high -speed downlink packet access · – 63 values per channelization code and modulation scheme...

3G Evolution3G Evolution3G Evolution3G EvolutionChapter:Chapter:Chapter:Chapter: 9999

High-Speed Downlink

Deepak DasalukunteDepartment of Electrical and Information Technology

High-Speed Downlink Packet Access

16-Apr-2009 3G Evolution - HSPA and LTE for Mobile Broadband 1

OutlineOutlineOutlineOutline

• Overview– Shared channel transmission– Channel dependent scheduling– Rate control– Hybrid ARQ and soft combining

• Details/Finer details of HSPA– Channels – downlink/uplink, data/control...– Channels – downlink/uplink, data/control...– MAC-hs and physical layer processing

• Scheduling • Rate control• Hybrid ARQ with soft combining

• In-sequence delivery to higher layers from MAC• CQI/ downlink quality• Uplink and Downlink control signalling


Part I : Overview Part I : Overview Part I : Overview Part I : Overview

• Major extension of WCDMA radio interface• Enhancing WCDMA packet data performance and capabilities

– Higher peak data rates– Reduced latency – Increased capacity

• Achieved through– Channel dependent scheduling


– Channel dependent scheduling– Higher order modulation– Rate conrol – Hybrid ARQ and soft combining

Shared channel transmissionShared channel transmissionShared channel transmissionShared channel transmission

• Resources in a cell are common to user and shared dynamically– Downlink radio resources– Channelization codes

• Configurable: 1-15• Remaining: control and other purposes

– Transmit power • Allocation depending on requirement.


• Allocation depending on requirement.• Power remaining after serving other channels is allocated to HS-DSCH• 2ms TTI (transmit time interval):

– reduces overall latency – also exploited by rate control – and channel dependent scheduling

• More SCHs and CCHs later

Figure courtesy: 3G evolution: HSPA and LTE for mobile broadband by Erik Dahlman, Stefan Parkvall et. al

Recap from chapter 7Recap from chapter 7Recap from chapter 7Recap from chapter 7

• Channel dependent scheduling– Scheduler decides which user at a given time instance gets the resource– effective channel variations as seen by NodeB is better.– Larger gains with larger channel variations and larger number of users

• Rate control and higher order modulation– QPSK, 16QAM– Higher bandwidth utilization in better channels– Data rate varies every TTI (2ms)


– Data rate varies every TTI (2ms)• Hybrid ARQ and soft combining

– Incremental redundancy– Chase combining

• Discussed in this chapter:– Parameters to be used

• Channelization codes, modulation schemes, coding rates, transport block sizes etc.

– when and what to use in a particular situation – Implementation details: specifications, actual numbers and examples


Architecture Architecture Architecture Architecture

• HSDPA techniques: adaptation to variations in radio conditions– Should be placed close to the radio interface ￫ NodeB

• Minimize architectural changes– Simplifies HSPA introduction in already deployed networks– Cells not upgraded to HSPA can co-exist

• A new MAC sub layer in NodeB: MAC-hs• A new MAC sub layer in NodeB: MAC-hs• At network side HSDPA introduction implies

– Enhancements to RNC– MAC-hs layer in NodeB

• UE can move out of the cell supporting HSDPA and vice versa.– Uninterrupted service to user (lower data rate)– Switch user to deidcated channel in non-HSDPA cell– To enter HSDPA cell: UE should be HSDPA-capable



Part II: Details/Finer details of HSDPAPart II: Details/Finer details of HSDPAPart II: Details/Finer details of HSDPAPart II: Details/Finer details of HSDPA

• Channels – downlink/uplink, data/control...• MAC-hs and physical layer processing• Scheduling, rate control• Hybrid ARQ with soft combining


• Finer details:– In-sequence delivery to higher layers– CQI/ downlink quality– Uplink and Downlink control signaling

Channels: Shared/Dedicated, Channels: Shared/Dedicated, Channels: Shared/Dedicated, Channels: Shared/Dedicated, Uplink/Downlink, …Uplink/Downlink, …Uplink/Downlink, …Uplink/Downlink, …

• HS-DSCH: transport channel, supports• Shared channel transmission• Channel dependent scheduling• Rate control• Hybrid-ARQ with soft combining

• Other channels• Control signaling• Circuit switched services

NodeB

UE

• Circuit switched services• Resource sharing mainly in time domain

• Exploit channel dependent scheduling advantages• Code domain also possible with channelization codes

• Constant Tx power with HS-DSCH• 2ms TTI result of tradeoff between

• Obtaining small end user delay• Reduce control signaling overhead



Channels: Shared/Dedicated, Channels: Shared/Dedicated, Channels: Shared/Dedicated, Channels: Shared/Dedicated, Uplink/Downlink, … (2)Uplink/Downlink, … (2)Uplink/Downlink, … (2)Uplink/Downlink, … (2)

• HS-SCCH(Shared Control CH): control signaling for DSCH – Notifies

• code tree used• Modulation scheme• Block size

– All users receive this, to find out if they have been scheduled or not.• HS-DPDCH: uplink user data.• HS-DPDCH: uplink user data.• HS-DPCCH (Dedicated Physical Control CH): Uplink control signaling

– ACK/NAK – CQI: Downlink channel conditions fed back to NodeB

• for channel dependent scheduling and rate control• DPCH (Dedicated Physical CH): power control commands (NodeB￫UE)

– Can also be used for user data– f-DPCH (fractional): reduce consumption of downlink channelization codes.


