Distributed Pilots for Channel Estimation in Single Frequency Networks

The instantaneous capacity of orthogonal frequency-division multiplexing (OFDM) in frequency-selective Rayleigh fading is evaluated, based on constrained capacity arguments, to account for the signal constellations used on the sub-carriers, and on a form of the central limit theorem suitable for correlated random variables. If the number of sub-carriers is large, the overall capacity is shown to be Gaussian-distributed, and its mean and variance are derived, as well as their dependence on the selected constellation, the signal-to-noise ratio and the channel parameters. The results show the separation between constrained and unconstrained capacity, and indicate the usefulness of adaptive modulation in exploiting all the signal-to-noise ratio range. Furthermore, the statistical characterization of the instantaneous capacity is instrumental in defining the outage capacity for OFDM systems.