1.
Modem
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A modem is a network hardware device that modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals to decode the transmitted information. The goal is attempting to produce a signal that can be transmitted easily, modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second. Modems can also be classified by their rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two frequencies, corresponding to two distinct symbols, to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols, news wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However, the function was incidental to the multiplexing function, so they are not commonly included in the history of modems. S. SAGE modems were described by AT&Ts Bell Labs as conforming to their newly published Bell 101 dataset standard, while they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101,110 baud modems. The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying, the famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines, frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems. For many years, the Bell System maintained a monopoly on the use of its phone lines, however, the seminal Hush-a-Phone v. FCC case of 1956 concluded it was within the FCCs jurisdiction to regulate the operation of the Bell System. The FCC found that as long as a device was not electronically attached to the system and this led to a number of devices that mechanically connected to the phone through a standard handset. Since most handsets were supplied by Western Electric and thus of a standard design and this type of connection was used for many devices, such as answering machines. Acoustically coupled Bell 103A-compatible 300 bit/s modems were common during the 1970s, well-known models included the Novation CAT and the Anderson-Jacobson, the latter spun off from an in-house project at Stanford Research Institute. An even lower-cost option was the Pennywhistle modem, designed to be built using parts from electronics scrap, in December 1972, Vadic introduced the VA3400, notable for full-duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit, in November 1976, AT&T introduced the 212A modem to compete with Vadic
2.
Telecommunication
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Telecommunication is the transmission of signs, signals, messages, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology and it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. The term is used in its plural form, telecommunications. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, and loud whistles. Zworykin, John Logie Baird and Philo Farnsworth, the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, and the Latin communicare, meaning to share. Its modern use is adapted from the French, because its use was recorded in 1904 by the French engineer. Communication was first used as an English word in the late 14th century, in the Middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could pass a single bit of information. One notable instance of their use was during the Spanish Armada, in 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris. However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres, as a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. Homing pigeons have occasionally used throughout history by different cultures. Pigeon post is thought to have Persians roots and was used by the Romans to aid their military, frontinus said that Julius Caesar used pigeons as messengers in his conquest of Gaul. The Greeks also conveyed the names of the victors at the Olympic Games to various cities using homing pigeons, in the early 19th century, the Dutch government used the system in Java and Sumatra. And in 1849, Paul Julius Reuter started a service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. Sir Charles Wheatstone and Sir William Fothergill Cooke invented the telegraph in 1837. Also, the first commercial electrical telegraph is purported to have constructed by Wheatstone and Cooke. Both inventors viewed their device as an improvement to the electromagnetic telegraph not as a new device, samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837
3.
Interferometry
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Interferometry is a family of techniques in which waves, usually electromagnetic waves, are superimposed causing the phenomenon of interference in order to extract information. Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes, in an interferometer, light from a single source is split into two beams that travel different optical paths, then combined again to produce interference. The resulting interference fringes give information about the difference in path length, waves which are not completely in phase nor completely out of phase will have an intermediate intensity pattern, which can be used to determine their relative phase difference. Most interferometers use light or some form of electromagnetic wave. Typically a single incoming beam of coherent light will be split into two beams by a beam splitter. Each of these beams travels a different route, called a path, the path difference, the difference in the distance traveled by each beam, creates a phase difference between them. It is this phase difference that creates the interference pattern between the initially identical waves. If a single beam has been split along two paths, then the difference is diagnostic of anything that changes the phase along the paths. This could be a change in the path length itself or a change in the refractive index along the path. As seen in Fig. 2a and 2b, the observer has a view of mirror M1 seen through the beam splitter. The fringes can be interpreted as the result of interference between light coming from the two virtual images S1 and S2 of the original source S, the characteristics of the interference pattern depend on the nature of the light source and the precise orientation of the mirrors and beam splitter. In Fig. 2a, the elements are oriented so that S1 and S2 are in line with the observer. Use of white light will result in a pattern of colored fringes, the central fringe representing equal path length may be light or dark depending on the number of phase inversions experienced by the two beams as they traverse the optical system. Interferometers and interferometric techniques may be categorized by a variety of criteria, In homodyne detection, the phase difference between the two beams results in a change in the intensity of the light on the detector. The resulting intensity of the light after mixing of two beams is measured, or the pattern of interference fringes is viewed or recorded. Most of the interferometers discussed in this article fall into this category, the heterodyne technique is used for shifting an input signal into a new frequency range as well as amplifying a weak input signal. A weak input signal of frequency f1 is mixed with a reference frequency f2 from a local oscillator. The nonlinear combination of the input signals creates two new signals, one at the sum f1 + f2 of the two frequencies, and the other at the difference f1 − f2 and these new frequencies are called heterodynes
4.
