5 Cell Architecture

7/31/2019 5 Cell Architecture

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Company Confidential 15/7/05 1

BTS

MS

BSC MSC

CELLRF Channels

Abis - 2 MBits/s

5MS

5BTS5Antenna5BSC5MSC

Cell Architecture


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Mobile Station Output Power

CLASS 2 8 watts (39 dBm) Vehicle and Portable

CLASS 3 5 watts (37 dBm) Hand-held

CLASS 4 2 watts (33 dBm) Hand-held

CLASS 5 0.8 watts (29 dBm) Hand-held

GSM 900

DCS 1800

CLASS 1 1 W (30 dBm) Handheld

CLASS 2 0.25 W (24 dBm) Hand-held

CLASS 3 4W (36 dBm) Hand-held

Mobile Station


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Sensitivity

-102 dBm

MS Antenna gains

Handheld Mobile Antenna Gain = 0 dbi

Vehicle Mounted Antenna Gain = 2 dbi

Mobile Station Antenna and Sensitivity


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BSC

TxRx A Rx B

Splitter

Processor Modules

T

S

B

T

R

X

C

P

PCMInterface

AlarmInterface

A1

TRX TRX1 2

B1

A2

B2

T

R

A

U

BTS

COM

RFUBPF

Fr

R

e

f

BTS Architecture 1


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TRX Logic

TRX RF

Power

Amplifier

TRX Unit

PSU & Climate Control

Antenna

ABIS

over G703

Backplane

BTS Cabinet. 3 Channel. Single Sector

I Q

RF

I QCombiner / Distribution Unit

Rx FilterLNA

Splitter

CombinerDuplexer

Coupler

Coupler

Interface

Frequency Reference

Unit

CPU

BTS

Alarms

NEM

Proprietary

Interface

BTS Architecture 2


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T

R

XC

Tx

RxA

RxB

MMI

Transmit Section

C

O

M

B

T

R

X

C

Tx

RxARxB

MMI

BPF

Tx Power

925 - 960 MHz

Lt Ar.

BTS - Transceiver


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BTS Tx maximum output power, measured at the input of the combiner is

according to its power class as shown below.

TRX

power

class

Maximum

output power

Tolerance

(dB)

1 55 dBm -0, +3

2 52 dBm -0, +3

3 49 dBm -0, +3

4 46 dBm -0, +3

5 43 dBm -0, +3

6 40 dBm -0, +3

7 37 dBm -0, +3

8 34 dBm -0, +3

TRX

power

class

Maximum

output power

Tolerance

(dB)

1 43 dBm -0, +3

2 40 dBm -0, +3

3 37 dBm -0, +3

4 34 dBm -0, +3

GSM 900 DCS 1800

BTS - Tx Power


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q Hybrid / Tunable Filter Combiners are used.q Hybrid Combiner can conventionally combine 2 TRX's with a loss in the

range of 3 -4 dBs.q For more than 2 TRX's, Hybrid Combiner's need to be cascaded

C

O

M

B

TRX1

TRX2

C

O

M

B

- 3 dB

TRX3

-6,-6,-3

To balance the output power, the power output from individual TRX's

should be regulated.

BTS - Transceiver Combiner


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q Tunable Combiners are narrowband combiners .

q Coventional tunable combiners can combine up to 16 TRX's with a

loss of 3 - 4 dB.

q Tuning of each channel can be done remotely.

BTS - Transceiver TunableCombiner


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Exercise !!

There are 4 TRX's in a cell. Plan the Hybrid Combiner

Cascading arrangement . 1 BTS can only support any

one TRX Power Class ?

BTS - Transceiver Combiner


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Tx

RxA

RxB

MMI

Receive Section

Tx

RxA

RxB

MMI

880 - 915 MHz

Lt Ar.

Rx

BPF

Pre-

Amp

TRX 1

TRX 2

A B

A B

BTS - Transeiver


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Types

5

Omni-directional antennas

5Directional antennas

BTS Antenna's


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q Omni-directional Antennas have a uniform horizontal radiation

patterns.

q These antenna's are constructed by co-linear arrays of dipoles.

q The Gain and the beamwidth of the antenna will be depend on the

number of dipole elements in the array and spacing between each

dipole.

q The Gain, since is dependent on number of elements, will also be

restricted to the size of the antenna.

q The Horizontal beamwidth though is 360 deg, but practically will have

some nulls , typically 5%

q The Vertical beamwidth is inversely proportional to the gain.

Omni-directional Antennas


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Omni Antennas are always mounted on top of the pole

Gain Beamwidth

Horizontal : 360 deg

Vertical = 101.5 deg

10e Gain/10

Ex: 12db gain antenna will have a

beamwidth of app. 7 deg and size

of 3.5m at 900 MHz

No ofelmements

Separation. wavlgth44

Separation. wavlgth33

4 . dB44 . dB44

4 . dB33 . dB44

4 . dB44 . dB33

4 . dB44 . dB44

4 . dB44 . dB44

44 . dB44 . dB444

44 . dB444 . dB444

Omnidirectional Antenna's


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q By reducing the directivity, we increase the gain.

q Directional antenna's are high gain , small size

antenna's.

q Application is to increase coverage and reduce

interference.

q Directional antenna's are achieved by using

reflectors.

q Typically 120 deg or 60 deg corner reflectors are

used.

