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.

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