Non-line-of-sight propagation
Encyclopedia
Non-line-of-sight or near-line-of-sight is a term used to describe radio
transmission across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone
.
Many types of radio transmissions depend, to varying degrees, on line of sight
(LOS) between the transmitter and receiver. Obstacles that commonly cause NLOS conditions include buildings, trees, hills, mountains, and, in some cases, high voltage electric power
lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget.
Lower power levels at receiver give less space for correcly picking the transmission. Low levels can be caused by at least three basic reasons: low transmit level (such as Wi-Fi
), far-away transmitter (such as 3G above 5 miles away or TV more than 50 km away), or obstruction between TX and RX leaving no decent path.
NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with better antennae, but Non Line Of Sight is usually dealt with alternative paths or multipath propagation methods.
How to achieve effective NLOS networking has become one of the major questions of modern computer networking. Currently, the most common method for dealing with NLOS conditions on wireless computer networks is simply to circumvent the NLOS condition and place relay
s at additional locations, sending the content of the radio transmission around the obstructions. Some more advanced NLOS transmission schemes now use multipath signal propagation, bouncing the radio signal off other nearby objects to get to the receiver.
Non-Line-of-Sight (NLOS) is a term often used in radio communications
to describe a radio channel or link where there is no visual
line of sight
(LOS) between the transmitting
antenna
and the receiving antenna
. In this context LOS is taken
There are many electrical characteristics of the transmission media that affect the radio wave propagation
and therefore the quality of operation of a radio channel, if it is possible at all, over an NLOS path.
The acronym NLOS has become more popular in the context of wireless local area networks
(WLANs) and wireless metropolitan area networks such as WiMAX
because the capability of such links to provide a reasonable level of NLOS coverage greatly improves their marketability and versatility in the typical urban
environments in which they are most frequently used. However NLOS contains many other subsets of radio communications.
The influence of a visual obstruction on a NLOS link may be anything from negligible to complete suppression. An example might apply to a LOS path between a television broadcast antenna and a roof mounted receiving antenna. If a cloud passed between the antennas the link could actually become NLOS but the quality of the radio channel could be virtually unaffected. If, instead, a large building was constructed in the path making it NLOS, the channel may be impossible to receive.
Beyond Line-Of-Sight (BLOS) is a related term often used in the military to describe radio communications capabilities that link personnel or systems which are too distant or too fully obscured by terrain for LOS communications. These radios utilize active repeaters, groundwave propogation, tropospheric scatter links, and ionospheric propogation to extend communication ranges from a few miles to a few thousand miles.
we find that radio waves, as they exist in free space in the far field
or Fraunhofer
region behave as plane waves. In plane waves the electric field
, magnetic field
and direction of propagation are mutually perpendicular
. To understand the various mechanisms that allow successful radio communications over NLOS paths we must consider how such plane waves are affected by the object or objects that visually obstruct the otherwise LOS path between the antennas. It is understood that the terms radio far field waves and radio plane waves are interchangeable.
to resolve a distant object. Our eyes are sensitive to light but optical wavelength
s are very short compared to radio wavelengths. Optical wavelengths range from about 400 nanometer (nm) to 700 nm but radio wavelengths range from approximately 1 millimetre
(mm) at 300 GHz to 30 kilometre
s (km) at 10 kHz. Even the shortest radio wavelength is therefore about 2000 times longer than the longest optical wavelength. For typical communications frequencies up to about 10 GHz the difference is more like 60,000 times so it is not always reliable to compare visual obstructions, such as might suggest a NLOS path, with the same obstructions as they might affect a radio propagation path.
NLOS links may either be simplex
(transmission is in one direction only), duplex (transmission is in both directions simultaneously) or half-duplex (transmission is possible in both directions but not simultaneously). Under normal conditions all radio links including NLOS are reciprocal
which means that the effects of the propagation conditions on the radio channel are identical whether it operates in simplex, duplex or half-duplex. Please remember that propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies is not necessarily reciprocal.
with multi-wavelength dimensions passing between the transmit and receive antennas could be a significant visual obstruction but is unlikely to affect the NLOS radio propagation much assuming it is constructed from fabric and full of hot air, both of which are good insulators. Conversely, a metal obstruction of dimensions comparable to a wavelength would cause significant reflections. When considering obstruction size we will assume its electrical properties are the most common intermediate or lossy type.
If the obstruction dimensions are much smaller than the wavelength of the incident plane wave, the wave will be essentially unaffected. For example, low frequency (LF) broadcasts, also known as long waves
, at about 200 kHz has a wavelength of 1500 m and is not significantly affected by most average size buildings which are of much smaller dimensions.
If the obstruction dimensions are of the same order as a wavelength, there will be a degree of diffraction
around the obstruction and possibly some transmission through it. The incident radio wave could be slightly attenuated and there might be some interaction between the diffracted wavefronts.
