Design and implementation of PSK signal modulation based on DSP

Digital modulation signal is also called keying signal. Its modulation process is to modulate the amplitude, frequency and phase of the carrier signal by the baseband signal using the keying method. There are three basic methods of this modulation: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). At the same time, it can be divided into binary and multi-base modulation (M-base) according to the different baseband signal bases. Compared with binary, multi-base digital modulation has higher spectrum utilization. Among them, QPSK (ie 4PSK) is a widely used modulation method in MPSK (multi-base phase shift keying). To this end, this paper studies the implementation method of BPSK and DPSK modulation circuits based on DSP, and gives the results of DSP modulation experiments.

1 Modulation implementation of BPSK signal

Binary Phase Shift Keying (BPSK) is the basis of Multi-Level Phase Shift Keying (MPSK). 2PSK is a digital modulation method in which the phase of the keyed carrier changes according to the rules of the baseband pulse sequence.

The 2PSK signal form can generally be expressed as:

The phase keying method that uses the 0 phase and π phase of the modulated signal carrier to represent the 1 and 0 of the binary digital baseband signal respectively is usually called the absolute phase shift method. The modulation block diagram of the 2PSK signal is shown in Figure 1. Figure 2 is the software implementation flow chart of the 2PSK signal.

Figure 1 2PSK signal modulation principle diagram

Figure 2 2PSK signal generation flow chart

When the recovered coherent carrier produces a 180° phase reversal, the demodulated digital baseband signal will be exactly opposite to the transmitted digital baseband signal, and the demodulator output digital baseband signals are all wrong at this time. This phenomenon is called "inverted π" or "reverse working" phenomenon. For this reason, the 2PSK method is generally not used, but a so-called relative (differential) phase shift (2DPSK) method is used.

2DPSK is a method of representing digital information by using the relative carrier phase values ​​of adjacent symbols. The change rule of the 2DPSK signal phase is: when the information code (absolute code) is "1", the initial phase of the 2DPSK signal in this symbol changes by 1800 relative to the final phase of the 2DPSK signal in the previous symbol; when the information code is "0", the initial phase of the 2DPSK signal in this symbol does not change relative to the final phase of the 2DPSK signal in the previous symbol. The implementation method of 2DPSK is to first differentially encode the binary digital baseband signal, transform the binary information represented by the absolute code into information represented by the relative code, and then perform absolute phase modulation to generate a binary differential phase keying signal. The waveform diagram of the 2DPSK signal modulation process is shown in Figure 3. The conversion mode between the absolute code and the relative code is shown in Figure 4:

Figure 3 2DPSK signal modulation process waveform

Figure 4: Absolute code and relative code conversion structure

In the demodulation process of 2DPSK, the phase ambiguity generated by the coherent carrier will cause the demodulated relative code to be inverted. However, after the code inverse converter, since the output absolute code will not be inverted, the "inverted π" phenomenon will not occur. Figure 5 shows the signal waveform comparison of 2PSK and 4DPSK.

Figure 5 Waveforms of 2PSK and 2DPSK signals

2 QPSK signal modulation implementation

Four-phase phase modulation is a type of multi-phase digital phase modulation. Among the multi-phase modulations, the most widely used are four-phase and eight-phase. This article mainly introduces the modulation implementation of QPSK signals. The general expression of multi-phase phase modulation is:

Among the multi-level phase modulation, the quaternary digital phase modulation (QPSK) signal is the most commonly used modulation method. When designing, one of the four possible phases can be selected, for example: 0, π/2, π, 3π/2. The generation of QPSK signals can be divided into phase modulation method and phase keying method.

The schematic diagram of generating QPSK signal by phase modulation method is shown in FIG6 .

Figure 6 QPSK signal modulation principle diagram

In FIG6, the serial/parallel converter can divide the input binary sequence into two parallel bipolar sequences in sequence. The two bipolar pulses can be used to perform two-phase modulation on the in-phase carrier and the orthogonal carrier respectively through two balanced modulators, and then the outputs are superimposed to obtain a four-phase phase shift signal. The QPSK modulation signal waveform is shown in FIG7. FIG8 is a software implementation flow chart of the QPSK signal.

Figure 7 QPSK modulation signal waveform

Figure 8 QPSK signal generation flow chart

The generation of QDPSK signal has been pointed out in the two-phase modulation. In order to obtain 2DPSK signal, the absolute code should be converted into relative code first, and then the relative code should be used to perform absolute phase shift on the carrier. Similarly, the generation of QDPSK signal can also adopt this method. That is, the input double bit is first converted into code type, and then the double bit output by the code converter is used to perform four-phase absolute phase shift. In this way, the output signal obtained is a four-phase relative phase shift signal. Therefore, the generation flow chart of QDPSK signal only has one more code converter than QPSK, and the implementation method of the code converter is shown in Figure 9.

Figure 9 QDPSK signal code converter

In fact, the function of the code converter is to convert the input double ratio *ab into a double ratio *cd, and it is required that the relationship between the QDPSK signal generated by cd and ab can meet the requirements listed in Table 1.

Table 1 QDPSK signal phase encoding logic relationship

It can be seen from Table 1 that when the input double-bit data is 00, the carrier phase of the phase-modulated signal does not change relative to the carrier phase of the previous double-bit element; when the input double-bit data is 01, the carrier phase of the phase-modulated signal changes by 90° relative to the carrier phase of the previous double-bit element, and so on.

Considering the "inversion" phenomenon in absolute phase shift, relative phase shift (QDPSK) is usually used to replace QPSK modulation. For multi-phase modulated signals, there are many ways to map k information bits to M=2k possible phases, among which Gray coding is preferred. In this coding method, adjacent phases differ by only one binary bit. If noise causes the transmission phase to select adjacent phases incorrectly, only a single bit error will be generated in the bit sequence encoded with Gray.

3 Conclusion

High-speed digital burst communications usually require fast and efficient estimation of the bit timing and carrier initial phase information of the received signal. The modulation method of BPSK and QPSK signals proposed in this article has very wide application value in both military and civilian fields, and can be applied to various digital communication fields.