Multipath propagation

In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.

Multipath propagation causes multipath interference, including constructive and destructive interference, and phase shifting of the signal; destructive interference causes fading. Where the magnitudes of the signals arriving by the various paths have a distribution known as the Rayleigh distribution, this is known as Rayleigh fading. Where one component (often, but not necessarily, a line of sight component) dominates, a Rician distribution provides a more accurate model, and this is known as Rician fading.

Interference

Coherent waves that travel along two different paths will arrive with phase shift, hence interfering with each other.

Multipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two (or more) components of the wave interfere. Multipath interference is a common cause of "ghosting" in analog television broadcasts and of fading of radio waves.

A diagram of the ideal situation for TV signals moving through space: The signal leaves the transmitter (TX) and travels through one path to the receiver (the TV set, which is labeled RX)
In this illustration, an object (in this case an aircraft) pollutes the system by adding a second path. The signal arrives at RX by means of two different paths which have different lengths. The main path is the direct path, while the second is due to a reflection from the plane.

The condition necessary is that the components of the wave remain coherent throughout the whole extent of their travel.

The interference will arise owing to the two (or more) components of the wave having, in general, travelled a different length (as measured by optical path length – geometric length and refraction (differing optical speed)), and thus arriving at the detector out of phase with each other.

The signal due to indirect paths interferes with the required signal in amplitude as well as phase which is called multipath fading.

Examples

In facsimile and (analog) television transmission, multipath causes jitter and ghosting, seen as a faded duplicate image to the right of the main image. Ghosts occur when transmissions bounce off a mountain or other large object, while also arriving at the antenna by a shorter, direct route, with the receiver picking up two signals separated by a delay.

Radar multipath echoes from an actual target cause ghosts to appear.

In radar processing, multipath causes ghost targets to appear, deceiving the radar receiver. These ghosts are particularly bothersome since they move and behave like the normal targets (which they echo), and so the receiver has difficulty in isolating the correct target echo. These problems can be minimized by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below the ground or above a certain height (altitude).

In digital radio communications (such as GSM) multipath can cause errors and affect the quality of communications. The errors are due to intersymbol interference (ISI). Equalizers are often used to correct the ISI. Alternatively, techniques such as orthogonal frequency division modulation and rake receivers may be used.

GPS error due to multipath

In a Global Positioning System receiver, Multipath Effect can cause a stationary receiver's output to indicate as if it were randomly jumping about or creeping. When the unit is moving the jumping or creeping may be hidden, but it still degrades the displayed accuracy of location and speed.

In wired media

Multipath propagation is similar in power line communication and in telephone local loops. In either case, impedance mismatch causes signal reflection.

High-speed power line communication systems usually employ multi-carrier modulations (such as OFDM or Wavelet OFDM) to avoid the intersymbol interference that multipath propagation would cause. The ITU-T G.hn standard provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring (power lines, phone lines, and coaxial cables). G.hn uses OFDM with a cyclic prefix to avoid ISI. Because multipath propagation behaves differently in each kind of wire, G.hn uses different OFDM parameters (OFDM symbol duration, Guard Interval duration) for each media.

DSL modems also use Orthogonal frequency-division multiplexing to communicate with their DSLAM despite multipath. In this case the reflections may be caused by mixed wire gauges, but those from bridge taps are usually more intense and complex. Where OFDM training is unsatisfactory, bridge taps may be removed.

Mathematical modeling

Mathematical model of the multipath impulse response.

The mathematical model of the multipath can be presented using the method of the impulse response used for studying linear systems.

Suppose you want to transmit a signal, ideal Dirac pulse of electromagnetic power at time 0, i.e.