If the obstruction has dimensions of many wavelengths, the incident plane waves will be heavily dependent on the electrical properties of the material forming the obstruction which are described below.
at one extreme to a perfect insulator
at the other. Most materials have both conductor and insulator properties. They may be mixed: for example, many NLOS paths result from the LOS path being obstructed by reinforced concrete
buildings constructed from concrete
and steel
. Concrete is quite a good insulator when dry and steel is a good conductor. Alternatively the material may be a homogeneous
lossy material.
The parameter which describes to what degree a material is a conductor or an insulator is known as
, or the loss tangent, given by
where
is the conductivity of the material in siemens
per metre (S/m)
is the angular frequency
of the RF plane wave in radian
s per second (rad/s) and
is its frequency in hertz
(Hz).
is the absolute permittivity
of free space in farad
s per metre (F/m)
and
is the relative permittivity
of the material (also known as dielectric constant
) and has no units.
the material is a good conductor or a poor insulator and will substantially reflect the radio waves that are incident upon it with almost the same power. Therefore virtually no RF power will be absorbed
by the material itself and virtually none will be transmitted, even if it is very thin. All metal
s are good conductors and there are of course many examples that cause significant reflections of radio waves in the urban
environment, for example bridges, metal clad buildings, storage warehouses, aircraft and electrical power transmission towers or pylons
.
the material is a good insulator (or dielectric) or a poor conductor and will substantially transmit waves that are incident upon it. Virtually no RF power will be absorbed but some could be reflected at its boundaries depending on its relative permittivity compared to that of free space, which is unity. This uses the concept of intrinsic impedance which is described below.There are few large physical objects that are also good insulators, with the interesting exception of fresh water iceberg
s but these do not usually feature in most urban environments. However large volumes of gas generally behave as dielectrics. Examples of these are regions of the Earths atmosphere
which gradually reduce in density at increasing altitude
s up to 10 to 20 km. At greater altitudes from about 50 km to 200 km various ionospheric layers also behave like dielectrics and are heavily dependent on the influence of the Sun
. Ionospheric layers are not gases but plasmas
.
Even if an obstruction is a perfect insulator, it may have some reflective properties on account of its relative permittivity
differing from that of the atmosphere. Electrical materials through which plane waves may propagate have a property called intrinsic impedance (
) or electromagnetic impedance which is analogous to the characteristic impedance
of a cable in transmission line theory. The intrinsic impedance of a homogeneous material is given by:
where
is the absolute permeability in henries per metre (H/m) and is a constant fixed at 
H/m
is the relative permeability (unitless)
is the absolute permittivity in farads per metre (F/m) and is a constant fixed at 
F/m
is the relative permittivity or dielectric constant (unitless)
For free space
and 
, therefore the intrinsic impedance of free space 
is given by
which evaluates to approximately 377
.
In an analogy
of plane wave theory and transmission line theory, the definition of reflection coefficient
is a measure of the level of reflection normally at the boundary when a plane wave passes from one dielectric medium to another. For example, if the intrinsic impedances of the first and second media were 
and 
respectively, the reflection coefficient of medium 2 relative to 1, 
, is given by:
The logarithmic measure in decibels (
) of how the transmitted RF signal over the NLOS link is affected by such a reflection is given by:
method because, on each reflection the plane wave undergoes a transmission loss which must be compensated for by a higher output power from the transmit antenna compared to if the link had been LOS. However the technique is cheap and easy to employ and passive random reflections are widely exploited in urban areas to achieve NLOS. Communication services which use passive reflections include WiFi
, WiMax
, WiMAX MIMO
MIMO, mobile
(cellular) communications and terrestrial broadcast to urban areas.
s may be used to achieve NLOS links by deliberately installing a precisely designed reflector at a critical position to provide a path around the obstruction. However they are unacceptable in most urban environments due to the bulky reflector requiring critical positioning at perhaps an inaccessible location or at one not acceptable to the planning authorities or the owner of the building. Passive reflector NLOS links also incur substantial loss due to the received signal being a 'double inverse-square law
' function of the transmit signal, one for each hop from the transmit antenna to the receive antenna. However, they have been successfully used in rural
mountainous areas to extend the range of LOS microwave links around mountains, thus creating NLOS links. In such cases the installation of the more usual active repeater was usually not possible due to problems in obtaining a suitable power supply.
the received signal and re-transmit it un-altered at either the same frequency or a different frequency. The former case is simpler and cheaper but requires good isolation between two antennas to avoid feedback
, however it does mean that the end of the NLOS link at A or C does not require to change the receive frequency from that used for a LOS link. A typical application might be to repeat or re-broadcast signals for vehicles using car radios in tunnels. A repeater that changes frequency would avoid any feedback problems but would be more difficult to design and expensive and it would require a receiver to change frequency when moving from the LOS to the NLOS zone. A communications satellite is a good example of an active repeater which does change frequency.
