GSoC 2025 · GNU Radio 4.0 Block-Set Expansion

Week-11 check-in (mentor — Josh Morman)

   
Date 24 July 2025, 17:00 IST
Duration 45 min
Participants Josh Morman · Krish Gupta

1 · Progress update

  • PR Review Status
    • Analog PR received positive feedback with only minor adjustments needed
    • Digital PR initial review highlighted documentation improvements
    • Math PR awaiting final resolution of CI Sonar issues
  • Timing Recovery
    • Completed implementation of all planned timing recovery blocks:
      • MuellerMullerClockRecovery.hpp
      • SymbolSync.hpp
      • CostasLoop.hpp
      • PFBClockSync.hpp (Polyphase filterbank clock sync)
    • All implementations thoroughly tested with comprehensive test cases
  • OFDM Components
    • Started implementation of key OFDM components:
      • CyclicPrefixer.hpp - Adds cyclic prefix to OFDM symbols
      • Serializer.hpp - Converts parallel subcarriers to serial
      • CarrierAllocator.hpp - Maps data to OFDM subcarriers

2 · Timing Recovery Innovations

Implemented several innovations in the timing recovery blocks:

PFB Clock Sync Implementation

template <typename T>
class PFBClockSync : public Block<PFBClockSync<T>> {
private:
    int nfilts_{32};          // Number of filterbank arms
    float rate_{1.0f};        // Resampling rate
    std::vector<std::vector<T>> filters_; // Filterbank taps
    
    // Minimal state with clear lifecycle
    float d_rate_f{0};
    float d_rate_i{0};
    float d_rate_error{0};
    
    // ... other implementation details

public:
    GR_MAKE_REFLECTABLE(PFBClockSync, nfilts_, rate_, filters_);
    
    void start() {
        d_rate_f = 0;
        d_rate_i = 0;
        d_rate_error = 0;
        // ... other initialization
    }
    
    template <typename InputIt, typename OutputIt>
    void processBulk(InputIt in, size_t n, OutputIt out, size_t& produced) {
        // Implementation of PFB clock synchronization
        // ...
    }
};

Key innovations:

  • Memory-efficient implementation with minimal state
  • Clear separation of configuration and runtime state
  • Optimized filter application for better performance
  • Robust error handling for edge cases

3 · OFDM Architecture

Designed a modular OFDM architecture with clear component separation:

ofdm/
├── CyclicPrefixer.hpp       # Add/remove cyclic prefix
├── Serializer.hpp           # Parallel<->Serial conversion
├── CarrierAllocator.hpp     # Data/pilot mapping
├── Equalizer/
│   ├── SimpleEqualizer.hpp  # One-tap equalization
│   └── LMSEqualizer.hpp     # Adaptive equalization
└── Sync/
    ├── ChannelEstimator.hpp # Pilot-based estimation
    └── FrameSync.hpp        # Frame synchronization

This structure:

  • Separates concerns for better maintainability
  • Enables focused testing of each component
  • Provides clear interfaces between subsystems
  • Follows the overall Digital blocks organization

4 · Testing Innovations

Developed several testing innovations to ensure robust implementations:

Innovation Benefit
End-to-end test chains Verify correct operation in realistic scenarios
Known-good reference data Compare against established implementations
Parameter sweep tests Ensure stability across configuration space
Edge case testing Verify behavior with extreme inputs

Example of parameter sweep testing:

"SymbolSync_ParameterSweep"_test = []{
    // Test across range of gain values
    for (float gain_mu : {0.01f, 0.05f, 0.1f, 0.2f, 0.5f}) {
        for (float gain_omega : {0.001f, 0.01f, 0.1f, 0.25f}) {
            SymbolSync<float> sync(1.0f, gain_mu, gain_omega);
            
            // Run stability test with these parameters
            // ...
            
            expect(that % stable == true);
        }
    }
};

5 · PR Review Feedback

Received and addressed key feedback on PRs:

PR Feedback Resolution
Analog Type casting in QuadratureDemod Implemented explicit casts with range checking
Analog Documentation examples needed Added concrete usage examples
Digital Directory structure questions Clarified rationale in PR description
Digital Test coverage gaps Added additional tests for edge cases

Josh mentioned:

“The Analog PR is very close to being ready for merge. Just a few minor type casting issues to fix. The Digital PR will need more time due to its size, but the core components look good so far.”

6 · Action items

Owner Task
Krish • Address final feedback on Analog PR
• Continue OFDM component implementation
• Add additional test coverage for Digital PR
Josh • Coordinate final approval of Analog PR
• Continue reviewing Digital PR components
• Provide guidance on OFDM implementation

Next sync: July 31st to review OFDM progress and prepare for final project phase.

7 · Reflection on Modern C++ Usage

Josh and I discussed how our implementation approach leverages modern C++23 features:

C++23 Feature How We Used It Benefit
std::span Non-owning buffer views Zero-copy processing
Concepts Type constraints Clearer errors, better documentation
Reflection Parameter configuration Automatic GUI/Python binding
std::simd Vectorized processing Performance without intrinsics

Our implementations demonstrate the power of modern C++ for DSP applications:

  • Type-safe yet flexible interfaces
  • Minimal runtime overhead
  • Clear parameter configuration
  • Maximum optimization opportunities