An explanation about DSSS Spread Spectrum Modulation and Demodulation.
In this video, Gregory explains the full topology of a Spread Spectrum demodulator, showing how the bitstream is recovered, how the code sequence is acquired, synchronized and carrier phase/offset is compensated.
Spread Spectrum modulation spreads the data signal in a larger bandwidth, using a PRBS sequence, with statistical noise like characteristics.
The process of demodulation needs precise synchronization of the local code sequence. The code phase is first acquired, in the acquisition step, and then tracked, aligning the local PRBS/LFSR using an early/late correlation topology.
Carrier phase/offset is compensated using phasor rotation in a Costas Loop like topology. The carrier phase/offset NCO is controlled by a PI Controller fed from an error detector, that forces corrections to align the constellation to the correct position.
The continuous recovered bitstream/IQ constellation can now be sampled, to determine the data symbol. A NCO running at the baudrate and a Gardner Time Error Detector in conjunction with a PI controller corrects the sampling interval/point.
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00:20 - Introduction
02:20 - Modulation details
03:30 - Theoretical Spread Spectrum receiver
05:38 - Real quadrature demodulator
06:20 - DSSS signal waveforms
08:28 - Code Acquisition
11:14 - How the correlator works
12:30 - Code Tracking
15:20 - Simulations
19:15 - Carrier phase/offset compensation
21:45 - Time recovery/synchronization
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