Non-line-of-sight (NLOS) radio propagation occurs when a radio signal travels between a transmitter and a receiver without a direct, unobstructed optical path (line-of-sight). Near-line-of-sight (also NLOS) conditions refer to partial obstruction by a physical object present in the innermost Fresnel zone.

Obstacles that commonly cause NLOS propagation 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 away or TV more than 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 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 eye 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 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.

(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 kilometers to a few thousand kilometers.

Background

Radio waves as plane electromagnetic waves

From Maxwell's equationsPozar, David M. (2005); Microwave Engineering, Third Edition (Intl. Ed.); John Wiley & Sons, Inc.; pp 5-9. . we find that radio waves, as they exist in free space in the far field or Fraunhofer region behave as plane waves.Ramo, Whinnery and Van Duzer;"Fields and Waves in Communication Electronics"; John Wiley & Sons, Inc; pp 322-324. Morton, A H; "Advanced Electrical Engineering"; Pitman Publishing Ltd.; pp 387-389. . In plane waves the electric field, magnetic field and direction of propagation are mutually perpendicular.A. J. Baden Fuller;"Microwaves, Second Edition"; Pergammon Press; p 47. . 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 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 millimeter (mm) at 300 GHz to 30 kilometers (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.Ramo, Whinnery and Van Duzer (op. cit); pp 717-719. However, propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies is not necessarily reciprocal.

Effect of obstruction size

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:

Much smaller than a wavelength The same order as a wavelength Much larger than a wavelength

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.

Effect of electrical properties of obstructions

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 , or the loss tangent, given by

where is the conductivity of the material in siemens per meter (S/m)

is the angular frequency of the RF plane wave in radians per second (rad/s) and is its frequency in hertz (Hz).

is the absolute permittivity of free space in farads per meter (F/m)

and

is the relative permittivity of the material (also known as dielectric constant) and has no units.

Good conductors (poor insulators) If the material is a good conductor or a poor insulator and substantially reflects the radio waves that are incident upon it with almost the same power.A. J. Baden Fuller (op. cit.); p152 Therefore, virtually no RF power is absorbed by the material itself and virtually none is transmitted, even if it is very thin. All metals 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.

Good insulators (poor conductors) 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 neutral gases but plasmas.

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 impedance of a cable in transmission line theory. The intrinsic impedance of a homogeneous material is given by:A. J. Baden Fuller (op. cit.); pp45-47

where

is the absolute permeability in henries per meter (H/m) and is a constant fixed at H/m

is the relative permeability (unitless)

is the absolute permittivity in farads per meter (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 .

Reflection losses at dielectric boundaries