UTRAN Radio Resource Management

Transcription

1 UTRAN Radio Resource Management BTS 3 BTS 1 UE BTS 2 Introduction Handover Control Soft/Softer Handover Inter Frequency Handover Power Control Closed Loop Power Control Open Loop Power Control Interference Management Load Control Call Admission Control Congestion Control Packet Data Transmission Packet Data Control Dynamic Scheduling

2 RRM High-Level Requirements Efficient use of limited radio resources (spectrum, power, code space) Minimizing interference Flexibility regarding services (Quality of Service, user behaviour) Simple algorithms requiring small signalling overhead only Stability and overload protection Self adaptive in varying environments Allow interoperability in multi-vendor environments Radio Resource Management algorithms control the efficient use of resources with respect to interdependent objectives: cell coverage cell capacity quality of service 2

3 RRM Components Core Network/ other RNCs Radio Resource Management Handover Control Load Control Packet Data Control typically in RNC Power Control Medium Access Control typically in NodeB Physical layer 3

4 Handover Control: Basics General: mechanism of changing a cell or base station during a call or session Handover in UMTS: UE may have active radio links to more than one Node B Mobile-assisted & network-based handover in UMTS: UE reports measurements to UTRAN if reporting criteria (which are set by the UTRAN) are met UTRAN then decides to dynamically add or delete radio links depending on the measurement results Types of Handover: Soft/Softer Handover (dedicated channels) Hard Handover (shared channels) Inter Frequency (Hard) Handover Inter System Handover (e.g. UMTS-GSM) Cell selection/re-selection (inactive or idle) All handover types require heavy support from the UMTS network infrastructure! 4

5 Macro Diversity & Soft Handover (Wrap-Up) NodeB 1 NodeB 2 UE Downlink: combining in the mobile station Uplink: combining in the base station and/or radio network controller 5

6 Soft/ Softer Handover In soft/softer handover the UE maintains active radio links to more than one Node B Combination of the signals from multiple active radio links is necessary Soft Handover The mobile is connected to (at least) two cells belonging to different NodeBs In uplink, the signals are combined in the RNC, e.g. by means of selection combining using CRC Softer Handover The mobile is connected to two sectors within one NodeB More efficient combining in the uplink is possible like maximum ratio combining (MRC) in the NodeB instead of RNC Note: In uplink no additional signal is transmitted, while in downlink each new link causes interference to other users, therefore: Uplink: HO general increase performance Downlink: Trade-off 6

7 Soft and Softer Handover in Practice 7

8 Soft Handover Control soft handover area NodeB 1 UE NodeB 2 Measurement Quantity CPICH 1 CPICH 2 d add d drop T link Measurement quantity, e.g. E C /I 0 on CPICH Relative thresholds d add & d drop for adding & dropping Preservation time T link to avoid ping-pong effects Event triggered measurement reporting to decrease signalling load Link to 1 Link to 1 & 2 Link to 2 time 8

9 Outage Probability (Blocking and Dropping) Soft Handover Simulation Results 25% 20% 15% 10% 1 link max 2 SHO links max 4 SHO links max 6 SHO links 5% 0% Offered Traffic [Erlang per site] Soft handover significantly improves the performance, but 9

10 Mean Number of Active Links Soft Handover Simulation Results (contd.) 2 1,5 1 0, Max. Active Set Size the overhead due to simultaneous connections becomes higher! 10

11 Inter-Frequency Handover Hierarchical cell structure (HCS) Hot-spot Macro Micro Macro f 1 f 2 f 1 Hot spot f 1 f 2 f 1 f 1 Handover f 1 f 2 always needed between layers Hard handover Handover f 1 f 2 needed sometimes at hot spot Inter-frequency measurements of target cell needed in both scenarios Mobile-assisted handover (MAHO) slotted (compressed) mode for inter-frequency measurements to find suitable target cell also supports GSM system measurements Database assisted handover (DAHO) no measurements performed on other frequencies or systems use cell mapping information stored in data base to identify the target cell 11

12 Power Control: Basics Controls the setting of the transmit power in order to: Keep the QoS within the required limits, e.g. data rate, delay and BER Minimise interference, i.e. the overall power consumption Power control handles: Path Loss (Near-Far-Problem), Shadowing (Log-Normal-Fading) and Fast Fading (Rayleigh-, Ricean-Fading) Environment (delay spread, UE speed, ) which implies different performance of the de-interleaver and decoder Uplink: per mobile Downlink: per physical channel Three types of power control: Inner loop power control Outer loop power control (SIR-target adjustment) Open loop power control (power allocation) Downlink power overload control to protect amplifier Gain Clipping (GC) Aggregated Overload Control (AOC) 12

