scheduling and harq - seminar lte: der mobilfunk der … · scheduling and harq seminar lte: der...

Scheduling and HARQScheduling and HARQSeminar LTE: Der Mobilfunk der ZukunftSeminar LTE: Der Mobilfunk der Zukunft

Tobias Schrage

University Erlangen-NürnbergChair of Mobile Communications

January 13, 2010

OutlineOutline

1 Scheduling

2 HARQ

3 Literature

Tobias SchrageTobias Schrage – Scheduling and HARQ– Scheduling and HARQ1 / 311 / 31

SchedulingScheduling

Why Scheduling?Why Scheduling?

SituationIn a LTE cell several UEs are logged in and each one wants toaccess the channel for data transfer. How can the eNodeBdetermine which one should be served?


SchedulingScheduling BasicsBasics

Scheduling BasicsScheduling Basics

What can be scheduled?

Space

Bandwidth

Code

Transmit Power

What has to be considered?

Maximum Delay

Fairness

Problem of “User Starvation”

Efficiency (overall throughput)

Overhead

Some of these aspects are contradictory


SchedulingScheduling AlgorithmsAlgorithms

AlgorithmsAlgorithms

The following scheduling algorithms will be discussed:

Round Robin

Maximum Rate

Proportional Fair Scheduling


SchedulingScheduling Round Robin AlgorithmRound Robin Algorithm

Round Robin IRound Robin I

Round Robin is the simplest known scheduling algorithm. Itallocates each UE equally spaced timeslots, completelyignoring channel quality or data rate.

t

Channel quality

UE #1

UE #2

UE #3 (selected)


SchedulingScheduling Round Robin AlgorithmRound Robin Algorithm

Round Robin IIRound Robin II

As we can see in the diagram, the Round Robin Algorithm hassome serious drawbacks:

It is not efficient

It is not fair in the sense of service quality

Therefore we need better scheduling algorithms.


SchedulingScheduling Maximum Rate AlgorithmMaximum Rate Algorithm

Maximum Rate IMaximum Rate I

The Maximum Rate Scheduler always selects the UE with thebest data rate for transfer. Therefore it needs informationabout the expected data rate (which can be derived from theSNR) of each UE.

t

Channel quality

UE #1

UE #2

UE #3 (selected)



Maximum Rate IIMaximum Rate II

Advantages

Very efficient in the sense of overall datarate

Easy implementation, only information about signalquality is needed.

Disadvantages

User starvation is very probable

Impossible to calculate maximum delay, very high delaytime possible

→ Practical application is pointless



Maximum Rate IIIMaximum Rate III

Improvements

To avoid “User Starvation” it is possible to add a minimal datarate rmin each user needs to receive. This data rate can beassigned to the users in a manner similar to the Round Robinalgorithm and thus we obtain a combination of bothalgorithms.


SchedulingScheduling Proportional Fair AlgorithmProportional Fair Algorithm

Proportional Fair Algorithm IProportional Fair Algorithm I

Basic IdeaMeasure the link quality of each UE over a defined timeinterval and select the UE with the best quality with respect toits average

i = argmaxi{ri ,n+1

θi ,n

}

t

Channel quality

UE #1

UE #2

UE #3 (selected)



Proportional Fair Algorithm IIProportional Fair Algorithm II

Advantages

“User Starvation” not very likely

Better overall throughput than the Round Robin Scheduler

Disadvantages

Information about the channel quality needed

Higher complexity

Lower throughput than Max Rate Scheduler



ComparisonComparison


SchedulingScheduling General RemarksGeneral Remarks

Remarks IRemarks I

Multiple Carriers

In LTE, these algorithms have to be extended to the amount ofavailable resource blocks (OFDM-Subcarriers).

Not part of the specification

Scheduling algorithms are not part of any specification. Thenecessary interfaces have to be specified (e.g. how to obtaininformation about channel quality) but not the scheduler itself.


SchedulingScheduling General RemarksGeneral Remarks

Remarks IIRemarks II

Quality of Service

Another important point the scheduler has to consider is QoS:Different services should have different priorities and maybesome users should be served preferential.

HARQRequested repetition of a packet should be able to prioritiseother scheduled packets.


SchedulingScheduling Uplink ChannelUplink Channel

Uplink ChannelUplink Channel

In principle, the same algorithms can be used for the uplinkchannel. However, some problems have to be considered:

Limited power of the UEs, hence it is unlikely that a singleterminal can use the whole link capacity


SchedulingScheduling Uplink ChannelUplink Channel

OutlineOutline

1 Scheduling

2 HARQ

3 Literature


HARQHARQ Error CorrectionError Correction

HARQHARQ

Error CorrectionLike every communication channel, the LTE channel is alsosubject to errors.These might occur due to:

Variations in channel quality (relatively slow varying)

Intracell interference and receiver noise (fast varying andunpredictable)


HARQHARQ FEC & ARQFEC & ARQ

FEC & ARQ IFEC & ARQ I

There are two different approaches to error correction:

FEC - Forward Error CorrectionIntroduces redundancy in the transmitted signal by addingparity bits.

