Effects of nonlinear transmit amplifiers in smart antennas in wireless systems

Signals and Systems Group Uppsala University e-mail:fmwjarjtog@signal.uu.se WWW: http://www.signal.uu.se Abstract | The use of array antennas in mobile telecommunication systems has proven to increase the spectral efficiency of the system. However, the use of an N element array antenna at the basestation site implies that the amount of radio hardware must be increasedN -fold. This will make basestations larger, more expensive and more power consuming. Much effort must be put into combine functionality as amplification, using a multicarrier amplifier (MCPA), capable to amplify a number of signals with different carrier frequencies in a single device. In this study we investigate by measurements and simulations the effects of a nonlinear MCPA in the downlink for a cellular system. The nonlinearity will introduce intermodulation distortion and also decrease the null-depth of the beamformer. This will have an impact on the carrier-to-interference ratio of the mobiles in the system. The simulations gives the CDF of the received carrier-to-interference ratio of the mobiles, which can be used in link budget analysis. The transmitted intermodulation distortion from an array will be spatially dispersed. This is shown by measurements in an anechoic chamber on a four element array using an analog Butler matrix as a beamformer.