Media and radio signal processing for mobile communications Mobile communications systems require a significant financial investment to obtain radio spectrum which consists of small but expensive frequency bands that are used to extend the networks over wide geographic areas. There are additional costs to operate and maintain those networks. Roaming agreements made between service providers can complement insufficient network coverage but financial constraints still dictate that existing assets such as the backbone networks be re used whenever possible. As a result new mobile communications systems are rarely designed without incorporating some elements of earlier systems