Exploiting multiuser diversity with capture in wireless networks

Justin Foo

    Research output: ThesisDoctoral Thesis

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    In a wireless network, owing to the time-varying nature of wireless channels, different mobile users typically experience peaks and troughs in channel quality at different times. This diversity in channel quality is known as multiuser diversity. With the aid of rate adaptation, multiuser diversity can be exploited by allowing the mobile user with the best channel to use the channel resource. However, in order to achieve this in most practical systems, the mobile users in the network must feed back their channel state information (CSI) to the transmitting user. In large networks, this feedback overhead can outweigh the multiuser diversity gain. In this thesis dissertation, a centralised wireless medium access control (MAC) scheme, namely Multiuser Diversity with Capture (MDC), is discussed as a solution to obviate the overhead problem. MDC explicitly employs the capture effect in radio receivers to reduce network overhead by allowing multiple mobile stations (MSs) with channels better than a nominal response threshold to simultaneously compete for the wireless channel. Owing to the capture effect, the base station (BS) can determine which MS has the best channel. In comparison with the Medium Access Diversity (MAD) scheme in the literature, the proposed MDC possesses the strong merit that the feedback overhead is independent of the number of MSs in the network. Several aspects of the MDC scheme are investigated in detail. An application of the MDC scheme based on the physical layer and parts of the MAC layer of the IEEE 802.11a standard is considered. A general analytical framework for the goodput performance of MDC is derived. Using this framework, the exact closed form solution for the expected goodput of MDC with rate adaptation over Rayleigh fading channels is calculated. The fairness performance of MDC in networks where some MSs experience better average channel conditions than others is also addressed. MSs with low average channel states tend to use the channel less often in MDC than MSs with high average channel states. This issue is tackled with Fairer Multiuser Diversity with Capture (FMDC), a variant of the MDC scheme designed to share the channel resource more equitably across all of the MSs in the network. In FMDC, instead of using the network-wide response threshold to decide whether to compete for the channel, each MS only competes for the channel when their channel state is greater than a threshold factor multiplied by their average channel state. Finally, the problem of adaptive optimisation of the response threshold for MDC and the threshold factor for FMDC is also considered. In the proposed solution, the response threshold and the threshold factor are adapted heuristically according to the estimated goodput performance of the system. The adaptive heuristic has importance in practical systems because the BS usually does not know the characteristics of the time varying channels of the MSs in the network.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2008


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