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Radio Frequency Components

Radio Frequency Components. RF System Components: Radiators Isotropic radiation Units of power level: Decibels, Decibels - milliwatt. Signal Level Unit. Telecommunications has a standard unit for measuring signal level This unit has been used for a very long time

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Radio Frequency Components

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  1. Radio Frequency Components RF System Components: Radiators Isotropic radiation Units of power level: Decibels, Decibels - milliwatt

  2. Signal Level Unit • Telecommunications has a standard unit for measuring signal level • This unit has been used for a very long time • Initially, it was drawn up by the study of the way that the human ear responds to sounds • The human ear respond to sound in a non-linear way, but rather a logarithmic way

  3. Signal Level Unit: the Decibel • Decibel is a logarithm of a ratio. • The ratio could be calculated between a number and an arbitrary reference number. • The reference number could be 1, for example. • The ratio could be calculated between the power measured in two different places.

  4. Signal Level Unit: the Decibel Telecom System OUT IN • When the ratio is calculated between the output of a system with respect the input the resulting decibel is a measure of either gain or loss of the system

  5. decibel dB • The decibel is the fundamental unit to express signal level or amplitude in telecommunications systems. • Signal level, signal loss (attenuation) and signal gain (amplification) are all expressed in decibels. • Loss is expressed in negative terms. • Gain is expressed in positive terms. • Decibels are added and subtracted. • Numbers smaller than 1 yield negative decibels.

  6. Decibel dB Telecom System OUT IN

  7. Decibel dB Telecom System 1 Watt IN 2 Watts OUT

  8. Decibel dB Telecom System 1 Watt IN 2 Watts OUT This telecom system increases the signal level twice in magnitude or + 3 dB

  9. Some decibel values

  10. Some decibel values

  11. Decibel Exercises • Model the system into function blocks • Convert power values to decibels • Add or subtract

  12. Example Microwave Radio Antenna Conductor Medium Transmitter

  13. Example Microwave Radio Antenna Gain Losses in coax cable Transmitter

  14. Example Microwave Radio Antenna Gain 10 dB + 7 dB Losses in coax cable - 3 dB Transmitter 0 dB

  15. Example Wireless Access Point Losses in the air Antenna Gain Transmitter

  16. Example Wireless Access Point Transmitter - 10 dB Losses in The air - 6 dB (Depends on distance and objects in path) PC receiver gets -16 dB

  17. Example Wireless Access Point Receiver Sensitivity - 70 dBm @ 54 Mbps Losses in the air ? Antenna Gain 5 dBi The IR transmits 20 dBm while a receiver with a sensitivity of – 70 dBm leaves a power budget of 90 dB with no reserve Transmitter 15 dBm @ 54 Mbps

  18. RF Components • Source • Transmitter of RF signal • Radiator • Interface between system and medium • Medium • Air or vacuum • Receiver • Accepts incoming signal

  19. The transmitter • Signal is delivered to medium by the interface (antenna) Modulator Signal Amplifier Input Information Local Frequency Oscillator

  20. Transmitters in 802.11 • 802.11 • DSSS or FHSS 2.4 GHz • 802.11a • OFDM 5 GHz • 802.11b • HR-DSSS 2.4 GHz • 802.11g • ERP 2.4 GHz • 802.11n • OFDM MIMO 5 GHz • Notice this specification from Linksys • 11b uses two spread spectrum techniques DSSS and CCK • But the specs describe PSK which is a modulation system

  21. Antenna • Transmission Function: radiates RF waves • Reception Function: absorb the RF waves • Reference: isotropic radiator

  22. Isotropic Radiator • A perfect (ideal) point source that radiates RF energy with the same intensity for all directions www.eetasia.com/ART_8800504656_590626_NT_4d77...

