TABLE OF CONTENTS In this final chapter, research and development trends, as well as unresolved problems related to digital microwave communications, are summarized.
A number of modulation and signal processing techniques which have been developed and successfully employed in line-of-sight terrestrial digital microwave systems will also find application in satellite and optical fiber systems. Present digital satellite systems use 4-phase QPSK modulation techniques. The next generation of digital communication satellites will be required to be more bandwidth-efficient than the theoretical limit of QPSK (2b/s/Hz). Previously, digital terrestrial microwave systems used, mostly, QPSK modems; but the trend now is to replace them with QPRS (correlative coded) 8-PSK and 16-QAM modulation techniques. It is expected that the same trend will continue for many new satellite systems applications.
As a result of frequency congestion problems in the lower frequency bands, a number of systems shall be designed to use frequencies in the 15 GHz range. Some of these systems shall be used for high-capacity long-haul systems (5000 or more digitized telephony channels); others will be designed for medium-capacity (100 to 1000 digitized voice channel) short-haul links.
Unfortunately, we do not foresee a short-term solution for the problems created by the different digital hierarchies. As explained in Chapter 1, the European, U.S./ Canadian, and Japanese hierarchies are different. These differences do not pose a major problem for terrestrial microwave and optical fiber systems since these systems are confined for digital transmission within the continent. However, when these systems are interfaced with...
