Understanding Out-of-Tree (OOT) Modules in GNU Radio

GNU Radio is a powerful, free, and open-source software development toolkit that provides signal processing blocks to build software-defined radio (SDR) applications. One of its most exciting features is the ability to create custom modules, known as Out-of-Tree (OOT) modules. In this article, I’ll guide you through the process of creating and using OOT modules in GNU Radio, providing you with a comprehensive understanding of this feature.

What is an OOT Module?

An OOT module is a custom module that you create to extend the functionality of GNU Radio. These modules can contain new blocks, new data types, or even new flow graphs. They are not part of the standard GNU Radio distribution, hence the name “Out-of-Tree.” Creating OOT modules allows you to tailor GNU Radio to your specific needs, adding functionality that is not available in the standard library.

Creating an OOT Module

Creating an OOT module involves several steps. Here’s a high-level overview of the process:

• Set up your development environment.
• Use the `grmodtool` to create a new module.
• Add new blocks to your module.
• Write the code for your new blocks.
• Compile and test your module.
• Use your module in GNU Radio flow graphs.

Setting Up Your Development Environment

Before you start creating an OOT module, you need to set up your development environment. This includes installing GNU Radio, CMake, and any other necessary tools. You can find installation instructions for GNU Radio on its official website.

Creating a New Module

Once your development environment is set up, you can create a new module using the `grmodtool`. This tool is part of the GNU Radio development suite and simplifies the process of creating new modules. Here’s an example command to create a new module named “my_module”:

grmodtool newmod my_module

This command creates a new directory for your module and sets up the necessary files.

Adding New Blocks

After creating your module, you can add new blocks to it. Blocks are the building blocks of GNU Radio flow graphs. You can create new blocks using the `grmodtool` or by manually creating the necessary files. Here’s an example command to add a new block named “my_block” to your module:

grmodtool add -t sync -l python my_block

This command creates a new Python block named “my_block” in your module.

Writing the Code for Your New Blocks

Once you’ve added a new block to your module, you need to write the code for it. The code for a block depends on the type of block you’re creating. For example, a Python block requires a Python script, while a C block requires a C++ source file. Here’s an example Python block that calculates the square of an input value:

import numpy as npfrom gnuradio import grclass my_block(gr.sync_block):    def __init__(self):        gr.sync_block.__init__(self, name="my_block", in_sig=[np.float32], out_sig=[np.float32])    def work(self, input_items, output_items):        for i in range(len(input_items[0])):            output_items[0][i] = input_items[0][i]  2        return len(input_items[0])

And here’s an example C++ block that calculates the square of an input value:

include include class my_block : public gr::block {private:    float input;    float output;public:    my_block() : gr::block("my_block", gr::io_signature::make(1, 1, sizeof(float)),                         gr::io_signature::make(1, 1, sizeof(float))) {}    int work(int noutput_items, gr_vector_const_void_star &input_items,             gr_vector_void_star &output_items) {        input = (float )input_items[0];        output = (float )output_items[0];        for (int i = 0; i < noutput_items; i++) {            output[i] = input[i]  input[i];        }        return noutput_items;    }