Enhancing Secrecy Capacity in Alternate AF Relaying Networks with Inter-Relay Interference

This paper exploits the physical layer security and signal processing to support confidentiality in two-path amplify-and-forward (AF) relay networks. In these systems, successive relaying enhances the capacity of wireless networks through simultaneous spectrum utilization by a source and relays which in turn leads to inter-relay interference (IRI). In the proposed signaling schemes, by controlling the IRI through power adjustment unique to relay-destination channel state information (CSI), IRI and fading channels are turned into a source of secrecy in a broadcasting environment where signal overhearing is unavoidable. Specifically, the IRI is fully mitigated at the intended receiver, while at the eavesdropper, without requiring additional system resources, the IRI is increasing the noise level. Conventional signaling and superposition coding (SC) schemes are considered when analyzing the secrecy capacity of different systems. Furthermore, network coding (NC) is investigated to improve the secrecy capacity. The impact of intended receiver and eavesdropper positions is examined using stochastic geometry principles. Numerical analysis and simulations show that with a single data stream, even though noise enhancement degrades the intended receiver's signal-to-noise ratio (SNR) in zero-forcing IRI cancelation, successive relaying system using the proposed scheme achieves higher secrecy capacity in deterministic environments and ergodic secrecy capacity in fading environments compared to the conventional half-duplex relaying system. Furthermore, SC scheme shows great promise in terms of providing different levels of secrecy rates for different data streams represented by different power allocations.