HSDPA Mobility

HSDPA/HSUPA In WCDMA Networks -Mobility IssuesDr. Tayeb SADIKI

01-Avril-2009

8301253 Advanced Topics in Radio Network Planning, TUT

Contents (1/2)Introduction to HSDPA/HSUPAMotivationPosition in UMTS systemDCH and DSCHHS-DSCHNew featuresPerformance

MAC-HSRadio Access Network ArchitectureMAC Layer Architecture (Release 6: HSDPA + HSUPA)MAC-hs and MAC-e/es


Contents (2/2)Mobility in HSDPAMobility in generalMobility in HSDPA

Mobility Studies (papers)HSDPA Handover StrategiesHSDPA Uplink Coverage ConsiderationsHSDPA Downlink Capasity ConsiderationsAntenna Diversity Simulations in HSDPAMulti-user Diversity Simulations in HSDPAHSUPA simulations (shortly)


Introduction to HSDPA/HSUPA


MotivationSophisticated UE applications need higher bit rates

Primary target of HSDPA/HSUPA is to enhance system throughput with minimum changes in network architecture

Is an extension to WCDMA Release (99)Release 4 (99)DCH + DSCH (current situation)Release 5HSDPA (High Speed Downlink Packet Access)Release 6HSUPA (High Speed Uplink Packet Access)


Position in UMTS systemCurrent WCDMA network can be upgraded to support HSDPA

Does not change network architecture dramatically

There can be HSDPA supported cells and regular cells

If HSDPA is not accessible at UE location it will use normal DCH to communicate at regular service speeds

HSDPA/HSUPA is more suitable to indoor environment


Release 4: DCH and DSCHPacket data transfer possibilities for downlink according to Release 4:DCH (Dedicated Channel)DSCH (Downlink-Shared Channel)FACH (Forward Access Channel)

DCH has fixed spreading factor (SF)DSCH has variable SFDSCH may be fast power controlled as DCH isDSCH does not support soft handoversDSCH has been designed to operate always together with a DCH

Data with tight delay budget (e.g. speech) DCHPacket data DSCH


HS-DSCHThe Release 5 with HSDPA concept includes a new channel High Speed Downlink Shared Channel (HS-DSCH)Two fundamental features of WCDMA are disabled:Variable SFFast Power ControlThese two features are replaced byAdaptive Modulation and Coding (AMC)Extensive multicoding operationFast retransmission strategy


New features(1/2)Shared channel transmissionHS-DSCH (high speed downlink shared channel)Supports up to 15 codes parallelFixed spreading factor (16)Works in parallel to DCH

Higher-order modulationQPSK16-QAM

Short transmission time interval (TTI)Dynamic channel code allocation interval of 2 ms


New features(2/2)Fast link adaptationAdjusts transmission parameters not TX power!Users near Node B: QPSK 16-QAM (for example)

Fast schedulingAllocates the use of shared channel to UEs with best radio conditions at certain time moment (Multi User Diversity)Scheduling is done at Node-B instead of RNC

Fast hybrid automatic-repeat-request (H-ARQ)Request and retransmit missing data (UE Node-B)Combine information from original transmission (Soft Combining)Signalling with ACKs and NACKs


PerformanceThe primary benefit of HSDPA is improved end-user experience (higher bit rates, reduced roundtrip times)

The benefit of HSDPA to operators is improved system capacity

H-ARQ and TTI will make small object TCP traffic faster (reduced roundtrip times)

Link adaptation maximizes channel usage and enables the base station to operate close to maximum cell power


Medium Access Control (MAC) layer


Radio Access Network Architecture (1/2)All Release 4 transport channels are terminated at the RNCRetransmission procedure is located in serving RNCIn Release 5 an additional HSDPA MAC layer (MAC-HS) is installed in the Node BRetransmissions will happen closer to air interface - in the Node BsFaster retransmissionsShorter delays with packet data operationFlow control mechanism is needed in Iub interface between Node B and RNC to ensure correct Node B data buffering (to prevent data losses at Node B)


Radio Access Network Architecture (2/2)RNC handles RLC (Radio Link Control) functionalitiesIf e.g. HS-DSCH TX from Node B fails for some reason


