scheschedulingduling, link adaption and , link adaption ...–combined tdm/fdm/cdm/sdm •tdm+cdm in...

3G EvolutionChapter: 77ScheSchedulingduling, Link adaption and , Link adaption and

Hybrid ARQHybrid ARQHybrid ARQHybrid ARQ

Department of Electrical and Information Technology

Ruiyuan Tian

p gy

2009-04-02 13G Evolution - HSPA and LTE for Mobile Broadband

Outline

• Introduction

• Link Adaption

• Scheduling

• Hybrid ARQ

S• Summary

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 2

Outline

• Introduction

• Link Adaption

• Scheduling

• Hybrid ARQ

S• Summary


From Wikipedia, the free encyclopedia…

• Link adaption, – … adaptive modulation and coding (AMC), modulation and coding p g ( ), g

scheme (MCS) … – … denote the matching of the modulation, coding and other signal

and protocol parameters to the conditions on the radio link (e.g. the p p ( gpathloss, the interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.) …

– … a rate adaptation algorithm that adapt MCS according to the quality of the radio channel, and thus the bit rate and robustness of data transmission …


AMC


MCS in 802.11n, by Meifang Zhu, MSc @ EIT

Link adaption

How to adapt to channel’s variation?p



• Scheduling– … in packet-switched computer networks, the notion of a scheduling p p , g

algorithm is used as an alternative to first-come first served queuing of data packets …

– … in advanced packet radio wireless networks such as HSDPA, p ,channel-dependent scheduling may be used to take advantage of favourable channel conditions to increase the throughput and system spectral efficiency …


Scheduling

http://users.ece.utexas.edu/~rheath/research/multihop/ofdm.php


Scheduling

How to manage the resource allocation?g



• Hybrid ARQ– … automatic repeat-request (ARQ) (or automatic repeat-query) is an p q ( ) ( p q y)

error control method for data transmission which uses acknowledgments and timeouts to achieve reliable data transmission over an unreliable service …

– … hybrid ARQ (HARQ) is a variation of the ARQ error control method. In standard ARQ, error-detection information bits are added to data to be transmitted (such as cyclic redundancy check, CRC). In Hybrid ARQ, forward error correction (FEC) bits are also added to the existing Error Detection (ED) bits (such as Reed-Solomon code or Turbo code) …


ARQ

http://www.ecst.csuchico.edu/~sim/547/Old547/notes/NOTE9_1.htm


Hybrid ARQ

How to request re-transmission smart?q


Motivation

• Rapid varying radio channel– Time-variant: coherence time (Doppler spread)( pp p )– Frequency-selective: coherence bandwidth (delay spread)– Interference

Exploit the channel variation prior to transmission• Exploit the channel variation prior to transmission– Link adaption

• Set transmission parameters to handle radio channel variation – Channel-dependent scheduling

• Efficient resource sharing among users

• Handle the channel variation after transmissionHandle the channel variation after transmission– Hybrid ARQ

• Retransmission request of erroneously received data packets


Outline

• Introduction

• Link Adaption

• Scheduling

• Hybrid ARQ

S• Summary


Link Adaption (1)

• Power control– Dynamically adjust the transmit power to compensate for the y y j p p

varying radio channel condition– Maintain a certain SNR at the receiver– Constant data rate regardless of the channel variationConstant data rate regardless of the channel variation

Desired for circuit-switched voice


By S. Parkvall

Link Adaption (2)

• Rate control– Packet-data traffic: constant rate not a strong desire for constant g

rate (as high rate as possible)– Dynamically adjust the data rate to compensate for the varying radio

channel condition– Full constant transmit power (desirable in multiuser systems)


By S. Parkvall

Link Adaption (3)

• Rate control– Adaptive Modulation and Coding (AMC) schemep g ( )– “Good” channel condition: Bandwidth limited

• High-order modulation + high-rate coding– “Poor” channel condition: Power limited– Poor channel condition: Power limited

