Cellular network planning_and_optimization_part7
- 1. Cellular Network Planning and Optimization Part VII: WCDMA RRM Jyri Hämäläinen, Communications and Networking Department, TKK, 1.2.2008
- 2. 2 Quality of Service (QoS)
- 3. 3 TE MT UTRAN CN Iu EDGE NODE CN Gateway TE UMTS End-to-End Service TE/MT Local Bearer Service UMTS Bearer Service External Bearer Service UMTS Bearer Service Radio Access Bearer Service CN Bearer Service Backbone Bearer Service Iu Bearer Service Radio Bearer Service UTRA FDD/TDD Service Physical Bearer Service Radio Access Bearer UE BS, RNC
- 4. 4 Radio Access Bearer Main task of the UTRAN is to create and maintain RAB for communication between UE and CN. RAB is build up in order to give for CN elements an illusion about fixed communication path to UE. The network builds up the end-to-end QoS connection from small pieces, which compose a complete chain without bottlenecks These pieces are called Bearers When the connection is set up, the network elements negotiate the QoS requirements of the bearers set up between them The result is a compromise, in which the QoS requirements and network’s capacity is taken into account
- 5. 5 UMTS QoS Classes File downloading, e-mailsBackground class Web surfingInteractive class Real-time streaming videoStreaming class Speech and video callsConversational class Example applicationTraffic Class
- 6. 6 UMTS QoS Classes Big variable delay, buffering allowed, asymmetric traffic, no guaranteed bit rate Background class Moderate variable delay, buffering allowed, asymmetric traffic, no guaranteed bit rate Interactive class Minimum variable delay, buffering allowed, asymmetric traffic, guaranteed bit rate Streaming class Minimum fixed delay, no buffering, symmetric traffic, guaranteed bit rate Conversational class PropertiesTraffic Class
- 7. 7 UMTS QoS Parameters QoS of some services are not negotiable (speech), packet data services admit various QoS classes QoS negotiable Set the limits for delay (>80ms)Allowed transfer delay Defines the bit rate that the UMTS bearer must carry between its end points Guaranteed bit rate Defines the maximum bit rate when delivering information between end points of UMTS bearer (<2Mbps) Maximum bit rate ExplanationParameter
- 8. 8 QoS Negotiation UE UTRAN (NB, RNC) CN UMTS bearer service: Request for UMTS QoS Class RAB assignment request QoS negotiationRadio bearer and radio link establishment UMTS Bearer service with negotiated QoS E2E service request Maximum bit rate Guaranteed bit rate Transfer delay QoS negotiable (y/n) Maximum bit rate Guaranteed bit rate Transfer delay QoS negotiable (y/n) RRM: Admission control RAB assignment response
- 9. 9 In early UMTS Release 99 all conversational and streaming class traffic were offered over the CS bearer Voice RT multimedia (e.g videotelephony) In early Release 99 only Interactive and background class traffic utilisises the PS bearer Release 4 capable networks introduce some streaming class traffic on PS bearer as well Release 5 brings along a full portfolio of PS bearers also utilised for conversational traffic QoS in UMTS
- 10. 10 The QoS over the air interface is implemented by matching each radio bearer with a transport channel whose format set defines the QoS parameters The mapping is performed during the establishment of the RAB RNC performs the mapping of RAB characteristics to actual resource requirements (vendor dependent) Example of mapping for web service, which belongs to the interactive class Parameters Interactive Class Radio Resource mapping Maximum bit rate 128 kbps SF=16 Maximum SDU size 1500 Map to Transport formats Residual BER 10^-6 1/3 turbo encoder Transfer Delay NA Interleaver = 40 or 80 msec Guaranteed bit rate 64 kbps Delivery order yes Use Acknowledged RLC SDU Error Ratio 1 % Set appropriate threshold for outer loop power control Delivery of erroneous SDU No Use Acknowledged RLC SF=16 QoS in UMTS
- 11. 11 Operators can define the wanted QoS profile (in HLR) per subscriber Users can be categorised (QoS differentiation) for various tariffing schemes Traffic handling priorities can be set (THP) Business Remote office Basic free time Traffic class All four allowed All four allowed Only converational (voice calls) and background Max bit rate 400 kbps 800 kbps 64 kbps Guaranteed bit rate 384 kbps 64 kbps 12 kbps Allowed THPs THP 1 (e.g. for e-mail downloads) THP 2 (e.g. for file transfer) THP 3 QoS in UMTS
