進階性無線網路區塊確認演算法與細胞切換機制技術

Abstract

論文的第一部分介紹進階性無線網路區塊確認演算法。IEEE 802.11n的標準利用封包合併機制和區塊確認技術達成高吞吐量的目標，而區塊確認技術中的傳統貪婪機制利用傳送端制定了起始序列編號，並用此建立確認窗來識別封包接收的正確與否；然而，對於那些在確認窗之外的封包，無論是否正確接收，都無可避免地必須重新傳送。因此，論文中提出以接收端制定起始序列編號的貪婪快速移動區塊確認演算法，不僅表示起始序列編號之前的封包全都正確接收，也給予起始序列編號之後的封包的接收狀況。論文中根據窗利用率對兩個不同的貪婪機制建立了分析模型，從模擬的結果可以看出貪婪快速移動區塊確認演算法因為在確認窗擁有快速移動的特性，可以得到較好的表現。

論文的第二部分提出一個考慮基地台可用頻寬的進階性細胞切換機制，移動終端負責選擇適合維持服務品質的基地台執行細胞切換。傳統的細胞切換機制會根據信號接收強度選擇基地台，然而這種機制忽略了與容量相關的基地台可用頻寬，因此，那些以最大信號接收強度來選擇基地台的移動終端並不一定能得到最佳的系統容量。為了達到更高的系統容量，論文中將細胞切換的問題闡述成部分性觀察馬可夫決策過程，用來預測非服務基地台的容量資訊，根據不同的考量因素，如：信號接收強度、系統容量、換手時間、移動端的移動性，訂立了不同的效用函數。根據模擬結果可得知：與傳統的細胞切換機制相比，論文中提出的進階性細胞切換機制能得到較高的系統容量。

In this thesis, an advanced technique of the block acknowledgement will be illustrated first. The techniques of frame aggregation and block acknowledgement are utilized in the IEEE 802.11n standard for achieving high throughput performance. Conventional greedy scheme for block acknowledgement adopts the transmitter-defined starting sequence number (SSN) to construct the acknowledgement window for recognizing the correctness of data packets. However, there exists correctly received packets that lie outside of the acknowledgement window which will unavoidably be retransmitted. Therefore, a greedy fast-shift (GFS) block acknowledgement mechanism is proposed to provide the receiver-defined SSN, which can both implicitly acknowledge the correctly received packets before the SSN and explicitly identify the correctness information for the packets after the SSN. In order to evaluate the effectiveness of the GFS scheme, the analytical models for these two mechanisms are proposed based on the window utilization. It is observed from the simulation results that the proposed GFS method can provide better performance owing to its fast-shift behavior on acknowledgement window.

In the second part of this thesis, an advanced cell selection mechanism is proposed with the consideration of available bandwidth of the base stations (BSs). The mobile station (MS) should execute the cell selection to choose a suitable BS for sustaining the quality of service (QoS) in the wireless networks. The conventional scheme of cell selection is based on the received signal strength (RSS) from the BS. However, the RSS-based scheme does not consider the available bandwidth allocation of the BSs, which is closely related to the capacity of the BSs. For those MSs that choose the serving BS (SBS) according to the maximum RSS criterion, there is no guarantee that maximum system capacity can be achieved. In order to acquire higher network capacity, the cell selection problem is formulated based on the partially observable Markov decision process (POMDP), which is designed to predict the unavailable capacity information from the non-serving BSs. Various utility functions are designed to consider different factors in the proposed POMDP-based cell selection (POCS) schemes, including the RSS, system capacity, handover time, and the MS's mobility. It is shown in the simulation results that the proposed POCS schemes can outperform existing RSS-based methods with higher system capacity.