M-ary Quadrature Amplitude Modulation (MQAM)
Introduction
M-ary Quadrature Amplitude Modulation -- or QAM -- is a modulation where data bits
select one of M combinations of amplitude and phase shifts that are applied to
the carrier.
The M possible waveforms may be described by M constellation points.
In this exercise, the M = 16 constellation points are confined to a square grid.
In this exercise, you will design an 16-QAM detector to process the data contained
in the file qam16data.mat
Textbook References
M-ary QAM: Section 5.3 (pp. 238 - 260),
discrete-time realizations: Section 5.3.2 (pp. 256 - 260),
partial response pulse shapes: Section A.2 (pp. 682 - 687).
Specifications
 |
| normalized sample rate: |
8 samples/symbol |
| normalized carrier frequency: |
0.25 cycles/sample |
| carrier phase: |
0 degrees |
| average energy: |
40 |
| pulse shape: |
SRRC (30% excess bandwidth, span = 16 symbols) |
| symbol clock offset: |
0 |
| input file |
qam16data.mat |
| input message length: |
322 symbols (1288 bits or 184 ASCII characters) |
|
Preliminary Design
Design the Detector
Design the detector, shown below, using blocks from the Simulink, DSP System,
and Communications System Toolboxes.
Test the Detector Design
You should test the detector you designed by constructing a modulator to produce
a test signal.
The following procedure steps you through this design process:
- Design the modulator shown below to meet the above specifications except
make the input the four symbol sequence 8 15 1 6.
-
Connect the output of your modulator to the input of your detector.
-
Connect the output of your detector to a To Workspace block
(be sure to open the Properties Dialog Window and set the Save format to matrix)
and a Scope block.
-
Set the simulation parameters as follows:
| Simulation Time |
| Start Time: |
0.0 |
| Stop Time: |
(16+4)*8-1 |
| Solver Options |
| Type: |
Fixed-step |
| Solver: |
discrete (no continuous states) |
| Fixed step size: |
auto |
| Tasking and sample time options |
| Periodic sample time constraint: |
Unconstrained |
| Tasking mode for periodic sample times: |
SingleTasking |
Note: the stop time is computed as follows: 16 = 2*(span/2) [the delay of the
pulse shaping filter plus the delay of the matched filter];
4 = the number of data symbols;
8 = the number of samples/symbol.
We subtract because the start time is t = 0.0.
- Run the simulation and plot the demodulator input and the matched filter
output on the same set of axes.
The simulation produces 19 downsampled matched filter outputs --
the last 5 correspond to the sequence (8 15 1 6).
Check the values in the workspace to see if they agree with input
sequence (8 15 1 6).
-
Adjust the offset of the Downsample block to obtain the proper
values.
Exercise
-
Replace the modulator blocks with the From File block and set the
Filename to qpskdata.mat and the sample time to 1.
-
Set the simulation parameters as follows:
| Simulation Time |
| Start Time: |
0.0 |
| Stop Time: |
(16+322)*8-1 |
| Solver Options |
| Type: |
Fixed-step |
| Solver: |
discrete (no continuous states) |
| Fixed step size: |
auto |
| Tasking and sample time options |
| Periodic sample time constraint: |
Unconstrained |
| Tasking mode for periodic sample times: |
SingleTasking |
-
Run the simulation.
-
The detector produces 338 symbol estimates.
The last 322 of these correspond to 184 7-bit ASCII characters.
Determine the message using either your Matlab script or an
ASCII Table.
-
Plot the eye diagram and signal space projections.