- 12. 12 QoS in UMTS
- 14. 14 General Radio Resource Management (RRM) is elementary part of WCDMA. RRM is responsible for efficient utilization of the air interface resources it is needed to Guarantee Quality of Service (QoS) Maintain the planned coverage area Optimize the cell capacity The importance of RRM is mostly due to the features of the UMTS system; interference limited nature and adaptive services
- 15. 15 Introduction to RRM/objectives Cell coverage Cell capacity QoS OPTIMISATION Objectives of RRM • Ensure planned coverage for each service • Ensure required connection quality • Ensure planned max blocking • Optimise the usage of system capacity resources
- 16. 16 Introduction to RRM/Functions Packet Scheduler Load Control Admission Control Power Control Handover Control
- 17. 17 Introduction to RRM/Logical model MS Node B RNC • Power Control • Power Control • Load Control • Power Control • Load Control • Handover Control • Admission Control (also in SGSN) • Packet Scheduler
- 18. 18 RRM algorithms Family of RRM algorithms: Power control Fast power control (Node B, UE) Outer loop power control (RNC) Handover control (RNC) Admission control (RNC) Load control (RNC) Fast load control (Node B) Packet scheduling (RNC)
- 19. 19 Power control
- 20. 20 Power control Objectives Maintain the link quality in uplink and in downlink by controlling the transmission powers Prevents near-far effect Minimise effects of fast and slow fading Minimises interference in network Accuracy of the power control is important No time-frequency separation of users, all use the same bandwidth Inaccuracy in power control immediately lifts the network’s interference level, which correspondingly lowers the capacity Due to users mobility the speed of power control is also a critical issue
- 21. 21 Near-far problem in uplink There can large path loss difference between UE1 (cell centre) and UE2 (cell edge) If both UEs are transmitting with the same power then UE1 will block UE2 (and other cell edge users too) Power control will drive transmission powers of UE1 and UE2 to the minimum level that is required to meet QoS In Node B received powers from UE1 and UE2 will be the same for same services UE1 UE2
- 22. 22 Power control Power Control on the common channels ensures that their coverage is sufficient both to set up UE-originating and UE- terminating calls. Power Control on the dedicated channels ensures an agreed quality of connection in terms of Block Error Rate (BLER), while minimizing the impact on other UEs. Uplink Power Control increases the maximum number of connections that can be served with the required Quality of Service (QoS), while reducing both the interference and the total amount of radiated power in the network. Downlink Power Control minimizes the transmission power of the NodeB and compensates for channel fading. Minimizing transmitted power maximizes the downlink capacity.
- 23. 23 Power control Main power control approaches Fast power control: Aim is to compensate the effect of fast fading Gain from fast power control is largest for slowly moving UEs and when fading is flat, i.e. there is multi-path diversity Fast power control drives the received power to a target SIR. This value is discussed more closely in connection with dimensioning. Outer loop power control Adjust the target SIR according to service QoS.
- 24. 24 PC mechanism Outer loop PC: RNC adjust the target SIR in order to meet target BLER Fast PC: Node B command terminal to change transmit power in order to meet target SIR Received SIR Outer loop power control Fast power control
- 25. 25 Uplink outer loop PC The goal is to control the target SIR in order to sustain the wanted QoS with minimum transmit power The target BLER is defined with the admission control algorithm The uplink algorithm is controlled in RNC Update frequency from 10 Hz up to 100 Hz Outer loop power control will raise or lower the target SIR according to step size, which is defined by radio network planning. The equipments’ performance defines the minimum value for target SIR
- 26. 26 Downlink outer loop PC Implemented in UE to set SIR target on DL traffic channels Quality target: BLER of each transport channel as set by RNC Admission control determines the value of DL BLER. No SIR target change if NodeB power reaches maximum or network congestion occurs.
