Bounds on the secrecy capacity with diversity combining techniques

This paper is concerned with the study of confidential communication through independent and identically distributed (i.i.d.) and evenly correlated Rayleigh fading channels, where a single antenna transmitter communicates with the corresponding receiver in the presence of an eavesdropper. Both the receiver and eavesdropper are equipped with multiple antennas and employ diversity combining receptions such as maximal ratio combining (MRC) and selection combining (SC) diversities. In the presence of channel diversity, we derive the upper and lower bounds on the secrecy capacity with full channel state information (CSI) at the transmitter. The closed-form expression for the upper bound is also derived at high signal-to-noise ratio (SNR) regime. The analysis of upper bound with the diversity combining techniques tells how the channel diversity enhances secrecy capacity. Moreover, as a result of deriving the expression in a closed-form, it is insightful to see, how the channel parameters such as correlation coefficient and the ratio of main and eavesdropper channel gains that will be called average channel gain ratio (CGR) affect the secrecy capacity.