Wireless Networking: Radio Propagation

1
Wireless Networking Radio Propagation

• Pravin Bhagwat

2
Understanding RF Propagation

• Goals
• Estimate radio coverage area
• Estimate link performance
• Estimate network design parameters
• Transmitters and their location
• Transmit power
• Antenna type

3
Conditions for correct reception
Pt
sensitivity threshold
Noise power
B
C
A

• S/N ratio should be above a certain threshold (a
  function of modulation scheme)
• Signal power should be above the sensitivity
  threshold
• What will happen if noise level increases?
• What will happen in presence of another
  transmitter?

4
Interesting Scenarios
At which locations will correct reception take
place?
5
Antenna Basics
High gain directional
Dipole
Isotropic
0 dBi
2.2 dBi
14 dBi
6
Free Space Propagation Model
PR
d
PT

Predict received signal strength when the
transmitter and receiver have a
clear line-of-sight path between them
Also known as Friis free space formula
7
Near Field and Far Field
PR
Pt
Good predictor when receiver is in far field (the
region beyond df)
df
near field

Example Find the near field distance for an
antenna with max dimension of 1m And operating
frequency of 900 Mhz
Answer ? at 900 Mhz 33 cm
8
Path Loss (relative measure)
PR
Pt
9
Path Loss (Example)
Assume that antennas are isotropic. Calculate
receive power (in dBm) at free space distance of
100m from the antenna. What is PR at 10Km?
PR
Pt
50 W 47 dBm
10
Path Loss (another example)
11
Path Loss (another example)
12
Quiz questions
B
C
A
Pt
2d
10d
d
PA
PB
PC

• Path loss between point A point B ?
• Path loss between point A point C ?
• Path loss between point B C ?
• If I use a 10dBi gain antenna at the transmitter
  end, by how much will the receive power increase
  at point A?

13
In real life
How signal attenuates with distance ?
14
Radio propagation path loss
near field
path loss in 2.4 Ghz band
Pr
r ? 8m
r gt 8m
Pt
near field
far field
r

Pr
path loss 10 log (4?r2/?)
r ? 8m 58.3 10 log (r3.3
/8) r gt 8m
15
Indoor Signal Measurement
16
Outdoor P2P Link Signal Measurement