On the Constrained Capacity of OFDM in Rayleigh Fading Channels

In this paper, the instantaneous capacity of Orthogonal Frequency-Division Multiplexing (OFDM) in frequencyselective 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 subcarriers is large, the overall capacity is shown to be Gaussiandistributed, and its mean and variance are derived, as well as their dependence on the selected constellation, the signalto- 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.