Improving wireless networks with cooperative arq protocols
Julián David Morillo Pozo · Dialnet (Universidad de la Rioja) · 2009
Data transmissions in Wireless Networks often suffer from bit errors that arise from the notoriously complex and timevarying signal propagation characteristics of the wireless medium. A number of physical factors such as attenuation and multipath can lead to high biterror rates at the link layer. These in turn lead to packet losses and low throughput observed at higher layers, degrading the performance of many applications. We use the notion of space diversity to develop an approach that improves data delivery efficiency and throughput in presence of transmission errors. Space diversity relies on multiple nodes receiving packets addressed to one of them from a transmitter. The hypothesis underlying this system is as follows: because frame errors are often pathdependent (e.g., due to multipath fading) and statistically independent between different transmittingreceiving radio pairs, transmissions are likely to succeed from at least one of the links between the transmitter and the multiple receiver nodes. Likewise, multiple nodes that all receive versions of the same transmission may together be able to correctly recover a frame, even when any individual node is not. Using these principles, we design mechanisms embedded into the wellknown Automatic Repeat reQuest (ARQ) protocol. The proposed techniques fall into the family of systems known as Cooperative ARQ (CARQ) which leverages the properties of space diversity to reduce frame loss rates in the linklayer, leading to increased throughput and packet delivery efficiency. We make an extensive study of CARQ (with and without some proposed techniques to work efficiently in a CARQ system) with analytical models, through simulation and through experimentation. We show, for example, that when M neighboring nodes cooperate using our proposed majority voting frame combiner, they can achieve the performance of an array of M' antennas provided that M = ?/2 x M' ? 1.57M', in AWGN channels or M = 2M'1 in Rayleigh channels. We also show that if the cooperating nodes exchange information about SNR during frame reception and we use a Maximum Likelihood decision rule, we can achieve full diversity. This result implies that we can achieve a similar BER versus SNR performance as in the case of cooperative transmission, but without any further hardware complexity. We study the FER of such a system, and propose a low coordination overhead frame exchange mechanism. Using this frame exchange and the majority voting frame combiner, we show how in channels with a strong Line of Sight (LOS) component and low SignaltoNoise ratio, the maximum achievable throughput is many times higher than for other ARQ schemes. For nonLOS scenarios, the cooperative ARQ without frame combiner achieves the best efficiency results. We also perform a practical evaluation through a real implementation of CARQ and present an integration of the protocol into a vehicular framework as an example of a practical application.