  Capacity of M-ary Amplitude and Phase-Shift Keying(M-APSK) over an Additive
White Gaussian Noise(AWGN) channel that also introduces an unknown carrier
phase rotation is considered. The phase remains constant over a block of L
symbols and it is independent from block to block. Aiming to design codes with
equally probable symbols, uniformly distributed channel inputs are assumed.
Based on results of Peleg and Shamai for M-ary Phase Shift Keying(M-PSK)
modulation, easily computable upper and lower bounds on the effective M-APSK
capacity are derived. For moderate M and L and a broad range of Signal-to-Noise
Ratios(SNR's), the bounds come close together. As in the case of M-PSK
modulation, for large L the coherent capacity is approached.
