Table of contents
1. Signal modulation and demodulation
2. ASK (Amplitude Shift Keying, amplitude keying)
(1) Keying method and simulation method
(2) 2ASK non-coherent demodulation
(3) 2ASK coherent demodulation
1. Signal modulation and demodulation
Most channels are not suitable for transmitting baseband signals because such low-frequency signals will produce great attenuation and distortion during transmission. In digital baseband transmission, we know that digital signals can be transmitted directly in the channel through digital encoding, but this is only suitable for short-range transmission (such as in the same local area network), and in complex network environments (such as the presence of different transmission media and channel type network), it cannot be transmitted directly in this way. Therefore, the baseband signal needs to be modulated and transformed into a form suitable for channel transmission. Modulation allows the baseband signal to control a certain (or certain) parameter of the carrier so that the parameter changes according to the rules of the information. This is "modulation" process. "Demodulation" is the reverse process of "modulation", that is, the process of recovering the original modulated signal from the modulated signal.
The most popular explanation of modulation is to use a "high energy" signal to carry (can be simply understood as "carrying") another "low energy" signal for propagation and transmission. The signal that carries another signal is called a carrier wave or carrier signal, and the carried signal is the signal we really want to spread or transmit.
However, modulation does not just add the carried signal to the carrier signal and everything will be fine, because in order to truly play a role in the transmission system, what is needed is the carried signal. The carrier signal is just a carrier, and the truly useful carried signal must be allowed to play its role. . At this time, the amplitude, frequency or phase of the carrier signal must be determined by the relevant characteristics of the signal being carried.
The modulated signal is usually a low-frequency signal, which can be an analog signal or a digital signal, so there are corresponding analog signal modulation (or analog modulation) and digital signal modulation (or digital modulation). There is essentially no difference between digital modulation and analog modulation. They are both sinusoidal modulations, but the source signals are different. The source signal in digital modulation is a discrete pulse digital signal, while the source signal in analog modulation is a continuous sine wave signal.
Only digital signal modulation technology is introduced here.
A carrier wave is a signal wave used to carry useful low-frequency modulated signals, usually a high-frequency signal. Through modulation technology, the modulated signal and the carrier signal can be superimposed, so that certain parameter characteristics of the carrier signal (such as signal amplitude, signal frequency or signal phase, etc.) change according to the modulated signal. Which parameter of the carrier will change with the modulation signal depends on the specific modulation type. If the parameter being controlled is amplitude, this modulation is called amplitude modulation; if the parameter being controlled is frequency, then the modulation is called amplitude modulation. This modulation is called frequency modulation; if the controlled parameter is phase, then this modulation is called phase modulation. After arriving at the receiving end, useful data can be separated from the modulated signal through demodulation technology.
So why use a carrier wave for modulation? Because usually the frequency of the digital signal we want to send is relatively low. If it is transmitted according to its own frequency, the attenuation of the signal will be serious, which is not conducive to long-distance transmission.
The reason why the modulation process must be carried out first is to facilitate the long-distance and efficient transmission of low-frequency signals in the channel.
When performing carrier modulation on digital signals, we also use the three modulation techniques of amplitude modulation, frequency modulation and phase modulation in analog carrier modulation. But they already have other names: ASK (Amplitude Shift Keying, amplitude keying), FSK (Frequency Shift Keying, frequency keying) and PSK (Phase Shift Keying, phase keying). The reason why it is called "keying" here means that these modulation techniques are controlled by electric keys, which is a term borrowed from telegraph transmission. They respectively correspond to using the amplitude, frequency or phase of the carrier wave (sine wave) to carry digital baseband signals, which can be regarded as special cases of analog linear modulation and angle modulation.
2. ASK (Amplitude Shift Keying, amplitude keying)
ASK (Amplitude Shift Keying), a digital amplitude modulation technology, refers to the digital adjustment of the amplitude of the sinusoidal carrier that changes with the baseband digital signal, also known as "on-off keying" (OOK) Or "on/off keying".
When the digital baseband signal is a binary code, it is called binary amplitude keying (2ASK), which is only suitable for unipolar digital signals . It uses baseband rectangular pulses representing digital information 0 or 1 to key a continuous carrier wave, causing the carrier wave to be output intermittently. Because 0 in the unipolar waveform is no level output. That is, when the source digital baseband signal is 1, a carrier signal is sent, and when it is 0, a zero level is sent, that is, no carrier signal is sent.
