This model shows how to use the ADALM-PLUTO Radio with Simulink® to implement a QPSK transmitter. The ADALM-PLUTO Radio in this model will keep transmitting indexed 'Hello world' messages at its specified center frequency. You can demodulate the transmitted message using the QPSK Receiver with ADALM-PLUTO Radio model. This example assumes that two ADALM-PLUTO Radios are attached to your computer.
The top-level structure of the model is shown in the following figure:
The transmitter includes the Bit Generation subsystem, the QPSK Modulator block, and the Raised Cosine Transmit Filter 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 100 'Hello world ###' messages and a header. The first 26 bits are header bits, a 13-bit Barker code that has been oversampled by two. The Barker code is oversampled by two in order to generate precisely 13 QPSK symbols for later use in the Data Decoding subsystem of the receiver model. The remaining bits are the payload. The payload correspond to the ASCII representation of 'Hello world ###', where '###' is a repeating sequence of '000', '001', '002', ..., '099'. The payload is scrambled to guarantee a balanced distribution of zeros and ones for the timing recovery operation in the receiver model. The scrambled bits are modulated by the QPSK Modulator (with Gray mapping). The modulated symbols are upsampled by two by the Raised Cosine Transmit Filter with a roll-off factor 0.5. The output rate of the Raised Cosine Filter is set to be 400k samples/second with a symbol rate of 200k symbols per second. Please match the symbol rate of the transmitter model and the receiver model correspondingly.
Before running the model, connect two ADALM-PLUTO Radios to the computer. Set the Center frequency parameter of the ADALM-PLUTO Radio Transmitter block and run the model. You can run the QPSK Receiver with ADALM-PLUTO Radio model to receive the transmitted signal. We suggest initialize two MATLAB® sessions to ensure real-time process.
Due to hardware variations among the ADALM-PLUTO Radios, a frequency offset will likely exist between the transmitter hardware and the receiver hardware. In that case, perform a manual frequency calibration using the companion frequency offset calibration transmitter and receiver models and examine the resulting behavior.
If the message is not properly decoded by the receiver model, you can vary the gain of the source signals in the ADALM-PLUTO Radio Transmitter block of this model, and that of the ADALM-PLUTO Radio Receiver block in the receiver model.