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
PTX version of the
L f ,TX Ga ,TX link budget.
Gain

Lp CRITERION
TO MEET:
Required
Loss

Ga , RX L f , RX C/N at
C receiver
input
N
Noise reference level

Antenna Noise
gain
Transmitter Antenna Propagation Receiver
gain loss
Transmit Feeder Feeder Received
power loss loss power
Slides for Wireless Communications Edfors, Molisch, Tufvesson
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 dimension-

  less.
 
The corresponding dB value is calculated as:

|  
 = 10 log
 
 |  

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 Power

We usually measure power in Watt (W) and milliWatt [mW]

The corresponding dB notations are dB and dBm
Non-dB dB
 |
  = 10 log = 10 log  
Watt:   1| 

|
  = 10 log = 10 log  
milliWatt:   1| 

|
 = 10 log = 10 log   + 30  =   + 30 
 0.001|    
RELATION:

Slides for Wireless Communications Edfors, Molisch, Tufvesson

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 ERP Effective

Radiated Power
Vacuum cleaner: 1600 W = 32 dBW or 62 dBm
Car engine: 100 kW = 50 dBW or 80 dBm

TV transmitter (Hrby, SVT2): 1000 kW ERP = 60 dBW or 90 dBm ERP

Nuclear powerplant (Barsebck): 1200 MW = 91 dBW or 121 dBm

Slides for Wireless Communications Edfors, Molisch, Tufvesson

Amplification and attenuation

(Power) Amplification: (Power) Attenuation:

   
! 1/#
Note: It doesnt
matter if the power
is in mW or W.
Same result!
  
 = ! ! =  = #=
 # 

The amplification is already The attenuation is already

dimension-less and can be converted dimension-less and can be converted
directly to dB: directly to dB:

! = 10 log%& ! # = 10 log%& #
 

Slides for Wireless Communications Edfors, Molisch, Tufvesson

Example: Amplification and attenuation

Ampl. Cable Ampl. Ampl.

Detector
A B
4 dB
30 dB 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

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Noise sources

The noise situation in a receiver depends on

several noise sources

Noise picked up
Wanted
by the antenna
signal

Analog Output signal

Detector with requirement
circuits
on quality
Thermal
noise

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Man-made noise

Copyright: IEEE

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Receiver noise: Equivalent noise source

To simplify the situation, we replace all noise sources

with a single equivalent noise source.

Wanted How do we determine

Noise free N from the other
signal
N sources?

C Analog Output signal

Detector with requirement
circuits
Noise free on quality

Same input quality, signal-to-noise

ratio, C/N in the whole chain.

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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 Boltzmanns constant (1.38 ( 10)*+ W/Hz) and T0 is the,
so called, room temperature of 290 K (17-).

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Receiver noise: Noise sources (2)

Antenna example

Na
Model

Noise temperature Noise free

of antenna 1600 K 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

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Receiver noise: System noise

Nsys

System System
Model
component 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

Dont 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
F1 F2
Na = kTa
Noise N1 = k ( F1 1)T0
N2 = k ( F2 1)T0
free
Na N1 N2

System 1 System 2
Noise Noise
free free

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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 G

Attenuator:
N N/L

1/L 1/L

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 The isotropic antenna

The isotropic antenna radiates Elevation pattern

equally in all directions
Radiation
pattern is
spherical

Azimuth pattern

This is a theoretical
antenna that cannot
be built.

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The dipole antenna

Elevation pattern

/ 2 -dipole
This antenna does not
radiate straight up or
down. Therefore, more
energy is available in
other directions.
Feed /2
THIS IS THE PRINCIPLE Azimuth pattern
BEHIND WHAT IS CALLED
ANTENNA GAIN.

A dipole can be of any length,

but the antenna patterns shown
are only for the /2-dipole. Antenna pattern of isotropic
antenna.
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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 The amount of increase
power! in input power to the
isotropic antenna, to
obtain the same maximum
radiation is called the
Isotropic, with increased
antenna gain!
input power.

Antenna gain of the /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 /2 dipole.
Be careful! Sometimes

G |dBi = G |dBd +2.15 it is not clear if the

antenna gain is given
in dBi or dBd.

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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.

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EIRP and the link budget

POWER [dB]

EIRP
GTX |dB
PTX |dB
Gain
Loss

9:;  = <=   !<= 

  

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Path loss

TX RX

*
@
Received power [log scale] ?= = <= !?= !<=
1/7 * 4B7

1/7 > * >

@ 7C
/D
?= = <= !?= !<=
4B7C
/D 7

Distance, d

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Fading margin

1. Fading
channel loss is time-variant (stochastic process)

2. Sometimes received power could be smaller than desired
3. Add some extra transmission power to decrease that probability
4. The extra transmission power
Fading margin

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Required C/N another central concept

Quality IN
Quality OUT
(C/N)
DETECTOR

DETECTOR CHARACTERISTIC The detector characteristic

Quality OUT is different for different
system design choices.

REQUIRED QUALITY OUT:

Audio SNR
Perceptive audio quality
Bit-error rate
Quality IN Packet-error rate
(C/N) etc.

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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 INTERFERENCE LIMITED

TX RX TX RX TX

Power Power

C C I

Min C/I
Min C/N

N N

Distance Distance
Max distance Copyright: Ericsson Max distance

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What is required distance between BSs?

Copyright: Ericsson

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