2ASK is the earliest and simplest, but its anti-noise performance is poor (because it only controls the amplitude of the output signal based on the modulation signal and cannot filter interference signals), so it is not widely used in practice, but it is often used as The basis for studying other digital modulation methods
(1) Keying method and simulation method
Two methods: keying method and simulation method
2ASK signal demodulation also has two methods: non-coherent demodulation (envelope detection method) and coherent demodulation (synchronous detection method)
(2) 2ASK non-coherent demodulation
【1】Yi(t) represents the modulated signal, and the a waveform is obtained after passing through the "bandpass filter" (mainly used to filter low-frequency and high-frequency interference signals)
[2] The a signal is subjected to an "envelope detector" for envelope detection (envelope: external manifestation. Envelope detection is to detect the amplitude change curve of the signal, whether it is frequency characteristics or phase characteristics) to obtain the b waveform.
In fact, it is a polarity filter, because the modulation signal is unipolar and only allows positive polarity waveform output.
[3] The c waveform is obtained after the b signal passes through a "low-pass filter" (filters the high-frequency components in the b signal and only allows the output of waveforms that conform to the frequency characteristics of the signal source).
【4】The continuous c waveform is sampled through the "sampling decision device" to restore the original discrete modulation signal.
(3) 2ASK coherent demodulation
【1】Same as non-coherent demodulation
【2】The a waveform and the carrier signal coswct are multiplied in the multiplier to obtain the b waveform. It is not only a polar filter, but also contains the frequency component of the carrier wave, making the signal stronger.
【3】Same as non-coherent demodulation
【4】Same as non-coherent demodulation
3. FSK frequency shift keying
FSK only works with bipolar digital signals. It treats the amplitude and phase of the signal as constants and the frequency as a variable.
For example, digital signal code 1 is transmitted using a carrier of frequency f1, and signal code 0 is transmitted using f2. It is equivalent to the carrier switching between two different frequencies, also known as "frequency shift keying"
The same as the generation of 2ASK signal, it can be realized by analog frequency modulation method or keyed frequency modulation method.
The 2FSK signal generated using the analog frequency modulation method corresponds to two frequency carriers. The two carriers can maintain continuous phase at the moment of symbol conversion, so it becomes a phase continuous signal (CPFSK, Continuous-Phase Frequency Shift Keying, continuous phase frequency shift keying).
Because only one carrier is used, the phases between the modulated S(t) signal elements are continuous.
Keyed frequency modulation is implemented using digital keying. Digital matrix pulses are used to control the electronic switch, so that the electronic switch switches between two independent oscillators (generating carriers f1 and f2 of different frequencies) to obtain different frequencies. Modulate the signal.
2FSK demodulation methods: frequency discrimination method, zero-crossing detection method, non-coherent demodulation method (envelope detection method) and coherent IOU (differential detection method)
2FSK non-coherent demodulation
Sampling and decision are performed on the sampling decision maker (v1 and v2 are used for sampling of the two signals respectively) and decision. If v1>=v2, the signal code 1 is output; if v1<v2, the signal code 0 is output.
2FSK coherent demodulation
zero crossing detection method
4. PSK (Phase Keying)
PSK (Phase Keying) is a modulation method that uses a bipolar digital baseband signal to control the phase of the carrier to transmit information, that is, using different phases of the carrier to represent the signal code of the signal.
Digital phase modulation is to switch the carrier wave between two two bits, so it is also called "phase shift keying".
PSK is divided into "Absolute Phase Shift Keying, APSK " and "Relative Phase Keying" (or "Differential Phase Keying," Differential Phase Keying, DPSK ).
5. APSK modulation
APSK modulation: Modulate using the phase of the carrier as the reference phase. When the symbol is 1, the modulated signal is the same as the carrier; when it is 0, it is opposite to the carrier. That is, the phase difference between 1 and 0 codes after modulation is 180 degrees.
APSK demodulation: When the modulated signal phase is the same as the reference phase, the signal code 1 is sent, otherwise 0 is sent;
6. DPSK modulation
DPSK modulation: First use a differential encoder to differentially encode the digital baseband signal, that is, convert the absolute code (original baseband signal number) into a relative code, and then perform absolute phase modulation. The relative code is a differential code obtained through differential encoding. It uses whether the level of the current symbol of the modulated signal changes from the level of the previous symbol to represent the transmitted digital code. That is, 0 code is transmitted when there is no change, and 1 code is transmitted when there is a change.
In order to solve the phase inversion problem in "absolute phase modulation", differential encoding is performed before digital phase modulation, and then binary digital phase modulation is performed on the differential code.
DPSK demodulation: Use the relative phase value between the preceding and following symbols (that is, the difference between the initial phase of this symbol and the initial phase of the previous symbol) to transmit digital information. The rule is: when the phase offset value is 0, 0 code is output, and when the phase offset value is 180 degrees, 1 code is output
