Transmission of a QPSK Waveform Using USRP E3xx
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 by using the model in the Reception of a QPSK Waveform Using USRP E3xx example 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.
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 Toolbox™ Simulink blocks for QPSK system design.
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.
Some additional steps may be required if you want to run two radios from a single host computer. See Setup for Two Radios Connecting to One Host for help.
2. Ensure that you have a suitable receiver. This example is designed to work in conjunction with the Reception of a QPSK Waveform Using USRP E3xx receiver example.
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:
Pulse shaping and upsampling
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 E3xx SDR Transmitter block.
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.
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 E3xx 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 E3xx SDR Transmitter block, and not by the simulation sample time.
This example describes the Simulink implementation of a QPSK transmitter with USRP E3xx 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 in Simulink 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 Common Problems and Fixes.
List of Example Helper Files
This example uses the following helper files:
usrpe3xxQPSKTxSL_init.m: returns a structure of variables used to control constant parameters the model.