Power Loading in Parallel Channels with Channel Distribution Information at the Transmitter

In this paper, the ergodic capacity of parallel channels is investigated with a view to designing the power allocation strategies that optimise this performance measure under the assumption that perfect, instantaneous channel knowledge is available at the receiver whereas the transmitter only has knowledge of the fading distributions and/or the first and second moments of the subchannel gains. Upper and lower bounds on the capacity are derived for general fading distributions, and low and high SNR optimal power allocation strategies for three parallel fading channels are given. The specific channels of interest are a singleinput single-output Rayleigh fading channel, a selection channel, and a receive diversity channel with maximum ratio combining (MRC). Moreover, it is shown that, for the selection and MRC channels, as the number of diversity branches M → ∞, the upper and lower bounds are tight, and the power loading that maximises mutual information for all fading distributions follows a waterfilling principle, but where the mean channel gains are used to calculate the power values instead of the instantaneous channel gains. Finally, practical examples using new measured ultrawideband channel data are provided.