Amplitude-Shift Keying (ASK)

The Principle of Amplitude-Shift Keying (ASK)

Amplitude-Shift Keying (ASK) is digital modulation in which a symbol selects one of several discrete carrier amplitudes. In binary ASK, the input bits select two levels of a high-frequency . During each symbol interval, the carrier frequency and phase remain fixed; the selected amplitude represents the digital symbol.

Binary ASK: On-Off Keying (OOK)

On-Off Keying assigns a nonzero carrier amplitude to logical '1' and zero, or a near-zero level, to logical '0'. The carrier is present during one symbol interval and absent during the other.

Logical '1': The carrier wave is transmitted at a fixed, high amplitude. (Signal is ON).
Logical '0': The carrier wave is not transmitted, or transmitted at a low amplitude. (Signal is OFF).

OOK and Morse Code

Radio Morse encodes a dot or dash with a keyed tone and a space with its absence. OOK applies the same two signal states over defined symbol intervals, which makes binary detection straightforward.

Multi-Level ASK (M-ASK)

At a fixed symbol rate and pulse-shaping bandwidth, ASK can use multiple amplitude levels to carry more bits per symbol. This is known as M-ary ASK or M-ASK, where 'M' is the number of distinct amplitude levels.

If a system uses $M$ levels and assigns every level a binary label, each symbol represents $k = \log_2(M)$ bits when $M$ is a power of two. For example, 4-ASK uses four amplitudes to represent 2 bits per symbol ('00', '01', '10', '11').

Detection Constraint

M-ASK increases at the same symbol rate. The amplitude levels become closer, so reliable operation requires a higher . Noise can move a received sample across a decision threshold and cause a bit error.

For coherent binary OOK with equiprobable bits, two-sided AWGN power spectral density $N_0/2$ , and average bit energy $E_b$ , optimum threshold detection gives $P_b = Q(\sqrt{E_b/N_0}) = \frac{1}{2}\operatorname{erfc}(\sqrt{E_b/(2N_0)})$ . This is 3 dB worse than coherent BPSK at the same average $E_b$ . A noncoherent envelope detector has a different BER.

If $P_1$ denotes the carrier-on power and the bits are equiprobable, the average OOK signal power before channel losses is $P_{avg} = P_1/2$ .

Advantages and Disadvantages of ASK

Advantages

Simplicity: ASK transmitters and receivers are relatively simple and inexpensive to implement. The demodulator, especially for OOK, can be as simple as an envelope detector.

Disadvantages

Susceptibility to Interference:Since information is encoded in the amplitude, ASK is sensitive to noise, interference, and signal fading, all of which directly affect the signal's amplitude and can cause errors.
Inefficient Power Usage:In OOK, carrier power is transmitted only for logical '1'. With equiprobable bits, the average power is half the carrier-on power, so a target average power requires a higher on-state power.

Practical Applications

ASK is used where simple transmitter and receiver implementations are acceptable:

Fiber Optic Communications: Used for short-range digital data transmission over fiber optic cables with intensity modulation and direct detection. A laser or LED is switched on and off to represent bits.
Low-Cost RF Systems: ASK is used in simple, low-data-rate wireless applications such as garage door openers, remote control systems for lights and gates, and tire pressure monitoring systems (TPMS).
Industrial Control: Used in some short-range industrial environments for simple control signaling.