Amplitude modulation
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Amplitude modulation is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude of the wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker and this technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, and phase modulation, in which its phase is varied. AM was the earliest modulation method used to transmit voice by radio and it was developed during the first two decades of the 20th century beginning with Roberto Landell De Moura and Reginald Fessendens radiotelephone experiments in 1900. It remains in use today in many forms of communication, for example it is used in two way radios, VHF aircraft radio, Citizens Band Radio, and in computer modems. AM is often used to refer to mediumwave AM radio broadcasting, when it reaches its destination, the information signal is extracted from the modulated carrier by demodulation. In amplitude modulation, the amplitude or strength of the oscillations is what is varied. For example, in AM radio communication, a continuous wave signal has its amplitude modulated by an audio waveform before transmission. The audio waveform modifies the amplitude of the wave and determines the envelope of the waveform. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency, each sideband is equal in bandwidth to that of the modulating signal, and is a mirror image of the other. Standard AM is thus sometimes called double-sideband amplitude modulation to distinguish it more sophisticated modulation methods also based on AM. One disadvantage of all amplitude modulation techniques is that the receiver amplifies and detects noise, increasing the received signal to noise ratio, say, by a factor of 10, thus would require increasing the transmitter power by a factor of 10. For this reason AM broadcast is not favored for music and high fidelity broadcasting, another disadvantage of AM is that it is inefficient in power usage, at least two-thirds of the power is concentrated in the carrier signal. The carrier signal contains none of the information being transmitted. However its presence provides a means of demodulation using envelope detection, providing a frequency. The receiver may regenerate a copy of the frequency from a greatly reduced pilot carrier to use in the demodulation process. Even with the carrier totally eliminated in double-sideband suppressed-carrier transmission, carrier regeneration is possible using a Costas phase-locked loop and this doesnt work however for single-sideband suppressed-carrier transmission, leading to the characteristic Donald Duck sound from such receivers when slightly detuned. Single sideband is used widely in amateur radio and other voice communications both due to its power efficiency and bandwidth efficiency
5.
IEEE 802.11
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They are created and maintained by the Institute of Electrical and Electronics Engineers LAN/MAN Standards Committee. The base version of the standard was released in 1997, and has had subsequent amendments, the standard and amendments provide the basis for wireless network products using the Wi-Fi brand. As a result, in the marketplace, each tends to become its own standard. The 802.11 family consists of a series of half-duplex over-the-air modulation techniques that use the basic protocol. 802. 11-1997 was the first wireless networking standard in the family, but 802. 11b was the first widely accepted one, followed by 802. 11a,802. 11g,802. 11n, and 802. 11ac. Other standards in the family are service amendments that are used to extend the current scope of the existing standard, which may also include corrections to a previous specification. 802. 11b and 802. 11g use the 2.4 GHz ISM band, operating in the United States under Part 15 of the U. S. Federal Communications Commission Rules and Regulations. Because of this choice of band,802. 11b and g equipment may occasionally suffer interference from microwave ovens, cordless telephones. Better or worse performance with higher or lower frequencies may be realized,802. 11n can use either the 2.4 GHz or the 5 GHz band,802. 11ac uses only the 5 GHz band. The segment of the frequency spectrum used by 802.11 varies between countries. In the US,802. 11a and 802. 11g devices may be operated without a license, as allowed in Part 15 of the FCC Rules and Regulations. Frequencies used by one through six of 802. 11b and 802. 11g fall within the 2.4 GHz amateur radio band. Licensed amateur radio operators may operate 802. 11b/g devices under Part 97 of the FCC Rules and Regulations, allowing increased power output but not commercial content or encryption. 802.11 technology has its origins in a 1985 ruling by the U. S. Federal Communications Commission that released the ISM band for unlicensed use, in 1991 NCR Corporation/AT&T invented a precursor to 802.11 in Nieuwegein, the Netherlands. The inventors initially intended to use the technology for cashier systems, the first wireless products were brought to the market under the name WaveLAN with raw data rates of 1 Mbit/s and 2 Mbit/s. Vic Hayes, who held the chair of IEEE802.11 for 10 years, in 1999, the Wi-Fi Alliance was formed as a trade association to hold the Wi-Fi trademark under which most products are sold. The original version of the standard IEEE802.11 was released in 1997 and clarified in 1999 and it specified two net bit rates of 1 or 2 megabits per second, plus forward error correction code. The latter two radio technologies used microwave transmission over the Industrial Scientific Medical frequency band at 2.4 GHz, some earlier WLAN technologies used lower frequencies, such as the U. S.900 MHz ISM band
6.