MAC architecture (UE side)Relates to older MAC (medium access control) architectureMAC-dHandles DCH trafficNew MAC entities are implemented according to latest Release 6 specificationMAC-hs (already in Release 5)To handle HS-DSCH traffic (HSDPA)MAC-e/es (in Release 6, not closed yet)To handle E-DCH traffic (HSUPA)

UE side MAC architecture


DCH

DCH

MAC-m

DTCH

DTCH

DCCH

CTCH

RACH

FACH

MAC-d

MAC-hs

MAC Control

MSCH

CPCH

( FDD only )

BCCH

CCCH

SHCCH

( TDD only )

PCCH

PCH

MTCH

MAC-es / MAC-e

HS-DSCH

Associated Uplink Signalling

Associated Downlink Signalling

E-DCH



MSCH

MTCH

MCCH

FACH

MAC-c/sh/m

FACH

MAC architecture (UTRAN side)Quite similar to UE side except:One MAC-d for each UEAll UEs in a cell will use one MAC-c/sh/mOne MAC-e (in Node-B) and MAC-es (in SRNC) entity is configured for each UE that uses E-DCH (HSUPA)MAC-c/sh/m handles common channels and DSCHUTRAN side MAC architecture


MAC-hs (UTRAN side)One MAC-hs entity in UTRAN for each cell that support HS-DSCH transmissionScheduling/priority handlingManages HS-DSCH resources between HARQ entities and data flows according to their priorityDetermines either to send a new transmission or a retransmission based on uplink signallingDetermines QueueID and TSN for each new MAC-hs PDUHARQOne HARQ entity per userMultiple instances of stop and wait HARQ protocols supported for each HARQ entityTFRC selectionSelection of appropriate transport format and resource for the dataTo UEFrom network


HARQ entity

Priority

Queue

Priority Queue

distribution

Priority Queue

distribution

MAC-hs


MAC Control

TFRC selection

Scheduling/Priority handling

Priority

Queue

HS-DSCH


Priority

Queue

Priority

Queue

MAC-d flows

MAC-hs (UE side)HARQHandles all the tasks that are required for hybrid ARQGenerates ACKs or NACKsConfigurations of the H-ARQ protocol is provided by RRC over the MAC-Control SAPRe-ordering queue distributionReorders incoming MAC-hs PDUs to ordering buffer based on queue IDReorderingReorders received PDUs according to received transmission sequence number (TSN)Delivers complete objects to disassembly functionPDUs are not delivered if PDUs with lower TSN are missingDisassemblyRemoves MAC-hs header and padding bits and passes the MAC-d PDU to higher layersUE side MAC-hs detailsFrom networkTo UE



Re-ordering queue distribution

Disassembly

HS-DSCH

MAC Control

Disassembly

Reordering

HARQ

Reordering

To MAC-d


MAC-hs

_1047110271.doc

HS-DSCH

MAC HS-DSCH

MAC Control

HS-DSCH

HARQ

RR handler

_1047184317.doc

HS-DSCH

MAC-hs

MAC Control

HS-DSCH

HARQ

RR handler

MAC-e (UTRAN side)Handles HSUPA specific functions in Node B

One MAC-e entity in Node B for each UE

One E-DCH scheduler function in Node B

HARQ entity generates ACKs and NACKs

E-DCH Scheduling function manages cell resources between UEs

De-multiplexer separates MAC-e PDUs to MAC-es PDUs and forwards them in the associated MAC-d flow (to SRNC)To networkFrom UE


HARQ entity

De-multiplexing

E-DCH

Scheduling (FFS)

MAC-e


MAC Control

E-DCH

Control (FFS)

E-DCH


MAC-d Flows

MAC-es (UTRAN side)MAC-es handles E-DCH specific functionalitiesOne MAC-es entity in SRNC for each UEReordering queue distribution routes PDUs to correct reordering buffers according to SRNC configurationMacro diversity selectionPerformed in MAC-es in case of SHO with multiple Node-BsReordering Queue Distribution entity receives all the MAC-d flows (DCHs) from all the Node-Bs (in SHO), and one MAC-es entity per UEExact implementation is not specifiedFrom Node-B


