Utilizing multiuser diversity in multicast transmissions and geographic communications

Multipath propagation is a unique characteristic of wireless communication channels that causes signal level fluctuations called fading. Initially, the fading was considered an impairment to communications, but it has recently been shown that fading instead can be exploited to improve performance, especially in wireless networks. For example, in network with many communicators, there exist multiple communication links between radio nodes, and the different links experience different fading gains. At any instant time, it is likely that there is a pairs of radios that has a good channel quality. The probability of having such links increases with the number of radios in the network, thus achieving multiuser diversity or network diversity . We propose to develop new approaches to improve the performance by exploiting network diversity or multiuser diversity over a wireless fading channel. We propose the use of superposition coding that can be applied for the downlink transmission and a geographic transmission scheme for the uplink of wireless mesh or sensor networks.;We first propose the use of superposition coding (SPC) over network coding for reliable multicast over time-varying fading channels. Two-level SPC is utilized in conjunction with two network-coded versions of the packets in order to take advantage of the diversity of fading gains that can exist during each message transmission. Through the use of SPC, a receiver who has good channel conditions can recover more copies of the network-coded packet. This approach can be utilized for the downlink transmission in wireless mesh network to improve the throughput performance. We compare the performance of reliable multicasting using network coding with and without 2-SPC. When the transmission rates of both messages in 2-SPC are equal to the transmission rate without SPC, and the rates are independent of the channel SNR, it is shown that 2-SPC may provide significant gains. However, when the communication parameters are optimized to maximize the multicasting throughput, 2-SPC shows little performance improvement over network coding without SPC. The results indicate that SPC is useful to improve the performance of network coding for reliable multicast if the transmission rates for the all of the message were fixed, and the channel statistics are not known exactly.;Next, we consider the use of simulcasting by employing 2-SPC to deliver a mix of multicast and unicast traffic to receivers over fading channels when the transmitter has channel state information (CSI), which may be delayed. The CSI allows the transmitter to adapt the parameters of the SPC to provide additional messages to the receiver who is expected to have a better channel condition in the next time slot. We show that simulcasting can provide additional performance gains over using just network coding for several different cases where the source has exact or delayed knowledge of the channel state.;In the third part of this work, we consider the uplink of wireless mesh or sensor networks. When the channels from the mobile radios or sensors, hereafter called "nodes", to the infrastructure nodes (the access points (APs) or sinks) suffer from fading, packet transmissions may need to be relayed through other nodes since direct transmissions to the APs may have a high probability of failure. We propose a geographic approach that can improve performance by using opportunistic reception, in which a relay is selected from those nodes that can receive the packet correctly and move it toward the AP. We assume that the packet may be delivered in at most two hops. We consider the design of a scheme to select which nodes should compete to relay the packet and how they should compete in a time-slotted protocol. We then show that this problem of relay selection can be cast as an optimization problem. The resulting protocol is called Geographic Transmission with Optimized Relaying (GATOR), we show that GATOR provides better performance gains than direct transmission, fixed routing, and other geographic transmission schemes.;Finally, we study the use of multi-hop GATOR to improve throughput and energy efficiency for the uplink transmission in mesh networks in which APs are spare and the dimension of the network is large that the packet may need to be delivered in multiple hops between the source and the AP. We propose two multi-hop GATOR with fixed and non-fixed hop distance, and compared those performances with ones of 2-hop GATOR, multi-hop routing, and direct transmission. We show that the multi-hop GATOR with fixed optimal hop distance offers large performance gains over 2-hop GATOR, multi-hop routing, or direct transmission, especially when the dimension of the network is large and APs are sparse.