Essential Wi-Fi

Determining the maximum range for 802.11 communication

Three major qualities determine whether an 802.11 station can receive a signal: the signal's initial transmit power, the distance between the transmitter and the receiver, and the data rate of the signal.  These qualities are interrelated, and a change in one has direct effects on the others.  This month, we'll explore how these qualities go together to determine the range and data rate of an 802.11 device.

Like every RF signal, an 802.11 signal is transmitted at some power level and then grows weaker as it travels (propagates) away from the transmitter. This power loss is called "free space path loss," and is abbreviated as "FSPL". Two conditions will cause the signal to become unreceivable. First, all 802.11 radios have a minimum power level, called the receive sensitivity, below which a signal cannot be received. More sensitive cards can receive weaker signals than less sensitive cards. Second, at a certain point, the signal will fade into the background RF radiation, at which point the receiver won't be able to make out the signal, no matter how sensitive the receiver is.

Free space path loss occurs even when the signal is travelling through open air. When the signal has to pass through objects such as walls, it loses additional energy. This loss is known as absorption.

Understanding FSPL and absorption, we can begin to understand the relationship between transmit power and the signal's range. The stronger the signal starts out, the more power it can afford to lose to FSPL before it becomes too weak to be received. The more power the signal can afford to lose, the more distance it can travel and the more obstacles it can push through. Therefore, as transmit power goes up, so does range.

Digital RF signals like 802.11 use various encoding methods to encode 1's and 0's onto an analog RF waveform. More complex methods can squeeze more 1's and 0's into the same space, resulting in a higher data rate, but the more complex the encoding method is, the more susceptible the signal is to corruption. Higher data rates are more easily corrupted, while lower data rates are more corruption-resistant. The effect of this is that higher data rates fade into the background RF radiation sooner than lower data rates. Since the lower data rates remain recognizable longer, they have greater range. Therefore, as data rate goes up, range goes down.

In summary, transmit power, range, and data rate have a reciprocal relationship. If transmit power increases and data rate remains the same, range also increases. If data rate increases and transmit power stays the same, range decreases. If you want to increase data rate while keeping range the same, you'll need to increase transmit power accordingly.

Next month, we'll take a more in-depth look at these three factors and other factors that might affect the range and data rate of a signal.

 Technology and Engineering

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. Joe Bardwell has written an essay building up the reasons for the Fresnel Zone from basic principles. You may download this essay [4.7Mb PDF] and read it in its entirety. 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.

To Infinity... and Beyond!

World Cup Soccer RFID

All of the 2.9 million tickets to the 2006 World Cup soccer tournament to be held in Germany in 2006 have embedded RFID chips, and that's how fans will gain entry to the sports venue. This is the largest use of RFID at a public event anywhere in the world and there's every expectation that the publicity will be a big boost to the use of smart tag technology.

Read the World Cup Soccer Article in InfoWorld

Real Time Location Services

Standard Wi-Fi networks can be used to lcoate assets and people when a real-time location service (RTLS) system is combined with long-range, active RFID technology. "Some 15 per cent of errors in hospitals are from misidentification in surgery", said David Morgan, consultant ear, nose, throat surgeon at Birmingham Heartlands hospital. Patients, wearing portable Wi-Fi tags, are identified, and their photographs are presented to the doctors, along with their medical records, to greatly reduce the percentage of errors.

Read about Wi-Fi and RFID in HospitalsRead about Brmingham Heartlands Hospital

Read about Wi-Fi active tagging available through Connect802 - Call Connect802 for more information