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Lecture 2: Introduction to case studies: Radiolink Anders Västberg vastberg@kth.se 08-790 44 55

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Digital Communication System Source of Information Source Encoder ModulatorRF-Stage Channel RF-Stage Information Sink Source Decoder Demodulator Channel Encoder Digital Modulator Channel Decoder Digital Demodulator [Slimane]

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The Radio Link Design considerations –The distance over which the system meets the performance objectives –The capacity of the link. Performance determined by –Frequency –Transmitted Power –Antennas –Technology used [Black et. al]

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Propagation between two antennas (not to scale) No Ground Wave for Frequencies > ~2 MHz No Ionospheric Wave for Frequencies > ~30 Mhz

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Radiation Only accelerating charges produce radiation [Saunders, 1999]

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Antennas The antenna converts a radio frequency signal to an electromagnetic wave An isotropic antenna radiates power in all directions equally – an ideal antenna Real antennas does not perform equally well in all directions

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Free Space Propagation

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Radiation Patterns Beam width Front-back ratio Side lobe level

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Antenna Gain (maximum gain or directivity) The antenna gain is defined by its relative power density

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Real antennas

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Antennas Isotropic antenna Omnidirectional Directional antenna [Stallings, 2005]

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Transmission media Microwaves 1 GHz-100 GHz Broadcast Radio 30 MHz-1 GHz HF 3-30 MHz Infrared

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Wave Propagation Reflection –Results in multipath propagation Diffraction –Radio waves propagates behind obstacles Scattering –Rough surfaces scatter radio wave in a multitude directions

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Reflection (R), Diffraction (D) and Scattering (S) [Stallings, 2005]

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Multipath propagation [Saunders, 1999]

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Diffraction [Saunders, 1999]

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Diffraction For radio wave propagation over rough terrain, the propagation is dependent on the size of the object encountered. Waves with wavelengths much shorter than the size of the object will be reflected Waves with wavelengths much larger than the size of the obstacle will pass virtually unaffected. Waves with intermediate wavelengths curve around the edges of the obstacles in their propagation (diffraction). Diffraction allows radio signals to propagate around the curved surface and propagate behind obstacles. [Slimane]

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Maxwell's Equations Electrical field lines may either start and end on charges, or are continuous Magnetic field lines are continuous An electric field is produced by a time-varying magnetic field A magnetic field is produced by a time-varying electric field or by a current

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Electromagnetic Fields Poyntings Vector: Power density:

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Impedance of Free Space Both fields carry the same amount of energy Free space impedance is given by The power density can be expressed as [Slimane]

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decibels The bel is a logarithmic unit of power ratios. One bel corresponds to an increase of power by a factor of 10 relative to some reference power, P ref. The bel is a large unit, so that decibel (dB) is almost always used: The above equation may also be used to express a ratio of voltages (or field strengths) provided that they appear across the same impedance (or in a medium with the same wave impedance): [Saunders, 1999]

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decibels UnitReference PowerApplication dBW1 WAbsolute power dBm1 mWAbsolute power P [dbW] = P [dBm] - 30 dB V1 V Absolute voltage, typically at the input terminals of a receiver dBanyGain or loss of a network dB V/m1 V/m Electric field strength dBiPower radiated by and isotropic reference antenna Gain of an antenna dBdPower radiated by a half-wave dipole Gain of an antenna 0 dBd = 2.15 dBi [Saunders, 1999]

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dB Problems Convert the following to linear scale: 3 dB, -6 dB, 10 dB, 20 dB, 23 dB, -30 dB Convert the following to dBm and mW: -3 dBW, 0 dBW, 20 dBW, -10 dBW. Convert 22 mW to dBW and 63 to dB. Convert 15 dB to linear scale. 23

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Uppgifter inför F2 Bestäm frekvens, vinkelfrekvens, periodtid och amplitud för följande sinuskurva 24

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Uppgifter inför F2 25

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