- 27. 27 Transmit Power Control (TPC) Ideal fast power control invert the channel In practice power control accuracy is reduced by feedback errors, Better figure, PC headroom etc Fast fading channel Transmitted power Note: It is usual to talk about ‘fast power control’ when power control is build up to mitigate fast fading. Transmit power control is WCDMA specific term
- 28. 28 Uplink TPC Update rate 1.5 kHz => fast enough to track and compensate fast fading up to x km/h mobile speed If received SIR > target SIR in Node B => UE is commanded to decrease its transmit power. Similarly UE is commanded to increase its transmission power if received SIR < target SIR Network planning defines the step size. Usual step size values are between 0.5dB and 2dB. Soft handover: UE can receive contradictory PC commands from different node Bs UE transmission power will be increased if all node B’s ask for it and decreased if at least one node B demands it
- 29. 29 Downlink TPC Similar as UL TPC: UE measures SIR on DL DPCCH during the pilot period (or use CPICH) UE maintains the QoS by sending fast power control commands (TPC bits) requesting power adjustment Power offsets can be used in DL in order to improve control reliability. Offsets are network parameters that can be set in planning phase
- 30. 30 TPC characters Main interference migitation means in UMTS TPC (1500Hz) is able to follow fast fading up to ~50km/h MS speed, after that the fading dips are averaged out In high MS speeds TPC can have even negative impact TPC lowers the required Eb/No, not so much tx-powers directly • Concerns in practise: •In SHO, DL powers can drift apart due to the inaccurate reception of uplink PC commands → Degraded SHO performance in case drift prevention not working •In SHO, DL PC commands cannot be combined in RAKE (because they contain different information). Data bits however can be combined → Worse reability for PC commands. => Can be improved by allocating more power to CCHs •Building corners in the urban areas • Average TPC headroom (4dB) must be assumed to pathloss.
- 32. 32 Handover types in WCDMA Inter-system HandoverIntra-system Handover UMTS -Handover Intra-frequency Handover Soft Hard UMTS<>GSM UMTS <> GPRSUMTS <> GPRSetc. HardSofter Inter-frequency Handover UMTS<>EDGE
- 33. 33 WCDMA Handover control Hard HO (HHO) All the old radio links of an UE are released before the new radio links are established. Real time bearers: short disconnection in transmission. Non real time bearers: HHO is lossless. Shared & common channels used for hard handover (cell reselection) Soft HO (SHO). MS always keeps at least one radio link to UTRAN. Soft HO: MS is simultaneously controlled by two or more cells belonging to diffetrent BTS of the same RNC or to different RNC. Softer HO. MS is controlled by at least two cells under one BTS. Dedicated channels (Cell_DCH state) used for SHO Handover can be either network or UE initiated Serving RNC makes the decisions in both cases
- 34. 34 WCDMA Handover control Core network RNC1 RNC2 RNC3 RNC4 BSC Node B Node B Node B Node B Node B BTS Iur Combining (Softer HO) Soft Handover Softer Handover Soft Handover Hard Handover Hard Handover Hard Handover
- 35. 35 Hard handovers Intra & Inter-frequency HHO’s Usually triggered to maintain mobility Not recommended in WCDMA unless there is an urgent need, because Hard HO increases interference easily, since the real-time user is disconnected temporarily and the used power must be re- evalueted This decreases the capacity in heavy traffic situations and can worsen the near-far effect Absence of Iur (connection between RNC’s) will cause hard HOs Compressed mode used in HOs between carriers and systems In compressed mode UE stop UL transmission for few milliseconds within a radio frame (10ms) in order to enable measurements of different carriers/systems
- 36. 36 Inter frequency handover IFHO can be used in planning to provide coverage (micro ֏ macro cell) provide capacity (reduce cell loading) 2nd carrier can be enabled on cell basis Not so straightforward to perform in UE due to need of compressed mode Most Network vendors’ equipment supports IFHO IFHO is generally seen as a means of optimisation as the traffic evolves, but can be used also e.g. to provide indoor coverage
- 37. 37 Soft Handover (SHO) SHO helps avoid near-far effect for real-time connection For high mobility users shadow fading + (slow) hard handovers would create near-far situations SHO is an essential interference mitigation tool in WCDMA
- 38. 38 DOWNLINK: SHO utilises two separate codes in DL (RAKE fingers in UE are assigned for reception) Maximum ratio combining done in UE for the signals Produced gain 1-3 dB, however... Gain depends on the difference of the signals’ strength Gain depends on channel conditions and accuracy of the received channel estimate → In some circumstanses the gain can be lost! The more multipath diversity is available the less SHO gain is achived Soft Handover
