Test of Downlink Joint Transmission with Distributed Antennas

Joint transmission techniques with base station cooperation are discussed in [1-4] as a good possibility to increase average mobile data rates and fairness of wireless systems and to reduce the intercell interference. Especially the cell edge spectral efficiency can be improved. The gain of multi-cell joint processing is limited by the imperfect channel knowledge at the base station. To improve the channel state information feedback, a rake compression algorithm has been studied [5-6]. This paper presents conducted lab trials which analyze different precoding algorithms like conjugate beamforming and zero-forcing in a joint transmission scenario, reducing the intercell interference. Furthermore, channel state feedback with adjustable rake compression is examined to provide the necessary channel knowledge at the transmitter in an efficient way. We investigate possible improvements of the spectral efficiency and cell edge throughput. We demonstrate with a practical trial setup that feedback rates of a few kilobits per second are already sufficient for the channel knowledge feedback in order to achieve a very high precoder performance.