Cooperative WLAN Protocols for Multimedia Communication

While the demand for multimedia traffic over WLANs increases rapidly, the existing WLANs approach their limits. It is known that multiuser cooperative communication can enhance performance of wireless networks. It can substantially increase the spectral efficiency of wireless networks by utilising interference rather than avoiding it. This paradigm shift has most impact on the medium access control (MAC) protocol. Most existing MAC protocols are designed to reduce the interference. In this project, the focus is on the WLAN and its limitations to utilise cooperative communication. The WLANs are originally designed for point-to-point links with data traffic and in order to support variety of applications with different requirements further enhancements are needed.

This project is divided into two main parts. In the first part, the fundamental bounds of the existing WLANs with respect to PHY and MAC layers are studied. Outage probability, throughput and delay have been chosen as the main performance metrics. The results in this part present some of the existing problems in WLANs including relatively small coverage range, low efficiency, and some fairness issues. Possible relaying methods as a way to improve the coverage range or the data rate of a WLAN are also considered in this part. In the second part of the project, the focus is on the possible enhancement to improve the WLAN performance. We begin this part by first looking into a multiuser single-hop scenario and afterwards studying three advanced PHY relaying methods: multiuser decode-and-forward, two-way, and multiuser zero-forcing relayings. It has been shown that these methods can improve the spectral efficiency. However, these methods as well as many other cooperative techniques require simultaneous multiuser transmissions and hence cannot be supported by the current IEEE 802.11 systems using carrier sense multiple access with collision avoidance (CSMA/CA). In this part, a novel cluster-based MAC as extension of CSMA/CA is proposed to facilitate these cooperative techniques by utilising the multiuser interference mitigation capability of PHY.

According to the proposed cluster-based CSMA/CA (CB-CSMA/CA) scheme, nodes in a network, including stations and relays, are allocated to different clusters. The nodes, which belong to the same cluster, are allowed to transmit at the same time, provided that there are enough degrees of freedom available to efficiently decode the desired stream at the destina- tions. We consider the impact of signal processing on the MAC protocol design, and explain the basics of CB-CSMA/CA and the required modifications of the existing IEEE 802.11 MAC to facilitate it. We apply the proposed CB-CSMA/CA to the above-mentioned relaying scenarios. The typical uplink/downlink transmissions in a WLAN with a multiple-antenna access point can also be considered as a subset of the first scenario, i.e. multiuser decode- and-forward relaying scenario. We analyse the network performance with respect to both MAC and PHY parameters. This leads to more comprehensive results compared to the cases where only PHY or MAC parameters are studied. The results show that the CB-CSMA/CA improves throughput and delay significantly compared to the current WLAN systems.

After studying the performance of CB-CSMA/CA with some ideal assumptions, we ex- tend our study to more realistic scenarios and take into account some cross-layer enhancements of the CB-CSMA/CA. We consider three possible backoff procedure models and compare their performances in the presence of PHY channel errors. Up to this point, we have only considered scenarios where all cluster members set their contention window to the same value, i.e. they are all perfectly event-synchronised, and therefore transmit simultaneously whenever they access the channel. We extend our study by looking into scenarios where stations in a cluster are not anymore perfectly event-synchronised and hence they may set their contention window to different values. We show analytically that in the worst case scenario, i.e., when all stations in all clusters are asynchronous, the CB-CSMA/CA throughput gets close to that of the IEEE 802.11 system. In fact, with the same data rate, header durations and backoff model, throughput of the CB-CSMA/CA is always better than that of the IEEE 802.11. We also consider different clustering methods, where stations are grouped into different clusters based on their channel conditions or higher layer requirements.

So far, only one- or two-hop communication links are considered. As a next step, an outlook on multihop communications and possible extensions of the CB-CSMA/CA for the multihop links are investigated. In this project we show that the proposed CB-CSMA/CA is a promising approach for a variety of network configurations including typical infrastructure WLANs as well as ad hoc cooperative networks with or without relays. The proposed scheme is simple but powerful and flexible. The network performance results show that throughput and delay of an IEEE 802.11 system can be significantly improved when the CB-CSMA/CA is applied. Analysis and discussion confirm that the performance can be further improved if we can take some cross-layer parameters into account while choosing the backoff model or forming the clusters.