Fixed and Random Access Cooperative Networks

Rich scattering of electromagnetic waves propagating through physical environments generates complex interference patterns. As such patterns go through maxima and minima, large variations in energy adversely affect wireless reception and thus deteriorate error probability performance of wireless communication systems. By providing multiple channels with independent (or at least uncorrelated) variations in time, frequency, and/or space, diversity techniques offer well-appreciated countermeasures mitigating such (so called fading) effects. With the deployment of multiple antennas effecting space diversity we create copies of the transmitted signal either at the receiver, at the transmitter, or both. In time or frequency (a.k.a. Doppler or multipath) diversity systems, we exploit the natural property of wireless channels to vary over time or frequency. The benefits of diversity are significant. In a typical (wireline) additive white Gaussian noise (AWGN) channel the error probability decays exponentially as the received signal-to-noise-ratio (SNR) increases; that is, error effects decrease as e −SNR. A wireless Rayleigh fading channel, however, exhibits errors decaying as SNR −1. A κth-order diversity channel entails κ uncorrelated channels and exhibits error probability which decreases as SNR −κ. Needless to say the gap between the exponential decay in wireline channels and the inversely linear decay in wireless channel is enormous. Considering that for sufficiently large κ the SNR −κ and e −SNR functions are not very different , the value of diversity is clear: it can close the error performance gap between wireline and wireless channels. Spatial and time-frequency diversity systems are at opposite ends of a deployment cost versus reliability curve. Spatial diversity is reliable but comes with hardware cost. Time-frequency diversity on the other hand exploits natural phenomena that may or may not be present in a particular link and is thus less reliable even if it comes for free when available. User cooperation is an alternative form of diversity which aims to strike a balance in this curve by providing diversity more reliable than natural time-frequency variations yet without requiring deployment of additional antennas. The basic idea is to have single-antenna