Ham LTE is an accident of international spectrum allocations.
And its future is different for each country.
I’ll talk primarily about USA hams for today, since I’m one.
LTE has specified bands. If your phone supports LTE band 3, and an eNodeB (base station) operates on band 3, it is expected to ‘just work’.
Ham radio has specified operating bands as well, encoded in law or regulation.
There are several LTE bands that have overlap with ham privileges - band 8 for instance has overlap with the US ham radio 900MHz band, as well as the 900MHz ISM band that is extremely popular. Unfortunately for us, band 8 is “FDD”, meaning it operates in full duplex, where phones transmit on one section of the band and the base station transmits on another.
Band 8 has a 45MHz duplex offset, wider than our entire privileges on 900MHz, so if you operate Band 8 such that your base station is operating under ham radio rules, by definition there is no way to legally operate the UE.
Band 30 is another example off limits to US hams. Band 30 has uplink in 2305 – 2315, and downlink 2350 – 2360. Back in the day, US hams had full access to 2300 MHz through at least 2400MHz. Now we don’t.
That 2.3GHz band was carved apart for the benefit of aviation uses, and satellite broadcasting like SiriusXM. European hams usually have full access to the entire 2300-2400MHz, so they can use Band 30. This is much easier to build or source hardware for, and could be practically put together with a decent SDR, some filters, and a good amplifier.
There’s another type of LTE band, which is TDD. The eNB and UE take turns transmitting, so there’s no frequency offset. This makes spectrum planning much easier, and means if we can get any device on a band that has overlap with our privileges, we can have a working LTE network without worrying about the duplex spacing giving us only half a band.
One gotcha here, is that TDD is much harder to use. It requires precise timing, and amplifying signals with that precise timing is tricky or expensive, if not both.
(Band 42 may have some overlap as well, but only by the footnote operations we have in that 3.3GHz band, and this is not common in UE or eNB hardware, nor will it become common.)
By pure luck, Band 40 (2300-2400 MHz TDD) has solid overlap with US ham privileges, at 2300-2310 and 2390-2400 MHz.
This LTE band is driven almost entirely by markets across Asia, but within that area it’s still a predominantly Chinese phenomenon.
So all we have to do is get working hardware from China.
Thanks to Licheng BA7JKX, we can!
Carrier Aggregation
Could we still use these other bands where we have partial overlap? Possibly! It would require carrier aggregation support on the eNBs, and for cost and availability reasons, this is hard to get a hold of at the moment.
A Plea
For future expansion, we can keep relying on getting hardware from China, but with trade wars and other concerns, this is not a long-term plan. It does let us prove the concept and show demand.
Please send me any leads - any leads at all - on macro cell Band 40 hardware, especially by western companies who haven’t realized there’s a market here. We can get grants to put these up as proper cell towers, but using Amateur spectrum for Amateur purposes.
Please make introductions to anyone who can help, e.g. Ericsson, Qualcomm,
Nokia, Airspan, etc. You can reach me at lte [at) tarxvf (dot] tech,
and I’m happy to get on the phone, a zoom call, or a plane as necessary.