Link Budget_DataCommunication_engineering.ppt

data communication

1. Link Budget No. 1Seattle Pacific University
Wireless Link Budgeting

2. Link Budget No. 2Seattle 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

3. Link Budget No. 3Seattle 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

4. Link Budget No. 4Seattle 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

5. Link Budget No. 5Seattle Pacific University
Path Loss Models
•Path loss is a reduction in the signal’s power, which is a direct result
of the distance between the transmitter and the receiver in the
communication path.
•There are many models used in the industry today to estimate the
path loss and the most common are:
•Free Space
•Hata
•Lee
•Each model has its own requirements that need to be met in order
to be utilized correctly.
•The free space path loss is the reference point for the rest of the
models used.

6. Link Budget No. 6Seattle Pacific University
Free Space
•Used as the foundation for all propagation models.
•Typically underestimates the path loss actually
experienced for mobile communications.
•Used extensively for predicting Point-to-Point, fixed, path
loss.
•It uses the following formula to calculate the loss
experienced by a signal:
56.147log20log20
1010  dfFree space path loss (dB):
F in Hz, d in meters

7. Link Budget No. 7Seattle Pacific University
Hata Model
•Hata Model used extensively in cellular
communications.
•Empirical Model based on Okumura’s data from Tokyo
•Better estimates the path loss experienced as
compared to Free Space.
•Basic model is for urban areas, with extensions for
suburbs and rural areas
•Valid only for these ranges
• Distance 1-20km
•Base height 30-200m
•Mobile height 1-10m
•150 MHz to 1500 MHz (note: some mobile bands
are at 1900 MHz, so be careful)

8. Link Budget No. 8Seattle Pacific University
Hata Model
•Hata formula for urban areas:
•L
H
= 69.55 + 26.16log
10
f
c
– 13.82log
10
h
b
– a(h
m
) +
(44.9 – 6.55log
10
h
b
)log
10
R
•h
b
is the base station antenna height in meters.
•h
m
is the mobile antenna height also measured in
meters.
•R is the distance from the cell site to the mobile in
km.
•f
c
is the transmit frequency in MHz.
•a(h
m
) is an adjustment factor for the type of
environment and the hieght of the mobile.
•a(h
m
) = 0 for urban environments with a mobile height of 1.5m.
•See textbook p. 88 for suburban and rural extensions

9. Link Budget No. 9Seattle Pacific University
Propagation Impairments
•Impairments result in signal loss that are added to
the path loss.
•Causes of impairments:
•Morphology (general environment)
•Obstructions (man made and natural)
•Some propagation models incorporate Morphology
impairments.

10. Link Budget No. 10Seattle Pacific University
Morphology
•Morphology describes the general type of
environment the signal will propagate through.
•Major Classifications of Morphology:
•Dense Urban
•Urban
•Suburban
•Rural

11. Link Budget No. 11Seattle Pacific University
Man-Made Obstructions
Man-made obstructions
result in blockages,
diffraction and reflection

12. Link Budget No. 12Seattle Pacific University
Natural Obstructions
Radio tower
•Numerous types of natural obstructions:
•Mountains
•Water
•Ravines
•Earth curvature
Obstructions result in both
blockages and diffraction

13. Link Budget No. 13Seattle Pacific University
Diffraction
•Results in bending of the wave.
•Can occur in different situations when waves:
•Pass through a narrow slit.
•Pass the edge of a reflector.
•Reflect off two different surfaces approximately one
wavelength apart.
•K-factor used in LOS calculations is a beneficial
effect of diffraction of radio waves.

14. Link Budget No. 14Seattle Pacific University
Received Signal Strength
•The purpose of transmission is to create a strong
enough signal at the receiver
•RSS – Received Signal Strength (usually in dBm)
•RSS determined by:
•+ Transmit power
•- Filter and feedline loss
•+ Transmit antenna gain
•- Path loss (various models)
•RSS can be measured as well

15. Link Budget No. 15Seattle 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

16. Link Budget No. 16Seattle Pacific University
Receiver Sensitivity
•Sensitivity describes the weakest signal power level
that the receiver is able to detect and decode
•Sensitivity is determined by 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

17. Link Budget No. 17Seattle 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
•N (dBm) = 10log
10
(kTB) + 30
•Thermal noise is usually very small for reasonable
bandwidths
•Noise introduced by the receiver pre-amplifier
•Noise Factor = SNR
in/SNR
out (positive because
amplifiers always generate noise)
•May be expressed linearly or in dB

18. Link Budget No. 18Seattle Pacific University
Receiver Sensitivity Calculation
•The smaller the sensitivity, the better the receiver
•Sensitivity (W) =
kTB * NF(linear) * minimum SNR required (linear)
•Sensitivity (dBm) =
10log
10(kTB*1000) + NF(dB) + minimum SNR required (dB)
10log
10(kTB) + 30 + NF(dB) + minimum SNR required (dB)

19. Link Budget No. 19Seattle 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 (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

20. Link Budget No. 20Seattle Pacific University
RSS and Receiver Sensitivity
•Transmit/propagate chain produces a received
signal has some RSS (Received Signal Strength)
•EIRP - 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 LNA of receiver
•This must be greater than the sensitivity of the
receiver
•If the receiver has sensitivity of -78dBm or lower, the
signal is successfully received.

21. Link Budget No. 21Seattle Pacific University
Link Budgeting
•By modeling the full transmit/receive system we
know:
•Given a transmit power and mobile distance, what the
power of the signal going into the receiver’s LNA will
be (RSS+receiver gains/losses)
•How much power is required at the receiver’s LNA
(Sensitivity)
•Link budgeting for a cellular system is the process of
achieving balance of these
•At the boundaries of the cell, received power should
equal receiver sensitivity
•Usually, a fading margin is added

22. Link Budget No. 22Seattle Pacific University
When we care about link budgets
•For simple systems, we don’t care
•Transmit at maximum legal power
•No coverage  Receiver moves
•For cell-based systems, we have to:
•Provide continuous coverage over the entire region
•Ensure transmitters in one cell don’t interfere with
those in the closest cell that uses the same frequency
•Consider reducing cell size in order to have more
capacity through a larger number of cells

23. Link Budget No. 23Seattle Pacific University
What we can change
•To achieve a balanced link budget within a given cell
size we can:
•Adjust transmit power
•Adjust transmit tower height
•Adjust transmit antenna gain
•Modify the receivers
•For planning a system we can use a link budget to:
•Determine the cell size
•Determine the frequency reuse ratio

24. Link Budget No. 24Seattle Pacific University
Forward and Reverse Paths
•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

25. Link Budget No. 25Seattle Pacific University
Forward/Reverse Link Budget Example
•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