7/30/2019 Mobile Networks 7
1/24
1
Link Budget
Prof. Johnson I Agbinya
University of the Western Cape
http://www.cs.uwc.ac.za/~jagbinya
7/30/2019 Mobile Networks 7
2/24
2
Link Budget
DefinitionThe aggregation of the signal losses from a TX
to its RX is called its link budget Is a product of gains and losses expressed in decibels or
decoupled logarithmically Sets limit for power levels at various points in the network
Preferred for calculating propagation losses instead of
prediction expressions
Link budgets required for different terrains
Should consider all sources of power losses in the link
7/30/2019 Mobile Networks 7
3/24
3
Product ParametersProduct parameters is a list of all power loss and
gain sources in the link resulting from
hardware.
Transmit power(Pm
)- the power one base
station transmits 30 to 35 dBm for base stations
30 dBm for handsets
Receiver sensitivityis the lowest signal level
a receiver needs to be able to recover the
voice signal with acceptable quality usually in the range of -102 to -110 dBm
Base station sensitivity is Sb
Mobile station sensitivity is Sm
7/30/2019 Mobile Networks 7
4/24
4
Product Parameters (1)
Antenna Gain Gis a measure of the ability of the
antenna to amplify the signal it receives fortransmission. It will do so by
directing the beam
Narrowing the beam (focusing)
usually in the range of 18dBi Base station antenna gain is Gb
Mobile station antenna gain is Gm
Feeder lossoccurs in the cable connecting the
antenna to the electronics in the base station Is usually about 3 dB
7/30/2019 Mobile Networks 7
5/24
5
Product Parameters (2)
Diversity techniquesare used to tolerate weak
signals in the network
Types of diversity
Space diversity
Time diversity
Frequency diversity
Diversity gain is usually in the range of 3 to 5 dB
Diversity gain of a base station is Gd
Combiner Lossoccurs when a single antenna is
used to combine signals at different frequencies
and from many sources
7/30/2019 Mobile Networks 7
6/24
6
Product Parameters (3)
Filter lossaccounts for losses in the
filters used in the base station
Is usually 2 to 3 dB
Duplexer loss Ldis used to account
for the device which duplexes the up
and down links
7/30/2019 Mobile Networks 7
7/24
7
Loss Margins In link budget
Margins need to be provided for propagation losses in
the links (i.e, environmental loss effects)
Interference marginaccounts for interference during the
busy hour Due to traffic, frequency reuse etc
is usually about 1 dB
Losses in Buildingsoccurs due penetration into
buildings and is a function of the type of building material Is around 5 to 20 dB (quite heavy or severe)
Vehicle Penetration:Losses due to RF penetrating intocar body and structure
Is usually around 6 dB
7/30/2019 Mobile Networks 7
8/24
7/30/2019 Mobile Networks 7
9/24
9
Power Balance Equations
Power balance equation Is a summary (sum) of the gains and losses in the
link Gains are positive, losses are negative
Use of decibels makes summing easy
Two power balance equations are used for uplink and downlink
Uplink Power Balance Equation
where
Lpu is the maximum allowable uplink path loss in dB
Lj
is the jumper loss in dB
bjddbmmpu SLLGGGPL
7/30/2019 Mobile Networks 7
10/24
10
Power Balance Equations (1)
Downlink Power balance equation
where
Lpd is the maximum allowable downlink path loss in dB Ltf is the BTS transmitter filter loss in dB P
b
is the setting of the power of the BTS in decibels
bbmtfjdbpd SGGLLLPL
7/30/2019 Mobile Networks 7
11/24
11
Summary of ProductParameters
Parameter Symbol Units Uplink Downlink
Frequency F MHz 1900 1900
MS TX Power Pm dBm 30
MS RX Power Sm dBm -101
MS antenna gain Gm dBi 20 2
MS feeder loss Lm dB 0 0BS Transmit power Pb dBm 47.0
BS RX sensitivity Sb dBm -107.0
BS Antenna gain Gb dBi 20 20
BS Diversity gain Gd dB 3.5
BS Duplexer loss Ld
dB 0.6 0.6
BS Jumper/connectorloss
Lj dB 0.9 0.9
BS TX filter loss Ltf dB 2.3
Product Path Loss Lp dB 161.0 166.2
7/30/2019 Mobile Networks 7
12/24
12
Example of Link Budget
Includes urban, suburban and rural
Only major losses of concern are included hereArea Classification
Urban Suburban Rural
Symbol Units
UL DL UL DL UL DL
Product path loss Lp dB 161.0 166.2 161.0 166.2 161.0 166.2BS antennaheight
Hb M 30 30 45 45 60 60
Feeder loss per m Lf/f dB/m
0.0646
0.0646
0.0646
0.0646
0.0477
0.0477
Total feeder loss Lf dB 1.938 1.938 2.907 2.907 2.862 2.862
Fade margin Mf dB 5.60 5.60 5.60 5.60 5.60 5.60
Body Attenuation Ab dB 2 2 2 2 2 2
Vehicleattenuation
Av dB 0 0 0 0 6 6
Buildingattenuation
Abld dB 15 15 12 12 0 0
Total Path Loss Lpt dB 136.5 141.7 138.5 143.7 144.5 149.8
7/30/2019 Mobile Networks 7
13/24
13
Receiver SensitivityNoise in a receiver and the networks on which it
operates affect how sensitive it can be.
