多封包接收無線網路之上鏈媒體存取控制協定設計

Abstract

本論文主要探討多封包接收（MPR）無線網路之上鏈媒體存取控制（MAC）協定設計，藉由一個簡單的旗標位元機制，吾人提出適用於多封包接收無線網路之多群優先佇列（MGPQ）媒體存取控制協定。多群優先佇列協定能夠克服現今多封包接收媒體存取控制協定設計的兩大瓶頸。首先，此法避免使用複雜的使用者狀態估測演算法，而依據封包接收狀況自動排序出使用者的可能活躍程度，進而大幅降低演算複雜度；其次，遷就使用者狀態估測演算法所加諸於使用者的封包阻絕（blocking）限制也隨之解除。因此，多群優先佇列協定不僅可以適用於異質使用者的環境，同時性能也優於現今的多封包接收媒體存取控制協定。模擬數據顯示，相對於常見的動態佇列（DQ）媒體存取控制協定，多群優先佇列協定最多可以提昇40%的系統吞吐量，平均而言，也有14%的改善。 本論文接著將同質（homogeneous）通道的假設推廣為異質（heterogeneous）通道環境。協力式的媒體存取控制協定在多封包接收無線網路的設計上為一具挑戰性的議題，也尚未見諸於文獻上；吾人提出協力式多群優先佇列（CMGP），以利用空間多樣性（diversity）來提昇系統吞吐量，並利用使用者空閒（idle）的時間作封包的中繼傳輸，因此沒有一般中繼方式造成部分使用者吞吐量下降的缺點。此外，吾人也以馬可夫鍊（Markov Chain）針對最差情況作分析，推導出直接傳輸受到中繼傳輸干擾所引起的吞吐量損失上界，及中繼傳輸對失敗傳輸所提供的吞吐量增益下界。藉由上述推導的封閉解，吾人將可以直接經由實體層的多封包接收矩陣，計算協力式多群優先佇列的吞吐量性能。 無線通道不可避免地受到各種衰落（fading）而惡化，儘管吾人可利用以機率密度函數為基礎的統計量，例如蜂巢邊緣可靠度、蜂巢區域可靠度等來量測其影響，但實際上由於傳統的多封包接收媒體存取控制協定必須分配通道資源給每一使用者，因此單一個具有較差通道狀況的使用者都可造成整個系統吞吐量的惡化。本論文最後從系統吞吐量最佳化的觀點，提出基於流量的動態使用者集合（DUST）演算法，進一步改善整體系統吞吐量並已模擬加以驗證。

This dissertation focuses on the medium access control (MAC) protocol design for the uplink of wireless networks with multi-packet reception (MPR) capability. Relying on a simple flag-assisted mechanism, a multi-group priority queueing (MGPQ) MAC protocol is proposed. The proposed MGPQ scheme is capable of overcoming two major performance bottlenecks inherent in the existing MPR MAC protocols. First, the proposed solution can automatically produce the list of active users by observing the network traffic conditions, remove the need of active user estimation algorithm, and thus can largely reduce the algorithm complexity. Second, the packet blocking constraint imposed on the active users for keeping compliant with prediction is relaxed. As a result, the proposed MGPQ is not only applicable to both homogeneous and heterogeneous (in traffic) cases, but also outperforms the existing MPR MAC protocols. Simulation results show that the network throughput can be improved by 40% maximum and 14% average as compared with the well-known dynamic queue (DQ) MAC protocol. Subsequently the homogeneous channel is generalized into heterogeneous channel. MAC protocol design for cooperative networks over MPR channels is a challenging topic, but has not been addressed in the literature yet. In this dissertation, we propose a cooperative multi-group priority (CMGP) based MAC protocol to exploit the cooperation diversity for throughput enhancement over MPR channels. The proposed approach can bypass the computationally-intensive active user identification process. Moreover, our method can efficiently utilize the idle periods for packet relaying, and can thus effectively limit the throughput loss resulting from the relay phase. By means of a Markov chain model, the worst-case throughput analysis is conducted. Specifically, we derive (i) a closed-form upper bound for the throughput penalty of the direct link that is caused by the interference of concurrent packet relay transmission; (ii) a closed-form lower bound for the throughput gain that a user with packet transmission failure can benefit thanks to cooperative packet relaying. The results allow us to investigate the throughput performance of the proposed CMGP protocol directly in terms of the MPR channel coefficients. Simulation results confirm the system-wide throughput advantage achieved by the proposed scheme, and also validate the analytic results. Wireless channel is inevitably degraded with many kinds of fading. Probability density function based statistics, e.g. cell edge reliability and cell area reliability, are used to measure the effect of shadowing. However, in practice even one user with poor link may severely degrade the system throughput, because the central controller (CC) needs to allocate channel resource for such an inefficient access. To overcome the above problem, we propose a dynamic user set based on traffic (DUST) algorithm aiming for uplink throughput optimization in wireless networks with multi-packet reception. Numerical results show significant improvement in the network throughput.