Interference
Interference will exist if there are other transmitters on the same channel or adjacent channels. This will lead to loss of frames and the frames will then have to be retransmitted, using up air time. Interference can be caused by micro waves, DECT phones and other devices depending on the band used.
Co-Channel Interference
Co-Channel Interference occurs when two or more transmitters use the same channel. The signals completely overlap and the whole 20 or 22 MHz channel bandwidth is affected. This is only a problem if they transmit simultaneously though. However they will contend for the same airtime and a channel can become very congested. When two signals interfere, it causes data corruption, rentransmitting of frames and that in turn uses up even more airtime.
It is common practice to only place a transmitter on a specific channel where received signals are much weaker. A margin of 19 dB is recommended. The margin depends on the coding and modulation scheme. BPSK may need less than 10 db but 64-QAM will require 19 dB. More advanced modulation such as 256-QAM may require between 31 to 50 dB.
Neighboring Channel Interference
In the 2.4 GHz band neighboring channels overlap. If one transmitter uses for example channel 6 and the other channel 7 then the signals will almost completely overlap and performance will be detrimental in both channels. In the 5 GHz band this is less of a problem since channels are 20 MHz wide and OFDM signals have a bandwidth of 20 MHz. It is still recommended to not place neighboring APs on neighboring channels to prevent raising the noise floor. Do note that sometimes the term adjacent channel interference is used which is technically incorrect because adjacent channels are channels that do not overlap. The correct term is therefore neighboring channel interference.
Non-802.11 Interference
The 2.4 GHz band is an ISM band. It’s likely that non-802.11 devices will be present there and they might not sit nicely on one channel, instead using FHSS and hopping around between channels. Some devices may not use a channel scheme at all. A microwave with RF leaking out is a classic interfering device. The solution is to have properly shielded devices and to have cordless phones and wireless video cameras use different frequencies that are outside of 802.11.
Free Space Path Loss
When an RF signal is transmitted from an antenna, the amplitude decreases as it travels through free space. This occurs even if there are no obstacles in the path and is known as free space path loss.
The RF signal travels through space as a wave, a three-dimensional curved shape that expands as it travels. This expansion or spreading is what causes the signal to weaken. Imagine a sphere where the sphere increases in size as the signal travels. The same amount of energy is then spread over a larger and ever expanding space which means the the concentration of the energy gets weaker as the distance from the antenna increases. This loss occurs regardless of the antenna used.
Free space path loss (FSPL) is calculated with the following formula:
FSPL (dB) = 20 x log10(d) + 20 x log10(f) + 32.44
D is the distance from the transmitter in km and f is the frequency in MHz. Note the following interesting facts:
- FSPL is an **exponential** function; the signal strength **decreases rapidly near the transmitter**, but **more slowly farther away**.
- The loss is a **function of distance and frequency only**.
As you can see in the formula, the higher the frequency, the higher the loss. For this reason, 2.4 GHz networks (b/g/n) have a better range than 5 GHz networks (a/ac/n) if the signal strength is the same. This effect can be quite noticable and tests have shown that with a RSSI of -67 dBm, a 2.4 GHz signal could reach 140 feet while the 5 GHz signal was reduced to 80 feet. Your mileage may vary though and there are other factors such as antenna size and receiver sensitivity.
Mitigating the Effects of Free Space Path Loss
Increasing the power of the transmitter or the antenna gain can boost the EIRP. A stronger signal before FSPL occurs will lead to a greater RSSI after the loss occurs. This can cause interference though if there are other transmitters in the area.
The more robust solution is to simply cope with FSPL. Wireless devices are often mobile and can move. When the receiver is closer to the transmitter, RSSI increases and hence also the SNR. The reverse is true as the receiver gets farther away from the transmitter. More basic modulation and coding schemes are needed for these situations because of the increase in noise and the need to repeat data.
802.11 devices can adjust their modulation and coding schemes based on the current RSSI and SNR conditions. Complex modulation and coding schemes can be used when the conditions are favorable and less complex schemes can be used when the conditions are less favorable, resulting in a greater range but lower data rates. This is often known as Dynamic Rate Shifting (DRS) and can be performed dynamically without manual intervention. DRS is not defined in the 802.11 standard and different manufacturers have different ways of doing it, as well as names. It can also be known as Link Adaption, Adaptive Modulation and Coding (AMC), rate adaption, and so on.
