In Hungary the digital changeover of the terrestrial television broadcasting is now in progress, it is assumed to be finished in 2015. The analog transmissions will be turned off in the next few years, and frequencies will be released for new services in the UHF band. The frequency is an expensive natural resource, engineers work for efficient utilization methods for a long time. The cognitive radio systems are good solutions, which look for spectrum holes, and enable efficient spectrum usage by adaptive modulation techniques. The simplest approach for cognitive radio developments can be achieved by software defined radio (SDR) environments. In my thesis the applied peripheral for the SDR system was a USRP (Universal Software Radio Peripheral).
The research group at the department is currently developing a prototype platform with SDR using a USRP, a few commercially available daughterboards, a UHF band upconverter, signal generator and special antennas. The receiver side can be implemented properly with these devices, but on the transmitter side there is a need for well-designed filter which can remove the undesired components of the sampled signal. We use a special multicarrier modulation in the radio transmission which has a large peak-to-average power ratio. We have to reduce the transmitter power if we only want to use the available instruments without any nonlinear distortions. Therefore the development of an amplifier was needed which matches in outgoing power to an external upconverter.
The parameters of the implemented devices were specified by the USRP hardware parameters and the used FBMC (Filter Bank Multicarrier) modulation. There was a strict criterion for both amplifiers that they must have a low nonlinear distortion, and the filter has to operate with a linear phase.