Secure Broadcasting with Multiuser Diversity

We investigate the problem of broadcasting secret information to one or more receivers over wireless links in the presence of potential eavesdroppers. A fast fading channel model is assumed, with perfect channel state information (of intended receivers) at the transmitter. Both the case of independent messages and common message are considered. For the case of independent messages we propose a scheme that achieves the sum capacity as the number of receivers goes to infinity. We note that in the limit of large number of intended users, capacity scales with the number of intended receivers, but not with power. For the case where a common message is broadcasted, we present a coding scheme that achieves a certain positive rate independently of the number of intended receivers.