• Low-order modulation + low-rate coding

• In HSDPA link adaptation– QPSK for noisy channels and 16QAM for clearer channels– 14 Mbps, on clear channels using 16-QAM and close to 1/1 coding

rate. – 2.4 Mbps, on noisy channels using QPSK and 1/3 coding rate

(14 Mbps x 1/2 x 1/3 )– This adaptation is performed up to 500 times per secondThis adaptation is performed up to 500 times per second


AMC+M

Adaptive Modulation Coding and MIMO Scheme in IEEE802 11n


Adaptive Modulation, Coding and MIMO Scheme in IEEE802.11n, by Meifang Zhu, MSc @ EIT

Link Adaption (4)

• Power control: constant rate– Desired for voice/video

• Short-term rate variation not an issue with constant average data rate– Inefficient use of transmit power

• Rate control: constant (max) transmit power• Rate control: constant (max) transmit power– Adaptive data rate– Efficient use of transmit power– Desired in multiuser systems to reduce variations in interference

power• [Chung & Goldsmith, 2001] Little spectral efficiency is lost[Chung & Goldsmith, 2001] Little spectral efficiency is lost

when the power or rate is constrained to be constant, with optimal adaption.


Outline

• Introduction

• Link Adaption

• Scheduling

• Hybrid ARQ

S• Summary


Scheduling

• The allocation of the shared resources among the users at each time instant– Whom ?– How ?

Joint function with link adaption• Joint function with link adaption• Channel dependent• Downlink schedulingDownlink scheduling

– Centralized resource• Uplink scheduling

– Distributed resource


Downlink Scheduling (1)

• Intra-cell (quasi) orthogonality– Combined TDM/FDM/CDM/SDM

• TDM+CDM in HSDPA• TDM+FDM in LTE

– No intra-cell interferenceNo intra cell interference• Assuming TDM-based DL with single user scheduled a time:

– Maximized resource utilization, if, at each time instant, all resources i d t th ith th b t i t t h l ditiassigned to the user with the best instantaneous channel condition

– Power control:• For a given rate, lowest possible TX power minimum interference• Inefficient use of TX power

– Rate control:• For a given TX power, highest rate• Highest link utilization



• Channel-dependent scheduling– Max-C/I (Max rate) scheduler argmaxk R=( )

• Schedule at the fading peaks– Independently varying radio links

• Multiuser diversity gain

argmax ii

k R=

Multiuser diversity gain– High system throughput but not fair

Exploit fading rather than combatStarve the poor

channel user


By S. Parkvall


• Round-robin scheduling– Regardless of channel conditionsg– Fair? … same amount of the radio resources – Unfair! … service quality (more resources needed for poor channel)

Simple but poor performance– Simple but poor performance


By S. Parkvall


• Two-fold requirement– Take advantage of the fast channel variations Rg– Ensure the same average user throughput

• Proportional-fair schedulerP ti b t th i t t d t t d th

argmax i

i i

RkR

=

– Proportion between the instantaneous data rate and the average data rate during a certain period

– High throughput and fairness

Schedule on fading peaksSchedule on fading peaks, regardless of the absolute quality


By S. Parkvall


• LTE – channel-dependent scheduling in time and frequency domainsp g q y


By S. Parkvall

Uplink Scheduling (1)

• Distributed resources• Limited available TX powerLimited available TX power

– Far user power limited small amount of bandwidth resource– Close user bandwidth limited large amount of bandwidth

• Orthogonal multiple access (TDMA, FDMA)– Similar to the downlink case– Limited TX power for the sake of inter-cell MA interferencep

• Non-orthogonal multiple access (CDMA)– Power control (Constant RX power, BER, data rate)

f ( )– Maximum tolerable interference level (intra / inter)


Uplink Scheduling (2)

• Max-Rate – Highest capacity (neglecting power limitation)g p y ( g g p )

• Greedy filling– Max-Rate + max tolerable interference level

Diff t d t t ( t i i f h l )– Different user data rates (no transmission for poor channel user)• Proportional-fair

– Compromise between Max-Rate and Greedy fillingp y g– Proportion between the instant and average rate

