Non-line-of-sight (NLOS) and near-line-of-sight are radio transmissions 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 a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than 5 miles (8.0 km) away or TV more than 31 miles (50 km) away; and obstruction between the transmitter and the receiver, leaving no clear path.
NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with using better antennas, but Non Line Of Sight usually requires 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 relays 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 where 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 too distant or too fully obscured by terrain for LOS communications. These radios utilize active repeaters, groundwave propagation, tropospheric scatter links, and ionospheric propagation to extend communication ranges from a few miles to a few thousand miles.
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 a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than 5 miles (8.0 km) away or TV more than 31 miles (50 km) away; and obstruction between the transmitter and the receiver, leaving no clear path.
NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with using better antennas, but Non Line Of Sight usually requires 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 relays 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 market
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 where 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 too distant or too fully obscured by terrain for LOS communications. These radios utilize active repeaters, groundwave propagation, tropospheric scatter links, and ionospheric propagation to extend communication ranges from a few miles to a few thousand miles.
From Maxwell's equations[1] we find that radio waves, as they exist in free space in the far field or Fraunhofer region behave as plane waves.[2][3] In plane waves the electric field, magnetic field and direction of propagation are mutually perpendicular.[4] 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.
By definition, line of sight is the visual line of sight, that is determined by the ability of the average human eye to resolve a distant object. Our eyes are sensitive to light but optical wavelengths 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 kilometres (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 on the order of 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 NLOSl 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.[5] However, propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies is not necessarily reciprocal.
In general, the way 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 balloon 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 assume its electrical properties are the most common intermediate or lossy type.
Broadly, there are three approximate sizes of obstruction in relationship to a wavelength to consider in a possible NLOS path—those that are:
If the obstruction dimensions are much smaller than the wavelength of the incident plane wave, the wave is 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 much smaller.
If the obstruction dimensions are of the same order as a wavelength, there is 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 depend heavily on the ele
By definition, line of sight is the visual line of sight, that is determined by the ability of the average human eye to resolve a distant object. Our eyes are sensitive to light but optical wavelengths 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 kilometres (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 on the order of 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 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 NLOSl 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.[5] However, propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies is not necessarily reciprocal.
In general, the way 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 balloon 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 assume its electrical properties are the most common intermediate or lossy type.
Broadly, there are three approximate sizes of obstruction in relationship to a wavelength to consider in a possible NLOS path—those that are:
If the obstruction dimensions are
Broadly, there are three approximate sizes of obstruction in relationship to a wavelength to consider in a possible NLOS path—those that are:
If the obstruction dimensions are much smaller than the wavelength of the incident plane wave, the wave is 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 much smaller.
If the obstruction dimensions are of the same order as a wavelength, there is 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 depend heavily on the electrical properties of the material that forms the obstruction.
The electrical properties of the material forming an obstruction to radio waves could range from a perfect conductor 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 that describes to what degree a material is a conductor or insulator is known as
If the material is a good insulator (or dielectric) or a poor conductor and substantially transmit waves that are incident upon it. Virtually no RF power is absorbed but some can 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 icebergs 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 altitudes 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,
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:[7]
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 impedance of the first and second media were
and
respectively, the reflection coefficient of medium 2 relative to 1,
.
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.
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 impedance of the first and second media were
and
respectively, the reflection coefficient of medium 2 relative to 1,
, is given by:
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 are 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 are often heterogeneous and comprise a mixture of 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 absorbs much less RF power from a normally incident wave than a building constructed from the same material.
Passive random reflections are achieved when plane waves are subject to one or more reflective paths around an object that makes an otherwise LOS radio path into NLOS. The reflective paths might be caused by various objects that could either be metallic (very good conductors such as a steel bridge or an airplane) or relatively good conductors to plane waves such as large expanses of concrete building sides, walls etc. Sometimes this is considered a brute force method because, on each reflection the plane wave undergoes a transmission loss that 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 that use passive reflections include WiFi, WiMax, WiMAX MIMO, mobile (cellular) communications and terrestrial broadcast to urban areas.
Passive repeaters 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 buildi
Passive repeaters 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.
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 amplify 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 an example of an active repeater that does change frequency. Communications satellites, in most cases, are in geosynchronous orbit at an altitude of 22,300 miles (35,000 km) above the Equator.
A communications satellite is an example of an active repeater that does change frequency. Communications satellites, in most cases, are in geosynchronous orbit at an altitude of 22,300 miles (35,000 km) above the Equator.
Application of the Poynting Vector to vertically polarized 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.
A tropospheric scatter 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.
Earth–Moon–Earth communication, Meteor burst communications, and Sporadic E propagation are also other methods of achieving communications past the radio horizon.
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