Behavior of outage probability in MISO systems with no channel state information at the transmitter

It is well known that, in terms of the ergodic capacity, the optimum transmit strategy is to allocate equal power to all available transmit antennas. We show that this transmit strategy is not the optimal one for minimizing the outage probability. The optimum strategy is then to avoid using all available antennas. The optimal number of active antennas depends on the required rate and the SNR. We propose an algorithm which computes the optimum number of active antennas for a given available number of transmit antennas, SNR, and rate. We compare the optimum strategy to equal power allocation.