13. Laboratory Work 3. Digital Modulation (BPSK/QPSK)¶

Goal¶

Move from analog modulation to digital modulation and demonstrate how information is represented in the complex plane.

This lab covers:

BPSK;
QPSK.

1. Learning idea¶

bit stream → modulation → IQ signal → RF → reception → synchronization → demodulation → BER

This is the first step toward real digital communication systems.

2. Experiment diagram¶

flowchart TB
    classDef dsp fill:#DCFCE7,color:#0F172A,stroke:#16A34A;
    classDef rf fill:#FFE4E6,color:#0F172A,stroke:#E11D48;
    classDef tool fill:#EDE9FE,color:#0F172A,stroke:#7C3AED;

    BITS["1. Bit stream"]:::dsp
    MAP["2. Symbol mapping<br/>BPSK/QPSK"]:::dsp
    SHAPE["3. Pulse shaping<br/>RRC"]:::dsp
    TX["4. RF TX"]:::rf
    CH["5. Channel"]:::rf
    RX["6. RF RX"]:::rf
    DDC["7. Baseband"]:::dsp
    SYNC["8. Synchronization"]:::dsp
    DEMOD["9. Demodulation"]:::dsp
    BER["10. BER estimation"]:::tool

    BITS --> MAP --> SHAPE --> TX --> CH --> RX --> DDC --> SYNC --> DEMOD --> BER

3. Core concepts¶

BPSK¶

1 bit per symbol;
phase 0 or π;
the two points are as far apart as the power allows, so at the same Eb/N0 no PSK scheme has a lower BER (QPSK matches it per bit).

QPSK¶

2 bits per symbol;
4 constellation points;
higher data rate.

4. Tasks¶

Generate a bit stream.
Perform modulation (BPSK or QPSK).
Transmit the signal.
Receive it with RTL-SDR.
Perform demodulation.
Estimate BER.

5. Expected observations¶

constellation diagram;
noise influence;
synchronization errors;
BER changes.

6. Expected result¶

The student should:

understand IQ representation;
observe constellation;
evaluate errors;
connect DSP and RF behavior.

7. Engineering conclusion¶

Digital modulation is the basis of modern communication systems, and SDR allows observing it simultaneously at the model, hardware, and RF levels.