Simulation and Implementation of a Switched Beam Antenna System for WLAN

Abstract
In WLAN, the main causes of degradation of quality of communication are multipath fading and interference. To overcome these problems, we implement a 4-beam switched beam antenna system operating in the 5 GHz frequency band. Switched beam antenna is one type of smart antennas. A switched beam antenna consists of an antenna array and a feeding network based on the Butler matrix, which allows producing 2n orthogonal narrow beams to cover the communication range. Each of these beams leads to high directivity and high power gain. Thus, the SNR of the system is enhanced, and the multipath fading is suppressed.
Because of operating in the 5 GHz band, we use RO4003 board ( = 3.38, = 0.002, thickness = 1.32 mm) for fabricating the antenna array and the Butler matrix. Due to its omni-directional property, we use a twin-line dipole antenna as the antenna element; we use a 4×4 Butler matrix circuit as the feeding network to form four orthogonal beams. These beams can cover a communication range of 120o angular spread. Equipped with the designed switched beam antenna, experimental measurements were carried out to examine its performance. Besides, a simple model of the indoor multipath propagation is presented. By using this model, a large number of indoor channel responses are generated for computer simulations. From the measurement results and the simulation results, switched beam antenna can reduce delay spread in WLAN, and lead to a flatter frequency response. That means the deep fading is in lower probability. However, if the direct wave is wholly or mostly blocked, switched beam technique cannot provide a better performance than omni-directional antenna, even worse.