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Scheduling, Link Adaptation and HARQ
 

Scheduling, Link Adaptation and HARQ

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    Scheduling, Link Adaptation and HARQ Scheduling, Link Adaptation and HARQ Presentation Transcript

    • 3G Evolution Outline 7 Chapter: • Introduction • Link Adaption Scheduling, Scheduling, Link adaption and • Scheduling Hybrid ARQ • Hybrid ARQ Ruiyuan Tian • S Summary Department of Electrical and Information Technology p gy 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 1 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 2 Outline From Wikipedia, the free encyclopedia… • Introduction • Link adaption, – … adaptive modulation and coding ( p g (AMC), modulation and coding ), g scheme (MCS) … • Link Adaption – … denote the matching of the modulation, coding and other signal and protocol p p parameters to the conditions on the radio link ( g the (e.g. • Scheduling pathloss, the interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.) … • Hybrid ARQ – … 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 … • S Summary 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 3 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 4
    • AMC Link adaption How to adapt to channel’s variation? p MCS in 802.11n, by Meifang Zhu, MSc @ EIT 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 5 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 6 From Wikipedia, the free encyclopedia… Scheduling • Scheduling http://users.ece.utexas.edu/~rheath/research/ – … in packet-switched computer networks, the notion of a scheduling p p , g multihop/ofdm.php 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 … 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 7 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 8
    • From Wikipedia, the free encyclopedia… • Hybrid ARQ – … automatic repeat-request (ARQ) ( automatic repeat-query) is an p q ( ) (or p q y) error control method for data transmission which uses Scheduling 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 How to manage the resource allocation? g 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) … 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 9 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 10 ARQ http://www.ecst.csuchico.edu/~sim/547/Ol d547/notes/NOTE9_1.htm Hybrid ARQ How to request re-transmission smart? q 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 11 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 12
    • Motivation Outline • Rapid varying radio channel • Introduction – Time-variant: coherence time (Doppler spread) ( pp p ) – Frequency-selective: coherence bandwidth (delay spread) • Link Adaption – Interference • Exploit the channel variation prior to transmission • Scheduling – Link adaption • Set transmission parameters to handle radio channel variation – Channel-dependent scheduling • Hybrid ARQ • Efficient resource sharing among users • Handle the channel variation after transmission • S Summary – Hybrid ARQ • Retransmission request of erroneously received data packets 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 13 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 14 Link Adaption (1) Link Adaption (2) • Power control • Rate control – Dynamically adjust the transmit p y y j power to compensate for the p – Packet-data traffic: constant rate not a strong desire for constant g varying radio channel condition rate (as high rate as possible) – Maintain a certain SNR at the receiver – Dynamically adjust the data rate to compensate for the varying radio – Constant data rate regardless of the channel variation channel condition – Full constant transmit power (desirable in multiuser systems) Desired for circuit-switched voice By S. Parkvall By S. Parkvall 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 15 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 16
    • Link Adaption (3) AMC+M • Rate control – Adaptive Modulation and Coding ( p g (AMC) scheme ) – “Good” channel condition: Bandwidth limited • High-order modulation + high-rate coding – “Poor” channel condition: Power limited Poor • 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 Adaptive Modulation Coding and MIMO Scheme in IEEE802 11n Modulation, IEEE802.11n, by Meifang Zhu, MSc @ EIT 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 17 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 18 Link Adaption (4) Outline • Power control: constant rate • Introduction – Desired for voice/video • Short-term rate variation not an issue with constant average data rate • Link Adaption – Inefficient use of transmit power • Rate control: constant (max) transmit power – Adaptive data rate • Scheduling – Efficient use of transmit power – Desired in multiuser systems to reduce variations in interference • Hybrid ARQ power • [Chung & Goldsmith, 2001] Little spectral efficiency is lost • S Summary when the power or rate is constrained to be constant, with optimal adaption. 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 19 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 20
    • Scheduling Downlink Scheduling (1) • The allocation of the shared resources among the users at • Intra-cell (quasi) orthogonality each time instant – Combined TDM/FDM/CDM/SDM – Whom ? • TDM+CDM in HSDPA – How ? • TDM+FDM in LTE – No intra cell interference intra-cell • Joint function with link adaption • Channel dependent • Assuming TDM-based DL with single user scheduled a time: – Maximized resource utilization, if, at each time instant, all resources • Downlink scheduling assigned t th user with th b t i t t i d to the ith the best instantaneous channel condition h l diti – Centralized resource – Power control: • Uplink scheduling • For a given rate, lowest possible TX power minimum interference – Distributed resource • 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 21 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 22 Downlink Scheduling (2) Downlink Scheduling (3) • Channel-dependent scheduling • Round-robin scheduling – Max-C/I (Max rate) scheduler ( ) k = arg max Ri – Regardless of channel conditions g • Schedule at the fading peaks i – Fair? … same amount of the radio resources – Independently varying radio links – Unfair! … service quality (more resources needed for poor channel) • Multiuser diversity gain – Simple but poor performance – High system throughput but not fair Starve the poor Exploit fading rather than combat channel user By S. Parkvall By S. Parkvall 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 23 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 24