Effects of Physical Objects
Physical objects in the path of an RF signal can affect the signal in different ways, mostly in a degrading or destructive fashion.
Reflection
When the RF signal travelling as a wave meets a dense reflective material, the signal can be reflected. Think of a bulb where the light emits in all directions, away from the bulb, some of the light may be reflected from objects in the room. The light might travel back towards the bulb or another part of the room, making that area brighter.
Indoor objects such as metal furniture, filing cabinets and metal doors can cause reflection. An outdoor wireless signal can be reflected by objects such as a body of water, reflective glass on a building, or even the surface of the earth.
All reflections are not necessarily bad, because it’s just a copy of the original signal. If both the copy and the original reach the receiver, they can arrive out of phase with each other. This is because the reflection takes a slightly different path, causing it to arrive slightly later. This is known as multipath. When the receiver combines the two signals, the result is a poor representation of the original signal. The data may end up corrupted due to the combined signal being weak and distorted.
There are two different outcomes though with multipathing:
- If the receiver only has a single radio chain, then the end result is bad because all of the signals are combined into a poor, error prone composite signal
- If the receiver has multiple radio chains and supports MIMO, the different signals will be received on different antennas and radios. By using processing, the signal quality can be increased to extract multiple data streams and making something good out of a bad situation.
Absorption
An RF signal can become attenuated if it passes through a material that can absorb some of its energy. The denser the material, the more attenuation. The classic example is a wireless signal passing through a wall. The thicker the wall and the denser material, the more energy is absorbed. A gypsum or drywall may cause -4 dBm attentuation and a solid concrete wall -12 dBm. Don’t take these numbers as absolute though.
An outdoor signal may travel through water, either through water contained in leaves in a tree along the path, or rain, snow, fog, hail etc.
The human body is mostly filled with water and therefore attenuates wireless signals as well. Often we have devices close to our body and depending on how the person is oriented with respect to the transmitter, the body could be positioned in between the transmitter and receiver, attenuating the signal. A hand covering a phone antenna can decrease the signal strength by 6-8 dB and a head could case up to 30 dB of attentuation! Classrooms and office spaces will have a lot of human bodies, also causing attentuation.
Scattering
Scattering occurs when an RF signal travels through a medium that is rough, uneven, or made up of very small particles. Scattering is when the signal travels in different directions. This can also occur in a dusty or sandy environment. Scattering as poosed to reflection means that the signal scatters away in several directions.
Refraction
Refraction occurs when the RF signal meets a boundary between media of two different densities. Think of reflection as the signal bouncing off the medium and refraction as the signal being bent while passing through the surface.
The angle of the signal will be different after it passes through the medium and the speed of the wave may also be affected. Refraction can occur when a signal travels through layers of air having different densities or through building walls with different densities, for example.
Diffraction
Diffraction is when a signal bends around an object and where the wave is rejoined after passing it but the signal is never quite the same. A signal can therefore be received even if there is a building in between the transmitter and the receiver but the signal will be distorted.
Fresnel Zones
If a standing object such as a building obstructs an RF signal, the signal can be adversely affected in the vertical direction. Diffraction along the top and front of a building can cause the signal to be bent and attenuated. This is especially important in narrow, line-of-sight wireless transmission. These signals are focused into a tight cone-shaped pattern. The path must be clear of obstacles such as trees, or other objects or have the antennas raised higher than the obstructions to get a clear path.
Over long distances, the curvature of the earth becomes an obstacle that can affect the signal. A signal can be affected by diffraction even if there isn’t an object directly blocking the signal. The Fresnel zone is an elliptical-shaped volume around the line of sight that must also remain free of obstructions. Objects penetrating the Fresnel zone may cause diffraction. That portion of the signal may then end up bent, causing it to be delayed or altered and effecting the overall qualityof the signal arriving at the receiver.
There are many Fresnel zones but the first one affects wireless signals the most. The zones are numbered incrementally as they increase in size. Not all Fresnel zones have a negative impact on the signal though, only odd numbered zones, even zones can in fact add to the signal’s power.
As a rule, the antennas should be raised so that even the bottom of the Fresnel zone is free of obrstructions. Over long distances, the curvature of the earth can enter the Fresnel zone and cause problems.
| Path Length | Fresnel Zone Radius at Path Midpoint |
|---|---|
| 0.5 mile | 16 feet |
| 1 mile | 23 feet |
| 2 miles | 33 feet |
| 5 miles | 52 feet |
| 10 miles | 72 feet |