Frequency modulation
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In telecommunications and signal processing, frequency modulation is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. This contrasts with amplitude modulation, in which the amplitude of the wave varies. This modulation technique is known as frequency-shift keying, FSK is widely used in modems and fax modems, and can also be used to send Morse code. Frequency modulation is used for FM radio broadcasting. For this reason, most music is broadcast over FM radio, frequency modulation has a close relationship with phase modulation, phase modulation is often used as an intermediate step to achieve frequency modulation. Mathematically both of these are considered a case of quadrature amplitude modulation. While most of the energy of the signal is contained within fc ± fΔ, the frequency spectrum of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems. Mathematically, a baseband modulated signal may be approximated by a continuous wave signal with a frequency fm. This method is also named as Single-tone Modulation. As in other systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. e. The maximum deviation of the frequency from the carrier frequency. For a sine wave modulation, the index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave. If h ≪1, the modulation is called narrowband FM, sometimes modulation index h<0.3 rad is considered as Narrowband FM otherwise Wideband FM. In the case of digital modulation, the carrier f c is never transmitted, rather, one of two frequencies is transmitted, either f c + Δ f or f c − Δ f, depending on the binary state 0 or 1 of the modulation signal. If h ≫1, the modulation is called wideband FM, if the frequency deviation is held constant and the modulation frequency increased, the spacing between spectra increases. The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals, for particular values of the modulation index, the carrier amplitude becomes zero and all the signal power is in the sidebands. Since the sidebands are on sides of the carrier, their count is doubled, and then multiplied by the modulating frequency to find the bandwidth. For example,3 kHz deviation modulated by a 2.2 kHz audio tone produces an index of 1.36. Suppose that we limit ourselves to only those sidebands that have an amplitude of at least 0.01
7.
Single-sideband modulation
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Amplitude modulation produces an output signal that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of increased device complexity, radio transmitters work by mixing a radio frequency signal of a specific frequency, the carrier wave, with the signal to be broadcast. That is, the signal has a spectrum with twice the bandwidth of the original input signal. In conventional AM radio, this signal is sent to the radio frequency amplifier. Due to the nature of the process, the quality of the resulting signal can be defined by the difference between the maximum and minimum signal energy. Normally the maximum signal energy will be the carrier itself, perhaps twice as powerful as the mixed signals, SSB takes advantage of the fact that the entire original signal is encoded in either one of these sidebands. It is not necessary to broadcast the entire mixed signal, a receiver can extract the entire signal from either the upper or lower sideband. This means that the amplifier can be used more efficiently. A transmitter can choose to only the upper or lower sideband. By doing so, the amplifier only has to work effectively on one half the bandwidth, as a result, SSB transmissions use the available amplifier energy more efficiently, providing longer-range transmission with little or no additional cost. The first U. S. patent for SSB modulation was applied for on December 1,1915 by John Renshaw Carson, the U. S. Navy experimented with SSB over its radio circuits before World War I. SSB first entered service on January 7,1927 on the longwave transatlantic public radiotelephone circuit between New York and London. The high power SSB transmitters were located at Rocky Point, New York and Rugby, the receivers were in very quiet locations in Houlton, Maine and Cupar Scotland. SSB was also used over long distance lines, as part of a technique known as frequency-division multiplexing. FDM was pioneered by telephone companies in the 1930s and this enabled many voice channels to be sent down a single physical circuit, for example in L-carrier. SSB allowed channels to be spaced just 4,000 Hz apart, amateur radio operators began serious experimentation with SSB after World War II. The Strategic Air Command established SSB as the standard for its aircraft in 1957. It has become a de facto standard for voice radio transmissions since then
8.
