Basic radio frequency communications - 1

Session 1

Contents

• Definition and basic properties of radio waves• Electromagnetic spectrum• Radio frequency spectrum• Modulation and demodulation• Bandwidth• Basic decibel computation

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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.

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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.

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Radio waves

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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)

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Radio waves

• Basic properties of electromagnetic waves (2)– Speed v – the velocity of propagation of the wave

through the medium (m/s)

is the magnetic permeability of the medium and is electric permittivity of the medium.For vacuum

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1

v

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c

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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)

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Radio waves

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Radio waves

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Radio waves

• Wave speed v in different media

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Radio waves

• The waveform need not be sine– Sine wave – analog wave– Other waveforms (typically square) – digital wave

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Electromagnetic spectrum

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Radio frequency spectrum

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Radio frequency spectrum

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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.)

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Modulation and demodulation

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Modulation and demodulation

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• 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)

Modulation and demodulation

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• DSSS

The chip rate of the PN sequence is much higher then the data rate

Modulation and demodulation

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• FHSS

Bandwidth

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• Various radio signals occupy different bandwidth

• With digital signals, this is mainly determined by the speed of transmission

Bandwidth

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• Classification of communications according to bandwidth– Baseband– Narrowband– Wideband– Broadband

Bandwidth

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• 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

Bandwidth

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• 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.

Basic decibel computation

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• 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

Basic decibel computation

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