LTE-U

Cell-TowerRecently, the NCTA asked the FCC to make sure that wireless carriers don’t interfere with WiFi spectrum. I wrote a blog a few weeks ago talking about all of the demands on WiFi, and the threat that the NCTA is warning about is another use of the already busy WiFi spectrum.

Cellular carriers are using LTE technology to deliver 4G data. Cellular carriers today deliver 4G data and voice using spectrum for which they have paid billions (at least in the US and Europe). But in urban areas the LTE spectrum is already stressed and the demand for the existing spectrum is growing far faster than the carriers can find new spectrum to offload the extra demand.

The cellular carriers have had their eye on the 5 GHz unlicensed band of spectrum that is used for WiFi. This is a big swatch of spectrum that in some markets is larger than the band that some carriers have for LTE. Recently, various carriers have been experimenting with using this public spectrum to deliver LTE. Huawei and NTT demonstrated this capability last August; Qualcomm showed this capability at the CES show earlier this year. It’s rumored that T-Mobile plans to run a trial of this technology this year.

This new technology is being called LTE-U (for Unlicensed). NCTA filed at the FCC on behalf of their cable company members who use this WiFi spectrum to deliver WiFi for various uses such as distributing data wirelessly around a home or to bring data to settop boxes. They are worried that if the cellular companies start using the spectrum that they will swamp it and make WiFi useless for everybody else, particularly in urban areas where WiFi is under the most pressure.

That certainly is a valid concern. As my recent blog noted, the list of companies and technologies that are planning on using WiFi spectrum is large and growing. And there is already notable stress on WiFi around crowded places like large hotels, convention centers, and stadiums. The fear is that if cellular carriers start using the spectrum this same crowding will spread to more places, making the spectrum useless to everyone.

The cellular carriers argue that the swath of WiFi is large enough to allow them to use it without hurting other users. They argue that nobody can use all of the 400 MHz of spectrum in that band all at once. While that is true, it doesn’t take a huge pile of LTE-U customers at one time to locally overdraw the WiFi spectrum in the same manner that they are overloading the cellular spectrum today.

Engineers tell me that LTE uses the spectrum more efficiently today than does most WiFi technologies. This is due to the fact that the LTE specifications very neatly limit the bandwidth that any one customer can draw while most WiFi applications will let a user grab all of the bandwidth if it’s available. This means you can fit a lot more LTE customers into the spectrum that might be assigned to one WiFi customer.

There is a characteristic of WiFi that makes it incompatible with the way that LTE works. WiFi has been designed to share spectrum. When one customer is using WiFi they can grab a huge swath of spectrum. But when another customer demands bandwidth the system dynamically decreases the first connected customer to make room for the second one. This is very different than how LTE works. LTE works more like a telephone network and if there is enough bandwidth available to handle a customer it will assign a band to the customer or else deliver a ‘busy signal’ (no bars) if there us not enough bandwidth. The problem with these two different operating systems is that LTE would continually grab spectrum until it’s all used and the WiFi users are shut out, much like what you might get in a busy hotel in the evening.

The LTE providers say they have handled this by introducing a new protocol called LAA (Licensed Assisted Access) which introduces the idea of coexistence into the LTE network. If it works properly, LAA ought to be able to coexist with WiFi in the same manner that multiple WiFi customers coexist. Without this change in protocol LTE would quickly gobble all of the free WiFi spectrum.

But this still doesn’t answer the concern that even with LAA there could be a lot of people trying to grab bandwidth in environments where the WiFi is already stressed. Such a network never shuts anybody out like an LTE system will, but rather will just keep subdividing the bandwidth forever until the amount each customer gets is too small to use.

It will be interesting to see what the FCC says about this. This was discussed years ago and the FCC never intended to let licensed cellular holders snatch the public WiFi spectrum. I will also be curious to see if wireless carriers try to charge customers for data usage when that data is being delivered over a free, unlicensed swath of spectrum. And how will customers even know that is where they are getting their data?

I hope the FCC doesn’t let the wireless carriers run rampant with this, because I think it’s inevitable that this is going to cause huge problems. There are already places today where WiFi is overloaded, and this new kind of data traffic could swamp the spectrum in a lot more places. The wireless carriers can make promises all day about how this won’t cause problems, but it doesn’t take a huge number of LTE-U users at a cell site to start causing problems.

Why Not 3.65 GHz?

Transmitter_tower_in_SpainAny company about deploying point-to-multipoint wireless data services ought to be thinking about using the 3.65 GHz spectrum. Unless you happen to own other licensed spectrum, this is probably your best alternative to using the normal unlicensed spectrum. But in many places the normal unlicensed bands of 900MHz, 2.4GHz, and 5.8GHz are congested, and are getting more so every day. I’ve written earlier blogs talking about how all of the cable companies and telcos are now using unlicensed spectrum routers at almost every home. And the Internet of Things is going to pile a ton of new uses onto unlicensed spectrum everywhere.

The FCC authorized the 3.65GHz – 3.70GHz frequency for public use in 2006, with some usage rules to maximize the utility of the spectrum. The rules are aimed to provide the most benefit to smaller markets and less densely populated areas. This can mean a cleaner signal for any carrier deploying a point-to-multipoint wireless services. A few of the rules include:

Restricted Locations. The spectrum cannot be used close to existing government installations or satellite earth stations that use the spectrum. So you can’t deploy around some of the larger air force bases and around a handful of remaining satellite earth stations. The FCC maintains a list of the restricted locations. It should be noted that the earthstation market has been consolidating and over the last few years a number of older earthstations have been decommissioned. This restriction does not block the spectrum in too many places.

Licensed Use. You can license the spectrum for a $280 fee. However, such a license is not exclusive and every holder of the spectrum is expected to coordinate with other users. This is not like a normal FCC license and it is not first come first serve. Everyone using the spectrum in a given area is expected to work with others to minimize interference. The FCC will act as the arbiter if parties can’t work things out. I would point out that in a point-to-multipoint deployment it I fairly easy to keep interference to a minimum.

Contention. There are different rules for using the spectrum depending upon how you deploy it. The rules promote using radios that deploy other spectrum in addition to 3.65 GHz. For radios that only use this spectrum the usage is limited to the 25 MHz band between 3.65 and 3.675 GHz. But radios that allow for a shift to other frequencies when there is contention can use the full 50 MHz channel within the frequency.

The frequency can support bandwidth on one channel up to a theoretical 37 Mbps download. But real life deployments are called somewhere around 25 Mbps close to the transmitter.

Radios for this frequency are readily available from most of the major point-to-multipoint radio manufacturers. The price of the base stations and customer CPE are very much in line with the cost of radios in the unlicensed bands.

One advantage of this spectrum is that it can go a significant distance. It can theoretically work to the horizon, but the throughput diminishes with distance. Life with most bandwidth, you can engineer to get good bandwidth at the outside of your range by sacrificing bandwidth close to the antenna, or you can alternately go for big bandwidth close to the tower with decreasing bandwidth with distance. It’s easy to engineer a system that can deliver 10 Mbps download at five miles. We’ve seen 3 Mbps at 9 miles.

This frequency is best used in a rural deployment, because the bandwidth from a given sector of a basestation is shared with all of the customers using that sector. Like with any shared bandwidth technology, the more customers you cram onto the system, the less bandwidth available for each customer, particularly at peak times.