在無線通訊中，若是接收端同時接收到兩個或以上的訊號，接收功率最高的訊號之訊號雜訊比(SINR)若是超過一定的門檻值(Threshold)，則此訊號可以成功被接收端所接收，這個現象稱為捕獲效應(Capture Effect)。本論文在IEEE 802.11 無線區域網路的系統架構下，提出一個結合傳送端功率控制(Transmit Power Control)及計數倒退機制(Backoff Mechanism)的創新設計，以彰顯捕獲效應並提高系統整體性能。

在本論文研究中，我們將一個無線區域網路分成三個等面積區域，並且給予各區域一個目標的接收功率。各用戶必須要使訊號傳送到AP時達到該區域對應的目標接收功率，因此AP同時接收到兩個或以上不同接收功率的訊號時，能以捕獲效應成功接收高功率訊號。由於捕獲效應作用，功率較高的用戶相對低功率的用戶會有較大的機會成功傳送訊號，影響用戶的公平性。為了降低此不公平的現象，本論文修訂計數倒退機制(Backoff Mechanism)，給予低功率的用戶比較佳的競爭參數，以提高其被成功接收的機率。經由這個創新的傳送端功率控制以及計數倒退機制，我們可以使網路系統的整體吞吐量(Throughput)大幅提升，並且解決捕獲效應所造成的公平性問題。

In wireless communication, when two or more signals are received by a receiver simultaneously, if the strongest signal’s SINR exceeds a threshold, it can still be successfully received. This phenomenon is called Capture Effect. In this thesis, we propose a joint transmit power control and backoff mechanism in IEEE 802.11 WLAN to exploit Capture Effect and enhance overall system performance.

In the proposed scheme, we divide the network into three non-overlapped areas and assign different target received powers for each areas. Stations in each area should tune their transmission powers to meet the target received power at Access Point (AP). Thus, when AP receives two or more signals with different power levels, it may receive correctly the signal with highest power level due to Capture Effect. Moreover, the backoff mechanism of IEEE 802.11 is adjusted in the scheme in consideration of fairness issue. The main idea is that we assign the stations with lower received power level a better contention probability to access the channel to compensate their loss due to Capture Effect. The simulation results show that the proposed design can get much higher throughput than the legacy IEEE 802.11 design with slightly loss on fairness.

References

[1] IEEE 802.11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. (2012 revision).

[2] G. Bianchi, “IEEE 802.11-saturation throughput analysis, “ IEEE Commun.
Letter., vol. 2, no. 12, pp. 318–320, Dec. 1998.

[3] Vukovic, I.N. ; Motorola Inc., IL ; Smavatkul, N. “Impact of capture and varying transmit power levels on saturation throughput in IEEE 802.11“ Personal, Indoor and Mobile Radio Communications, 2005. PIMRC 2005. IEEE 16th International Symposium on (Volume:3 )

[4] Patras, P. ; Hamilton Inst., Nat. Univ. of Ireland Maynooth, Maynooth, Ireland ; Hanghang Qi ; Malone, D. “Exploiting the capture effect to improve WLAN throughput” World of Wireless, Mobile and Multimedia Networks (WoWMoM), 2012 IEEE International Symposium

[5] J. Yee and H. Pezeshki-Esfahani, “Understanding Wireless Lan Performance Tradeoffs,” Communication Systems Design, Nov. 2002.

[6] Jain, R.; Chiu, D.M.; Hawe, W. (1984). "A Quantitative Measure of Fairness and Discrimination for Resource Allocation in Shared Computer Systems". DEC Research Report TR-301.

[7] Jahanzeb Farooq, Bilal Rauf “An Overview of Wireless LAN Standards IEEE 802.11 and IEEE 802.11e”

[8] Z. J. Haas and J. Deng, ”On Optimizing the Backoff Interval for Random Access Schemes”, IEEE Transactions on Communications, vol. 51, no. 12, 2003

[9] X. Li and Q. Zeng, “Performance Analysis of the IEEE 802.11 MAC Protocol over a WLAN with Capture Effect,” IPSJ Digital Courier, 2005.

[10] S. Ganu, K. Ramachandan, M. Gruteser, I. Seskar and J. Deng, “Method for restoring MAC layer fairness in IEEE 802.11 networks with physical layer capture,” in Proc. ACM REALMAN’06, 2006.

[11] S. Han, T. Nandagopal, Y. Bejerano and H. Choi, “Analysis of spatial unfairness in wireless LANs,” in Proc. IEEE INFOCOM’09, 2009.

[12] M. F. Uddin, C. Rosenberg, W. Zhuang, and A. Girard, “Joint configuration of routing and medium access parameters in wireless networks,” in Proc. IEEE Globecom, pp. 1-8, Dec. 2009.

[13] A. Iyer, C. Rosenberg and A. Karnik, “What is the right model for Wireless channel Interference?” IEEE/ACM Trans. on Networking, vol. 8, no. 5, May 2009.