${\displaystyle x(t)=\delta (t)}$

At the receiver, due to the presence of the multiple electromagnetic paths, more than one pulse will be received, and each one of them will arrive at different times. In fact, since the electromagnetic signals travel at the speed of light, and since every path has a geometrical length possibly different from that of the other ones, there are different air travelling times (consider that, in free space, the light takes 3 μs to cross a 1 km span). Thus, the received signal will be expressed by

${\displaystyle y(t)=h(t)=\sum _{n=0}^{N-1}{\rho _{n}e^{j\phi _{n}}\delta (t-\tau _{n})}}$

where ${\displaystyle N}$ is the number of received impulses (equivalent to the number of electromagnetic paths, and possibly very large), ${\displaystyle \tau _{n}}$ is the time delay of the generic ${\displaystyle n^{th}}$ impulse, and ${\displaystyle \rho _{n}e^{j\phi _{n}}}$ represent the complex amplitude (i.e., magnitude and phase) of the generic received pulse. As a consequence, ${\displaystyle y(t)}$ also represents the impulse response function ${\displaystyle h(t)}$ of the equivalent multipath model.

More in general, in presence of time variation of the geometrical reflection conditions, this impulse response is time varying, and as such we have

${\displaystyle \tau _{n}=\tau _{n}(t)}$
${\displaystyle \rho _{n}=\rho _{n}(t)}$
${\displaystyle \phi _{n}=\phi _{n}(t)}$

Very often, just one parameter is used to denote the severity of multipath conditions: it is called the multipath time, ${\displaystyle T_{M}}$, and it is defined as the time delay existing between the first and the last received impulses

${\displaystyle T_{M}=\tau _{N-1}-\tau _{0}}$
Mathematical model of the multipath channel transfer function.

In practical conditions and measurement, the multipath time is computed by considering as last impulse the first one which allows receiving a determined amount of the total transmitted power (scaled by the atmospheric and propagation losses), e.g. 99%.

Keeping our aim at linear, time invariant systems, we can also characterize the multipath phenomenon by the channel transfer function ${\displaystyle H(f)}$, which is defined as the continuous time Fourier transform of the impulse response ${\displaystyle h(t)}$

${\displaystyle H(f)={\mathfrak {F}}(h(t))=\int _{-\infty }^{+\infty }{h(t)e^{-j2\pi ft}dt}=\sum _{n=0}^{N-1}{\rho _{n}e^{j\phi _{n}}e^{-j2\pi f\tau _{n}}}}$

where the last right-hand term of the previous equation is easily obtained by remembering that the Fourier transform of a Dirac pulse is a complex exponential function, an eigenfunction of every linear system.

The obtained channel transfer characteristic has a typical appearance of a sequence of peaks and valleys (also called notches); it can be shown that, on average, the distance (in Hz) between two consecutive valleys (or two consecutive peaks), is roughly inversely proportional to the multipath time. The so-called coherence bandwidth is thus defined as

${\displaystyle B_{C}\approx {\frac {1}{T_{M}}}}$

For example, with a multipath time of 3 μs (corresponding to a 1 km of added on-air travel for the last received impulse), there is a coherence bandwidth of about 330 kHz.

References

An adaptive equalizer is an equalizer that automatically adapts to time-varying properties of the communication channel. It is frequently used with coherent modulations such as phase shift keying, mitigating the effects of multipath propagation and Doppler spreading.

Adaptive equalizers are a subclass of adaptive filters. The central idea is altering the filter's coefficients to optimize a filter characteristic. For example, in case of linear discrete-time filters, the following equation can be used:

${\displaystyle \mathbf {w} _{opt}=\mathbf {R} ^{-1}\mathbf {p} }$

where ${\displaystyle \mathbf {w} _{opt}}$ is the vector of the filter's coefficients, ${\displaystyle \mathbf {R} }$ is the received signal covariance matrix and ${\displaystyle \mathbf {p} }$ is the cross-correlation vector between the tap-input vector and the desired response. In practice, the last quantities are not known and, if necessary, must be estimated during the equalization procedure either explicitly or implicitly.

Many adaptation strategies exist. They include, e.g.:

A well-known example is the decision feedback equalizer, a filter that uses feedback of detected symbols in addition to conventional equalization of future symbols. Some systems use predefined training sequences to provide reference points for the adaptation process.

