liquid-dsp documentation

Software-Defined Radio Digital Signal Processing Library - https://liquidsdr.org

liquid-dsp is a free and open-source digital signal processing (DSP) library designed specifically for software-defined radios on embedded platforms. The aim is to provide a lightweight DSP library that does not rely on a myriad of external dependencies or proprietary and otherwise cumbersome frameworks. All signal processing elements are designed to be flexible, scalable, and dynamic, including filters, filter design, oscillators, modems, synchronizers, complex mathematical operations, and much more.

// get in, process data, get out
#include <liquid/liquid.h>
int main() {
    unsigned int M  = 4;     // interpolation factor
    unsigned int m  = 12;    // filter delay [symbols]
    float        As = 60.0f; // filter stop-band attenuation [dB]

    // create interpolator from prototype
    firinterp_crcf interp = firinterp_crcf_create_kaiser(M,m,As);
    float complex x = 1.0f;  // input sample
    float complex y[M];      // interpolated output buffer

    // repeat on input sample data as needed
    {
        firinterp_crcf_execute(interp, x, y);
    }

    // destroy interpolator object
    firinterp_crcf_destroy(interp);
    return 0;
}

For more information, please refer to the documentation online.

Installation and Dependencies

liquid-dsp only relies on libc and libm (standard C and math) libraries to run; however liquid will take advantage of other libraries (such as FFTW if they are available. Starting with version 1.7.0, liquid-dsp has moved to the CMake <https://cmake.org> build system which can be installed with brew install cmake on macOS, sudo apt-get install cmake on Debian variants.

Installation

The recommended way to obtain the source code is to clone the entire repository from GitHub:

git clone git://github.com/jgaeddert/liquid-dsp.git

Building and installing the main library is a simple as

mkdir build
cd build
cmake ..
make
sudo make install

If you are installing on Linux for the first time, you will also need to rebind your dynamic libraries with sudo ldconfig to make the shared object available. This is not necessary on macOS.

Run all test scripts

Source code validation is a critical step in any software library, particularly for verifying the portability of code to different processors and platforms. Packaged with liquid-dsp are a number of automatic test scripts to validate the correctness of the source code. The test scripts are located under each module's tests/ directory and take the form of a C source file. When configured with BUILD_AUTOTESTS enabled, these tests are parsed, compliled, and linked into an executable which will run the tests.

./xautotest
# ...
# autotest seed: 1738416409
# ==================================
#  PASSED ALL 716450 CHECKS
# ==================================

There are currently more than 700,000 checks across 1,316 tests to verify functional correctness. Drop me a line if these aren't running on your platform.

Testing Code Coverage

In addition to the full test suite, you can configure gcc to export symbol files to check for code coverage and then use gcovr to generate a full report of precisely which lines are covered in the autotests. These symbol files aren't generated by default and need to be enabled at compile-time through a CMake option:

cmake -DBUILD_AUTOTESTS=ON -DCOVERAGE=ON ..

A coverage report can be generated by running the autotests and running gcovr:

make -j4 xautotest
./xautotest -q -o autotest.json
cd ..
gcovr --filter="src/.*/src/.*.c" --print-summary
# ...
# ------------------------------------------------------------------------------
# TOTAL                                      20730   17014    82%
# ------------------------------------------------------------------------------
# lines: 82.1% (17014 out of 20730)
# functions: 62.9% (1742 out of 2770)
# branches: 64.0% (5676 out of 8874)

Examples

Nearly all signal processing elements have a corresponding example in the examples/ directory. Most example scripts generate an output .m file for plotting with GNU octave All examples are built as stand-alone programs and can be compiled with the BUILD_EXAMPLES CMake flag:

cmake -DBUILD_EXAMPLES=ON ..
make
./examples/modem_example -m qpsk
# <liquid.modemcf, scheme="qpsk", order=4>
#    0 :   0.70710677 + j*  0.70710677
#    1 :  -0.70710677 + j*  0.70710677
#    2 :   0.70710677 + j* -0.70710677
#    3 :  -0.70710677 + j* -0.70710677
# num sym errors:    0 /    4
# num bit errors:    0 /    8
# results written to modem_example.m.

Sometimes, however, it is useful to build one example individually. This can be accomplished by directly targeting its binary (e.g. make examples/modem_example). The example then can be run at the command line, viz. ./examples/modem_example.

Benchmarking Tool

Packaged with liquid are benchmarks to determine the speed each signal processing element can run on your machine. Initially the tool provides an estimate of the processor's clock frequency and will then estimate the number of trials so that each benchmark will take between 50 and 500 ms to run. You can build and run the benchmark program with the following command:

make bench

Linking from C++

Compiling and linking to C++ programs is straightforward. Just include <complex> before <liquid/liquid.h> and use std::complex<float> in favor of float complex. Here is the same example as the one above but in C++ instead of C:

// get in, process data, get out
#include <complex>
#include <liquid/liquid.h>
int main() {
    unsigned int M  = 4;     // interpolation factor
    unsigned int m  = 12;    // filter delay [symbols]
    float        As = 60.0f; // filter stop-band attenuation [dB]

    // create interpolator from prototype
    firinterp_crcf interp = firinterp_crcf_create_kaiser(M,m,As);
    std::complex<float> x = 1.0f;   // input sample
    std::complex<float> y[M];       // interpolated output buffer

