RF Propagation No. 1Seattle Pacific University
Basic RF Transmission Concepts
RF Propagation No. 2Seattle Pacific University
Radio Systems
Information Modulator Amplifier
Ant
Feedline
Transmitter
Information Demodulator Pre-Amplifier
Ant
Feedline
Receiver
Filter
Filter
RF Propagation
This presentation concentrates on the propagation portion
RF Propagation No. 3Seattle Pacific University
Waves from an Isotropic source propagate spherically
• 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
RF Propagation No. 4Seattle Pacific University
Real antennas are non-isotropic• Most real antennas do not
radiate spherically
• The wavefront will be only a portion of a sphere
• The surface area of the wave 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
Gain in this area
RF Propagation No. 5Seattle Pacific University
Transmitted Power
• Radiated power often referenced to power radiated by an ideal antenna
ttGPEIRP Pt = power of transmitter
Gt = gain of transmitting antenna system
• The isotropic radiator radiates power uniformly in all directions
• Effective Isotropic Radiated Power calculated by:
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)
The exact same formulas andprinciples apply on the receiving side too!
RF Propagation No. 6Seattle Pacific University
Propagation Models
• 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:
22Dd f 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
RF Propagation No. 7Seattle Pacific University
Propagation ModelsFor Free Space (no buildings, trees, etc.)
2
2
2
2 )4()4()(
c
fdd
P
PlinlossFree
r
t
dBdfc
fddBlossFree 56.147log20log20
4log10)( 1010
2
10
f = frequencyd = distance (m)= wavelength (m)c = speed of light
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
)3)()(loglog55.60.44(
)(log82.13)6)((log16.2655.69)(
1010
1010
dh
hahfdBlossHata
b
mb
For Urban environments, use the Hata model
RF Propagation No. 8Seattle Pacific University
Calculation of Received Signal Strength
1. Confirm that far-field metrics can be used: Use Fraunhofer distance
2. Calculate EIRP = Transmit Power * Antenna Gain3. Calculate propagation loss (free space or Hata)4. Received signal strength (RSS) = EIRP – propagation
loss
RF Propagation No. 9Seattle Pacific University
Applying formulas to real systemsA transmission system transmits a signal at 960MHz with a power of 100mW usinga 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna.1. Confirm that far-field metrics can be used.
= 3.0*108 m/s / 960MHz = 0.3125 meters
Fraunhofer distance = 2 D2/ = 2(0.16m)2/0.3125 = 0.16m
2. Calculate EIRP.
Method 1: Convert power to dBm and add gainPower(dBm) = 10 log10 (100mW / 1mW) = 20dBmEIRP = 20dBm + 2.15dB = 22.15dBm
Method 2: Convert gain to linear scale and multiplyGain(linear) = 102.15dB/10 = 1.64EIRP = 100mW x 1.64 = 164mW
Checking work: 10 log10 (164mW/1mW) = 22.15dBm
RF Propagation No. 10Seattle Pacific University
Applying formulas to real systemsA 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.3. Calculate propagation loss (use free space in this example).
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 = -75.85dBmReceived 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 – 20.41 – 0 + (35.22)(0.30) = 137.7 dB Received power = 22.15dBm – 137.7dB = -115.55dBm
4. Calculate RSS.
RF Propagation No. 11Seattle Pacific University
Link Budget Analysis
Information Modulator Amplifier
Ant
Feedline
Transmitter
Information Demodulator Pre-Amplifier
Ant
Feedline
Receiver
Filter
Filter
RF Propagation
Gain
Gain
Loss
• A Link Budget analysis determines if there is enough power at the receiver to recover the information
RF Propagation No. 12Seattle Pacific University
Transmit Power Components• 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
Information Modulator Amplifier
Ant
Feedline
Transmitter
Filter
RF Propagation
RF Propagation No. 13Seattle Pacific University
Calculating EIRP
dBi12Antenna gain
dB(1.5)150 ft. at 1dB/100 footFeedline loss
dB(1)Jumper loss
dB(0.3)Filter loss
dBm4425 WattsPower Amplifier
ScaleValueComponent
dBm53Total
All values are example values
RF Propagation No. 14Seattle Pacific University
Receiver System Components
InformationDemodulatorPre-Amplifier
Ant
Feedline
Receiver
Filter
• 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
RF Propagation No. 15Seattle Pacific University
Receiver Sensitivity• 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
RF Propagation No. 16Seattle Pacific University
Receiver Noise Sources• 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
RF Propagation No. 17Seattle Pacific University
Receiver Sensitivity Calculation• 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)
RF Propagation No. 18Seattle Pacific University
Sensitivity Example
• 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 (dBm) = -121dBm + 1.2dB + 15dB = -104.8dB• Sensitivity (W) = (8.006 x 10-16 W )(1.318)(31.62) = 3.33 x 10-14 W
• Sensitivity decreases when:• Bandwidth increases• Temperature increases• Amplifier introduces more noise
RF Propagation No. 19Seattle Pacific University
RSS and Receiver Sensitivity
• Transmit/propagate chain produces a received signal has some RSS (Received Signal Strength)
• EIRP minus path loss
• For example 50dBm EIRP – 130 dBm = -80dBm
• Receiver chain adds/subtracts to this
• 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.
RF Propagation No. 20Seattle Pacific University
Link Budget Analysis
Information Modulator Amplifier
Ant
Feedline
Transmitter
Information Demodulator Pre-Amplifier
Ant
Feedline
Receiver
Filter
Filter
RF Propagation
EIRP
Prop Loss
RSS
Sensitivity
RF Propagation No. 21Seattle Pacific University
Link Budgets• 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
RF Propagation No. 22Seattle Pacific University
Forward/Reverse Link Budget Example
• Forward (Tower to Mobile)• Amplifier power 45dBm• Filter loss -2dB• Feedline loss -3dB• TX Antenna gain +10dB• Path loss X• Vehicle Penetration -12dB• RX Antenna gain +3dB• Feedline loss -3dB
• RSS at mobile = 38dBm – X (path loss)
• If Mobile Sensitivity is -100dBm• Maximum Path loss = 138dB
• Reverse (Mobile to Tower)• Amplifier power 28dBm• Filter loss -1dB• Feedline loss -3dB• TX Antenna gain +3dB• Vehicle Penetration -12dB• Path Loss X• RX Antenna gain +10dB• Feedline loss -3dB
• RSS at Tower = 22dBm – X (path loss)
• If Tower Sensitivity is -105dBm• Maximum Path loss = 127dB
Unbalanced – Forward path can tolerate 11dB more loss (distance) than reverse.Reduce Tower transmit power by 11dB.