Base Station Antenna Design in WiMAX system..

To increase the capacity and coverage in broadband data communication according to the IEEE 802.16e WiMAX standard, intelligent base station antenna with beam- and nullsteering over a full circle is developed and optimised.

Conventional base station antennas in existing operational systems are either omnidirectional 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 optimised or by null steering. In rural areas a 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 be used for direction finding.

The antenna array consists of 8 vertical dipoles equally spaced on a ring with diameter d. This building block is vertically stacked many times to achieve enough gain in the horisontal plane. The number of side lobes and the side lobe attenuation depends on the radio d/λ which is ranged in 1/2 and 3/2. It is possible to provide m different and imdependent beams in m directions concurrently. Each dipole is fed by an amplitude and phase actuator and isolated power dividers. In principle this can also be achieved by means of a digital signal processor.

With the circular array of vertical dipoles it is possible to steer the beam in the direction of any user and to provide an optimal side lobe attenuation for m different directions simultaneously. The number of coexistent beams is only limited in practice by the capabilities of the power dividers.

It is also possible to provide a full nullsteering in every direction around the base station. This can be done by phase shifters and attenuators in the RF-region or by DSP after linear down conversion. The results are achieved by the numerical EM-solver FEKO.

With this antenna array it is possible to enhance either the coverage in rural areas or to enhance the capacity of the network.