What Is Amplitude-Shift Keying?

The modulation method that uses the baseband digital signal to control the amplitude change of the carrier is called amplitude shift keying (ASK), also known as digital amplitude modulation. Each characteristic state of a digitally modulated signal is modulated by a specific value of the amplitude of the sinusoidal oscillation. Amplitude shift keying refers to the digital signal 1 and 0 by changing the amplitude of the carrier signal. The carrier signal A1 represents the digital signal 1, the carrier signal A2 represents the digital signal 0 and the carrier signal. and are constant.

Chinese name: Amplitude Shift Keying

English name: amplitude-shift keying; ASK

definition: Each characteristic state of a digitally modulated signal is modulated by a specific value of the amplitude of the sinusoidal oscillation.

Applied discipline: Communication technology (first-level discipline), communication principles and basic technology (second-level discipline)

The above content was published by the National Science and Technology Terminology Examination Committee.

Chinese name: Amplitude Shift Keying

Foreign name: Amplitude Shift Keying
Applied discipline: Communication

1. Amplitude Shift Keying 1. Introduction

ASK refers to the amplitude keying method. This modulation method is to adjust the amplitude of the sine wave according to the different signals.
Amplitude keying can be implemented by multipliers and switching circuits. The carrier is turned on or off under the control of digital signal 1 or 0. When the signal is 1, the carrier is turned on. At this time, the carrier appears on the transmission channel. When the signal is 0, the carrier is turned off. No carrier transmission. Then at the receiving end we can restore the 1 and 0 of the digital signal according to the presence or absence of the carrier. For a binary amplitude keyed signal, the frequency bandwidth is twice the width of the binary baseband signal.
The amplitude of the carrier of the amplitude shift keying method (ASK) changes with the modulation signal. The simplest form is that the carrier is switched on and off under the control of the binary modulation signal. . The multilevel MASK modulation method is a relatively efficient transmission method, but because of its poor anti-noise capability, especially its ability to resist fading, it is generally only suitable for use in constant parameter channels.

2. ASK Amplitude shift keying 2. ASK signal and power spectrum

Digital modulation is implemented using a multiplier. A binary digital amplitude modulation (2ASK) signal can be expressed as a unipolar pulse sequence multiplied by a sinusoidal carrier, that is:

Formula 1

among them:

A signal is formed for the baseband, and g ( t ) is the impulse response that forms the filter.

Is a data sequence, while a random variable

The value is shown in Figure 1.

The transmission schematic diagram and output waveform of ASK are shown in Figure 2.

Assuming that the Fourier transform of the rectangular wave g ( t ) is G ( f ), the power spectral density of e ( t ) in rectangular amplitude modulation is:

From this, it can be seen that the power spectral density of the 2ASK signal is composed of a continuous spectrum and a discrete spectrum. The continuous spectrum is determined by the modulated double-sided band of the continuous spectrum of the baseband spectrum G ( f ), and the discrete spectrum is determined by the discrete spectrum in G ( f ). The bandwidth of the 2ASK signal is twice the bandwidth of the baseband signal.

3. Amplitude shift keying 3. Double-sideband signal and power spectral density with suppressed carrier frequency

Multiplying the carrier with a bipolar non-return-to-zero code can achieve the purpose of suppressing the carrier frequency. The amplitude-modulated signal still satisfies Equation 1, and the values of a _n are 1 and 1, and they are still random variables. The voltage waveform of the bipolar non-return-to-zero code with amplitude is equivalent to multiplying the voltage waveform of the unipolar non-return-to-zero code with amplitude A by removing the DC component. Therefore, under the condition that the probability of "0" and "1" of the 2ASK signal is independent and the preceding and following symbols are independent, the power spectral density is:

At the receiving end, a coherent carrier is used to recover the baseband signal through a multiplicative demodulator.

4. Amplitude shift keying 4. Single sideband and residual sideband modulation

The ASK signal has two sidebands, both of which contain the same information.

In order to improve channel utilization, only one sideband is needed to transmit information. Because the baseband signal has rich low-frequency components, a sharp-cut filter must be used in order to filter out one of the sidebands, but this requires a higher filter. Usually, some processing is performed on the baseband signal so that the DC is zero and the low frequency component is as small as possible, so that there is a clear boundary between the upper and lower sidebands of the modulated ASK signal. As long as the signal is multiplied and modulated, its spectrum has neither a carrier frequency nor a clear boundary between the upper and lower sidebands. In this way, a common filter can be used to cut off a component, so that single-sideband transmission is achieved, and the spectrum utilization rate is twice that of double-sideband transmission.

Residual band modulation is a modulation method between double-sideband and single-sideband. It allows the modulated double-sideband signal to pass through a residual sideband filter, so that only the majority of one sideband and the other A small portion of the band passes, forming a so-called residual sideband modulated signal. Its spectrum utilization is slightly smaller than that of single sideband modulation.

