Implementing QPSK modulation and Demodulation

I have implemented QPSK modulation and demodulation system but does not give the 4 constellation points from the qpsk modulator block. The output is displayed on to discrete-time scatter plot scope and shows only 2 constellation points when I run the simulation. I have used Bernoulli binary generator for ease of analyzing and to generate a 2stage square wave.

Answer 1

[Rob Graessle]

The Bernoulli generator block is only going to generate a square wave with a value of 0 or 1. This explains why you are only seeing two constellation points at the output of the QPSK modulator. For QPSK, you want to have a message signal with possible values of 0, 1, 2 or 3 as an input to the QPSK modulator block. The Random Integer Generator block should do what you're looking for.

Or, you could still use the Bernoulli generator block with the output set to 2 samples per frame, and configure the QPSK modulator block to accept Bit input instead of Integer input.

[Nauka Fernando]

Thankx rob.. it solved my problem.i set the bernoulli generator to 2 samples per frame and configured the modulator and demodulator blocks as you said. but i can't get the display out to a scope to see how it modulated because it gives me errors as below,

"Invalid frame-based input with frame size 2 specified at port 1 of scope 'modulationwiterros/Scope'. For the case of frame-based inputs, the Scope requires that the frame size be 1. Alternatively, you may use the Vector Scope provided by the Signal Processing Blockset"

i need to take the display out at every output from each block for my analyzing..

need help please..

[Rahool]

how to implement qpsk modulator and demodulator using square root raised cosine function as shaping pulse