In many telecommunications systems, it is
necessary to represent an information-bearing signal with a waveform that can
pass accurately through a transmission medium. This assigning of a suitable
waveform is accomplished by modulation, which is the process by which some
characteristic of a carrier wave is varied in accordance with an information
signal, or modulating wave. The modulated signal is then transmitted over a
channel, after which the original information-bearing signal is recovered
through a process of demodulation.
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Modulation
is applied to information signals for a number of reasons, some of which are outlined below.
1. Many transmission channels are characterized by limited pass bands--that is, they will pass only certain ranges of frequencies without seriously attenuating them (reducing their amplitude). Modulation methods must therefore be applied to the information signals in order to “frequency translates” the signals into the range of frequencies that are permitted by the channel. Examples of channels that exhibit pass band characteristics include alternating-current-coupled coaxial cables, which pass signals only in the range of 60 kilohertz to several hundred megahertz, and fiber-optic cables, which pass light signals only within a given wavelength range without significant attenuation. In these instances frequency translation is used to “fit” the information signal to the communications channel.
2. In many instances a communications channel is shared by multiple users. In order to prevent mutual interference, each user’s information signal is modulated onto an assigned carrier of a specific frequency. When the frequency assignment and subsequent combining is done at a central point, the resulting combination is a Frequency-division multiplexed signal, as is discussed in Multiplexing. Frequently there is no central combining point, and the communications channel itself acts as a distributed combine. An example of the latter situation is the broadcast radio bands (from 540 kilohertz to 600 megahertz), which permit simultaneous transmission of multiple AM radio, FM radio, and television signals without mutual interference as long as each signal is assigned to a different frequency band.
3. Even when the communications channel can support direct transmission of the information-bearing signal, there are often practical reasons why this is undesirable. A simple example is the transmission of a three-kilohertz (i.e., voice band) signal via radio wave. In free space the wavelength of a three-kilohertz signal is 100 kilometers (60 miles). Since an effective radio antenna is typically as large as half the wavelength of the signal, a three-kilohertz radio wave might require an antenna up to 50 kilometers in length. In this case translation of the voice frequency to a higher frequency would allow the use of a much smaller antenna.
Analog modulationAs is noted in analog-to-digital conversion, voice signals, as well as audio and video signals, are inherently analog in form. In most modern systems these signals are digitized prior to transmission, but in some systems the analog signals are still transmitted directly without converting them to digital form. There are two commonly used methods of modulating analog signals. One technique, called amplitude modulation, varies the amplitude of a fixed-frequency carrier wave in proportion to the information signal. The other technique, called frequency modulation, varies the frequency of a fixed-amplitude carrier wave in proportion to the information signal.
1. Many transmission channels are characterized by limited pass bands--that is, they will pass only certain ranges of frequencies without seriously attenuating them (reducing their amplitude). Modulation methods must therefore be applied to the information signals in order to “frequency translates” the signals into the range of frequencies that are permitted by the channel. Examples of channels that exhibit pass band characteristics include alternating-current-coupled coaxial cables, which pass signals only in the range of 60 kilohertz to several hundred megahertz, and fiber-optic cables, which pass light signals only within a given wavelength range without significant attenuation. In these instances frequency translation is used to “fit” the information signal to the communications channel.
2. In many instances a communications channel is shared by multiple users. In order to prevent mutual interference, each user’s information signal is modulated onto an assigned carrier of a specific frequency. When the frequency assignment and subsequent combining is done at a central point, the resulting combination is a Frequency-division multiplexed signal, as is discussed in Multiplexing. Frequently there is no central combining point, and the communications channel itself acts as a distributed combine. An example of the latter situation is the broadcast radio bands (from 540 kilohertz to 600 megahertz), which permit simultaneous transmission of multiple AM radio, FM radio, and television signals without mutual interference as long as each signal is assigned to a different frequency band.
3. Even when the communications channel can support direct transmission of the information-bearing signal, there are often practical reasons why this is undesirable. A simple example is the transmission of a three-kilohertz (i.e., voice band) signal via radio wave. In free space the wavelength of a three-kilohertz signal is 100 kilometers (60 miles). Since an effective radio antenna is typically as large as half the wavelength of the signal, a three-kilohertz radio wave might require an antenna up to 50 kilometers in length. In this case translation of the voice frequency to a higher frequency would allow the use of a much smaller antenna.
