Wireless line-of-sight, non-line-of-sight, beyond-line-of-sight propagation
One of the most important factors in designing a wireless system is how the radio frequency (RF) signals will propagate between the transmitter and receiver. While this may seem obvious to many, real-life deployments often aren’t that simple. Ideally, a clear line of sight (LOS) between two end points is the desired goal, but this is impractical, particularly in an industrial setting, crowded urban environment, or even an office. Rural areas present unique seasonal problems that affect propagation. This can flummox even a seasoned technician.
It is not always possible to provide a clear LOS transmission. This results in reflections, called multipath propagation, which require specialized methods of transmission and reception. Non-LOS (NLOS) and beyond-LOS (BLOS) are other cases of propagation that can be successfully dealt with to provide a robust and secure link.
Visual LOS vs. radio LOS
Line of sight is exactly what it states; the transmitter can see the receiver, or at least, the antennas of each can see each other. It is the visual line of sight. This is, again, an ideal case. It is important to remember the shortest signal wavelength is several thousand times longer than the longest optical wavelength. This means a visually clear LOS does not necessarily translate into radio LOS, and vice-versa.
To achieve a reliable RF link, careful planning, including a radio path study must be performed, along with an informed selection of equipment and antenna locations. The transmitter may use an omnidirectional antenna that is transmitting in all directions. The receiving antenna also may be an omni, but in many cases, and to increase the likelihood of receiving a usable signal, a directional antenna may be used.
For a dedicated link between two points—a point-to-point link will use a directional antenna to narrow the beam-width to avoid interference and increase the effective strength of the signals. All of these factors must be considered prior to final system design. Designers also should be aware of several possible impairments.
The first possible impairment is the Fresnel Zone (pronounced Fren-nel), which is a football-shaped area between the two tapered link end points that must be kept clear of obstructions to ensure a quality link. Area of concern here is the first Fresnel Zone (there are several); technically the area is a "prolate ellipsoid" that surrounds the transmitter and receiver and the area between them.
Obstructions within the first Fresnel Zone are not necessarily in the LOS between the end points, but they will cause a degradation of the signal strength and intermittent impairment. Behavior of the signal will differ based upon antenna polarization: a vertically polarized signal encountering an object in the first Fresnel Zone will invert and arrive at the antenna out of phase, degrading the signal. The opposite will happen with a horizontally polarized signal. The distance between the link endpoints and the wavelength of the transmitted signal determines the area of the Fresnel Zone.
Ground, water RF reflections
The next impairment to LOS are the reflections from the ground or water local to the transmitter. Without getting too far into the weeds of antenna theory, the reflections from what is essentially a ground plane cause multipath interference and degrade the signal. In short range microwave transmission, the multipath phenomenon is dealt with by using diversity antennas and complex algorithms to combine or reject signals based on whether they are received in or out of phase (constructive and destructive multipath). For longer-range links, raising the height of the antenna is the most common way to deal with reflections from the ground plane. The improvement in signal quality is called "height gain."
One other parameter affecting LOS propagation is the earth’s curvature. The rule of thumb is a transmitter at sea level has a LOS of seven miles if unobstructed, which is referred to as an "earth bulge." Another factor is the effect of atmosphere on propagation. Since the signal does not travel at a uniform height above the earth, the effects of varying atmospheric conditions will affect LOS. The most pronounced effect of declining atmospheric pressure is the signal will be bent towards the earth, effectively increasing propagation by a factor of around 4/3, or about 15%.
NLOS describes a link without a clear line-of-sight. Obstructions are either in the path of the link or within the first Fresnel Zone. The effect of an obstruction in a NLOS situation can range from negligible to complete obstruction. Radio waves are considered "plane waves" in that the magnetic and electric fields propagate in two distinct planes perpendicular to each other. Plane waves are affected by obstructions in several ways and the effect is dependent upon wavelength.
Obstructions fall into three broad categories: Smaller than the incident wavelength, the same size as the incident wavelength and larger than the incident wavelength. When an obstruction is smaller than the incident wavelength, there is negligible, if any, interference. When an obstruction is the same size as the incident wavelength, the plane wave will diffract around and through it with minor attenuation.
If an obstruction is larger than the incident wavelength, the signal will be obstructed to varying degrees depending upon the obstruction’s materials and their electrical characteristics.
BLOS, beyond NLOS
Beyond-line-of-sight (BLOS) propagation is a special case of NLOS often encountered in very long-distance communication links blocked by earth bulge, terrain, or other obstructions. BLOS and NLOS are virtually identical conditions with BLOS being used by the military to describe much the same conditions as NLOS.
Methods for overcoming these conditions use the same technology to achieve stable communication links. The most common method for medium to long-range links are passive and active repeaters, which receive the signal from the originating transmitter and repeat it to increase range. Passive repeaters do not amplify the signal; they reflect it into the desired area. Passive repeaters are used to beam signals into areas isolated by terrain such as a community in a valley or a hollow surrounded by hills or mountains.
A passive repeater is useful if the original signal is strong enough to sustain the loss of transmission (propagation loss), the propagated signal diminishes according to the "inverse square rule," which states the signal strength is inversely proportional to the square of the distance from the transmitter-the signal attenuates by a factor of four as the distance from the transmitter doubles.
Active repeaters receive, amplify, and then re-transmit the signal. In most cases of NLOS propagation mitigation. Active repeaters are more commonly used to increase range while preserving signal quality.
Other methods of dealing with NLOS/BLOS are troposphere scatter (troposcatter) ionospheric propagation, which use the earth’s atmosphere as a reflector to propagate RF over the horizon. Troposcatter can increase range up to 300 miles; ionospheric propagation can cover more than 2,000 miles. Both methods are vulnerable to atmospheric conditions and suffer greatly during magnetic storms, such as CMEs.
Do a radio path study
The first step in determining the quality of the link between the endpoints is to conduct a radio path study. This study is done by specialists who use a variety of resources to accurately map the path between endpoints to determine the best path, the Fresnel Zone obstructions and their effect on propagation, the need for, and location of, any ancillary equipment such as repeaters, the required signal strength at the transmitter, and receiver sensitivity.
The report typically contains visual depictions of the path on a topographic map and identifies any potential obstructions. When designing a link, it is advisable to contact the local building department to determine if any new high-rise buildings for other towers are being planned for the area within the path.
Planning for a communication system cannot be done on the fly or by putting components together without a plan or professional guidance. As with most things, one dollar spent on proper planning will save many dollars later.
Daniel E. Capano is senior project manager with Gannett Fleming Engineers and Architects, based in New York City. He is also the vice-chairman of the Stamford Water Pollution Control Authority (SWPCA) and chairs the SWPCA Technical Committee. Capano is a member of the Control Engineering Editorial Advisory Board. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, firstname.lastname@example.org.
KEYWORDS: Industrial wireless, wireless propagation, RF signal integrity
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