Bounds on the capacity of ofdm under spread frequency selective fading channels

In this paper we derive novel bounds on the capacity of OFDM under spread frequency selective fading channels without prior knowledge of the channel (non-coherent channel). The derived bounds are characterized by a newly defined parameter termed effective coherence time, which characterize the number of effective number of samples that can be used for channel estimation for a given SNR. We also show that the effective coherence time is a non-increasing function of the SNR. The presented bounds are tight for any SNR as long as the effective coherence time is large enough.