MSK is called minimum frequency shift keying and is an improved type of frequency shift keying (FSK). Here "minimum" refers to the ability to obtain quadrature signals with a minimum modulation index (ie, 0.5), which can deliver a higher bit rate than PSK.
The expression of the binary MSK signal can be written as:
When ka=+1, the frequency of the signal is: 
When ka=-1, the frequency of the signal is: 
The resulting frequency difference is: 
The MSK signal waveform is shown in Figure 2.1:
In order to maintain the continuity of the phase, the following formula should be established within t=kTs:
If 
=0, then 
=0 or ±π, this formula shows that the phase constant in this bit is not only related to the input of this bit interval, but also related to the input and phase constant in the previous bit interval.
For ease of understanding as shown in Figure 2.2:
According to the above description, an MSK modulator can be constructed, and its block diagram is shown in Figure 2.3.
The input data NRZ, and then through the CPLD circuit to achieve differential encoding and serial / parallel conversion, Ik, Qk two-way data. The waveform selection address generator is an address selected based on the received data (Ik or Qk) output waveform. EEPROM (a variety of waveform data is stored in it) outputs the corresponding data according to the address output by the CPLD, then through the D / A converter to get the baseband waveform we need, and finally through the multiplier modulation, the op amp summation has got what we need MSK modulation signal.
The MSK baseband waveform consists of only two waveforms, as shown in Figure 2.4:
In MSK modulation, the principle of shaping signal extraction is that since only two waveforms are selected for the shaping signal, the shaped signal taken out of the current data is only related to its previous data. If the current data is the same as the previous data, the output shaping signal is reversed (if the previous data corresponds to waveform 1, then the current data corresponds to waveform 2); if the current data is opposite to the previous data, the output shaping signal is the same ( If the previous data corresponds to waveform 1, the current data still corresponds to waveform 1).
FSK modulation principleIn binary frequency shift keying, the frequency of the carrier signal with a constant amplitude changes as the input code stream changes (called treble and bass, representing binary 1 and 0).
Non-continuous phase FSK modulation
The simplest way to generate an FSK signal is to switch between two independent oscillators depending on whether the input data bit is 0 or 1. Waveforms generated using this method are discontinuous at the time of the switch, so this FSK signal is called a discontinuous FSK signal.
Because the phase discontinuity will make the spectrum spread, this kind of FSK modulation method is used more in the traditional communication equipment. With the continuous development of digital processing technology, continuous phase FSK modulation technology is increasingly used.
Continuous Phase FSK Modulation Signal
At present, the method for generating the FSK signal is to generate the FSK baseband signal first, and use the baseband signal to perform frequency modulation on a single carrier oscillator.
The power spectral density function of the phase-continuous FSK signal eventually fades in the negative fourth power of the frequency offset. If the phase is not continuous, the power spectral density function declines in the negative quadratic power of the frequency offset.
FSK signal spectrum
In the communication theory integrated experimental system, the modulation scheme of FSK is as follows: FSK signal: S(t)=cos(ω0t+2πfi·t)
In the baseband signal of the communication channel FSK mode, the frequency is fH, and the space number is fL. In the FSK mode, Hamming error correction coding technology is not used. The data sources provided by the modulators are:
FSK quadrature modulator structure
1, external data input: can be from the synchronous data interface, asynchronous data interface and m sequence;
2, all 1 code: The test frequency can be sent when the frequency (high);
3, all 0 yards: The test frequency can be sent when the frequency (low);
4,0/1 code: 0101. . Alternating pattern, used as a general test;
5, special code sequence: a code sequence of 7 cycles for observation by a conventional oscilloscope;
6, m sequence: used to test the channel performance;
FSK modulator with processing structure
Differences and relations between MSK modulation and FSK modulationDifference: In the FSK mode, the phase is usually not continuous. MSK is a type of modulation that keeps its phase constant all the time.
Contact: MSK is making some improvements to the FSK signal based on FSK.
In FSK mode, the frequency of adjacent symbols does not change or jump to a fixed value. Between two adjacent frequency hopping symbols, the phase is usually discontinuous. MSK is a kind of modulation that makes some improvement to the FSK signal so that its phase remains constant at all times.
MSK is also called Fast Frequency Shift Keying (FFSK). Here "minimum" means that the quadrature signal can be obtained with a minimum modulation index (ie, 0.5); and "rapid" means that for a given frequency band, it can transmit a higher bit rate than the PSK.
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