Department of Electrical and Information TechnologyJohan Löfgren

3G Evolution

Chapter:

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15LTE Radio Interface

Architecture

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Outline

• Architecture Basics• Protocol Architecture Overview• Short Description of Layers• Longer Descriptions

– Radio Link Control (RLC)– Medium Access Control (MAC)– Physical Layer (PHY)

• Channels and Channel Mapping– Logical Channels– Transport Channels– Physical Channels– Channel Mapping

• Data Flow Example• Chapter Summary

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Architecture Basics

• LTE is strucutred into different protocol layers– Lower protocol layers provide services for upper layers

• The data from/to higher layers is known as a Service Data Unit (SDU)

• The data from/to lower layers is known as a Protocol Data Unit (PDU)

– Each layer is responsible for different tasks• The upper layers should not have to care about the details

• This structure is common to most modern systems– Some layers are almost universal

– Others are LTE specific

• In LTE, IP packets enters through on of the SAE bearers– SAE = System Architecture Evolution

– The IP packets are then processed by the protocols

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Protocol Architecture Overview

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Short Description of Layers (1/2)• Packet Data Convergence Protocol (PDCP)

– Performs IP header compression• Reduces the number of bits to transmit over the radio

interfaced• Based on Robust Header Compression (ROHC)

• Radio Link Control (RLC)– Responsible for

• Segmentation/concatenation• Retransmission handling• In-sequence delivery to higher layers

– Located in the eNodeB since no higher layers exists in LTE• In WCDMA this was handled higher in hierarchy

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Short Description of Layers (2/2)• Medium Access Control (MAC)

– Responsible for• Uplink/downlink scheduling• Hybrid-ARQ retransmissions• Choice of modulation• Resoucre assignment

• Physical Layer (PHY)– Responsible for

• Coding/decoding• Modulation/demodulation• Resource mapping

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Radio Link Control (RLC)

• The RLC splits and concatenate the incoming data to fit the system– The RLC PDU sizes are dynamically varied to fit the amount of data

• Much data = long RLC PDUs to reduce RLC header overhead• Little data = short RLC PDUs to avoid too much padding

• Responsible for error-free delivery to higher layers– Accomplished with retransmission protocol– Re-sequencing of data may be requiered– Complementary to Hybrid-ARQ in lower layers

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Medium Access Control (MAC) (1/2)• Scheduling

– The basic operation is so-called dynamic scheduling• Downlink/Uplink are independently schedueled• Time-frequency resources dynamically shared between users• Each 1 ms TTI a new decision is taken• Each mobile unit scheduled by the eNodeB

– However each mobile still decides what radio bearer(s) to transmit

– Scheduling strategy is not specified by 3GPP• Implementation specific• Normally aiming at taking advantage of channel variations• Channel status report from mobile unit to eNodeB

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Medium Access Control (MAC) (2/2)

• Hybrid ARQ with soft combining controlled by MAC– Different simultaneous processes running– When error in one part, only that process needs to retransmit– Soft combining with adaptive coding leads to better chance of

reception

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Physical Layer (PHY)

• Responsible for– Coding– PHY Hybrid ARQ processing– Modulation

• Controlled by MAC– Transport Format (TF)

• Tells PHY modulation et c.

Simplified PHY processing

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Logical Channels

• The MAC offers services to the RLC as logical channels– Broadcast Control Channel (BCCH)

• System information to all mobile terminals– Paging Control Channel (PCCH)

• Paging information when searching a unit– Common Control Channel (CCCH)

• Random access information (setting up a connection)– Dedicated Control Channel (DCCH)

• User-specific control information (power/handover et c)– Multicast Control Channel (MCCH)

• Information needed for multicast reception– Dedicated Traffic Channel (DTCH)

• Transmission of user data – Multicast Traffic Channel (MTCH)

• Transmission of multicast data

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Transport Channels

• The PHY offers services to the MAC as transport channels– Broadcast Channel (BCH)

• Maps to BCCH

– Paging Channel (PCH)• To convey the PCCH

– Downlink Shared Channel (DL-SCH)• Main channel for downlink data transfer, used by many logical

channels

– Multicast Channel (MCH)• Used to transmit MCCh information to set up multicast

transmission

– Upink Shared Channel (UL-SCH)• Main channel for uplink data transfer, used by many logical

channels

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Physical Channels

• The PHY transmit the data over the physical channels– Physical Downlink Shared Channel (PDSCH)

• Used for unicast transmission and paging– Physical Broadcast Channel (PBCH)

• Carries system information for accessing the network– Physical Multicast Channel (PMCH)

• Carries system information for multicast– Physical Downlink Control Channel (PDCCH)

• Carries mainly scheduling information– Physical Hybrid ARQ Indicator Channel (PHICH)

• Reports Hybrid ARQ status– Physical Control Format Indicator Channel (PCFICH)

• Information to enable the terminalse to decode the PDSCH– Physical Uplink Shared Channel (PUSCH)

• Uplink counterpart of PDSCH– Physical Uplink Control Channel (PUCCH)

• Sends Hybrid ARQ acknowledgement– Physical Random Access Channel (PRACH)

• Used for random access

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Channel Mapping

Downlink Channel Mapping

Uplink Channel Mapping

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Data Flow Example

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Chapter Summary

• LTE is structured into different protocol layers– RLC is responsible for splitting and concatenation of data– MAC controls data and Hybrid ARQ– PHY is responsible for actual transmission

• Different Channels are interfaces to higher layers– Logical Channels– Transport Channels– Physical Channels