Wireless propagation fundamentals
Industrial wireless tutorials: Understanding wireless propagation fundamentals will help you understand what can happen to a wireless signal and how that can influence wireless signal reliability. See 6 diagrams to illustrate wireless propagation. Learn the 4 results of multipath.
Wireless reliability requires, in part, that the signal be transmitted reliably. Ideally, radio waves will travel from the transmitter to the receiver with no loss in signal strength; this is, unfortunately, not possible in the real world. As soon as the radio frequency (RF) energy leaves the transmitting antenna, the signal begins to degrade. Any object will variously reflect, refract, diffract, or absorb the signal's energy. You will notice that RF often behaves very much like another well-known electromagnetic phenomenon, light.
Absorption is a common propagation characteristic. Water is a very efficient absorber of RF energy; it follows that the human body, which is over 70% water, will attenuate RF. Paper, cardboard, wood, and aquariums all will absorb RF energy to varying degrees. Concrete and brick absorb RF to a very large degree; in fact, all materials will absorb RF to a greater or lesser extent.
Another characteristic of RF propagation is reflection. Reflection is the property of a wave or beam to bounce off of a smooth surface and proceed in a direction different from that which was intended. Technically, the angle of incidence of a wave equals the angle of reflection; if one were to draw a line perpendicular to the reflecting surface, the incoming (incident) signal would strike the surface at an angle to the line (for you wonks, it's called the normal) and the outgoing (reflected) signal angle would equal the incident angle. Under about 1GHz, RF will reflect form the ionosphere, a layer of the atmosphere containing charged particles; this is called skip and can transmit signals thousands of miles farther than would normally be possible with ordinary line-of- sight transmissions. Above 1 GHz, wavelengths become so small that RF will reflect off of just about anything smooth, for example, doors, filing cabinets, buildings, even roads. The ocean is a very efficient reflector of radio energy. Reflection is the primary cause of a behavior called multipath, which caused serious signal degradation in pre-IEEE 802.11n Wi-Fi systems. Reflection can also be used to advantage: large smooth-surfaced buildings have been used to great effect to bounce signals to an otherwise unreachable receiver. Reflection always results in some signal loss.
Scattering, or scatter, is a form of reflection and can happen in several ways. The most common method is when a wave strikes an uneven surface such as foliage and reflects in many different directions. Scattering could be thought of as multiple reflections. Scattering can also occur when RF energy passes through a material and encounters reflective entities within it. Scattering can seriously attenuate a radio signal by essentially breaking it into many smaller, weaker signals, even resulting in a complete loss of the signal.
Refraction is a property of a wave to change direction when traveling between different mediums. Each medium has a characteristic refractive index: a dimensionless number comparing the speed of light in a vacuum to the speed of light in a material. Water has a refractive index of 1.33, meaning that light travels 1.33 times slower in water than in a vacuum, which is why a stick appears bent when it is in water. The refractive index is an indicator of how much the wave will be bent from its original path. This principle can also be applied to RF energy. As RF passes through the boundary between heavy fog and clear air, its speed slows in relation to the change in density, and the wave bends. Typically, the wave is partially refracted and partially reflected.
Diffraction is another property of RF that bends the signal. Whenever a wave encounters an obstacle, it tends to travel around it, much like water flowing around the pier of a bridge. This phenomenon is most pronounced at long distances and at lower wavelengths. At shorter distances, an object can create an RF shadow, while at longer distances the object has a negligible effect on the signal. Diffraction occurs at a greater degree as the edge of an obstacle becomes sharper, as in the ridge of a mountain range.
On long-distance links, heavy rain will attenuate a signal as much as 10 dB/km on top of normal free space path loss. We briefly touched on free space path loss (FSPL) in the last segment. FSPL is one characteristic of the behavior of a transmitted wave. To review, the signal will attenuate (decrease in amplitude or intensity) at a rate equal to the inverse of the square of the distance between the two antennas (the Inverse Square Law). As the distance doubles, the power decreases to one quarter of its original intensity. Figure 4 gives a graphical representation of the Inverse Square Law.
