MOB TME Subject 1 – Signal Transmission

Master of Computer Science 1 - MOB Mobile Internet

1/13 Baey, Fladenmuller – Subject 1

MOB TME Subject 1 – Signal Transmission

SIGNAL AND WAVES PROPAGATION

Data transmission on a physical support is the result of the propagation of a wave. The type of wave

depends on the physical support used:

 Electric wave: in the case of the voltage or electrical current ;

 Electromagnetic wave: in the case of a coupled electric and magnetic field (light, radio

wave, etc).

A wave is a function of both time variable and spaces. As an example, the electric field of a

electromagnetic wave that is propagating along an axis (x) can be represented by a sinusoidal

function of the form:

The parameter c represents the propagation speed of the wave, (c is equal to 300.000 km/s for an

electromagnetic wave which is propagated in the vacuum or the air). x is distance, x is measured by

metre (10

-3

km).

For a fixed value of x, this wave is thus represented by a sinusoidal signal Asin(20t +  ),

characterized by three parameters :

 Amplitude A : the peak value of the signal in the time, the average power is equal to

2

2

A

 Frequency f0 : the speed in which the signal repeats, expressed in cycles per second or in Hertz

(Hz) (the inverse of the frequency is called period T of signal and measured in seconds

;

 Phase φ : a measure of relative position in the time with a period of the signal, expressed in

radians (rad).

In practice, the propagating signal consists of several sinusoidal components of different

frequencies, amplitudes and phases.

MULTIPLE TRAFFIC PROPAGATION

As the waves propagate, some of them will be reflected or diffracted by obstacles in their path. The

received signal is thus composed of several copies of the emitted wave, with different delays and

attenuations compared to each other. This is called multi-path transmission. The combination of