Transcript Radio Propagation In A Nutshell

Ron Milione Ph.D.
W2TAP
This presentation concentrates
on the propagation portion
Ant
Transmitter
Information
Modulator
Amplifier
Filter
Feedline
RF Propagation
Ant
Receiver
Information
Demodulator
Pre-Amplifier
Filter
Feedline

As the wave propagates, the
surface area increases
 The power flux density
decreases proportional to
1/d2
• At great enough distances
from the source, a portion of
the surface appears as a
plane
• The wave may be modeled
as a plane wave
• The classic picture of an EM
wave is a single ray out of
the spherical wave

Most real antennas do not
radiate spherically
 The wavefront will be
only a portion of a sphere
• The surface area of the wave
Gain in
this area
is reduced
• Power density is increased!
• The increase in power
density is expressed as
Antenna Gain
• dB increase in power along
“best” axis
• dBi = gain over isotropic
antenna
• dBd = gain over dipole
antenna

Radiated power often referenced to power
radiated by an ideal antenna
• The isotropic radiator radiates power uniformly in all
directions
• Effective Isotropic Radiated Power calculated by:
EIRP Pt Gt
The exact same formulas and
principles apply on the
receiving side too!
Pt = power of transmitter
Gt = gain of transmitting antenna system
Gt = 0dB = 1 for isotropic antenna
This formula assumes power and gain is expressed linearly. Alternatively,
you can express power and gain in decibels and add them: EIRP = P(dB) + G(dB)
• Large-scale (Far Field) propagation model
• Gives power where random environmental effects
have been averaged together
• Waves appear to be plane waves
• Far field applies at distances greater than the
Fraunhofer distance:
df 
2D 2

D = largest physical dimension of antenna
 = wavelength
• Small-scale (Near Field) model applies for shorter
distances
• Power changes rapidly from one area/time to the next
For Free Space (no buildings, trees, etc.)
Pt (4d ) 2 (4fd ) 2
lossFree(lin)  

2
Pr

c2
f = frequency
d = distance (m)
= wavelength (m)
c = speed of light
 4fd 
lossFree(dB)  10log10 
  20log10 f  20log10 d  147.56dB
 c 
2
For Urban environments, use the Hata model
lossHata(dB)  69.55  26.16(log10 f  6)  13.82log10 hb  a(hm )
 (44.0  6.55log10 hb )(log10 d  3)
hb = base station antenna height (m)
hm = mobile antenna height (m)
a(hm) is an adjustment factor for the type of environment and the
height of the mobile.
a(hm) = 0 for urban environments with a mobile height of 1.5m.
Note: Hata valid only with d in range 1000-20000, hb in range 30-200m
A transmission system transmits a signal at 960MHz with a power of 100mW using
a 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna.
At what distance can far-field metrics be used?
 = 3.0*108 m/s / 960MHz = 0.3125 meters
Fraunhofer distance = 2 D2/  = 2(0.16m)2/0.3125 = 0.16m
What is the EIRP?
Method 1: Convert power to dBm and add gain
Power(dBm) = 10 log10 (100mW / 1mW) = 20dBm
EIRP = 20dBm + 2.15dB = 22.15dBm
Method 2: Convert gain to linear scale and multiply
Gain(linear) = 102.15dB/10 = 1.64
EIRP = 100mW x 1.64 = 164mW
Checking work: 10 log10 (164mW/1mW) = 22.15dBm
A transmission system transmits a signal at 960MHz with a power of 100mW
using a 16cm dipole antenna system with a gain of 2.15dB over an isotropic
antenna.
What is the power received at a distance of 2km (assuming free-space
transmission and an isotropic antenna at the receiver)?
Loss(dB) = 20 log10(960MHz) + 20 log10(2000m) – 147.56dB
= 179.6dB + 66.0dB – 147.56dB = 98.0dB
Received power(dBm) = EIRP(dB) – loss
= 22.15dBm – 98.0dB = -75.85dBm
Received power(W) = EIRP(W)/loss(linear)
= 164mW / 1098.0dB/10 = 2.6 x 10-8 mW = 2.6 x 10-11 W
Checking work: 10 -75.85dBm/10 = 2.6x 10-8 mW
What is the power received at a distance of 2km (use Hata model with base
height 30 m, mobile height 1.5 m, urban env.)?
Loss(dB) = 69.55+26.16(log(f)-6) – 13.82(log(hb)) – a(hm)+ 44.9-6.55(log(hb))(log(d)-3)
=69.55 + 78.01 – 27.79 – 0 + (35.22)(0.30)
= 130.34 dB  Received power = 22.15dBm – 130.34dB = -108.19dBm
Gain
Ant
Transmitter
Loss
Information
Modulator
Amplifier
Filter
Feedline
RF Propagation
Ant
Receiver
Information
Demodulator
Pre-Amplifier
Filter
Feedline
Gain

A Link Budget analysis determines if there is
enough power at the receiver to recover the
information


