廣播多播架構下，以往常被用來增加資料可靠度的重傳技術卻很難被實現。因為在廣播多播的架構中，會存在著大量的用戶，在這樣子的情形下單播所使用的重傳技術將會造成下傳頻寬的浪費。在本篇論文提出了一個結合回饋通道以及將行動用戶群組化的重傳機制，以求在行動用戶的資料區塊錯誤率及觸發重傳的機率上做一個平衡。接著，我們也提出了兩種的回饋通道配置方式讓回饋通道可以有更靈活的使用方式，其一是每一群組都有特定通道；另一是有兩群組分享一個通道。另外，我們使用了兩種方法讓基地台判定是否需要重傳，其一是基於閾值上的判定，只要所收到的NACK超過閾值，重傳就會被觸發。另一是基於機率上的判定，當基地台收一個NACK，將會有一定的機率會觸發重傳。在模擬中，本論文的方法確實在可接受的行動用戶的資料區塊錯誤率上大幅降低觸發重傳的機率。除此之外，實際吞吐量(goodput)也遠大於我們所比較的對象。

Due to the continuously increasing number of mobile multimedia uses, packet switch streaming would make much downlink resource be occupied. In order to make downlink resource be taken advantage more efficiently, multimedia broadcasting/multicasting service (MBMS) is proposed. In LTE, single frequency network can be further utilized to reinforce the coverage and the power of transmitting signal of MBMS. However, retransmission technique which is usually used to increase data robustness is hard to realize in broadcasting/multicasting. If unicast retransmission technique works in broadcasting/multicasting, it would result in the waste of downlink resource. In this thesis, we propose the retransmission scheme which takes advantage of feedback channel and grouping of mobile users to strike a balance between data block error rate and retransmission rate. Additionally, we propose two kinds of feedback channel assignment to pursue more flexible assignment. We have two ways to let base station judge whether the retransmission will be performed or not. First one is based on threshold, and the second one is based on probability. In simulation, the proposed scheme indeed decreases retransmission rate with acceptable data block error rate. Besides, goodput of proposed is larger than the compared schemes.