Directional Antenna's


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Gain: 8 db

Beamwidth = 120 deg

Horizontal Pattern for DirectionalAntenna


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Gain: 8 db

Beamwidth = 31 deg

Vertical Pattern for DirectionalAntenna


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q

Two receive antenna's are used at the cell-site to receive the samesignal with different fading envelopes, one at each antenna.

q The degree of correlation between two fading envelopes is determined

by the degree of separation between two receiving antenna's.

D

h

Space Diversity

Cell- Site Receiving Antenna's


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n = h = 11

D

Separation

n = correlation factor

h = effective antenna height (m)

D = Antenna Separation(m)

q For SD to be effective the below correlation equation should be

maintained.q The only easily variable factor in the equation is "D"

Ex: For an antenna height of 30m, D 8

For an antenna height of 50m, D 14

Cell-Site Receiving Antenna


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q Objective is to balance the uplink and downlink receive signal levels

q Since, MS and BTS has different RF architecture, the receive signal

and sensitivity will be different .

q BTS power can be adjusted to balance the link, so this becomes ourmain objective to decide what should be the BTS output power.

q RF balance calculation will also decide the cell range.

RF Link Budget Calculation


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MS sensitivity

Sensitivity = -174 + 10 log (BW) + NF + Ec/No

Rx RF-input Sensitivity = - 102 dbm

BW = Bandwidth in Hz ( 271 Khz)NF = 10 db

Ec/No = 8 db which includes 2 db implementation

margin to maintain 0.4 % type II BER

without interference.



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BTS sensitivity

Sensitivity = -174 + 10 log (BW) + NF + Ec/No

BW = Bandwidth in Hz ( 271 Khz)

NF = 8 db

Ec/No = 8 db which includes 2 db implementation

margin to maintain 0.4 % type II BER

without interference.

Rx RF-input Sensitivity = - 104 dbm



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Uplink Budget & Cell Range

Transmitting endEIRP = Pms - Mantloss + Gms

Receiving endMin-Rx- lev = EIRP - Ploss - Intmargin - Lmargin - Bantloss + Gbs

Pms = MS peak output power ( 2W)

Mantloss = Antenna Feeder /Connector loss 0 db

Gms = 0 dbi

Min-Rx-lev= BTS sensitivity (-104 dbm)

Ploss = Propogation Loss + 3db ( ant/body loss )

Imargin = Interference Degradation margin ( 3db)

Bantloss = BTS Antenna Cable and Connector loss

Lmargin = Log normal shadow margin for 90% coverage area (5db)

Gbs = BTS Rx Antenna Gain.



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Uplink Budget & Cell Range

With the help of the previous equation, we can calculate the

maximum affordable Path Loss in the uplink

The max affordable path loss , when substituted in HATA model

will give the cell range in the uplink.

Add, 15 db to the path loss and again substitute in the HATA

model , this will give the indoor coverage range.



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Exercise

MS = 33 dbm

Terrain = Urban

MS Antenn loss = 0 db

MS Antenna Gain = 0dbi

MS Antenna Height = 1.5 m

BTS Rx Antenna Gain = 12 dbBTS Rx Antenna Loss = 4 db

BTS Antenna Height = 50m

Interference Margin = 3 db

Lognormal Margin = 5 db

Indoor Loss = 15 db

Calculate the max affordable path loss and cell range for both

outdoor as well as indoor ?



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Solution

EIRP = Pms - Mantloss + Gms

= 33 - 0 - 0

= 33 dbm

Min-Rx- lev = EIRP - Ploss - Intmargin - Lmargin - Bantloss + Gbs

-104 = 33 - Ploss - 3 - 5 - 4 + 12

Max affordable Path Loss = 137 db

Max affordable Propagation loss = 137 - 3 (ant/body loss) = 134 db

Cell Range (from HATA ) = 2 km for outdoor= 0.7 km for indoor

What do you think is the most important parameter in

deciding cell range ?



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Downlink Budget & Cell Range

Transmitting end

EIRP = Pbs - Closs - Bantloss + Gbs

Receiving endMin-Rx- lev = EIRP - Ploss - Intmargin - Lmargin - Mantloss + Gms

Pbs = BTS peak output power

Closs = Combiner/Filter/Isolator Loss

Bantloss = Tx Antenna Feeder /Connector loss

Gbs = BTS Tx Antenna Gain

Min-Rx-lev= MS sensitivity (-102 dbm)

Ploss = Propogation Loss + 3db ( ant/body loss )

Imargin = Interference Degradation margin ( 3db)

Mantloss = MS Antenna Cable and Connector loss (0 db)

Lmargin = Log normal shadow margin for 90% coverage area (5db)

Gms = MS Antenna Gain.



