Link Budget
PROF. MICHAEL TSAI
2011/9/22
What is link budget?
• Accounting all losses and gains from the transmitter, the
medium, to the receiver.
• Therefore the word “budget”.
• Generally, − = .
• There is a minimum required ,
associated with the minimum required “service quality”.
• How much you can spend on the channel loss?
• Range
• How much transmission power do you need?
• Energy
• How much sensitivity do you need?
• Cost
SINGLE LINK
The link budget – a central concept
”POWER” [dB]
This is a simple
version of the
link budget.
PTX
Gain
L f ,TX Ga ,TX
Loss
Lp
Ga , RX L f , RX
C
CRITERION
TO MEET:
Required
C/N at
receiver
input
N
Noise reference level
Antenna Propagation
gain
loss
Transmitter
Transmit Feeder
power
loss
Antenna Noise
gain
Receiver
Feeder
loss
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Received
power
dB in general
When we convert a measure X into decibel scale, we always divide by a
reference value Xref:
Independent of the
dimension of X (and
), this value is
always dimensionless.
The corresponding dB value is calculated as:
| = 10 log
| Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Power
We usually measure power in Watt (W) and milliWatt [mW]
The corresponding dB notations are dB and dBm
Non-dB
Watt:
milliWatt:
RELATION:
= 10 log
dB
| 
= 10 log = 10 log
1|
|
= 10 log = 10 log
1|
|
= 10 log 0.001|
+ 30
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
=
+ 30
Example: Power
Sensitivity level of GSM RX: 6.3x10-14 W = -132 dBW or -102 dBm
Bluetooth TX: 10 mW = -20 dBW or 10 dBm
GSM mobile TX: 1 W = 0 dBW or 30 dBm
GSM base station TX: 40 W = 16 dBW or 46 dBm
Vacuum cleaner: 1600 W = 32 dBW or 62 dBm
ERP – Effective
Radiated Power
Car engine: 100 kW = 50 dBW or 80 dBm
TV transmitter (Hörby, SVT2): 1000 kW ERP = 60 dBW or 90 dBm ERP
Nuclear powerplant (Barsebäck): 1200 MW = 91 dBW or 121 dBm
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Amplification and attenuation
(Power) Attenuation:
(Power) Amplification:
!
= ! ⇒ ! =
The amplification is already
dimension-less and can be converted
directly to dB:
!
= 10 log%& !
1/#
Note: It doesn’t
matter if the power
is in mW or W.
Same result!
=
⇒#=
#
The attenuation is already
dimension-less and can be converted
directly to dB:
#
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
= 10 log%& #
Example: Amplification and attenuation
Ampl.
A
Cable
Ampl. Ampl.
B
4 dB
30 dB
Detector
10 dB 10 dB
The total amplification of the (simplified)
receiver chain (between A and B) is
GA, B |dB = 30 − 4 +10 +10 = 46
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Noise sources
The noise situation in a receiver depends on
several noise sources
Noise picked up
by the antenna
Wanted
signal
Analog
circuits
Thermal
noise
Detector
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Output signal
with requirement
on quality
Man-made noise
Copyright: IEEE
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Receiver noise: Equivalent noise source
To simplify the situation, we replace all noise sources
with a single equivalent noise source.
Wanted
signal
How do we determine
N from the other
sources?
Noise free
N
C
Analog
circuits
Noise free
Detector
Same “input quality”, signal-to-noise
ratio, C/N in the whole chain.
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Output signal
with requirement
on quality
Receiver noise: Noise sources (1)
The power spectral density of a noise source is usually given in one
of the following three ways:
This one is
1) Directly [W/Hz]:
Ns
sometimes
given i dB and
2) Noise temperature [Kelvin]:
Ts
called noise
figure.
3) Noise factor [1]:
Fs
The relation between the tree is
Ns = kTs = kFsT0
where k is Boltzmann’s constant (1.38 ( 10)*+ W/Hz) and T0 is the,
so called, room temperature of 290 K (17-).
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Receiver noise: Noise sources (2)
Antenna example
Na
Model
Noise temperature
of antenna 1600 K
Noise free
antenna
Power spectral density of antenna noise is
0/ = 1.38 ( 10)*+ ( 1600 = 2.21 ( 10)*& 3/45 = −196.6 783/45
and its noise factor/noise figure is
./ = 1600 / 290 = 5.52 = 7.42 dB
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Receiver noise: System noise
Nsys
System
component
Model
System
component
Noise factor F
Noise free
Due to a definition of noise factor (in this case) as the ratio of noise
powers on the output versus on the input, when a resistor in room
temperature (T0=290 K) generates the input noise, the PSD of the
equivalent noise source (placed at the input) becomes
Nsys = k ( F −1)T0 W/Hz
Don’t use dB value!
