Tools4SDR : Transmission of linearly modulation of sequences of QPSK and 16-QAM symbols. The received constellation.

Tutorial : Transmission of a linearly modulated sequence of symbols

1. The "hello world !" of the communication with two USRPs, matlab and windows
2. Transmission of linearly modulation of sequences of QPSK and 16-QAM symbols. The received constellation.
3. Frequency offset estimation with linearly modulated sequence of symbols
4. Channel impulse response estimation with a linearly modulated sequence of symbols
5. Transmission of a binary signal with a linearly modulated sequence of symbols
6. Transmission of an image with the USRP (matlab/windows)

Structure of this article
1 Goals of this tutorial 1-1 The transmitted signal 1-2 Signal processing at the receiver 2 Using a QPSK constellation 2-1 Without frequency offset compensation 2-2 With frequency offset compensation 3 Using a 16-QAM constellation

More about SDR4Teaching >> Linear modulations
Frequency offset estimation with linearly modulated sequence of symbols Transmission of an image with the USRP (matlab/windows) Estimation of the RF impairments of the USRP throught channel impulse response estimation Channel impulse response estimation with a linearly modulated sequence of symbols Transmission of a binary signal with a linearly modulated sequence of symbols

See also
SDR4Teaching >> Linear modulations SDR4Teaching >> OFDM Signals SDR4Teaching >> RF impairments

Transmission of linearly modulation of sequences of QPSK and 16-QAM symbols. The received constellation.

1. Goals of this tutorial

This tutorial is about the transmission of linearly modulated sequence of symbols. Two USRP are therefore used. Each one is connected to a computer running windows and matlab.

The transmission system: 2 USRP and 2 PCs with matlab

The objective of this demonstration is to show how the transmission through the USRP modify the constellation at the receiver.

1.1. The transmitted signal

On the first computer, a linearly modulated sequence of symbols (QPSK or 16-QAM) have been generated. These symbols have then been modulated using a square root raised cosine filter with a bandwith of 0.2.

The transmitted signal is a linearly modulation of sequences of symbols

1.2. Signal processing at the receiver

This signal is then transmitted with the USRP to the second computer. As it is done thanks to the matlab interface proposed on this website, the received signal is composed of bursts, each being a replica of the signal of interest:

The received signal is composed of bursts, each one being a replica of the signal of interest

At the received, the processing is the following one:

Selection of one burst.
Sampling the signal at the symbol rate.
Plotting the signal in a complex plan.

2. Using a QPSK constellation

When transmitting QPSK symbols, the constellation of the received signal is the following one:

The received constellation of QPSK symbols

The received signal is almost of constant modulus. The points at the origin correspond to the beginning and the end of the signal. To go further in the study of this signal, the time evolution of the received constellation is shown below thanks to some video:

2.1. Without frequency offset compensation

The first video has been computed without additionnal treatments. As shown, the QPSK constellation can hardly be recognized.

2.2. With frequency offset compensation

The second idea is to compensate the frequency offset before plotting the constellation. The rough estimation method based on the autocorrelation function of the received signal has therefore been used. The received constellation is then the following one:

The QPSK constellation can now be easily recognized. Note that the constellation is moving despite the fact the channel impulse response is not changing.

3. Using a 16-QAM constellation

To conclude, a linear modulation of 16-QAM symbols has been transmitted. The constellation of the received signal is the following one:

The received constellation of 16-QAM symbols

The frequency offset has also been compensated for this experimentation. As shown on the video, despite this preliminary treatment does not allow to easily recognize the transmitted constellation: