MATLAB Examples

Transmission of a QPSK Waveform Using USRP® E310

This example shows how to use the USRP® Embedded Series Radio Support Package with Simulink® to implement a QPSK transmitter. The SDR device in this model will continuously transmit indexed 'Hello world' messages that are QPSK modulated onto a carrier with a specified center frequency. You can demodulate the transmitted message using the Reception of a QPSK Waveform Using USRP® E310 model if you have a second SDR platform.

Refer to the Guided Host-Radio Hardware Setup documentation for details on configuring your host computer to work with the Support Package for USRP® Embedded Series Radio.

Contents

Introduction

This example transmits a QPSK signal over the air using SDR hardware. It has two main objectives:

  • Implement a prototype QPSK-based transmitter in Simulink® using Simulink blocks from the USRP® Embedded Series Radio Support Package.
  • Illustrate the use of key Communications System Toolbox™ Simulink blocks for QPSK system design.

Setup

Before running the example, ensure you have performed the following steps:

1. Configure your host computer to work with the Support Package for USRP® Embedded Series Radio. See Guided Host-Radio Hardware Setup for help.

2. Ensure that you have a suitable receiver. This example is designed to work in conjunction with any of the following possible receiver examples:

Running the Example

Start the transmitter and then your companion receiver. Once both are running, you should see "Hello world" messages in the MATLAB command window where the receiver is running.

Transmitter Design: System Architecture

The top-level structure of the model is shown below.

The system performs four major processes:

  1. Bit generation
  2. Baseband modulation
  3. Pulse shaping and upsampling
  4. Sending baseband data to SDR hardware

Each process is explored in more detail in the following sections. The result of this processing is a single channel of QPSK data for transmission by the E310 SDR Transmitter block.

Bit Generation

The Bit Generation subsystem uses a MATLAB workspace variable as the payload of a frame, as shown in the figure below.

Each frame contains 200 bits. The first 26 bits are a frame header, and the remaining 174 bits represent a data payload.

  • The 26 header bits result in a 13-symbol Barker code to use as a preamble. The preamble is used to aid in overcoming channel impairments in the receiver.
  • The first 105 bits of the payload correspond to the ASCII representation of 'Hello world ###', where '###' is a repeating sequence of '001', '002', '003',..., '099'.
  • The remaining payload bits are random.

The payload is scrambled to guarantee a balanced distribution of zeros and ones for the timing recovery operation in the receiver.

Baseband Modulation

The QPSK Modulator Baseband block modulates pairs of bits from the output of the Bit Generation subsystem to QPSK constellation points using Gray mapping. Each QPSK symbol is represented by one complex sample.

Pulse Shaping and Upsampling

The Raised Cosine Transmit Filter block performs root raised cosine pulse shaping with a roll off factor of 0.5. It also upsamples the baseband signal by a factor of 4.

Sending Baseband Data to SDR Hardware

The USRP® E310 Transmitter block sends baseband data to the SDR hardware over Ethernet. The FPGA sends the baseband data to match the AD9361 baseband sample rate, at which point the AD9361 further upsamples the signal to RF and transmits it over the air. It is important to note that the real world rate at which the model runs is determined by the Baseband sample rate in E310 SDR Transmitter block, and not by the simulation sample time.

Alternative Implementations

This example describes the Simulink implementation of a QPSK transmitter with USRP® E310 SDR platform.

You can also explore a non-hardware QPSK transmitter and receiver example that models a general wireless communication system using an AWGN channel and simulated channel impairments with the QPSK Transmitter and Receiver example.

Troubleshooting the Example

If you run the example and you get the message WARNING: SDR hardware Tx data buffer underflow! in the command window, then the simulation ran slower than real time. You can try using burst mode.

If you still fail to get the example to work, see USRP® Embedded Series Radio Processing Errors and Fixes.

Copyright Notice

USRP® is a trademark of National Instruments Corp.

List of Example Helper Files

This example uses the following helper files: