CSMA/CA無線網路系統上空間重複使用率與傳輸可靠度之整合研究

Abstract

在CSMA/CA架構與多重速率、多點跳躍的無線網路環境之下，我們可以藉由根據環境的變化動態調整載波偵聽門檻(carrier-sensing threshold, Tcs)，並進一步平衡空間重複使用率(spatial reuse)及傳輸可靠度(transmission reliability)來達到整體網路效能(network capacity)的最佳化，較低的Tcs會使偵聽範圍(carrier-sensing range, dcs)擴大，而在此情況下能夠經由降低因並行傳輸(concurrent transmissions)造成的封包碰撞以提高資料傳輸的可靠度，然而，整體的網路效能(network capacity)卻會因為節點對於資料傳輸的嘗試太過於保守而下降；反之，較高的Tcs則會使dcs縮小，這樣一來空間重複利用率會得到顯著的提高，但相對的潛在訊號干擾亦會隨著增加而導致封包碰撞更頻繁的發生，因此於建立在CSMA/CA基礎上的無線網路之下根據當下環境情況適當的調節Tcs是非常重要的。在本篇論文中，我們提出一個於分散式的無線環境之下可在合理的計算時間內用以動態調節Tcs之值的適應性機制，模擬結果也證實了我們所提出的演算法所求得的網路效能較其他現有的載波偵聽調節相關演算法要來得佳。

In CSMS/CA-based, multi-hop, multi-rate wireless ad-hoc networks, we can balance the interplay of spatial reuse and transmission reliability by dynamicly tuning carrier-sensing threshold (Tcs) according to environmental situations in order to reach the maximum network capacity. Lower carrier-sensing thresholds generally lead to larger carrier-sensing ranges (dcs), which ensure higher transmission reliability. In this manner, we actually increase the transmission reliability by reducing packet collisions due to mitigated interference produced by concurrent transmissions. However, the total network capacity suffers because of too preservative channel access attempts. On the other hand, higher carrier-sensing thresholds usaually result in smaller carrier-sensing ranges, which encourage better spatial reuse yet transmission collisions will happen more frequently due to potentially increased interference. Therefore, it is important to make adequate adjustments of carrier-sensing thresholds according to environmental conditions in a CSMA/CA-based wireless network. In this thesis, we address the above issue by proposing an adaptive mechanism to dynamicly adjust carrier-sensing thresholds used by wireless nodes in a distributed manner within reasonable computation time. Simulation results show that our proposed algorithm outperforms other implemented carrier-sensing tuning approaches in terms of aggregate network capacity.