RF Propagation No. 1 Seattle Pacific University Basic RF Transmission Concepts

RF Propagation No. 1Seattle Pacific University

Basic RF Transmission Concepts


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


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


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


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!


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


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


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


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


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.


Link Budget Analysis


Ant

Feedline

Transmitter


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


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


Ant

Feedline

Transmitter

Filter

RF Propagation


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


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


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


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


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)


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


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.


Link Budget Analysis


Ant

Feedline

Transmitter


Ant

Feedline

Receiver

Filter

Filter

RF Propagation

EIRP

Prop Loss

RSS

Sensitivity


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


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.

Documents

RF Propagation No. 1 Seattle Pacific University Basic RF Transmission Concepts