According to news reports, Sprint is on the hunt for new funding to build out its WiMAX network and ease investor concern about the costs of such a capital-intensive project. Sprint is exploring options such as forming a partnership with cellular pioneer Craig McCaw or seeking more funds from the cable operators in its Pivot joint venture, the Wall Street journal reported today, citing unnamed sources.

Sprint has publicly stated it plans to spend about $3 billion building the new mobile WiMAX network in major markets, creating a high-capacity broadband wireless overlay to its core 3G cellular networks. However, poor operating performance in that core cellular business has raised hackles with Sprint’s investors, a group not normally prone to embrace large-scale capital expenditures in the first place. And some shareholders have begun questioning the wisdom of Sprint’s move into 4G.

According to Skyline Marketing Group, Sprint is already leading all U.S. carriers in declared 2007 capital expenditures at $7 billion. Only $800 million of that is for its initial WiMAX markets. WiMAX expenditures will grow to $1.8 billion in 2008 and $1.2 billion in 2009, said Skyline President John Celentano. Considering Sprint’s current financial problems, it would have trouble going to the equity markets for more capital, so it may rely on a partner, Celantano said.

“I don’t know if Clearwire has the financial wherewithal for that kind of transaction,” Celantano said. “The cable operators definitely have that kind of money, though.”

A partnership with McCaw’s Clearwire group would have several immediate benefits beyond relieving Sprint of some of its capital burden. Though the companies have two different strategies—Clearwire is focusing on landline replacement while Sprint is committing to mobility—they both operate in the same spectrum and can use the same equipment. Their licenses also appear to be complementary. Many of Clearwire’s licenses, however, appear to be in small- and mid-sized markets, where many of its initial deployments have been, while Sprint is focusing on major metropolitan areas. Sprint and Clearwire also share the same vendor. Motorola bought Clearwire’s wholly owned vendor, NextNet last year and made an investment in service provider itself, ensuring that Clearwire will migrate over to the same Mobile WiMAX platform that Motorola is providing for Sprint.

The merits of a WiMAX partnership with the cable providers are not so obvious. Comcast, Time Warner Cable, Cox Communications and Newhouse want to get into the wireless business to cement their quadruple play offerings, and they’ve taken numerous steps toward that goal, such as forming Pivot with Sprint and bidding collectively on spectrum in the Advanced Wireless Services auction. But WiMAX may fundamentally compete with the cable broadband services they are offering. Sprint, however, has stressed that it is not positioning its WiMAX offering as a wireline replacement offering, which may ease concerns from the MSOs. Sprint is pursuing the concept of “personal broadband,” linking all sorts of electronic appliances and consumer digital hardware to a pervasive network. Such a strategy might complement the cable operators’ broadband ambitions well, allowing them to promote a broadband service that originates in the home but extends far beyond it.