The planning process of fixed micro-wave radio links consists mainly of three parts:
- Link budget calculation
- Performance and availability calculation
- Interference calculation
With the link budget calculation the purpose is to ensure that you have line-of-sight between the sites and that the antennas, feeders and transmitters give you an input power in the receiver that is high enough. The fading margin between receiving input power and the receivers threshold for BER = 10-6 must be large enough to be able to meet the planning objective target in the performance calculation. The picture below shows the link budget planning in PROGIRA®plan.
In the performance calculation you set your planning objectives. The planning objectives are given in the ITU recommendations G.821, G.826 and G.828. You calculate the performance and availability and compare the result with the planning objectives. What you calculate is the probability that the fading margin is reduced due to multi-path fading, selective fading and rain fading. The probability calculation is given in ITU-R P.530.
In the interference calculation you calculate the interference level from other fixed micro-wave radio links. For the interference calculation you set the CIR (Carrier-over-interference ratio) value. CIR is often set to 15-20 dB below the receivers threshold level for BER = 10-6 . If the interference level is below the sum of receivers threshold level for BER = 10-6 + CIR than the interference will not affect your performance calculation.
For all calculation needed for proper planning of fixed micro-wave radio link a planning tool is needed. PROGIRA®plan have now a module for complete planning of fixed micro-wave radio links. The calculations are all based on the latest ITU recommendations.