Wi-Fi is growing in popularity despite (or because) it operates in unlicenced spectrum. LTE-U (using LTE technology in the Wi-Fi spectrum bands) has become another recent hot topic.

Taking things a step further, several backhaul vendors are proposing use of unlicensed (Wi-Fi) spectrum to connect Urban Small Cells or remote radio heads back into the network. Is this a sensible fit with delivery of consistent and high quality cellular service?

Licenced vs Unlicenced spectrum

Certain frequency bands are allocated exclusively for use by specific organisations. These include the precious sub 3GHz frequencies allocated for mobile phone use, where each mobile phone network often pays substantial amounts for exclusive use.

Other frequencies are set aside for public use with restrictions of the RF transmit power and certain other characteristics to avoid conflicts with other bands and users. Think of it like sharing a village common in olden times, where anyone had the right to let their cattle graze rather than in dedicated fields owned by each farmer. It requires some good manners and co-operation to avoid abuse, but on the whole works quite well.

A further set of frequencies may be lightly licenced, in that they are intended for organisations to make use of within their networks but in an appropriate manner. These include frequencies such as those at 3.6GHz and the 60GHz V-Band. Often little or no licence fee is required but the regulator may insist on being notified about each link deployed.

Using unlicenced as an extension, augmenting licenced bands

LTE-U is designed to always require a standard cellular connection over licenced spectrum. Additional capacity is provided by using Carrier Aggregation on unlicenced frequencies. The argument is that should anything untoward happen on the uncontrolled/unlicenced bands, then the network can continue to communicate and resolve the situation.

The same principle is also relevant for some of the quality driven Wi-Fi solutions, which offload to Wi-Fi where possible. These continuously measure the quality of service being received and would switch back to the cellular network if needed.

Lightly licenced bands are popular

Several companies have successfully marketed 60GHz V-Band products for Small Cell backhaul, with data rates of 100Mbps and up, benefitting from the low cost of lightly licenced spectrum. These are ideal for the short distances required for Urban Small Cells, typically connecting back into a nearby hub on a rooftop or at street level. Lightly licenced bands are also available to support Non-Line-of-Sight, such as at 3.65GHz.

Other companies continue to argue that traditional microwave frequencies are a worthwhile option. Spectrum choices vary widely in different countries and there are no hard and fast rules. Hardware using these frequencies may benefit from many years of development for a larger market, reducing costs. However, the products needed for street level deployment have to be much more compact and lower cost than those designed for longer range macrocell backhaul.

Licenced TD-LTE spectrum could be used for backhaul too

There's quite a lot of spectrum allocated for LTE at 2.6GHz, of which some has been auctioned off to operators. In many jurisdictions, there are no constraints about what radio technology is used (as long as it doesn't interfere with other networks or users). Several NLoS backhaul vendors point to this as an extremely cost effective and useful option. The capacity of a fixed backhaul link can be much greater than that of the same spectrum used to communicate with a smartphone, simply because neither end is moving and it's not shared with hundreds of other devices. Doubling the capacity isn't unrealistic, which means a single 10MHz TDD band could support the traffic from a 10+10MHz FDD band.

However if this is shared by several small cells within the same geographic area/sector then the backhaul links wouldn't benefit from the same spectrum reuse that Small Cells do.

Small Cell backhaul using unlicenced spectrum

Some companies have been touting unlicenced spectrum for Small Cell backhaul. The frequency proposed is in the 5GHz range, typically allocated to Wi-Fi. Use of these frequencies may be subject to Dynamic Frequency Selection and/or Transmit Power Control limits. The regulations vary between continents and countries, and is documented quite well on Wikipedia

Two unrelated approaches come from:

• Fastback Networks, a US startup who offer their own Non-Line-of-Sight technology operating in the unlicenced 5GHz band. They claim speeds of up to 500Mbps and use extensive noise and interference cancelling algorithms to cater for the most hostile RF environments. Having overcome the difficulties of achieving NLoS service in urban areas, they believe that link reliability isn't much further affected by other signals interfering on the same frequencies.
• eBlink, a French startup, take a completely different approach. Rather than deploy Small Cells, they promote the use of remote radio heads connected over their wireless links using unlicenced 5.8GHz spectrum. This means transmitting much higher bandwidth (a CPRI datalink can be 50x the user data rate, so typically 2Gbps or more is needed). Their latest design handles up to 3x CPRI links, so could drive 3 LTE sectors at 20MHz each. The physical size of their transmission unit seemed a little larger to me than other small cell backhaul products. Unlike Small Cell deployments where the baseband processing is included in the radio head, the total system cost would have to include the serving macrocell basestation equipment. This extra cost may be justified for some by being able to tightly co-ordinate LTE-Advanced features and synchronise across multiple radio heads.

The choice of using exclusively unlicensed spectrum is for some a step too far. Mobile network operators want to differentiate from free, best effort Wi-Fi service by providing a consistent and higher quality service. Some would see the risk of using shared spectrum as too high, and look for protection from uncontrolled and unexpected interference.

Some expect unlicensed spectrum to be used in adjacent markets such as for fibre extension, by cable companies and fixed wireless access by providers without spectrum who want to increase their existing service footprint.

Perhaps accommodating that view, Cambium Networks has added a licensed 3GHz option to their PMP450 product, which had previously used unlicensed bands at 2.4GHz and 5.8GHz. Scott Imhoff, VP Product management, said this "allowed network operators around the world to expand their service offerings with confidence".

Tarana Wireless, another NLoS start-up, offers both licenced and unlicensed options. Rakesh Tiwari, Director of Product Management, told me he "believes unlicensed spectrum will be used by some Tier 1 operators for small cell backhaul, primarily for sparse point-to-point links. He didn't see operators using 5.x GHz for dense urban small cell environments co-located with LTE small cells with integrated 802.11ac Wi-Fi access points."

CCS, another start-up but this time offering a mesh based multi-point to multi-point architecture, have chosen to launch using traditional microwave bands in the 28-43GHz range with their first offering capable of 960Mbps throughput per hub at 28GHz. Steve Greaves, their CEO, said they weren't married to any particular frequency. They've looked at a wider range of solution requirements, from physical form factor, deployment time/cost through to minimising subsequent site visits. As their technology evolves, they may offer it in other bands but he also felt that lightly licenced bands would be a better choice.

Summary

There are undoubtedly use cases for fixed wireless links operating on unlicenced spectrum. Some believe these extend to short range Small Cell backhaul, where the latest technology can deal with the potential of high levels of interference. This may be easier when point-to-point links are used, but there is potential to add risk to system availability and performance.

Others are more cautious and would prefer to choose lightly licenced bands, which offer the benefit of lower (or zero) ongoing licence fees with access to quite significant chunks of as yet unused spectrum.

A number of vendors offer both options, and many would be able to adapt their technology to either.

As with many options in the telecom industry, there are no hard and fast rules about what is best. The correct answer will vary between countries and even between operators. I expect that use of Wi-Fi as an access technology will continue to grow and become more closely intertwined with cellular data service. That will involve improving the consistency and predictability of the quality of service delivered. In areas of peak capacity, where Wi-Fi is also heavily used, I'd expect the backhaul would be more effective where it avoids congestion of those shared channels.