Investigating Unexpected Power Increase with Analogue FDM

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SUMMARY

The forum discussion focuses on the implementation of Analogue Frequency Division Multiplexing (FDM) on top of single carrier modulation techniques such as Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK). The user models the system in MATLAB, observing a significant increase in Signal-to-Noise Ratio (SNR) from 16 dB to 22 dB when multiplexing two carriers at different frequencies (2.2 GHz and 2.7 GHz). The discussion highlights the need for higher energy in the multiplexed signal and suggests that the increase in SNR is due to the difference in modulation schemes, specifically between PSK and 4-QAM.

PREREQUISITES
  • Understanding of Analogue Frequency Division Multiplexing (FDM)
  • Familiarity with Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK)
  • Proficiency in MATLAB for signal modeling
  • Knowledge of Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER) concepts
NEXT STEPS
  • Explore MATLAB functions for simulating FDM systems
  • Research the impact of modulation schemes on SNR and BER
  • Investigate techniques for optimizing power efficiency in multiplexed signals
  • Learn about the differences between PSK and QAM modulation techniques
USEFUL FOR

Engineers and researchers working in telecommunications, particularly those involved in signal processing, modulation techniques, and system optimization for frequency division multiplexing.

thavamaran
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Hi guys, I am trying to implement Analogue FDM on top of my single carrier modulation (eg: QAM and QPSK) of Multi carrier modulation (eg: OFDM). For an example, I have two different pair of I and Q channels, where first pair is upconverted at 2.2 GHz and the next pair at say 2.7 GHz and them MUX them together as in FDM.

I model them in matlab. Then when I downconvert them, it is possible to downconvert them without Bandpass filter as its ideal case in matlab, with using correlator receiver and correlate them at exact upconverted frequency to remove the frequency component.

This all works perfectly fine, the problem is, when I transmit a signal say at 2.2 GHz without any MUX, I get SNR of 16 dB at the BER that I want, but when I MUX two different carriers and downconvert them, my SNR reaches almost 22 dB at the same BER.

Im really curious to know what would be the problem, why it needs higher energy, is there anyway that I can solve this 6 dB of extra power. Please advice.
 
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I think you will find that it is just the difference between PSK and 4-QAM. You are putting twice as many bits into the spectrum.
 

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