Research On Ultraviolet Subcarrier Modulation Diversity Technology In Weak Turbulence

At present, Ultraviolet (UV) communication system mostly adopts the intensity modulation / direct detection system (IM/DD). The main modulation modes are on-off keying(OOK),pulse position modulation(PPM),differential pulse position modulation(DPPM),digital pulse interval modulation (DPIM)and so on. OOK is easily influenced by atmospheric turbulence in the transmission process, which increases the bit error rate(BER), and seriously affects the performance of the system. Although PPM has better anti-interference ability and higher power efficiency compared with OOK, it needs to solve the problem of clock synchronization in the demodulation process. Because DPPM and DPIM evolved from the PPM,their performance is similar. It has been shown that subcarrier intensity modulation is an effective way to resist atmospheric turbulence. In this paper, we mainly study the subcarrier modulation diversity technique and analyze the performance of the subcarrier modulation in weak turbulence. The specific research contents are as follows:(1) Firstly, this paper introduces present situation and progress at home and abroad of UV communication and modulation diversity technology. Secondly, the influence factors of UV communication and the average transmit power, bandwidth requirement, and error slot rate of several modulation methods are analyzed. Finally, the principle of subcarrier binary phase shift keying(2PSK), binary differential phase shift keying (2DPSK), binary frequency shift keying(2FSK) modulation and demodulation are introduced in detail.(2)Aiming at the model of wireless UV Line-of-Sight (LOS)and Non-Line-of-Sight(NLOS) communication, the expressions for BER using subcarrier BPSK, Quadrature Phase Shift Keying (QPSK)?8 Phase Shift Keying(8PSK) in logarithmic normal (log-normal)distribution are derived. The effects of transmitted power, range, transmitter (Tx) beam angle,receiver (Rx) field of view (FOV)angle, Tx and Rx elevation angle on BER of subcarrier modulation are simulated under different scintillation index. The results show that BER m decreases with the increase of transmitted power, Tx beam angle and Rx FOV angle,while BER increases with the increase of range, Tx and Rx elevation angle. Under the same condition, the performance of BPSK is the best, but 8PSK is the worst in the three subcarrier modulation.(3) Discussing the model of wireless UV LOS and NLOS diversity reception, the expressions for BER using Maximal-Ratio Combiner (MRC),Equal Gain Combiner (EGC),Selective Combiner (SC) in log-normal distribution were derived.The performance of subcarrier modulation diversity technology are simulated under different scintillation index. When the signal-to-noise ratio(SNR) is the same, the performance of MRC is the best, but SC is the worst in the three methods.In summary, the subcarrier diversity technology can effectively reduce BER of UV communication system in the turbulent channel and improve the performance of UV communication system, so it has broad application prospects.