MACMACMACMAC----hs and PHY layer processinghs and PHY layer processinghs and PHY layer processinghs and PHY layer processing

• Changes in MAC-hs, reflects some changes in PHY layer• MAC-hs

• Scheduling• Priority handling• Transport-format selection (block size)• Hybrid-ARQ mechanism

• PHY layerPHYMAC

• PHY layer• Rate 1/3 turbo coding• Rate matching (RM) – to obtain code rate selected by rate control mechanism

– Puncturing/repetition• RM as a part of hybrid-ARQ

– Generate different redundancy versions -IR• Constellation rearrangement if 16-QAM is used.



MACMACMACMAC----hs: Schedulinghs: Schedulinghs: Schedulinghs: Scheduling

• Implementation is not specified• Information required for scheduling

– Instantaneous channel conditions at UE– Buffer status and priorities of data flow

• Channel Quality Indicator(CQI) – 5 bits of information fed back to nodeBs (HS-DPCCH)– Calculated at UE using the received pilot symbols– Calculated at UE using the received pilot symbols– Converted to transport block size, also accounting Rx performance

• For same channel, a more advanced UE receiver reports higher CQI• Important signals are put on higher priority level in the scheduler

– Radio resource control signaling information about change of cell.– Streaming services also can tolerate only a marginal delay

• For this a priority queue is included which the scheduler makes use of.


MACMACMACMAC----hs: Rate controlhs: Rate controlhs: Rate controlhs: Rate control

• Adjusting data rate to match channel conditions• Modulation• Channel coding rate

• MAC-hs sets transport format independently• Transport block:

– 254 different possibilities– 63 values per channelization code – 63 values per channelization code and modulation scheme– 13 – 27952 bits– coding rate: 1/3 to 1

• The block size also depends on the traffic situation– Better channel conditions implies larger block size– Relatively smaller block size enough at low traffic conditions



HybridHybridHybridHybrid----ARQ with soft combiningARQ with soft combiningARQ with soft combiningARQ with soft combining

• Faster compared to RLC based retransmissions• No signaling between nodeB and RNC• RLC (higher layer within MAC) configured with infrequent status reports• 1 transport block per TTI and entire block re-transmitted

– Lesser uplink signalling• Short TTI ensures static channel during transmission of one transport

blockblock• Incremental redundancy during re-transmissions is achieved through

the rate matching block in PHY• Soft combining happens through new data indicator for the same

transmitted block


HybridHybridHybridHybrid----ARQ with soft combining (2)ARQ with soft combining (2)ARQ with soft combining (2)ARQ with soft combining (2)

• One hybrid-ARQ entity with multiple hybrid-ARQ processes– To allow continuous transmission– No. of processes is configurable– Configured according to roundtrip time b/w nodeB and UE – Up to 8 processes configurable, typical is 6 as it provides 2.8ms of

processing time for nodeB– Each process has its own soft buffer in UE


– Each process has its own soft buffer in UE– Soft buffer in UE configured by nodeB by downlink signaling– Transport blocks out of sequence:

• reordering before passing it to RLC


Changes in PHY layer due to MACChanges in PHY layer due to MACChanges in PHY layer due to MACChanges in PHY layer due to MAC----hshshshs

• MAC-hs• Scheduling• Rate control – modulation scheme selection• Transport-format selection – block size using no. of channelization codes and

modulation scheme• Hybrid-ARQ mechanism – multiple Hybrid-ARQ processes

• PHY layer• Rate matching (RM) – to obtain code rate selected by


• Rate matching (RM) – to obtain code rate selected by rate control mechanism

– Puncturing/repetition• RM as a part of hybrid-ARQ

– Generate different redundancy versions -IR• Constellation rearrangement if 16-QAM is used.

HybridHybridHybridHybrid----ARQ : PHY layer processingARQ : PHY layer processingARQ : PHY layer processingARQ : PHY layer processing

• Rate matching: • Corresponds rate control and hybrid-ARQ in MAC

• Rate chosen by MAC-hs layer independently• Output bits from Turbo coder different• RM works in 2 stages

– 1st stage: limits to match soft buffer in UE– 2nd stage: to match physical channel block size

Rate matching block

– 2nd stage: to match physical channel block size• Depends on modulation and channelization code• Help in generating different sets of coded bits (r,s)

– s: systematic bits, r: retransmission

• Example:• MAC-hs configures block size = 3840 (QPSK, 4

channelization codes)• Transport block to be transmitter = 2404• UE soft buffer size for one hybrid-ARQ process is

7000


Figure courtesy: 3G evolution: HSPA and LTE for mobile broadband by Erik Dahlman, Stefan Parkvall et . al

(size=7000)

(size=3840)

HybridHybridHybridHybrid----ARQ : PHY layer processing (2)ARQ : PHY layer processing (2)ARQ : PHY layer processing (2)ARQ : PHY layer processing (2)