- 39. 39 UPLINK: More complex situation than in DL During softer HO, same procedure in node B’s RAKE than in DL case Produced gain 1-3 dB Better performance in terms of strenght differences, since the signals come from the same source During Soft HO, the combining of signals is done in the RNC Selection combining performed for baseband signal Based on selecting the frame with better FER or BER Better frame send to be used in open loop PC (target SIR estimation) Gain achieved through more stable UE tx-powers (1-2dB) → No actual gain to the radio link Softer/Soft Handover
- 40. 40 The cells in a WCDMA RAN are, from UE point of view, divided in different mutual excluding sets defined by 3GPP: Active Set The cells involved in soft handover and measured by the UE Monitored Set The cells only measured by the UE and not part of the Active Set. The monitored set can consist of intra-frequency, Inter-Frequency and Inter-RAT cells The cells measured by the UE are the sum of the Active Set and the Monitored Set. The number of Intra-frequency cells in the Monitored Set + the Active Set cells is limited by 3GPP to 32. The number of Inter-Frequency cells in the Monitored set is limited to 32. The number of Inter-RAT cells in the Monitored set is limited to 32. Soft Handover
- 41. 41 Active Set As UE moves, node Bs are continuously added to and removed from the active set. When added, they are also updated to the neighbor cell list. UE measures the monitored set of cells and Handover Control evaluates if any node B should be added to, removed from or replaced in the active set Maximum Active Set Size parameter is used to determine the maximum allowed number of SHO connections (varies between 1-5, typical default 3) Too high value decreases capacity (signalling increases and multiple connections occur too often) Too low value degreases the SHO performance (best candidate cells may be excluded in some situations) Soft Handover
- 42. 42 The handover measurements for Intra-Frequency HO are based on P-CPICH Ec/Io Ec/Io is the received signal code power divided by the total received power. It is calculated from signal before the signal de-spreading operation while Eb/No is calculated after de-spreading. Ec/Io can be be determined for the signal ”in the air” Eb/No depends on the service (bit rate, CS/PS, receiving end) and Ec/Io is service independent The accuracy of the Ec/Io measurements is essential for HO performance Depends on filtering lenght and mobile speed Filter length for slowly moving & stationary UE’s should be just long enough to avoid Fast Fading errors Too long filter length for will cause HO delays to fast moving UE Soft Handover
- 43. 43 Event based triggered measurements and reporting Basic reporting events 1A, 1B and 1C (Ref. 3GPP) 1A: Primary CPICH enters the reporting range 1B: P-CPICH leaves the reporting range 1C: Non-active P-CPICH becomes better than an active P- CPICH 1D: Change of current best cell with new P-CPICH Handover decision Done by RNC based on measurements and available resources Soft Handover
- 44. 44 Picture of events 1A and 1B. Example: The terminal sends an event 1A report to the RNC, if the new cell belongs to the monitored cells list and Active Set is not full. Then new cell is proposed to be added to the Active Set. If the Active Set is full, the cell is proposed as a replacement of the worst cell in the Active Set (1C) Soft Handover
- 45. 45 Picture of event 1C. Example: The terminal sends an event 1C report to the RNC if the new cell belongs to the monitored cells list and Active Set is not full. Then new cell is proposed to be added to the Active Set. If the Active Set is full, theen new cell is proposed as a replacement of the worst cell in the Active Set Soft Handover
- 46. 46 Picture of event 1D. Example: The terminal sends an event 1D report to the RNC if the cell belongs to the monitored cells list and Active Set is not full. Then cell is proposed to be added to the Active Set. If the Active Set is full, the cell is proposed as a replacement of the strongest cell in the Active Set Soft Handover
- 48. 48 Inter-Frequency Handover is a hard handover where the UE is ordered by the network to tune to another frequency. This means that there will be small interuptions in the data flow to and from the UE • When Inter-Frequency HO is considered, first the UE measures the conditions to start Compressed Mode • Usually Ec/Io of current carrier • Events 2d and 2f defined for IFHO • Time to trigger used Inter-Frequency HO (IFHO)
