Ac RE56 33 GSM TDMA Engineering

8/7/2019 Ac RE56 33 GSM TDMA Engineering

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Alexandre CAMINADA, UTBM

3 -2006

Contents

1.Spectrum use2. Frequency assignment

3. Frequency hopping


4 -2006

Communication 1

Communication 2

Communication 3

Time

Spec

trum

Frequency Division Multiple Access (FDMA)

1G analogue systems

Plus: easy to do

Minus: interference, fading

1/ Spectrum use

2/ Frequency assignment

3/ Frequency hopping


3/20


5 -2006

Time

Spect

rum

Frequency-Time DMA (F-TDMA)

Communication 1

Communication 2

Communication 3

2G numerical systems: GSM, DECT, D-AMPS

Plus: gain in capacity

Minus: synchronisation, fading

1/ Spectrum use




6 -2006

Time

Spec

trum

Slow Frequency Hopping F-TDMA

GSM

Plus: gain in interference, gain in fading

Minus: complex to evaluateCommunication 1

Communication 2

Communication 3

1/ Spectrum use




4/20


7 -2006

Spectrum reuse in mobile networks1/ Spectrum use




8 -2006

Interference brings by spectrum reuse

Reuse is depending on system ability for interference management

It is not possible to use the same frequency in adjacent cells: co-channel

interference between 2 mobiles

Interference is C/(I+N), where

C, power of expected signal

I, set of interference, often limited to co-channel

N, white noise, whereN


5/20


9 -2006

Downlink interference formulation

Let Pei,j the emitted power from BSj to MS i, andLi,j the global loss from BSj to MS i

Then

With TDMA, cells circuits are rightly orthogonal (=0) then there is no intra-cellinterference

0

i

0

, , ,

', , , ' , 'intra inter' , ' ' , '

, ,

, tot', , , ' , '

' , ' ' , '

orthogonality factoret ,

/

i j i j i j

i j i j i j i ji ii i i C j j j BS

i j i j

i ji j i j i j i j

i i i C j j j BS

C Pe L

Pe L Ptot L

Pe L

Pe L Ptot L

I I

C I

BS0

BS1BSk

BSk+1

Li,0

Li,1Li,k

Li,k+1

Iintra

Iinter

Iinter

Iinter

1/ Spectrum use




10 - 2006

Distance of reuse between cells

Lower required C/I means shorter reuse distance and higher capacity

Analogue system: C/I 18 dB

GSM: C/I 9dB

Reuse separation distance ranges from 4 to 6 times the cell radius (W.C.Y. LEE)

D

f1

R

f1

R

D R SeuilR: cell radiusD: frequency reuse distance

1/ Spectrum use




6/20


11 - 2006

Frequency reuse pattern (k=3, 7, 12)

Hypothesis

Regular network (grid)

Regular traffic demand

Regular propagation

Graph-coloring problem

Advantages

Easy to do

No propagation model

Inherent problem

High traffic demand requires small patterns

Small patterns produce interference

1

3

3

2

1

4

2

7

K=3

1

1

1

2

2

2

3

3

3

44

4

4

5

5

5

6

6

6

77

7

7

K=7

Frequency reuse pattern between cells1/ Spectrum use




12 - 2006

The big problem of real cell coverage

Nominal cell boundaries

The cell

Radio link

Transmitter

T-antenna

Propagation &

Environmental effects

R-antenna

Receiver

Coverage:Blue: field strength > -100Yellow: field strength > -90

FrequencyDistanceWeatherObstacledependent

1/ Spectrum use




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13 - 2006

The big problem of real cell coverage

Theory Reality

1/ Spectrum use




14 - 2006

The model is built on ideal scenario

Regular plane surface: uniform propagation (no obstacles)

Each station located at a node on a regular grid

No vacancy on node

All stations parameters settings identical (omni directionaldiagram)

Each station has a regular traffic

Co-channel interference is only considered (no adjacentinterference)

The real networks are far from theory1/ Spectrum use




8/20


15 - 2006

Contents

1. Spectrum use

2.Frequency assignment3. Frequency hopping


16 - 2006

Definition: frequency reuse consists inusing the same frequency channel onareas that are separated enough toavoid co-channel interferenceproblems

It is a graph colouring problem: thefrequency are assigned to cells asthe colours are assigned to areas

This concept is fundamental to get thegap between low bandwidth and highcapacity one need to catch a lot ofcustomers

