demod_process

Complete demodulation chain implementation (from a RF or IF signal to a binary sequence).

Calling sequence

[demod,b] = demod_process(demod,x);
[demod,b,y] = demod_process(demod,x);
[demod,b,y,dbg] = demod_process(demod,x);

Parameters

demod:

Demodulation object (can be created with the function demod_init)

x:

Input RF or IF signal

b:

Decoded binary sequence (hard-decoding)

y:

Decoded symbols, which can be used for soft-decoding (optionnal output parameter)

dbg:

Debug structure, containing the intermediate results of the demodulation process (optionnal output parameter)

Description

Proceed to demodulation of the input signal. The demodulation consists in the following steps:

  1. Downconversion to baseband (if the configured intermediate frequency is not null), including image frequency filtering.

  2. Carrier recovery: detection and removing of carrier offset and residual frequency shift (with phase detector automatically choosen according to the currently used modulation scheme, and a second order loop)

  3. Matched filtering: filter matched to the pulse shaping filter (e.g. moving average for NRZ, or RC/SRRC filter for RC/SRRC pulses, etc.)

  4. Clock recovery: using an interpolator and a timing error detector automatically choosen according to the modulation being used.

  5. Symbol demapping: conversion of the symbol sequence to a binary sequence using hard decoding (this step may not be used in the case of soft-decoding after the demodulation stage).

In case of frequency modulation (FSK, MSK, ...),

Example

// QPSK demodulator (NRZ matched filter),
// sampling frequency = 1 MHz, IF = 200 KHz, FSYMB = 20 KHz
demod = demod_init('qpsk', 1e6, 200e3, 20e3);
//
// Create a modulator to simulate RF signal
mod = mod_init('qpsk', 1e6, 200e3, 20e3);
b1 = prbs(100); // Random bit vector
[mod,x] = mod_process(mod,b1);
//
// Proceed to demodulation
[demod,b2] = demod_process(demod, x);
//
// Alignment of the two bit vectors (and phase ambiguity resolution)
[b1,b2,nerrs,ber] = cmp_bits(b1,b2,'p',demod.wf.k);
//
// Verification
if(nerrs ~= 0)
error("No noise case and error(s) are detected!");
end;


See also