Spread spectrum
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This is a technique in which a telecommunication signal is transmitted on a bandwidth considerably larger than the frequency content of the original information. Frequency hopping is a modulation technique used in spread spectrum signal transmission. This technique decreases the potential interference to other receivers while achieving privacy, spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband band of frequencies. The receiver correlates the received signals to retrieve the information signal. Frequency-hopping spread spectrum, direct-sequence spread spectrum, time-hopping spread spectrum, chirp spread spectrum, ultra-wideband is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless standard IEEE802.11 uses either FHSS or DSSS in its radio interface, techniques known since the 1940s and used in military communication systems since the 1950s spread a radio signal over a wide frequency range several magnitudes higher than minimum requirement. DS is good at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming, by contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth. Multiple access capability, known as code-division multiple access or code-division multiplexing and their approach was unique in that frequency coordination was done with paper player piano rolls - a novel approach which was never put into practice. A synchronous digital system is one that is driven by a signal and. In fact, a clock signal would have all its energy concentrated at a single frequency. It has become a technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds, since these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. Active EMI reduction techniques such as spread-spectrum clocking are needed in these cases, however, spread-spectrum clocking, like other kinds of dynamic frequency change, can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew, note that this method does not reduce total radiated energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths, typical measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to all its energy in a narrow bandwidth. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits, FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, the spread-spectrum functionality may be disabled by the user in some cases and this might be considered a loophole, but is generally overlooked as long as spread-spectrum is enabled by default
9.
Phase-shift keying
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Phase-shift keying is a digital modulation scheme that conveys data by changing the phase of a reference signal. The modulation occurs by varying the sine and cosine inputs at a precise time and it is widely used for wireless LANs, RFID and Bluetooth communication. Any digital modulation scheme uses a number of distinct signals to represent digital data. PSK uses a number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes a number of bits. Each pattern of bits forms the symbol that is represented by the particular phase and this requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent. Alternatively, instead of operating with respect to a constant reference wave, changes in phase of a single broadcast waveform can be considered the significant items. In this system, the demodulator determines the changes in the phase of the signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying, in exchange, it produces more erroneous demodulation. In the case of PSK, the phase is changed to represent the data signal, a convenient method to represent PSK schemes is on a constellation diagram. This shows the points in the plane where, in this context. Such a representation on perpendicular axes lends itself to straightforward implementation, the amplitude of each point along the in-phase axis is used to modulate a cosine wave and the amplitude along the quadrature axis to modulate a sine wave. By convention, in-phase modulates cosine and quadrature modulates sine, in PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption and they are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the numbers they represent will be the same. Two common examples are binary phase-shift keying which uses two phases, and quadrature phase-shift keying which uses four phases, although any number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is designed with the number of constellation points being a power of 2. It is a form of the complementary Gaussian error function
10.
Line code
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In telecommunication, a line code is a code chosen for use within a communications system for transmitting a digital signal down a line. Line coding is used for digital data transport. Some line codes are digital baseband modulation or digital baseband transmission methods, Line coding consists of representing the digital signal to be transported, by a waveform that is appropriate for the specific properties of the physical channel. The pattern of voltage, current or photons used to represent the data on a transmission link is called line encoding. The common types of line encoding are unipolar, polar, bipolar, after line coding, the signal is put through a physical channel, either a transmission medium or data storage medium. Sometimes the characteristics of two very different-seeming channels are similar enough that the line code is used for them. The most common physical channels are, the signal can directly be put on a transmission line. The line-coded signal undergoes further pulse shaping and then modulated to create an RF signal that can be sent through free space, the line-coded signal can be used to turn on and off a light source in free-space optical communication, most commonly used in an infrared remote control. The line-coded signal can be printed on paper to create a bar code, the line-coded signal can be converted to magnetized spots on a hard drive or tape drive. The line-coded signal can be converted to pits on an optical disc, some of the more common or binary line codes include, Each line code has advantages and disadvantages. The running disparity is the total of the disparity of all previously transmitted words. Unfortunately, most long-distance communication channels cannot reliably transport a DC component, the DC component is also called the disparity, the bias, or the DC coefficient. The simplest possible line code, unipolar, gives too many errors on such systems, most line codes eliminate the DC component – such codes are called DC-balanced, zero-DC, or DC-free. There are three ways of eliminating the DC component, Use a constant-weight code, for example, Manchester code and Interleaved 2 of 5. In other words, the transmitter has to make sure that every word that averages to a negative level is paired with another code word that averages to a positive level. Therefore, it must keep track of the running DC buildup, the receiver is designed so that either code word of the pair decodes to the same data bits. For example, AMI, 8B10B, 4B3T, etc, for example, the scrambler specified in RFC2615 for 64b/66b encoding. This is important if the signal must pass through a transformer or a transmission line
11.
