Distributed Opportunistic Scheduling for Wireless Ad-Hoc Networks with Block-Fading Model

In this paper, we study a distributed opportunistic scheduling problem to exploit the channel fluctuations in wireless ad-hoc networks. In this problem, channel probing is followed by a transmission scheduling procedure executed independently within each link in the network. We study this problem for the popular block-fading channel model, where channel dependencies are inevitable between different time instances during the channel probing phase. Different from existing works, we explicitly consider this type of channel dependencies and its impact on the transmission scheduling and hence the system performance. We use optimal stopping theory to formulate this problem, but at carefully chosen time instances at which effective decisions are made. The problem can then be solved by a new stopping rule problem where the observations are independent between different time instances. Since the stopping rule problem has an implicit horizon determined by the network size, we first characterize the system performance using backward induction. We develop one recursive approach to solve the problem and show that the computational complexity is linear with respect to network size. Due to its computational complexity, we present an approximated approach for performance analysis and develop a metric to check how good the approximation is. We characterize the achievable system performance if we ignore the finite horizon constraint and design stopping rules based on the infinite horizon analysis nevertheless. We present an improved protocol to reduce the probing costs which requires no additional cost. We characterize the performance improvement and the energy savings in terms of the probing signals. We show numerical results based on our mathematical analysis with various settings of parameters.This research is partially supported by the NSF under grant CNS-1018346, by the U.S. AFOSR under MURI grant award FA9550-09-1-0538, and by DARPA under grant award SA00007007 for the Multi-Scale Systems Center (MuSyC), through the FCRP of SRC and DARPA

Throughput optimal random medium access control for relay networks with time-varying channels

The use of existing network devices as relays has a potential to improve the overall network performance. In this work, we consider a two-hop wireless relay setting, where the channels between the source and relay nodes to the destination node are time varying. The relay nodes are able to overhear the transmissions of the source node which may have a weak connection to the destination, and they help the source node by forwarding its messages to the destination on its behalf, whenever this is needed. We develop a distributed scheme for relay selection and channel access that is suitable for time-varying channels, and prove that this scheme is throughput optimal. We obtain the achievable rate region of our proposed scheme analytically for a relay network with a single source and a single relay node. Meanwhile, for a more general network with more than one relay nodes, we perform Monte-Carlo simulations to obtain the achievable rate region. In both cases, we demonstrate that the achievable rate region attained with our distributed scheme is the same as the one attained with centralized optimal scheme