Although all three beamforming technologies provide SNR gain, they are quite different. In addition, all types of beamforming can exacerbate the following problems:

• Sticky client problem: The sticky client problem occurs when stations (STAs) remain associated with a distant AP (i.e., when they are “stuck” to an AP), even when a closer AP can offer the STA a better data rate or voice call quality.
• Hidden node problem: The hidden node problem typically occurs when one or more STAs on the same channel cannot “hear” one or more of the other STAs, resulting in channel interference. In some beamforming instances, this can also be true of STAs not hearing the AP.

A static beamforming array, such as the Xirrus product, is useful in dense deployments such as a large conference room. In addition, a directional antenna can be used to form a static beam to direct energy down a hallway or toward the interior of a building.

The extent to which transmit beamforming (TxBF) will reliably improve SNR is dependent on many factors such as the effectiveness of sounding the channel between the AP and the STA. The WLAN industry is in the early stages of TxBF deployment and I expect that the industry will gain useful real-world experience with Cisco’s proprietary TxBF solution (ClientLink). I also anticipate that silicon vendors will eventually integrate standards-based TxBF mechanisms once the IEEE ratifies implicit and/or explicit beamforming.

Dynamic beamforming is useful in many deployments because it can provide SNR gain for all STA types in the uplink and downlink directions. Since dynamic and transmit beamforming operate in ways that are complimentary, it is conceivable that future enterprise products will combine both techniques to provide even greater SNR gain. Dynamic beamforming will not likely become widely deployed unless WLAN silicon vendors support this feature, thus driving down development cost and complexity.

Refer to the chart for a more detailed comparison of each beamforming method.

Beamforming is not new.  At the most basic level, beamforming affects the radiation pattern of a wireless signal.  The radiation pattern refers to the way in which the electromagnetic waves propagate outward from the antenna element. For example, the most commonly deployed antenna is the omnidirectional antenna. The radiation pattern for the omnidirectional antenna is in the shape of a doughnut (see Figure 1). This type of antenna is a good choice for hotspots or any environment where the intent is to propagate the WLAN signal in a broadly dispersed pattern.

Figure 1: OMNI radiation pattern (source: Cisco Systems)

WLAN venders currently offer three types of beamforming.  Each type affects radiation patterns in different ways.

Static beamforming involves the use of internal or external antennas that have a fixed radiation pattern (such as a YAGI antenna) that emits a directional radiation pattern (see Figure 2).  A directional pattern is a good choice in environments where the intent is to propagate the WLAN signal in a particular direction, such as down a hallway or toward the interior of a building.

Figure 2: Directional radiation pattern (source: Cisco Systems)

Transmit beamforming propagates two or more phase-shifted copies of a signal on a frame-by-frame basis, so that they will be in-phase at particular points in space where the transmitter believes the receiver to be, thereby increasing SNR.

Dynamic beamforming changes the antenna radiation pattern on a frame-by-frame basis using a central processing unit (CPU) controlled antenna array in order to increase the SNR at the receiver.

In subsequent posts we will look at each of the beamforming types.  Next time, we will look at static beamforming.