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Chapter: 7 Introduction Link Adaption Sche Scheduling duling, - - PowerPoint PPT Presentation

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3G Evolution Outline Chapter: 7 Introduction Link Adaption Sche Scheduling duling, Link adaption and , Link adaption and Scheduling Hybrid ARQ Hybrid ARQ Hybrid ARQ Hybrid ARQ Hybrid ARQ Summary S Ruiyuan Tian

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SLIDE 1

3G Evolution

Chapter: 7

Sche Scheduling duling, Link adaption and , Link adaption and Hybrid ARQ Hybrid ARQ Hybrid ARQ Hybrid ARQ

Department of Electrical and Information Technology

Ruiyuan Tian

p gy

2009-04-02 1 3G 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

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

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 ( g pathloss, 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 …

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 4

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SLIDE 2

AMC

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 5

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

Link adaption

How to adapt to channel’s variation? p

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 6

From Wikipedia, the free encyclopedia…

  • 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

  • f 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 …

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Scheduling

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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 8

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SLIDE 3

Scheduling

How to manage the resource allocation? g

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From Wikipedia, the free encyclopedia…

  • 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

  • r Turbo code) …

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ARQ

http://www.ecst.csuchico.edu/~sim/547/Ol d547/notes/NOTE9_1.htm

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Hybrid ARQ

How to request re-transmission smart? q

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 12

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SLIDE 4

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 transmission

Handle the channel variation after transmission

– Hybrid ARQ

  • Retransmission request of erroneously received data packets

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 13

Outline

  • Introduction
  • Link Adaption
  • Scheduling
  • Hybrid ARQ

S

  • Summary

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 14

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 variation Constant data rate regardless of the channel variation

Desired for circuit-switched voice

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 15

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)

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 16

By S. Parkvall

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SLIDE 5

Link Adaption (3)

  • Rate control

– Adaptive Modulation and Coding (AMC) scheme p 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 second This adaptation is performed up to 500 times per second

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 17

AMC+M

Adaptive Modulation Coding and MIMO Scheme in IEEE802 11n

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 18

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

  • ptimal adaption.

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Outline

  • Introduction
  • Link Adaption
  • Scheduling
  • Hybrid ARQ

S

  • Summary

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 20

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SLIDE 6

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 scheduling

Downlink scheduling

– Centralized resource

  • Uplink scheduling

– Distributed resource

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 21

Downlink Scheduling (1)

  • Intra-cell (quasi) orthogonality

– Combined TDM/FDM/CDM/SDM

  • TDM+CDM in HSDPA
  • TDM+FDM in LTE

– No intra-cell interference No 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 diti assigned 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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 22

Downlink Scheduling (2)

  • Channel-dependent scheduling

– Max-C/I (Max rate) scheduler

argmax k R =

( )

  • Schedule at the fading peaks

– Independently varying radio links

  • Multiuser diversity gain

argmax

i i

k R =

Multiuser diversity gain

– High system throughput but not fair

Exploit fading rather than combat Starve the poor channel user

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 23

By S. Parkvall

Downlink Scheduling (3)

  • Round-robin scheduling

– Regardless of channel conditions g – Fair? … same amount of the radio resources – Unfair! … service quality (more resources needed for poor channel) Simple but poor performance – Simple but poor performance

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 24

By S. Parkvall

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SLIDE 7

Downlink Scheduling (4)

  • Two-fold requirement

– Take advantage of the fast channel variations

R

g – Ensure the same average user throughput

  • Proportional-fair scheduler

P ti b t th i t t d t t d th

argmax

i i i

R k R =

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

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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 25

By S. Parkvall

Downlink Scheduling (5)

  • LTE

– channel-dependent scheduling in time and frequency domains p g q y

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 26

By S. Parkvall

Uplink Scheduling (1)

  • Distributed resources
  • Limited available TX power

Limited 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 interference p

  • Non-orthogonal multiple access (CDMA)

– Power control (Constant RX power, BER, data rate) f ( ) – Maximum tolerable interference level (intra / inter)

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 27

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 filling p y g – Proportion between the instant and average rate

  • Round-robin

S ( ) – Simple (no uplink channel knowledge required) – Poor performance

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 28

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SLIDE 8

On Channel State Information (CSI)

  • CSI

– Needed at TX for link adaption and channel-dependent scheduling p 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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 29

Traffic Behavior and Scheduling (1)

  • Load

– Significant different performance of schedulers in high load system g p g y

  • Traffic behavior

– Significant impact on the trade-off between the service fairness and the system throughput the 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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 30

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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 31

PF: robust and good performance

Outline

  • Introduction
  • Link Adaption
  • Scheduling
  • Hybrid ARQ

S

  • Summary

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 32

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SLIDE 9

Hybrid ARQ (1)

  • Forward Error Correction (FEC)

– Add redundancy for error correction y

  • 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 ARQ

Hybrid ARQ

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

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Hybrid ARQ (2)

  • Hybrid ARQ with soft combining

– Erroneously received packet stored in a buffer memory y p y – Later combined with the retransmission – Soft-combining improved performance

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 34

combining By S. Parkvall

Hybrid ARQ (3)

  • Hybrid ARQ with soft combining

– Chase Combining

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

g

By S. Parkvall Constant coding rate

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 35

Hybrid ARQ (4)

  • Hybrid ARQ with soft combining

– Incremental Redundancy

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

By S. Parkvall Varying (reducing) coding rate

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coding rate

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SLIDE 10

Hybrid ARQ (5)

  • Hybrid ARQ with soft combining

– Implicit link adaption p p

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

g

– Drawbacks

  • Delay: unacceptable from the end-user QoS perspective

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 37

Outline

  • Introduction
  • Link Adaption
  • Scheduling
  • Hybrid ARQ

S

  • Summary

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 38

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

2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 39

3G Evolution

Chapter: 7

Sche Scheduling duling, Link adaption and , Link adaption and Hybrid ARQ Hybrid ARQ Hybrid ARQ Hybrid ARQ

Department of Electrical and Information Technology

Ruiyuan Tian

p gy

2009-04-02 40 3G Evolution - HSPA and LTE for Mobile Broadband