Cooperative Communication and QoS in Infrastructure WLANs
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IEEE 802.11 wireless LANs operating in the infrastructure mode are extremely popular and have seen widespread deployment because of their convenience and cost eﬃciency. A large number of research studies have investigated the performance of DCF, the default MAC protocol in 802.11 WLANs. Previous studies have pointed out several performance problems caused by the interaction of DCF in infrastructure-based WLANs. This thesis addresses a few of these issues. In the ﬁrst part of the thesis, we address the issue of head-of-line (HOL) blocking at the Access Point (AP) in infrastructure WLANs. We use a cooperative ARQ scheme to resolve the obstruction at the AP queue. We analytically study the performance of our scheme in a single cell IEEE 802.11 infrastructure WLAN under a TCP controlled ﬁle download scenario and validate our analysis by extensive simulations. Both analysis and simulation results show considerable increase in system throughput with the cooperative ARQ scheme. We further examine the delay performance of the ARQ scheme in the presence of both elastic TCP traﬃc and delay sensitive VoIP traﬃc. Simulations results show that our scheme decreases the delay in the downlink for VoIP packets signiﬁcantly while simultaneously providing considerable gains in the TCP download throughput. Next, we propose a joint uplink/downlink opportunistic scheduling scheme for maximising system throughput in infrastructure WLANs. We ﬁrst solve the uplink/downlink unfairness that exists in infrastructure WLANs by maintaining a separate queue and a backoﬀ timer at the AP for each mobile station (STA). We also increase the system throughput by making the backoﬀ timer a function of the channel gains. We analyse the I performance of our scheme under symmetric UDP traﬃc with i. i. d. channel conditions. Finally, we discuss several opportunistic scheduling policies which aim to increase the system throughput while satisfying certain Quality of Service (QoS) objectives. The standard IEEE 802.11 DCF protocol only oﬀers best-eﬀort services and does not provide any QoS guarantees. Providing QoS in 802.11 networks with time varying channel conditions has proven to be a challenge. We show by simulations that by an appropriate choice of the scheduling metric in our opportunistic scheduling scheme, diﬀerent QOS objectives like maximizing weighted system sum throughput, minimum rate guarantees and throughput optimality can be attained.