Rapid growth and modernization of microwave communications is currently taking place in both the commercial and military environments. Much of this growth is being accomplished through digital transmission, and calls for efficient utilization of frequency spectra allocated for LOS communications. As the bit efficiencies and data rates of digital radio systems have increased, the problems associated with multipath propagation phenomena have become more evident. This is due to the fact that the transmitter signal is randomized before transmission and the system behaves as though it has full loading at all times. To minimize outages and to meet path availability objectives, diversity protection alone, in many instances, is not sufficient. However, diversity protection, coupled with an acaptive equalizer, will meet the objective in all but a few severe cases. This paper describes a digital radio system operating in the 8 GHz band with 8 PSK modulation and nominal data rates of 45 Mb/s or 90 Mb/s. Multipath propagation and its effects on digital transmission are discussed. Improvements in bit error rate threshold obtained through the use of an adaptive equalizer designed into the receiver are also presented.
A comparative study is presented showing the relative merits of four modulation techniques for use in line-of-sight digital radio systems operating at 45 Mbith in a bandwidth of 10 MHz. Candidate modulations are: conventional WQAM, reduced-bandwidth lSQAM, reduced-bandwidth 44QPRS, conventional 81 -QPRS.Comparisons are made from three points of bit-error-rate performance with unfaded robustness to multipath fading, sensitivity to timing and phase errors.Extensive numerical calculations show that none of the above signaling techniques is uniformly better than the others. Conventional 64-QAM seems the best solution to combat selective fading. On the other hand, reduced bandwidth 16-QAM and 44QPRS as well as conventional 81-QPRS all have the advantage of lower bit-errorrates and less sensitivity to synchronization errors. view: channels,
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