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The carrier M&A rumor mill is again spinning at full speed. This time, according to the Wall Street Journal, Sprint(s s) is revisiting the idea of acquiring of T-Mobile(s tmus), something Sprint wasn’t in position to do before SoftBank took control of the company.

The Journal’s unnamed sources said Sprint is only weighing a bid and hasn’t decided whether the deal would meet the regulatory test. Since the Federal Communications Commission and the U.S. Department of Justice shot down AT&T-Mo(s t) in 2011, regulators have let big acquisitions through, such as T-Mobile’s acquisition of MetroPCS, but no one has attempted to combine any of the big four nationwide operators.

When asked about mobile consolidation in a recent interview with Gigaom, new FCC chair Tom Wheeler didn’t come out and say he wouldn’t tolerate any mergers among the top tier carriers, but he did make it very clear he wanted to preserve the current level of mobile competition in the U.S. You could interpret that two ways, though. T-Mobile and Sprint are both smaller than Verizon(s vz)(s vod) and AT&T by half. The FCC might feel they could be more competitive combined than separate.

On paper, a combined Sprint-T-Mobile might look appealing, but in operationally it would be a mess. T-Mobile uses GSM and HSPA+ technologies for its 2G voice and 3G data networks, while Sprint is a CDMA shop. A year or two ago I would have said there was be no way Sprint would even consider such a merger, especially since Sprint is just now recovering from the eight-year fiasco that was its Nextel acquisition (a deal Sprint CEO Dan Hesse now acknowledges was a mistake).

But carrier merger math has changed in the last few years. Operators are no longer consolidating to combine customer bases or infrastructure. They’re after spectrum. T-Mobile bought CDMA operator MetroPCS to cannibalize its networks of airwaves, and AT&T will do the same with Leap Wireless(s leap).

Still, T-Mobile is an awfully big company to buy if you’re merely harvesting airwaves, and Sprint would see a mass exodus of customers if it tried to shut down T-Mobile’s network. Chances are Sprint and T-Mobile would focus their consolidation efforts on the technology they share in common — LTE — and look to aggressively move customers onto 4G. That would be no picnic either. A combined, Sprint and T-Mobile would be running three different LTE networks built by four different vendors and running on four different frequency bands.

If Sprint does pursue this deal, one of its biggest obstacles will be justifying its enormous spectrum holdings to the FCC. Since it fully took over Clearwire, Sprint has the largest untapped repository for 4G spectrum in the country. Adding T-Mobile’s airwaves to the mix would give it far greater spectrum stores than either AT&T or Verizon, even though both operators would have larger customer bases. Lately Sprint has been trying to downplay the value of its high-frequency 4G spectrum, and this potential merger bid may explain why.