M-pulse amplitude modulation of a flat-topped beam for aeronautical laser communications.

Using the Rytov method, bit error rate (BER) performances of M-pulse amplitude modulation (M-PAM) are investigated for a flat-topped beam when such a beam is employed in an aeronautical laser communication system operating in vertical paths having weak atmospheric turbulence. By using the on-axis scintillation index and the log-normal distributed intensity, the average BER (⟨BER⟩) is evaluated for M-PAM when M=2,4,8. The scintillation indices of collimated flat-topped beams of various flatness orders N are compared against propagation lengths, source sizes, and zenith angle for laser communication vertical paths, including uplink and downlink. Also, the ⟨BER⟩ versus the average signal to noise ratio (⟨SNR⟩) is examined for various beam flatness orders. It is shown that as the flatness order increases, the scintillation index decreases. Taking one of the best flatness order values, N=15, for reducing the scintillations, ⟨BER⟩ versus propagation lengths, source sizes, zenith angle, and ⟨SNR⟩ are found for various values of M. When M is increased, the ⟨BER⟩ is found to deteriorate.