  This paper evaluates the bit error rate (BER) performance of underlay relay
cognitive networks with decode-and-forward (DF) relays in arbitrary number of
hops over Rayleigh fading with channel estimation errors. In order to
facilitate the performance evaluation analytically we derive a novel exact
closed-form representation for the corresponding BER which is validated through
extensive comparisons with results from Monte-Carlo simulations. The proposed
expression involved well known elementary and special functions which render
its computational realization rather simple and straightforward. As a result,
the need for laborious, energy exhaustive and time-consuming computer
simulations can be ultimately omitted. Numerous results illustrate that the
performance of underlay relay cognitive networks is, as expected, significantly
degraded by channel estimation errors and that is highly dependent upon of both
the network topology and the number of hops.
