Amplitude-shift keying (ASK) is a form of
amplitude modulation that represents
digital data as variations in the
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of a
carrier wave
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
.
In an ASK system, a
symbol, representing one or more
bit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
s, is sent by transmitting a fixed-amplitude carrier wave at a fixed frequency for a specific time duration. For example, if each symbol represents a single bit, then the carrier signal could be transmitted at nominal amplitude when the input value is 1, but transmitted at reduced amplitude or not at all when the input value is 0.
Any digital modulation scheme uses a
finite
Finite is the opposite of infinite. It may refer to:
* Finite number (disambiguation)
* Finite set, a set whose cardinality (number of elements) is some natural number
* Finite verb, a verb form that has a subject, usually being inflected or marke ...
number of distinct signals to represent digital data. ASK uses a finite number of amplitudes, each assigned a unique pattern of
binary digit
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that ta ...
s. Usually, each amplitude encodes an equal number of bits. Each pattern of bits forms the
symbol that is represented by the particular amplitude. The
demodulator, which is designed specifically for the symbol-set used by the modulator, determines the amplitude of the received signal and maps it back to the symbol it represents, thus recovering the original data.
Frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
and
phase of the carrier are kept constant.
Like
AM, an ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in
PSTN
The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
, etc. 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 short pulse of light and binary 0 by the absence of light. Laser transmitters normally have a fixed "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. This type of modulation is called
on-off keying (OOK), and is used at radio frequencies to transmit Morse code (referred to as continuous wave operation),
More sophisticated encoding schemes have been developed which represent data in groups using additional amplitude levels. For instance, a four-level 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 signal-to-noise ratio for their recovery, as by their nature much of the signal is transmitted at reduced power.
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. The following notation is used:
* ''h
t''(f) is the carrier signal for the transmission
* ''h
c''(f) is the impulse response of the channel
* ''n''(t) is the noise introduced by the channel
* ''h
r''(f) is the filter at the receiver
* ''L'' is the number of levels that are used for transmission
* ''T''
s is the time between the generation of two symbols
Different symbols are represented with different voltages. If the maximum allowed value for the voltage is A, then all the possible values are in the range
��A, Aand they are given by:
:
the difference between one voltage and the other is:
:
Considering the picture, the symbols v
are generated randomly by the source S, then the impulse generator creates impulses with an area of v
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.
Out of the transmitter, the signal s(t) can be expressed in the form:
:
In the receiver, after the filtering through hr (t) the signal is:
:
where we use the notation:
:
where * indicates the convolution between two signals. After the A/D conversion the signal z
can be expressed in the form:
: