A comparative study of 2.667 Gb/s OOK, DPSK, and PPM modulation formats for FSO applications

We have compared experimentally the transmission performance of on-off keying (OOK), differential phase-shift keying (DPSK), and pulse position modulation (PPM) optical waveforms at 2.5 Gb/s for free-space optical communication applications. We show that each technique is advantageous depending on the desired system complexity, cost, transmission data rate, optical aperture used (single mode vs. multimode), optical receiver used, availability of optical preamplifier, and ambient conditions. RZ format offers a 1- to 2-dB receiver sensitivity advantage over NRZ for both OOK and DPSK. With an optical preamplifier, an APD receiver offers no advantage over a PIN receiver. If the light can be collected efficiently in single-mode fiber without phase noise and an optical preamplifier is available, RZ-DPSK has definite performance advantages over other data formats. We measured an unoptimized balanced receiver sensitivity of 58 photons/bit for 10(-9) BER for RZ-DPSK. For multimode received optical signals, RZ-OOK combined with an APD receiver is an optimum choice for 2.5 Gb/s.