Tuesday, December 15, 2015

RF Fundamentals

Adjacent Channel Interference vs Co-Channel Congestion

  • In WLAN industry, an adjacent channel is considered to be the next or previous numbered channel. Devices in adjacent channels will corrupt each other packets hence, adjacent channel interference is dangerous.
  • When all devices are in same channel then each device will cooperate using CSMA/CA. This can create congestion but devices will not corrupt each others packet hence, it is much better than adjacent channel interference.

Side Band Emissions

Guard Band

Active Gain vs Passive Gain

  • Amplifiers provide active gain
  • Antennas provide passive gain and is achieved by focusing the RF signal in specific direction.

Free Space Path Loss: 
  • Loss of signal strength (due to broadening of wave) as the signal moves away from the source.
  • It is logarithmic and not linear. Thus loss is lesser as we move away from the source.
  • It depends on "Frequency of operation" & "Distance". Higher the frequency, bigger is the loss. Hence, 5 GHz signal decays faster than 2.4 GHz. 
  • 6 dB rule: Doubling the distance will result in a loss of amplitude of 6dB.
  • For 2.4 GHz signal, this loss is nearly 80 dB at 100 meters (87 dB for 5.8 GHz). For next 100 meters, additional loss will be 6 dB. Hence, if the radiated power is 30 dBm (1W), then received signal at 100 meters will be -50 dBm (10 nW). Therefore, in 100 metres, the RF signal loses  99.9999% of the power.
  • Calculator: http://www.changpuak.ch/electronics/calc_10.php 

  • Loss of signal strength due to the absorption by the propagation medium.
  • It depends on type of medium and length of medium. E.g. 
    • Water bodies absorb a large amount of signal. However, rain, snow, and fog attenuation are very small for frequencies under 10 GHz. Moisture in the propagation medium will increase this loss.
    • Human body which has large water content also absorbs (3 dB for 2.4 GHz and 5 dB for 5.8 GHz). Hence, due to large number of people around (e.g full stadium vs empty stadiums) there will be good amount of absorption.
    • Material of wall (wooden or concrete) will also impact it. 
    • Thickness of wall will directly impact the loss due to absorption.

Receive Sensitivity
  • Indicates the weakest signal the receiver can reliably decode.
  • It depends on modulation and bit rate.

Link Budget Analysis

  • Radiated Power (EIRP) = Radio Transmit Power (dBm) – Cable/Connector/Switch Loss (dB) at Transmitter + Transmit Antenna Gain (dBi)
  • Received Power = Radiated Power/EIRP – Path Loss (free space + absorption + others) + Receiver Gain
  • Receiver Gain = Receive Antenna Gain (dBi) - Cable/Connector/Switch Loss (dB) at Receiver
  • Link Budget = Received Power – Receive Sensitivity
  • Note: Design should take care that link budget for highest bit rate (desired) is at least 0 dB. Leaving a margin of 10 dB for variations is a good practice.
  • Every smooth surface reflects some part of the signal falling on it. 
  • Anything made of metal will absolutely cause reflection.
  • Glass is a highly reflective material.
  • Reflection & Multi-path plays a negative role in 802.11 a/b/g radios however, they are constructively used in MIMO.

  • When wave strikes an uneven surface (such as tree foliage, rocky terrain) it gets scattered in multiple directions. 
  • It can cause substantial signal downgrade. 

  • Change of wave direction when passing through a medium
  • RF refraction mainly occurs as a result of atmospheric conditions (such as water vapors, changes in air temperature & changes in air pressure).
  • Only in long distance outdoor wireless bridge deployments, refraction can be an issue.
  • Diffraction is mainly caused by partial blockage of the RF signal.
  • Wave that encounters the obstruction bend around the object.
  • Analogy: Rock sitting in the middle of the river. 

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