Equipment vendors are quietly working on a WiMax technology that could challenge options for cellular telecoms networks, by allowing the technology out of its small spectrum "ghetto," into the main 3G band due to be allocated this year.

The WiMax Forum is making a profile for mobile WiMax that uses "paired" FDD (frequency division duplex) signalling, with separate channels for uplink and downlink. Telcos and regulators such as the ITU prefer FDD, and most of the spectrum for 3G and 4G networks requires it. WiMax standards and equipment have focussed on TDD (time division duplex), in which uplink and downlink signals have separate time slots on a single channel - but which is limited to smaller bands of spectrum.

"The WiMax forum will have an FDD profile for Mobile WiMax inside six months. We've been working on it for the last twelve months," Paul Senior, chief technology officer of WiMax vendor Airspan told Telecoms.com.

The ITU endorsed WiMax for use in the IMT 2000 2.6GHz spectrum in May 2007, but operators have expected to limit its deployment to a smaller part of this spectrum which would be designated for TDD technologies: 50MHz in the middle of the band, sandwiched between two paired 70MHz chunks for FDD.

In the UK, the regulator Ofcom will auction the 2.6GHz spectrum this year. Ofcom operates a "technology neutral" policy, and is expected to find a way round the ITU band plan, but operators may nevertheless have to fall in step with the rest of the world, for international roaming to take place.

The Forum seems to have been playing a strategic game, gaining ITU approval for WiMax, before announcing the more aggressive move to create an FDD profile: "We've been a bit quiet about it because we wanted to get the IMT 2000 decision," said Senior. "And if we had gone to IMT with an FDD profile, we probably couldn't have got it through. We decided to go for something that was a little less threatening, which was a TDD profile. We didn't talk too much about the FDD work which we've been doing for the last 18 months. There will be an FDD profile, it will sit at 2.5GHz FDD allocations just as well as any other technology."

TDD technologies have traditionally been the poor relation in telecoms, with small amounts of spectrum allocated to it, but even these have not been well used, since TDD versions of 3G networking have never taken off. By approving WiMax, the ITU thought it was only allowing it into these TDD ghettos, but now the FDD profile may allow it into wider swathes of spectrum.

Although the WiMax industry has kept a lid on FDD WiMax, "that cat is now truly out of the bag and is now frolicking amongst the pigeons," said Dean Bubley of Disruptive Analysis. "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 U.S. auction."

FDD WiMax could well be pitched head-on against HSPA, EVDO, LTE and other 3G technologies in the 2.5Ghz band, according to Bubley. I wonder if that means that Ofcom and 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."