mobile communications topic 12 link budget interference
TRANSCRIPT
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Mobile Com-
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Jose AntonioPortilla
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Mobile Communications
Jose Antonio Portilla Figueras
September 29, 2009
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Agenda
Topic 1: Fundamental of Cellular Radio Networks
1 General Concepts of Telecommunication Networks2 Link Budget and Interference
3 Experimental Propagation Models for Cellular Radio
Networks
4 Basic Traffic Concepts
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Environment is not perfect....
There are several effects that could affect the qualityexperienced by the end user in a mobile communication system
Noise
From external sources.
From the receiver.
Interference experienced at the receiver
Co-channel interferer: other MS transmit at same frequency (frequency reuse) as the receiver listens.
Adjacent interferer: other MS uses adjacent frequencies for transmissio.
Fading
Due to mobile channel effects: houses .
Multipath Shadowing.
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Link Budget
Link budgets are used to calculate the coverage or cell range inthe UL and in the DL.
BTS use higher power that mobile.
TOTAL Coverage= Minimum (coverage in DL, coverage in UL)
For this reason, the coverage in UL is always the limiting factor.
The minimum performance requirements regarding the BTSreceiver and the MS receiver are fixed by the standardizationbodies.
This minimum performance requirements determine the MS andBTS receiver sensitivity.
Example from 3GPP for TU50iFH for EDGE in 900Mhz bandand Coding MCS5 BLER 10%
BTS receiver sensitivity= -97dBm.
MS receiver sensitivity= -94dBm
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Link Budget
Receiver sensitivity (minimum accepted received signal level) for BTSand MS are specified in the standard 3GPP:
For frequency bandFor different channels Typical urban, rural areaFor the queality levelSNR.
Receiver Sensibility=SNR+Noise.Noise=Thermal Noise+Noise Bandwidth+Fr
Thermal Noise = -174dBm / Hz.
Noise Bandwidth (depends on the receiver filter) typical value forGSM 52.6 dB.
Noise Figure, typicall value 5 dB.
Typical noise value at the receiver filter output = -116.4 dBm
The receiver sensitivity determine the maximum allowed pathloss and
hence, the maximum cell range
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Link Budget
General Link Budget Equation
PTX+
G L LPathloss=SRXWhere
PTX: Transmitter Power.
G: Gain of the diff
erent equipment (dB).L: Losses of the different equipment (dB).
SRX: Receiver Sensibility.
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Link Budget
Free Space Pathloss
LFreePathloss= 32 + 20Log(f) + 20Log(d)
Where
f: Frequency in MHz.
d: Distance in Km.
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Propagation Models
We have obtained previously the values for the total pathloss and our first estimation of the range using the freespace formulation.
However it is straight forward to see that we are not in afree space environment so we have to study some other
methods.Propagation methods may be classified into two maincategories:
Theoretical Models: They come from theoreticalsimulations and mathematical studies.
Simulation of Wave Propagation.Ray Theory.
Experimental models: They come from measures in realworld
COST 231 Hata ModelCOST 231 Walkfish Ikegami.PCS
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COST 231 - Hata Model
Path loss estimation is performed by empirical models ifland cover is known only roughly, and the parametersrequired for semi-deterministic models cannot bedetermined.
One of the most extended model is the Okumura Hata.This model was developed by Y. Okumura using a set ofequation which required to go to a large set of curvesobtained from empirical measurements.
M. Hata performs the first extension of this equations.
The model is based in four parameters
1 Frequency f (MHz).2 Distance d (Km).3 BTS antenna height hBTS (m).4 Mobile antenna height hMobile (m).
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COST 231 - Hata Model
The basic transmission loss Lbcan be calculated asfollows.
