85 How to choose the right target SIR?

(1)

84 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

(2)

85 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?

(3)

86

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 attachment if quality better than required then

decrease SIR_target = SIR_target – ∆_down else

increase SIR_target = SIR_target + ∆_up

Radio Network Controller (RNC)

target SIR adjustment frame reliability

information

(4)

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

WS 2012/2013 Drahtlose Kommunikation ‐Zellulare Netze 87

(5)

88 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 GSM

capacity 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 F1

F2 F2

handover at hot spot

F1 F1 F1 F1

F2 F2 F2 F2 F2 F2 F2

handover to support macro and micro layers

(6)

89 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

(7)

90 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

(8)

91 Soft handover: the uplink perspective



Selection combining (SC) in the RNC



Target SIR decided after SC

NodeB1

NodeB2

NodeB1 NodeB2 SC

frame with CRC

frame with CRC RNC

(9)

92 Softer handover

 Sectored antenna

 Downlink: similar to soft handover

 Uplink: the more effective MRC instead of SC is possible and used NodeB

(10)

93 Ingredients of the soft handover procedure

cell 1

cell 2

cell 3

CPICH E_c/I₀  Measurement quantity, e.g.

CPICH E_c/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

(11)

94 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

(12)

95 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

(13)

96 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

(14)

Zusammenfassung und Literatur

• Zellgeometrie

• Frequency‐Reuse

• Übliche Systemfunktionen

• Ausbreitungsmodelle

• Traffic‐Engineering

• Beispiel GSM

• Beispiel UMTS

(15)

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

(16)

Literatur

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

Kapitel 4.1.3: Luftschnittstelle Kapitel 4.1.8: Neue Datendienste Kapitel 4.4: UMTS

[Rappaport2002] Theodore Rappaport, „Wireless Communications, Principles and Practice“, Second Edition, Prentice Hall, 2002

10.1 Principles of Cellular Networks 10.3 Second‐Generation TDMA 10.4 Second‐Generation CDMA 10.5 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.