Lecture 2

1. Lecture 2 RF fundamentals

2. Objectives • Explain the principals of radio wave transmissions • Describe the basic types of modulation

3. Radio Wave Transmission Principles • Understanding principles of radio wave transmission is important for: • Troubleshooting wireless LANs • Creating a context for understanding wireless terminology

4. What Are Radio Waves? • Electromagnetic wave: Travels freely through space in all directions at speed of light • Radio wave: When electric current passes through a wire it creates a magnetic field around the wire • As magnetic field radiates, creates an electromagnetic radio wave • Spreads out through space in all directions • Can travel long distances • Can penetrate non-metallic objects

5. What Are Radio Waves? (continued) Table 3-1: Comparison of wave characteristics

6. Analog vs. Digital Transmissions Figure 3-2: Analog signal Figure 3-4: Digital signal

7. Analog vs. Digital Transmissions (continued) • Analog signals are continuous • Digital signals are discrete • Modem (MOdulator/DEModulator): Used when digital signals must be transmitted over analog medium • On originating end, converts distinct digital signals into continuous analog signal for transmission • On receiving end, reverse process performed • WLANs use digital transmissions

8. Frequency Figure 3-5: Long waves Figure 3-6: Short Waves

9. Frequency (continued) • Frequency: Rate at which an event occurs • Cycle: Changing event that creates different radio frequencies • When wave completes trip and returns back to starting point it has finished one cycle • Hertz (Hz): Cycles per second • Kilohertz (KHz) = thousand hertz • Megahertz (MHz) = million hertz • Gigahertz (GHz) = billion hertz

10. Frequency (continued) Figure 3-7: Sine wave

15. Frequency (continued) Table 3-2: Electrical terminology

16. Frequency (continued) • Frequency of radio wave can be changed by modifying voltage • Radio transmissions send a carrier signal • Increasing voltage will change frequency of carrier signal

17. Frequency (continued) Figure 3-8: Lower and higher frequencies

18. Modulation • Carrier signal is a continuous electrical signal • Carries no information • Three types of modulations enable carrier signals to carry information • Height of signal • Frequency of signal • Relative starting point • Modulation can be done on analog or digital transmissions

19. Analog Modulation • Amplitude: Height of carrier wave • Amplitude modulation (AM): Changes amplitude so that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bit • Frequency modulation (FM): Changes number of waves representing one cycle • Number of waves to represent 1 bit more than number of waves to represent 0 bit • Phase modulation (PM): Changes starting point of cycle • When bits change from 1 to 0 bit or vice versa

20. Carrier frequency • A carrier frequency is an electronic wave that is combined with the information signal and carries it across the communications channel.

21. Modulation

22. Frequency (continued) • Radio transmissions send a carrier signal

23. Analog Modulation (continued) Figure 3-9: Amplitude

24. Analog Modulation (continued) Figure 3-10: Amplitude modulation (AM)

25. Analog Modulation (continued) Figure 3-11: Frequency modulation (FM)

26. Analog Modulation (continued) Figure 3-12: Phase modulation (PM)

27. Digital Modulation • Advantages over analog modulation: • Better use of bandwidth • Requires less power • Better handling of interference from other signals • Error-correcting techniques more compatible with other digital systems • Unlike analog modulation, changes occur in discrete steps using binary signals • Uses same three basic types of modulation as analog

28. Digital Modulation (continued) Figure 3-13: Amplitude shift keying (ASK)

29. Digital Modulation (continued) Figure 3-14: Frequency shift keying (FSK)

30. Digital Modulation (continued) Figure 3-15: Phase shift keying (PSK)

31. Radio Frequency Behavior: Gain • Gain: Positive difference in amplitude between two signals • Achieved by amplification of signal • Technically, gain is measure of amplification • Can occur intentionally from external power source that amplifies signal • Can occur unintentionally when RF signal bounces off an object and combines with original signal to amplify it

32. Online simulation • http://williams.comp.ncat.edu/Networks/modulate.htm

33. Online simulation • http://digitalmodulation.net/fsk.html

34. Simulation • http://digitalmodulation.net/psk.html

35. More simulations • http://sem.mosaic-service.com/electron2/amplitude_shift_keying.htm

36. Radio Frequency Behavior: Gain (continued) Figure 3-16: Gain

40. Radio Frequency Behavior: Loss • Loss: Negative difference in amplitude between signals • Attenuation • Can be intentional or unintentional • Intentional loss may be necessary to decrease signal strength to comply with standards or to prevent interference • Unintentional loss can be cause by many factors

41. Radio Frequency Behavior: Loss (continued) Figure 3-18: Absorption

43. Radio Frequency Behavior: Loss (continued) Figure 3-19: Reflection

45. Radio Frequency Behavior: Loss (continued) Figure 3-20: Scattering

47. Radio Frequency Behavior: Loss (continued) Figure 3-21: Refraction

49. Radio Frequency Behavior: Loss (continued) Figure 3-22: Diffraction