Cyclic prefix

In telecommunications, the term cyclic prefix refers to the prefixing of a symbol, with a repetition of the end. The receiver is typically configured to discard the cyclic prefix samples, but the cyclic prefix serves two purposes:

It provides a guard interval to eliminate intersymbol interference from the previous symbol.

It repeats the end of the symbol so the linear convolution of a frequency-selective multipath channel can be modeled as circular convolution, which in turn may transform to the frequency domain via a discrete Fourier transform. This approach accommodates simple frequency domain processing, such as channel estimation and equalization.For the cyclic prefix to serve its objectives, it must have a length at least equal to the length of the multipath channel. The concept of a cyclic prefix is traditionally associated with OFDM systems, however the cyclic prefix is now also used in single carrier systems to improve the robustness to multipath propagation.

Digital Radio Mondiale (DRM; mondiale being Italian and French for "worldwide") is a set of digital audio broadcasting technologies designed to work over the bands currently used for analogue radio broadcasting including AM broadcasting, particularly shortwave, and FM broadcasting. DRM is more spectrally efficient than AM and FM, allowing more stations, at higher quality, into a given amount of bandwidth, using various MPEG-4 audio coding formats.

Digital Radio Mondiale is also the name of the international non-profit consortium that has designed the platform and is now promoting its introduction. Radio France Internationale, TéléDiffusion de France, BBC World Service, Deutsche Welle, Voice of America, Telefunken (now Transradio) and Thomcast (now Ampegon) took part at the formation of the DRM consortium.

The principle of DRM is that bandwidth is the limited element, and computer processing power is cheap; modern CPU-intensive audio compression techniques enable more efficient use of available bandwidth, at the expense of processing resources.

Diversity scheme

In telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity is mainly used in radio communication and is a common technique for combatting fading and co-channel interference and avoiding error bursts. It is based on the fact that individual channels experience different levels of fading and interference. Multiple versions of the same signal may be transmitted and/or received and combined in the receiver. Alternatively, a redundant forward error correction code may be added and different parts of the message transmitted over different channels. Diversity techniques may exploit the multipath propagation, resulting in a diversity gain, often measured in decibels.

Dynamic single-frequency networks

Dynamic Single Frequency Networks (DSFN) is a transmitter macrodiversity technique for OFDM based cellular networks.

DSFN is based on the idea of single frequency networks (SFN), which is a group of radio transmitters that send the same signal simultaneously over the same frequency. The term originates from the broadcasting world, where a broadcast network is a group of transmitters that send the same TV or radio program. Digital wireless communication systems based on the OFDM modulation scheme are well-suited to SFN operation, since OFDM in combination with some forward error correction scheme can eliminate intersymbol interference and fading caused by multipath propagation without the use of complex equalization.

The concept of DSFN implies the SFN grouping is changed dynamically over time, from timeslot to timeslot. The aim is to achieve efficient spectrum utilization for downlink unicast or multicast communication services in centrally controlled cellular systems based on for example the OFDM modulation scheme. A centralized scheduling algorithm assigns each data packet to a certain timeslot, frequency channel and group of base station transmitters. DSFN can be considered as a combination of packet scheduling, macro-diversity and dynamic channel allocation (DCA). The scheduling algorithm can be further extended to dynamically assign other radio resource management parameters to each timeslot and transmitter, such as modulation scheme and error correction scheme, in view to optimize the efficiency.

DSFN makes it possible to increase the received signal strength to a mobile terminal in between several base station transmitters in comparison to non-macrodiversity communication schemes. Thus, DSFN can improve the coverage area and lessen the outage probability. Alternatively, DSFN may allow the same outage probability with a less robust but more efficient modulation and error coding scheme, and thus improve the spectral efficiency in bit/s/Hz/base station transmitter in comparison to a non-macrodiversity communication scheme.

DSFN resembles the CDMA downlink soft handover. A difference is that in the CDMA case, co-channel interference from transmissions to other users are more efficiently avoided by giving the other users other spreading codes.

A special form of DSFN is Continuous Transmission DSFN, where all base station transmitters always transmit at full power, without blocking of non-utilized transmitters, and without power control. This concept is very similar to so called Virtual cellular networks (VCNs), where a virtual cell is a group of base stations sending using the same spreading code, or a group of OFDM transmitters form a Single Frequency Network.

