Product Focus

The Ruckus BeamFlex Beam Steering Antenna

As the user of a mobile device moves from room to room in a home or office the degree to which an access point's signal is attenuated (reduced) varies from room to room. The amount of signal power being sent in any particular direction by an access point's typical omnidirectional "rubber duck" style antenna doesn't change. The result is that the user encounters areas of reduced signal strength, reduced throughput, and "dead spots" where they lose connectivity.

The Ruckus Wireless antenna system is different. By incorporate multiple internal antenna elements in an antenna array the circuitry in a Ruckus access point is able to dynamically focus the antennas strongest beam. It's a technology called beamsteering and it avoids costly and unsightly external antennas. It's like a directional antenna that tracks the movements of the mobile devices, always pointing in the right direction. This tracking occurs on a packet-by-packet basis and stabilizes the wireless network's performance.

BeamFlex technology focuses up to 7 dB of directional gain into an effective 360-degree horizontal and 360-degree vertical beamwidth. A conventional 7 dB antenna that could be attached to another manufacturer's access point would limit the vertical beamwidth to less than 20 degrees (hence, you can't simply add a big antenna to another manufacturer's access point to emulate the behavior and performance a a BeamFlex radio).

A stable, directional connection helps reduce weak coverage areas, dead spots, and poor performance. This provides picture-perfect video streaming and crystal-clear audio and voice communications in addition to maintaining the highest possible throughput to each individual client.

Essential Wi-Fi

Signal to Noise Ratio
"Signal to noise ratio," or SNR, is commonly defined by 802.11 administrators as "the ratio of the power of the data signal to the power of the ambient RF energy," where "ambient RF energy" refers essentially to any RF emitter that is not an 802.11 transmitter. Although this definition could be correct under some interpretations of the term, "signal to noise ratio," it's important for an 802.11 administrator to realize that slightly different interpretations of that term are used in other areas, and confusion may arise if it's not clear exactly what kind of “signal to noise” is being discussed.

In general, "signal to noise ratio" refers to the power level of an incoming signal relative to some type of background noise. The definition of "noise" can vary, depending on the field in which "noise" is being measured. For example, audio engineering refers to the SNR of an analog recording medium­that is, the strength of the loudest undistorted signal the medium can carry relative to the background "hiss" that would be heard if a blank medium were played back. Audio amplifiers also have an SNR rating, which essentially measures the level of "hiss" that would be heard if the amplifier were turned up all the way without a signal being fed through it (you can try this at home with your stereo if you want). Although we have used audio examples, since everyone has some experience with them, the same "hiss" exists in any analog circuit, including the RF receiver in your 802.11 card. The point here is that, because the definition of “noise” can vary, so does the definition of SNR. In each case, the "signal" is the power level of the incoming signal, but the “noise” changes depending on your perspective.

From the perspective of an RF chipset engineer or an electrical engineer, "signal to noise" ratio probably refers to the strength of the incoming signal relative to the "noise" within the RF chipset itself. Electronic circuits are subject to a type of noise known as Boltzmann noise, which is caused by thermal effects. Essentially, the heat within the chipset causes a certain amount of electrical distortion that manifests itself as noise. Electrical circuits are also subject to induced noise from outside sources­for example, it's common to find noise in the 60 Hz range (the frequency of AC current in the U.S). Notice that this definition of "noise" and "signal to noise ratio" is completely different from the 802.11 administrator's definition mentioned previously. This definition focuses on the strength of the signal and noise within the electrical circuit itself, as opposed to in the air.  Measurement of SNR, then, depends on the definition of the “noise floor,” which is the level of “noise” in the environment being measured.

In summary, to an RF chipset engineer, the "noise" might be the background noise within the circuit of the chipset, while, to an 802.11 administrator, "noise" might be the ambient, non-802.11, RF energy in the environment. To avoid a double-use of the term "noise" and "signal to noise ratio," we propose the term, "interference" to refer to the ambient, RF energy in the environment, and the term, "signal to interference ratio," or "SIR," to refer to the strength of the signal relative to the ambient, RF energy in the environment. This leaves "noise" and "SNR" to refer exclusively to the thermal noise within the chipset and avoids confusion when 802.11 administrators talk to RF engineers or electrical engineers. Finally, bear in mind that SIR is probably not what the card is measuring when it reports "signal quality," even though it's common to (incorrectly) equate signal quality with SIR.

This topic is discussed in more depth in Connect802's paper, "You Believe You Understand What You Think I Said -- The Truth About 802.11 Signal and Noise Metrics," available from our white papers page.

 Technology and Engineering

Line-of-Sight and Fresnel Zone Clearance

The Fresnel Zone can be conceptualized as a football-shaped or oval-shaped volume of space centered around the line of sight between any two RF antennas. RF energy propagates through space in such a way that any obstruction of the Fresnel Zone will affect the signal, even if the line of sight remains clear. This is contrary to most untrained people's intuition, which would suggest that if the line of sight is clear from one antenna to the other, everything should be fine.