A communications satellite is an example of an active repeater which, in most cases, is positioned in geosynchronous orbit
at an altitude of 22,300 miles (35,000 km) above the Equator
.
to vertically polarised plane waves at LF
(30 kHz to 300 kHz) and VLF (3 kHz to 30 kHz) indicates that a component of the field is propagated a few metres into the surface of the Earth. The propagation is very low loss and communications over thousands of miles over NLOS links is possible. However, such low frequencies by definition (Nyquist–Shannon sampling theorem
) are very low bandwidth, so this type of communication is not widely used.
NLOS link typically operates at a few gigahertz using potentially very high transmit powers (typically 3 kW to 30 kW, depending on conditions), very sensitive receivers and very high gain, usually fixed, large reflector antennas. The transmit beam is directed into the troposphere
just above the horizon with sufficient power flux density that gas and water vapour molecules cause scattering in a region in the beam path known as the scatter volume. Some components of the scattered energy travel in the direction of the receiver antennas and form the receive signal. Since there are very many particles to cause scattering in this region, the Rayleigh fading
statistical model may usefully predict behaviour and performance in this kind of system.
itself, such as would exist if the other end of the link was beyond the optical horizon. A very useful property of the Earth's atmosphere
is that, on average, the density of air gas molecule
s reduces as the altitude
increases up to approximately 30 km. Its relative permittivity or dielectric constant reduces steadily from about 1.00536 at the Earth's surface. To model the change in refractive index with altitude, the atmosphere may be approximated to many thin air layers, each of which has a slightly smaller refractive index than the one below. The trajectory
of radio waves progressing through such an atmosphere model at each interface, is analogous to optical beams passing from one optical medium to another as predicted by Snell's Law
. When the beam passes from a higher to lower refractive index it tends to get bent or refracted away from the normal at the boundary according to Snell's Law. When the curvature of the Earth is taken into account it is found that, on average, radio waves whose initial trajectory is towards the optical horizon will in fact follow a path which does not return to the Earth's surface at the horizon, but slightly beyond it. The distance from the transmit antenna to where it does return is approximately equivalent to the optical horizon, had the Earth's radius been 4/3 of its actual value. The '4/3 Earth's radius' is a useful rule of thumb
to the radio communication engineers when designing such a NLOS link.
The 4/3 Earth radius rule of thumb is an average for the Earth's atmosphere assuming it is reasonably homogenised
, absent of temperature inversion layers or unusual meteorological conditions. NLOS links which exploit atmospheric refraction will typically operate at frequencies in the VHF and UHF bands including FM
sound and TV terrestrial broadcast services.
with increasing height. Air density is also a function of temperature which ordinarily also reduces with increasing height. However, these are only average conditions; local meteorological conditions can create phenomena such as temperature inversion layers where a warm layer of air settles above a cool layer. At the interface between them exists a relatively abrupt change in refractive index from a smaller value in the cool layer to a larger value in the warm layer. By analogy with the optical Snell's Law
, this can cause significant reflections of radio waves back towards the Earth's surface where they are further reflected, thus causing a ducting effect
. The result is that radio waves can propagate well beyond their intended service area with less than normal attenuation. This effect is only apparent in the VHF and UHF spectra and is often exploited by amateur radio
enthusiasts to achieve communications over abnormally long distances for the frequencies involved. For commercial communication services it cannot be exploited because it is unreliable (the conditions can form and disperse in minutes) and it can cause interference well outside of the normal service area.
Temperature inversion and anomalous propagation can occur at most latitudes but they are more common in tropical climates than temperate
climates, usually associated with high pressure areas (anticyclones).
in supporting NLOS links is similar to that for atmospheric refraction but, in this case, the radio wave refraction occurs, not in the atmosphere but in the ionosphere at much greater altitudes. Like its tropospheric counterpart, ionospheric propagation can sometimes be statistically modelled using Rayleigh fading
.
The ionosphere
extends from altitudes of approximately 50 km to 400 km and is divided into distinct plasma
layers denoted D, E, F1 and F2 in increasing altitude. Refraction of radio waves by the ionosphere rather than the atmosphere can therefore allow NLOS links of much greater distance for just one refraction path or 'hop' via one of the layers. Under certain conditions radio waves that have undergone one hop may reflect off the Earth's surface and experience more hops, so increasing the range. The positions of these and their ion
densities are significantly controlled by the Sun's incident radiation and therefore change diurnally
, season
ally and during Sun spot
activity. The initial discovery that radio waves could travel beyond the horizon by Marconi
in the early 20th century prompted extensive studies of ionospheric propagation for the next 50 years or so which have yielded various HF link channel prediction tables and charts.
Frequencies that are affected by ionospheric propagation range from approximately 500 kHz to 50 MHz but the majority of such NLOS links operate in the 'short wave' or high frequency
(HF) frequency bands between 3 MHz and 30 MHz.
In the latter half of the twentieth century, alternative means of communicating over large NLOS distances were developed such as satellite communications and submarine optical fiber, both of which potentially carry much larger bandwidths than HF and are much more reliable. Despite their limitations, HF communications only need relatively cheap, crude equipment and antennas so they are mostly used as backups to main communications systems and in sparsely populated remote areas where other methods of communication are not cost effective.
