User Capacity of Fading Multi-User Channels with a Minimum Rate Constraint

In this paper, a power allocation scheme is proposed to maximize the number of active terminals in fading multi-user channels in which a minimum rate must be maintained for all active users. It is assumed that receivers and transmitters have perfect channel state information. Both uplink and downlink scenarios are considered and under the assumption of independent Rayleigh fading channels for different terminals, the optimal number of active terminals is asymptotically obtained as the total number of users, n, is large enough. For broadcast channels with successive interference cancellation at receivers and multiple- access channels with joint decoding at the receiver, the maximum number of active terminals is shown to be arbitrarily close to In ( P_total/sigma² In n) /R_min with probability approaching one, where P_total and R_min denote total transmit power and the minimum rate respectively, and sigma² represents the noise variance.