13 Near-Far Problem Power Control (Wrap-Up) UE 1 Near-Far Problem: Spreading sequences are not orthogonal (multi-user interference) Near mobile dominate Signal to interference ratio is lower for far mobiles and performance degrades NodeB The problem can be resolved through dynamic power control to equalize all received power levels AND/OR UE 2 By means of joint multi-user detection 13

14 Closed Loop Power Control Closed loop power control is used on channels, which are established in both directions, such as DCH There are two parts Inner Loop Power Control (ILPC): receiver generates up/ down commands to incrementally adjust the senders transmit power Outer Loop Power Control (OLPC): readjusts the target settings of the ILPC to cope with different fading performance SIR > SIR target? target adjustment BLER target UE Inner Loop (1500 Hz) Outer Loop ( 100Hz) RNC control command: Up/Down NodeB Example: Uplink Closed Loop Power Control 14

15 power/ fading [db] E b /N 0 [db] Impact of Power Control 8 speed = 3 km/h time [sec] Example: UMTS Closed Loop Power Control in the slow fading channel 15

16 Required UL SIR [db] Power Control Performance PedA VehA Velocity [km/h] SIR requirement strongly depends on the environment (due to different fast fading conditions Jakes models) outer loop power control needed to adapt target SIR 16

17 Open Loop Power Control Open loop power control is used on channels that cannot apply closed loop power control, e.g. RACH, FACH The transmitter power is determined on the basis of a path loss estimate from the received power measure of the opposite direction To avoid excessive interference, probes with incremental power steps until a response is obtained: power ramping UE NodeB Open Loop Power Control on RACH 17

18 CDMA Overload CDMA systems tend to become unstable More traffic increases the interference More interference requires higher power More power increases the interference Methods are required to limit the system load Restrict the access to the system Overcome overload situations 18

19 Cell Breathing CDMA systems: cell size depends on the actual loading Additional traffic will cause more interference If the interference becomes too strong, users at the cell edge can no more communicate with the basestation CDMA interference management Restriction of the users access necessary Cell breathing makes network planning difficult Example: cell brething with increasing traffic 19

20 Cell Breathing (contd.) Coverage depending on load: load causes interference, which reduces the area where a SIR sufficient for communication can be provided coverage low load coverage medium load coverage high load shadowed area: connection maybe lost 20

21 Interference in CDMA Networks Interference Inter-Symbol Interference (ISI) Problem Delayed components from the same user signal interfere due to multipath propagation Multiple Acces Interference MAI Different user signals interfere dependent on the access scheme Intra-Cell Interference Inter-Cell Interference Interference caused by the users belonging to same cell Interference caused by the users belonging to neighbor cells. Frequency reuse factor is one CDMA is subject to high multiple access interference Soft capacity: CDMA capacity (e.g. number of users) determined by the interference is soft Handling of interference is the main challenge in designing CDMA networks 21

22 Interference in CDMA Networks (contd.) Uplink Downlink Multi access channel: access of the same medium but with different channel conditions Intra-cell interference: sum of unsynchronized signals Broadcast channel: signals are summed up and then broadcast Intra-cell interference: sum signal (synchronized) with same channel condition Inter-cell interference: sum of signals not assigned to BS Inter-cell interference: signals from BS not assigned 22

23 Coverage vs. Capacity Capacity depends on: QoS of the users (data rate, error performance (bit-error-rate)) User behaviour (activity) Interference (out of cell) Number of carriers/ sectors Coverage (service area) depends on: Interference (intra- & inter-cell) + noise Pathloss (propagation conditions) QoS of the users (data rate, error performance (bit-error-rate)) Thus, trade-off between capacity and coverage 23

24 Maximum cell radius (km) Coverage vs. Capacity kbps circuit switched service capacity versus maximum cell radius Downlink Uplink Erlangs (2% GOS) Downlink limits capacity while uplink limits coverage Downlink depends more on the load (users share total transmit BS power) 24

25 coverage map best server map Example of Coverage and Best Server Map Application: RF engineering (cell layout) Legend: violet indicates high signal level, yellow indicates low level Application: HO decision Legend: color indicates cell with best CPICH in area 25

26 Load Control: Basics Main objective: Avoid overload situations by controlling system load Monitor and controls radio resources of users Call Admission Control (CAC) Admit or deny new users, new radio access bearers or new radio links Avoid overload situations, e.g. by means of blocking the request Decisions are based on interference and resource measurements Congestion Control (ConC) Monitor, detect and handle overload situations with the already connected users Bring the system back to a stable state, e.g. by means of dropping an existing call 26

