Simple approximate expression for error performance of coherent M-ary orthogonal signals in generalised fading channels

Presented is a simple tight upper-bound and quite accurate expression for the average bit error probability of coherent M-ary orthogonal signals over generalised fading channels. The derived expression is applicable for various diversity combining techniques, and it includes the cases in which the diversity branches are arbitrarily correlated, non-identically distributed, and/or following any fading distribution. The results clarify the tightness of the derived expression, and illustrate the impacts of non-identical and correlated branches on the system power performance.