(WLANs) often use finite absorption NLOS links to communicate between a router and clients
in the typical office environment. The radio frequencies used, typically a few gigahertz (GHz) will normally pass through a few thin office walls and partitions with tolerable attenuation. After many such walls though or after a few thick concrete or similar (non-metallic) walls the NLOS link will become unworkable.
based localization system, the emitted signal can only arrive at the receiver through its NLOS paths. The NLOS error is defined as the extra distance travelled by the received signal with respect to the LOS path. The NLOS error is always positively biased with the magnitude dependent on the propagation environment.
"Technical Planning Parameters and Methods for Terrestrial Broadcasting" (April 2004); Australian Broadcasting Authority. ISBN 0-642-27063-5
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
transmission across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone
Fresnel zone
In optics and radio communications , a Fresnel zone , named for physicist Augustin-Jean Fresnel, is one of a number of concentric ellipsoids which define volumes in the radiation pattern of a circular aperture...
.
Many types of radio transmissions depend, to varying degrees, on line of sight
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
(LOS) between the transmitter and receiver. Obstacles that commonly cause NLOS conditions include buildings, trees, hills, mountains, and, in some cases, high voltage electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget.
Lower power levels at receiver give less space for correcly picking the transmission. Low levels can be caused by at least three basic reasons: low transmit level (such as Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
), far-away transmitter (such as 3G above 5 miles away or TV more than 50 km away), or obstruction between TX and RX leaving no decent path.
NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with better antennae, but Non Line Of Sight is usually dealt with alternative paths or multipath propagation methods.
How to achieve effective NLOS networking has become one of the major questions of modern computer networking. Currently, the most common method for dealing with NLOS conditions on wireless computer networks is simply to circumvent the NLOS condition and place relay
Radio relay
Radio stations that cannot communicate directly due to distance, terrain or other difficulties sometimes use an intermediate radio relay station to relay the signals. Examples include airborne radio relay, microwave radio relay, and communications satellite...
s at additional locations, sending the content of the radio transmission around the obstructions. Some more advanced NLOS transmission schemes now use multipath signal propagation, bouncing the radio signal off other nearby objects to get to the receiver.
Non-Line-of-Sight (NLOS) is a term often used in radio communications
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
to describe a radio channel or link where there is no visual
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
line of sight
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
(LOS) between the transmitting
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
and the receiving antenna
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. In this context LOS is taken
- either as a straight line free of any form of visual obstruction, even if it is actually too distant to see with the unaided human eyeHuman eyeThe human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
- as a virtual LOS i.e. as a straight line through visually obstructing material, thus leaving sufficient transmission for radio waves to be detected.
There are many electrical characteristics of the transmission media that affect the radio wave propagation
Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
and therefore the quality of operation of a radio channel, if it is possible at all, over an NLOS path.
The acronym NLOS has become more popular in the context of wireless local area networks
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
(WLANs) and wireless metropolitan area networks such as WiMAX
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
because the capability of such links to provide a reasonable level of NLOS coverage greatly improves their marketability and versatility in the typical urban
Urban area
An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...
environments in which they are most frequently used. However NLOS contains many other subsets of radio communications.
The influence of a visual obstruction on a NLOS link may be anything from negligible to complete suppression. An example might apply to a LOS path between a television broadcast antenna and a roof mounted receiving antenna. If a cloud passed between the antennas the link could actually become NLOS but the quality of the radio channel could be virtually unaffected. If, instead, a large building was constructed in the path making it NLOS, the channel may be impossible to receive.
Beyond Line-Of-Sight (BLOS) is a related term often used in the military to describe radio communications capabilities that link personnel or systems which are too distant or too fully obscured by terrain for LOS communications. These radios utilize active repeaters, groundwave propogation, tropospheric scatter links, and ionospheric propogation to extend communication ranges from a few miles to a few thousand miles.
Radio waves as plane electromagnetic waves
From Maxwell's equationsMaxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
we find that radio waves, as they exist in free space in the far field
Near and far field
The near field and far field and the transition zone are regions of the electromagnetic radiation field that emanates from a transmitting antenna, or as a result of radiation scattering off an object...
or Fraunhofer
Joseph von Fraunhofer
Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...
region behave as plane waves. In plane waves the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
, magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
and direction of propagation are mutually perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
. To understand the various mechanisms that allow successful radio communications over NLOS paths we must consider how such plane waves are affected by the object or objects that visually obstruct the otherwise LOS path between the antennas. It is understood that the terms radio far field waves and radio plane waves are interchangeable.
What is line-of-sight?
By definition, line of sight is the visual line of sight, that is determined by the ability of the average human eyeHuman eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
to resolve a distant object. Our eyes are sensitive to light but optical wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s are very short compared to radio wavelengths. Optical wavelengths range from about 400 nanometer (nm) to 700 nm but radio wavelengths range from approximately 1 millimetre
Millimetre
The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....