The theoretical basis of frequency assignment1/ Spectrum use




9/20


17 - 2006

Cells are overlapping each others

Cell overlap is measured from

Propagation simulation

Field and neighbor measurement reports

On one pixel, currently are 40 to 70 significant signals

6 or 7 good signals are needed (HO)

Others are multiple radio interference:I = I1+I2++In

Best server Interference

Good signals

1/ Spectrum use




18 - 2006

CIM [i,j] = surface with single radio interference between stations i (carrier) andj

(interference) at all C/I level

Computed from cell overlap

Pixels restricted to single radio interference

Cover from A

Interference from B

C/I

PixelCIM [A,B]

Carrier-to-Interference matrix computation1/ Spectrum use




10/20


19 - 2006

OM [i,j] = surface with single radio interference between stations i (carrier) andj

(interference) for a given C/I compatibility threshold for co-channel and adjacent

channel

Computed from C/I matrix

Threshold per cell, per channel, per network layer.

C/I

Pixel Pixel

Threshold

OM [A,B]





20 - 2006

Co-channel and adjacent channel interference rating for cell pairs are specified in terms of

affected areas

Specification are cell planned ; it supposes that TRX in a cell use the same technology and

the same transmission power, and emit from the same antenna ; or several cells have to be

defined

0,15

0

0,18

0,12

D

0,34

0,08

C

0,12

0

B

0,25

0,15

0,30

0,12

A

DCBAStations





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21 - 2006

Matrix of channel separations between cells

Additional separations required for engineering constraints

Co-station separation: 3 channels (>= 3)

Co-site separation: 2 channels (>= 2) ; A and C are co-located

SM [i,j] = channel separation requirement between frequency assigned to stations i

and j to avoid any interference from j on i

Computed from overlapping matrix for (i,j) where i j

3102D

2302C

0031B

2223A

DCBAStations

Etc.

0,12

0

B

0,25

0,15

0,30

0,12

A

DCBAStations

1/ Spectrum use




22 - 2006

Major FAP problems for operators

Assigning frequency to cell is computing afrequency plan followingone of the problems below

Problem 1: Minimize Spectrum FAPA number of frequencies is available for the network

Objective is to minimize the number of frequencies used while satisfying allcompatibility constraints and demand constraints

Problem 2: Minimum Span FAPSpan of an assignment is the difference between the largest channel used and thesmallest channel used

Objective is to minimize the span needed to satisfy all EMC and demandconstraints

Problem 3: Minimize Interference FAPFinite, fixed number of frequencies available for the network

Objective is to satisfy all demands constraints (its increases the reuse factor!) andto minimize some measure of interference (e.g. EMC constraints violation) with thegiven frequencies

1/ Spectrum use




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23 - 2006

Evaluating the quality of frequency plan

Networks

stations

InterferenceComputation

Frequency

plan

Cell

coverage

1/ Spectrum use




24 - 2006

Evaluating the quality of frequency plan

Radio interference C/I+N ; N


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25 - 2006

Communications quality thresholds

C/I thresholds depend on the engineering on frequency planning

Most of the time radio interference are considered around 14 dB on non hoppingnetwork

Several FP evaluation are available on one pixel

C/I worst case on the pixel; non hopping

C/I mean value on the pixel; average of all frequencies; band base hopping

C/I worst case among the best frequency per cell; BCCH

C/I minimum threshold depends on channel separation between communications

C/I = - 49 dB3rd adjacentC/I = -9 dB1st adjacent

C/I = - 41dB2nd adjacentC/I = 9 dBCo-channel

1/ Spectrum use




26 - 2006

Contents

1. Spectrum use

2. Frequency assignment

3.Frequency hopping


14/20


27 - 2006

Why Frequency Hopping?

Frequency Hopping stands for the dynamic changing of frequency during communications

On each hop, only a burst of information is transmitted on one frequency

The transmitter and the receiver must have the foreknowledge of the correct sequence offrequency changes

Advantages on jamming

The jamming frequency is not always the same, sometime jamming sometime not

Spread Spectrum ability (FH-SS): the total transmission, viewed over a long period such 1 sec,appears to occupy the entire bandwidth (spreading of spectrum)

We are not trying to eliminate interference with channelization, interference levels will risegradually with the number of mobiles

Advantage on multi-path fading

Deep fades tend to be frequency selective

If the hops are separated by a given distance (coherence bandwidth = 600 KHz at 900 MHz), twosuccessive hops are not faded

The average fade on the whole frequency range is much less: equivalent of about 2-3 dB insteadof 20 dB

