ratio becomes. There is no easy way to 1248KHz Voice 1 E 8 1248KHz avoid this phenomenon or to remove the accumulated noise and 1E - 6 2438KHz distortion. It is commonly assumed that one of the great advantages of digital 1 E -4 microwave/multiplex (and digital communications systems, in 1 E 2 general) is that, unlike analogue systems, noise does not accumu- late. Within limits, this is true, however, digital microwave/multi- plex systems will be shown to have other noise phenomenon to consider. Received level (dBm) Noise phenomena may be used to compare analogue and digital microwave systems on a static basis which is the 'normal' operating condition, and also on a dynamic basis where microwave signal Figure 50.24 Average Voice-To-Noise and 32 QAM data fading reduces system availability. channel BER vs. received level on a 600 channel analog FM system (with emphasis) 50.3.2 Voice Either analogue or digital microwave may be used to transmit voice The digital system equivalent to baseband frequency slot alloca- communications effectively. However, the technical jargon used to tion is time slot allocation, which makes absolutely no difference in describe system performance differs somewhat for analogue and channel performance, unlike the analogue FM system where the digital microwave systems, thus tending to make comparisons based highest baseband frequency slot usually has the worst noise. In the on voice communications difficult. analogue system, serious consideration must be given to the assign- In this chapter, we will use the concept of Average Voice Power- ment of baseband frequencies, especially when assigning such sen- To-Noise ratio as a common parameter to compare the voice chan- sitive devices as the 256 QAM T-1 baseband modem. This type of nel performance of analogue and digital equipment. baseband modem may begin to make bit errors for received levels as much as 20dB above the analogue microwave FM receiver 50.3.3 General comparisons of analogue and threshold, unless it is assigned to the low end of th