Übersicht• Zellgeometrie• Frequency‐Reuse• Übliche Systemfunktionen• Ausbreitungsmodelle• Traffic‐Engineering• Beispiel GSM• Beispiel UMTS

– 3G‐Systeme– Diskussion von CDMA‐Systemen– Übersicht über das UMTS‐System– Power Control– Handover Control

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The near-far problem of CDMA

Large area may become blocked Need to balance emitted power Assume for now a target SIR for each UE Goal: minimum TX power to keep the SIR

NodeB

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Fast fading spoils our plans

Figure copied from: Harri Holma and Antti Toskala, “WCDMA for UMTS”,3rd Edition, WILEY, 2004, ISBN 0-470-87096-6

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The solution: fast close loop power control

NodeB

execute in NodeB at rate 1.5kHz:foreach UE i assigned to NodeB

estimate SIRest after rake combiningif SIRest > SIRtarget then

generate TPC “DOWN” command for iif SIRest ≤ SIRtarget then

generate TPC “UP” command for i

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Compensates a fading channel

Figure copied from: Harri Holma and Antti Toskala, “WCDMA for UMTS”,3rd Edition, WILEY, 2004, ISBN 0-470-87096-6

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Further remarks

And the downlink? basically the same…

A short reflection: closed loop power control• Tight interaction between sender and receiver• Useful for an interaction period

What if sender and receiver are not connected so far?

Example random access on RACH for• Initial access• Short packages

Open loop power control…

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Open loop power control

Transmit power needs to be known to UE Inaccurate! Fast fading between uplink and

downlink is uncorrelated in WCDMA FDD Does not consider interference at receiver (Use power ramping to avoid excessive

interference)

NodeB

• estimate path loss• adapt power

• estimate path loss• adapt power

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How to choose the right target SIR?

Adjust target SIR to meet the link quality

Consider quality as BER or BLER

SIR for quality depends on• Mobiles speed• Multipath profile

Adjust SIR to the worst case?• Unnecessary high SIR wastes capacity• Desirable: minimal SIR which fulfils the quality requirement

How to find such SIR?

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Finding the target SIR: outer loop power control

Similar method for the downlink Downlink method resides in UE Why is uplink handled in RNC? Soft handover combining! …

NodeB

execute in RNC at rate of max 100Hz:foreach UE i assigned to a NodeB

determine the quality from CRC attachmentif quality better than required then

decrease SIRtarget = SIRtarget – ∆downelse

increase SIRtarget = SIRtarget + ∆up

Radio NetworkController (RNC)

target SIRadjustment

frame reliabilityinformation

Übersicht• Zellgeometrie• Frequency‐Reuse• Übliche Systemfunktionen• Ausbreitungsmodelle• Traffic‐Engineering• Beispiel GSM• Beispiel UMTS

– 3G‐Systeme– Diskussion von CDMA‐Systemen– Übersicht über das UMTS‐System– Power Control– Handover Control

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WCDMA Handover types

Inter-system (e.g. WCDMA and GSM)

Inter-frequency (needed at different cell layers or at hot spots)

Intra-frequency (what we look at here)• Soft handover• Softer handover

GSM GSM GSM GSM

WCDMA WCDMA WCDMA

GSM GSMcapacity extension coverage extension

Figures inspired from: Harri Holma and Antti Toskala, “WCDMA for UMTS”,3rd Edition, WILEY, 2004, ISBN 0-470-87096-6

F1 F1 F1 F1F2 F2

handover at hot spot

F1 F1 F1 F1F2 F2 F2 F2 F2 F2 F2

handover to support macro and micro layers

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The idea of soft handover

Exploiting multi path/antenna diversity (Macro diversity)

Uplink• No additional signal is transmitted• In principal, always increases performance

Downlink• Each link causes interference at other users• Trade-off

NodeB1

NodeB2

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Soft handover: the downlink perspective

