Performance evaluation of a cellular base station multibeam antenna

Abstract

Experimental test results are used to determine the performance that can be achieved from a multibeam antenna array, with fixed-beam azimuths, relative to a traditional dual-diversity three-sector antenna configuration. The performance tradeoffs between the hysterisis level, switching time, and gain improvement for a multibeam antenna are also examined. The multibeam antenna uses selection combining to switch the signals from the two strongest directional beams to the base station's main and diversity receivers. To assess the impact of beamwidth on overall system performance, the following two multibeam antennas were tested: a 12-beam 30/spl deg/ beamwidth array and a 24-beam 15/spl deg/ beamwidth array. Both multibeam antennas were field-tested in typical cellular base station sites located in heavy urban and light urban environments. Altogether, the system performance is evaluated by investigating three fundamental aspects of multibeam antenna behavior. First, the relative powers of the signals measured in each directional beam of the multibeam antenna are characterized. Then, beam separation statistics for the strongest two signals are examined. Gain improvements achievable with a multibeam antenna compared to the traditional sector configuration are determined in the second phase of the analysis. Results indicate that in excess of 5 dB of gain enhancement can be achieved with a 24-beam base station antenna in a cellular mobile radio environment. Finally, the effects of hysterisis level and switching time are characterized based on gain reductions relative to a reference case with no hysterisis and a 0.5-s switching decision time. Useful approximations are developed for the gain effects associated with varying hysterisis levels and switching times. The resulting design curves and empirical rules allow engineers to quantify multibeam antenna performance while making appropriate tradeoffs for parameter selection.