  The ergodic capacity of underlay cognitive (secondary) dual-hop relaying
systems is analytically investigated. Specifically, the amplify-and-forward
transmission protocol is considered, while the received signals undergo
multipath fading and shadowing with non-identical statistics. To efficiently
describe this composite type of fading, the well-known generalized-$K$ fading
model is used. New analytical expressions and quite accurate closed-form
approximations regarding the ergodic capacity of the end-to-end communication
are obtained, in terms of finite sum series of the Meijer's-$G$ function. The
analytical results are verified with the aid of computer simulations, while
useful insights are revealed.