Reordering Queue

Distribution

Reordering Queue

Distribution

Disassembly

MAC-es

From

MAC-e in

NodeB #k

MAC Control

Reordering/

Combining

MAC-d flow #n

Reordering/

Combining

MAC-d flow #1

From

MAC-e in

NodeB #1

Disassembly

Reordering/

Combining

Disassembly

To MAC-d

MAC-e/es (UE side)MultiplexingResponsible for concatenating and multiplexing PDUsMAC-d MAC-es MAC-e Sets TSN for each PDUHARQHandles MAC functions relating to HARQ protocolStores MAC-e payloads and retransmits them when neededProvides E-TFC, retransmission sequence number (RSN) and power offset used by L1Configurations of the H-ARQ protocol is provided by RRC over the MAC-Control SAP

To networkFrom UE


Associated Uplink Signalling E-TFC

(E-DPCCH)

Multiplexing and TSN setting

Associated Scheduling Downlink Signalling

(E-AGCH / E-RGCH(s))

E-DCH

MAC Control

E-TFC Selection

HARQ

To MAC-d

Associated ACK/NACK

signaling

(E-HICH)

MAC-es/e

Mobility in HSDPA


Mobility in generalMobility:Due to user movement in networkUE speedUE travel distanceWhat is the best way to make a handover when UE changes between cells?Hard handover is simple but maybe not so effectiveSoft handover is more complicated in network point of view but usually saves capasityHSDPA does not support soft handovers


Mobility in HSDPA (1/7)

No soft handovers in HS-DSCHHSDPA control channels are sent via only one of the radio links assigned to UE (from serving HS-DSCH cell)UTRAN determines the serving HS-DSCH cell for an HSDPA capable UESynchronized change of serving HS-DSCH cell is supported between UTRAN and UE connectivity is achieved if UE moves from one cell to anotherServing HS-DSCH cell change is triggered by UE measurement reports and determined by UTRANUTRAN (RNC) dictates the time moment when serving cell is changedThis gives full mobility and coverage to exploit the advantages for HSDPA over Release 4 channelsServing HS-DSCH cell can be changed: Without changing users active set for Release 4 dedicated channelsIn combination with establishment, release or reconfiguration of Release 4 dedicated channelsA new UE measurement event is needed to provide these properties


Mobility in HSDPA (2/7)Reports the best serving HS-DSCH cell to the serving RNCBased on P-CPICH Ec/Io or RSCP measurementsDecision can be made using:Whole cell candidate setCan be restricted to use only the cells which are in users active set for dedicated channelsHysteresis margin can be used to avoid fast change of serving HS-DSCH cellCell Individual Offset value (CIO) can be used to favour certain cells (to increase cells coverage area)Measurement event for best serving HS-DSCH cell (1d)


Mobility in HSDPA (3/7)Three handover typesIntra-Node B HS-DSCH to HS-DSCH handover Inter-Node B HS-DSCH to HS-DSCH handoverHS-DSCH to DCH handover


Mobility in HSDPA (4/7)Node-B and UE are informed about incoming handover by SRNCTime moment for the handover is specified by the SRNCAll transmission from the source cell stops at that specified time and the packet scheduler in the target cell is then allowed to control transmission to UEMAC-hs preservation:Buffered data (for H-ARQ protocol: data waiting acknowledgement or new PDUs) in source cell is moved to target cell inside the Node B no data loss!H-ARQ manager will continue without breaks or retransmissionsNo higher layer retransmissions (e.g. from RLC protocol)If MAC-hs preservation is not supported same as inter-Node B handover case

Intra-Node B HS-DSCH to HS-DSCH Handover


Mobility in HSDPA (5/7)Node Bs are potentially under different RNCNode-Bs, target cell RNC and UE are informed about incoming handover by current SRNCAt the time of cell change (usually 300-500ms from the RNC decision []), the MAC-hs for the user in the source cell is reset (user data in buffers is deleted)At the same time the MAC-hs flow control unit in target cell starts to request PDUs from the new SRNC

Inter-Node B HS-DSCH to HS-DSCH Handover

Higher layer retransmissions (RLC) are needed to recover the data that was destroyed in buffer resetIf RLC is used in unacknowledged mode and if user application does not have retransmission mechanisms, some data is lost forever when a handover occurs!