Analog modulationAs is noted in analog-to-digital conversion, voice signals, as well as audio and video signals, are inherently analog in form. In most modern systems these signals are digitized prior to transmission, but in some systems the analog signals are still transmitted directly without converting them to digital form. There are two commonly used methods of modulating analog signals. One technique, called amplitude modulation, varies the amplitude of a fixed-frequency carrier wave in proportion to the information signal. The other technique, called frequency modulation, varies the frequency of a fixed-amplitude carrier wave in proportion to the information signal.
In order to transmit computer data and other
digitized information over a communications channel, an analog carrier wave can
be modulated to reflect the binary nature of the digital baseband signal. The
parameters of the carrier that can be modified are the amplitude, the
frequency, and the phase.
Three
methods of digital signal modulation digital signal, representing the binary
digits 0 and 1 by a series of on and off amplitudes, is impressed onto an
analog carrier wave of constant amplitude and frequency. In amplitude-shift
keying (ASK), the modulated wave represents the series of bits by shifting
abruptly between high and low amplitude. In frequency-shift keying (FSK), the
bit stream is represented by shifts between two frequencies. In phase-shift
keying (PSK), amplitude and frequency remain constant; the bit stream is
represented by shifts in the phase of the modulated signal. Encyclopedia
Britannica, Inc.
Amplitude-shift keying
If amplitude is the only parameter of the
carrier wave to be altered by the information signal, the modulating method is
called amplitude-shift keying (ASK). ASK can be considered a digital version of
analog amplitude modulation. In its simplest form, a burst of radio frequency
is transmitted only when a binary 1 appears and is stopped when a 0 appears. In
another variation, the 0 and 1 are represented in the modulated signal by a
shift between two preselected amplitudes.
Frequency-shift keying
If frequency is the parameter chosen to be a
function of the information signal, the modulation method is called
frequency-shift keying (FSK). In the simplest form of FSK signaling, digital
data is transmitted using one of two frequencies, whereby one frequency is used
to transmit a 1 and the other frequency to transmit a 0. Such a scheme was used
in the Bell 103 voice band modem, introduced in 1962, to transmit information
at rates up to 300 bits per second over the public switched telephone network.
In the Bell 103 modem, frequencies of 1,080 +/- 100 hertz and 1,750 +/- 100
hertz were used to send binary data in both directions.
Phase-shift keying
When phase is the parameter altered by the
information signal, the method is called phase-shift keying (PSK). In the
simplest form of PSK a single radio frequency carrier is sent with a fixed
phase to represent a 0 and with a 180° phase shift—that is, with the opposite
polarity—to represent a 1. PSK was employed in the Bell 212 modem, which was
introduced about 1980 to transmit information at rates up to 1,200 bits per
second over the public switched telephone network.
In addition to the elementary forms of digital
modulation described above, there exist more advanced methods that result from
a superposition of multiple modulating signals. An example of the latter form
of modulation is quadrature amplitude modulation (QAM). QAM signals actually
transmit two amplitude-modulated signals in phase quadrature (i.e., 90° apart),
so that four or more bits are represented by each shift of the combined signal.
Communications systems that employ QAM include digital cellular systems in the
United States and Japan as well as most voice band modems transmitting above
2,400 bits per second.
A form of modulation that combines
convolutional codes with QAM is known as trellis-coded modulation (TCM), which
is described in Channel encoding. Trellis-coded modulation forms an essential
part of most of the modern voice band modems operating at data rates of 9,600
bits per second and above, including V.32 and V.34 modems.
Multiplexing
Because of the installation cost of a
communications channel, such as a microwave link or a coaxial cable link, it is
desirable to share the channel among multiple users. Provided that the
channel’s data capacity exceeds that required to support a single user, the
channel may be shared through the use of multiplexing methods. Multiplexing is
the sharing of a communications channel through local combining of signals at a
common point. Two types of multiplexing are commonly employed:
frequency-division multiplexing and time-division multiplexing.