DSFN schemes can be described as a form of "virtual" power control.

In telecommunication, the term fade margin (fading margin) has the following meanings:

A design allowance that provides for sufficient system gain or sensitivity to accommodate expected fading, for the purpose of ensuring that the required quality of service is maintained.

The amount by which a received signal level may be reduced without causing system performance to fall below a specified threshold value. It is mainly used to describe a communication system such as satellite, for example a system like globalstar operates at 25-35 dB Fade margin

In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a communication channel that experiences fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath-induced fading, weather (particularly rain), or shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.

A GPS navigation device, GPS receiver, or simply GPS is a device that is capable of receiving information from GPS satellites and then to calculate the device's geographical position. Using suitable software, the device may display the position on a map, and it may offer directions. The Global Positioning System (GPS) is a global navigation satellite system (GNSS) made up of a network of a minimum of 24, but currently 30, satellites placed into orbit by the U.S. Department of Defense.The GPS was originally developed for use by the United States military, but in the 1980s, the United States government allowed the system to be used for civilian purposes. Though the GPS satellite data is free and works anywhere in the world, the GPS device and the associated software must be bought or rented.

A GPS device can retrieve from the GPS system location and time information in all weather conditions, anywhere on or near the Earth. A GPS reception requires an unobstructed line of sight to four or more GPS satellites, and is subject to poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit multipath propagation where signals bounce off structures, or are weakened by meteorological conditions. Obstructed lines of sight may arise from a tree canopy or inside a structure, such as in a building, garage or tunnel. Today, most standalone GPS receivers are used in automobiles. The GPS capability of smartphones may use assisted GPS (A-GPS) technology, which can use the base station or cell towers to provide a faster Time to First Fix (TTFF), especially when GPS signals are poor or unavailable. However, the mobile network part of the A-GPS technology would not be available when the smartphone is outside the range of the mobile reception network, while the GPS aspect would otherwise continue to be available.

The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s. GLONASS can be added to GPS devices to make more satellites available and enabling positions to be fixed more quickly and accurately, to within 2 meters.

Gregory Raleigh

Gregory “Greg” Raleigh (born 1961 in Orange, California), is an American radio scientist, inventor, and entrepreneur who has made contributions in the fields of wireless communication, information theory, mobile operating systems, medical devices, and network virtualization. His discoveries and inventions include the first wireless communication channel model to accurately predict the performance of advanced antenna systems, the MIMO-OFDM technology used in contemporary Wi-Fi and 4G wireless networks and devices, higher accuracy radiation beam therapy for cancer treatment, improved 3D surgery imaging, and a cloud-based Network Functions Virtualization platform for mobile network operators that enables users to customize and modify their smartphone services.

Incremental frequency keying

Incremental frequency keying, also known as IFK or IFK+, is a modified type of MFSK modulation where the data to be transmitted is represented by the difference in frequency between the previously received tone and the currently received tone.This modulation produces a signal which is much more tolerant of receiver mis-tunings and frequency drift than MFSK modulation. Additionally, IFK modulation is more resistant to multipath interference and intersymbol interference caused by multipath propagation than traditional MFSK.

This combination of features makes IFK modulation well suited for High Frequency communications.

This modulation is used in the amateur radio data-modes DominioEX and THOR.

Intersymbol interference

In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable. The spreading of the pulse beyond its allotted time interval causes it to interfere with neighboring pulses. ISI is usually caused by multipath propagation or the inherent linear or non-linear frequency response of a communication channel causing successive symbols to "blur" together.

The presence of ISI in the system introduces errors in the decision device at the receiver output. Therefore, in the design of the transmitting and receiving filters, the objective is to minimize the effects of ISI, and thereby deliver the digital data to its destination with the smallest error rate possible.

Ways to alleviate intersymbol interference include adaptive equalization and error correcting codes.

Log-distance path loss model

The log-distance path loss model is a radio propagation model that predicts the path loss a signal encounters inside a building or densely populated areas over distance.