(mm) at 300 GHz to 30 kilometre
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
s (km) at 10 kHz. Even the shortest radio wavelength is therefore about 2000 times longer than the longest optical wavelength. For typical communications frequencies up to about 10 GHz the difference is more like 60,000 times so it is not always reliable to compare visual obstructions, such as might suggest a NLOS path, with the same obstructions as they might affect a radio propagation path.
NLOS links may either be simplex
Simplex communication
Simplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
(transmission is in one direction only), duplex (transmission is in both directions simultaneously) or half-duplex (transmission is possible in both directions but not simultaneously). Under normal conditions all radio links including NLOS are reciprocal
Reciprocity (electromagnetism)
In classical electromagnetism, reciprocity refers to a variety of related theorems involving the interchange of time-harmonic electric current densities and the resulting electromagnetic fields in Maxwell's equations for time-invariant linear media under certain constraints...
which means that the effects of the propagation conditions on the radio channel are identical whether it operates in simplex, duplex or half-duplex. Please remember that propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies is not necessarily reciprocal.
How are plane waves affected by the size and electrical properties of the obstruction?
In general, the way in which a plane wave is affected by an obstruction depends on the size of the obstruction relative to its wavelength and the electrical properties of the obstruction. For example, a hot air balloonHot air balloon
The hot air balloon is the oldest successful human-carrying flight technology. It is in a class of aircraft known as balloon aircraft. On November 21, 1783, in Paris, France, the first untethered manned flight was made by Jean-François Pilâtre de Rozier and François Laurent d'Arlandes in a hot air...
with multi-wavelength dimensions passing between the transmit and receive antennas could be a significant visual obstruction but is unlikely to affect the NLOS radio propagation much assuming it is constructed from fabric and full of hot air, both of which are good insulators. Conversely, a metal obstruction of dimensions comparable to a wavelength would cause significant reflections. When considering obstruction size we will assume its electrical properties are the most common intermediate or lossy type.
Obstruction Size
Broadly, there are three approximate sizes of obstruction in relationship to a wavelength to consider in a possible NLOS path. Those which are:- (a) much smaller than a wavelength
- (b) of the same order as a wavelength
- (c) much larger than a wavelength
If the obstruction dimensions are much smaller than the wavelength of the incident plane wave, the wave will be essentially unaffected. For example, low frequency (LF) broadcasts, also known as long waves
Longwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
, at about 200 kHz has a wavelength of 1500 m and is not significantly affected by most average size buildings which are of much smaller dimensions.
If the obstruction dimensions are of the same order as a wavelength, there will be a degree of diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
around the obstruction and possibly some transmission through it. The incident radio wave could be slightly attenuated and there might be some interaction between the diffracted wavefronts.
If the obstruction has dimensions of many wavelengths, the incident plane waves will be heavily dependent on the electrical properties of the material forming the obstruction which are described below.
The electrical properties of obstructions that may cause NLOS
The electrical properties of the material forming an obstruction to radio waves could range from a perfect conductorElectrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
at one extreme to a perfect insulator
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
at the other. Most materials have both conductor and insulator properties. They may be mixed: for example, many NLOS paths result from the LOS path being obstructed by reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
buildings constructed from concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
and steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
. Concrete is quite a good insulator when dry and steel is a good conductor. Alternatively the material may be a homogeneous
Homogeneity (physics)
In general, homogeneity is defined as the quality or state of being homogeneous . For instance, a uniform electric field would be compatible with homogeneity...
lossy material.
The parameter which describes to what degree a material is a conductor or an insulator is known as
, or the loss tangent, given bywhere
is the conductivity of the material in siemensSiemens (unit)
The siemens is the SI derived unit of electric conductance and electric admittance. Conductance and admittance are the reciprocals of resistance and impedance respectively, hence one siemens is equal to the reciprocal of one ohm, and is sometimes referred to as the mho. In English, the term...
per metre (S/m)
is the angular frequencyAngular frequency
In physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...
of the RF plane wave in radian
Radian
Radian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit...
s per second (rad/s) and
is its frequency in hertzHertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(Hz).
is the absolute permittivityPermittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
of free space in farad
Farad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
s per metre (F/m)
and
is the relative permittivityPermittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
of the material (also known as dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
) and has no units.
Good conductors (poor insulators)
If the material is a good conductor or a poor insulator and will substantially reflect the radio waves that are incident upon it with almost the same power. Therefore virtually no RF power will be absorbedAbsorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
by the material itself and virtually none will be transmitted, even if it is very thin. All metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s are good conductors and there are of course many examples that cause significant reflections of radio waves in the urban
Urban area
An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...
environment, for example bridges, metal clad buildings, storage warehouses, aircraft and electrical power transmission towers or pylons
Electricity pylon
A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...
.
