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Impact on System Performance
For operations managers, the impact of migration on system reliability is very important.
In this section, we will answer these questions related to fade margin, availability and path obstruction clear- ance. Fade Margin and Availability Questions related to fade margin and availability are best answered by reviewing an example, single-hop, path calculation. For illustration purposes, let's assume that the hop will be migrating from 1920 MHz to 6650 MHz and has a fixed length of 20 miles. An 8-foot diameter antenna will be used at each end of the hop for both the 2-GHz and the 6.5-GHz systems. Other parameters needed for path calculations, such as trans- mission line loss, transmitter power, receiver threshold, are noted in Figure 9. Figure 9 - System Performance, Fade Margin and Availability - System Parameters
* Digital RF equipment parameters will differ. Figure 10 displays the path calculation results for opera- tion at both 1920 and 6650 MHz. These calculations reveal that the fade margin at 6650 MHz increases by |
13.1 dB compared with the fade margin of the original 1920 MHz system. Associated with this increased fade margin is an improvement in system availability from 99.999847% (48.7 seconds/year outage) to 99.999972% (8.2 seconds/year outage). Figure 10 - System Performance, Fade Margin and Availability - 20-Mile Path
The reason for this performance improvement is that the total antenna gain is increasing by a factor of 40*Log (frequency) where the path loss is increasing by only 20*Log (frequency). The net result is an increase in fade margin and an improvement in system availabil- ity. Please note that this is only an example and that each system should be evaluated. In some cases, where an existing analog system is upgraded to digital, space diversity may be required to achieve the desired system Bit-Error-Rate (BER) performance. Remember that addi- tional antennas and transmission lines will create added tower wind loading. Obstruction Clearance (Fresnel Zone Clearance) When migrating, you should also analyze the clearance between your path and obstructions along the path. In some situations, excessive clearance may cause degrad- ed system performance. The required antenna height is based on path profile and clearance from obstructions. The criterion used to deter- mine the required antenna height is the first Fresnel zone. Reflections of signal from obstructions that are near the first, or other odd-numbered Fresnel zone bound- ary(ies) could negatively impact performance. Fresnel zone calculations are frequency dependent. The highter the frequency, the smaller the Fresnel zone. |