MIMO

In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wireless communication standards including IEEE 802.11n (Wi-Fi), IEEE 802.11ac (Wi-Fi), HSPA+ (3G), WiMAX (4G), and Long Term Evolution (4G LTE). More recently, MIMO has been applied to power-line communication for 3-wire installations as part of ITU G.hn standard and HomePlug AV2 specification.At one time, in wireless the term "MIMO" referred to the use of multiple antennas at the transmitter and the receiver. In modern usage, "MIMO" specifically refers to a practical technique for sending and receiving more than one data signal simultaneously over the same radio channel by exploiting multipath propagation. MIMO is fundamentally different from smart antenna techniques developed to enhance the performance of a single data signal, such as beamforming and diversity.

MIMO-OFDM

Multiple-input, multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) is the dominant air interface for 4G and 5G broadband wireless communications. It combines multiple-input, multiple-output (MIMO) technology, which multiplies capacity by transmitting different signals over multiple antennas, and orthogonal frequency-division multiplexing (OFDM), which divides a radio channel into a large number of closely spaced subchannels to provide more reliable communications at high speeds. Research conducted during the mid-1990s showed that while MIMO can be used with other popular air interfaces such as time-division multiple access (TDMA) and code-division multiple access (CDMA), the combination of MIMO and OFDM is most practical at higher data rates.MIMO-OFDM is the foundation for most advanced wireless local area network (wireless LAN) and mobile broadband network standards because it achieves the greatest spectral efficiency and, therefore, delivers the highest capacity and data throughput. Greg Raleigh invented MIMO in 1996 when he showed that different data streams could be transmitted at the same time on the same frequency by taking advantage of the fact that signals transmitted through space bounce off objects (such as the ground) and take multiple paths to the receiver. That is, by using multiple antennas and precoding the data, different data streams could be sent over different paths. Raleigh suggested and later proved that the processing required by MIMO at higher speeds would be most manageable using OFDM modulation, because OFDM converts a high-speed data channel into a number of parallel lower-speed channels.

Modal dispersion

Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenomenon include multimode distortion, multimode dispersion, modal distortion, intermodal distortion, intermodal dispersion, and intermodal delay distortion.In the ray optics analogy, modal dispersion in a step-index optical fiber may be compared to multipath propagation of a radio signal. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle. Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than rays that enter at a steeper angle (which reflect many more times off the boundaries of the core as they travel the length of the fiber). The arrival of different components of the signal at different times distorts the shape.Modal dispersion limits the bandwidth of multimode fibers. For example, a typical step-index fiber with a 50 µm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km. Modal dispersion may be considerably reduced, but never completely eliminated, by the use of a core having a graded refractive index profile. However, multimode graded-index fibers having bandwidths exceeding 3.5 GHz·km at 850 nm are now commonly manufactured for use in 10 Gbit/s data links.

Modal dispersion should not be confused with chromatic dispersion, a distortion that results due to the differences in propagation velocity of different wavelengths of light. Modal dispersion occurs even with an ideal, monochromatic light source.

A special case of modal dispersion is polarization mode dispersion (PMD), a fiber dispersion phenomenon usually associated with single-mode fibers. PMD results when two modes that normally travel at the same speed due to fiber core geometric and stress symmetry (for example, two orthogonal polarizations in a waveguide of circular or square cross-section), travel at different speeds due to random imperfections that break the symmetry.

ODOP

The ODOP (Offset DOPpler) radar tracking system is essentially the same as the UDOP system used for many years at the Atlantic Missile Range, but ODOP operates at different frequencies. It is a phase-coherent, multistation Doppler tracking system which measures position of a vehicle equipped with the ODOP transponder. ODOP stations are located at and around Cape Kennedy. The ODOP transponder is carried in the first stage (S-IB or S-IC) of the Saturn vehicles and, therefore, ODOP tracking data is limited to the flight of the first stage only. The ODOP tracking system provides data immediately following lift-off while other tracking systems cannot "see" the vehicle or their accuracy is reduced by multipath propagation during the early phase of the flight.