Frequency-division multiplexing
In frequency-division multiplexing (FDM), the
available bandwidth of a communications channel is shared among multiple users
by frequency translating, or modulating, each of the individual users onto a
different carrier frequency. Assuming sufficient frequency separation of the
carrier frequencies that the modulated signals do not overlap, recovery of each
of the FDM signals is possible at the receiving end. In order to prevent
overlap of the signals and to simplify filtering, each of the modulated signals
is separated by a guard band, which consists of an unused portion of the
available frequency spectrum. Each user is assigned a given frequency band for
all time.
Analog
multiplexing, as employed in the North American telephone system In
frequency-division multiplexing (FDM), 12 separate voice signals, each of 4-kilohertz
bandwidth, are modulated onto carrier waves in the 60–108-kilohertz range.
These modulated signals are combined to form a single complex group signal.
Groups are further combined to form a hierarchy of increasing bandwidth and
voice-carrying capacity. Encyclopedia Britannica, Inc.
While each user’s information signal may be
either analog or digital, the combined FDM signal is inherently an analog
waveform. Therefore, an FDM signal must be transmitted over an analog channel.
Examples of FDM are found in some of the old long-distance telephone
transmission systems, including the American N- and L-carrier coaxial cable
systems and analog point-to-point microwave systems. In the L-carrier system a
hierarchical combining structure is employed in which 12 voice band signals are
frequency-division multiplexed to form a group signal in the frequency range of
60 to 108 kilohertz. Five group signals are multiplexed to form a super group
signal in the frequency range of 312 to 552 kilohertz, corresponding to 60 voice
band signals, and 10 super group signals are multiplexed to form a master group
signal. In the L1 carrier system, deployed in the 1940s, the master group was
transmitted directly over coaxial cable. For microwave systems, it was
frequency modulated onto a microwave carrier frequency for point-to-point
transmission. In the L4 system, developed in the 1960s, six master groups were
combined to form a jumbo group signal of 3,600 voice band signals.
Time-division multiplexing
Multiplexing also may be conducted through
the interleaving of time segments from different signals onto a single
transmission path—a process known as time-division multiplexing (TDM).
Time-division multiplexing of multiple signals is possible only when the
available data rate of the channel exceeds the data rate of the total number of
users. While TDM may be applied to either digital or analog signals, in
practice it is applied almost always to digital signals. The resulting
composite signal is thus also a digital signal.
Digital
multiplexing, as employed in the North American telephone system In
time-division multiplexing (TDM), 24 digitized voice signals, each at 64
kilobits per second, are assigned successive time slots in a
1.544-megabits-per-second signal. Combined signals are further combined to form
data streams of increasing bit-rate and voice-carrying capacity. Encyclopedia
Britannica, Inc.
In a representative TDM system, data from
multiple users are presented to a time-division multiplexer. A scanning switch
then selects data from each of the users in sequence to form a composite TDM
signal consisting of the interleaved data signals. Each user’s data path is
assumed to be time-aligned or synchronized to each of the other users’ data
paths and to the scanning mechanism. If only one bit were selected from each of
the data sources, then the scanning mechanism would select the value of the
arriving bit from each of the multiple data sources. In practice, however, the
scanning mechanism usually selects a slot of data consisting of multiple bits
of each user’s data; the scanner switch is then advanced to the next user to
select another slot, and so on. Each user is assigned a given time slot for all
time.
Most modern telecommunications systems employ
some form of TDM for transmission over long-distance routes. The multiplexed
signal may be sent directly over cable systems, or it may be modulated onto a
carrier signal for transmission via radio wave. Examples of such systems
include the North American T carriers as well as digital point-to-point
microwave systems. In T1 systems, introduced in 1962, 24 voice band signals (or
the digital equivalent) are time-division multiplexed together. The voice band
signal is a 64-kilobit-per-second data stream consisting of 8-bit symbols
transmitted at a rate of 8,000 symbols per second. The TDM process interleaves
24 8-bit time slots together, along with a single frame-synchronization bit, to
form a 193-bit frame. The 193-bit frames are formed at the rate of 8,000 frames
per second, resulting in an overall data rate of 1.544 megabits per second. For
transmission over more recent T-carrier systems, T1 signal is often further
multiplexed to form higher-data-rate signals—again using a hierarchical scheme.
