Basic radio frequency
communications - 1
Session 1
Contents
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Definition and basic properties of radio waves
Electromagnetic spectrum
Radio frequency spectrum
Modulation and demodulation
Bandwidth
Basic decibel computation
2/27
Radio waves
• Wireless communications are based on
electromagnetic phenomena
• An oscillating electric field E(t) generates an
oscillating magnetic field B(t) and vice versa,
i.e. these two fields are alternate.
• Together, these oscillating fields produce an
electromagnetic wave.
3/27
Radio waves
• Maxwell (1864) mathematically predicted
existence of such waves (Maxwell’s equations)
and Hertz effectively first produced them.
• Electromagnetic radiation includes radio
wave, microwave, infrared light, visible light,
ultraviolet light, X-rays and gamma rays.
4/27
Radio waves
5/27
Radio waves
• Basic properties of electromagnetic waves (1)
– Amplitude A – the height of the wave – (V/m)
• The amplitude of the electric field is dominant, since
B=E/v, v is the wave speed (light speed c in vacuum)
– Frequency  – the number of oscillations per unit
of time (the SI unit is Hertz (Hz) – 1 oscillation per
second, i.e. s-1); period T=1/
– Wavelength – the distance the wave travels
during one period (m)
6/27
Radio waves
• Basic properties of electromagnetic waves (2)
– Speed v – the velocity of propagation of the wave
through the medium (m/s)
1
v

 is the magnetic permeability of the medium and
 is electric permittivity of the medium.
For vacuum 0  4 107 NA2  0  8,854 1012 C2 N1A 2 
c
1
0 0
 3 108 ms 1
7/27
Radio waves
• Basic properties of electromagnetic waves (3)
– Phase - The lag or lead of a wave to a reference
wave (rad)
– Polarization - The orientation of a wave’s electrical
field ()
– Intensity I – The energy propagated in a wave
(Wm-2)
8/27
Radio waves
9/27
Radio waves
10/27
Radio waves
• Wave speed v in different media
11/27
Radio waves
• The waveform need not be sine
– Sine wave – analog wave
– Other waveforms (typically square) – digital wave
12/27
Electromagnetic spectrum
13/27
Radio frequency spectrum
14/27
Radio frequency spectrum
15/27
Radio frequency spectrum
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Modulation and demodulation
• Today’s communication is mostly digital
– A digital information sequence (mostly binary) has
to be transformed into an analog signal in order to
be transmitted through a radio channel
– Such a transformation is called modulation
– Modulation techniques that are used for digital
signal transmission can be the same as for analog
signal transmission (AM – ASK, FM – FSK, PM –
PSK etc.)
17/27
Modulation and demodulation
18/27
Modulation and demodulation
• To use the radio channels in an optimal way
and to prevent jamming, spread-spectrum
techniques are often used in transmission of
digital signals.
• The most often used spread-spectrum
techniques
– Direct sequence spread-spectrum (DSSS)
– Frequency hopping spread-spectrum (FHSS)
19/27
Modulation and demodulation
• DSSS
The chip rate of the PN sequence is much higher then the data rate
20/27
Modulation and demodulation
• FHSS
21/27
Bandwidth
• Various radio signals occupy different
bandwidth
• With digital signals, this is mainly determined
by the speed of transmission
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Bandwidth
• Classification of communications according to
bandwidth
– Baseband
– Narrowband
– Wideband
– Broadband
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Bandwidth
• Baseband
– A type of data transmission in which single digital
or analog data signal is sent over the whole
available channel.
• Narrowband
– A type of data transmission, whose transmission
rate is up to 1.5 Mbit/s
– Examples: dial-up networking, fax machines
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Bandwidth
• Wideband
– Wideband transmission uses multiple channels of
a medium to provide high speed transmission
– Wideband operates between narrowband and
broadband with speed between 1.5 Mbps and
45 Mbps
• Broadband
– This is also a multiple-channel transmission, at
even higher speeds.
25/27
Basic decibel computation
• Decibel
– Logarithmic measure for comparing power levels
(example: output/input)
– N=10log10(P2/P1) [dB]
• Sometimes, the power level is compared to a
standard power level
dBm – the power relative to 1mW
• 0 dBm is the power of 1 mW
• 1 dBm is the power of 1,259 mW
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Basic decibel computation
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