Microwave chain availability and protection on a rural backhaul
A regional operator backhauls 14 rural 4G/5G sites over a seven-hop Nokia 9500 MPR microwave chain. Hops are 8–14 km at 15/18 GHz with 56 MHz channels. Hop 4 is 11 km at 18 GHz, 256QAM, 420 Mbit/s, and drops to 64QAM for 40–90 minutes during heavy rain, halving 4G cell throughput and causing X2 handover failures. The chain has no protection. The site is in ITU rain zone K (42 mm/h at 0.01%). Plans are a second 18 GHz radio on hop 4 for 1+1 HSB and a 10 Gbit/s fibre spur at site 7 terminating on a 7250 IXR-R6 with an 1830 PSS-4. The SLA is 99.95% availability and under 50 ms protection switching. Site 4 has 48 V DC, 6U free and a 300 W budget. All sites take 1588v2 timing from the core.
1. Estimate the rain fade margin hop 4 needs for 99.95% availability. Does 1+1 HSB on the same antenna deliver it?
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Use ITU-R P.837/P.530 methods as the check, not as a substitute for the tool.
At 18 GHz, specific attenuation at 42 mm/h is roughly 3.4 dB/km (k ≈ 0.08, α ≈ 1.0), so 11 km gives about 37 dB at 0.01% of the year. For 99.95% you need the 0.05% rainfall rate, which is lower - planning figures put the required fade margin at roughly 24–27 dB. Verify with the local rain-rate map and the actual antenna gain and feeder loss.
256QAM needs an SNR of about 30 dB, so the link will drop modulation well before the 25 dB fade is reached. That is expected; ACM is doing its job. The problem is that the outage lasts 40–90 minutes, so the low-modulation state still cannot carry the traffic.
1+1 HSB on the same antenna, same frequency band and same path gives no rain-diversity gain, because rain fading is highly correlated across the two radios. It protects against radio and feeder hardware failure only. It does not deliver 99.95%.
2. What protection architecture would you build with the fibre spur at site 7, and how do you guarantee sub-50 ms switching?
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Close the ring. Use the site 7 fibre spur to provide a second path from the far end of the chain back to the core, converting the chain into a ring. Site 7 then has two diverse routes: the microwave chain on one side and fibre on the other.
Protection layer. Run G.8032/ERPS on the microwave and fibre ring, or MPLS-TP 1:1 with BFD. The important property is pre-provisioned protection with a pre-computed backup path - not IP/MPLS reconvergence, which will not meet 50 ms on seven hops.
Sub-50 ms budget. With 10 ms BFD × 3 (30 ms) plus about 10 ms repair, you land at roughly 40 ms. Configure the RPL owner and verify that the fibre spur is genuinely diverse from the microwave route; a shared trench or a common duct destroys the protection argument.
Keep 1+1 HSB for hardware. It is still worth installing on hop 4 for equipment failures, but position it as hardware protection and let the ring carry the rain-fade availability target. For link budget, consider a lower band (15 GHz has meaningfully lower rain attenuation than 18 GHz) on the second hop-4 path if the ring design allows.
3. Design the 1588v2 timing chain across the ring. What breaks when the ring protects, and how do you prevent it?
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Grandmaster placement. Put the grandmaster at the core, on a PRTC-quality source, and distribute PTP over the fibre spur where delay is symmetric and well behaved.
Across microwave. Each 9500 MPR hop adds packet delay variation and asymmetry. Use boundary clocks at aggregation sites to regenerate the timing rather than chaining seven transparent hops, and check that each hop supports 1588v2 timestamping at the required accuracy class. If any hop is not PTP-aware, the end-to-end budget will not close.
Ring protection interaction. When ERPS or MPLS-TP switches, the PTP path changes and BMCA may re-converge slowly or pick a worse source. Configure G.8275.1-style static master/slave relationships and alternate BMCA, so path changes do not trigger a timing re-election storm.
Holdover. Size OCXO holdover at the far sites for at least the expected repair time (target 4 hours), and test that 4G/5G air-interface sync holds through a protected switch and a fibre cut. A timing outage shows up as cell loss, not as a transport alarm, so include PTP in the protection test plan.