Hoping to build what they call 'credible WiMax networks' and provide for life after 3G, network equipment maker Airspan and operator Freedom4 have shown a laptop roaming seamlessly between WiMax networks on different spectrum bands.

Airspan and Freedom4 say this will help operators build credible WiMax networks using limited radio spectrum, and give users an alternative when 3G networks run out of steam.

"To have a high capacity WiMax network in a market such as the UK, you need devices to do what ours does," said Graham Currier, chief operating officer of Freedom4, formerly known as Pipex Wireless.

In the UK's much-delayed 2.6GHz spectrum auction, operators may only get 30 or 40MHz of spectrum, he said, which would not be enough to deliver the kinds of sustained data rate that users will expect. Dual-band roaming would allow these operators to buy wholesale data from other operators (in particular Freedom4, which has a large block of spectrum around 3.6GHz) and move users on to that spectrum as required.

Dual-band WiMax dongles are already available, but the Airspan demonstration shows that users can be placed on different spectrum under the control of the network operator, Currier explained: "We must consolidate lots of spectrum, to allow a network that delivers the kind of capacity. You need one device that can move to other spectrum without the awareness of the customer."

"This capability changes the ground rules for carriers," said Paul Senior, chief technical officer at Airspan.

"Any prospective WiMax carrier can now create a large spectrum allocation for their deployment by combining smaller pieces of spectrum in different bands. This also allows carriers the ability to use larger allocations of spectrum per cell site (say 60 MHz, or even 120 MHz), and create sites with 100-200Mbps of wide-area mobile broadband capacity."

"This would be roughly 10 times the capacity of a typical 3G HSPA site," said Senior.

The technology should make European roaming easier said Currier: "Europe has a lot of historic, odd spectrum, and is very fragmented. If you want to have Europe-wide data mobility, almost every network has to work like ours."

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