5. Amplitude shift keying 5. Quadrature amplitude modulation

Quadrature amplitude modulation, also known as quadrature double-sideband modulation (QAM), is an amplitude modulation method that improves the utilization of frequency bands.

1 Amplitude shift keying (1) basic principles

Quadrature AM is composed of two double-band AMs that suppress the carrier frequency orthogonally in the frequency spectrum. The schematic diagram of modulation is shown in Figure 3:

Among them, the carrier signal used by channel A is:

, And the carrier signal used by B is:-

. Because the correlation coefficient of these two carrier signals is 0, they are orthogonal signals.

Assuming that the baseband signal of the A channel is S _A ( t ) and the baseband signal of the B channel is S _B ( t ), the output signal of the entire modulation system is:

In this modulation method, both A and B signals are double-sideband modulation, but the two signals are in the same frequency band. Although the double-sideband doubles the bandwidth, because the two signals can be transmitted, the power spectrum utilization rate is Same as single-sideband transmission, but there are no special requirements for the transmit filter.

The QAM signal can be received using coherent modulation. For the sake of simplicity, it is assumed that the channel is an ideal channel without distortion, unlimited bandwidth, and noise. The signal received by the receiver is e ( t ), which can be demodulated by the method shown in FIG.

It can be known from the formula in FIG. 4 that the first term is the original baseband signal, and the rest are high-frequency signals, so the baseband signal can be recovered through a low-pass filter to correctly distinguish the two signals. Its demodulation principle block diagram is shown as in Fig. 5.

2 Amplitude shift keying (2) representation method of orthogonal codes

Quadrature modulation signals can be represented by vector and constellation representation.

Vector representation

Orthogonal modulation can be described by a vector method. The quadrature modulated signal e ( t ) can be written as a composite wave:

In the formula:

In this way, the quadrature amplitude-modulated signal can be represented by a vector. For example, when A sends a "1" code, the A modulator output is

When B sends 1 code, the modulator output of B is

, Its synthesized signal is:

; Similarly, you can get four types: 0 for A and 0 for B, 1 for B and 0 for B, and 1 for A and B for 0. The situation is represented by a vector as shown in FIG. 6.

The output signal has four different phases, each representing a pair of binary codes (AB). There are 4 combinations of binary codes, namely 00,01,11,10, which are represented by 0,1,2,3 according to the phase rotation order (see the numbers in parentheses in Figure 3).

If a quadrature amplitude modulation system is used to transmit the data code stream, the odd-numbered bits of the data stream can be sent to the A channel and the even-numbered bits to the B channel through the serial-parallel conversion circuit at the transmitting end. In this way, the receiver end restores the serial code output through the parallel-serial conversion circuit after the sampling decision.

Constellation notation

The so-called constellation representation is a method that uses the endpoints of a vector to represent a signal. therefore. In vector representation, if only the endpoints of the vector are drawn, this graph is a constellation representation of orthogonal modulation. For example, the constellation representation in FIG. 5 is shown in FIG. 7. Because there are four points (constellations) in the picture, this quadrature amplitude modulation signal is also called 4QAM.

If two-way four-level codes are sent to the modulators of A and B, the spectrum utilization rate can be further improved. Due to the use of a four-level baseband signal, each channel has 4 points on the constellation to form a constellation diagram of 16 points. This orthogonal modulation is called 16QAM. Similarly, two-way eight-level codes can also be sent to the A and B modulators, and a constellation of 64QAM points can be obtained, and so on. Figure 7 shows the constellation diagrams for 32QAM and 64QAM.

The distance between the constellations (points) in the constellation chart indicates the ability to resist bit errors, and the number of points in the constellation indicates the frequency band utilization. The larger the distance, the stronger the anti-error capability; the larger the number of points, the higher the frequency band utilization. However, the two are often contradictory to each other. The greater the distance between the constellations (points), the fewer the points in the constellation diagram. Therefore, although its anti-error capability is high, the frequency band utilization is not high. Conversely, if the number of stars is more, it indicates that the frequency band utilization is higher, but the distance between the constellations is getting smaller and smaller, so the anti-interference ability and anti-error characteristics are worse.

6. Amplitude shift keying 6. Power spectral density and spectrum utilization of quadrature amplitude-modulated signals

The quadrature AM signal is a combination of two double-sideband signals. Assuming that the two random data codes are independent of each other, the power spectral density of the quadrature AM signal is the sum of the A and B channels, and they are in the same frequency band .

Table 1 shows the spectrum utilization of QAM.

7. Amplitude shift keying 7. Differential coding in quadrature amplitude modulation

The quadrature amplitude modulation requires a carrier with the same frequency and phase as the carrier of the transmitter when demodulating at the receiving end. If the carrier signal used at the receiving end is out of phase with the carrier signal at the transmitting end (phase blur), a demodulation error will occur because In the quadrature amplitude modulation system, the coherent carrier at the receiving end is extracted from the received signal, and its coherent carrier has 4 possible phases. Differential coding can be used to eliminate this error.