  23. Antenna Output • Antenna output can be manipulated by: • Power Gain • Beam forming

  24. Intentional Radiator • FCC Code Federal Regulations Part 15: • …”A device that generates and emits radio frequency energy by radiation”… • IR includes: transmitter, connections, grounding, amplifiers, but not the antenna. • Power level reference is typically the dBm

  25. Equivalent Isotropically Radiated Power • FCC defines EIRP as…”the product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna”…

  26. Units of Power Level • Watts, milli-watts • Decibel dB • Decibel isotropic dBi • Decibel Dipole dBd

  27. dBi • The gain of an antenna as compared to an ideal isotropic radiator

  28. dBd • The reference of the gain is a Dipole Antenna • Standard dipole has a gain of 2.14 dBi • To convert dBd to dBi just add 2.14 • dBi = dBd + 2.14

  29. Dipole Antenna • Check this out: www-antenna.ee.titech.ac.jp/.../index.html

  30. EXERCISES

  31. Typical Model For WLAN • The typical model for a complete telecommunications system includes a source, a path, and a receiver end. • If an obstacle is present in the path, then it is added to the model. Path Loss RF out Obstacle

  32. Typical Model For WLAN • The typical model for a complete telecommunications system includes a source, a path, and a receiver end. Path Loss RF out Receiver Antenna Intentional Radiator IR Signal out Obstacle

  33. Typical Model For WLAN • This is the simplified model of the telecom system in functional blocks. • Now, the whole system can be evaluated in terms of gain and losses in decibels. Source (Transmitter) Path Obstacles Receiver

  34. Data for Exercises

  35. Data for Exercises SpecificationsThe receive sensitivity of the Aironet 4800 100mW card is: • -93dBm @ 1Mbps -90dBm @ 2Mbps -83dBm @ 5.5Mbps -80dBm @ 11Mbps • The receive sensitivity of the Aironet 4800B 30mW card is: • -90 dBm @ 1 Mbps -88 dBm @ 2 Mbps -87 dBm @ 5.5 Mbps -83 dBm @ 11 Mbps

  36. Exercise • The Signal out is 100 mW • The antenna has a gain of 10 dBi • What is the value of RF out in dB? RF out Antenna Intentional Radiator IR Signal out

  37. Exercise • The Signal out is 200 mW • The antenna has a gain of 10 dBd • What is the value of Antenna Gain in dBi? • What is the value of RF out in dBm? RF out Antenna Intentional Radiator IR Signal out

  38. Exercise • Aironet PCI4800 (Cisco 340) wireless card • The IR output power is 100mW • The card has a 2dBi rubber antenna • What is the value of RF out in dB and dBm?

  39. Exercise • The Signal out is 31 dBm • The antenna has a gain of 2 dBi • What is the value of RF out in dBm? RF out Antenna Intentional Radiator IR Signal out

  40. Exercise • Frequency = 2.412 GHz • Signal out is 30 mW • Antenna gain is 3 dBi • Find RF out in dBm RF out Antenna Intentional Radiator IR Receiver in Signal out

  41. Exercise • Frequency = 2.412 GHz • Signal out is 30 mW • Antenna gain is 3 dBi • Receiver sensitivity is -83 dBm @ 11 Mbps • Find maximum distance for link to work RF out Receiver in Antenna Intentional Radiator IR Signal out

  42. Obstacles

  43. Exercise • Frequency = 2.412 GHz • IR Signal out is 30 mW • Antenna gain is 3 dBi • Receiver sensitivity is -83 dBm @ 11 Mbps • Add a brick wall • Find maximum distance for link to work RF out Antenna Intentional Radiator IR Receiver in Signal out

  44. rssi

  45. Radio Signal Strength Indicator RSSI • PHY sub-layer energy • RSSI arbitrary, optional parameter • Vendors can do whatever they want with their RSSI value • A value from 0 to a maximum

  46. Power Levels in WLAN • In Wireless communications, strong power is not the issue. It is never that strong. • Small received power is the main concern. • Wireless clients can detect and work with extremely low power (nanowatts and picowatts)

  47. Small Power • From 0 dBm to – 100 dBm there is a delta power of only 0.00099 watts • (1 mW – 0.1 picoW = 0.0009999999999 Watts) • Most receivers have no problem listening to signal power higher than 1 mW

  48. Atheros • dBm= RSSI – 95 • Atheros makes – 95 dBm their 0% RSSI • -95 dBm is their card sensitivity • Atheros has a MAXRSSI of 60 • Source: http://www.wildpackets.com/elements/whitepapers/Converting_Signal_Strength.pdf

  49. Cisco • Cisco assigns 101 values from 0 to 100 • -10 dBm is considered 100 % RSSI • -113 dBm is considered 0 % RSSI

  50. Cisco RSSI 101 values

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