    // repeat on input sample data as needed
    {
        firinterp_crcf_execute(interp, x, y);
    }

    // destroy interpolator object
    firinterp_crcf_destroy(interp);
    return 0;
}

PlatformIO

Cross-compling for embedded platforms is most easily achieved with platformio. Just add liquid-dsp to your platform.io list of dependencies:

[env:native]
platform = native
lib_deps = https://github.com/jgaeddert/liquid-dsp.git

To test this, compile the example program for a Raspberry Pi Pico microcontroller:

# create a virtual environment, install platformio, and compile an example
virtualenv pio
source pio/bin/activate
pip install platformio
pio ci --lib="." --board=pico examples/platformio_example.c
# ...
# Generating UF2 image
# elf2uf2 ".pio/build/pico/firmware.elf" ".pio/build/pico/firmware.uf2"
# Checking size .pio/build/pico/firmware.elf
# Advanced Memory Usage is available via "PlatformIO Home > Project Inspect"
# RAM:   [==        ]  15.5% (used 41820 bytes from 270336 bytes)
# Flash: [          ]   0.2% (used 5196 bytes from 2097152 bytes)
# Building .pio/build/pico/firmware.bin
# ===================== [SUCCESS] Took 23.63 seconds =====================

Build

Here is a table of CMake options available for configuring liquid:

Option	Default	Description
BUILD_EXAMPLES	ON	Compile example programs
BUILD_AUTOTESTS	ON	Parse and compile autotests into executable binary
BUILD_BENCHMARKS	ON	Parse and compile benchmarks into executable binary
ENABLE_SIMD	ON	Enable use of single instruction, multiple data (SIMD) extensions
BUILD_SANDBOX	ON	Compile sandbox (testing) programs
BUILD_DOC	OFF	Generate documentation
COVERAGE	OFF	Set flags to enable code coverage testing

For example, if you want to benchmark how fast a vector dot product runs without SIMD extensions, you could run the following:

cmake -DENABLE_SIMD=OFF -DBUILD_BENCHMARKS=ON ..
make
./benchmark -s dotprod_rrrf

Available Modules

• agc: automatic gain control, received signal strength
• audio: source audio encoders/decoders: cvsd, filterbanks
• buffer: internal buffering, circular/static, ports (threaded)
• channel: additive noise, multi-path fading, carrier phase/frequency offsets, timing phase/rate offsets
• dotprod: inner dot products (real, complex), vector sum of squares
• equalization: adaptive equalizers: least mean-squares, recursive least squares, semi-blind
• fec: basic forward error correction codes including several Hamming codes, single error correction/double error detection, Golay block code, as well as several checksums and cyclic redundancy checks, interleaving, soft decoding
• fft: fast Fourier transforms (arbitrary length), discrete sin/cos transforms
• filter: finite/infinite impulse response, polyphase, hilbert, interpolation, decimation, filter design, resampling, symbol timing recovery
• framing: flexible framing structures for amazingly easy packet software radio; dynamically adjust modulation and coding on the fly with single- and multi-carrier framing structures
• math: transcendental functions not in the C standard library (gamma, besseli, etc.), polynomial operations (curve-fitting, root-finding, etc.)
• matrix: basic math, LU/QR/Cholesky factorization, inversion, Gauss elimination, Gram-Schmidt decomposition, linear solver, sparse matrix representation
• modem: modulate, demodulate, PSK, differential PSK, QAM, optimal QAM, as well as analog and non-linear digital modulations GMSK)
• multichannel: filterbank channelizers, OFDM
• nco: numerically-controlled oscillator: mixing, frequency synthesis, phase-locked loops
• optim: (non-linear optimization) Newton-Raphson, evoluationary algorithms, gradient descent, line search
• quantization: analog/digital converters, compression/expansion
• random: (random number generators) uniform, exponential, gamma, Nakagami-m, Gauss, Rice-K, Weibull
• sequence: linear feedback shift registers, complementary codes, maximal-length sequences
• utility: useful miscellany, mostly bit manipulation (shifting, packing, and unpacking of arrays)
• vector: generic vector operations

License

liquid projects are released under the X11/MIT license. By default, this project will try to link to FFTW if it is available on your build platform. Because FFTW starting with version 1.3 is licensed under the GNU General Public License v2 this unfortunately means that (and I'm clearly not a lawyer, here) you cannot distribute liquid-dsp without also distributing the source code if you link to FFTW. This is a similar situation with the classic libfec which uses the GNU Lesser GPL. Finally, liquid-dsp makes extensive use of GNU autoconf, automake, and related tools. These are fantastic libraires with amazing functionality and their authors should be lauded for their efforts. In a similar vain, much the software I write for a living I give away for free; however I believe in more permissive licenses to allow individuals the flexibility to use software with fewer limitations. If these restrictions are not acceptible, liquid-dsp can be compiled and run without use of these external libraries, albeit a bit slower and with limited functionality.

Short version: this code is copyrighted to me (Joseph D. Gaeddert), I give you full permission to do whatever you want with it except remove my name from the credits. Seriously, go nuts! but take caution when linking to other libraries with different licenses. See the license for specific terms.