Synergy MAC: A Cooperative MAC Protocol
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Since the advent of IEEE's 802.11 standard, Wireless Local Area Networks (WLAN) have gained widespread acceptance in providing broadband wireless access to portable devices. The performance of any WLAN depends largely on the nature of the wireless environment available to its participating nodes. This is because radio waves experiencing interference and fading effects severely affect the overall throughput achieved by the wireless network. Although spatial diversity is known to minimize the ill effects of channel fading, realizing this form of diversity generally requires incorporation of newer technologies such as Multiple Input Multiple Output (MIMO) systems. But it is impractical to equip every node in a WLAN with multiple antennas, primarily due to their size and energy constraints. Recent research on cooperative communication demonstrates that spatial diversity can also be achieved by exploiting some unique properties of the underlying wireless medium. The inherent broadcast nature of the wireless channel suggests that any signal transmitted on the medium can be overheard by all nodes within the receiving range. If such nodes were to retransmit the overheard signal towards the destination rather than discarding it completely, they would effectively provide the destination with extra observations of the source signal. These observations at the destination are all dispersed in space and are akin to the observations resulting from MIMO systems. In short, one can think of a cooperative system as a virtual antenna array, where each antenna in the array corresponds to one of the assisting neighbors. However, to exploit the spatial diversity realized at the physical layer, the idea of node cooperation needs to be extended to other layers of the protocol stack, especially the MAC sub-layer. Further, if such cooperation aware MAC sub-layer is designed to be backward compatible with 802.11b, then even devices with legacy hardware could potentially derive its benefits. In addition to interference and fading effects, nodes in WLANs can also suffer from fairness problems resulting from multi-rate modulation scheme employed by IEEE's 802.11b standard. Studies have shown that when all nodes in a WLAN have uniform traffic to/from the access point (AP), the lower data rate nodes will use much more channel time than the higher data rate nodes resulting in two negatives: not only do the lower data rate nodes get poor service, they also reduce the bandwidth of the higher data rate nodes. This in turn reduces the effective throughput of the entire network. Researchers have proposed a multi-hop extension to IEEE's 802.11b infrastructure mode to mitigate such fairness problems. In this work, we focus on the design of a new IEEE 802.11b compatible cooperative MAC protocol that also incorporates multi-hop techniques to counter fairness problems discussed above. With the studied protocol, labeled Synergy MAC, low data rate nodes transmit their packet first to an intermediate node which in turn forwards the packet to the destination at rates higher than otherwise possible. By ensuring that the destination receives two copies of the original transmission, Synergy MAC is able to realize spatial diversity. Performance of Synergy MAC is validated by means of extensive simulations using ns-2 network simulator. Results show that the proposed protocol is able to deliver a superior performance in comparison to IEEE's 802.11b.