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