Ultraviolet communication, as a new type of communication, has the advantages of strong anti-interference ability, non-line-of-sight(NLOS), and all-weather operation, so it has been emphasized and become one of the research focuses in the field of wireless communication. And the light source beam angle directly determines the distance and performance of communication, but most of the current studies are limited to simulation to analyze the influence of the beam angle. In this paper, in order to verify the effect of light source beam angle on the performance of line-of-sight(LOS) and NLOS communication systems, four types of collimator lenses for controlling the light source beam angle are designed and molded, and comparative analyses of simulations and measurements are carried out. The result shows that in LOS communication, the smaller the beam angle is, the better the system performance is. In NLOS communication, the change of beam angle has less change on the path loss and almost no effect on the system performance. The conclusion are of guiding significance in promoting the practical application of ultraviolet communication.
In order to solve the problem that the severe inter-symbol interference (ISI) introduced by faster-than-Nyquist (FTN) strong filtering leads to the degradation of the equalization capability of the traditional receiver side (Rx) IQ imbalance compensation, a two-stage Rx IQ imbalance compensation algorithm is designed and demonstrated in this paper. The algorithm uses a time domain equalizer based on the decision directed-least mean square (DD-LMS) algorithm cascaded with a 4×2 frequency domain (FD) equalizer based on the radius directed equalization (RDE) algorithm, which can effectively improve the compensation range compared to using only a conventional 4×2 FD equalizer based on the RDE algorithm. 128Gbaud-FTN-PM-16QAM system simulation results show that with an acceleration factor as low as 0.85, the designed algorithm has a 14.3%, 100% and 40% improvement in the compensation range of Rx IQ amplitude, phase and skew imbalance compared to using only the 4×2 FD equalizer based on the RDE algorithm. The results validate the performance improvement of the proposed algorithm.
In fast-than-Nyquist wavelength division multiplexing (FTN-WDM) systems, the severe shaping filtering introduces serious inter-symbol-interference, and the long-distance fiber optical transmission also leads to more complex link impairments. Traditional chromatic dispersion (CD) estimation algorithms face the problems of difficulty in balancing complexity and estimation accuracy, and sensitivity to polarization mode dispersion (PMD) impairments. Aiming at the problem of large CD estimation error when traditional CD estimation algorithm has large PMD damage, a PMD-tolerant CD estimation algorithm for FTN coherent optical transmission systems is proposed. The algorithm achieves coarse CD estimation by stacking training sequences insensitive to PMD damage, and completes finer CD estimation by using the consistency parameter of clock phase detection. 64GBaud PM-16QAM FTN-WDM simulation results show that when the acceleration factor is 0.85 accumulated CD is 15360ps/nm and the CD estimation error is 200ps/nm, the PMD tolerance is about 100ps, and the complexity and PMD tolerance are 0.24 and 2.5 times of the peak average power ratio (PAPR) blind CD estimation algorithm, respectively.
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