Link Budget - NTU

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

Slides for “Wireless Communications” © Edfors, Molisch, Tufvesson

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:


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 �| � ��| � ��

� ��

� ��


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 �

��


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


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%& #


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


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


Man-made noise

Copyright: IEEE


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?


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.


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


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!


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


N

N

G

1/L

NG

N/L


The isotropic antenna

The isotropic antenna radiatesequally in all directions

Radiationpattern isspherical

Elevation pattern

Azimuth pattern

This is a theoreticalantenna that cannot

be built.


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 .


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.


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.


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


GTX |dBPTX |dB

EIRP and the link budget

”POWER” [dB]

EIRP

9:;� ��

= �<= ��

� !<= ��


Path loss

Distance, d

TX RX

Received power [log scale]

∝ 1/7*

∝ 1/7>

�?= = �<=!?=!<=@

4B7*

�?= = �<=!?=!<=@

4B7C/D

* 7C/D7

>


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


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)


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


What is required distance between BSs?

Copyright: Ericsson

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