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 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 (AMC), modulation and coding p g ( ), g scheme (MCS) … • Link Adaption – … 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 • Scheduling 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 … • Summary S 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/ multihop/ofdm.php – … 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 … 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) (or automatic repeat-query) is an p q ( ) ( 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 Exploit the channel variation prior to transmission • Scheduling – Link adaption • Set transmission parameters to handle radio channel variation • Hybrid ARQ – Channel-dependent scheduling • Efficient resource sharing among users • Handle the channel variation after transmission Handle the channel variation after transmission • Summary S – 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 power to compensate for the y y j p 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 channel condition – Constant data rate regardless of the channel variation Constant data rate regardless of the channel variation – 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 (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 Adaptive Modulation, Coding and MIMO Scheme in IEEE802.11n, Adaptive Modulation Coding and MIMO Scheme in 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 • Rate control: constant (max) transmit power • Scheduling – Adaptive data rate – Efficient use of transmit power • Hybrid ARQ – 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 • Summary S 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 • TDM+CDM in HSDPA – Whom ? • TDM+FDM in LTE – How ? – No intra-cell interference No intra cell interference • Joint function with link adaption Joint function with link adaption • Assuming TDM-based DL with single user scheduled a time: • Channel dependent – Maximized resource utilization, if, at each time instant, all resources • Downlink scheduling Downlink scheduling assigned to the user with the best instantaneous channel condition i d t th ith th b t i t t h l diti – Centralized resource – Power control: • Uplink scheduling • For a given rate, lowest possible TX power � minimum interference • Inefficient use of TX power – Distributed resource – 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 ( ) – Regardless of channel conditions g k k R R argmax argmax i • Schedule at the fading peaks – Fair? … same amount of the radio resources i – Independently varying radio links – Unfair! … service quality (more resources needed for poor channel) • Multiuser diversity gain Multiuser diversity gain – Simple but poor performance 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
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