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.

 Technology and Engineering

How do directional antennas become directional? Why is it that amplifiers have separate receive and transmit gain, but antenna gain occurs equally well for both transmission and reception. The principle at work is called the "Law of Antenna Reciprocity" and it's what allows a high-gain antenna to not only send RF energy out over longer distances, but to pick up weak signals arriving from long distances, even though the weak transmitter has no particularly special antenna.

Antenna Gain

Imagine an antenna that radiated an equal amount of energy in all directions. The coverage pattern of this antenna could be visualized as a sphere. The signal strength that was perceived by a client would be independent of the orientation of the antenna relative to the client. No matter which way the antenna was pointed, the client would see the same signal strength. The only thing that would change the signal strength perceived by the client would be if the distance between the client and the antenna changed.

This type of antenna is called an isotropic radiator. A good example of an isotropic radiator is the sun. The amount of energy the earth receives from the sun doesn’t depend on which “side” of the sun we’re facing. There’s no “front” or “back” side to the sun. It puts out equal energy in all directions.

Real antennas are never isotropic radiators. They always “shape” or “focus” the energy that they transmit, putting more energy in some directions and correspondingly less energy in other directions. A “dipole” is a very common type of antenna (the antennas on most wireless access points are dipoles). To get a mental picture of how radiation propagates outwards from a dipole antenna, consider a fluorescent light. Light (electromagnetic energy) propagates outwards from the sides of the tube, and not from the top or bottom. This is similar to the way electromagnetic energy propagates away from a dipole antenna. Instead of a sphere the energy volume of a dipole antenna more closely resembles a 3-dimensional doughnut shape, known as a toroid.

It’s important to realize that, all other things being equal, the dipole antenna doesn’t put out any less energy than an isotropic antenna would. The dipole antenna is simply focusing the energy more in one area and less in another. Consider the analogy of an adjustable-spray nozzle for a garden hose. The water pressure coming through the hose and the volume of water coming out of the hose may be constant, but you can have a wide, fine spray or a narrow jet. This is, in essence, how antennas work.

The degree to which an antenna is “focused” can be expressed in terms of gain. An antenna’s gain is the strength of that antenna’s output at its most focused point, relative to the output of an isotropic radiator. The unit used to measure antenna gain is "dBi" (decibel gain relative to isotropic). The details of decibels are outside the scope of this article, but suffice it to say that the more focused an antenna is, the higher its gain will be.

An isotropic radiator would have 0 dBi of gain, but true isotropic antennas aren’t practical to build. Real antennas always have some amount of gain relative to an isotropic antenna. The most simple dipole antenna (as would be found on a typical 802.11 access point) manifests 2.15 dBi. This means that, at its most focused point, the antenna is 2.15 dB stronger than an isotropic antenna would be. The most focused area of a dipole antenna is a plane that extends perpendicular to the antenna like a disc, with the antenna stuck through the middle of the disc like an axle. Correspondingly, at its least focused point, the antenna is 2.15 dB weaker than an isotropic radiator would be. The least focused area of a dipole antenna is directly above and below the top and bottom point of the antenna.

The Law of Antenna Reciprocity

Having defined the concept of gain, we can now ask the question, “Where does antenna gain come from?” The answer is that antenna gain is directly related to the physical shape and structure of the antenna. Consider a paddle dipped into the surface of a lake. Imagine that the paddle is waggled back and forth such that it creates waves. This is analogous to a transmitting antenna. Now, imagine that there is a second paddle at the other side of the lake. As the waves pass the second paddle, they cause it to waggle back and forth. This is analogous to a receiving antenna. The analogy is actually much closer than you might think—the main difference being that the antennas are not physically “waggling”. Rather, the electrons in the antennas are “waggling”.

Continuing the “paddle” analogy, we can imagine that different shapes of paddle would make larger or smaller waves. For example, a paddle with teeth cut out of it like a comb would make smaller waves than a solid one. Here’s where things get interesting: the solid paddle would also “waggle” more than the comb-shaped one when waves passed by it. In other words, the same characteristics that would cause the solid paddle to transmit big waves would also cause it to be more sensitive to the reception of waves.

This principle holds true for RF antennas. The Law of Antenna Reciprocity that states that the same physical characteristics that make an antenna “focus” its transmissions in a given way also cause the antenna’s reception to be “focused” in the same way. This is a real boon for wireless network designers because it’s usually the case that the access point’s antenna can be changed, but the client’s antenna is fixed because it’s built into the client device. If you put a higher-gain antenna on your access point in order to increase its range, you don’t have to worry about putting a corresponding antenna on your client devices. The Law of Antenna Reciprocity says that the additional gain of the antenna will make the AP talk louder and hear better. The client device will simply perceive a larger coverage area.

Conclusion