Subcarrier noncoherent and differentially coherent modulated optical communications in strong atmospheric turbulence

We study the error rate performance of subcarrier noncoherent and differentially coherent modulated optical wireless communication systems over the K-distributed turbulence channels. Using a moment generating function approach along with a series expansion of the modified Bessel function, we obtain a unified highly accurate series bit-error rate expression for a family of noncoherent and differentially coherent modulations. We also analyze the symbol-error rate performance of subcarrier M-ary differential phase-shift keying (DPSK) and M-ary noncoherent frequency-shift keying (NCFSK) modulated systems using the same frequency domain approach. We compare the error rate performance of subcarrier DPSK and adaptive on-off keying (OOK) using direct detection. It is shown that subcarrier DPSK is only 0.91 dB worse than OOK in strong turbulence.