**4. Future research in power amplifiers for wireless communications**

Needs for high data rates anywhere and anytime while the spectrum resource is limited will be a great challenge for future mobile and wireless communications. In order to utilize the bandwidth efficiently, new approaches on the network layers are being standardized and conceived including opportunistic, software defined and white space radio. The challenge of such frequency agile concepts will be not only be on the network and system layers e.g. spectrum sensing for vacant frequency slots, but also on the physical layer regarding the need of transmitters which can cope with extremely wide band or can be reconfigured for dynamic band migration. Regarding efficiency and power management, issues on every layer must be taken into consideration which would lead to interlayer optimization from network over system to physical layers. Active antenna and multiple inputs, multiple outputs (MIMO) concept will also be important topics which will require co-design and integration of amplifiers and antennas.

Energy saving is and will be a big issue not only in automotive and electrical power areas but also in wireless communications. To fulfil the intention for the "green transmission", high efficiency must be provided by all infrastructure components e.g. base stations. Also, the trend of modern wireless communication standards is going in the direction of low power and small base stations will small cell size. This means that not only the mobile devices e.g. smart phone or tablets require aesthetic design but also the infrastructure components which should be well integrated into the environment. High efficiency will contribute to this requirement by offering small size of base stations. Regarding efficiency, research and development efforts will be spent in high efficiency signal transmission including design of switched-mode high efficiency power amplifiers with modulated input for improved efficiency e.g. class S amplifiers for delta sigma modulated signal (Pivit et al, 2008). Considering the demand of wide bandwidth and the capability to deliver high switching speed at high power, GaN-based devices are promising device technology for future wireless communications.
