Frequency spread

5 mins read

WiFi is being pushed to higher speeds and longer ranges and, in the process, is acquiring more spectrum to help it on its way.

The work never seems to stop on the IEEE 802.11 standards committees. It's a testament to WiFi's market success, with more than 13 billion devices installed, that the work just keeps going on pushing the wireless communications standard to higher and higher bandwidths.

Following the standard's approval this time last year, the hardware for WiFi-6 is only just arriving but work is already advancing on its even-faster successor. Currently known as 802.11be, this variant will likely end up being called WiFi-7.

If the work is successful, devices will achieve aggregate data rates as high as 30Gbit/s per access point for indoor networks. Its development continues to cross over with the cellular standards as the quest for more bandwidth on the move continues.

One of the design aims of 802.11ax or WiFi-6 was to make the protocol perform better in large public spaces. Better support for more than one high-demand user is similarly on the cards for 802.11be, which is slated to appear as a draft later this year, improving on some of the features found for the first time in WiFi-6.

A big change in the current version was the adoption of the 6GHz band, which the US Federal Communications Commission opened up for licence-free use similar to that for the existing 2.4GHz and 5GHz bands a little under two years ago. South Korea quickly followed by clearing the full 1.2GHz of bandwidth. The UK and the 19 members of the European Conference of Postal and Telecommunications also made spectrum available but only the lower 500MHz initially and there are cut-outs in Europe for rail and transport network applications.

Though one advantage of the additional 6GHz band is access to what is today a relatively uncluttered piece of spectrum without the line-of-sight and range issues that plagued attempts to extend WiFi into the 60GHz range the bigger difference will probably come in the way 802.11be access points manage it.

Above: This shows one of the configurations for multilink operation where control, uplink and downlink are split across the bands.

The presence of a 1.2GHz-wide band in the US and South Korea at least makes it possible to aggregate many channels into one that in 802.11be could be as wide as 320MHz, double that of the maximum in 802.11ax. Such bonding can be a problem in congested spectrum though a technique called preamble puncturing can address it. In this situation, clients have access to subcarriers, called resource units in the standard, that are scheduled to prevent them clashing when sending data to the access point. The preamble puncturing clears space used by older forms of WiFi and other transmissions so that WiFi-6 clients do not try to use the affected subcarriers.

A big advantage of the resource-unit system is that the spectrum can be chopped up in many ways to suit the needs of different kinds of users instead of trying to deliver large chunks of bandwidth for traffic that cannot fully use it.

One of the potential features that 802.11be may deliver is smarter use of that management, using not just subcarriers within a larger band but splitting the traffic for the same client across multiple bands. As we can expect 802.11be-capable clients to be able to transmit on the 2.4GHz, 5GHz and 6GHz bands it makes sense to give them the option to use more than one at once, as long as they can tolerate the increased power consumption this implies.

Multilink operation

There are a number of possibilities for 802.11be's multilink operation. One is to perform relatively straightforward load balancing across the channels with the clients reassembling the data as needed. Other approaches might be to take advantage of the different transmission properties of the individual bands by reserving, say, the wide channels in the 6GHz band for downlink traffic and one of the others for uplink.

A more sophisticated variation on that is to put short control messages onto the heavily congested 2.4GHz band, which can more readily cope with those short messages, with the uplink going on 5GHz.

Another way to provide high bandwidth to more users at once in congested spaces is to have access points cooperate with each other more. They can do that through spatial multiplexing, which should receive a boost with the future standard, which like 802.11ax will have access to up to 16 spatial streams. Though beamforming is tightly controlled in the WiFi space to avoid breaching severe limits on power, the access points will be able to use it to steer beams away from each other. For example, an access point may deliver high-capacity traffic in the direction away from another nearby access point, which itself directs its channels towards users in the opposite direction.

Another option is joint transmission, borrowed from the techniques developed for the LTE-Advanced cellular standards, in which neighbouring access points cooperate to become, in effect, a super-base station. It is tricky to achieve because of the need for tight timing synchronisation between the access points but it allows the access points to do a form of beamforming and direct more energy to devices that lie between the cooperating access points.

Though much of the market's attention is focused on faster WiFi, the 802.11 workgroups are attempting to move the core protocol into the low-power, longer-range environments that characterise the internet of things (IoT).

One proposed standard, 802.11bf, would make WiFi operate as a passive motion sensor. Though many systems use signal-strength measurement already for positioning, 802.11bf aims to bolster that with phase and Doppler-effect measurements and cooperation between devices to improve overall accuracy. For widespread motion sensing to be worthwhile, portable devices will need to be able to operate with lower-power versions of WiFi.

Published in 2017, IEEE 802.11ah or HaLow, was the IEEE workgroup's second attempt, following the 802.11af version intended for TV ‘white space’ spectrum, to extend WiFi in another direction: down the frequency chart so it could fill in the apparent gap between low-power wide-area networks and local-area networks.

Above: APs use beamforming to direct transmissions to specific clients, use coordination to avoid interference and coordinate jointly to send data to the clients in range of more than one AP

The protocol uses licence-exempt sub-gigahertz bands to push the achievable range up to 1km. It trades energy against data rate and includes power-saving modes so that devices running off small batteries can use it. HaLow carries over the use orthogonal frequency division multiplexing from the old 802.11a and .11g specifications that drove early use of WiFi, and which continues to be at the core of the newer standards. The use of quadrature-amplitude modulation with as many as 256 symbols (256-QAM) can, in principle, deliver data at up more than 200Mbit/s though far lower rates are more realistic for low-power devices. At the high end, 802.11be will push QAM to 4K symbols.

Recognising that a significant energy consumer in wireless networks is the need for receivers to stay active just in case a message is sent to that node, HaLow took a leaf out of the operating manual for networks like LoRaWAN. These divide the reception window into time slices synchronised with a basestation. A device does not have to listen outside its designated window, so that it can put to sleep not just the host processor but the entire RF subsystem until the internal clock signals that the next window has opened. Gateways will cache data destined for a device with a restricted access window until it is scheduled to wake up and activate its receiver.

The problem with time-slot allocations is an inevitable increase in average latency. The 802.11ba wake-up radio proposal may provide a fix. In this mode, a receiver stays on but can turn off most of its circuit in favour of one that that listens for a specific wakeup packet type, based on a simple on-off key sequence transmitted at low datarates over multiple carriers. The target active power consumption for such a receiver is less than 1mW.

In truth, the 802.11 workgroups have spawned a forest of standards, many of which have not been anywhere near as successful as the comparatively small collection of protocols that remain centred on the traditional home- and office-networking market. The current supply crunch will likely extend the life of the currently mainstream forms of WiFi and push the adoption of WiFi-6 and WiFi-7 further into the future.