Begin with the power output of the transmit amplifier
 Subtract (in dB) losses due to passive components in the transmit
chain after the amplifier
 Filter loss
 Feedline loss
 Jumpers loss
 Etc.
 Add antenna gain
 dBi
Result is EIRP
Ant
Transmitter
Information
Modulator
Amplifier
Filter
Feedline
RF Propagation
All values are example values
Component
Value
Scale
44
dBm
Filter loss
(0.3)
dB
Jumper loss
(1)
dB
(1.5)
dB
Antenna gain
12
dBi
Total
53
dBm
Power Amplifier
Feedline loss
25 Watts
150 ft. at 1dB/100 foot


The Receiver has several gains/losses
 Specific losses due to known environment around the receiver
 Vehicle/building penetration loss
 Receiver antenna gain
 Feedline loss
 Filter loss
These gains/losses are added to the received signal strength
 The result must be greater than the receiver’s sensitivity
Ant
Receiver
Feedline
Filter
Pre-Amplifier
Demodulator
Information

Sensitivity describes the weakest signal power
level that the receiver is able to detect and decode
Sensitivity is dependent on the lowest signal-to-noise
ratio at which the signal can be recovered
 Different modulation and coding schemes have different
minimum SNRs

 Range: <0 dB to 60 dB


Sensitivity is determined by adding the required
SNR to the noise present at the receiver
Noise Sources
Thermal noise
 Noise introduced by the receiver’s pre-amplifier


Thermal noise

N = kTB (Watts)
 k=1.3803 x 10-23 J/K
 T = temperature in Kelvin
 B=receiver bandwidth


Thermal noise is usually very small for reasonable
bandwidths
Noise introduced by the receiver pre-amplifier


Noise Factor = SNRin/SNRout (positive because
amplifiers always generate noise)
May be expressed linearly or in dB



The smaller the sensitivity, the better the receiver
Sensitivity (W) =
kTB * NF(linear) * minimum SNR required (linear)
Sensitivity (dBm) =
10log10(kTB*1000) + NF(dB) + minimum SNR
required (dB)



Example parameters
 Signal with 200KHz bandwidth at 290K
 NF for amplifier is 1.2dB or 1.318 (linear)
 Modulation scheme requires SNR of 15dB or 31.62 (linear)
Sensitivity = Thermal Noise + NF + Required SNR
 Thermal Noise = kTB =
(1.3803 x 10-23 J/K) (290K)(200KHz)
= 8.006 x 10-16 W = -151dBW or -121dBm
 Sensitivity (W) = (8.006 x 10-16 W )(1.318)(31.62) = 3.33 x 10-14 W
 Sensitivity (dBm) = -121dBm + 1.2dB + 15dB = -104.8dBm
Sensitivity decreases when:
 Bandwidth increases
 Temperature increases
 Amplifier introduces more noise

Transmit/propagate chain produces a received
signal has some RSS (Received Signal Strength)



Receiver chain adds/subtracts to this


EIRP minus path loss
For example 50dBm EIRP – 130 dBm = -80dBm
For example, +5dBi antenna gain, 3dB feedline/filter
loss  -78dBm signal into receiver’s amplifier
This must be greater than the sensitivity of the
receiver

If the receiver has sensitivity of -78dBm or lower, the
signal is successfully received.
EIRP
Ant
Transmitter
Information
Modulator
Amplifier
Filter
Feedline
RF Propagation
Prop Loss
Ant
Receiver
RSS
Information
Demodulator
Pre-Amplifier
Filter
Sensitivity
Feedline




A Link Budget determines what maximum path loss a system can
tolerate
 Includes all factors for EIRP, path loss, fade margin, and
receiver sensitivity
For two-way radio systems, there are two link budgets
 Base to mobile (Forward)
 Mobile to base (Reverse)
The system link budget is limited by the smaller of these two
(usually reverse)
 Otherwise, mobiles on the margin would have only one-way
capability
The power of the more powerful direction (usually forward) is
reduced so there is no surplus
 Saves power and reduces interference with neighbors



Forward (Base to Mobile)
 Amplifier power 45dBm
 Filter loss
(2dB)
 Feedline loss
(3dB)
 TX Antenna gain 10dBi
 Path loss
X
 Fade Margin
(5dB)
 Vehicle Penetration
(12dB)
 RX Antenna gain 3dBi
 Feedline loss
(3dB)
Signal into mobile’s LNA has
strength 33dBm – path loss
If Mobile Sensitivity is -100dBm
 Maximum Path loss = 133dB
•
•
•
Reverse (Mobile to Base)
• Amplifier power
28dBm
• Filter loss
(1dB)
• Feedline loss
(3dB)
• TX Antenna gain
3dBi
• Fade Margin
(5dB)
• Vehicle Penetration (12dB)
• Path Loss
X
• RX Antenna gain
10dBi
• Feedline loss
(3dB)
Signal into base’s LNA has
strength 17dBm – path loss
If Base Sensitivity is -105dBm
• Maximum Path loss = 122dB
Unbalanced – Forward path can tolerate 11dB more loss (distance) than reverse