He said AT&T already has enough spectrum to build out a third major competitor in the prepaid market but its attempt to offer its own branded prepaid service have had lackluster results. "It's easy to say that you plan to enhance competition in the market but our concern is AT&T will say, 'Thanks very much for the spectrum,' and then abandon what is a less profitable niche."

Feld also said the Leap deal's opponents are concerned about spectrum concentration nationally. "The universe of spectrum in the secondary market is limited and continues to go to AT&T and Verizon Communications Inc. through these transactions. It's true the regulators have said previously that Leap is not a competitor to AT&T, but the trouble is when you have one of these smaller companies acquired is when you ask where the next competitors to AT&T and Verizon going to come from."

Feld said he is encouraged by recent DOJ's to challenge mergers in highly concentrated markets, namely the agency's successful effort to stop AT&T's planned takeover of T-Mobile in 2011 and this month's legal challenge to US Airways Group Inc.'s attempt to buy American Airlines parent AMR Corp. out of bankruptcy. "In the US Air/American deal the DOJ said there's a point when we have too much concentration," he noted. "I'd say we're pretty much at that point in wireless too."

Even if the Leap deal isn't challenged, Feld said a long, in-depth review will likely temper enthusiasm for more deals, particularly one like AT&T's possible interest in buying Dish Network ( DISH).

Indeed, telecom analysts continue to predict that AT&T's planned acquisition of Leap will be approved by both the FCC and the DoJ. Guggenheim Securities LLC analyst Paul Gallant noted in a recent report that in its application to the FCC, made public in mid-August, AT&T stressed that the merger would put AT&T over the FCC's spectrum cap in only 38 of 356 designated wireless markets. In 17 of those 38 markets, AT&T would be over by less than 5 MHz. A company is considered over the spectrum cap if it controls more than one-third of the wireless spectrum in a market.

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