Mobility in HSDPA (6/7)Needed when user moves from HSDPA capable cell to a cell that does not support HSDPANode Bs, target cell RNC and the user are informed about incoming handover eventBuffers in Node B are reset as in previous case and similar retransmissions are required because of lost buffer data

HS-DSCH to DCH Handover

Transmission continues in DCH


Mobility in HSDPA (7/7)


References (1)[1]Holma, Toskala, WCDMA for UMTS[2]3GPP TS 25.321 V.6.3.0, Medium Access Control (MAC) protocol specification, Release 6[3]3GPP TS 25.308 V6.3.0, High Speed Downlink Packet Access (HSDPA) Overall description, Release 6[4]Ericsson, WCDMA Evolved, The first step - WCDMA, White paper, 2004


Mobility StudiesStudied papers:

[5] Pedersen, Toskala, Mogensen, Mobility Management and Capasity Analysis for High Speed Downlink Packet Access in WCDMA,[6]Pedersen, Lootsma, Stttrup, Frederiksen, Kolding, Mogensen, Network Performance of Mixed Traffic on High Speed Downlink Packet Access and Dedicated Channels in WCDMA[7]Ramiro-Moreno, Pedersen, Mogensen, Network Performance of Transmit Diversity in HSDPA under Different Packet Scheduling Strategies[8]Bertinelli, Malkamki, HARQ FOR WCDMA ENHANCED UPLINK: LINK LEVEL PERFORMANCE IN SHO


HSDPA Handover Strategies Two types of handover strategies are studied

HS-DSCH to DCH if active set > 1No HSDPA coverage in soft handover areas

Direct change from HS-DSCH to target cell HS-DSCHFull HSDPA coverageHSDPA users will receive data on HS-DSCH independently of their position


HSDPA Uplink Coverage Considerations (1/3)It makes sense to allow HSDPA users to receive data on HS-DSCH only if there is sufficiently good uplink coverage

High Speed Dedicated Physical Control Channel (HS-DPCCH) carries Layer 1 ACKs/NACKs and the Channel Quality Indicator (CQI) report

This means that HS-DPCCH should be hearable at the Node B to correctly receive ACKs and NACKs


HSDPA Uplink Coverage Considerations (2/3)The Simulation Case: UE in SHO and downloading data on HS-DSCH over TCPParallel ongoing speech call (12.2kbps) on DCH DCH return channel is also used for TCP Acks and HS-DPCCHLink level simulations with user in 3-way SHORepeated with: Different power offsets between HS-DPCCH and DPCCHDifferent degrees of Ack/Nack repetitions for HS-DPCCH


HSDPA Uplink Coverage Considerations (3/3) Detection error probabilities listed in table can be fulfilled for a user in SHO when ACKs/NACKs are sent twice before decoding and still the max user TX power is not reachedHence, UL coverage is not a problem for users in SHO area

HS-DPCCH detection error probabilities

User in SHOUser in non-SHOPr(Ack|Nack)0.01%0.01%Pr(Nack|Ack)1%1%


Downlink Capasity Considerations (1/4)HSDPA Bearer Gain:

PDCH: average required TX power to serve a user on DCH with given average bit rate PHSDPA : average required TX power to serve a user on HSDPA with same bit ratePHSDPA includes TX power of both HS-DSCH and associated DPCHDPCH is assumed to carry only 3.4kbps L3 signallingConstant TX power is assigned to HS-DSCH and it is time shared between HSDPA users average TX power to serve a user depends from scheduling strategyTwo cases are consideredBlind schedulingUser is scheduled independently of radio channel conditions (Round Robin schduler)Intelligent schedulingUser is only scheduled in good fast fading conditions (Proportional Fair scheduler)Scheduling frequency is higher for blind scheduling


Downlink Capasity Considerations (2/4)G-factor is defined as: at the user (DL)

HSDPA bearer gain is slightly lower in SHO case (with average branch power ratio 0 dB) because the DCH benefits from additional diversity that SHO providesIn branch power ratio of 3 dB case, the SHO gain on the DCH turns into a loss since the DCH is assumed to be transmitted with equal power from two cells even though there is an unbalance of 3 dB between SHO legs ACTIVE SET SIZE = ONE USERS DCH IS IN 2-WAY SHO


