Mobile TV will have 140 million subscribers by 2011, according to Screen Digest's "Mobile TV: Business Models and Opportunities."

Screen Digest said that North America would see the greatest mobile TV growth, with 28.8 million subscribers by 2011, more than 20 times the current number.

Yet free services will account for most mobile TV viewing, making paid subscriptions a tough sell.

"The free-to-air services are the success stories for subscriber uptake, yet business models for mobile pay-TV are still to be proven," said Ronan de Renesse, author of the report, in a statement.

Asia has more than 15 million mobile TV subscribers. The majority of broadcast networks there are offered free-to-air.

As a result, the research company predicted that the Asian market would generate less revenue than Europe and the US by 2011.