Physical Layer Security for Channel-Aware Random Access With Opportunistic Jamming

We study the secure transmissions for channel-aware (CA) random access based on the notion of physical layer security in this paper, which might be suitable for machine type communications. Thanks to the availability of channel state information in CA random access, opportunistic jamming can be employed to improve the secrecy rate. As a result, a positive secrecy rate can be obtained even if the signal-to-noise ratio at an eavesdropper is higher than that at a legitimate receiver. Since a powerful eavesdropper equipped with multiple antennas can suppress the interference due to opportunistic jamming once jamming users are identified, it is important to know the number of received signals that allows Eve to identify them. Using the notion of unicity distance, we study this issue. Based on analysis, we find that it is necessary to keep the number of jamming users relatively small (compared with the number of ordinary active users) and use a fraction of the coherence time for secure transmissions when the coherence time is long.