Downlink Capasity Considerations (3/4)HSDPA cell capasity gain over a cell with only DCH users:

Q: a fraction of total Node B power allocated to HSDPA (Q = 0.75 here)P: probability of users in SHO or non-SHO situations (got from SHO statistics figure, 0.6, 0.4)W: HSDPA bearer gains (got from the figures shown before, 2.7, 2.6)

If proportional fair (PF) scheduler is used, the cell capasity gain is:

C: additional gain from given higher bit rates to users near the Node B (C = 1.3 here)


Downlink Capasity Considerations (4/4)ResultsCase 1: direct change of best serving HS-DSCH cell (all users using HS-DSCH)GainPF = 2.59Case 2: users in SHO area uses DCH instead of HS-DSCHCorresponds to WSHO = 1 GainPF = 1.96Case 2 results indicate decreased HSDPA cell capasity gain if mobility for HSDPA users is supported via channel switching to DCH as users move to SHO areaAccording to these simulations the best solution is to use direct change of the serving HS-DSCH cell during handoverIf switching to DCH is needed for some reason, one should minimize the SHO areas by using small SHO window to keep the HSDPA capasity gain at high levels


Antenna Diversity Simulations in HSDPA (1/2)PF scheduler is usedPedestrian-A is usedSTTD: space-time transmit diversityCLTD: closed loop mode-1 transmit diversity2Rake: dual Rake receivers at UE1Tx-1Rx: single antenna transmission and receptionIdea: CQI reports are subject to signalling delays so the system performance is sensitive to the UE speed


Antenna Diversity Simulations in HSDPA (2/2)Result: Cell capasity decreases when UE speed is increasedCapasity degradation is slower if antenna diversity techniques are usedSystem becomes more robust to signalling delaysSTTD provides throughput gain for UE speeds more than 6 kmphNote that the throughput degradation is dominated by the CQI signalling delay, since the feedback delay for CLTD is smaller


Multi-user Diversity GainMulti-user diversity gain of PF scheduler over RR scheduler for HSDPA with 7W and 5 HS-PDSCH codes6 HSDPA usersConclusion:Multi-user diversity gain is marginal with UE speeds greater than 15 kmphThis is due to the fact that MAC-hs can no longer accurately track Ec/No variations for the usersThis is due to the delay from the time where the user terminal estimates the CQI until the time where the HS-DSCH is transmittedHowever, HSDPA still provides a gain over DCH because of the L1 HARQ mechanism


HSUPA simulationsSimulation parameters:Channel model :Pedestrian A (3km/h)Vehicular A (30km/h)Data rates 32/144/384kbpsOutband info. bits 14 or 3

Macro Diversity Combining (MDC) at RNC is compared to case without MDC


HSUPA simulationsIn Veh A case, the MDC gain is more limited than in Pedestrian A case, ranging from 0.5 to 1.5dB. This fact can be explained considering that VehA model includes much more diversity than PedA. The gain due to macro diversity (exploited at the RNC), still present, is therefore more limited than in the Ped A case.


HSUPA simulationsIt is clear from figure on the left that the best choice, in the example considered, is using 144kbps, with a resulting lower received SNR (and then increased capacity for the system).ConclusionThe use of HARQ cannot quarantee high margin of gain over pure ARQ for low data rate services: its use is much more convenient for higher data rates (e.g. 144/384kbps or even higher).


References (2)[5] Pedersen, Toskala, Mogensen, Mobility Management and Capasity Analysis for High Speed Downlink Packet Access in WCDMA,[6]Pedersen, Lootsma, Stttrup, Frederiksen, Kolding, Mogensen, Network Performance of Mixed Traffic on High Speed Downlink Packet Access and Dedicated Channels in WCDMA[7]Ramiro-Moreno, Pedersen, Mogensen, Network Performance of Transmit Diversity in HSDPA under Different Packet Scheduling Strategies[8]Bertinelli, Malkamki, HARQ FOR WCDMA ENHANCED UPLINK: LINK LEVEL PERFORMANCE IN SHO


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HSDPA Mobility