The Secrecy Capacity of Block Fading Multiuser Wireless Networks

The resilience of block fading wireless orthogonal frequency division multiple access (OFDMA) networks to passive eavesdroppers is investigated. The network secrecy capacity is evaluated in scenarios involving a base station and several terminals, some of which constitute passive eavesdroppers. Assuming a block fading Rayleigh channel, the probability of a secrecy outage during a transmission frame is evaluated with respect to a target secrecy rate tau in the following cases: (i) in the absence of any cooperation between the network nodes, and, (ii) when the full multi-user diversity is exploited both by the legitimate users as well as by the eavesdroppers. Remarkably, it is demonstrated that in a network of as few as 12 legitimate users and a single eavesdropper it is possible to transmit 1 bit/sec/Hz with a probability of secrecy outage less than 1%. Furthermore, the delay constrained secrecy capacity of this network is evaluated when the full channel state information (CSI) is available both at the base station and at all receiving nodes. A secure waterfilling scheme is discussed, satisfying a short-term power constraint.