Good insulators (poor conductors)
If the material is a good insulator (or dielectric) or a poor conductor and will substantially transmit waves that are incident upon it. Virtually no RF power will be absorbed but some could be reflected at its boundaries depending on its relative permittivity compared to that of free space, which is unity. This uses the concept of intrinsic impedance which is described below.There are few large physical objects that are also good insulators, with the interesting exception of fresh water icebergIceberg
An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice...
s but these do not usually feature in most urban environments. However large volumes of gas generally behave as dielectrics. Examples of these are regions of the Earths atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
which gradually reduce in density at increasing altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
s up to 10 to 20 km. At greater altitudes from about 50 km to 200 km various ionospheric layers also behave like dielectrics and are heavily dependent on the influence of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. Ionospheric layers are not gases but plasmas
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
.
Plane waves and intrinsic impedance
Even if an obstruction is a perfect insulator, it may have some reflective properties on account of its relative permittivity
differing from that of the atmosphere. Electrical materials through which plane waves may propagate have a property called intrinsic impedance () or electromagnetic impedance which is analogous to the characteristic impedanceCharacteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
of a cable in transmission line theory. The intrinsic impedance of a homogeneous material is given by:
where
is the absolute permeability in henries per metre (H/m) and is a constant fixed at H/m is the relative permeability (unitless) is the absolute permittivity in farads per metre (F/m) and is a constant fixed at F/m is the relative permittivity or dielectric constant (unitless)For free space
and , therefore the intrinsic impedance of free space is given bywhich evaluates to approximately 377
.Reflection losses at dielectric boundaries
In an analogy
Analogy
Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...
of plane wave theory and transmission line theory, the definition of reflection coefficient
Reflection coefficient
The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave...
is a measure of the level of reflection normally at the boundary when a plane wave passes from one dielectric medium to another. For example, if the intrinsic impedances of the first and second media were and respectively, the reflection coefficient of medium 2 relative to 1, , is given by:The logarithmic measure in decibels (
) of how the transmitted RF signal over the NLOS link is affected by such a reflection is given by:Intermediate materials with finite conductivity
Most materials of the type affecting radio wave transmission over NLOS links are intermediate: they are neither good insulators nor good conductors. Radio waves incident upon an obstruction comprising a thin intermediate material will be partly reflected at both the incident and exit boundaries and partly absorbed, depending on the thickness. If the obstruction is thick enough the radio wave might be completely absorbed. Because of the absorption, these are often called lossy materials, although the degree of loss is usually extremely variable and often very dependent on the level of moisture present. They will often be heterogeneous and comprise a mixture materials with various degrees of conductor and insulator properties. Such examples are hills, valley sides, mountains (with substantial vegetation) and buildings constructed from stone, brick or concrete but without reinforced steel. The thicker they are the greater the loss. For example a wall will absorb much less RF power from a normally incident wave than a building constructed from the same material.Means of achieving non-line-of-sight transmission
The non-line-of sight transmission means the transmission of building blocking.Passive random reflections
Passive random reflections are achieved when plane waves are subject to one or more reflective paths around an object which makes an otherwise LOS radio path into NLOS. The reflective paths might be caused by various objects which could either be metallic (very good conductors such as a steel bridge) or relatively good conductors to plane waves such as large expanses of concrete building sides, walls etc. Sometimes this is considered a brute forceBrute force
Brute force may refer to any of several problem-solving methods involving the evaluation of multiple possible answer for fitness. The term has also been used as a stage name, book title, etc.In mathematics:...
method because, on each reflection the plane wave undergoes a transmission loss which must be compensated for by a higher output power from the transmit antenna compared to if the link had been LOS. However the technique is cheap and easy to employ and passive random reflections are widely exploited in urban areas to achieve NLOS. Communication services which use passive reflections include WiFi
WIFI
WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...
, WiMax
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
, WiMAX MIMO
WiMAX MIMO
WiMAX MIMO refers to the use of Multiple-input multiple-output communications technology on WiMAX, which is the technology brand name for the implementation of the standard IEEE 802.16.-WiMAX:...
MIMO, mobile
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
(cellular) communications and terrestrial broadcast to urban areas.
Passive repeaters
Passive repeaterPassive repeater
A passive radio link deflection, or passive repeater is a plant for the implementation of a microwave link, in which because of an obstacle in the signal path no direct line of sight microwave link is possible...
s may be used to achieve NLOS links by deliberately installing a precisely designed reflector at a critical position to provide a path around the obstruction. However they are unacceptable in most urban environments due to the bulky reflector requiring critical positioning at perhaps an inaccessible location or at one not acceptable to the planning authorities or the owner of the building. Passive reflector NLOS links also incur substantial loss due to the received signal being a 'double inverse-square law
Inverse-square law
In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity....
' function of the transmit signal, one for each hop from the transmit antenna to the receive antenna. However, they have been successfully used in rural
Rural
Rural areas or the country or countryside are areas that are not urbanized, though when large areas are described, country towns and smaller cities will be included. They have a low population density, and typically much of the land is devoted to agriculture...
mountainous areas to extend the range of LOS microwave links around mountains, thus creating NLOS links. In such cases the installation of the more usual active repeater was usually not possible due to problems in obtaining a suitable power supply.
