| Spread-spectrum technology, especially direct-sequence spread-spectrum (DSSS) modulation, is extensively used in modern military and commercial communication systems. One reason for its popularity is the ability of interferece suppression, in particular for multipath components which are intractable in common systems. DSSS technology is applied to both data links of the researched Wireless Image and Command Transmission System for anti-multipath. However, experimental results with long-distance communication in the channel of ground-air or sea surface showed that both the sensitivity and bit error rate (BER) of the command data link were worse than theoretical values apparently. Thus, it is meaningful to systematically study the performance of the DSSS system under various channels and with different parameters. Through this way, proper compensation methods will be proposed for improving the anti-multipath capability of the command data link in the system.The channel of ground-air or sea surface is firstly modeled as a time-nonselective Rician channel based on analyzing the general characteristics of multipath channels. Rician factor K is used to differentiate the communication distance which induces the unexpected errors. Then a two-path model, a tap delay line Rician model, and a Rician model based on Matlab/Simulink Communication Toolkits are established for simulations in the next Chapters.Secondly, the performance of DSSS system under multipath channels and its influential parameters are systematically analyzed. This study is conducted in two aspects. For a theoretical sytem with ideal chip synchronization, effects of multipaths with time delay longer or shorter than one chip interval on DSSS system are mathematically and experimentally researched in a typical Rician channel, and the influences of different chip rate and different code are also simulated. Then the impact of Rician factor K is also studied. The researches above illustrate that in the presence of multipaths with time delay shorter than one chip interval, or in a channel with small Rician factor K, the DSSS system has a poor anti-multipath performance which will lead to unexpected BERs. For a practical sytem without ideal chip synchronization, the effects of multipaths on the acquisition as well as the tracking loop are studied, which indicate that it's essential to choose a proper threshold in acquisition algorithm, and that the correlation width or early-late spacing has important impact on the tracking error caused by multipaths.Finally, algorithm of the practical DSSS receiver is applied to a DSSS simulation platform on MATLAB/Simulink. The platform is used in one way to discover the reason for bit errors in multipath channels, and in the other way to validate the improvements of the system performance due to employing compesation methods. Compesation methods are broungt forward in three ways, which are respectively the optimization of system parameters, the acquisition algorithm and the tracking algorithm. Employing the proposed method of high sample rate, the algorithms of double-threshold multiple-dwell acquisition as well as the bidirectionally sliding acquisition, the probability of unlocking in acquisition loop will decrease apparently; and using the method of increasing the chip rate and the technique of narrow correlator, the BER performance will moderately ameliorate. In conclusion, with the algorithms proposed here applied to the DSSS system design of the command data link, the performance under multipath channels in long distance communication will be commendably improved. |
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