The 700 MHz wireless broadband auctions have started to heat up, with bids for the leading nationwide C band license approaching the $4.6 billion reserve initially set by the Federal Communications Commission (FCC) on Tuesday.

The big question remains, however: Which operators and other companies are bidding for the bandwidth?

The FCC keeps which companies are bidding on what a secret in order to prevent collusion between the interested parties. It is no secret, however, that the big-name U.S. carriers, AT&T Inc. (NYSE: T) and Verizon Wireless , along with search giant Google (Nasdaq: GOOG), are the companies that are most interested in the C band -- though it is still not clear what Google would do with the spectrum if it ended up acquiring the rights to the radiowaves.

Meanwhile, the rest of the secret bidding suggests a pattern of large and small carriers looking to add capacity around the U.S. -- from a $179 million bid for bandwith in the New York area to $252,000 for a Guam license.

The D band, which is intended for public safety services, has so far garnered a paltry $472 million bid for a nationwide license. With the exit of Frontline Wireless, LLC from the process it is not clear who might be bidding on the D block.

Meanwhile, there are still plenty of wild-card bidders, such as petrol firm Chevron, that have signed up to bid. It could be weeks, however, before we fully know which companies bid on what.

The auction will end when there are no new bids and all the spectrum blocks have been sold. This could take weeks yet if the Advanced Wireless Services (AWS) auctions in 2006 are any indication.

— Dan Jones, Site Editor, Unstrung