1. HOME
  2. Resources
  3. Radio technology
  4. Digital modulation: Phase shift keying(PSK)

技術情報

  • RF design guide
  • Modem evaluation
  • Calculation tools
  • Radio technology
  • Technical literature
  • Modem evaluation
 

Introduction

下線 トップへ戻る

This Java applet allows you to experience the basics of digital phase modulation and demodulation visually.
It’s interesting to look at the spectrogram and try changing the various parameters. You can understand the background to the characteristics of the signal.

To run the applet, click the picture. *Java Runtime Environment version 5.0 or higher is required to run the applet. Download:http://www.java.com/en/download/index.jsp

An outline of digital phase modulation and demodulation

下線 トップへ戻る

Analog phase modulation is used less commonly than frequency modulation, but digital phase modulation is used in many radios.
Types of digital phase modulation include BPSK, QPSK and 8PSK. BPSK stands for Binary Phase Shift Keying. QPSK stands for Quadrature Phase Shift Keying.

Digital phase modulation has a low error rate, but its drawback is that it requires synchronous detection for demodulation, which involves high technology and significant hardware cost. However, with recent advances in digital technology, the circuits can easily be incorporated in a chip, making it relatively cheap.

With digital phase modulation, the phase of the carrier wave is modified by being assigned appropriately to one bit of the information bits. Types of digital phase modulation include BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 8PSK, and π/4 Shift QPSK.
BPSK has 1-symbol 1-bit composition, and the level of the 1 and 0 information signal is changed to a 1 or -1 dipolar NRZ signal and the phase of the carrier wave is assigned 0 or π.
QPSK has 1-symbol 2-bit composition, and 2 bits and 4 statuses are assigned to the carrier wave phases π/4, 3π/4, 5π/4, and 7π/4. If this line of thinking is extended, PSK with eight or sixteen values is possible.
The constellation of each phase modulation and phase transition is as shown in the figure below.

psk01.gif

The theory of digital phase modulation and demodulation

下線 トップへ戻る

◆BPSK modulation
With BPSK modulation, the level of the modulating signal (code 0, 1) is changed to a dipolar NRZ signal, and the signal and carrier wave are multiplied with a mixer. The modulating signal spectrum is shifted directly to the carrier frequency band.


BPSK modulation changes the phase of the carrier wave C(t) proportionally to the information signal. The carrier wave C(t) is a sine wave with the following properties.
eq001.gif

BPSK changes the phase of the carrier wave C(t) by 0 degrees or 180 degrees with regard to the 1-bit 2-status information, resulting in the following formula.
eq018.gif

When the initial phase of the carrier wave is Φc = 0;
eq019.gif

If p(t) is;
eq023.gif

which is the modulated wave, and the BPSK modulated wave Spsk(t) is multiplied by a squarer as follows.

BPSK_01.gif

eq020.gif

The modulating signal p(t) follows the information signal m(t)(0 or 1), taking the values 1 and -1.

With BPSK, 1 bit of the information signal can be expressed with 1 symbol. As shown in the figure below, information is assigned to the 0 and π phases of the carrier wave as 0 and 1 respectively.

BPSK_03.gif

BPSK_04.gif

◆BPSK demodulation
The BPSK modulating signal is demodulated with a synchronous detection system. The synchronous detection system uses a modulator to multiply the received signal and regenerated carrier wave. The frequency and phase of the regenerated carrier wave must match (synchronize with) the carrier wave used on the transmitting end. If multiplication is performed with a regenerated carrier wave that is not synchronized, the amplitude level may vary, the signal polarity may be reversed, and many errors may occur, making it unusable. Frequency multiplication and other methods are used to regenerate the carrier wave.

When the received signal is regenerated, the result is as follows.
eq021.gif

BPSK_02.gif

The second term in the braces is an unwanted component, so LPF processing is performed to recover the signal component only.

eq022.gif

*Note: This applet does not actually regenerate the carrier wave.

With QPSK, 2 bits of the information signal can be expressed with 1 symbol. The constellation of QPSK modulation and phase transition is as shown in the figure below. As shown in the figure below, information is assigned to the π/4, 3π/4, -π/4 and -3π/4 phases of the carrier wave as 00, 01, 10 and 11 respectively.

constaQPSK01.gif

QPSK_03.gif

QPSK constellation and transition of the information signal is shown in the diagram below. The information signal stream input to the modulator is assigned so that two bits represent one symbol. On the right is the QPSK modulated waveform, with one symbol and two wave lengths.

QPSK_02.gif

QPSK modulation can be expressed as changing the phase of the I and Q carrier wave C(t) proportionally to the information signal. The carrier waves Ci(t) and Cq(t) are sine waves with the following properties.



As in the figure below, QPSK modulation is achieved by multiplication of the I and Q components of the dipolar NRZ modulating signal and the I and Q carrier waves with a mixer based on the information, and adding the two signals. The information signal is divided into an I component signal and Q component signal by the P/S conversion unit and both are input in the mixer.


QPSK_01.gif

◆QPSK demodulation
With QPSK too, it is necessary regenerate the carrier wave and perform synchronous detection.

QPSK_04.gif

BPSK modulation and demodulation applet

下線 トップへ戻る

An BPSK modulation and demodulation applet
Click the “Explain” button at the bottom right of the applet for how to use it.

BPSK_00.gif

Click the image: BPSK modulation and demodulation applet

QPSK modulation and demodulation applet

下線 トップへ戻る

A QPSK modulation and demodulation applet
Click the “Explain” button at the bottom right of the applet for how to use it.

QPSK_00.gif

Click the image: QPSK modulation and demodulation applet

back
このページのトップへ