This post continues my discussion on beamforming which is a method of concentrating radio frequency (RF) energy in order to improve the signal to noise ratio (SNR) at the receiver, thereby improving network performance and predictability. In this post, we discuss static beamforming.
Static beamforming provides a fixed radiation pattern by using a directional antenna. Virtually every vendor provides APs with removable antennas so it is easy to swap an OMNI antenna for a directional antenna. Some vendors, such as Xirrus, use an array of static beamforming antennas to create densely packed multiple channels in much the same way that a cellular tower uses directional antennas to create cellular sectors. By assigning a different channel to each beam, many non-overlapping channels can be densely packed together within a single array (see Figure).
Figure 1: Static beamforming (source: Xirrus Networks)
Static beamforming can provide an signal-to-noise (SNR) benefit to both legacy wirless protocols like 802.11a/b/g and newer 802.11n stations. Since the antenna propagation pattern is static, the AP cannot adjust the radiation pattern on a frame-by-frame basis in order to track a station as it moves through the enterprise. Therefore, unlike transmit beamforming and dynamic beamforming, it does not take advantage of knowledge of the WLAN channel between the AP and the station in order to further optimize signal propagation. An array of directional antennas, such as the Xirrus Array, can improve SNR in 360 degrees.
Next time we’ll look at Transmit beamforming.
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