There is an ever-increasing demand on mobile wireless operators to provide voice and high-speed data services. At the same time, these operators want to support more users per basestation to reduce overall network costs and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities are a pressing need.
Interference Limited Systems
Unfortunately, because the available broadcast spectrum is limited, attempts to increase traffic within a fixed bandwidth create more interference in the system and degrade the signal quality.
Smart Antenna Technology—Beamforming
Smart antenna technology offers a significantly improved solution to reduce interference levels and improve the system capacity. With this technology, each user’s signal is transmitted and received by the basestation only in the direction of that particular user. This drastically reduces the overall interference in the system. A smart antenna system, as shown in Figure bellow, consists of an array of antennas that together direct different transmission/reception beams toward each user in the system. This method of transmission and reception is called beamforming and is made possible through smart (advanced) signal processing at the baseband.
Smart Antenna System—Beamforming
In beamforming, each user’s signal is multiplied with complex weights that adjust the magnitude and phase of the signal to and from each antenna. This causes the output from the array of antennas to form a transmit/receive beam in the desired direction and minimizes the output in other directions.
Switched and Adaptive Beamforming
If the complex weights are selected from a library of weights that form beams in specific, predetermined directions, the process is called switched beamforming. Here, the basestation basically switches between the different beams based on the received signal strength measurements. On the other hand, if the weights are computed and adaptively updated in real time, the process is called adaptive beamforming. Through adaptive beamforming, the basestation can form narrower beams towards the desired user and nulls towards interfering users, considerably improving the signal-to-interference-plus-noise ratio.