GSoC 2025 · GNU Radio 4.0 Block-Set Expansion

Week-6 check-in (mentor — Josh Morman)

   
Date 19 June 2025, 17:00 IST
Duration 50 min
Participants Josh Morman · Krish Gupta

1 · Progress update

  • Analog Blocks
    • Completed demodulator implementations:
      • QuadratureDemod.hpp for FM demodulation
      • FmDet.hpp (slope detector) as an alternative implementation
      • AmDemod.hpp with configurable IIR smoothing
    • Implemented AGC family:
      • Agc.hpp - basic automatic gain control
      • Agc2.hpp - dual-rate AGC with separate attack/decay rates
    • All implementations thoroughly tested with Boost.UT
  • Testing Approach
    • Created end-to-end tests that verify mod→demod chains
    • Added edge case testing for extreme signal levels
    • Verified correct behavior with rapidly changing signals

2 · Implementation Highlights

AmDemod Implementation

template <typename T>
class AmDemod : public Block<AmDemod<T>> {
private:
    T alpha_{0.001}; // Smoothing factor
    T last_env_{0};  // Minimal state

public:
    // Configuration via reflection
    GR_MAKE_REFLECTABLE(AmDemod, alpha_);
    
    void start() { last_env_ = 0; }
    
    void processOne(const std::complex<T>& in, T& out) {
        // Envelope detection with IIR smoothing
        T env = std::abs(in);
        last_env_ = alpha_ * env + (1.0 - alpha_) * last_env_;
        out = last_env_;
    }
};

GR_REGISTER_BLOCK(AmDemod<float>);
GR_REGISTER_BLOCK(AmDemod<double>);

Agc2 Implementation

Implemented dual-rate AGC with separate attack/decay rates for better performance in varying signal conditions.

3 · Challenges Resolved

Challenge Solution
State management Used minimal state variables with clear lifecycle through start()/stop()
Phase unwrapping Implemented quadrature demodulation with proper phase difference calculation
Memory pressure in tests Split test suites into smaller files to avoid CI memory issues

4 · Digital Blocks Planning

Discussed the digital blocks implementation strategy:

  • Directory structure by use-case rather than block type
  • Core implementation priorities:
    • LFSR (Linear Feedback Shift Register)
    • CRC (Cyclic Redundancy Check)
    • Scrambler/Descrambler
    • Constellation representation

Josh and I agreed on a modular approach with distinct subdirectories:

digital/
├── core/          # Fundamental components (LFSR, CRC, etc.)
├── mapping/       # Symbol mapping/slicing
├── timing/        # Clock recovery, synchronization
├── equalizer/     # Channel equalization
├── ofdm/          # OFDM-specific components
└── compat/        # GR3 compatibility layers

This structure allows for focused development and testing.

5 · Porting Guide Updates

Added key sections to the porting guide:

  • State management patterns - When to use member variables vs. start()/stop()
  • Error handling approaches - How to handle edge cases gracefully
  • Performance considerations - Guidelines for SIMD-friendly implementations
  • Reflection best practices - When and how to expose parameters

6 · Action items

Owner Task
Krish • Create PR for completed Analog blocks
• Begin implementation of Digital core components
• Finalize initial version of porting guide
Josh • Review Analog block implementations
• Provide feedback on Digital directory structure
• Connect with other maintainers about repository transition

Next sync: June 26th to review Analog PR and discuss Digital block implementation details.

7 · Repository Strategy

With the repository transition to gnuradio/gnuradio4 in progress, we decided on the following approach:

  1. Submit the Analog PR directly to the new repository
  2. Eventually rebase the Math PR or submit a fresh PR to the new repository
  3. Structure the Digital blocks PR for easier review given its larger scope

Josh mentioned:

“The Digital blocks PR will be larger and more complex. We should structure it for incremental review, focusing first on the core components that other blocks will depend on.”