  Expressions for the BER of M-ary PPM & biphase DPSK modulations in the
presence of noise are derived using analytical, statistical methods. The PPM
expression is verified via Poisson statistics based simulation. BER expressions
are then applied to a representative set of receiving telescope & sky spectral
radiance parameters in order to assess performance of PPM & DPSK relative to
one another. Finally, efficiency & additional considerations are discussed.