Amplitude-shift keying
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Amplitude-shift keying is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In an ASK system, the binary symbol 1 is represented by transmitting a carrier wave. If the signal value is 1 then the signal will be transmitted, otherwise. Any digital modulation scheme uses a number of distinct signals to represent digital data. ASK uses a number of amplitudes, each assigned a unique pattern of binary digits. Usually, each amplitude encodes a number of bits. Each pattern of bits forms the symbol that is represented by the particular amplitude, frequency and phase of the carrier are kept constant. Like AM, an ASK is also linear and sensitive to noise, distortions, propagation conditions on different routes in PSTN. Both ASK modulation and demodulation processes are relatively inexpensive, the ASK technique is also commonly used to transmit digital data over optical fiber. For LED transmitters, binary 1 is represented by a pulse of light. Laser transmitters normally have a bias current that causes the device to emit a low light level. This low level represents binary 0, while a higher-amplitude lightwave represents binary 1, the simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. For instance, an encoding scheme can represent two bits with each shift in amplitude, an eight-level scheme can represent three bits, and so on. These forms of amplitude-shift keying require a high ratio for their recovery. ASK system can be divided into three blocks, the first one represents the transmitter, the second one is a linear model of the effects of the channel, the third one shows the structure of the receiver. These impulses are sent to the filter ht to be sent through the channel, in other words, for each symbol a different carrier wave is sent with the relative amplitude. After the A/D conversion the signal z can be expressed in the form, z = n r + v g + ∑ n ≠ k v g In this relationship, the second term represents the symbol to be extracted. The others are unwanted, the first one is the effect of noise and it is shown in cyan for just one of them
12.
Orthogonal frequency-division multiplexing
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Orthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM is a frequency-division multiplexing scheme used as a digital modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels, each sub-carrier is modulated with a conventional modulation scheme at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe conditions 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. e, the following list is a summary of existing OFDM-based standards and products. For further details, see the Usage section at the end of the article, High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc. Can easily adapt to severe conditions without complex time-domain equalization. This greatly simplifies the design of both the transmitter and the receiver, unlike conventional FDM, a filter for each sub-channel is not required. The orthogonality requires that the spacing is Δ f = k T U Hertz, where TU seconds is the useful symbol duration. Therefore, with N sub-carriers, the passband bandwidth will be B ≈ N·Δf. The orthogonality also allows high spectral efficiency, with a symbol rate near the Nyquist rate for the equivalent baseband signal. Almost the whole available frequency band can be utilized, OFDM generally has a nearly white spectrum, giving it benign electromagnetic interference properties with respect to other co-channel users. A simple example, A useful symbol duration TU =1 ms would require a sub-carrier spacing of Δ f =11 m s =1 k H z for orthogonality, N =1,000 sub-carriers would result in a total passband bandwidth of NΔf =1 MHz. For this symbol time, the bandwidth in theory according to Nyquist is B W =1 / =0.5 M H z. If a guard interval is applied, Nyquist bandwidth requirement would be even lower, the FFT would result in N =1,000 samples per symbol. If no guard interval was applied, this would result in a base band complex valued signal with a rate of 1 MHz. However, the passband RF signal is produced by multiplying the signal with a carrier waveform resulting in a passband bandwidth of 1 MHz. A single-side band or vestigial sideband modulation scheme would achieve almost half that bandwidth for the symbol rate
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