Microwave backhaul is a wireless link, between two dishes mounted on towers, that carries a mobile tower's traffic to the rest of the network where no fibre cable reaches the tower. Backhaul means the link from a tower to the operator's core network. Microwave here means radio waves of high frequency, from about 6 GHz up to about 80 GHz, sent in a narrow beam from one dish to another.
How does it work? A dish on one tower points straight at a dish on another tower. The two dishes must see each other, with no hill, building or tree in between. This is called line of sight. Each dish has a radio unit behind it. The radio unit turns the tower's data into a radio signal, and the far dish turns it back into data. Lower frequencies, around 6 to 18 GHz, can cross about 10 to 50 kilometres but carry less data. The highest frequencies, called E-band, around 70 and 80 GHz, can carry about 10 gigabits a second, but only over a few kilometres. Heavy rain weakens these high frequencies. So engineers plan each link with a margin for the monsoon, and the radio can switch to a slower but sturdier mode while the rain lasts.
Here is a typical case. A tower serves a village in the hills of Uttarakhand. The nearest fibre point is a tower in a town 18 kilometres away, across a valley. Laying fibre down one slope and up the other would take many months and cost lakhs of rupees. Instead, the operator mounts a dish on each tower. On the first day it is switched on, the link carries all the village tower's 4G traffic to the town. There, the traffic moves onto fibre and continues through the optical transport network to the packet core.
The traffic on the link comes from the radio units on the village tower. Several towers can also be joined in a chain, each passing its neighbours' traffic along, until the chain reaches fibre.
In India, a large share of towers have long used microwave, because laying fibre is slow and costly. Operators are now moving more towers to fibre for 5G, which needs more capacity. But microwave remains common in hills, villages and other places that fibre does not reach. Indian start-ups are also building links with much higher capacity than before.
How does microwave differ from fibre? Fibre carries far more data, and rain does not affect it. But fibre must be dug into the ground along the whole route, while microwave needs only two towers. The trade-off is explained in bandwidth and latency.