The ODOP system is a radar interferometer tracking system which determines the position of a vehicle-borne transponder. The ground transmitter radiates a CW signal of 890 MHz to the transponder in the vehicle. The transponder shifts the received signal in frequency by 70 MHz and retransmits it to the receiving stations (R1, R2, R3). The signal from the transponder received at the ground stations contains a 2-way Doppler shift fD which is extracted by mixing the received signal (fi = 960 MHz + fD) with the reference frequency (fR = 960 MHz) derived from the transmitter frequency. Actually, a reference frequency of 53.33 MHz is transmitted over a VHF link to each transmitter station and then multiplied by a factor of 18, yielding 959.94 MHz. When this frequency is combined with the signal received from the transponder, the Doppler shift is obtained with a 60 kHz bias frequency (60 kHz + fD). The UDOP system used a transmitter frequency of 450 MHz which was doubled in the transponder (900 MHz). The higher frequency in the ODOP system (890 MHz versus 450 MHz) is less affected by the ionosphere and the result is increased tracking accuracy.

The Doppler frequencies, fD, (including the bias frequency) from all receiving stations are transmitted to the central station and recorded on magnetic tape. Integration of the Doppler frequency received at a particular station provides the range sum, i.e., the distance transmitter-transponder receiver. At least three range sums (for three different stations) are necessary to compute the position of the vehicle (transponder). The ODOP system uses 20 receiver stations around Cape Kennedy for redundancy and optimum tracking geometry. ODOP tracking data is not available in real time but is obtained from post-flight evaluation.

Orthogonal frequency-division multiplexing

In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, used in applications such as digital television and audio broadcasting, DSL internet access, wireless networks, power line networks, and 4G mobile communications.

In coded orthogonal frequency-division multiplexing (COFDM), forward error correction (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted. This is done to overcome errors in mobile communication channels affected by multipath propagation and Doppler effects. COFDM was introduced by Alard in 1986 for Digital Audio Broadcasting for Eureka Project 147. In practice, OFDM has become used in combination with such coding and interleaving, so that the terms COFDM and OFDM co-apply to common applications. OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. OFDM was introduced by Chang of Bell Labs in 1966. Numerous closely spaced orthogonal sub-carrier signals with overlapping spectra are emitted to carry data. Demodulation is based on Fast Fourier Transform algorithms. OFDM was improved by Weinstein and Ebert in 1971 with the introduction of a guard interval, providing better orthogonality in transmission channels affected by multipath propagation. Each sub-carrier (signal) is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate. This maintains total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

The main advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and use echoes and time-spreading (in analog television visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs) where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be re-combined constructively, sparing interference of a traditional single-carrier system.

Single-frequency network

A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.

Analog AM and FM radio broadcast networks as well as digital broadcast networks can operate in this manner. SFNs are not generally compatible with analog television transmission, since the SFN results in ghosting due to echoes of the same signal.

A simplified form of SFN can be achieved by a low power co-channel repeater, booster or broadcast translator, which is utilized as gap filler transmitter.

The aim of SFNs is efficient utilization of the radio spectrum, allowing a higher number of radio and TV programs in comparison to traditional multi-frequency network (MFN) transmission. An SFN may also increase the coverage area and decrease the outage probability in comparison to an MFN, since the total received signal strength may increase to positions midway between the transmitters.

SFN schemes are somewhat analogous to what in non-broadcast wireless communication, for example cellular networks and wireless computer networks, is called transmitter macrodiversity, CDMA soft handoff and Dynamic Single Frequency Networks (DSFN).

SFN transmission can be considered as a severe form of multipath propagation. The radio receiver receives several echoes of the same signal, and the constructive or destructive interference among these echoes (also known as self-interference) may result in fading. This is problematic especially in wideband communication and high-data rate digital communications, since the fading in that case is frequency-selective (as opposed to flat fading), and since the time spreading of the echoes may result in intersymbol interference (ISI). Fading and ISI can be avoided by means of diversity schemes and equalization filters.

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