• Constraints on r and s in 2nd stage of RM – more details– Initial transmission: s=1 and code rate<1 for good performance– s=1 and r=0 for all re-transmission attempts– Same set of bits in re-transmission: chase combiningchase combiningchase combiningchase combining– s=1 and r=0 initial Tx; s=0 and r>0 in re-Tx: incremental redundancyincremental redundancyincremental redundancyincremental redundancy– More possibilities using ssss and rrrr


Interleaving and constellation Interleaving and constellation Interleaving and constellation Interleaving and constellation rearrangementrearrangementrearrangementrearrangement

• Systematic bits more important than parity bits in Turbo decoding• While using 16QAM

– Systematic bits mapped to more reliable positions in 16 QAM symbol– Two interleaver scheme

• QPSK – Single interleaver

• 16QAM with hybrid-ARQ with chase combining– Performance gain with constellation rearrangement + re-transmissions– Gains not significant with incremental redundancy

• Constellation rearrangement: – bit manipulations/reordering– Essentially selecting one out of 4 constellations for 16QAM– ?? Which is which is not evident (supposedly color coded)



Mobility Mobility Mobility Mobility

• Change of serving cell• RRC signaling

• Measurements from UE reported to RNC • RNC reassigns the UE to corresponding NodeBs• Several measurement mechanisms• Measurement event 1D is one such

• Common pilot strength from neighboring cell • Common pilot strength from neighboring cell is reported to be stronger than current cell

• Reconfiguration of UE– Synchronous

• Pre-defined activation time, all nodes involved switch at this time• Packet losses during handover, taken care of by RLC protocol

– Asynchronous • Involved nodes reconfigure as soon as they receive the message• Can result in data loss, due to delays in one of the nodes not updating quickly



Downlink control signaling: Downlink control signaling: Downlink control signaling: Downlink control signaling: HSHSHSHS----SCCH; FSCCH; FSCCH; FSCCH; F----DPCHDPCHDPCHDPCH

• UE to properly despread, demodulate, and decode HS-DSCH data• Every HS-DSCH TTI, one HS-SCCH carries PHY layer signaling• Several SCCHs in parallel can exist

• UE should be able to decode 4 HS-SCCHs in parallel • HS-SCCH carries

• Transport format– Channelization code– Channelization code– Modulation scheme– Transport block size info

• Hybrid ARQ– Hybrid ARQ process #– Redundancy version– New data indicator

• UE ID that identifies which UE the HS-SCCH is intended• Power control commands for UE

• Adjust uplink transmission power• When UE is moving towards/away from the NodeB


Uplink control signaling: HSUplink control signaling: HSUplink control signaling: HSUplink control signaling: HS----DPCCHDPCCHDPCCHDPCCH

• Hybrid-ARQ protocol• For NodeB to get instantaenous channel conditions• Separate physical channel using channelization code • One bit for ACK/NAK

− Repetition coded to 10 bits to fit first slot of HS-DPCCH sub-frame− Possibility to repeat ACK/NAK in 10 subsequent ACK/NAK slots

• Cannot receive HS-DSCH in those consecutive TTIs


• Cannot receive HS-DSCH in those consecutive TTIs• But can be helpful in soft handover situations / very large cells

• Much more protocol specific information related to control signaling

Other topics Other topics Other topics Other topics

• In-sequence delivery– Multiple hybrid-ARQ processes does not ensure delivery of packets in

sequence.– Reordering required as the upper layer assumes in-sequence delivery.– Reordering queue: store all data blocks until all data blocks with lower

sequence number have been delivered.– Timer based mechanism to determine lost data blocks.– Timer based mechanism to determine lost data blocks.

• Resource control for HS-DSCH– Parts of Radio resource management handled by NodeB– RNC can set max Tx power for NodeB to use for HSDPA related transmissions– Admission control: allowing a new user enter the cell (Tx power availability)


Others topics (2) Others topics (2) Others topics (2) Others topics (2)

• UE categories– used by network to select a configuration– Soft buffer memory (14 400-172 800 soft bits)– Capability to de-spread physical channels (5,10,15)

• Data flow– Flow of user data in different layers– Flow of user data in different layers

• MAC-hs header– Reordering of higher layer (MAC-d) PDUs

• CQI and other means to assess the downlink quality– Tables showing what each UE category supports and so on


Chapter summaryChapter summaryChapter summaryChapter summary

• Channels: data/control, uplink/downlink... for signaling and data• New MAC-hs sub layer introduced in NodeB

– Scheduling– Rate control:

• modulation schemes• block size choice through CQI feed back

– Hybrid-ARQ with soft combining– Hybrid-ARQ with soft combining– Control signaling

• PHY layer– Rate control from MAC-hs resulted in rate matching

• Puncturing/repetition to match UE soft buffer, transmit block size– Hybrid-ARQ: rate matching also used here for incremental redundancy and

chase combining• Reordering due to multiple hybrid-ARQ processes• Mobility: handover


Questions?Questions?


high -speed downlink packet access · – 63 values per channelization code and modulation scheme...

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