Wave Propagation including Free Space Path Loss and Delay Spread
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Wave Propagation
- Wave propagation is the way in which the RF wave moves as it moves away from an antenna.
- An RF signal broadens and spreads as it travels farther away from the antenna.
- How the wave is propagated can vary drastically depending on the materials in the signal’s path; for example, drywall will have a much different effect on an RF signal than metal or concrete.
- An excellent analogy is an earthquake. In an earthquake, the concentric seismic rings that propagate away from the epicenter of the earthquake.
- RF waves behave in much the same fashion.
Free Space Path Loss (FSPL)
- An electromagnetic signal will attenuate as it travels, despite the lack of attenuation caused by obstructions, absorption, reflection, diffraction, and so on.
- Free space path loss (FSPL) is the loss of signal strength caused by the natural broadening of the waves.
- RF signal energy spreads over larger areas as the signal travels farther away from an antenna, and as a result, the strength of the signal attenuates.
- Loss in signal strength is logarithmic and not linear; thus the amplitude does not decrease as much in a second segment of equal length as it decreases in the first segment.
- A 2.4 GHz signal will change in power by about 80 dB after 100 meters but will lessen only another 6 dB in the next 100 meters.
- Formulas to calculate free space path loss (not required for CWNA, for reference only):
Miles
FSPL = 36.6 + (20log10( f)) + (20log10( D))
FSPL = path loss in dB
f = frequency in MHz
D = distance in miles between antennas
Kilometers
FSPL = 32.44 + (20log10( f)) + (20log10( D))
FSPL = path loss in dB
f = frequency in MHz
D = distance in kilometers between antennas
6 dB rule
- A simple way to estimate free space path loss (FSPL) is called the 6 dB rule.
- The 6 dB rule states that doubling the distance will result in a loss of amplitude of 6 dB. The table below shows estimated path loss and confirms the 6 dB rule.
Receiver Sensitivity
- All radio devices have what is known as a receive sensitivity level. The radio receiver can properly interpret and receive a signal down to a certain fixed amplitude threshold.
- If a radio receives a signal above its amplitude threshold, the signal is powerful enough for the radio to sense and interpret the signal.
- If the amplitude of a received signal is below the radio’s receive sensitivity threshold, the radio can no longer properly sense and interpret the signal.
- The signal must be louder than any background noise.
- In addition to the radio being able to receive and interpret a signal, the received signal must be not only strong enough to be heard but also strong enough to be heard above any RF background noise, typically referred to as the noise floor.
- You must make sure that the RF signal will not attenuate below the receive sensitivity level of your WLAN radio simply because of free space path loss, and you must make sure that the signal does not attenuate near or below the noise floor.
Delay Spread
- The difference in time between the primary signal and the reflected signals arriving at the receiver is known as delay spread.
- Delay spread occurs when the phenomena known as multipath is present.
- The delay spread is the time differential between multiple paths of the same signal. Normal delay spread is 50 nanoseconds to 100 nanoseconds, and a maximum delay spread is about 200 nanoseconds.
- Delay spread may also be too significant and cause data bits to be corrupted, resulting in excessive layer 2 retransmissions.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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