多媒體群播廣播服務下回饋量縮減與吞吐量最大化

隨著對於無線的群播與廣播服務需求的增加，MBMS (Multimedia Broadcast Multicast Services) 為此類傳輸提供了更有效率的方式，在LTE (Long-Term Evolution) 標準中亦使用MBMS作為群播與廣播服務的協定。對於群播與廣播的傳輸，因為系統希望能盡可能地服務訂閱相同服務的用戶，理論上每個用戶需要回傳其完美通道狀態資訊 (CSI, Channel-State Information) 或其他資訊，以便讓基地台能夠選擇傳輸時最適當的調變與編碼方式 (MCS, Modulation and Coding Scheme) 而能盡可能地提高傳輸速率與吞吐量 (throughput)。然而，隨著同一群的用戶數增加，此要求用戶回傳的回饋量同時地產生線性增加的負荷。但在MBMS的特性下，即系統希望能盡可能的服務訂閱相同服務的用戶，許多的回饋量並不會影響到系統效能，但卻顯著地浪費了許多回傳所需的功率與頻寬。在同一群中通道狀況較好的用戶，相對於通道狀況較差的用戶較不容易影響到整個系統的參數決定與效能。在這篇論文中，我們探討了在同一群用戶中，根據猜測自身排名的方式決定需不需要回傳通道狀態資訊給基地台。對用戶而言，若猜測自身排名較低則回傳，較高則不予回傳，進而達到回饋量縮減的目的。在實作上，我們先根據通道統計特性制定出一系列的門檻值 (threshold)，並根據特定的準則，挑選出一個門檻值作為用戶決定是否回傳的標準。從模擬結果顯示，我們所提出的架構的確能有效減低回饋量，並能達到接近於常數的回饋量。另一方面，我們也探討了實際群播與廣播服務系統上的吞吐量最大化。首先，我們從單一用戶的角度出發，探討了若要達成吞吐量最大化所應該滿足的條件，且根據此一條件提出相對應的吞吐量最大化之適應性演算法，並更進一步將此演算法放入MBMS的架構中。從模擬結果顯示，我們所提出的適應性演算法相較於傳統架構的確能有效地提升吞吐量。

關鍵字

多媒體群播廣播服務；回饋量縮減；速率最大化；鏈結適應技術；調變與編碼方式；吞吐量最大化

並列摘要

As the increasing demand of wireless multimedia services, the multimedia broadcast multicast service (MBMS) is an emerging technology which is adopted in various wireless communication standards to enhance multimedia data transmission. In order to provide best-effort service, all users in an MBMS system need to feed back its current channel condition to the eNodeB, so that the eNodeB can choose a proper modulation and coding scheme (MCS) for transmission. Clearly, the feadback load will increase linearly as the number of users in the MBMS system increases. As a result, the spectrum efficiency is degraded because of the accordingly increased feedback bandwidth. In this thesis, we propose a threshold-based feedback scheme based on the order statistics of users in the system. From the simulation results, a nearly constant feedback load is achieved when the proposed scheme is used, and the performance approaches the conventional feedback scheme. Besides, in an MBMS system, all users subscribing the specific service receive multimedia data via the broadcasting channel, which makes the link adaptation (LA) for the system more difficult. In this thesis, we propose an adaptive threshold-adjustment algorithm in order to maximize the MBMS system throughput. This algorithm enhances the performance of the adaptive modulation and coding (AMC) according to the instaneous channel condition. Moreover, this low-complexity algorithm is easy to implement on the user side. From the simulation results, we show that the proposed method can effectively improve the MBMS system performance.

並列關鍵字

Multimedia broadcast multicast service (MBMS) ； feedback reduction ； rate maximization ； link adaptation (LA) ； adaptive modulation and coding (AMC) ； throughput maximization

參考文獻

[1] Multimedia Broadcast/Multicast Service (MBMS); Stage 1 (Release 11). 3GPP TS 22.146 Version 11.1.0, May 2013.

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[4] W. Yu and J. Cioffi. Sum capacity of gaussian vector broadcast channels. IEEE Trans. Inf. Theory, 50(9):1875–1892, September 2004.

[5] T. Yoo and A. Goldsmith. On the optimality of multiantenna broadcast scheduling using zero-forcing beamforming. IEEE J. Sel. Areas Commun., 24(3):528–541, March 2006.

[6] T. Yoo, N. Jindal, and A. Goldsmith. Multi-antenna downlink channels with limited feedback and user selection. IEEE J. Sel. Areas Commun., 25(7):1478–1491, September 2007.

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多媒體群播廣播服務下回饋量縮減與吞吐量最大化

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