The channel family of objects add signal impairments due to various channel conditions commonly found in wireless communications systems, including

  • additive white Gauss noise (AWGN)
  • channel gain
  • carrier phase and frequency offsets
  • multi-path fading
  • slowly time-varying shadowing

Shown below is an example of the channel object

Figure [fig-channel]. channel example of QPSK


Figure [fig-channel-time]. channel output constellation


Figure [fig-channel-freq]. Power spectral density

Here is a basic example program demonstrating how to use the object:

#include <liquid/liquid.h>

int main() {
    // sample buffer
    unsigned int  buf_len = 256;    // number of input samples
    float complex buf_in [buf_len]; // complex input
    float complex buf_out[buf_len]; // output buffer
    unsigned int  num_written;      // number of values written to buffer

    // create channel object
    channel_cccf channel = channel_cccf_create();

    // additive white Gauss noise impairment
    float noise_floor   = -60.0f;   // noise floor [dB]
    float SNRdB         =  30.0f;   // signal-to-noise ratio [dB]
    channel_cccf_add_awgn(channel, noise_floor, SNRdB);

    // carrier offset impairments
    float dphi          =   0.00f;  // carrier freq offset [radians/sample]
    float phi           =   2.1f;   // carrier phase offset [radians]
    channel_cccf_add_carrier_offset(channel, dphi, phi);

    // multipath channel impairments
    float * hc          = NULL;     // defaults to random coefficients
    unsigned int hc_len = 4;        // number of channel coefficients
    channel_cccf_add_multipath(channel, hc, hc_len);

    // time-varying shadowing impairments (slow flat fading)
    float sigma         = 1.0f;     // standard deviation for log-normal shadowing
    float fd            = 0.1f;     // relative Doppler frequency
    channel_cccf_add_shadowing(channel, sigma, fd);

    // print channel internals

    // fill buffer and repeat as necessary
        // apply channel to input signal
        channel_cccf_execute_block(channel, buf_in, buf_len, buf_out);

    // destroy channel

For a more detailed example, refer to examples/channel_cccf_example.c located under the main liquid project source directory.