Design & Optimization of an antenna array for WIMAX base station

Abstract

Conventional base station antennas in existing operational systems are either omni-directional or sectorized. The greater part of the transmitted signal power is radiated into directions other than toward the specific user. This causes interference, reduces efficiency and the range of coverage. Especially in new broadband services as WiMAX, where the user front-end is very simple, it becomes necessary to provide every user with a specific beam offering enough gain to increase the range. It is also important to reduce interference by other users or services by means of beam forming in a way, that either the side lobe attenuation of the base station antenna array as a whole is optimized or by null steering. In rural areas whole 360° degrees coverage around the base station is desired. This leads to the solution introduced here of circular antenna arrays, a setup which can also be used for direction finding. The project is to design an eight array dipole antenna. The antenna array consists of n vertical dipoles equally spaced on a ring with diameter d. A power-combiner/ divider network is connected to the feed point of every dipole with m inputs/outputs according to the number of wanted coinstantaneous beams or null directions. For each independent beam direction the weights of the amplitudes and phases have to be calculated and optimized. The adjustment can be performed directly by phase shifters and attenuators in the RF-region or after linear down-conversion and analog to digital conversion by a DSP.