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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