18 GHz Microwave Path Obstructed by a New Building
A state utility operates an 18 GHz point-to-point microwave link over a 22 km path between two hill sites in Rajasthan. The link uses 0.6 m dishes, vertical polarisation, 20 dB fade margin and a required availability of 99.99%. A new 35 m building is approved at 17.5 km from the transmit end; its roof is 5 m above the current line of sight. The path's first Fresnel zone at that point has a radius of about 7.7 m, and design practice requires 60% clearance. The link currently carries 8×E1 plus Ethernet and cannot be down for more than 30 minutes during migration. Rain statistics show a 1-minute rain rate of 42 mm/h for 0.01% of the time; the 18 GHz rain attenuation is estimated at 14 dB. The project team asks whether to raise the tower, increase transmit power by 6 dB, or change band.
1. Assess the obstruction using the Fresnel-zone figures. Determine the clearance and state whether the link should be redesigned.
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The required clearance is 60% of 7.7 m = 4.6 m. The building roof is 5 m above the geometric line of sight, so the clearance is −5 m, not +4.6 m; the path is obstructed and the first Fresnel zone is substantially blocked. Diffraction loss will be several dB and will vary with the building profile, so the 20 dB fade margin cannot be assumed intact. The link should be redesigned, not merely re-tuned. To restore 4.6 m clearance the line of sight at the obstacle must rise by 5 + 4.6 = 9.6 m. Because the obstacle is 17.5 km from the transmit end and 4.5 km from the receive end, raising the receive tower by h raises the line at the obstacle by about h x 17.5/22 = 0.795h; a 12 m raise at the receive end would be needed. Raising the transmit end would require about 47 m because its lever arm is only 4.5/22. Moving the building or the path is therefore worth evaluating first.
2. Evaluate the three options against the fade margin and availability requirement. Include the effect of the 14 dB rain attenuation.
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With 20 dB fade margin and 14 dB rain attenuation at 0.01%, only 6 dB is left for multipath, obstruction and interference. If the building adds even 6–10 dB diffraction loss, the effective margin is zero or negative, so the link cannot meet 99.99%. Raising power by 6 dB raises the margin to 26 dB and leaves 12 dB after rain, but it does not remove the obstruction and may exceed the authorised EIRP or saturate the receiver; it is at best a temporary measure. Changing to a lower band, for example 7 or 8 GHz, reduces rain attenuation but increases the Fresnel radius (the radius scales as the square root of wavelength), so the building obstruction becomes harder to clear and larger dishes are needed. A tower raise at the receive end plus retention of 18 GHz, or a move to 11 GHz with a fresh path design, is the more defensible option. Space diversity or a protected path can also help availability.
3. Propose a migration plan that respects the 30-minute outage limit and lists pre-cutover tests.
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Treat the existing link as the service-protection path until the new path is proven. Build the new tower/antenna or new band in parallel, then commission it with a path-loss measurement, RF sweep, BER test at the required capacity, ATPC and fade-margin check, 24 h error-free run, and a rain-fade test if possible. Pre-cutover checks: receive level within 3 dB of design, Fresnel clearance, VSWR, frequency coordination, synchronisation and 8xE1 plus Ethernet BER, RFC 2544 throughput, and a rollback procedure. Cut over in a low-traffic window with the old link hot; the actual outage should be limited to the protected switchover, well under 30 minutes. Keep the old link for at least one monsoon season if the licence and space permit.