Expressions are derived for the average probability of block error

, i.e., the probability of more than

errors in a block of

symbols. The average is formed over the inatantaneous receiver signal-to-noise ratio after combining. Binary frequency-shift keying with noncoherent detection is transmitted over a very slow nonselective Rayleigh fading channel and received in additive Gaussian noise. Diversity is used and the branch fading is assumed independent. Formulas for

are derived for both ideal selection combining and maximal ratio combining.