1/ Spectrum use




28 - 2006

Family of Frequency Hopping

Slow Frequency Hopping (SFH): GSM Speed: 1733 times per second (at every burst)

Base band hopping: few frequency are used

Synthesized hopping: all spectrum can be used

Fast Frequency Hopping (FFH): military system A burst is a very few bits: frequency hopping each n electric symbols

(eventually n=1) where 1 electric symbol = 1, 2 or 4 bits

The length of the burst must be lower than the propagation time from thetransmitter to the receiver (typically 10-100 microseconds)

The time the jammer detects the signal, the transmitter has already shifted to anew frequency

The sequences are randomized

1/ Spectrum use




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29 - 2006

SFH Base band hopping

time

Base band hopping

21 3 4 5 6 7

TDMA frame

4.62 ms

TRX0

(f0)

TRX1

(f1)

0

f0 f0 f0f0

f1f1 f1 f1f1

f0

0.577ms

TRX2

(f2) f2f2 f2 f2 f2

1/ Spectrum use




30 - 2006

SFH Synthesized hopping

Synthesized hopping

f0 f0 f0f0 f0 f0

f1 f2 f1f1 f2 f1

time21 3 4 5 6 7

TDMA frame

4.62 ms

TRX0

(f0)

TR

X1

(f1

,f2)

0

0.577ms

TRX2

(f1,f

2)

f2 f1 f2

1/ Spectrum use




16/20


31 - 2006

Power

Time

Freque

ncy

Interferencethreshold

Carrier

Interferer 1, low power

Interferer 2, high power

SFH Synthesized hopping1/ Spectrum use




32 - 2006

SFH Synthesized hopping parameters

Implementing synthesized frequency hopping allows the planner to assign much

more frequency than TRX

Gain in frequency diversity (quality of radio path is frequency dependent)

Gain in interference diversity (successive bursts suffer from varying sources ofinterference)

In TU50, diversity gains are low

New parameters: MAL, HSN and MAIO

Size ofMobile Allocation Lists (number of frequency channels) per station

Frequency to assign toMobile Allocation Lists per station

Hopping Sequence Numberto assign to stations or sites (station versus site driven)

Mobile Allocation Index Offset to assign to TRX

New evaluation criteria: FER

Frame Erasure Rate: number of erased vocal frame, that is after FEC application

1/ Spectrum use




17/20


33 - 2006

SFH Frequency diversity gains

Gain at 2% FER from random hopping in test conditions

(Ref: GSM, GPRS and EDGE performance, WILEY, 2002)

1/ Spectrum use




34 - 2006

Synthesized hopping Parameters setting

SITE DRIVEN

3 BCCH = 3 channels

MAL TCH = 1 for all stations

Size of MAL: greater than the number

of TRX TCH on the site

STATION DRIVEN

3 BCCH = 3 channels

MAL TCH = 1 per station

Size of MAL: greater than the numberof TRX TCH on the station

BCCH 1

BCCH 2

BCCH 3

1 MAL TCH

BCCH 1

BCCH 2

BCCH 3

3 MAL TCH

1/ Spectrum use




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35 - 2006

Synthesized hopping Parameters setting

Let MAL = N frequencies (


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37 - 2006

Synthesized hopping Interfering cells

Non hopping Cell are interfering continuously

But interfering powers: low

Synthesized hopping Cell are interfering with intermittence

But interfering powers: high (HO areas)

NB: further interfering cells (second circleof neighbours) are still present but alsointermittently and with a higher loss =>not a problem

1/ Spectrum use




38 - 2006

SFH Quality thresholds in FER

FER evaluation with synthesized hopping: thresholds to 4% and 7%

C/I mean: 12 dB on base band hopping network

C/I mean: 8 dB on synthesized hopping network on theoretical conditions

SFH quality measurement is complex

Traffic load is needed

Go from C/I to FER needs to estimate error corrections process between BER and FER

NB: BER is calculated before the decoding with no gain from FH, so the BER is the same for allhopping configuration

Simulated quality tables are required

SFH gain is strong for TU3 and week for TU50 because of the natural diversity of the

channel (fast variations)

3%6%TU50

3%21%TU3

FER with SFHFER without SFHAt C/I = 9 dB

1/ Spectrum use




20/20


39 - 2006

SFH FER(C/I) estimation

TU3 full hopping link with 6 interferers for different loads in the case of power control

(Ref: GSM, GPRS and EDGE performance, WILEY, 2002)

1/ Spectrum use



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Ac RE56 33 GSM TDMA Engineering