27 Resource Consumption Service/BLER-dependent resource consumption Uplink example: Service I: Voice R b = 12.2kbps, E b /N t = 5dB a I = 0.99% Service II: Data R b = 144kbps, E b /N t = 3.1dB a II = 7.11% a In downlink there is additional dependency on the location of the user Cell center low consumption Cell edge high consumption 27

28 Admission/ Congestion Control Basic algorithm Admission control is triggered when load thr_cac New users are blocked Existing users are not affected as long as load < thr_conc Congestion Control is triggered when load thr_conc Reduce consumption of one or several users Simple action: drop the user Repeat until load < thr_conc 28

29 Blocking Probability Dropping Probability Call Admission Control: Simulation Results I Tradeoff between blocking and dropping Example: 64k per user, urban 50% 45% 40% thr_cac = 50% thr_cac = 75% thr_cac = 90% 20% 18% 16% thr_cac = 50% thr_cac = 75% thr_cac = 90% 35% 14% 30% 12% 25% 10% 20% 8% 15% 6% 10% 4% 5% 2% 0% % Offered Traffic [Erlang per site] Offered Traffic [Erlang per site] 29

30 Cell Loading Call Admission Control: Simulation Results II Cell load depending on CAC threshold Example: 64k per user, urban 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Offered Traffic [Erlang per site] thr_cac = 50% thr_cac = 75% thr_cac = 90% 30

31 Packet Data Control: Channel Switching Flexibility of packet services Asymmetrical data rates Very low to very high data rates Control information/user information Efficient transmission making good use of CDMA characteristics Dedicated channel (DCH) Minimise transmission power by closed-loop power control Independence between uplink and downlink capacity Common channel Random access in the uplink (RACH) Dynamic scheduling in the downlink (FACH) Adaptive channel usage depending on traffic characteristics Infrequent or short packets Common channel (Cell_FACH) Frequent or large packets Dedicated channel (Cell_DCH) No packet transmission UE stand by modus (URA_PCH) 31

32 Channel Switching Example CELL_DCH CELL_FACH CELL_DCH DCH Active Time Page Download Time Reading Time Chatty Applications Example: Web service Chatty apps.: keep alive message, stock tickers, etc. (e.g. 100 bytes every 15 sec) Second stage: when no activity in CELL_FACH then switch to URA_PCH 32

33 Power Control vs. Rate Adaptation NodeB high data rate area UE 1 low data rate area UE 2 Power Control: Balances user received quality (BLER, SIR) Users at cell center get less share of BTS transmit power assigned than at cell edge Occurrence of power overload Rate Adaptation: Transmit power is proportional to data rate Users at cell edge get lower data rate assigned than at cell center Reduces also power overload On DCH combination of power control and rate adaptation Rate assignment at begin of a transmission based on load and user location Rate adaptation when ongoing transmission according to power consumption and overload Based on RRC-signaling (time horizon: 100msec 10sec) 33

34 Outage Probability Mean Delay [sec] Rate Adaptation Performance 40% 35% 30% 25% UMTS_urban, 50 kbyte 384k 64k adaptive k 64k UMTS_urban, 50 kbyte adaptive 20% 4 15% 3 10% 2 5% 1 0% Cell Throughput [kbit/sec] Cell Throughput [kbit/sec] Rate adaptation significantly improves the RRM performance on DCH. 34

35 Dynamic Scheduling NodeB Flow #3 Sample Flow Flow #1 Flow #2 UE 2 UE 1 Statistical multiplexing of data packets from different data flows on one shared medium, e.g. on DSCH or HSDPA Scheduling with time-horizon of 2msec 1sec Optimised usage of radio resources Exploitation of the short-term variations on the radio channels (opportunistic scheduling) Can provide certain degree of QoS UE 3 35

36 References H. Holma, A. Toskala (Ed.), WCDMA for UMTS, 5 th edition, Wiley, 2010 H. Kaaranen, et.al., UMTS Networks: Architecture, Mobility and Services, 2 nd edition, Wiley, A. Viterbi: CDMA: Principles of Spread Spectrum Communications, Addison Wesley, J. Laiho, A. Wacker, T. Novosad (ed.): Radio Network Planning and Optimisation for UMTS, 2 nd edition, Wiley, 2005 T. Ojanperä, R. Prasad, Wideband CDMA for Third Generation Mobile Communication, Artech House, R. Prasad, W. Mohr, W. Konhäuser, Third Generation Mobile Communications Systems, Artech House, March GPP standards: TS : Physical Layer Procedures (esp. power control) TR : Radio Resource Management Strategies TR : RF System Scenarios 36