Forward link capacity with linear receivers and multiple transmit antennas

The asymptotic growth in forward-link sum capacity is characterized as a function of the number of users and transmit antennas with linear receivers. We assume a single cell with frequency-selective fading channels, which are known at the transmitter, and a single receive antenna at each mobile. We first show that when the channel matrices are circulant, multi-carrier transmission maximizes the sum mutual information. A particular subchannel is allocated to the user with the largest channel gain with maximum-ratio combining at the transmitter, and the optimal power allocation across subchannels is determined by water pouring over those gains. By applying results from extreme value theory, we then show that when the channel consists of a large number of i.i.d. Rayleigh fading subchannels, the sum capacity grows as O (log(root N log U + N)) where N is the number of transmit antennas, and U is the number of users. Numerical results are presented, which show that the asymptotic results are valid for moderate size systems.