Maximal ratio combining (MRC) in the rake receiver

Recall: MRC used to exploit multi path diversity

Difference: rake receiver fingers use different codes

NodeB1

NodeB2

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Soft handover: the uplink perspective

Selection combining (SC) in the RNC Target SIR decided after SC

NodeB1

NodeB2

NodeB1

NodeB2

SC

framewith CRC

framewith CRC

RNC

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Softer handover

Sectored antenna Downlink: similar to soft handover Uplink: the more effective MRC

instead of SC is possible and used

NodeB

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Ingredients of the soft handover procedure

cell 1

cell 2

cell 3

CPICH Ec/I0 Measurement quantity, e.g. CPICH Ec/I0

Active set: soft handover connection of UE

Neighbor/monitored set: set of cells that UE can measure

In the following example the active set size is 2

time

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Adding a cell to the active set

cell 1

cell 2

cell 3

Event 1A(add cell2)

add add = reporting_range –hysteresis_event1A

= window_add

Active set is not full

Best pilot

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Replacing a cell in the active set

cell 1

cell 2

cell 3

Event 1A(add cell2)

Event 1C(replace cell1 with cell3)

Worst pilot in full active set

Best candidate pilot

replace

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Removing a cell from the active set

cell 1

cell 2

cell 3

Event 1C(replace cell1 with cell3)

Event 1B(remove cell2)

Event 1A(add cell2)

remove

Best pilot

remove = reporting_range +hysteresis_event1B

= window_drop

Zusammenfassung und Literatur

• Zellgeometrie• Frequency‐Reuse• Übliche Systemfunktionen• Ausbreitungsmodelle• Traffic‐Engineering• Beispiel GSM• Beispiel UMTS

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Zusammenfassung• Generelle Idee zellularer Netze: räumlich verteilte Basisstationen wegen 

beschränkter Bandbreite und limitierter Übertragungsreichweite• Erfordert: Leistungskontrolle, Handover‐Mechanismen, aufwendige 

drahtgebundene Infrastruktur (drahtlos nur „auf der letzten Meile“)• Bemerkung: das Thema schnurlose Telefone (z.B. DECT) wurde hier nicht 

betrachtet• Vereinfachte Darstellung von Zellen mittels Hexagonen• Zwei Varianten zur Aufteilung der Bandbreite: Zuweisung von Frequenzen, 

CDMA• Alte Mobilefunkgenerationen: der Schwerpunkt ist hier die 

Sprachübertragung. (Eine Verbindung pro aktivem Nutzer)• In der Mobiltelefonie spricht man von Evolution von alten Generationen 

hin zu neuen Generationen• Neue Generationen: Datendienste werden immer wichtiger

– Evolution von leitungsvermittelnden zu paketorientiertem Netz (näher am Internet‐Modell)

• Beispiele: GSM und UMTS

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Literatur[Schiller2003] Jochen Schiller, „Mobilkommunikation“, 2te überarbeitete Auflage, 2003

Kapitel 4.1.3: LuftschnittstelleKapitel 4.1.8: Neue DatendiensteKapitel 4.4: UMTS

[Rappaport2002] Theodore Rappaport, „Wireless Communications, Principles andPractice“, Second Edition, Prentice Hall, 200210.1 Principles of Cellular Networks10.3 Second‐Generation TDMA10.4 Second‐Generation CDMA10.5 Third‐Generation Systems

Weiterführende Literatur zum Thema UMTS (nicht unbedingt erforderlich zur Nachbearbeitung dieser Folien)

• H. Holma, A. Toskala (Ed.), “WCDMA for UMTS”, Wiley, 3rd edition, Wiley, 2004.• R. Prasad, W. Mohr, W. Konhäuser (Ed.), “Third Generation Mobile 

Communications Systems”, Artech House, March 2000.• J. P. Castro, “The UMTS Network and Radio Access Technology”, Wiley, 2001.• 3GPP standards: TR 25.922: “Radio Resource Management Strategies”, 2007.

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