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The analysis of the channel capacity in the absence of prior channelknowledge (noncoherent channel) has gained increasing interest in recent years,but it is still unknown for the general case. In this paper we derive bounds onthe capacity of the noncoherent, underspread complex Gaussian, orthogonalfrequency division multiplexing (OFDM), wide sense stationary channel withuncorrelated scattering (WSSUS), under a peak power constraint or a constrainton the second and fourth moments of the transmitted signal. These bounds arecharacterized only by the system signal-to-noise ratio (SNR) and by a newlydefined quantity termed effective coherence time. Analysis of the effectivecoherence time reveals that it can be interpreted as the length of a block inthe block fading model in which a system with the same SNR will achieve thesame capacity as in the analyzed channel. Unlike commonly used coherence timedefinitions, it is shown that the effective coherence time depends on the SNR,and is a nonincreasing function of it. We show that for low SNR the capacity isproportional to the effective coherence time, while for higher SNR the coherentchannel capacity can be achieved provided that the effective coherence time islarge enough.

Bounds on the capacity of OFDM underspread frequency selective fading channels