Sunday 10 July 2022

5GHz: The Ad Hoc Link Band

Here in the UK, 5GHz link bands are being used for point-to-point back-haul communications, most cities and many towns have systems for providing internet access and links to enable camera surveillance systems (CCTV). In this post I outline what is going on.

The 2.4GHz WIFI system has long been available and in use for domestic and business Local Area Networks (LANs). However there is a problem with this system and it is linked to the reason that the microwave oven works. Such ovens work by using radio frequency (RF) power at around 2.4GHz, at this frequency the H-O bond of water (H2O) is excited, which warms the molecule, and as your food contains water it warms the food. However this presents a problem outside the home, when it rains the signal is absorbed by the rain and the link tends to degrade in performance or totally fail.

At 5GHz this effect does not happen, and in this frequency band an allocation has been made for LANs, but these LANs can use point-to-point antennae to give distances of hundreds of metres for a link. This allocation is license-free, unlike links at other frequencies which need to be licensed, here in the UK OFCOM manages those licenses.

Above is a picture of a cluster of such licensed links, mounted on a tower block, most of these are used to connect cellphone base stations into the network that connects them to switching centres. Because of the need for reliability of service the operators pay for an allocated channel that they alone have the right to use in this location. The equipment used in such systems is massively more expensive than alternate WIFI systems, but the range of the WIFI systems is less.

Unlicensed bands are a different matter, they use channels which have standards governing the power that can be used and performance of equipment, but otherwise can be used by anyone, So there is no assurance that another user won't interfere with you. Wikipedia gives a run down of channels here. Below is an example of a CCTV camera in a city in England.

The black nodule on the top of the central pole is a camera unit, with pan, tilt, zoom capabilities, purposefully designed not to be clear where it is looking (panopticon style). Attached to this pole by two struts is a smaller pole on which two off-the-shelf 5GHz WIFI antennae are mounted. These antennae are planar arrays (not steerable) and produced a beamed signal radiating from the front of the unit. Note that one is horizontal, one angled upwards.

The horizontal antenna aims along a street to a similar CCTV set up.

So the signal from, and control signals to, this site are routed via the previous camera system. Both of these are then linked to a node on top of an office block.

In the above shot two planar array antennae can be seen. 

Below is a mixture of cellular equipment (large rectangular cellular phone antenna to the right.) and sector antennae for the WIFI system (smaller rectangular antennae top left). The sector antenna allow a number of out-stations to connect across a wide horizontal area.

Using a WIFI app on a mobile phone these systems can be spotted as hidden SSID systems.

As noted above, there is a growth of use of the 5GHz WIFI band for point to point linking often, but not exclusively, within large urban areas in the UK. I expect this to be reflected across the world. They can be used to link together systems such as CCTV but are also being used to provide a cheap form of internet access. As an alternative to the sort of backhaul systems used by cellular phone companies they present a cheap option with a technology that is now robust and mature. 

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