| Free space optical(FSO) communication, one kind of optical wireless communications, is conducted by the transmissions of laser optical signals through atmosphere turbulence channels. FSO communication has the advantages such as high bandwidth, high security, no electromagnetic interference and spectrum license-free. FSO is an effective supplement for modern wireless communication networks, and can also be used to fix “the last mile” problem or to build high performance core networks. FSO is an important component of next generation ultra-high bandwidth wireless communication networks.The disturbances and attenuations from atmosphere turbulence channel are the main challenges for the investigations and applications of FSO system. Severe weather conditions and scintillations degrade FSO system performance seriously. Among kinds of turbulence fading mitigation technologies, advanced modulation and coding is the basic scheme that can improve FSO system performance. In recent years, multi-dimensional hybrid modulations have been proposed in optical communications by researchers. In order to improve the bandwidth utilization efficiency and decrease BER, several traditional modulation formats are combined, so that the multi-dimensional freedoms of optical waves can be jointly modulated. In this way, the performances of optical communication systems are enhanced.In this paper, we combined the polarization division multiplexing(PDM), quadrature phase shift keying(QPSK) and multi-pulse pulse position modulation(MPPM), and proposed the multi-dimensional hybrid modulation called PDM-QPSK-MPPM. The transmitter and receiver models are presented. The bandwidth utilization efficiency and bit error performance for the proposed PDM-QPSK-MPPM modulation are theoretically analyzed, and the mathematical expressions of bit error rate(BER) and symbol error rate(SER) for PDM-QPSK-MPPM are derived. In the numerical analyses, the BERs and SERs for PDM-QPSK-MPPM, BPSK and MPPM are compared. It can be seen that the proposed PDM-QPSK-MPPM multi-dimensional hybrid modulation has better bandwidth utilization efficiency and reliability. In addition, compared with hybrid modulations like PQP and POLMUX-QPSK, the proposed PDM-QPSK-MPPM modulation has its flexibility, and can compromise the bandwidth utilization efficiency and BER through adjusting the number of transmitting pulses in one symbol duration. Thus, PDM-QPSK-MPPM modulation has a promising application perspective in adaptive FSO transmission systems, because it can adjust the transmission rate according to different channel conditions to maintain the objective bit error rate performance.In order to decrease the system complexity, we further propose the PDM-DPSK-MPPM multi-dimensional hybrid modulation based on the concept of PDM-QPSK-MPPM. And the expressions of upper bound BER and lower bound BER for PDM-QPSK-MPPM modulation are derived. In PDM-DPSK-MPPM modulation, in order to decrease system complexity and improve practicability for FSO systems, we adopt Mach-Zehnder interferometers to demodulate DPSK signals, and the employments of local oscillator laser and phase-locked loop circuit can be avoided. Numerical analyses results show that PDM-DPSK-MPPM has a slightly worse BER performance than PDM-QPSK-MPPM with coherent demodulation, but still has obvious better BER performances and turbulence mitigation abilities than traditional MPPM and BPSK modulations. |
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