Digital modulation in many forms has
proven to be an effective means of delivering voice, data, and video through
the limited channel bandwidths of modern wireless communications networks.
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Simply
put, digital modulation has made the modern wireless communications revolution
possible. As users of wireless communications devices seek greater capacity to
send and receive voice-, data- and video-laden signals, network operators have
come to rely on communications standards based on high-capacity digital
modulation formats.
Modulation of any form relies on controlled
changes in one or more of a periodic waveform's three basic parameters:
amplitude, frequency, and phase. By changing or modulating a carrier signal in
one or more of these parameters, information can be added or modulated to the
carrier at the transmit end of a system, then recovered or demodulated at the
receive end of the system. Traditional analog forms of modulation relied on simple
changes to amplitude, frequency, or phase. But bandwidth is limited, and modern
communications systems are required to transport increasing amounts of
information over channels with relatively narrow bandwidths.
Just like their analog counterparts, digital modulation formats manipulate
the three basic carrier signal parameters, but do so in discrete states
representing digital bits. Basic forms of digital modulation include amplitude
shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), and
modulation in which several of the parameters are combinedsuch as quadrature
amplitude modulation (QAM), in which at least two discrete phase states and
amplitude states are used to transfer information.
Digitally modulated signals are typically generated
through use of in-phase (I) and quadrature (Q) rectangular signal coordinates,
which can be added to a carrier via a pair of frequency mixers, one of which is
90 deg. offset from the other. Binary phase shift keying (BPSK) is a simple
form of digital modulation, keeping amplitude constant but shifting the phase
between 0 and 180 deg. In quadrature phase shift keying (QPSK), four phase
states are used.
Digital
modulation formats are characterized in terms of their symbol rates, or the
number of bits that are transmitted per modulation state. In BPSK, for example,
one bit per symbol is transmitted. Because the modulation is either at 0 or 180
deg., there is one I signal state and one Q signal state. In QpSK, in which two
I and two Q values are transmitted, the modulation format sends two bits per
symbol. As a result, QPSK is potentially twice as bandwidth efficient as BPSK.
As modulation formats increase in complexity, they can send an increasing
amount of data over a given portion of the RF/microwave spectrum. Additional
variants of pSK include eight-state PSK (8PSK), 16-state PSK (16PSK),
differential PSK (DPSK), differential quadrature PSK (DQPSK), offset quadrature
pSK (OQPSK), and p-4 quadrature PSK (p-4-QPSK).
For example, in 16QPSK, there are four I
signal states and four Q signal states, so that a total of 4 x 4 or 16 signal
states can be transmitted. In this modulation format, four bits per symbol are
transmitted; that is, the symbol rate is one quarter the bit rate, making
16QPSK more spectrally efficient than a less complex form of this modulation
format (such as simple QPSK, with two bits per symbol).
Varying signals with time, or multiplexing,
is also often used with digital modulation formats to achieve efficient use of
available frequency spectrum. Typical multiplexing formats include
frequency-division multiple access (FDMA), time-division multiple access
(TDMA), and code-division multiple access (CDMA).
The symbol rate, also known as the baud rate, determines the amount of
bandwidth required by a carrier of a given modulation format. The symbol rate
is equal to the bit rate divided by the number of bits transmitted per symbol.
For example, a system with bit resolution of 8 b operating at a sampling rate
of 1 MHz produces a bit stream of 8 b x 1 mHz = 8 MHz.
With more complex forms of digital modulation
come greater linearity and noise requirements in a system, as an increased
number of bits and symbols are packed closer together. Sometimes increased
transmission power might be needed to keep the symbols of a complex modulation
format properly spaced for demodulation.
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are modulators both in digital and analog modulators, amplifier and combiner.
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Source：https://www.mwrf.com/content/delivering-data-digital-modulation
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