Performance Evaluation of Optically Preamplified M-ary PPM Systems for Free-Space Optical Communications

Abstract

M-ary pulse position modulation (M-ary PPM) has been widely considered as an attractive solution for increasing the bit rates in free-space optical (FSO) communications. Besides increasing the bit rate, M-ary PPM increases the power efficiency of FSO systems. Hence, better performance can be achieved at lower Eb=N0 values when compared to on-off keying (OOK). M-ary PPM systems can be implemented using optically preamplified, direct detection receivers. Furthermore, M-ary PPM can be combined with polarization division multiplexed-quadrature phase shift keying (PDM-QPSK) or with PDM-binary phase shift keying (PDM-BPSK) and then detected using optically preamplified coherent detection receivers based on phase/polarization diversity techniques. In this thesis, the performance of optically preamplified, direct detection 16-ary and 64- ary PPM systems in terms of the bit error ratio (BER) is evaluated. Simulation techniques were used to evaluate the BER without the need to assume that the noise at the decision sample is Gaussian. The combined effects of the dual polarized amplifier noise, the Fabry- Perot optical filter, the extinction ratio (ER) of the optical transmitter, and the electrical filter at the receiver are all considered in the evaluation. The bandwidths of the optical and electrical filters at the receiver were optimized to obtain the best performance. In addition, the penalties due to frequency drift and timing jitter are also calculated. Simulation results provide the Eb=N0 values at a target BER of 10^-3 due to the availability of FEC codes that can reduce the input BER down to 10^-12. Four systems with different pulse shapes under an ER value of 20 dB were considered. In each case, the optimum filters were used. For 16-ary PPM systems, those values are 8 dB for the rectangular pulse and 8.2 dB for the sin2 pulse, while for 64-ary PPM systems, those values are 8.35 dB for the rectangular pulse and 9.6 dB for the sin2 pulse, respectively. This result indicates that under an ER value of 20 6 dB, 16-ary PPM systems require smaller values of Eb=N0 when compared to 64-ary PPM systems to achieve the same BER. In addition, 16-ary PPM systems have better bandwidth efficiency compared to 64-ary PPM systems. Also, the Eb=N0 performance of optically preamplified coherent M-ary PPM combined with PDM-BPSK and with PDM-QPSK systems is evaluated. The performance penalties due to finite ER values are also evaluated. For PDM-QPSK 8-ary PPM systems, the penalty at 20 dB ER is 0.5 dB and for PDM-QPSK 16-ary PPM systems is 1.5 dB, while for PDMBPSK 8-ary and 16-ary PPM systems, the penalty is 1 dB and 2.6 dB, respectively. For 64-ary PPM systems combined with PDM-QPSK or PDM-BPSK, the penalty at 20 dB ER is much larger than 7 dB, which indicates the impracticality of these systems.