Active repeaters
An active repeater is a powered piece of equipment essentially comprising a receiving antenna, a receiver, a transmitter and a transmitting antenna. If the ends of the NLOS link are at positions A and C, the repeater is located at position B where links A-B and B-C are in fact LOS. The active repeater may simply amplifyAmplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
the received signal and re-transmit it un-altered at either the same frequency or a different frequency. The former case is simpler and cheaper but requires good isolation between two antennas to avoid feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
, however it does mean that the end of the NLOS link at A or C does not require to change the receive frequency from that used for a LOS link. A typical application might be to repeat or re-broadcast signals for vehicles using car radios in tunnels. A repeater that changes frequency would avoid any feedback problems but would be more difficult to design and expensive and it would require a receiver to change frequency when moving from the LOS to the NLOS zone. A communications satellite is a good example of an active repeater which does change frequency.
A communications satellite is an example of an active repeater which, in most cases, is positioned in geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
at an altitude of 22,300 miles (35,000 km) above the Equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
.
Groundwave propagation
Application of the Poynting VectorPoynting vector
In physics, the Poynting vector can be thought of as representing the directional energy flux density of an electromagnetic field. It is named after its inventor John Henry Poynting. Oliver Heaviside and Nikolay Umov independently co-invented the Poynting vector...
to vertically polarised plane waves at LF
Lf
Lf or LF may stand for:* FlyNordic IATA airline designator* Nippon Broadcasting System, a radio station in Tokyo, Japan - JOLF* Lactoferrin, a protein* Laissez-faire* LeapFrog, an educational toy company...
(30 kHz to 300 kHz) and VLF (3 kHz to 30 kHz) indicates that a component of the field is propagated a few metres into the surface of the Earth. The propagation is very low loss and communications over thousands of miles over NLOS links is possible. However, such low frequencies by definition (Nyquist–Shannon sampling theorem
Nyquist–Shannon sampling theorem
The Nyquist–Shannon sampling theorem, after Harry Nyquist and Claude Shannon, is a fundamental result in the field of information theory, in particular telecommunications and signal processing. Sampling is the process of converting a signal into a numeric sequence...
) are very low bandwidth, so this type of communication is not widely used.
Tropospheric scatter links
A tropospheric scatterTropospheric scatter
Tropospheric scatter is a method of transmitting and receiving microwave radio signals over considerable distances – often up to 300 km...
NLOS link typically operates at a few gigahertz using potentially very high transmit powers (typically 3 kW to 30 kW, depending on conditions), very sensitive receivers and very high gain, usually fixed, large reflector antennas. The transmit beam is directed into the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
just above the horizon with sufficient power flux density that gas and water vapour molecules cause scattering in a region in the beam path known as the scatter volume. Some components of the scattered energy travel in the direction of the receiver antennas and form the receive signal. Since there are very many particles to cause scattering in this region, the Rayleigh fading
Rayleigh fading
Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices.Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission medium will vary randomly, or fade, according to a...
statistical model may usefully predict behaviour and performance in this kind of system.
Refraction through the Earth's atmosphere
The obstruction that creates an NLOS link may be the EarthEarth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
itself, such as would exist if the other end of the link was beyond the optical horizon. A very useful property of the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
is that, on average, the density of air gas molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s reduces as the altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
increases up to approximately 30 km. Its relative permittivity or dielectric constant reduces steadily from about 1.00536 at the Earth's surface. To model the change in refractive index with altitude, the atmosphere may be approximated to many thin air layers, each of which has a slightly smaller refractive index than the one below. The trajectory
Trajectory
A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...
of radio waves progressing through such an atmosphere model at each interface, is analogous to optical beams passing from one optical medium to another as predicted by Snell's Law
Snell's law
In optics and physics, Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass...
. When the beam passes from a higher to lower refractive index it tends to get bent or refracted away from the normal at the boundary according to Snell's Law. When the curvature of the Earth is taken into account it is found that, on average, radio waves whose initial trajectory is towards the optical horizon will in fact follow a path which does not return to the Earth's surface at the horizon, but slightly beyond it. The distance from the transmit antenna to where it does return is approximately equivalent to the optical horizon, had the Earth's radius been 4/3 of its actual value. The '4/3 Earth's radius' is a useful rule of thumb
Rule of thumb
A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination...
to the radio communication engineers when designing such a NLOS link.
The 4/3 Earth radius rule of thumb is an average for the Earth's atmosphere assuming it is reasonably homogenised
Homogeneity (physics)
In general, homogeneity is defined as the quality or state of being homogeneous . For instance, a uniform electric field would be compatible with homogeneity...
, absent of temperature inversion layers or unusual meteorological conditions. NLOS links which exploit atmospheric refraction will typically operate at frequencies in the VHF and UHF bands including FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
sound and TV terrestrial broadcast services.
