Gaussian fading channel with secrecy outside a bounded range

The Gaussian fading channel is studied, in which the channel from the transmitter to the receiver is corrupted by a multiplicative fading coefficient H and an additive Gaussian random noise. It is assumed that the channel is experiencing block fading, and the transmitter does not know the channel state information (CSI). The receiver is assumed to have full knowledge of the CSI. If the channel state is better, then more information is required to be decoded by the receiver, and if the channel state is worse, more information is required to be secure from the receiver. Furthermore, the information intended to be decoded by the receiver with a better state (e.g., |H| ≥ |h₀|) is required to be secure from the receiver if it has a state worse than |h₀| by Δ (i.e., |H| ≤ |h₀| - Δ), which is referred to as secrecy outside a bounded range. A (layered) broadcast approach is studied for this problem, which views the fading channel as a degraded broadcast channel with a number of receivers each experiencing a different fading coefficient. The achievable scheme designates one superposition layer to each message with binning employed to protect all upper-layer messages from lower-layer receivers. Furthermore, the scheme allows adjacent layers to share rates so that part of the rate of each message can be shared with its upper-layer messages to enlarge the rate region. The achievable secrecy rate region via the broadcast approach is characterized. The developed scheme can adapt the transmission rate to the actual unknown channel state without exploiting the CSI at the transmitter.