DSSS Modulation and Demodulation Direct sequence spread spectrum (DSSS or often just "spread spectrum") is a variation of the DSBSC modulation scheme with a pulse train (called a pseudo-noise sequence or just PN sequence) for the carrier instead of a simple sinewave. This may sound radical until you remember that pulse trains are actually made up of a theoretically infinite number of sinewaves (the fundamental and harmonic). That being the case, spread spectrum is really the DSBSC modulation of a theoretically infinite number of sinusoidal carrier signals. The result is a theoretically infinite number of pairs of tiny sidebands about a suppressed carrier.
Pseudo Noise (PN) code With DSSS, the message signal is used to modulate a bit sequence known as a Pseudo Noise (PN) code; this PN code consists of a radio pulse that is much shorter in duration (larger bandwidth) than the original message signal. This modulation of the message signal scrambles and spreads t he pieces of data, and thereby resulting in a bandwidth size nearly identical to that of the PN sequence.
Fig 1 : DSSS Modulation and Demodulation block diagram
Spread spectrum signals are demodulated in the same way as DSBSC signals using a product detector. Importantly, the product detector's local carrier signal must contain all the sine waves that make up transmitter's pulse train at the same frequency and phase. If this is not done, the tiny demodulated signals will be at the wrong frequency and phase and so they won't add up to reproduce the original message. Instead, they'll produce a garbage signal t hat looks like noise.
Equipments needed for this experiment
Emona Telecoms-Trainer 101 (plus power-pock)
Dual channel 20MHz oscilloscope
Two Emona Telecoms-Trainer 101 oscilloscope leads
Assorted Emona Telecoms-Trainer 101 patch leads
Part A - Generating a DSSS signal using a simple message As DSSS is basically just DSBSC with a pulse train for the carrier instead of a simple sinusoid, it can be generated, by implementing the mathematical model for DSBSC. Setup the Emona board according to the block diagram on Fig 2.
Fig 2 : DSSS Modulation block diagram
Locate the Sequence Generator module and set its dip-switches to 00. The multiplier multiplies the 2kHz sine wave message from master signal with a PN sequence modelled by the Sequence Generator's 32-bit pulse train output whose clk is 100Khz digital signal which is also taken from the master signal.
The graphs obtained are:
Fig 3 : The message signal (2kHz sine) is red and the red a nd the DSSS signal is blue
Fig 4: Sequence (red) and modulated (blue) signal.
Question 1 What feature of the Multiplier module's output suggests that it's basically a DSBSC Signal?
Answer: As the DSSS signal exists on both sides of about center. Both positive and negative envelopes are formed.
Question 2 Why is the DSSS signal so large when it’s supposed to be small and indistinguishable from noise?
Answer: This is because usually the PN sequence is made of mor e bits, as the PN sequence here is used for 31-bit code, it’s more distinguishable. If larger no. of bits were used than it wouldn’t be recognizable. .
Part B -Generating a DSSS signal using speech So far, this experiment has generated a DSSS signal using a sine wave for the message. The next part of the experiment lets you see what a DSSS signal looks like when modulated by speech. The block diagram is the same but now the message signal is taken from the speech mo dule not the sequence generator. The graphs obtained are:
Fig 5: Speech (blue) and modulated (red) DSSS signal
Question 3 Why isn't there any signal out of the DSSS modulator when you're not t alking, etc?
Answer: There will be no input signal is in the speech module and since the PN sequence has nothing to multiply with, so there is no output signal.
Part C - Using the product detector to recover the message Setup the Emona board according to the block diagram on Fig 6 below. In the Tunable Low-pass Filter module set its Gain control to about the middle of its travel. Turn the Tunable Low-pass Filter module's Cut-off Frequency Adjust control fully anticlockwise.
Fig 6 : Demodulation block diagram
The Multiplier and Tunable low-pass Filter modules implement a product Detector which recovers the original message from the DSSS signal. To facilitate this, the PN sequence used for the modulator's carrier is "stolen" for the product detector's local carrier (though it's stolen from the modules X output but the sequence is the same). The graphs obtained are:
Fig 7: Message (red) and Demodulated (blue) signal
Using a different PN sequence and observe the output given below,
Fig 8: Message (red) and Demodulated (blue) signal
Question 4 What does the signal out of the low-pass filter look like?
Answer: The message signal, 2 kHz sine wave.
Question 5 Why does using the wrong PN sequence for the local carrier cause the product detector's output to look like this?
Answer: It will be distorted as only the PN sequence used during modulation has to be used in product detector to get back the message signal.
Part D - DSSS and deliberate interference (jamming) Interference occurs when on unwonted electrical signal gets added to the transmitted signal (typically in the channel) and changes it enough to change the recovered message. Electrical noise is a significant source of unintentional interference. However, sometimes noise is deliberately added to the transmitted signal for the purpose of interfering or "jamming" it. The next port of the experiment models deliberate interference to show how spread spectrum signals are highly resistant to it.
Fig 9: Message (red) and Demodulated (blue) signal
Move the patch lead from the Sequence Generator to its X output. The VCO produces different frequencies. Set the Adder module's adds the different frequency to the modulated signal. The product detector is recovering the message again. This modification forces the VCO module’s output to sweep continuously through a wide range of Frequencies. The graphs obtained are:
Fig 10: Message signal (red) and Adder output signal (blue)
Fig 11: Message signal (red) and demodulation output aft er jamming (blue)
An even more sophisticated approach to jamming involves using jamming signals at once to increase the chances of upsetting the transmitted signal. The next part of the experiment let’s you see how spread spectrum handles this. Noise is added to modulated signal.
Fig 12: Noise is taken from Noise Generator and added to modulated signal.
The graphs obtained are:
Fig 13: Message signal (red)and noise (0 dB) (blue)
Fig 14: Message signal (red) and Demodulation output (f or 0 dB noise) (blue)
Fig 15: Message signal (red) and noise (-6 dB) (blue)
Fig 16: Message signal (red) and Demodulation output ( for -6 dB noise) (blue)
Fig 17: Message signal (red) and noise (-20 dB) (blue)
Fig 18: Message signal (red) and Demodulation output ( for -20 dB noise) (blue)
Question 6 Why doesn't the jamming signal interfere with the recovery of the message?
Answer: Due to PN sequence, information is spread over a wide range of frequencies, it is not possible to jam the signal by a certain frequency. Even if tried , the message can be easily recovered.
Discussion In our lab we learned in practice, not all of these sidebands have any energy of significance. However, the fact that the message information is distributed across so many of them makes spread spectrum signals difficult to deliberately interfere with o r "jam". Due to this spreading It’s protected from jamming and the addition of noise (the information is well encrypted). This is one of the reasons it is usually used in the military. Here we used a product detector (with a stolen carrier) to reproduce the message. Here we should remember that the leads were connected to the Multiplier module's AC inputs and not its DC inputs.
United International University EEE 456 Digital Communication Lab
Group: 04
Experiment-6 Experiment name: DSSS Modulation and Demodulation Submitted by:
