Radio Hacking Village - Technical Diagram

Welcome to the Radio Hacking Village

Never touched an SDR before? Perfect. Start here, we'll guide you through 3 hands-on activities that take you from "What's a Signal?" to actually decoding radio.

📚 The Basics

Before we dive into hands-on activities, let's understand the fundamentals of how information travels through radio waves.

🌟 The Big Picture: Encoding vs Modulation

Message → ENCODING (digital) → Bits → MODULATION (analog) → Radio Waves

🔢 ENCODING

What: How we represent information as bits (digital)

Examples:

  • Text → ASCII → Binary
  • Audio → MP3 → Binary
  • Error correction codes

📡 MODULATION

What: How we put those bits onto a radio wave (analog)

Examples:

  • Amplitude changes (AM)
  • Frequency changes (FM)
  • On/Off switching (OOK)
🔑 Key Concept: Encoding is choosing a language (English, Morse, Binary). Modulation is choosing how to transmit it (speaking, light signals, hand signals).

🌊 What is Modulation?

Radio waves have 3 properties we can change to encode information:

1. Amplitude

Height of the wave

   /\      /\
  /  \    /  \
 /    \  /    \
       \/

2. Frequency

How fast it oscillates

/\/\/\/\/\/\  (fast)

/\    /\    (slow)
  \  /  \  /
   \/    \/

3. Phase

Where in cycle it starts

/\      /\     (0°)
  \    /  \
   \  /    \

  /\    /\   (shifted)
 /  \  /  \
💡 Modulation = Changing one of these properties to encode information
Carrier wave → Wiggle it → Receiver detects wiggling → Extract information

📻 Common Modulation Schemes

AM (Amplitude Modulation)

Basic

Changes: Height of wave

Used for: AM radio, aviation

Note: Simple but noise-sensitive

FM (Frequency Modulation)

Basic

Changes: Oscillation speed

Used for: FM radio, two-way radios

Note: More noise-resistant than AM

OOK (On-Off Keying)

⭐ Important

Changes: Signal ON or OFF

Used for: Key fobs, garage doors, IoT

Note: Simplest digital modulation

FSK (Frequency Shift Keying)

Intermediate

Changes: Frequency hops between values

Used for: Pagers, old modems

Note: Good noise resistance

PSK (Phase Shift Keying)

Advanced

Changes: Phase at symbol boundaries

Used for: Satellite, WiFi, GPS

Note: Efficient but complex

🔢 What is Encoding?

Encoding = Representing information as bits (happens before modulation)

Source Encoding

  • Text: ASCII, UTF-8
  • Images: JPEG, PNG
  • Audio: MP3, WAV

Channel Encoding

(Error Correction)

  • Parity bits
  • Hamming codes
  • Reed-Solomon
  • LDPC

Line Coding

  • NRZ: 0=low, 1=high
  • Manchester: Mid-bit transition
  • Differential: Change=1

🔗 The Complete Signal Chain

📤 Transmission

Message
↓
Source Encoding
↓
Channel Encoding
↓
Modulation
↓
Transmission

📥 Reception

Reception
↓
Demodulation
↓
Channel Decoding
↓
Source Decoding
↓
Message
🎯 In our hands-on activities: We'll focus primarily on Reception → Demodulation. You'll learn to extract the original signal from radio waves!

🔧 Hands-On Activities

Time to get your hands dirty! These 3 activities will take you from zero to decoding real signals.

1

📻 Activity 1: Decode FM with GQRX

🟢 Easy
⏱️ Time: 10 minutes 🎯 Goal: Hear your first decoded signal

What you'll do:

  1. Step 1: Download and install GQRX (see Resources below)
  2. Step 2: Download the FM radio IQ file:
    mumbai-10s-10M-92.3-8-10-25.iq

    10-second recording from Mumbai FM broadcast

    Sample Rate
    10 MSPS
    (10 million samples/second)
    Center Frequency
    92.3 MHz
    (FM radio band)
    Download File
  3. Step 3: Open the file in GQRX (set sample rate to 10 MSPS)
  4. Step 4: Select FM demodulator (Wideband FM)
  5. Step 5: Listen to the decoded audio!
✅ Success: You hear clear audio from the IQ file

What you learned: GQRX automatically performs all the demodulation steps for you. But what's happening under the hood?

