Übersicht. WS 2012/2013 Drahtlose Kommunikation Zellulare Netze 78

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1 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 78

2 The near-far problem of CDMA NodeB 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 79

3 Fast fading spoils our plans Figure copied from: Harri Holma and Antti Toskala, WCDMA for UMTS, 3rd Edition, WILEY, 2004, ISBN

4 The solution: fast close loop power control NodeB execute in NodeB at rate 1.5kHz: foreach UE i assigned to NodeB estimate SIR est after rake combining if SIR est > SIR target then generate TPC DOWN command for i if SIR est SIR target then generate TPC UP command for i 81

5 Compensates a fading channel Figure copied from: Harri Holma and Antti Toskala, WCDMA for UMTS, 3rd Edition, WILEY, 2004, ISBN

6 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 83

7 Open loop power control estimate path loss adapt power estimate path loss adapt power NodeB 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) 84

8 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? 85

9 Finding the target SIR: outer loop power control frame reliability information NodeB target SIR adjustment Radio Network Controller (RNC) 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 Similar method for the downlink Downlink method resides in UE Why is uplink handled in RNC? Soft handover combining! 86

10 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

11 WCDMA Handover types Inter-system (e.g. WCDMA and GSM) GSM GSM GSM GSM GSM GSM capacity extension WCDMA WCDMA WCDMA coverage extension Inter-frequency (needed at different cell layers or at hot spots) 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 Intra-frequency (what we look at here) Soft handover Softer handover Figures inspired from: Harri Holma and Antti Toskala, WCDMA for UMTS, 3rd Edition, WILEY, 2004, ISBN

12 The idea of soft handover NodeB1 NodeB2 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 89

13 Soft handover: the downlink perspective NodeB1 NodeB2 Maximal ratio combining (MRC) in the rake receiver Recall: MRC used to exploit multi path diversity Difference: rake receiver fingers use different codes 90

14 Soft handover: the uplink perspective frame with CRC NodeB1 NodeB1 NodeB2 RNC SC frame with CRC NodeB2 Selection combining (SC) in the RNC Target SIR decided after SC 91

15 Softer handover NodeB Sectored antenna Downlink: similar to soft handover Uplink: the more effective MRC instead of SC is possible and used 92

16 Ingredients of the soft handover procedure cell 1 cell 3 cell 2 CPICH E c /I 0 Measurement quantity, e.g. CPICH E c /I 0 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 93

17 Adding a cell to the active set cell 1 cell 3 Best pilot cell 2 Active set is not full add add = reporting_range hysteresis_event1a = window_add Event 1A (add cell2) 94

18 Replacing a cell in the active set cell 1 cell 3 cell 2 Best candidate pilot replace Worst pilot in full active set Event 1A (add cell2) Event 1C (replace cell1 with cell3) 95

19 Removing a cell from the active set cell 1 cell 3 cell 2 Best pilot remove = reporting_range + hysteresis_event1b = window_drop Event 1A (add cell2) Event 1C (replace cell1 with cell3) Event 1B (remove cell2) 96

20 Zusammenfassung und Literatur Zellgeometrie Frequency Reuse Übliche Systemfunktionen Ausbreitungsmodelle Traffic Engineering Beispiel GSM Beispiel UMTS WS 2012/2013 Drahtlose Kommunikation Zellulare Netze 97

21 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 WS 2012/2013 Drahtlose Kommunikation Zellulare Netze 98

22 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, 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, R. Prasad, W. Mohr, W. Konhäuser (Ed.), Third Generation Mobile Communications Systems, Artech House, March J. P. Castro, The UMTS Network and Radio Access Technology, Wiley, GPP standards: TR : Radio Resource Management Strategies, WS 2012/2013 Drahtlose Kommunikation Zellulare Netze 99