The simplest forms of digital modulation are of second order because they can transmit only two symbols ( usually denoted as " 0 " and " 1 " or as "-1 " and " 1 " ).
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Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to symbol rate and thus to bandwidth are not linear due to significant innovations in digital signal processing and digital modulation methods.
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In a digital modulation method provided by a modem, each symbol is typically a sine wave tone with certain frequency, amplitude and phase . Symbol rate, baud rate, is the number of transmitted tones per second.
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The messages are either represented by a sequence of pulses by means of a line code ( baseband transmission ), or by a limited set of continuously varying wave forms ( passband transmission ), using a digital modulation method.
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"' Orthogonal Frequency-Division Multiple Access "'( "'OFDMA "') is a multi-user version of the popular orthogonal frequency-division multiplexing ( OFDM ) digital modulation scheme.
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Some textbooks also consider passband data transmission using a digital modulation method such as ASK, PSK and QAM, i . e . a sinewave modulated by a digital bit-stream, as analog transmission and as an analog signal.
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As in many digital modulation schemes, the constellation diagram is useful for QAM . In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible ( e . g.
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A digital modulation method ( or passband transmission scheme ) using 2 " N " different symbols, for example 2 " N " amplitudes, phases or frequencies, can transfer " N " bit / symbol.
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The aim of "'digital modulation "'is to transfer a channel, for example over the public switched telephone network ( where a bandpass filter limits the frequency range to 300 3400 Hz ) or over a limited radio frequency band.
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Classic Bluetooth also uses frequency hopping but the details are different; as a result, while both FCC and ETSI classify Bluetooth technology as an FHSS scheme, Bluetooth Smart is classified as a system using digital modulation techniques or a direct-sequence spread spectrum.