Thermal noise is the major culprit.The sensitivity of the receiver is:
where C/N is carrier to noise ratio In sub-Saharan Africa, thermal noise is a major concern
Neither thermal nor carrier to noise ratio are within thecontrol of the engineer
The noise figure is however within her control and isnormally reduced to improve the sensitivity of the
receiver The receiver contributes a noise figure into the system
and can be improved with good system design
Initial stages of the receiver contribute most to its noisefigure
NCNNFS tRX /
7/30/2019 Mobile Networks 7
14/24
14
Noise Figure
The noise figure of the receiver is:
G1 and G2 are the gains of the early stages of
the receiver
For a cellular base station they are the low
noise amplifier (LNA) on the mast and the
first stages of the receiver electronics
LNA is usually deployed at the top of the mast
to provide a low noise figure first in the chain
of noise figures
The gains of LNAs are usually very low to
start the chain
211
2
1
131
GG
NF
G
NFNFNFtotal
7/30/2019 Mobile Networks 7
15/24
15
Noise Figure (1) MountingLNA
Where should the LNA be mounted to reduce noise figure?
Cabinet Mounting:
Place the LNA at the edge of the BTS cabinet.
Assume the lengths of the feeder is 50 metres
of type 7/8-in air dielectric coaxial cable
with 6.06 dB/100 metre loss.
Gain of the LNA is 12 dB and the
noise figure (NF) is 2 dB
7/30/2019 Mobile Networks 7
16/24
16
Cabinet / Tower Mounting
NF1 = 3.03 dB = 2 NF2 = 2 dB = 1.58 G1 = -3.03 dB = 0.5 The total noise figure: NFTotal = 10xlog[2 + ((1.58 - 1)/0.5)] =
4.99 dBTower Mounted LNA:
NF1 = 2 dB = 1.58
NF2 = 3.03 dB = 2
G1 = 12 dB = 15.85 The total noise figure is NFTotal = 10xlog[1.58 + ((2 - 1)/15.85)]
= 2.15 dB.
The tower mounted LNA has reduced the total noise figure
by 2.84 dB in this case.
7/30/2019 Mobile Networks 7
17/24
17
Tower Mounted LNA
The LNA provides the first noise figure in the chain
Exercise
If the antenna mast height becomes 80metres, by how much is
the total noise figure reduced by mounting the LNA at the
top of the mast?
BTS
LNA
NF2
NF1,
G1
7/30/2019 Mobile Networks 7
18/24
18
Diversity
Diversity techniques are used to tolerate weak
signals and reduce the effects of interference
Types of Diversity
time space
polarisation
frequency
Frequency diversity Frequency hoping
In GSM a user hops from one frequency to another every time
slot to avoid interference
Hops through a group of frequencies
7/30/2019 Mobile Networks 7
19/24
19
Frequency Diversity
Probability of fading occurring in all the sets
of frequencies is low
Is used to reduce Rayleigh fading
Clyclic hoping with a small set of frequenciesRandom hoping is preferred with a large set of
frequencies Is used to reduce co-channel interference
All traffic channels can hop but control channels cannot
Frequency hoping improves frequency reuse by an orderof 7 to 4 thereby increasing capacity
7/30/2019 Mobile Networks 7
20/24
20
Time Diversity
Is used by Rake receivers to provide gain
Signals separated in time to ensure they do not correlate
Separation should be more than coherence time
Polarisation Diversity
Orthogonal signals have very low correlation
Horizontal and vertically polarised signals are used in a two-branch
diversity
7/30/2019 Mobile Networks 7
21/24
21
Space or Receive Diversity
Popular in antenna systems for decorrelation of signals in
the antenna
Separation of 10 to 20 times the signal wavelength is used
in suburban areas
In receive diversity two or more antennas are used for
reception
Most cellular receivers use 2 antennas and onetransmission antennas (3 per sector)
7/30/2019 Mobile Networks 7
22/24
22
Diversity Processing
Diversity processing is mostly exploitation ofthe spatial dimension more effectively (eg.)
(i) Smart Antenna Technology
(ii) Transmit Diversity Schemes (iii) Spatial Multiplexing and
(iv) Space-Time Coding.
7/30/2019 Mobile Networks 7
23/24
23
Smart Antennas
Are used for beam focusing from basestations (or beam forming), for example:
adaptive phased array antennasSmart antennas can provide up to 4 times
improved system capacity
7/30/2019 Mobile Networks 7
24/24
Top Related