Multiuser diversity-enhanced geographic transmissions in wireless channels.

摘要

In wireless multi-hop packet radio networks, the conventional packet forwarding scheme is to pre-select the next-hop receiver for a packet based on knowledge of the network topology. However, when the nodes experience fading that changes on the order of the packet duration, the conventional routing approach will often offer poor performance because the pre-selected receiver may not be able to recover the packet because of fading. An alternative approach is geographic transmission, in which the packet is transmitted in the direction of the destination, but the next-hop receiver is not pre-selected. Multiuser diversity benefit can be exploited in such a scenario because the different receivers in the direction of the destination are likely to experience independent fading channels. This approach could significantly improve the probability of the packet being correctly received by the next-hop receiver.;In the first part of this work, we show that such a benefit can maximize the expected value of the maximum transmission distance, which is one routing metric we consider for geographic transmissions. To provide an application of our findings, we design geographic transmission schemes that provide multiuser diversity gain in a Rayleigh fading channel.;However, this approach places significant burden on the energies of the receiving nodes if the forwarding scheme requires that all of the next-hop neighbors of the transmitter (that are in the direction of the destination) attempt to receive a packet. This is because in a wireless multihop packet radio network, the nodes are limited in battery life. Thus, in the second part of this dissertation, we consider geographic transmission schemes that provide multiuser diversity with a fixed energy constraint. Towards that end, our approach is to provide energy efficiency by limiting the energy used in reception (which depends on the number of nodes that activate to receive a transmission).;In determining which nodes should activate, an intuitive approach is to turn off all nodes located either very close to the transmitter or those very far away. This is because nodes very close to the transmitter are likely to decode a packet successfully but do not achieve a large transmission distance. On the other hand, nodes at very large transmission distances have low probabilities of decoding a packet successfully. Thus we propose node-activation-based-on-link-distance (NA-BOLD) schemes in which the probability that a node will activate/turn on to try to receive a packet is a function of its distance from the transmitter. With the goal of maximizing transmission distance, we analyze the optimum NA-BOLD scheme under a constraint on the number of nodes that activate. We also consider the maximization of transport capacity---another useful metric used in geographic transmissions. Transport capacity can be considered to be maximum transmission distance weighted by the maximum achievable rate of information transmission. Under a total energy constraint, i.e., a constraint on the sum of the energies used in transmission and reception, we consider the joint design of node-activation functions and transmission rates to maximize transport capacity. We optimize the allocation of energy between transmission and reception when nodes activate using our NA-BOLD approach. We have evaluated our NA-BOLD approach in the Nakagami-m fading channel, where the parameter m is used to indicate fading severity and can model a large variety of wireless channels.