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Downlink Budget & Cell Range

q Since Path Loss is same in both U/L and D/L

q

Substitute the Max-affordable Path loss value, calculated inUplink Budget , in the equation , and get the EIRP

q From the value of EIRP, compute the Pbts.

q This the Transmit Power output at the input of the combiner.



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Exercise

Using the same last exercise parameters, calculate the

BTS ouput power ,with combiner loss as 3 db ?



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Solution

BTS Output Power = 38 dbm



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q In our previous case we have seen that there is an imbalance of 5 db

between u/l and d/l and to compensate that we need to bring up theBTS power by 5 db.

q Practically there may be other areas of imbalance also, like the

diversity gain at the BTS receiver , which is typically 4-6 db and hence

will make the UL more strong.

q So we need to add this value to the BTS output power ,so that the

balance is maintained in the DL also.

q The RF Link Balance depends on the following parameters

BTS Tx PowerBTS Combiner Loss

BTS Rx Diversity Gain

RF Link Balance


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Mtx - PropLoss = -104

when,

BTX - PropLoss = - 102

Once the link is balanced;

q Since Propagation Loss is same in both directions,q Any change in Downlink level = Change in Uplink levelq Uplink , we need a fixed level for power control.q So till Power control threshold is reached , MS will be Transmitting at Max powerq When UL Rxlev exceeds Power control threshold, then MS will decrease powerq This means , Downlink Signal Level indicates what should be the MTx Power

Estimating UpLink Imbalancefrom Downlink


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Example :q Downlink Level = -90 dbm. UL Power Control thresh=-80 dbm

What should be Mtx ???

q DL = -90 means , Proploss reduced by 12 dB

q So Mtx should reduce by 12 dB !!!

q NO !! since Power control threshold is -80 dbm,

q When DL = -78 dbm, Prop loss reduces by 24 dB

q This means UL Level is now -80 dbm( Power control threshold)

q After this any DL signal level increase should reduce the MS powerproportionately with an accuracy of +/- 3 dB

q IF THIS DOES NOT HAPPEN, then there is a UL IMBALANCE !!

Estimating UL Link Imbalancefrom Downlink Level


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q Apart from terrain, in an open area the maximum cell range is

limited by Timing Advance.

q Timing Advance is a mandatory operation feature by which theinter- timeslot interference is avoided.

q Let us first understand Timing Advance in depth.

Maximum Cell Range


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TDMA approach requires signals to arrive at BTS at

the correct time. They must not overlap.

B

Ex RCH

BTS

BTS

Reference

B Delay( n bits )

MS

Reference

(delayed)

RACHSame Delay( n bits )

RACH

Reception Delay of 2n bits(but prop delay is still n bits only )

Ex: If an MS receives BCH , 10 bits delayed due to propagation , it takes the time of reception

as synch reference, and transmits the RACH , which is also delayed due to propagation by 10

bits. So at the BTS TS0 reference receives RACH delayed by 20 bits.

But the propagation delay still remains 10 bits, i.e the MS is not 20 bits but 10 bits away from

the BTS

Timing Advance


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So, What to do ?

Ex RCH

BTS

BTS

Reference

B Delay( n bits )MS

Reference

(delayed)

RACHSame Delay( n bits )

RACH

Reception Delay of 2n bits(but prop delay is still n bits only )

B

Ask the MS to transmit earlier by 2n bits !!

SDCCH2n

SDCCHAdvanced by 2n bits

TCH

2n

TCHAdvanced by 2n bits

Timing Advance


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T

BSynch Seq

41 bitsEncrypted

bits 36

T

B

63 bits

Timing Adv

5.25 GP

BSS calculates access delay from RACH in terms of bits

Informs Mobile to delay its timing in terms of bits

Maximum Timing Advance which the MS

can do is of 63 bits.

RACH Burst

Guard Period

68.25 bp

Timing Advance


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MS_max_Range = Max Propagation delay x velocity

Range = Distance between Mobile to Base Station

Timing Advance = Delay of Bits ( 0 -- 63 )

Bit period = 577 / 156.25 = 3.693 usecs = 3.693 x 10e-6 secs

Velocity = 3 x 10e5

Range = ( 63 ) x ( 3.693 x 10e-6 ) x (3 x 10e5)

2= 34.9 kms

Max Propagation Delay = Max propagation delayed bits x Bit period

Delayed Bits = Max Timing Advance bits

2

Mobile Maximum Range


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q Cell range can be extended to take care of extra propagation delay ( in

open area or over water ).

q This can be done by not using the timeslot after the RACH timeslot

and later allocated SDCCH and TCH timeslot. This is because MS

cannot increase timing beyond 63 bits.

153 bits of additional delay accepted by BTS

Total accepted delay at BTS

63 + 153 = 216 bits

Max allowed propagation delay108 bits

This is 120 Km!!!

R X T T X T T

R

kms44R

TCH

TCH

Extended Cell Range

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5 Cell Architecture