Smart antennas and performance in mobile communications

Abstract

The term smart antennas is used for antenna systems with several antenna elements which are combined in some special way. Two such systems are MIMO and Beamforming. MIMO systems use several widely spaced elements to get diversity benefits and the option to send multiple simultaneous data streams by spatial multiplexing.

A Beamformer uses several closely spaced antenna elements to focus the array response into a narrow beampattern which can be steered in desired directions. In addition to this steering ability the beampattern will also reduce co-channel interference generated by cellular networks.

This interference reduction is not trivial to compare to the diversity combing and spatial multiplexing abilities of the MIMO system. Therefore, it is developed a simulation tool for this purpose.

This tool simulates a cellular network with co-channel interference and MIMO and Beamformer antennas. There are defined two distinct network scenarios in which to perform the comparisons, where one of them emulates a system with low power and small cells while the other uses tower based Access Points and has large cells.

To utilize the extra signal-to-noise ratio gained by using the smart antennas an adaptive modulation scheme will be used to adapt the data rates to the signal-to-interference-andnoise ratio for the various users.

Furthermore, it is defined two performance metrics for comparing the two smart antenna systems. One metric is based on the spectrum efficiency of the systems and the other is based on the maximum coverage the systems are capable of.

In this environment it is done simulation with varying degrees of users with line-of-sight with the Access Point.

We find that the MIMO system outperforms the Beamformer up to a LOS percentage of over 90% in both coverage and spectrum efficiency. For LOS percentages below 20% the MIMO system has twice the spectrum efficiency and coverage as the Beamformer.