    • Downlink Scheduling (4) Downlink Scheduling (5) • Two-fold requirement • LTE – Take advantage of the fast channel variations g – channel-dependent scheduling in time and frequency domains p g q y R – Ensure the same average user throughput k = arg max i • Proportional-fair scheduler i Ri – P Proportion b t ti between th i t t the instantaneous d t rate and th average data t d the data rate during a certain period – High throughput and fairness Schedule on fading peaks peaks, regardless of the absolute quality By S. Parkvall By S. Parkvall 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 25 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 26 Uplink Scheduling (1) Uplink Scheduling (2) • Distributed resources • Max-Rate • Limited available TX power – Highest capacity ( g g p y (neglecting p g power limitation) ) – Far user power limited small amount of bandwidth resource • Greedy filling – Close user bandwidth limited large amount of bandwidth – Max-Rate + max tolerable interference level • Orthogonal multiple access (TDMA, FDMA) – Diff Different user d t rates ( t t data t (no transmission f poor channel user) i i for h l ) – Similar to the downlink case • Proportional-fair – Limited TX power for the sake of inter-cell MA interference p – Compromise between Max-Rate and Greedy filling p y g • Non-orthogonal multiple access (CDMA) – Proportion between the instant and average rate – Power control (Constant RX power, BER, data rate) • Round-robin – Maximum tolerable interference level ( f (intra / inter) ) – S Simple ( uplink channel knowledge required) (no ) – Poor performance 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 27 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 28
    • On Channel State Information (CSI) Traffic Behavior and Scheduling (1) • CSI • Load – Needed at TX for link adaption and channel-dependent scheduling p p g – Significant different p g performance of schedulers in high load system g y • Downlink • Traffic behavior – Pilot signal – Significant impact on the trade-off between the service fairness and • e.g., C Correlation channel estimator l ti h l ti t the system throughput – Measured channel conditions reported to BS • Scheduler • Outdated if high mobility – Round-robin (RR): fair, regardless of channel – Channel prediction – Max-Rate (Max-C/I): absolute best instantaneous channel • Additional complexity and constraint – Proportional-fair (PF): balanced compromise … – Link adaption based on “long-term” average channel • Traffic • Uplink – Full buffer scenario: always data at BS for all terminals – TDD: reciprocity – Bursty packet: e.g., web browsing – HSPA: Chap 10 Chap. – LTE: Chap. 17 By S. Parkvall 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 29 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 30 Traffic Behavior and Scheduling (2) Outline • Introduction • Link Adaption • Scheduling By S. Parkvall • Hybrid ARQ RR: similar but poor performance • S Summary MAX C/I: poor with full buffer; Improved with bursty buffer PF: robust and good performance 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 31 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 32
    • Hybrid ARQ (1) Hybrid ARQ (2) • Forward Error Correction (FEC) • Hybrid ARQ with soft combining – Add redundancy for error correction y – Erroneously received p y packet stored in a buffer memory y • Automatic Repeat Request (ARQ) – Later combined with the retransmission – Compatible with TCP behavior for packet data – Soft-combining improved performance – E Error-detecting code b C li R d d d t ti d by Cyclic Redundancy Ch k (CRC) Check – 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 – Combination of FEC and ARQ – FEC: correct a subset of errors – ARQ: if still error detected combining By S. Parkvall 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 33 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 34 Hybrid ARQ (3) Hybrid ARQ (4) • Hybrid ARQ with soft combining • Hybrid ARQ with soft combining – Chase Combining g – Incremental Redundancy y • Retransmission of the same set of data, i.e., additional repetition coding • Multiple sets of the information • Maximum-ratio combining: (re-transmission diversity?) • Retransmission of a different set • Accumulated increasing SNR g • Combine to recover the same information By S. Parkvall By S. Parkvall Constant Varying (reducing) coding rate coding rate 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 35 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 36
    • Hybrid ARQ (5) Outline • Hybrid ARQ with soft combining • Introduction – Implicit link adaption p p • Instead of channel estimation, adapt to the channel based on results • Link Adaption • Additional redundancy added only when needed • Robust regardless of channel’s variation g – Drawbacks • Scheduling • Delay: unacceptable from the end-user QoS perspective • Hybrid ARQ • S Summary 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 37 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 38 Chapter summary 3G Evolution 7 • Link adaption Chapter: – AMC+M for rate control • Channel-dependent scheduling – Exploit diversity vs QoS fairness • Hybrid ARQ – Smart retransmission Scheduling, Scheduling, Link adaption and • Adapt to and Exploit Hybrid ARQ – Variations in radio channel quality – Variations in traffic pattern ff Ruiyuan Tian …instead of combating them! By S. Parkvall Department of Electrical and Information Technology p gy 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 39 2009-04-02 3G Evolution - HSPA and LTE for Mobile Broadband 40