Although there is a bunch of articles and videos on the internet about the transmission of simple sinusoidal signals over various Radio Frequency(RF) propagation channels, there was almost no proper documentation related to modelling transmission of voice signals. So I thought to write this article which describes a simple model designed by me.

This article is a part of a complete document related to Modelling Propagation Losses and MANETs which was composed by me and one of my colleagues as a continuous assessment of EN2053-Communication Systems and Networks module. The full documentation related to this post is available here and reasons for the values used below are well explained there. All the MATLAB codes, Simulink models and voice samples can be found at the repository on GitHub linked at the end of this post.

Logic and Assumptions Used to implement the Propagation Model

Following figure shows the variation of the Total Path Loss with the Frequency in a modulated Voice Signal.

By inspecting the above figure, it can be observed that the total Path Loss of the Frequency Modulated Voice Signal, over its frequency range( from 50e9 - 4e3 to 50e9 + 4e3 )Hz changes only by a value less than 0.000025 dB which is almost Zero. Therefore in a real world application it can be assumed that the total path loss of the modulated voice signal is as same as that of the carrier wave(50 GHz) alone. That is, path loss variation due to the frequency variation in the above range can be neglected and can be assumed as a constant of 214.6240 dB which is the total path loss corresponding to the 50 GHz wave. Therefore for the following model, total path loss of the signal was taken as 214.6240 dB and it was included in the Free Space Loss block.

In addition to that since the attenuation affects only to the amplitude of the signal, actual scenario of the Frequency Modulation of the voice signal was not considered and instead the voice signal was FM modulated using the FM Modulator Baseband block which is an ideal option for simulation purposes.

RF Propagation Model - Simulink

Steps of the simulation are as follows:

  1. Input the voice.wav file of 8000Hz sampling rate using From Multimedia File block.
  2. Normalization of the voice signal using Normalization block.
  3. Frequency modulating the signal using FM Modulator Baseband block.
  4. Addition of the -3 dB Cable loss at Transmitter using dB Gain block.
  5. Addition of the 30 dB transmitter gain using the Transmitter block.
  6. Addition of the effect of noise using AWGN channel block.
  7. Addition of the -214.6240 dB propagation loss using Free Space Path Loss block.
  8. Addition of the 24.77 dB receiver gain using Receiver Preamp block.
  9. Addition of the -4 dB Cable loss at the Receiver using dB Gain block.
  10. Demodulating the signal using FM Demodulator Baseband block.
  11. Saving the output voice signal using To Multimedia File block.
  12. Convert the output signal into audio using Audio Device Writer block.

Before run the simulation directories related to the voice input and output must be set correctly.

RF Propagation Model - Simulink

Frequency Spectrum of the signal at Various States

Find this on GitHub