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 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)  is the magnetic permeability of the medium and  is electric permittivity of the medium. For vacuum 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 16/27

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 19/27 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 20/27 DSSS The chip rate of the PN sequence is much higher then the data rate

Modulation and demodulation 21/27 FHSS

Bandwidth 22/27 Various radio signals occupy different bandwidth With digital signals, this is mainly determined by the speed of transmission

Bandwidth 23/27 Classification of communications according to bandwidth – Baseband – Narrowband – Wideband – Broadband

Bandwidth 24/27 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 25/27 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 26/27 Decibel – Logarithmic measure for comparing power levels (example: output/input) – N=10log 10 (P 2 /P 1 ) [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 27/27