"It lets us get more spectral capacity in the market sooner than any other industry alternative," Stankey said.

Both AT&T and T-Mobile have accumulated frequencies over the years from various auctions of regional spectrum licenses. Their spectrum holdings vary from city to city, but Marshall estimates that AT&T has between 85MHz and 90MHz of total spectrum, on average, across the country. T-Mobile has between 50MHz and 55MHz, he said. So, assuming it isn't forced to divest some licenses, the combined company would have between 135MHz and 145MHz in an average city.

That would put the new carrier far ahead of both Verizon (with 85-90MHz) and Sprint (with 50-55MHz) in an average market, according to Marshall's estimate. Only Clearwire, with 120-150MHz, might rival the combined company in some areas. And Clearwire lacks the capital to take advantage of that spectrum.

If the acquisition is approved and AT&T takes control of T-Mobile, it plans to consolidate the 3G traffic of both carriers into a frequency band around 1900MHz. This will free up their AWS (Advanced Wireless Services) frequencies for LTE, Stankey said. Combining the AWS spectrum with some of T-Mobile's frequencies will create a band 20MHz wide that can be used to bring LTE to many rural and suburban areas, he said.

Meanwhile, AT&T's spectrum in the 700MHz range, which is prized for long reach and good in-building penetration, exists mainly in cities and will be used for LTE there, the company said. AT&T also continues to pursue an acquisition of a band in the 700MHz range that Qualcomm has used for its FLO TV service.