GSoC 2025 · GNU Radio 4.0 Block-Set Expansion

Final Week Wrap-up and Project Retrospective

   
Date 19 August 2025
Participants Josh Morman, John Sally, Krish Gupta
Topics Project completion, PR status, lessons learned, and future plans

1 · Project Deliverables Summary

Successfully delivered three major block families for GNU Radio 4.0:

  • Math Blocks (PR fair-acc/gnuradio4#595)
    • ~500+ LOC across headers and tests
    • Binary operations (Add, Subtract, Multiply, Divide)
    • Reductions (Max, Min)
    • Bitwise operations (And, Or, Xor)
    • Unary operations (Negate, Not, Abs)
    • Special functions (Log10, Integrate, Argmax)
  • Analog Blocks (PR gnuradio/gnuradio4#5)
    • ~1k+ LOC across headers and tests
    • Modulators (FrequencyMod, PhaseModulator)
    • Demodulators (QuadratureDemod, FmDet, AmDemod)
    • Gain Control (Agc, Agc2)
  • Digital Blocks (PR gnuradio/gnuradio4#6)
    • ~4k+ LOC across ~45 files
    • Core components (LFSR, CRC, Scrambler, Constellation)
    • Mapping utilities (ChunksToSymbols, Differential coding)
    • Timing recovery (Mueller&Muller, Symbol Sync, Costas Loop)
    • OFDM components (Cyclic Prefixer, Carrier Allocator)
    • Packet processing framework

2 · PR Status Overview

PR Status Next Steps
Math Blocks CI mostly green; Sonar check on forked PRs blocked Resolve CI issue or resubmit to new repository
Analog Blocks Functionally complete; minor CI nits pending Fix remaining type casting issues and merge
Digital Blocks Structure and core components landed; review in progress Continue addressing reviewer feedback

Josh’s assessment:

“The PRs represent significant contributions to GNU Radio 4.0. The Analog PR is nearly ready to merge, and the Math PR just needs a CI fix. The Digital PR, due to its size, will undergo review over the coming months, but the foundation is solid.”

3 · Technical Achievements

Modern C++ Implementation

Successfully implemented blocks using modern C++23 features:

template <typename T>
requires std::floating_point<T> || gr::concepts::ComplexValue<T>
class FrequencyMod : public Block<FrequencyMod<T>> {
private:
    T sensitivity_{1.0};

public:
    GR_MAKE_REFLECTABLE(FrequencyMod, sensitivity_);
    
    void processOne(const T& in, T& out) {
        // Clean, optimizable implementation
        out = std::sin(sensitivity_ * in);
    }
};

// Type-safe instantiation
GR_REGISTER_BLOCK(FrequencyMod<float>);
GR_REGISTER_BLOCK(FrequencyMod<double>);

Testing Innovation

Developed efficient testing patterns using Boost.UT:

"FrequencyModulation"_test = []{
    "BasicModulation"_test = []{
        FrequencyMod<float> mod(1.0f);
        expect(that % mod.processOne(1.0f) == approx(0.15915f, 0.0001f));
    };
    
    "ComplexModulation"_test = []{
        FrequencyMod<std::complex<float>> mod(1.0f);
        auto result = mod.processOne({1.0f, 0.5f});
        expect(that % std::real(result) == approx(0.15915f, 0.0001f));
    };
};

CI Optimization

Resolved memory constraints in GitHub Actions runners with focused test matrices:

// Prioritized testing on most common types
using PriorityTypes = std::tuple<
    float, double,
    std::complex<float>, std::complex<double>
>;

// Separate test for integer variants
using IntegerTypes = std::tuple<
    int32_t, int16_t
>;

4 · Documentation Contributions

Created comprehensive documentation to support future development:

  • GR3→GR4 Porting Guide
    • Detailed patterns for translating blocks
    • Examples for each block family
    • Common pitfalls and solutions
    • Best practices for testing
  • Block-Specific Documentation
    • Implementation notes
    • Configuration parameters
    • Usage examples
    • Performance considerations

John Sally commented:

“The porting guide is a valuable resource that will help onboard new contributors and accelerate the transition to GR4. The clear examples and patterns make it accessible even to those new to the codebase.”

5 · Challenges and Solutions

Successfully navigated several technical challenges:

Challenge Solution
CI memory constraints Implemented focused test matrices and selective instantiation
Template diagnostics Used concepts for clearer error messages and documentation
Repository transition Coordinated PR submissions across repositories
Complex type handling Created specialized templates for complex I/O types
State management Developed clear lifecycle patterns with minimal state

These solutions have established patterns that future contributors can follow.

6 · Lessons Learned

Key lessons from the GSoC project:

  1. C++23 Power: Modern C++ features enable cleaner, more efficient code with less boilerplate
  2. Template Discipline: Careful template design prevents memory and compilation issues
  3. State Minimization: The less state a block maintains, the easier it is to optimize and reason about
  4. Testing Strategies: Strategic test matrices provide good coverage without exhausting CI resources
  5. Documentation Importance: Clear documentation is critical for complex template-based code

Josh highlighted:

“The most valuable outcome is not just the blocks themselves, but the patterns and practices established for future development. The clean CRTP implementations, strategic testing, and clear documentation set a high bar for GNU Radio 4.0 contributions.”

7 · Future Plans

Post-GSoC commitment to ensure successful completion:

Priority Plans
PR Merging Continue supporting PRs through review process until merged
Additional Blocks Potential future contributions to expand the block set further
Documentation Maintain and expand the porting guide as needed
Community Support Remain available to help new contributors build on this work

John Sally suggested:

“Consider creating a ‘Block of the Month’ program where you and other contributors tackle one GR3 block family at a time, using the patterns established during GSoC.”

8 · Acknowledgments

I’m deeply grateful to everyone who supported this project:

  • Josh Morman - Primary mentor who provided invaluable guidance and technical direction
  • John Sally - Secondary mentor offering insights and community perspective
  • GNU Radio Community - For feedback, testing, and maintaining an open, collaborative environment
  • Element (Matrix) Contributors - For technical discussions and support
  • Google Summer of Code - For making this opportunity possible

9 · Final Thoughts

This GSoC project has been an incredible learning journey. Working on GNU Radio 4.0 has deepened my understanding of:

  • Modern C++ template programming
  • Signal processing algorithms
  • Open-source collaboration
  • Software architecture design
  • Testing strategies for complex systems

The strategic pivot from Audio I/O to core DSP blocks (Math, Analog, Digital) proved to be the right decision, maximizing the impact on GNU Radio 4.0 readiness and unblocking core DSP workflows for users.

I’m proud of what we’ve accomplished and excited to see these contributions become part of GNU Radio 4.0’s foundation. The work doesn’t end with GSoC—I remain committed to seeing these PRs through to completion and continuing to support the GNU Radio community.

Thank you to everyone who made this possible!

— Krish Gupta