Capacity of a Bandwidth Constrained Constant Envelope Signal

The capacity of constant envelope signals with bandwidth constraints has been an open problem for many years. In this paper we find a lower bound on this capacity, for second-moment bandwidth constrained signals. The bound is found by utilizing classic results on optimum angle modulation. We found that the capacity of constant envelope signals, in the range investigated, was about two-thirds to three-quarters of the capacity of signals with no amplitude constraints. This indicates that it should be possible to find constant envelope modulations, with spectral efficiencies, which more closely approach the efficiencies of QAM signals.