Equivalent noise temperature
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Receiver noise: Sev. noise sources (1)
A simple example
Ta
System 1
System 2
F2
F1
Na = kTa
N1 = k ( F1 −1)T0
Noise
free
Na N1
N2 = k ( F2 −1)T0
N2
System 1
Noise
free
System 2
Noise
free
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Receiver noise: Sev. noise sources (2)
After extraction of the noise sources from each component, we need to
move them to one point.
When doing this, we must compensate for amplification and attenuation!
Amplifier:
N
NG
G
Attenuator:
G
N
N/L
1/L
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
1/L
The isotropic antenna
The isotropic antenna radiates
equally in all directions
Elevation pattern
Radiation
pattern is
spherical
Azimuth pattern
This is a theoretical
antenna that cannot
be built.
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
The dipole antenna
Elevation pattern
λ / 2 -dipole
Feed
λ/2
This antenna does not
radiate straight up or
down. Therefore, more
energy is available in
other directions.
THIS IS THE PRINCIPLE
BEHIND WHAT IS CALLED
ANTENNA GAIN.
A dipole can be of any length,
but the antenna patterns shown
are only for the λ/2-dipole.
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Azimuth pattern
Antenna pattern of isotropic
antenna.
Antenna gain (principle)
Antenna gain is a relative measure.
We will use the isotropic antenna as the reference.
Radiation pattern
Isotropic and dipole,
with equal input
power!
Isotropic, with increased
input power.
The amount of increase
in input power to the
isotropic antenna, to
obtain the same maximum
radiation is called the
antenna gain!
Antenna gain of the λ/2 dipole is 2.15 dB.
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
A note on antenna gain
Sometimes the notation dBi is used for antenna gain (instead of dB).
The ”i” indicates that it is the gain relative to the
isotropic antenna (which we will use in this course).
Another measure of antenna gain frequently encountered
is dBd, which is relative to the λ/2 dipole.
G |dBi = G |dBd +2.15
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Be careful! Sometimes
it is not clear if the
antenna gain is given
in dBi or dBd.
EIRP: Effective Isotropic Radiated Power
EIRP = Transmit power (fed to the antenna) + antenna gain
9:;
= <=
!<=
Answers the questions:
How much transmit power would we need
to feed an isotropic antenna to obtain the
same maximum on the radiated power?
How ”strong” is our radiation in the maximal direction of the antenna?
This is the more important
one, since a limit on EIRP
is a limit on the radiation in
the maximal direction.
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
EIRP and the link budget
”POWER” [dB]
EIRP
GTX |dB
Loss
Gain
PTX |dB
9:;
= <=
!<=
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Path loss
TX
RX
?= = <= !?= !<=
Received power [log scale]
∝
1/7 *
∝ 1/7 >
?= = <= !?= !<=
Distance, d
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
@
4B7
@
4B7C
*
*
/D
7C
7
/D
>
Fading margin
1.
2.
3.
4.
Fading channel loss is time-variant (stochastic process)
Sometimes received power could be smaller than desired
Add some extra transmission power to decrease that probability
The extra transmission power Fading margin
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Required C/N – another central concept
Quality IN
(C/N)
Quality OUT
DETECTOR
DETECTOR CHARACTERISTIC
Quality OUT
The detector characteristic
is different for different
system design choices.
REQUIRED QUALITY OUT:
Quality IN
(C/N)
Audio SNR
Perceptive audio quality
Bit-error rate
Packet-error rate
etc.
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
Example:
Mobile radio system
• Consider a mobile radio system at 900-MHz carrier frequency,
and with 25-kHz bandwidth.
• It is affected only by thermal noise (temperature of the environment
E = 300F).
• Antenna gains at the TX and RX sides are 8 dB and -2 dB, respectively.
• Losses in cables, combiners, etc. at the TX are 2 dB.
• The noise figure of the RX is 7 dB.
• The 3-dB bandwidth of the signal is 25 kHz.
• The required operating SNR is 18 dB and the desired range of
coverage is 2 km.
• The breakpoint is at 10-m distance; beyond that point, the path loss
exponent is 3.8.
• The fading margin is 10 dB.
• What is the minimum TX Power?
• Textbook p42 (example 3.2)
Noise and interference limited links
NOISE LIMITED
TX
INTERFERENCE LIMITED
RX
TX
RX
TX
Power
Power
C
C
I
Min C/I
Min C/N
N
N
Distance
Max distance
Copyright: Ericsson
Distance
Max distance
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson
What is required distance between BSs?
Copyright: Ericsson
Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson