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35-400: It is the unit for symbol rate or modulation rate in symbols per second or pulses per second . It is the number of distinct symbol changes (signalling events) made to the transmission medium per second in a digitally modulated signal or a bd rate line code . Baud is related to gross bit rate , which can be expressed in bits per second (bit/s). If there are precisely two symbols in

70-441: A constellation diagram , showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol. PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal I + jQ (where j is the imaginary unit ). The resulting so called equivalent lowpass signal or equivalent baseband signal

105-488: A digital signal consisting of a sequence of binary digits (bits), a bitstream , on the carrier, by means of mapping bits to elements from a discrete alphabet to be transmitted. This alphabet can consist of a set of real or complex numbers , or sequences, like oscillations of different frequencies, so-called frequency-shift keying (FSK) modulation. A more complicated digital modulation method that employs multiple carriers, orthogonal frequency-division multiplexing (OFDM),

140-513: A challenging topic in telecommunication systems and computer engineering. Such systems have many civil and military applications. Moreover, blind recognition of modulation type is an important problem in commercial systems, especially in software-defined radio . Usually in such systems, there are some extra information for system configuration, but considering blind approaches in intelligent receivers, we can reduce information overload and increase transmission performance. Obviously, with no knowledge of

175-534: A cosine waveform) and a quadrature phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite number of amplitudes and then summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK. In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits . Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises

210-406: A different television channel , are transported through a single cable to customers. Since each carrier occupies a different frequency, the channels do not interfere with each other. At the destination end, the carrier signal is demodulated to extract the information bearing modulation signal. A modulator is a device or circuit that performs modulation. A demodulator (sometimes detector )

245-418: A large antenna is not practical. In radio communication , the modulated carrier is transmitted through space as a radio wave to a radio receiver . Another purpose of modulation is to transmit multiple channels of information through a single communication medium , using frequency-division multiplexing (FDM). For example, in cable television (which uses FDM), many carrier signals, each modulated with

280-627: A message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. According to one definition of digital signal , the modulated signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion . Most textbooks would consider digital modulation schemes as a form of digital transmission , synonymous to data transmission; very few would consider it as analog transmission . The most fundamental digital modulation techniques are based on keying : In QAM, an in-phase signal (or I, with one example being

315-429: A narrowband analog signal over an analog baseband channel as a two-level signal by modulating a pulse wave . Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e., as a quantized discrete-time signal ) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example, some line code . These are not modulation schemes in

350-597: A proper class. Another recent approach is based on feature extraction. Digital baseband modulation changes the characteristics of a baseband signal, i.e., one without a carrier at a higher frequency. This can be used as equivalent signal to be later frequency-converted to a carrier frequency, or for direct communication in baseband. The latter methods both involve relatively simple line codes , as often used in local buses, and complicated baseband signalling schemes such as used in DSL . Pulse modulation schemes aim at transferring

385-427: A sequence of binary digits, a bitstream from a computer. This carrier wave usually has a much higher frequency than the message signal does. This is because it is impractical to transmit signals with low frequencies. Generally, to receive a radio wave one needs a radio antenna with length that is one-fourth of wavelength. For low frequency radio waves, wavelength is on the scale of kilometers and building such

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420-425: Is synchronous modulation . The most common digital modulation techniques are: MSK and GMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one-symbol-time duration (total response signaling). OFDM

455-529: Is "impressed" on the carrier. Examples are amplitude modulation (AM) in which the amplitude (strength) of the carrier wave is varied by the modulation signal, and frequency modulation (FM) in which the frequency of the carrier wave is varied by the modulation signal. These were the earliest types of modulation , and are used to transmit an audio signal representing sound in AM and FM radio broadcasting . More recent systems use digital modulation , which impresses

490-417: Is a circuit that performs demodulation , the inverse of modulation. A modem (from mod ulator– dem odulator), used in bidirectional communication, can perform both operations. The lower frequency band occupied by the modulation signal is called the baseband , while the higher frequency band occupied by the modulated carrier is called the passband . In analog modulation , an analog modulation signal

525-444: Is a complex-valued representation of the real-valued modulated physical signal (the so-called passband signal or RF signal ). These are the general steps used by the modulator to transmit data: At the receiver side, the demodulator typically performs: As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because

560-425: Is based on the idea of frequency-division multiplexing (FDM), but the multiplexed streams are all parts of a single original stream. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The modulated sub-carriers are summed to form an OFDM signal. This dividing and recombining help with handling channel impairments. OFDM

595-657: Is capitalized for another reason, such as in title case. It was defined by the CCITT (now the ITU ) in November 1926. The earlier standard had been the number of words per minute, which was a less robust measure since word length can vary. The symbol duration time, also known as the unit interval , can be directly measured as the time between transitions by looking at an eye diagram of the signal on an oscilloscope . The symbol duration time T s can be calculated as: where f s

630-407: Is considered as a modulation technique rather than a multiplex technique since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the orthogonal frequency-division multiple access (OFDMA) and multi-carrier code-division multiple access (MC-CDMA) schemes, allowing several users to share

665-479: Is represented by one symbol, and binary digit "1" by another symbol. In more advanced modems and data transmission techniques, a symbol may have more than two states, so it may represent more than one bit . A bit (binary digit) always represents one of two states. If N bits are conveyed per symbol, and the gross bit rate is R , inclusive of channel coding overhead, the symbol rate f s can be calculated as By taking information per pulse N in bit/pulse to be

700-421: Is the process of varying one or more properties of a periodic waveform , called the carrier signal , with a separate signal called the modulation signal that typically contains information to be transmitted. For example, the modulation signal might be an audio signal representing sound from a microphone , a video signal representing moving images from a video camera , or a digital signal representing

735-462: Is the symbol rate. There is also a chance of miscommunication which leads to ambiguity. The baud is scaled using standard metric prefixes , so that for example The symbol rate is related to gross bit rate expressed in bit/s. The term baud has sometimes incorrectly been used to mean bit rate , since these rates are the same in old modems as well as in the simplest digital communication links using only one bit per symbol, such that binary digit "0"

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770-469: Is used in WiFi networks, digital radio stations and digital cable television transmission. In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include: In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion and

805-411: The symbol that is represented by the particular phase, frequency or amplitude. If the alphabet consists of M = 2 N {\displaystyle M=2^{N}} alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate ) is f S {\displaystyle f_{S}} symbols/second (or baud ),

840-520: The QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers. Automatic digital modulation recognition in intelligent communication systems is one of the most important issues in software-defined radio and cognitive radio . According to incremental expanse of intelligent receivers, automatic modulation recognition becomes

875-441: The base-2- logarithm of the number of distinct messages M that could be sent, Hartley constructed a measure of the gross bit rate R as Here, the ⌈ x ⌉ {\displaystyle \left\lceil x\right\rceil } denotes the ceiling function of x {\displaystyle x} , where x {\displaystyle x} is taken to be any real number greater than zero, then

910-403: The bit rate is N = log 2 (64) = 6 times the baud rate. In a line code, these may be M different voltage levels. The ratio is not necessarily an integer; in 4B3T coding, the bit rate is ⁠ 4 / 3 ⁠ of the baud rate. (A typical basic rate interface with a 160 kbit/s raw data rate operates at 120 kBd.) Codes with many symbols, and thus a bit rate higher than

945-407: The ceiling function rounds up to the nearest natural number (e.g. ⌈ 2.11 ⌉ = 3 {\displaystyle \left\lceil 2.11\right\rceil =3} ). In that case, M = 2 different symbols are used. In a modem, these may be time-limited sinewave tones with unique combinations of amplitude, phase and/or frequency. For example, in a 64QAM modem, M = 64 , and so

980-442: The corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet ). A simple example: A telephone line is designed for transferring audible sounds, for example, tones, and not digital bits (zeros and ones). Computers may, however, communicate over a telephone line by means of modems, which are representing

1015-407: The data rate is N f S {\displaystyle Nf_{S}} bit/second. For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate. In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on

1050-439: The digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or 1000 baud . Since each tone (i.e., symbol) represents

1085-541: The same physical medium by giving different sub-carriers or spreading codes to different users. Of the two kinds of RF power amplifier , switching amplifiers ( Class D amplifiers ) cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA , but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often

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1120-411: The symbol rate, are most useful on channels such as telephone lines with a limited bandwidth but a high signal-to-noise ratio within that bandwidth. In other applications, the bit rate is less than the symbol rate. Eight-to-fourteen modulation as used on audio CDs has bit rate ⁠ 8 / 17 ⁠ of the baud rate. Modulation In electronics and telecommunications , modulation

1155-457: The system (typically 0 and 1), then baud and bits per second are equivalent. The baud unit is named after Émile Baudot , the inventor of the Baudot code for telegraphy , and is represented according to the rules for SI units . That is, the first letter of its symbol is uppercase (Bd), but when the unit is spelled out, it should be written in lowercase (baud) except when it begins a sentence or

1190-496: The transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information, etc., blind identification of the modulation is made fairly difficult. This becomes even more challenging in real-world scenarios with multipath fading, frequency-selective and time-varying channels. There are two main approaches to automatic modulation recognition. The first approach uses likelihood-based methods to assign an input signal to

1225-549: The transmitter-receiver pair has prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations. Asynchronous methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier signal . In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred. The opposite

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