Sprint is very near finishing a deal with the LTE Network company, LightSquared, that would see LightSquared paying Sprint close to $20 Billion dollars over the next 15 years, and allow the two to partner up toward a 4G LTE network for Sprint. Here's a quote from the Bloomberg article,
Sprint would receive as much as $2 billion annually from LightSquared in the first stages of the contract to help pay for the network equipment and construction costs, said one of the people. Payments thereafter would vary annually based on the number of users on the LightSquared network and usage patterns, said the person. The full value of the contract will depend on those payments.
Earlier, on several occasions, we reported that it looked like Sprint was moving toward switching over to 4G LTE, and now it appears it is only a matter of time. Interestingly, Sprint's stock is doing very well right now. It is up about 40% just this year.

Source: BGR and Bloomberg