 first four steps. 9.2.3.2 Setting Performance Requirements. As we remember from Chapter 6, the performance of a digital system is expressed in a bit error rate (bit error ratio) (BER). In our case here, it will be expressed as a BER with a given time distribution. A time Optical Radio horizon horizon Figure 9.3 Radio and optical horizon (smooth earth). 9.2 RADIO SYSTEMS distribution tells us that a certain BER value is valid for a certain percentage of time, percentage of a year, or percentage of a month. Often a microwave link is part of an extensive system of multiple links in tandem. Thus we must first set system requirements based on the output of the far-end receiver of the several or many links in tandem. If the system was transmitting in an analog format, typically FDM using frequency modulation (FM), the requirement would be given for noise in the derived voice channel; if it were analog video, a signal-to-noise ratio specification would be provided. In the case we emphasize here, of course it will be BER or a derivative unit such as errored second (ES) on the far-end receiver digital bit stream. The requirements should be based on existing standards. If the link (or system) were to be designed as part of the North American PSTN, we would use a Bellcore2 standard (Ref. 1). In this case the BER at the digital interface level shall be less than 2 X 10-10. excluding burst error seconds. Another source is CCIR/ITU-R. For example CCIR Rec. 594-4 (Ref. 2) states that the errored second ratio (ESR) should not exceed 0.0032 of any month. The ESR is defined in this reference as the ratio of errored seconds (ES) to total seconds in available time during a fixed measurement interval. These specifications are considerably more stringent than in previous editions of the reference, the more stringent value being driven by demands of data users. Available time in this context is defined in the context of "unavailable time" as follows: A period of unavailable time begins when the bit error ratio (BER) in each second is 