ECEn 485: QPSK Phase Shift Keying

Quadri-Phase Shift Keying or Quaternary Phase Shift Keying, QPSK, is one of the most popular digital carrier modulations in use today. The QPSK signal set consists of four waveforms that differ in phase: they are each 90-degrees apart. For this reason, the QPSK constellation consists of four points equally spaced on a circle. In this exercise, you will design QPSK detector to process the data contained in the file qpskdata.mat

Specifications

normalized symbol rate:
normalized carrier frequency:
carrier phase:
symbol clock offset:
pulse shape:

average energy:
input file:
input message length:

1/8 bit/sample
1/4 cycles/sample
0°
0 sec
square-root raised cosine
with roll-off = 0.5
Lp = 6 symbols
9 Joules
qpskdata.mat
49 symbols (98 bits)

Preliminary Design

Detector Design

Design the QPSK detector using blocks from the SIMULINK Block Library and the Communications Blockset.

Test the Detector Design

Test your design using the following procerure:

Design a modulator (below) to meet the above specifications except make the input the four symbol sequence 0 2 1 3 (See notes on non-binary sources.)

2	Connect the output of your modulator to the the input of your detector.

Set the simulation parameters as follows:

Start Time:
Stop Time:
Solver options

Fixed step size:

0.0
(12+4+1)*8
Type: Fixed-step
discrete (no continuous states)
1

Note: The stop time is computed as follows: 12 = 2*Lp (the delay of the pulse shaping filter plus the delay of the matched filter) ; 4 = the number of data symbols; 1 = the one bit (or symbol) delay to allow proper phasing of the downsample operation in the detector; 8 = the number of samples/symbol.

4	Run the simulation. The first 14 decisions are invalid. The last 4 decisions are the ones you want.

5	Does the detector output agree with the input sequence (`0 2 1 3`)?

Exercise

1	Replace the modulator and AWGN channel blocks with the From File block and set the Filename to `qpskdata.mat`

Set the simulation parameters as follows:

Start Time:
Stop Time:
Solver options

Fixed step size:

0.0
(12+49+1)*8
Type: Fixed-step
discrete (no continuous states)
1

3	Run the simulation.

4	The detector produces approximately 63 estimates. 49 of these estimates correspond to 14 7-bit ASCII characters. Determine the message using either your matlab script or the on-line ASCII table.

5	Email your answer to ee485ta "at" byu.eduAttach to your email message, the simulink model file (.mdl) that contains your detector design.

6	Plot the eye diagram and signal space projections. You may either attach the plots to your email message (preferred method) or turn in the plots at the beginning of the next class period.

Back to the ECEn 485 Simulink Exercise Page.