link budget - ntu
TRANSCRIPT
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
Gai
nLo
ss
Transmitter
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N
SINGLE LINKThe link budget – a central concept
PTX
”POWER” [dB]
L f ,TX Ga ,TX
Noise reference level
Antenna Propagation
gain lossTransmit Feeder
power loss
Lp
Antenna Noise
Receiver
Ga , RX L f , RX
C
gain
Feeder Receivedloss power
RequiredC/N at
receiverinput
This is a simpleversion of thelink budget.
CRITERIONTO MEET:
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dB in general
When we convert a measure X into decibel scale, we always divide by a
reference value Xref:Independent of thedimension 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
Watt:
milliWatt:
RELATION:
� ����
= 10 log �|�1|�
= 10 log � ��
� ����
= 10 log �|��1|��
= 10 log � ���
� ����
= 10 log �|�0.001|�
= 10 log � ��
+ 30 ���= � �
���+ 30 �
��
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GSM base station TX: 40 W = 16 dBWor 46 dBm
Vacuum cleaner: 1600 W = 32 dBWor 62 dBm
Example: Power
Sensitivity level of GSM RX: 6.3x10-14 W = -132 dBWor -102 dBm
Bluetooth TX: 10 mW= -20 dBWor 10 dBm
GSM mobile TX: 1 W = 0 dBWor 30 dBm
Car engine: 100 kW = 50 dBWor 80 dBm
TV transmitter (Hörby, SVT2): 1000 kW ERP = 60 dBWor 90 dBm ERP
Nuclear powerplant (Barsebäck): 1200 MW = 91 dBWor 121 dBm
ERP – EffectiveRadiated Power
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Amplification and attenuation
(Power) Amplification:
The amplification is alreadydimension-less and can be converteddirectly to dB:
(Power) Attenuation:
The attenuation is alreadydimension-less and can be converteddirectly to dB:
Note: It doesn’tmatter if the power
is in mW or W.Same result!
�� ��� �� ��� ! 1/#
��� = !�� ⇒ ! = ��� ��
��� = �� # ⇒ # = ��
���
! ���
= 10 log%& ! # ���
= 10 log%& #
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Example: Amplification and attenuation
30 dB4 dB
10 dB 10 dB
Detector
Ampl. Ampl. Ampl.Cable
A B
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 onseveral noise sources
Detector
Noise picked upby the antenna
AnalogcircuitsThermal
noise
Output signalwith requirementon quality
Wantedsignal
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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 sourceswith a single equivalent noise source.
DetectorOutput signalwith requirementon quality
Wantedsignal
C
Noise free
N
Analogcircuits
Noise free
Same “input quality”, signal-to-noiseratio, C/N in the whole chain.
How do we determineN from the other
sources?
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Receiver noise: Noise sources (1)
The power spectral density of a noise source is usually given in oneof the following three ways:
1) Directly [W/Hz]:
2) Noise temperature [Kelvin]:
3) Noise factor [1]:
N s
Ts
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-).
This one issometimes
given i dB andcalled noise
figure.
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Receiver noise: Noise sources (2)
Antenna example
Noise temperatureof antenna 1600 K
Power spectral density of antenna noise is
and its noise factor/noise figure is
./ = 1600 / 290 = 5.52 = 7.42 dB
Noise freeantenna
Na
Model
0/ = 1.38 ( 10)*+ ( 1600 = 2.21 ( 10)*&3/45 = −196.6783/45
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Systemcomponent
Noise factor F
Model Systemcomponent
Noise free
Receiver noise: System noise
Nsys
Due to a definition of noise factor (in this case) as the ratio of noisepowers on the output versus on the input, when a resistor in roomtemperature (T0=290 K) generates the input noise, the PSD of theequivalent noise source (placed at the input) becomes
Nsys = k ( F −1)T0 W/Hz
Equivalent noise temperatureDon’t use dB value!
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System 1 System 2
F1 F2
Receiver noise: Sev. noise sources (1)
A simple example
Ta
1
Na = kTa
N1 = k ( F −1)T0
N2 = k ( F2 −1)T0
Noisefree
System 1
Noisefree
System 2
Noisefree
N2Na N1
Receiver noise: Sev. noise sources (2)
After extraction of the noise sources from each component, we need tomove them to one point.
When doing this, we must compensate for amplification and attenuation!
G
Amplifier:
Attenuator:
1/L
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N
N
G
1/L
NG
N/L
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The isotropic antenna
The isotropic antenna radiatesequally in all directions
Radiationpattern isspherical
Elevation pattern
Azimuth pattern
This is a theoreticalantenna that cannot
be built.
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Azimuth patternλ / 2Feed
A dipole can be of any length,but the antenna patterns shownare only for the λ/2-dipole. Antenna pattern of isotropic
antenna.
The dipole antenna
Elevation pattern
λ / 2 -dipole
This antenna does notradiate straight up ordown. Therefore, moreenergy is available inother directions.
THIS IS THE PRINCIPLEBEHIND WHAT IS CALLEDANTENNA GAIN .
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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 inputpower!
Isotropic, with increasedinput power.
The amount of increasein input power to theisotropic antenna, toobtain the same maximumradiation is called theantenna gain!
Antenna gain of the λ/2 dipole is 2.15 dB.
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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 theisotropic antenna (which we will use in this course).
Another measure of antenna gain frequently encounteredis dBd, which is relative to the λ/2 dipole.
G |dBi = G |dBd +2.15Be careful! Sometimesit is not clear if theantenna gain is givenin dBi or dBd.
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EIRP: Effective Isotropic Radiated Power
EIRP = Transmit power (fed to the antenna) + antenna gain
Answers the questions:
How much transmit power would we needto feed an isotropic antenna to obtain thesame maximum on the radiated power?
How ”strong” is our radiation in the maximal direction of the antenna?
This is the more importantone, since a limit on EIRPis a limit on the radiation in
the maximal direction.
9:;� ���
= �<= ���
� !<= ���
Gai
nLo
ss
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GTX |dBPTX |dB
EIRP and the link budget
”POWER” [dB]
EIRP
9:;� ���
= �<= ���
� !<= ���
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Path loss
Distance, d
TX RX
Received power [log scale]
∝ 1/7*
∝ 1/7>
�?= = �<=!?=!<=@
4B7*
�?= = �<=!?=!<=@
4B7C/D
* 7C/D7
>
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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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DETECTOR CHARACTERISTIC
Quality OUT
Quality IN(C/N)
Required C/N – another central concept
DETECTOR
Quality IN(C/N) Quality OUT
The detector characteristicis different for differentsystem design choices.
REQUIRED QUALITY OUT:
Audio SNRPerceptive audio qualityBit-error ratePacket-error rateetc.
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)
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Noise and interference limited links
C
N
Distance
Min C/N
Power
Max distance
C
N
Distance
Power
Min C/I
I
Max distance
TX TX TXRX RX
NOISE LIMITED INTERFERENCE LIMITED
Copyright: Ericsson
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What is required distance between BSs?
Copyright: Ericsson