無線通訊系統吞吐量最大化之研究

在無線通訊系統中，發射機通常都會在一個時變的通道環境下傳送訊號給接收機。為了在時變通道環境下達到最佳的系統效能，鏈接調適(Link Adaptation)是一個重要的機制。在現代的無線通訊系統中，可適性調變編碼(Adaptive Modulation and Coding，AMC)與混合自動重傳(Hybrid Automatic Repeat Request，HARQ)機制是兩個重要的鏈接調適方法。可適性調變編碼可根據通道狀況來調整調變編碼組合(Modulation and Coding Scheme，MCS)來使系統吞吐量最大化，而混合自動重傳機制則可以提高通訊系統的可靠度。然而，在實際的系統實作上，可適性調變編碼會根據一些事先訂定好的閥值來進行調變編碼組合的切換，但這些事先訂定好的閥值在時變通道下不一定是最佳值，如此一來便會造成系統吞吐量的損失。本論文提出了一個在時變系統下可調整可適性調變編碼的閥值之方法，來達成系統吞吐量最大化的目的。此方法藉由可適性調變編碼與混合自動重傳的特性與回饋訊號，來統計系統吞吐量並調整閥值，以期追蹤最佳之系統閥值，使系統吞吐量最大化。本論文所提出之方法可在各種通訊系統下實作，本論文亦提出兩個實施例：在無線中繼(Relay)系統與多媒體廣播群播服務系統(Multimedia Broadcast Multicast Service，MBMS)下，都可以顯著地提升系統效能。

關鍵字

吞吐量最大化；可適性調變編碼；混合自動重傳；閥值調整；無線中繼系統；多媒體廣播群播服務系統

並列摘要

In wireless communication systems, transmitters always have to transmit data to receivers under time-varying channels. To achieve optimal performance under time-varying channels, link adaptation is an important mechanism in wireless communication systems. Adaptive modulation and coding (AMC) and hybrid automatic repeat request (HARQ) are two important schemes for link adaptation in modern wireless communication systems. AMC can adjust modulation and coding schemes (MCS) according to immediate channel condition so that the system throughput can be maximized, and HARQ can improve system reliability by retransmitting data packet when the packet is incorrectly decoded. However, in practical system design, the AMC is usually implemented with fixed switching thresholds for MCS, and suffers from performance degradation when channel is varying. Although HARQ is designed to offer robust transmissions and hence alleviate the performance degradation due to the use of unfavorable MCS, it also introduces the problem of retransmission latency. In this dissertation, the problem of throughput maximization is investigated for common wireless communication systems. In addition, The method adaptively adjusts the AMC thresholds according to the HARQ acknowledge (ACK) feedback information in each transmission round so that the AMC mechanism can select the best MCS with nearly optimal thresholds. To verify the capability of the method proposed in this dissertation, this method is applied in two emerging technologies. The first one is wireless relay systems. The wireless relay system contains broadcast and multiple access channels, and therefore the throughput maximization method is difficult to design. After carefully investigating the channel characteristics, the proposed method is modified for wireless relay systems. The second one is Multicast Broadcast Services (MBS), or Multimedia Broadcast Multicast Services (MBMS) in 3GPP Long Term Evolution (LTE) systems. The properties of the MBS are considered in detail, and then the proposed method is modified to fit the technology so that the system throughput can be optimized. Simulations for different system settings are conducted to verify the performance of the proposed method. Simulation results show that the throughput of the systems are indeed improved by the proposed method under a variety of system assumptions. The proposed method is very easy to implement, and can perform dynamic optimization each time the channel statistics change.

並列關鍵字

Throughput maximization ； adaptive modulation and coding (AMC) ； hybrid automatic repeat request (HARQ) ； threshold adjustment ； wireless relay system ； multimedia broadcast multicast system (MBMS)

參考文獻

[1] High Speed Downlink Packet Access: Physical Layer Aspects. 3GPP TSG RAN WG1 Std. TR 25.858 V5.0.0, 2002-2003.

[2] Air Interface for Fixed Broadband Wireless Access Systems. IEEE Std. 802.16-2004, 2004.

[3] Feasibility Study for Orthogonal Frequency Division Multiplexing (OFDM) for UTRAN Enhancement. 3GPP TR 25.892 V6.0.0, June 2004.

[4] Physical Layer Standard for cdma2000 Spread Spectrum Systems. 3GPP2 Std. C.S0002-D V1.0, 2004.

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

國際替代計量

無線通訊系統吞吐量最大化之研究

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