  This paper considers secrecy of a three node cooperative wireless system in
the presence of a passive eavesdropper. The threshold-selection
decode-and-forward (DF) relay is considered, which can decode the source
message correctly only if a predefined signal-to-noise ratio (SNR) is achieved.
The effects of channel state information (CSI) availability on secrecy outage
probability (SOP) and ergodic secrecy rate (ESR) are investigated, and
closed-form expressions are derived. Diversity is achieved from the direct and
relaying paths both at the destination and at the eavesdropper by combinations
of maximal-ratio combining (MRC) and selection combining (SC) schemes. An
asymptotic analysis is provided when each hop SNR is the same in the balanced
case and when it is different in the unbalanced case. The analysis shows that
both hops can be a bottleneck for secure communication; however, they do not
affect the secrecy identically. While it is observed that CSI knowledge can
improve secrecy, the amount of improvement for SOP is more when the required
rate is low and for ESR when the operating SNR is also low. It is also shown
that the source to eavesdropper link SNR is more crucial for secure
communication.