Multiple Accesses
Multiplexing is defined as the sharing of a
communications channel through local combining at a common point. In many
cases, however, the communications channel must be efficiently shared among
many users that are geographically distributed and that sporadically attempt to
communicate at random points in time. Three schemes have been devised for
efficient sharing of a single channel under these conditions; they are called
frequency-division multiple access (FDMA), time-division multiple access
(TDMA), and code-division multiple access (CDMA). These techniques can be used
alone or together in telephone systems, and they are well illustrated by the
most advanced mobile cellular systems.
Frequency-division multiple access
In FDMA the goal is to divide the frequency
spectrum into slots and then to separate the signals of different users by
placing them in separate frequency slots. The difficulty is that the frequency
spectrum is limited and that there are typically many more potential
communicators than there are available frequency slots. In order to make
efficient use of the communications channel, a system must be devised for
managing the available slots. In the advanced mobile phone system (AMPS), the
cellular system employed in the United States, different callers use separate
frequency slots via FDMA. When one telephone call is completed, a
network-managing computer at the cellular base station reassigns the released
frequency slot to a new caller. A key goal of the AMPS system is to reuse
frequency slots whenever possible in order to accommodate as many callers as
possible. Locally within a cell, frequency slots can be reused when corresponding
calls are terminated. In addition, frequency slots can be used simultaneously
by multiple callers located in separate cells. The cells must be far enough
apart geographically that the radio signals from one cell are sufficiently
attenuated at the location of the other cell using the same frequency slot.
Time-division multiple access
In TDMA the goal is to divide time into slots
and separate the signals of different users by placing the signals in separate
time slots. The difficulty is that requests to use a single communications
channel occur randomly, so that on occasion the number of requests for time
slots is greater than the number of available slots. In this case information
must be buffered, or stored in memory, until time slots become available for
transmitting the data. The buffering introduces delay into the system. In the IS54
cellular system, three digital signals are interleaved using TDMA and then
transmitted in a 30-kilohertz frequency slot that would be occupied by one
analog signal in AMPS. Buffering digital signals and interleaving them in time
causes some extra delay, but the delay is so brief that it is not ordinarily
noticed during a call. The IS54 system uses aspects of both TDMA and FDMA.
Code-division multiple access
In CDMA, signals are sent at the same time in
the same frequency band. Signals are either selected or rejected at the
receiver by recognition of a user-specific signature waveform, which is
constructed from an assigned spreading code. The IS95 cellular system employs
the CDMA technique. In IS95 an analog speech signal that is to be sent to a
cell site is first quantized and then organized into one of a number of digital
frame structures. In one frame structure, a frame of 20 milliseconds’ duration
consists of 192 bits. Of these 192 bits, 172 represent the speech signal
itself, 12 form a cyclic redundancy check that can be used for error detection,
and 8 form an encoder “tail” that allows the decoder to work properly. These
bits are formed into an encoded data stream. After interleaving of the encoded
data stream, bits are organized into groups of six. Each group of six bits
indicates which of 64 possible waveforms to transmit. Each of the waveforms to
be transmitted has a particular pattern of alternating polarities and occupies
a certain portion of the radio-frequency spectrum. Before one of the waveforms
is transmitted, however, it is multiplied by a code sequence of polarities that
alternate at a rate of 1.2288 megahertz, spreading the bandwidth occupied by
the signal and causing it to occupy (after filtering at the transmitter) about
1.23 megahertz of the radio-frequency spectrum. At the cell site one user can
be selected from multiple users of the same 1.23-megahertz bandwidth by its
assigned code sequence.
CDMA is sometimes referred to as
spread-spectrum multiple access (SSMA), because the process of multiplying the
signal by the code sequence causes the power of the transmitted signal to be
spread over a larger bandwidth. Frequency management, a necessary feature of
FDMA, is eliminated in CDMA. When another user wishes to use the communications
channel, it is assigned a code and immediately transmits instead of being
stored until a frequency slot opens.
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Source: britannica
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