Lb= 66.95 + 26.16Log(f)13.82Log(hBTS)a(hMobile)+(44.95
6.55Log(hBTS))Log(d)
Where:
a(hMobile) = (1.1logf 0.7)hMobile (1.56log(f) 0.8)
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COST 231 - Hata Model
The model is restricted to:
f : 150...1000MHzhBTS : 30...200m.hMobile : 1...10m.d : 1...20Km
COST 231 has extended Hatas model to the frequencyband 1500
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COST 231 - Hata Model
Lb= 46.3 + 33.9Log(f)
13.82
Log(hBTS)a(hMobile)+(44.95 6.55Log(hBTS))Log(d) +Cm
Where a(hMobile) is defined as in the previous model and Cm:
Cm =
0dB for medium sized cities and suburban centres3dB for metropolitan centres
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COST 231 - Hata Model
The model is restricted to:
f : 150...2000MHzhBTS : 30...200m.hMobile : 1...10m.d : 1...20Km
The application of the COST-Hata-Model is restricted to
large and small macro-cells,
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COST 231 - WI Model
COST 231 proposed a combination of the Walfisch andIkegami model that is called theCOST-Walfisch-Ikegami-Model (COST-WI).It calculates improved path-loss estimation byconsideration of more data to describe the character of the
urban environment:Heights of buildings hRoof.Widths of roads wbuilding separation broad orientation with respect to the direct radio path .
Note that although we use some special values this modelis still statistical because we only use characteristic values.
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COST 231 - WI Model
The model distinguishes between line-of-sight (LOS) andnon-line-of-sight (NLOS) situations.
In the LOS case -between base and mobile antennas
within a street canyon - a simple propagation loss formuladifferent from free space loss is applied.
The loss is based on measurements performed in the cityof Stockholm
COST 231 - WI for LOS
Lb= 42.6 + 26Log(d) + 20Log(f) for d 20m
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COST 231 - WI Model
In the NLOS-case the basic transmission loss Lb iscomposed of the terms:
Free space loss L0,
Multiple screen diffraction loss LmsdRoof-top-to-street diffraction and scatter loss Lrts.
COST 231 - WI for LOS
Lb=
L0+Lrts+Lmsd Lrts+Lmsd>0L0 Lrts+Lmsd 0
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COST 231 - WI Model
Lets study the diff
erent terms that composes the model
The term Lrtsdescribes the coupling of the wave along themultiple path into the street where the mobile terminal is
located, that is, the roof-top-to-street diffraction andscatter loss.
Lrts= 16.9 10Log(w)+10Log(f) + 20Log(hMobile) +LOri
hMobile=hRoof hMobilehBase=hBase hRoof
The values for LOriand its explanation are in the following
slide
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COST 231 - WI Model
LOri=
10 + 0.354 0 352.5 + 0.075( 35) 35 554.0 + 0.114
( 55) 55 90
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COST 231 - WI Model
Now we have to deal with the effect of the multi screendiffraction, that is, Lmsd.
This part of the model is usually used when the antennaheight of the BTS is below the roof-top.
Lmsd=Lbsh+ka+kdLog(d) +kf Log(f) 9Log(b)
Where:
Lbsh =
18Log(1 + hBase) hBase>hRoof0 hBase hRoof
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COST 231 - WI Model
The term ka represents the increase of the path loss forbase station antennas below the roof tops of the adjacentbuildings.
ka =
54 hBase>hRoof
54 0.8 hBase d 0.5 hBase hRoof54 0.8 hBase d0.5 d
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COST 231 - WI Model
The terms kd and kfcontrol the dependence of themulti-screen diffraction loss versus distance and radiofrequency, respectively.
kd =
18 hBase>hRoof18 15 hBase
hRoofhBase hRoof
kf =
0.7( f925 1) Medium sized cities and suburban
1.5( f925 1) Metropolitan centers
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COST 231 - WI Model: We are finishing
An important problem in this method is to obtain values of thedifferent parameters.
Note that is most cases these models are used to perform studies inseveral and quite different cities, and hence not all parameters areknow.
If the data on the structure of buildings and roads are unknown thefollowing default values are recommended:
Recommended Values
hRoof = 3(Number of floors) + (roof height)
(roof height) =
3 pitched0 flat
b= 20...50.w=b/2= 90
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COST 231 - WI Model: The last slide
Conditions
The model is restricted to:f : 800...2000MHzhBTS : 4...50m.hMobile : 1...4m.d : 0.02...5Km
C
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Interference Calculations
When number of users start to increase (higher traffic) itis needed to calculate the interference in the system inorder to know if the quality criteria are fulfil.
