e baseband digital performance spectrum. It is informative to note that in the digital microwave/multiplex 50.3.4 System performance vs. size system, both voice and data channels are affected similarly by microwave signal fading to the receiver threshold below which Further insight may be gained by comparing digital and analogue performance is unsatisfactory (see Figure 50.23). This is in contrast multi-hop systems. Table 50.1 shows the estimated Average Voice- V/N(dB) To-Noise ratio of a voice channel on a hypothetical multi-hop analogue FM microwave system for various numbers of hops and 60.00 Data channel 55.00 parameters given in Table 50.2. The Average Voice-To-Noise ratio on a digital system, however, is limited by the A/D conversion 50.00 1 E-10 45.00 DSM simulcast voice channel technique used in the channel equipment; for companded PCM this 40.00 PCM voice channel is about 37dB. In this example, the number of hops where the 35.00 analogue system Average Voice-To-Noise ratio has diminished to 30.00 1 E-6 37dB due to accumulating noise is 13. Where this crossover point 25.00 occurs is very strongly affected by the NPR per hop assumed, as will 20.00 be explained below; therefore, the number 13 should only be used 15.00 as a rough generalisation. 10.00 It would appear that system performance of the hypothetical 13 hop analogue and digital systems are quite similar even when one Received signal level in dB with respect to threshold of the hops experiences signal fading. This is because the Average Voice-To-Noise ratio of the analogue system will remain relatively Figure 50.23 Average voice-to-noise and data channel unchanged, until the system noise level is overcome by the noise of BER VS. received level on a digital microwave system the fading hop. This is illustrated in Figure 50.25, for the worst channel of the analogue system. For larger systems, the all digital 50/12 Line of sight radio systems V/N (dB) Table 50.1 Voice channel performance on analogue microwave 40.00 Digital (PCM) 35.00 Average (Total) 30