In 3GPP standards, there is an expectation that the 2.5GHz band (which is actually from 2.50-2.69GHz) would be split into a mix of paired spectrum (FDD) - in which you have separate chunks of frequency for uplink and downlink channels - and unpaired spectrum (TDD). In TDD you use a single chunk, but have up- and down- timeslots. (Hence either frequency- or time-division duplex).
The general consensus expectation is that there will be a band-plan which goes 70-50-70MHz, with paired 70MHz FDD chunks sandwiching 50MHz of TDD in the middle.
Historically in Europe, FDD has generally meant the established GSM/UMTS family of technologies. In 3G bands, especially 2.1GHz, chunks of unpaired spectrum have been rather under-utilised but theoretically dedicated to the TDD flavour of UMTS 3G, long but unsuccessfully championed by companies like IPWireless.
The advent of WiMAX (another TDD technology) has changed that perception, as unlike UMTS-TDD, it appears to stand a chance of being commercially practical on a broad scale. Unsurprisingly, the 3G FDD advocates aren't too keen on the competition, which is why 2007 was full of legal and regulatory wrangling about whether or not WiMAX counted as a 3G / IMT-2000 technology & could therefore be used in what were previously thought to be the sole preserve of UMTS-TDD.
There have been various other angles on this around technology neutrality, legal arguments about the semantics of whether "designated" means "exclusively designated" and so on. But in the end, the ITU defined WiMAX as a species of 3G anyway, so it's good-to-go in newly-available chunks of TDD spectrum like that in the middle of the 2.5GHz band.
Some regulators - especially Ofcom - have appeared keen to see WiMAX evolve as a competitor for established UMTS FDD 3G. Indeed, Ofcom has been working very hard over the last year on an innovative and controversial structure for the upcoming 2.5GHz auction. Basically, this involved a complex way of 'flexing' the aforementioned 70-50-70 bandplan for FDD & TDD in 2.5GHz, to allow WiMAX operators to get more TDD spectrum if they bid more money. So depending on a horribly complicated set of auction procedures, there might be 60-(50+10)-60-(10) allocation, or 50-(50+20)-50-20, with more WiMAX-suitable spectrum in the middle & top of the band.
This has cause lots of controversy in terms of auction design, assessing whether there would be more interference, the need for guard bands and whether existing handset & base station silicon designs could deal with the filtering requirements and so on. Ofcom appears to be pushing ahead regardless.
But in the background was something that I'd been wondering about for a while - and which the WiMAX industry has (mostly) kept a lid on..... basically an FDD flavour of WiMAX. Well, that cat is now truly out of the bag & is now frolicking amongst the pigeons, courtesy of some commentary from Airpan's CEO. I'd had some hints about this before, but I'd thought the main aim was to get WiMAX working in paired-spectrum 700MHz bands in the upcoming US auction.
Looks like actually they're pitching it head-on against HSPA, EVDO, LTE etc in the 2.5Ghz band. I wonder if that means that Ofcom & other regulators need to go back to the drawing boards and re-work their interference assumptions for a possible cellular/WiMAX mix across the whole band.....