Another concept we touched upon was fade margin. The fade margin is calculated as the signal level in dB above what is required for a reliable signal. For example, if the receiver sensitivity is -85 dBm, a good fade margin would be 20-25 dB, requiring the received signal to be in the range of -110 to -80 dBm. In no instance should the fade margin be less than 10 dB.
The Fresnel (pronounced FRUH-nel) zone is an elliptical area surrounding a theoretical point-to-point transmission. While this phenomenon is not typically a factor in short-range propagation, it becomes a significant factor in long-distance point-to-point transmissions. Theoretically, there are an infinite number of Fresnel zones; each is concentric to the first Fresnel ellipse and has diminishing effect on the signal as they increase. The first Fresnel zone contains the transmitted signal and is susceptible to obstructions within the zone, such as trees or buildings. Obstruction of the first zone by more than 40% will result in an unreliable link. The diameter of the Fresnel zone is determined solely by the frequency and the distance of the link and can be calculated for any point along the link; narrowing the beamwidth of the antenna has no effect. Even numbered Fresnel zones are out of phase with the transmitted signal; odd numbered are in phase.
The link budget (see prior post on "Radio math") is determined by summing all the gains and losses in the system. This will give the designer an idea of how much transmit power is needed to provide a usable signal to the receiving radio. Each component in a radio system will either provide gain or exhibit loss. If it is determined that a specific signal level is required to provide a usable signal to the receiver, it is incumbent on the designer to calculate the link budget to ensure that the link will be successful. Each of the behaviors described in this article has a material effect on the signal strength and successful propagation.
In any radio system, the power output of a transmitter is applied to an antenna; in many instances, the antenna is connected to the transmitter by a transmission line. Every transmission line has a characteristic loss per foot of cable. Every connector or coupling also causes attenuation, called insertion loss. The antenna will provide passive gain. At the antenna, the signal strength is the sum of all the gains and losses from the transmitter to the antenna output; you will remember this is called the EIRP, or equivalent isotropically radiated power. We discussed FSPL, free space path loss, and attenuation of the signal by obstruction of the Fresnel zone, refraction and diffraction, and absorption. These factors must all be considered for any radio link.
Finally, an important concept to understand is multipath. Multipath is a transmitted signal arriving at a receiver at different times via different paths due to reflections from objects in the transmission path. This causes signals to arrive out of phase, resulting in corruption or cancellation of signals. The time differential between these signals is measured in nanoseconds and is called the "delay spread." Early versions of Wi-Fi suffered greatly from the effects of multipath; however, newer standards are capable of using the effects of this phenomenon using a technology called MIMO, or multiple input, multiple output. MIMO uses multiple radio chains to take advantage of multipath.
4 results of multipath
There are four possible results of multipath:
1. Upfade — This is an increase in signal strength that occurs when multiple signals are received within the phase angles of 0-120 degrees and is considered constructive multipath. Signal amplitude is combined to varying degrees according to the phase angle. Because of free space path loss, the signal amplitude can never be greater than the amplitude of the transmitted signal.
2. Downfade — This is a decrease in signal strength resulting from received signals in the 121-179 phase angle and is considered destructive multipath. When the phase angle of the signal is opposed to another signal, that is, opposite in phase by some angle, the net signal amplitude will decrease proportionate to the phase angle.
3. Nulling — This is complete signal cancellation that occurs when signals arrive 180 degrees out of phase with the primary signal and is obviously a destructive combination of signals.
4. Data Corruption — When signals exhibit different delay spreads, the differential can cause demodulation problems and overlapping bits, causing a condition known as inter-symbol interference, or ISI. This is the most common type of destructive multipath.
Multipath is a very important concept to understand when deploying a Wi-Fi system, particularly pre-802.11n systems. Multipath will be covered in greater detail in a future segment.
- Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Radio antenna types
Comparative modulation: Spread spectrum modulation terms and definitions for wireless networking
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.