The key to more spectrum and higher speeds may be Clearwire, the WiMax carrier that has supplied Sprint's existing 4G network for years. Softbank and Sprint didn't say much about Clearwire on Monday, but analysts think they eventually will tap into that company's massive reserves of spectrum. Clearwire, which Sprint helped form through a joint venture in 2008, has well over 100MHz of frequencies in most markets. Some of those originally belonged to Sprint.

The match between Clearwire and the merged company, to be called New Sprint, looks even better in the light of Softbank's current business. In Japan, it operates an LTE network in the same band as Clearwire's, using the same type of technology, called TDD (time-division duplex) LTE. That's different from the system most LTE carriers use, so Softbank would have a lot to gain from running two such networks, Ovum analyst Daryl Schoolar said. It could give the company more clout with manufacturers to get smartphones built for the networks, plus more economy of scale to lower handset costs, he said.

There's probably enough money in that $8 billion pot for Sprint to buy Clearwire outright, Entner said. Clearwire is worth about $4.3 billion including its cash and debt, and Sprint already owns nearly half the company, he said. "The long-term plan for Sprint was always to buy out Clearwire," Entner said.

Even if it doesn't buy the company, Sprint will want to help Clearwire, Ovum's Schoolar said. "I would be surprised if they weren't using this money at least to make Clearwire stronger," he said. The two companies already plan to have Clearwire's future LTE network add capacity to Sprint's. With new technologies for smoothly shifting customers to the stronger of the two networks, they could bring much more of Clearwire's spectrum to bear, Schoolar said.

Entner believes a combined Sprint and Clearwire could use an emerging LTE technology called carrier aggregation to bundle spectrum from both networks, in different bands, into one very high-capacity system. That would make them much more competitive against AT&T and Verizon. "You're talking about a sleeping giant," he said.