• Round-robinS ( )– Simple (no uplink channel knowledge required)

– Poor performance


On Channel State Information (CSI)

• CSI– Needed at TX for link adaption and channel-dependent schedulingp p g

• Downlink– Pilot signal

C l ti h l ti t• e.g., Correlation channel estimator– Measured channel conditions reported to BS

• Outdated if high mobility– Channel prediction

• Additional complexity and constraint– Link adaption based on “long-term” average channel

• Uplink– TDD: reciprocity

HSPA: Chap 10– HSPA: Chap. 10– LTE: Chap. 17


Traffic Behavior and Scheduling (1)

• Load– Significant different performance of schedulers in high load systemg p g y

• Traffic behavior– Significant impact on the trade-off between the service fairness and

the system throughputthe system throughput• Scheduler

– Round-robin (RR): fair, regardless of channel– Max-Rate (Max-C/I): absolute best instantaneous channel– Proportional-fair (PF): balanced compromise …

• Traffic• Traffic – Full buffer scenario: always data at BS for all terminals– Bursty packet: e.g., web browsing


By S. Parkvall

Traffic Behavior and Scheduling (2)

RR: similar but poor performance

By S. Parkvall

MAX C/I: poor with full buffer;Improved with bursty buffer


PF: robust and good performance

Outline

• Introduction

• Link Adaption

• Scheduling

• Hybrid ARQ

S• Summary


Hybrid ARQ (1)

• Forward Error Correction (FEC)– Add redundancy for error correctiony

• Automatic Repeat Request (ARQ)– Compatible with TCP behavior for packet data

E d t ti d b C li R d d Ch k (CRC)– Error-detecting code by Cyclic Redundancy Check (CRC)– CRC used as a checksum to detect errors

• Division of polynomials in Galois field GF(2) … remainder …– No error? Positive acknowledgement (ACK)– Error? Negative acknowledgement (NAK)

• Hybrid ARQHybrid ARQ– Combination of FEC and ARQ– FEC: correct a subset of errors– ARQ: if still error detected


Hybrid ARQ (2)

• Hybrid ARQ with soft combining– Erroneously received packet stored in a buffer memoryy p y– Later combined with the retransmission– Soft-combining improved performance


combining By S. Parkvall

Hybrid ARQ (3)

• Hybrid ARQ with soft combining– Chase Combiningg

• Retransmission of the same set of data, i.e., additional repetition coding• Maximum-ratio combining: (re-transmission diversity?)• Accumulated increasing SNRg

By S. Parkvall

Constant coding rate


Hybrid ARQ (4)

• Hybrid ARQ with soft combining– Incremental Redundancyy

• Multiple sets of the information• Retransmission of a different set• Combine to recover the same information

By S. Parkvall

Varying (reducing) coding rate


coding rate

Hybrid ARQ (5)

• Hybrid ARQ with soft combining– Implicit link adaptionp p

• Instead of channel estimation, adapt to the channel based on results• Additional redundancy added only when needed• Robust regardless of channel’s variationg

– Drawbacks• Delay: unacceptable from the end-user QoS perspective


Outline

• Introduction

• Link Adaption

• Scheduling

• Hybrid ARQ

S• Summary


Chapter summary

• Link adaption– AMC+M for rate control

• Channel-dependent scheduling– Exploit diversity vs QoS fairness

• Hybrid ARQ– Smart retransmission

• Adapt to and Exploit– Variations in radio channel quality

ff– Variations in traffic pattern…instead of combating them! By S. Parkvall


3G EvolutionChapter: 77ScheSchedulingduling, Link adaption and , Link adaption and

Hybrid ARQHybrid ARQHybrid ARQHybrid ARQ

Department of Electrical and Information Technology

Ruiyuan Tian

p gy

2009-04-02 403G Evolution - HSPA and LTE for Mobile Broadband

scheschedulingduling, link adaption and , link adaption ...–combined tdm/fdm/cdm/sdm •tdm+cdm in...

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