ot uncommon. Another type of hybrid DFM=30dB system is formed by the use of T-1 baseband modems, which permit the transmission of DS-1 signal over analogue microwave facilities. Certain of these configurations will also be considered. 50.3.1 Noise The distance over which one hop of microwave radio can provide reliable communications is limited essentially by line-of-sight dis- FFM (dB) tance, i.e. the antennas of the two stations must be within visible range of each other. Within this limitation, the distance is further limited by multipath fading due to atmospheric effects which Figure 50.22 CFM VS. FFM becomes rapidly more severe as the path length increases, and is worse for higher frequencies. For even higher microwave frequen- Digital and analogue microwave systems 50/11 cies the phenomenon of rain attenuation becomes more important to the difference in voice channel and data channel performance in than multipath fading. an analogue system, where voice channel performance remains In short, the typical microwave path is relatively short, shorter usable (above 15dB Average Voice-To-Noise) when the received than line-of-sight, and is designed for the desired path availability signal level has faded below the level which will cause data channel (these topics have been analysed in earlier sections). Thus, in order outage. Figure 50.24 illustrates this for a 32 QAM 12Kbit/s data to extend a communications system to reasonable sizes, multiple channel which occupies a voice channel on an analogue microwave hops, or relays, of microwave are required. system. The concept of cumulative noise in analogue systems is well known. For analogue microwave/multiplex systems, the basic noise V/N (dB) level is determined by the Frequency Division Multiplex channel 1 E 14 equipment, and the microwave radios over which the channel is 534KHz 1 E 12 conducted adds to this noise. The noise of each microwave hop adds 2438KHz to what is already present, thus, the longer or larger the system, the 1 E 10 534KHz worse the signal-to-noise 