Physical layer secrecy for wireless communication systems using adaptive HARQ with error contamination

This paper proposes a physical layer coding scheme to provide reliability and security in wireless communication systems. An Adaptive Granular Hybrid Automatic Repeat re- Quest (AG-HARQ) scheme based on Low-Density-Parity-Check (LDPC) codes is proposed. In the AG-HARQ protocol, the whole codewords are first transmitted (by Alice) to the intended receiver (Bob) and potentially intercepted by an unauthorized receiver or eavesdropper (Eve). Whenever the legitimate receiver (Bob) fails to decode the correct message, the received codewords are splitted into sub-packets and Bob computes a decoding confidence index for each of these sub-packets. Bob can then request a retransmission of those sub-packets having the lowest confidence indexes. These sub-packets are requested by Bob until correct decoding or until a maximum number of retransmissions is reached. An error contamination (EC) mechanism is also proposed which spreads the errors in the current frame with a scrambler and to the other frames by use of block interleavers. The proposed scheme ensures reliable and secure communication even in the case of negative Security Gap (SG) where the eavesdropper's channel benefits from better channel conditions than the legitimate channel.