Anomalous propagation
The phenomenon described above that the atmospheric refractive index, relative permittivity or dielectric constant gradually reduces with increasing height is on account of the reduction of the atmospheric air densityDensity
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
with increasing height. Air density is also a function of temperature which ordinarily also reduces with increasing height. However, these are only average conditions; local meteorological conditions can create phenomena such as temperature inversion layers where a warm layer of air settles above a cool layer. At the interface between them exists a relatively abrupt change in refractive index from a smaller value in the cool layer to a larger value in the warm layer. By analogy with the optical Snell's Law
Snell's law
In optics and physics, Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass...
, this can cause significant reflections of radio waves back towards the Earth's surface where they are further reflected, thus causing a ducting effect
Atmospheric duct
In telecommunication, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals are guided or ducted, tend to follow the curvature of the Earth, and experience less attenuation in the ducts than they would if...
. The result is that radio waves can propagate well beyond their intended service area with less than normal attenuation. This effect is only apparent in the VHF and UHF spectra and is often exploited by amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
enthusiasts to achieve communications over abnormally long distances for the frequencies involved. For commercial communication services it cannot be exploited because it is unreliable (the conditions can form and disperse in minutes) and it can cause interference well outside of the normal service area.
Temperature inversion and anomalous propagation can occur at most latitudes but they are more common in tropical climates than temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
climates, usually associated with high pressure areas (anticyclones).
Ionospheric propagation
The mechanism of ionospheric propagationIonospheric reflection
Ionospheric reflection is a bending, through a complex process involving reflection and refraction, of electromagnetic waves propagating in the ionosphere back toward the Earth....
in supporting NLOS links is similar to that for atmospheric refraction but, in this case, the radio wave refraction occurs, not in the atmosphere but in the ionosphere at much greater altitudes. Like its tropospheric counterpart, ionospheric propagation can sometimes be statistically modelled using Rayleigh fading
Rayleigh fading
Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices.Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission medium will vary randomly, or fade, according to a...
.
The ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
extends from altitudes of approximately 50 km to 400 km and is divided into distinct plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
layers denoted D, E, F1 and F2 in increasing altitude. Refraction of radio waves by the ionosphere rather than the atmosphere can therefore allow NLOS links of much greater distance for just one refraction path or 'hop' via one of the layers. Under certain conditions radio waves that have undergone one hop may reflect off the Earth's surface and experience more hops, so increasing the range. The positions of these and their ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
densities are significantly controlled by the Sun's incident radiation and therefore change diurnally
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
, season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
ally and during Sun spot
Sun SPOT
Sun SPOT is a wireless sensor network mote developed by Sun Microsystems. The device is built upon the IEEE 802.15.4 standard...
activity. The initial discovery that radio waves could travel beyond the horizon by Marconi
Marconi Company
The Marconi Company Ltd. was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal Company...
in the early 20th century prompted extensive studies of ionospheric propagation for the next 50 years or so which have yielded various HF link channel prediction tables and charts.
Frequencies that are affected by ionospheric propagation range from approximately 500 kHz to 50 MHz but the majority of such NLOS links operate in the 'short wave' or high frequency
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
(HF) frequency bands between 3 MHz and 30 MHz.
In the latter half of the twentieth century, alternative means of communicating over large NLOS distances were developed such as satellite communications and submarine optical fiber, both of which potentially carry much larger bandwidths than HF and are much more reliable. Despite their limitations, HF communications only need relatively cheap, crude equipment and antennas so they are mostly used as backups to main communications systems and in sparsely populated remote areas where other methods of communication are not cost effective.
Finite absorption
If an object which is responsible for changing a LOS link to NLOS is not a good conductor but an intermediate material it will absorb some of the RF power incident upon it. However, if it has finite thickness the absorption will also be finite and the resulting attenuation of the radio waves may be tolerable and an NLOS link may be set up using radio waves that actually pass through the material. As an example, wireless local area networksWireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
(WLANs) often use finite absorption NLOS links to communicate between a router and clients
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
in the typical office environment. The radio frequencies used, typically a few gigahertz (GHz) will normally pass through a few thin office walls and partitions with tolerable attenuation. After many such walls though or after a few thick concrete or similar (non-metallic) walls the NLOS link will become unworkable.
How is positioning accuracy affected by NLOS conditions?
In most of the recent localization systems, it is assumed that the received signals propagate through a LOS path. However, infringement of this assumption can result in inaccurate positioning data . For Time of ArrivalTime of arrival
Time of arrival , also called time of flight , is the travel time of a radio signal from a single transmitter to a remote single receiver. By the relation between light speed in vacuum and the carrier frequency of a signal the time is a measure for the distance between transmitter and receiver...
based localization system, the emitted signal can only arrive at the receiver through its NLOS paths. The NLOS error is defined as the extra distance travelled by the received signal with respect to the LOS path. The NLOS error is always positively biased with the magnitude dependent on the propagation environment.
Further reading
Bullington, K.; "Radio Propagation Fundamentals"; Bell System Technical Journal Vol. 36 (May 1957); pp 593–625."Technical Planning Parameters and Methods for Terrestrial Broadcasting" (April 2004); Australian Broadcasting Authority. ISBN 0-642-27063-5