Mobile broadband technologies such as LTE and WiMAX pose a unique set of opportunities and challenges at 700 MHz — particularly in the C Block, which at 22 MHz is the widest block of spectrum in the recent auction.

One potential advantage of WiMAX, LTE and UMB is that all three will support wider carrier channels compared with earlier-generation technologies. WiMAX today uses 8 MHz of bandwidth, and LTE and UMB can support channels up to 20 MHz wide. In comparison, CDMA uses 1.25 MHz channels, and wideband CDMA uses 5 MHz channels.

Newer technologies also will offer more flexibility in channel width. LTE, for example, can support channels in increments of 1.25 MHz.

Although all 700 MHz bands are appropriate for mobile broadband, the C Block will enable the highest data rates, said Danny Locklear, director of wireless network product marketing for Nortel Networks. By using 4G technologies at the wider channel bandwidths that the C Block can support, Locklear said: “You could offer services that would be more conducive to heavy video. The same thing could be done in 3G but not as cost-effectively.”

When used in the 22 MHz C Block, the broader carriers supported by 4G technologies will generate spectral efficiencies, Mukerjee said. In narrower spectrum blocks such as the A, B and E blocks, he said, “You won't get the spectral efficiency gains. There the current 3G and next-generation technologies begin to approach each other. The difference becomes more pronounced at broader bands.”

If 700 MHz winners devote a large portion of their spectrum to broader carriers, however, it will reduce the number of carriers and capacity in each cell site, which could require that cell sites be located just a few blocks apart.

“When you end up with small cells in urban areas, you can get self-interference between the cell sites themselves,” said Tom Flak, senior vice president of company operations for Soma Networks. Soma offers pre-standard WiMAX equipment operating at 700 MHz, which has been deployed by some small U.S. telcos that won spectrum in a previous auction in the lower 700 MHz band.

Flak believes developers will be able to address self-interference through careful antenna design but said that “it's an extra complication — no one has built an urban high-speed network yet.”

Another potential complication relates to multiple input/multiple output (MIMO) technology, which is used with WiMAX, LTE and UMB. MIMO increases spectral efficiency by using multiple antennas.

But as Gee Rittenhouse, vice president of research for Bell Labs explained, “To support multiple channels you have to be able to distinguish the signals coming from the various antennas. This is normally done through diversity — by separating the antennas more than a few wavelengths apart.”

But achieving that separation could be challenging at 700 MHz because the wavelengths are considerably longer than at higher frequencies.

Rittenhouse doesn't expect this to be a serious concern, however. “Diversity isn't a step function, where either you have it or you don't,” he said. “It's a continuum. If you don't have full spacing, there's not quite as much diversity, but that's not necessarily an issue.” Other possible solutions could involve using polar diversity instead of antenna diversity or increasing signal-processing power.

“The problem is complex but not insurmountable,” Rittenhouse said.

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