Lecture 2: Introduction to case studies: Radiolink

1. Lecture2: Introductiontocase studies: Radiolink Anders Västberg vastberg@kth.se 08-790 44 55

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

3. 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]

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

5. Radiation Only accelerating charges produce radiation [Saunders, 1999]

6. 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

7. Free Space Propagation

8. Radiation Patterns • Beam width • Front-back ratio • Side lobe level

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

10. Real antennas • Directivity, D, is equal to the maximum gain • The actual power gain of the antenna is where is the efficiency of the antenna (<1).

11. Antennas • Isotropic antenna • Omnidirectional • Directional antenna [Stallings, 2005]

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

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

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

15. Multipathpropagation [Saunders, 1999]

16. Diffraction [Saunders, 1999]

17. 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]

18. 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

19. Electromagnetic Fields Poyntings Vector: Power density:

20. 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]

21. 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, Pref. • 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]

22. decibels [Saunders, 1999]

23. 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.

24. Uppgifterinför F2 • Bestämfrekvens, vinkelfrekvens, periodtidochamplitudförföljandesinuskurva

25. Uppgifterinför F2 • Plotta följandeFourierserieochbestämtypavperiodiskfunktion. • Plottaocksåamplitudspektrum