Amplitude-shift keying
[1] In an ASK system, a symbol, representing one or more bits, is sent by transmitting a fixed-amplitude carrier wave at a fixed frequency for a specific time duration.
[citation needed] 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.
ASK uses a finite number of amplitudes, each assigned a unique pattern of binary digits.
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.
Like AM, an ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc.
Laser transmitters normally have a fixed "bias" current that causes the device to emit a low light level.
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.
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.
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.
After the A/D conversion the signal z[k] can be expressed in the form: In this relationship, the second term represents the symbol to be extracted.
If the filters are chosen so that g(t) will satisfy the Nyquist ISI criterion, then there will be no intersymbol interference and the value of the sum will be zero, so: the transmission will be affected only by noise.
The total probability of making an error can be expressed in the form: We now have to calculate the value of
We are in a situation like the one shown in the following picture: it does not matter which Gaussian function we are considering, the area we want to calculate will be the same.
Putting all these results together, the probability to make an error is: from this formula we can easily understand that the probability to make an error decreases if the maximum amplitude of the transmitted signal or the amplification of the system becomes greater; on the other hand, it increases if the number of levels or the power of noise becomes greater.
