|其他題名: ||Study and analysis of hybrid automatic retransmission request(harq) mechanism in multi-cell co-channel environment in wireless communication system|
|作者: ||王庭堅;Wang, Ting-chien|
|關鍵詞: ||混合自動請求重傳;干擾;同頻帶干擾;追縱結合;高移動性;HARQ;Interference;Co-Channel;Chase Combining;High Mobility|
|上傳時間: ||2010-01-11 07:15:52 (UTC+8)|
|摘要: ||近年來由於IEEE 802.11a/b/g 無線區域網絡 (WLAN) 的成功發展，無線寬頻上網的普及化又向前一大步，但隨著時代的進步，新規格的積極發展，讓無線網路朝向可移動之行動通訊服務，並且朝著具有高速移動之行動台來做為研究和發展；例如常聽到的WiMAX為IEEE 802.16介面標準，就具有高頻寬、高傳輸速率和涵蓋範圍大等優點。雖然無線通訊有很多優點，但是伴隨而來的卻是各種的干擾和來自通道的衰減，因而會造成封包傳送的失敗，為了確保傳送時的準確性，所以有了不同的重傳機制的提出。|
重傳的機制，一般主要有二種，一種是自動回覆請求重傳(Automatic Repeat Request，ARQ)，但是ARQ的問題是他回傳的狀態報告(Status Report)，並不會在封包遺失的第一時間回傳，所以等到ARQ發現封包遺失需要重傳時已過了不小的延遲時間；所以我們探討的是另一種新的重傳機制，那就是混合式自動回覆請求(Hybrid ARQ，HARQ)。HARQ 是一種結合了前饋式錯誤修正(Forward Error Correction)與ARQ 方法的技術，可以由前一個失敗的嘗試中存下有用的資訊，供之後的解碼使用。在通訊進行時，基地台會透過每個傳輸時間間隔回傳的肯定或否定性的確認(ACK/NACK)與通道品質指標(ChannelQuality Indication，CQI)獲得目前通道的狀況資訊，進而用不同的編碼方式重新傳送，以提高重傳的成功機率。
在本篇論文建構的系統中，會探討無線通訊從低速到高速，在同通道干擾的影響之下，所必需要採行的混合式自動回覆請求的機制為何；本論文使用的自動回覆請求機制，首先是以原始的Type I型配合AMC和新型的Type I型(Chase Combining)的HARQ為主，前者在重傳的時候，會傳和原本Packet一樣大的資料，而舊的傳送資料，則予以刪除，重傳的資料配合較低的調變，可以達到減少錯誤發生的可能；至於Chase Combining，他重傳的Packet，也是和原本的一樣大，但是不同的地方在於舊的資料不會被刪去，會保留在Symbol Level的地方做Soft Combining，來增加解碼的效能；而最終重傳機制選擇的方式為何，是根據模擬時系統規格的要求，像是重傳次數限制、重傳暫存器大小限制，PER(Packet Error Rate)小於10%等限制為我們選擇的要件。
In recent years due to the successful development of wireless local area network such as IEEE 802.11-a,-b and -g, the applications and services of wireless broadband Internet have become widely populated. As time evolves and extensive development of new specifications the wireless network services have been expanded toward the mobile communication services. Its evolution is in the research and development of the services when the mobile is moving in the high mobility environment; for example, the development of WiMAX interfaces for the IEEE 802.16 standards that possess the advantages of high-bandwidth, high transmission speed and wide area coverage. Although there are many advantages of wireless communication, it is accompanied by a variety of interferences and attenuation influences when the data is transmitted through the communication channel that would cause the failure detection at the receiver end of the packet transmitted, and in order to ensure an acceptable system performance several packets retransmission techniques have been proposed.
In this study we consider a basic cellular communication system that the system performance is seriously affected by the Co-channel interference (CCI) from other cells operating at the same frequency and propose a complete system simulation model that it not only includes the common simulation model as discussed in the wireless communication system such as the shadowing fading due to large-scale attenuation, path-loss attenuation and small- scale Rayleigh fading etc. but also includes the model due to Co-channel interference effect.
There are two kinds of retransmission methods, one is the automatic repeat request (ARQ) technique, and in this retransmission method if a packet is lost this packet lost information will not be reached at the transmitting side instantaneously because of the propagation delay of the transmission of this information in the feedback channel. It needs to have some packets ‘re-arrangement and management’ processes at the transmitter/receiver side. The other retransmission mechanism, hybrid automatic repeat request (HARQ), is a technique that combines the forward error (FEC) correction technique and the ARQ process. In this retransmission technique when a packet is failed in the decoding process it is not deleted as in the ARQ retransmission but is saved to be used with the later received retransmitted new data Furthermore in HARQ retransmission, the adaptive modulation and coding technique (AMC) is exploited that different modulation method and/or different coding rate can be adopted in the packets initial transmission and retransmissions depending on the channel quality and system performance requirement to possibly improve the system throughput.
In this study we propose the system architecture for HARQ retransmission in a wireless communication system when a mobile moves in low and high mobility and suffers co-channel interference during the packet transmission. Two retransmission mechanisms are considered in the architecture, one is the combination of the conventional Type I retransmission with AMC and the other is the new Type I retransmission, Chase Combining. In the first HARQ retransmission mechanism the retransmitted packet will have the same size as the initially transmitted packet and when the packet is retransmitted it will be transmitted at lower modulation level to enhance the retransmitted packet quality; meanwhile the original failed decoded packet will be deleted at the receiver side. In the Chase Combing retransmission the retransmitted packet will also have the same size as the initially transmitted packet the failed transmitted packet will not be deleted but be kept at the symbol level for later used in the performing of Soft Combining to improve the decoding efficiency. It depends on the system requirements and system specifications such as the maximum retransmission number allowed, the buffer size in the store of the retransmitted packets and packet error rate (PER) etc. to determine which retransmission mechanism will be exploited.