In fact, there are multiple "Fresnel Zones" and, in common discussion, the term "Fresnel Zone" is used to refer to the 1st Fresnel Zone. It's this zone that can cause the most dramatic problems if it's more than 40% obstructed. This zone also has the largest radius.

	The 1st Fresnel Zone is a football-shaped valume of space centered around the line of sight between any two RF antennas. The picture depicts not only this Fresnel Zone, but a number of other antenna characteristics that are described fully on the Antenna System Designer page.

The reasons for the Fresnel Zone are complex and require an understanding of some basic physics. But at a basic level, the WLAN engineer doesn't need to understand the reasons for the Fresnel Zone. All he or she needs to know is how to make sure that the Frenel Zone is unobstructed.

The size of the Fresnel Zone depends on just two factors: the wavelength of the signal and the distance between the two antennas. Formulae for the size of the Fresnel Zone typically express the result as the radius of a circle that has the line of sight between the two antennas running directly through the center of the circle, like a pencil stuck through a plate. The Fresnel Zone radius is smallest at the two antennas and grows to its maximum at a point halfway between them, resulting in an oval or "football" shape.

A common guideline states that the Fresnel Zone must be at least 60% clear in order to have a viable radio link. For example, if the Fresnel Zone between two antennas had a radius of 10 feet at the midpoint between the two antennas, the antennas would have to be at least 6 feet above any obstruction at the midpoint to clear the Fresnel Zone. Of course, obstructions can occur at any point between the two antennas, not just the midpoint, and the Fresnel Zone size changes as you get closer or further from the midpoint, so determining whether the Fresnel Zone is clear can be challenging. Connect802's RF system designer simplifies this task by performing the requisite calculations automatically.

You can use the free Connect802 Antenna System Designer to calculate the size of the Fresnel Zone, along with a large number of other antenna installation and radio range characteristics. While the calculation of the Fresnel Zone is relatively simple, in practice, it can be easy to overlook its effects. For example, imagine two buildings that are 1,500 feet away from each other with a third building in between. All buildings are equal height with flat roofs. An inexperienced RF engineer might assume that the clear line of sight between the two outside buildings would guarantee good RF signal, but if the antennas are mounted too low, the third building will impinge on the Fresnel Zone between them, destroying the link. For a 1,500 foot link at 2.4 GHz, the Fresnel Zone is about 10 feet in radius at the midpoint. This means that the two antennas would have to be mounted at least six feet above the plane of the roofs in order to have the Fresnel Zone be 60% clear and have a viable link. Ideally, the antennas would be at least 10 feet above the plane of the roofs, giving 100% Fresnel Zone clearance.

Fresnel Zone obstruction can be more subtle than that, however. What if the middle building in the previous example weren't there? Now, there's no obstruction at the midpoint between the antennas, but remember that we have to consider obstruction anywhere along the line of sight. Imagine that the RF designer decides to install the antennas on the roof, but for whatever reason, he mounts them in the middle of the roof. Now, the Fresnel Zone may be obstructed close to the antennas even though the midpoint is clear--again, the link will be destroyed. In this situation, the best thing to do would be to mount the antennas as close to the facing walls of the buildings as possible, so that there is as little roof as possible to obstruct the Fresnel Zone.

We believe that reason that the Fresnel Zone is sometimes overlooked is that many RF installations get away with violating the obstruction rules. For example, a typical plywood roof might cause approximately 6 dB of attenuation to a signal--a tolerable amount. In this case, even if the antenna was installed too close to the roof and the Fresnel Zone was obstructed, the link might work. The RF designer might learn to downplay the importance of the Fresnel Zone. Even a full 40% obstruction of the Fresnel Zone may typically cause 20 dB of loss, which may be tolerable in some situation.

We at Connect802 recently witnessed the deleterious effects of that conclusion. The antennas had been installed approximately 20 inches above the roof of a large aircraft hanger. Link budget calculations showed that the link should have about 40 dB more signal strength than it needed--a substantial buffer--but connectivity was sporadic, if it existed at all. The reason was that the metal roof of the hangar was causing much more attenuation than a typical plywood roof--40 to 50 dB vs. about 6 dB! The extra attenuation of the metal roof was absorbing all of the extra signal strength in the link budget. In this case, the designer's experience--that you could mount antennas close to the roof and "get away with" obstructing the Fresnel Zone--did not apply. Raising the antennas so that the Fresnel Zone was unobstructed resulted in a solid link.

Two final comments: First, people are sometimes surprised to learn that the size of the Fresnel Zone has nothing to do with the beamwidth of the antennas. Intuitively, it seems like an antenna with a narrower beamwidth should be able to "shoot" the beam over an obstruction, while an antenna with a wider beamwidth might "hit" the obstruction. Although beamwidth does affect a client's ability to receive signal, it is completely independent of the Fresnel Zone affects, and must be considered separately. Second, people sometimes wonder how the Fresnel Zone comes into play in an indoor environment, where line of sight to the antenna is often blocked. In that case, Fresnel Zone still applies, but the Fresnel Zone is often 100% obstructed. This means that a significant portion of the RF energy is absorbed, which is why indoor range is always so much shorter than outdoor range.