- 49. 49 Soft handover window 21 Soft handover margin, which is defined by radio network planning Received signal level in Node B SHO margin in planning tools Some 3G planning tools use one single SHO planning parameter (=SHO margin/SHO gain) Default Value varies between 2 and 6 dB Value for this parameter should be defined as: Reporting range1a + Reporting range1b 2 Handover margin =
- 50. 50 HO related topics in network planning Network topology: How sites are located relative to each other, how many sectors/site Node B antenna radiation patterns Overlapping patterns => more softer HOs Antenna tilt => Number of potential Node B’s in Active Set Path loss and shadow fading characteristics The average number of Node B’s that a UE can synchronise to HO parameter adjustments is part of the network optimization
- 51. 51 Admission and Congestion control
- 52. 52 Congestion and Admission Control Congestion/Load Control’s general responsibility is to remain the network in a stable state and prevent overloading Congestion/Load control is in close co-operation with functions of admission control and packet scheduler Load control operates in RNC: Admission control Packet scheduler Load control Information of network loading level Loading status NRT (Non-Real Time traffic) load
- 53. 53 Admission control If air interface loading is allowed to increase too much the coverage of the cell will be reduced below the planned value. Admission control decides whether to accept the terminal’s request for new radio access bearer by calculating how much interference new bearer would create to the cell in both UL and DL Congestion control Responsible of returning the network back into desired target load in case of overload Target load is set in network planning and overload should be an exceptional situation Admission and Congestion Control
- 54. 54 Load Interference power Maximum interference level defined by radio network planning ? Estimated growth in interference when new UE arrives to the cell New bearer’s load factor Maximum load level defined by radio network planning There are predefined UL and DL thresholds for interference power. Thresholds are set in network planning. If either UL or DL threshold is exceeded the RAB is not admitted. For decision AC may derive the transmitted bit rate, processing gain, radio link initial quality parameters, target BER, BLER, Eb/No, SIR target. Admission Control
- 55. 55 In case of congestion the use of resources are scaled down to reach normal loading status The priorisation and order of congestion control actions is based on vendor algorithms. Actions that can be carried out in order to decrease the load Deny power control commands received from UE Reduce the UL Eb/No target used in UL fast power control Reduce the throughput of packet data traffic Handover to other WCDMA carrier or to GSM Decrease bit rates in real time services Drop low priority data calls Congestion control
- 56. 56 Determines the available radio resources for NRT radio bearers Share the available radio resources between NRT radio bearers. Monitor the allocations for NRT radio bearers. Initiate the switching between common, shared and dedicated channels when necessary. Monitor the system loading. Perform load control actions for the NRT radio bearers when necessary. Packet scheduler
- 57. 57 Capacity can be divided between non-controllable and controllable traffic Load caused by real time traffic, interference from other cell users and noise together is called non- controllable load The part of the available capacity that is not used for non-controllable load can be used for NRT radio bearers on best effort basis (= controllable load). PS is implemented for dedicated (DCH) as well as common control transport channels (RACH/FACH). PS takes care of filling the controllable capacity with NRT traffic • The amount of scheduled capacity depends on: • UE and BTS capabilities, • the current load in the cell, • the availability of physical resources. Packet Scheduler
- 58. 58 For dimensioning purposes radio network planning can define packet access features per service, e.g.by next parameters: Amount of packet bursts per session Reading time between bursts Size of packets Arrival rate packets Amount of packets per burst Number of retransmission Packet Scheduler
- 59. 59 Load InterferenceAdmission control Congestion control Packet Scheduler Target level for interference Target level for interference+offset Threshold Maximum level Decrease bit rates and NRT Bearers are dropped Overload actions No actions New RT bearers are admitted normally NRT bearers are increased NRT load is not increased, but bit rate changes are allowed No actions No new bearers are admitted Preventive load control actions Decrease bit ratesNo new bearers are admitted Control summary