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cover_picture_qpsk_v3_small_zoom15This example has been updated. Find the latest version at QPSK Transceiver.

 

 

 

 

 

Phase-shift keying (PSK) is a digital modulation scheme that conveys message information by modulating the phase of the carrier wave. Quadrature Phase-Shift Keying (QPSK) can encode two bits per symbol by using four different phases.

 

In this example, we use INTERCONNECT solutions to study the QPSK transceiver. In this example, you will learn how to:

Generate and modulate QPSK signals (QPSK transmitter)

Demodulate QPSK signals (QPSK receiver)

Measure signal qualities

Map signals

 

Modeling Instruction

Start a new INTERCONNECT project. You can start a new project by pressing Ctrl+N, or by selecting New in the File menu.

To generate QPSK modulated signal, from the Element Library drag and drop 2 PRBS Generators (Element Library\ Sequence Generator), 2 NRZ Pulse Generators (Element Library\ Pulse Generators\ Electrical), 2 Mach-Zehnder Modulators (Element Library\ Modulators\ Optical), 2 1xN Forks (Element Library\ Tools), an Optical Phase Shift (Element Library\ Passive\ Optical) and a CW Laser (Element Library\ Sources\ Optical), set the FORK number of port to be 2 and the phase shift  to be pi/2, then connect the elements as follows:

qpsk_step1_zoom82

To generate a QPSK demodulator (receiver), drag and drop a CW Laser (local oscillator), 4 Waveguide Couplers (Element Library\ Waveguides\ Couplers), 4 PIN Photodiode (Element Library\ Photodetctors), 1 Optical Phase Shift, 2 Electrical Constant Multipliers and 2 Electrical Adder (Element Library\ Math\ Electrical), set the optical phase shift to be pi/2 and the electrical gain of Constant Multipliers to be -1, connect the elements as following:

qpsk_receiver_zoom75

To analyze and visualize results at different stages in the system, add some analyzers at different points. The whole system design diagram is as following:

QPSK_System

Set the "modulation type" of VSA as QPSK with a rotation of π/4, the build in "symbol map table" is as following:

Matrix Editor


1

2

Symbol

1

1

0

00

2

0

1

01

3

0

-1

10

4

-1

0

11

 

Discussion and Results

Optical Modulator

The relative phase shift variations between a single light source derived beams can be determined by the Mach–Zehnder interferometer, an interferometer is used to measure the phase shifts between the two beams caused by a sample or a change in length of one of the paths.

mzm_explain_zoom57

The properties of the MZM are given above, set the single drive MZM bias voltage at 0 with a swing of 2V.

 

Optical Hybrid Receiver

The optical hybrid receiver makes the received signal interferes with a local oscillator (which has the same central frequency as the carrier CW laser); with a 90 degree hybrid, the detected current can be separated to the in-phase part and quadrature phase part, so that the encoded signal information can be retrieved.

hybrid_receiver_zoom96

Analyzer Visualizations

Analyzers can be inserted at different points of the circuit for detailed analysis of the signal evolution from the transmitter to the receiver.

 

At the modulation stage, two independent bit streams are generated to drive the two MZMs, then the in-phase and quadrature-phase beams add together for transmission. The two electrical NRZ signals and the optical light waveform are shown as following:

QPSK_Visualize_Modulation

At the receiver stage, the detected signals are separated into the in-phase and quadrature-phase parts as when they are modulated.

QPSK_Visualize_Demodulation

The modulator modulates the pseudo-random bit sequences (which is a random bit stream with combination of 1s and 0s) to four levels of phases (π/4, 3π/4, 5π/4 and 7π/4) of the carrier lightwave. The VSA generates the decision points of the data according to the QPSK modulation type and then the received data can be mapped according to the decision regions.

qpsk_constellation_map_zoom80

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