Can only correct a certain number of errors

ARQ - Automated Repeat reQuest

Receiver performs Cyclic Redundancy Check check of thereceived data. If the received packet is error-free, thereception will be acknowledged and the packet accepted,otherwise a retransmission will be requested.

Adds delay for every erroneusly received bit


HARQHARQ FEC & ARQFEC & ARQ

FEC & ARQ IIFEC & ARQ II

Combination of FEC & ARQThe best of both worlds

1 FEC: Corrects a certain maximum of errors in the packet

2 ARQ: Performs CRC check on the possibly alreadycorrected packet and requests retransmission onerroneous reception.

→ HARQ (Hybrid-ARQ)


HARQHARQ HARQ with Chase CombiningHARQ with Chase Combining

HARQ with Chase Combining IHARQ with Chase Combining I

R = 3/4

1st Transmission 1st Retransmission 2nd Retransmission 3rd Retransmission

Input to Decoder

Accumulated Energy E 2E 3E 4E

Resulting Code Rate R = 3/4 R = 3/4 R = 3/4 R = 3/4

Info.

R = 1/4

Puncturing

Coded Bits


HARQHARQ HARQ with Chase CombiningHARQ with Chase Combining

HARQ with Chase Combining IIHARQ with Chase Combining II

Same Symbols

If the reception fails, the same symbols will be send again→ Accumulated Energy increases (and thus SNR improves)

Code RateCode Rate is untouched, only determined by the FEC Code


HARQHARQ HARQ with Soft CombiningHARQ with Soft Combining

HARQ with Soft Combining IHARQ with Soft Combining I



HARQ with Soft Combining IIHARQ with Soft Combining II

1 The information bits are encoded with a Mother Code,typically a low-rate code (R=1/4 in the example).

2 The encoded bits are either punctured or repeated to fitthe desired code rate.

3 The receiver decodes the bits and upon error-freedecoding accepts the block and notifies the sender,otherwise the next version is requested from the sender.



HARQ with Soft Combining IIIHARQ with Soft Combining III

4 The sender will transmit the next version with redundancybits and the receiver will try to decode the block, usingthe information from the old and the new block.

5 If the decoding fails again, in the example it would bepossible to request a third block.

6 If the decoding is still impossible, repetition will start. Thefirst block will be send again, thus the accumulatedenergy will be increased and hopefully allow decoding.


HARQHARQ Benefits of HARQBenefits of HARQ

Benefits of HARQBenefits of HARQ

FEC can correct a small amount of errors

ARQ can rely on FEC for packets with few errors and onlyhas to treat packets with many errors


HARQHARQ Benefits of HARQBenefits of HARQ

Comparison of ARQ and Link AdaptationComparison of ARQ and Link Adaptation

ARQCan automaticallyadapt code rate

Reacts immediatelyupon occurrence of anerror

Increases delay timewhile correcting

Link Adaptation

Adapts code rateexplicitly

Reacts relatively slow

Does not affect delay


HARQHARQ Implementation in LTEImplementation in LTE

Implementation in LTEImplementation in LTE

Multiple parallel HARQ processes run in the UE and theeNodeB.

How do receiver and sender know when to perform SoftCombining and when to clear their buffers for a newblock?→ new-data indicator has to be send



Downlink ChannelDownlink Channel

Asynchronous, adaptive HARQ

Asynchronous: No strict timing, retransmission packetis scheduled like any other packet, normally with higherpriority.

Adaptive: The retransmission can use other resources(frequency, modulation) than the original packet.

For each block, the HARQ process-id and the redundanyversion number has to be send via the PDCCH (PhysicalDownlink Control Channel). The ACK/NAK message is sentthrough the uplink channel.



Uplink Channel IUplink Channel I

Synchronous, non-adaptive HARQ

Synchronous: Retransmission after a fixed interval(8 packets)

non-adaptive: Same resources have to be used again.

The only necessary feedback to the sender is a single bit,with information whether to resend the packet or not.

There is also a possibility to explicitly define theretransmission for another frequency block, but this is notthe usual behaviour.



Uplink Channel IIUplink Channel II

How to support both modes?

There are two ways to request a retransmission:

PHICH (Physical Hybrid-ARQ Indicator Channel): Thereceiver can request the retransmission using the sameresources.

PDCCH: The eNodeB can use the PDCCH to allocateanother frequency for the retransmission or to request acertain redundancy version of the packet.


LiteratureLiterature

LiteratureLiterature

[Dah08] E. Dahlman et al.: 3G Evolution – HSPA and LTE forMobile Broadband , 2nd Edition, 2008.

[Kon09] M. Konrad: Scheduling Algorithms and theirApplication in Wireless Communications , 2009.

[Kwa08] R. Kwan et al.: Multiuser Scheduling on the Downlinkof an LTE Cellular System, 2008


Scheduling and HARQScheduling and HARQSeminar LTE: Der Mobilfunk der ZukunftSeminar LTE: Der Mobilfunk der Zukunft

Tobias Schrage

University Erlangen-NürnbergChair of Mobile Communications

January 13, 2010

scheduling and harq - seminar lte: der mobilfunk der … · scheduling and harq seminar lte: der...

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