Gearing up for 700 MHz

At least three different 4G wireless technologies have been proposed for mobile broadband use at 700 MHz. These include WiMAX; long-term evolution (LTE), which was developed by the same companies that built GSM cellular networks and equipment; and ultra mobile broadband (UMB), which has its roots in the CDMA development camp headed by Qualcomm. All three technologies appear capable of supporting mobile broadband connectivity at speeds in the tens of megabits.

At first glance, WiMAX might appear to have a time-to-market advantage over the other alternatives, as Sprint and others already are deploying it in the 2.5 GHz band. But adopting the technology for use at 700 MHz poses some significant challenges.

The 2.5 GHz spectrum band is somewhat unique in that it is unpaired spectrum, making it well suited for supporting time division duplexing (TDD), in which a single channel carries both upstream and downstream traffic — the approach used for WiMAX. But most cellular spectrum, including all but one of the five blocks in the recent 700 MHz auction, is paired. Paired spectrum lends itself well to frequency division duplexing (FDD), in which the upstream and downstream signals are carried on separate channels.

The WiMAX Forum has announced plans to develop an FDD version of WiMAX, but that will take time — and that's not the only area where WiMAX is being tweaked. The forum also is working on improving the uplink performance of WiMAX, which, according to Wright, is not as good as that of LTE.

“WiMAX is two and a half years ahead of LTE,” Wright said. “But by the time you modify WiMAX standards to improve the uplink performance and get a commercialized product, it will be neck and neck with LTE.”

Verizon Wireless and AT&T Wireless have already indicated plans to use LTE. That reality, coupled with GSM's worldwide predominance, could help ensure LTE's position as the preferred 700 MHz technology.

“The potential licensees are concerned about volume and scale and making sure they select a technology that others will likely deploy so they don't end up in a situation that would cause them to have a high infrastructure and device cost and few selections in terms of the vendors they get to deal with,” said Wright.

That logic favors LTE over WiMAX and UMB, added Wright, who questioned how much demand there could be for the FDD version of WiMAX. “It all comes back to spectrum allocation,” he said. “Around the world, WiMAX is generally deployed on spectrum that is well-suited for TDD.”

Wright added that Motorola expects to leverage the work it already has done with WiMAX in developing its LTE solutions. “We're expecting as a development company that about 75% to 80% of WiMAX software is reusable for LTE,” he said. “At this point we're focusing everything on LTE. I don't think UMB will get global traction, but we're ready to build it if people ask for it.”

Another consideration is that it will take a while for network operators to deploy 700 MHz infrastructure nationwide, which means operators likely will seek dual-mode devices supporting voice and data connectivity on 3G or 4G networks. Here, again, economies of scale will come into play.

“You'll see the device community working on dual-mode UMTS/LTE devices before UMTS/WiMAX,” said Peter Jarich, research director for Current Analysis.

(continued on next page)