Fundamentals of Microwave Technologies

1. Fundamentals of Microwave Technologies

2. Historical Perspective • Founded during WWII. • Used for long-haul telecommunications. • Displaced by fiber optic networks. • Still viable for right-of-way bypass and geographic obstruction avoidance.

3. Wireless Transmission • Transmission and reception are achieved by means of an antenna . • Directional : • Transmitting antenna puts out focused beam . • Transmitter and receiver must be aligned . • Omnidirectional “Isotropically” : • Signal spreads out in all directions . • Can be received by many antennas .

4. Wireless Examples • Terrestrial microwave transmission . • Satellite transmission . • Broadcast radio . • Infrared .

5. Terrestrial Microwave • Used for long-distance telephone service . • Uses radio frequency spectrum, from 2 to 40 GHz . • Parabolic dish transmitter, mounted high . • Used by common carriers as well as private networks . • Requires unobstructed line of sight between source and receiver . • Curvature of the earth requires stations (repeaters) ~30 miles apart .

6. Microwave Applications • Television distribution . • Long-distance telephone transmission . • Private business networks .

7. Microwave • Advantages : • No cabling needed between sites . • Wide bandwidth . • Multichannel transmissions . • Disadvantages : • Line of sight requirement . • Expensive towers and repeaters . • Subject to interference -e.g. passing airplanes, rain .

8. Satellite Microwave Transmission • A microwave relay station in space . • Can relay signals over long distances .

9. Wireless Technologies Microwave • Microwave systems transmit voice and data through the atmosphere as super-high-frequency radio waves • One particular characteristic of the microwave system is that it cannot bend around corners; therefore microwave antennas must be in "line of sight" of each other • The following are some of the characteristics of the microwave system: • High Volume • Long distance transmission • Point to point transmission • High frequency radio signals are transmitted from one terrestrial transmitter to another • Satellites serve as a relay station for transmitting microwave signals over very long distances. See image next slide

10. Wireless Technologies Low-Orbit Satellite and Microwave Transmission

11. Microwave Spectrum • Range is approximately 1 GHz to 40 GHz • Total of all usable frequencies under 1 GHz gives a reference on the capacity of in the microwave range.

12. Microwave Impairments • Equipment, antenna, and waveguide failures. • Fading and distortion from multipath reflections. • Absorption from rain, fog, and other atmospheric conditions. • Interference from other frequencies.

13. Microwave Engineering Considerations • Free space & atmospheric attenuation. • Reflections. • Diffractions. • Rain attenuation.

14. Microwave Engineering Considerations • Skin affect • Line of Sight (LOS) • Fading • Range • Interference

15. Free Space & Atmospheric Attenuation • Free space & atmospheric attenuation is defined by theloss the signal undergoes traveling through the atmosphere. Changes in air density and absorption by atmospheric particles.

16. Reflections • Reflections can occur as the microwave signal traverses a body of water or fog bank; cause multipath conditions

17. Diffraction • Diffraction is the result of variations in the terrain the signal crosses

18. Rain Attenuation • Raindrop absorption or scattering of the microwave signal can cause signal loss in transmissions.

19. Skin Affect • Skin Affect is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it any great distance. Skin Affect determines the properties of microwave signals.

20. Line of SightFresnel Zone Clearance • Fresnel Zone Clearance is the minimum clearance over obstacles that the signal needs to be sent over. Reflection or path bending will occur if the clearance is not sufficient.

21. LOS & FZC-cont’d Fresnel Zone D2 D1 D1 X D2 F x D 72.2 secret formula

22. Microwave Fading Normal Signal Reflective Path Caused by multi-path reflections and heavy rains

23. Range • The distance a signal travels and its increase in frequency are inversely proportional. • Repeaters extend range: • Back-to-back antennas. • Reflectors.

24. Range-cont’d • High frequencies are repeated/received at or below one mile. • Lower frequencies can travel up to 100 miles but 25-30 miles is the typical placement for repeaters.

25. Interference • Adjacent Channel Interference. • Digital not greatly affected. • Overreach • Caused by signal feeding past a repeater to the receiving antenna at the next station in the route. Eliminated by zigzag path alignment or alternate frequency use between adjacent stations.

26. Components of a Microwave System • Digital Modem. • Radio Frequency (RF) Unit. • Antenna.

27. Digital Modem • The digital modem modulates the information signal (intermediate frequency or IF).

28. RF Unit • IF is fed to the RF unit which is mounted as close physically to the antenna as possible (direct connect is optimal).

29. Antenna • The antenna is a passive device that radiates the modulated signal. It is fed by direct connect of the RF unit, coaxial cable, or waveguides at higher frequencies.

30. Waveguides Waveguidesare hollow channels of low-loss material used to direct the signal from the RF unit to the antenna.

31. Modulation Methods • Primarily modulated today with digital FM or AM signals. • Digital signal remains quiet until failure threshold bit error rate renders it unusable.

32. Bit Error Rate (BER) • The BER is a performance measure of microwave signaling throughput • 10 or one error per million transmitted bits of information. • Data fail over is at 10 ; voice traffic can withstand this error rate.

33. Diversity • Space Diversity • Frequency Diversity • Hot Standby • PRI

34. Space Diversity Normal Signal Faded Signal Transmitter Receiver

35. Space Diversity-cont’d • Space Diversity protects against multi-path fading by automatic switch over to another antenna place below the primary antenna. This is done at the BER failure point or signal strength attenuation point to the secondary antenna that is receiving the transmitted signal at a stronger power rating.

36. Frequency Diversity RCVR Frequency #1 Active XTMR Frequency #1 RCVR Frequency #2 Protect XTMR Frequency #2 Transmitter Receiver

37. Frequency Diversity-cont’d • Frequency Diversity uses separate frequencies (dual transmit and receive systems); it monitors primary for fail over and switches to standby. Interference usually affects only one range of frequencies. Not allowed in non-carrier applications because of spectrum scarcity.

38. Hot Standby* Active RCVR #1 System XTMR Primary #1 Standby RCVR #2 System XTMR Standby #2 failure switch Transmitter Receiver *Hot standby is designed for equipment failure only

39. PRI System Transmission Facilities System Receiver Facilities Connect to PRI interface & PSTN Connect to PRI interface & PSTN Transmitter Receiver To PSTN To PSTN

40. Availability Formula Percent Availability equals: 1 – (outage hours/8760 hours per year) Private microwaves have 99.99% availability

41. Microwave Path Analysis • Transmitter output power • Antenna gain • proportional to the physical characteristics of the antenna (diameter) • Free space gain • Antenna alignment factor • Unfaded received signal level

42. Microwave Radio Applications