Spread Spectrum Modulation and Demodulation

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2 жыл бұрын

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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Пікірлер: 43

@AllElectronicsChannel 2 жыл бұрын

Support the channel becoming a Patron: patreon.com/allelectronics

@dominikgs 2 жыл бұрын

Very nice explanation! Especially the recovery of the phases which is often left out on literature. Thanks for explaining the concept of phase recovery 👍 Perhaps you should make another video orthogonality of the spreading sequences. Multiplexing in Frequency only works if your spreading codes are orthogonal to each other. In the end it is the same principle of mixing one signal with sine and one with cosine of the same frequency. The signals can only be recovered out of the sumsignal because sine and cosine are orthogonal to each other. Nevertheless a brilliant video 👍👍👍 Learned something.

@AllElectronicsChannel 2 жыл бұрын

Thanks!! A video about orthogonality and how the LFSR works is a good idea!

@camodudeA51 2 жыл бұрын

I've always wondered how correlation worked in DSSS, with your great explanation I now know how they work...a major thank you. Now its on to the Costas Loop.🙂

@AllElectronicsChannel 2 жыл бұрын

Thank you!!

@asharnajid1957 Ай бұрын

I wish that my College lecturer would have taught like this...... Hat's off to you Sir

@AllElectronicsChannel Ай бұрын

Thanks!

@joeyglasser2574 Ай бұрын

Great video!

@AllElectronicsChannel Ай бұрын

THanks!

@BalticLab 2 жыл бұрын

Great video, Gregory!

@AllElectronicsChannel 2 жыл бұрын

Let's thing in a topic to make a colab! 😄😄 I follow you since your beginning

@BalticLab 2 жыл бұрын

@@AllElectronicsChannel I sent you an eMail ;)

@CallistoPili Жыл бұрын

fantastic explanation, well done. I always loved to understand this such topics in deep. Even if I studied this stuff I've never find the way to use as practical daily basis. I want to make from scratch one on these intricated spread spectrum systems!!!!

@AllElectronicsChannel Жыл бұрын

Excellent!

@laurentcadet6523 2 жыл бұрын

Your videos are mindblowing !!! The way you explain things is absolutely marvelous !! Moreover, I would like to know in which case the SSM is used, based on its complexity, it should be somewhat interesting (or it would not exists....). But why is it used ? If the PRBS signal is acting like a noise, what happens if you add real white noise on top of it ? will it be still recoverable ?

@AllElectronicsChannel 2 жыл бұрын

Thanks!! The spread Spectrum modulation has some magical aspects. Think with me: what happens if we have an interfering signal close to our signal? The process of despreading in the receiver will transform this signal into noise!!! And our desired signal will come from noise and appear clear! So, non linear distortion, IP3, spurious, all affect the receiver equal no thermal noise! Over all that, we can have multiple transmitter with different code sequences, transmitting at the same frequency. The receiver will despread only the signal of interest. GPS uses spread Spectrum. All satelites transmits at the same frequency, each one with a different code! And it even more fun, it connects to multiple satellites using paralel receivers, despreading each one with the correct code. The phase different of the codes relative to each other is the difference distance between you and the satelites!! This is what is used to decode your position!

@laurentcadet6523 2 жыл бұрын

@@AllElectronicsChannel very interesting ! Thank you for your reply ! And I like this format, don't change it (simulation and signal path explanation) !!!

@Mr_ASIC 2 жыл бұрын

thank you very much

@AllElectronicsChannel 2 жыл бұрын

🤓🤓🤓

@Confucij 3 ай бұрын

Great video! Unfortunately for me, I'm still missing the point at which the spreading sequence applies. Will it matter if we apply the spreading sequence to the data stream - as what was shown on a slide at 6:26 - first and then mix it with the carrier or we should first get the modulated data stream signal and then apply the spreading sequence? As I see it it should produce the same result, shouldn't it?

@AllElectronicsChannel 3 ай бұрын

Yes, absolutely!

@southpark4151 4 ай бұрын

Nice vid! The incoming pn sequence in the spread spectrum signal is running at chip rate. For your correlation procedure with the local pn sequence, what sampling frequency do you use? eg. 10 times the chip rate? Thanks!!

@AllElectronicsChannel 4 ай бұрын

Hey! For this example, I don't remember. I think anything higher than 5x should work fine. Probably the first systems used 2x...

@southpark4151 4 ай бұрын

@@AllElectronicsChannel Thanks very much for your reply, and for mentioning higher than 5X should work. Best regards.

@varjagg 8 ай бұрын

Thanks for the educating video. What's the advantage of re-mixing a BPSK with spreading sequence instead of doing that earlier in digital domain?

@AllElectronicsChannel 8 ай бұрын

Today all the topology is commonly implemented in the digital domain! I have a video where I design a DSSS beacon, take a look. All the steps are done inside the AVR uC.

@varjagg 8 ай бұрын

@@AllElectronicsChannel Fantastic, thanks!

@Sebas_Cba_82 2 жыл бұрын

Excelente explicación, (debo traducirlo), sobre temas poco difundidos. Que libro recomienda que hable de estos temas? Ya que algunos son muy generales como el de Wayne Tomassi. Saludos desde Córdoba, Argentina

@AllElectronicsChannel 2 жыл бұрын

Thank you!

@mjs825 2 жыл бұрын

Let's assume that the receiver PRBS generator is not the same as the transmitter's, how would you change the receiver's PRBS in real time?

@AllElectronicsChannel 2 жыл бұрын

You only need to change the PRBS generator. If it is a table you change it, if it is a LFSR you change the taps

@mancio92M 2 жыл бұрын

what are the advantages in a spread spectrum system?

@AllElectronicsChannel 2 жыл бұрын

The spread Spectrum modulation has some magical aspects. Think with me: what happens if we have an interfering signal close to our signal? The process of despreading in the receiver will transform this signal into noise!!! And our desired signal will come from noise and appear clear! So, non linear distortion, IP3, spurious, all affect the receiver equal no thermal noise! Over all that, we can have multiple transmitter with different code sequences, transmitting at the same frequency. The receiver will despread only the signal of interest. GPS uses spread Spectrum. All satelites transmits at the same frequency, each one with a different code! And it even more fun, it connects to multiple satellites using paralel receivers, despreading each one with the correct code. The phase different of the codes relative to each other is the difference distance between you and the satelites!! This is what is used to decode your position!