2

🔗 Activity 2: Decode FM with GNURadio Flowgraph

🟡 Intermediate
⏱️ Time: 20 minutes 🎯 Goal: Build your own FM decoder

What you'll do:

  1. Step 1: Install GNURadio Companion (see Resources)
  2. Step 2: Build a flowgraph: File Source → Low Pass Filter → Quadrature Demod → Audio Sink
  3. Step 3: Use the same FM IQ file from Activity 1:
    mumbai-10s-10M-92.3-8-10-25.iq

    Same 10-second recording from Mumbai FM broadcast

    Sample Rate
    10 MSPS
    (10 million samples/second)
    Download File
  4. Step 4: Set File Source sample rate to 10 MSPS
  5. Step 5: Run your flowgraph and hear the SAME audio!
📹 Follow along: Mike Ossmann - "Receiving FM" [Great Scott Gadgets SDR Tutorial]
✅ Success: Your flowgraph decodes the same audio as GQRX
💡 Insight: You just built what GQRX does automatically! Now you understand the signal processing chain.
3

🐍 Activity 3: OOK Signals with Python

🟡 Intermediate
⏱️ Time: 25 minutes 🎯 Goal: Generate and decode OOK signals with code

What you'll do:

  1. Step 1: Install Python, NumPy, Matplotlib, Jupyter (see Resources)
  2. Step 2 - Part A: Run the generation notebook to create an OOK signal
    generate_ook_signal.ipynb

    Jupyter notebook to generate OOK signals with hidden messages

    Type
    Jupyter Notebook
    (Python + NumPy)
    Output
    Complex32 IQ file
    Download Notebook
    • This creates a complex32 IQ file with a hidden message
  3. Step 3 - Part B: Run the decoder notebook to extract the message
    decode_ook_signal.ipynb

    Jupyter notebook to decode OOK signals and extract hidden messages

    Type
    Jupyter Notebook
    (Python + SciPy)
    Techniques
    DSP + Demodulation
    Download Notebook
    • Learn threshold detection, baud rate estimation, bit extraction
✅ Success: You decoded the hidden message from the OOK signal you generated!
💡 What you learned: DSP is just math on arrays! IQ data = complex numbers, demodulation = array operations. You now understand signal processing at the code level.

🏆 Capture The Signal CTF

Ready to test your skills? Join our Capture The Flag competition!

📡 Connect to the CTF Network

📶 WiFi SSID: c0c0n_Grand_1
🔐 Password: c0c0n@Grand#1
🖥️ CTFD Server: http://10.1.0.24/
🚀 Access CTF Platform
📋 Note: All challenge details, IQ files, and flag submissions are on the CTF platform. No hints here - use your skills and what you've learned!
🎯 Our Goal: Get at least 1 flag. That's all we ask. Every challenge you solve builds real RF skills!

🛠️ Resources

Essential tools and learning resources for your RF/SDR journey.

📦 Essential Tools

📊 GQRX

Software Defined Radio receiver with real-time spectrum analysis

Official Site GitHub Download

🔍 Inspectrum

Offline signal analysis and parameter measurement tool

GitHub Documentation

🔗 GNURadio Companion

Visual signal processing framework for building custom demodulators

Official Site GitHub Install Guide

🚀 FutureSDR

Next-generation async SDR runtime built in Rust

Official Site GitHub Documentation

🔧 Specialized GNURadio Modules

Advanced out-of-tree (OOT) modules for specific protocols - useful for CTF challenges!

📡 gr-rds

RDS (Radio Data System) decoder for FM radio - extracts text, station info, and metadata

GitHub

💡 Useful for decoding FM radio data streams

🛰️ gr-dvbs2rx

DVB-S2 receiver for satellite TV signals - advanced digital video broadcasting

GitHub

💡 For satellite signal demodulation challenges

🔧 Custom DSP Tooling

As you advance, you can build custom tools for specialized signal processing:

🐍 Python Ecosystem

Best for: Rapid prototyping, learning, analysis

Key libraries:

  • NumPy: Array operations and math
  • SciPy: Signal processing functions
  • Matplotlib: Visualization
  • PySDR: SDR-specific tools
NumPy SciPy PySDR

⚡ CUDA (GPU Acceleration)

Best for: High-bandwidth processing, real-time applications

Use cases:

  • FFT on large datasets
  • Parallel filtering operations
  • Real-time spectrum analysis
  • ML-based signal classification
CUDA Toolkit CuPy (NumPy for GPU)

🔌 FPGA Development

Best for: Hardware-accelerated processing, ultra-low latency

Use cases:

  • Custom hardware demodulators
  • High-speed signal capture
  • Deterministic real-time processing
  • SDR hardware development
Xilinx Vivado Intel Quartus
💡 Choosing the right tool:
Python for learning and prototyping → CUDA for performance → FPGA for dedicated hardware

📚 Learning Resources

🎓 Video Courses

Mike Ossmann's SDR Course

The definitive introduction to SDR

Watch Full Course
Balint Seeber's SDR Tutorials

Advanced SDR techniques and demonstrations

YouTube Channel
GNURadio Tutorials

Official tutorials and examples

GNURadio Wiki

📖 Written Resources

PySDR Online Textbook

Python-focused SDR learning

Read Online
Daniel Estevez's Blog

Satellite decoding, deep technical analysis, signal processing

Visit Blog
The Scientist and Engineer's Guide to DSP

Comprehensive DSP reference

Free Online
RTL-SDR Blog

Tutorials and hardware guides

Visit Blog

💬 Community & Events

GRCon (GNURadio Conference)

Annual conference for SDR community

GitHub Archive Official Site
Reddit r/RTLSDR

Active community for SDR enthusiasts

Join Subreddit
GNURadio Mailing Lists

Technical discussions and support

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