QPSK modulation/demodulation spectra

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In summary, the conversation is about digital modulation and the equivalence of three different structures in terms of their resulting spectrum. The speaker has tried to design different spectra shapes to understand the spectrum of a received signal in the case of an analytic bandpass filter, but has encountered some issues with the Q-path signal and the I-path signal. They are seeking clarification on why the spectrum of quadrature modulated signals has asymmetric sidebands and why the receiver chain with a complex filter does not give the same result as the other two structures. They are also asking for book recommendations for further understanding.
  • #1
Hi everybody,
I'm reading about digital modulation (in particular on Lee-Messerschmitt's Digital Communications) and I have this picture (RX.jpeg attached). These three structures are equivalent: so, at the end of each structure, I should have the same spectrum.
I tried to think what can happen to the spectrum of a received signal in the third case (analytic bandpass filter) and I tried to design some spectra shapes ( like they do @pag.13 of this article http://www.eumus.edu.uy/eme/cursos/dsp/material/Lyons_Quadrature.Signals.pdf ) .

In the case with a complex filter I found those spectra pictured in the attached figure RXchain.jpg, but if I multiply the Q-path signal by j and then sum to the I-path signal (like they do in the paper of Lyons) I DON'T obtain the same signal that I have from the structure with two mixers+low pass filter.

So, here my questions:
1) First of all, why the spectrum of quadrature modulated signal have the two sidebands asymmetric? How do they create such signals?
(I have found this paper http://www.google.it/url?sa=t&rct=j...sg=AFQjCNG_YGRcmbHMX2i7-KkwVyFHAjJtkA&cad=rja where in Fig. 1(a) they represent some baseband spectra QPSK but I don't know if they are correct..)
2) Why the receiver chain with the complex filter doesn't give the same result as the other two structures?

Thank you if you can help me or suggest me some book that can explain!



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  • #2

1. What is QPSK modulation/demodulation and how does it work?

QPSK (Quadrature Phase Shift Keying) is a digital modulation technique used in communication systems to transmit and receive data. It works by dividing the input data into two streams, each of which is then modulated onto a separate carrier signal. These two signals are then combined and transmitted. At the receiving end, the signals are separated and demodulated, and the original data is reconstructed.

2. What is the difference between QPSK and other modulation techniques?

QPSK differs from other modulation techniques like BPSK (Binary Phase Shift Keying) and ASK (Amplitude Shift Keying) in that it uses two carriers with a phase difference of 90 degrees. This allows for four unique phase states, which can represent two bits of data at a time. This results in a higher data transmission rate compared to other modulation techniques.

3. How is the spectrum of a QPSK signal affected by different factors?

The spectrum of a QPSK signal is affected by the data rate, modulation index, and the presence of noise. As the data rate increases, the spectrum becomes wider, and the individual spectral lines get closer together. The modulation index also affects the spectrum, with higher values resulting in a wider spectrum. Finally, noise in the transmission can cause the spectral lines to spread out and become less defined.

4. What are the advantages of using QPSK modulation?

QPSK has several advantages over other modulation techniques. It offers a higher data transmission rate, making it more efficient for communication systems. It also has a higher tolerance for noise, allowing for better signal quality in noisy environments. Additionally, QPSK is a more spectrally efficient modulation technique, meaning it uses the available bandwidth more efficiently.

5. How is QPSK demodulated at the receiver?

At the receiver, the QPSK signal is first passed through a filter to remove any noise. Then, it is separated into two streams, each of which is multiplied by a local oscillator signal to shift the phase. The two resulting signals are then compared to the original carrier signals to determine the phase shift and retrieve the original data.

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