7 GHz Microwave Link Fade Margin Under Rain and Multipath
A BSNL JTO is designing a 7 GHz microwave link between two exchanges 32 km apart. The transmitter output is 28 dBm, each antenna has 36 dBi gain, each end has 2.5 dB feeder loss and 1 dB branch loss. The receiver threshold for BER 10^-6 is -76 dBm. Site survey data show a 4 dB rain attenuation exceeded for 0.01% of the time and a required multipath fade margin of 20 dB. Use the free-space loss formula FSL(dB)=92.45+20log10(f_GHz)+20log10(d_km). The link must satisfy both the multipath and rain requirements simultaneously, with at least 3 dB additional margin. The JTO also has to decide whether to accept the vendor’s proposal to reduce antenna size to 32 dBi to cut tower loading.
1. Calculate the free-space loss and the received signal level. Determine the available fade margin and decide whether the link closes with the required multipath, rain, and additional 3 dB margin.
Read the worked answer
FSL = 92.45 + 20log10(7) + 20log10(32) = 92.45 + 16.90 + 30.10 = 139.45 dB.
Received power: P_rx = Tx + G_tx + G_rx - feeder losses - branch losses - FSL = 28 + 36 + 36 - (2.5 + 2.5) - (1 + 1) - 139.45 = 28 + 72 - 5 - 2 - 139.45 = -46.45 dBm.
Fade margin = P_rx - receiver threshold = -46.45 - (-76) = 29.55 dB. Required margin = multipath 20 dB + rain 4 dB + additional 3 dB = 27 dB.
29.55 dB > 27 dB, so the link closes with 2.55 dB spare above the stricter requirement. It also satisfies the simpler simultaneous 24 dB requirement. The design is acceptable, but the spare margin is modest; verify the rain and multipath assumptions and ensure the antenna alignment and feeder losses are as predicted.
2. Evaluate the vendor’s proposal to use 32 dBi antennas instead of 36 dBi. Show the effect on received level and margin, and state your decision with two possible alternatives if the link must still close.
Read the worked answer
Reducing each antenna from 36 dBi to 32 dBi removes 4 dB at each end, i.e. 8 dB total. The new received level is: P_rx = -46.45 - 8 = -54.45 dBm. New fade margin = -54.45 - (-76) = 21.55 dB.
This is below the 24 dB simultaneous rain-plus-multipath requirement and well below the 27 dB requirement with the additional 3 dB. Therefore reject the 32 dBi proposal as it stands; the link would not meet the design objective.
If tower loading must be reduced, alternatives are: (1) increase transmitter power or use a higher-gain antenna at only one end and verify tower loading on that side; (2) shorten the path by adding a repeater or selecting a nearer site; (3) use space diversity or a more robust modulation/coding scheme if the radio supports it. Each option must be re-evaluated against the 27 dB margin, not just the 24 dB minimum.