So we have to two factors to analyze.
Receiver sensibility that comes from the SNR of the serviceand the Noise.Carrier to Interference ratio C/I.
In urban environments the C/I will be often the limitingfactor
I f C l l i
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Interference Calculations
We would like to evaluate C/I en certain position.
C= carrier power.
I= Interference power.
We may have two situations
Interference downlink: Received by the MS and generatedby transmission of other BTS using same frequencies(cochannel).Interference uplink : Received by BTS listening to certainMS. Interference generated by other MS (located in othercells) using same frequencies (cochannel).
It is straight forward to see that the most dominant one will be the
downlink.There are two ways to evaluate that we fulfill the quality criteria:
C/I> C/I threshold where C/I threshold is the designcriteria at the end of the cell that result in certain quality.Prob(C/I>C/I threshold),
p example: 90 % (More
Exact)
I f C l l i
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Interference Calculations
This illustration shows the C over I calculation scenario.
I t f C l l ti
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Interference Calculations
Interference Equations
I = 6 PTD
+ 6 PT(
3D)+ 6 PT(2D)
C = PTR
Where:
I: Interference power.
C: Carrier power
D: Reuse distance.
: Power depravement ratio.
R: Cell Range.
Only considered three circumferences over the target cell.
I t f C l l ti
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Interference Calculations
The minimum physical distance between two transmittersusing the same frequency required to achieve certain linkquality is denoted at Reuse Distance, D.
Note the influence ofD
with
.Values of
Free space propagation = 2.
In mobile communications = 3 5
= 3 correspond with rural areas.
= 5 correspond with high-rise buildings areas.
R s All ti
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Resource Allocation
Our most scarce resource is the frequency.
In figure above the coverage area is organized in 4 groupsof 7 cells marked in different colors Cluster.
Each MS will connect with a BTS depending on thelocation
In this case each BTS has been located in the middle ofthe cell, using Omni directional antenna.
Resource Allocation
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Resource Allocation
Each BTS is able to use n frequencies
The total number of frequencies is 7n
Frequencies are reused in neighboring clusters
Example number of frequencies
In the 900 MHz band the bandwidth for GSM is 25 MHz,divided by 3 operators it is about 8 MHz.
This means 40 different frequencies (GSMBW= 200KHz)for each operator.
Hence about 5 frequencies will be assigned for each cell.
Resource Allocation
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Resource Allocation
If we have Nfrequencies available, each cell can use N/Kwhere K is the size of the group
We define frequency reuse factor as Reuse= K where
K is number of cells that belong to same cluster.
Reuse factor or cluster size is a compromise betweencapacity and quality
Small reuse factors result in high interference & highcapacity (more channels)High reuse factors result in low interference & low capacity(less channels)
Resource Allocation
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Resource Allocation
If we have Cavailable channels, we divide them into Kgroups of approximately equal size.
Each BTS will have assigned a group ofnf = CK.The number nf is orientation of the capacity of the system
Channel could not be reused in a cell that is to closebecause is needed to maintain quality requirements and we
saw that Reuse Distance=f(S.I.R required))
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Resource Allocation
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Resource Allocation
Symmetric cell plans exist for K = 1, 3, 4, 7, 9, 12, .
Formulation for the Cluster definition
K = (i+j)2 i
j i,j= 0, 1, 2, 3...
It is possible to demonstrate that the distance D betweentwo cells with the same channels is.
6 Closest channels, distance D.Next circumference, other 6 cochannels
3D
Next circumference, 2D.
Resource Allocation additional information
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Resource Allocation, additional information
In the previous examples the BTS was located in themiddle of the cell using an omni directional antenna.
In urban environments and indoor coverage is more
complicated and BTS usually use directive antennas.
In this way the cell is divided typically in three Sectors
Each antenna reaching 120.
Each sector has its own frequency set and it is basically a
cell.In highways typically cells with 2 sectors are used (180).