Channels and Physical Layer Processing

Chapter 3Chapter 3Chapter 3Chapter 3

LTE Ch l & Ph i lLTE Ch l & Ph i lLTE Channels & PhysicalLTE Channels & Physical--Layer ProcessingLayer ProcessingLayer ProcessingLayer Processing

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OutlineOutlineOutlineOutline

2.1 Radio Protocol Architecture2.2 Channel Mapping2.3 DL Physical Layer Processingy y g2.4 UL Physical Layer Processing

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Overall System ArchitectureOverall System Architecture

C: control-plane

U: user traffic-plane

X2: multi-cell radio resource

management (RRM)

Core Network Radio Access Network (RAN)

(e.g., Inter-Cell Interference

Coordination (ICIC))( )

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Overall RAN Protocol ArchitectureOverall RAN Protocol Architecture

PHY(Physical Layer): handle physical-layer (MOD, CODEC)MAC(Media Access Control): handle multiplexing, HARQ, and scheduling functionsRLC(Radio Link Control): handle segmentation and in-sequence delivery functionsPDCP(P k t D t C P t l) f IP h d i &PDCP(Packet Data Convergence Protocol): perform IP header compression & cipheringRRC(Radio Resource Control): handle RAN-related (SI broadcast, paging) proceduresproceduresNAS(Non-Access Stratum): handle non-RAN-related (authentication, security,…)

procedures from UE to MME 4

LTE Protocol Architecture (downlink)LTE Protocol Architecture (downlink)

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Example of LTE Data FlowExample of LTE Data Flow

An example of DL data flow with three IP packets, two on one radio bearer and one on another

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Radio Link ControlRadio-Link Control

RLC is responsible for segmentation & concatenation of IP packets (RLC SDU) from the PDCP into suitably sized RLC PDU (packet data unit)It also handles retransmission of erroneous PDUsFinally RLC ensures in-sequence delivery of SDUs to

lupper layers

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Medium Access ControlMedium-Access ControlMAC layer handles logical-channel multiplexing, HARQ retransmission, uplink, and downlink schedulingA logic channel is defined by the type of information it carries and is generally classified as control channels and trafficand is generally classified as control channels and traffic channelsMAC layer uses service in the form of transport channel from y pPHY layerA transport channel is defined by how and with what characteristics the information is transmittedData on a transport channel is organized into TBs (transport blocks) In each TTI at most one TB is to be transmitted overblocks). In each TTI, at most one TB is to be transmitted over the radio interface (SISO case)Associated with each TB is a TF (transport format), specifying ( p ), p y ghow the TB is transmitted

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MAC header and PDU multiplexing

MAC operation in multiple component carriers

Control element: time-advance command & random-access response 9

Scheduler is part of MAC layer and controls the assignment of uplink and downlink resources, i.e., resource-block pairs (Resource block : time-frequency

it (1 180kH ))unit (1ms x 180kHz))

10Transport-format selection in DL and UL

Downlink Channel MappingDownlink Channel Mapping

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Downlink Channel SummaryDownlink Channel Summary

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Physical Channels to Support DLPhysical Channels to Support DL

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Uplink Channel MappingUplink Channel Mapping

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Uplink Channel SummaryUplink Channel Summary

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Physical Channels to Support ULPhysical Channels to Support UL

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Physical Layer Processing of DL SCHPhysical-Layer Processing of DL-SCH

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CRC Insertion & SegmentationCRC Insertion & Segmentation

The Turbo-coder only allows a limited number of code-block sizes. Thus transport block is needed to be segmented into smaller code block.CRC per code block allows a early detection of erroneous code block

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Turbo EncoderTurbo Encoder

Turbo coding is used for DL-SCH, PCH & MCH

A maximum contention free QPP (QuadraturePermutation Polynominal) interleaver is used

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Rate Matching & HARQRate Matching & HARQ

The task of RM & physical-layer HARQ is to fit the blocks of code bits into the exact set of code bits in a given TTI/subframe

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Bit level ScramblingBit-level Scrambling

The purpose of bit-level scrambling is to avoid the interference from neighboring cellsBit-level scrambling is performed by multiplying (exclusive-or operation) the coded bits with a scrambling sequence

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Modulation Scheme for LTEModulation Scheme for LTE

DL Channel

UL Channel

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Adaptive Modulation for Data ChannelAdaptive Modulation for Data Channel

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DL L1/L2 Control RegionDL L1/L2 Control Region

The control region always occupies the first one, two, or three OFDM symbolsDL L1/L2 consists of

PCFICH: inform the size of control regionPDCCH: DL scheduling assignments & UL scheduling

tgrantPHICH: HARQ acknowledgement

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Numbering of resource-element groups in the control region

PCIFICH (1/2)PCIFICH (1/2)

PCIFICH (Physical Control Format Indicator Channel) consists of 2 bits, indicating the control size regionPCIFICH is divided into 4 REG (Resource Element Group) and is mapped to the first symbol of each subframe

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PCIFICH (2/2)PCIFICH (2/2)

4 REGs are spread to achieve frequency diversity

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PHICHPHICH

Used for transmitting HARQ ACK in response to UL-SCHEach PHICH is code spread on multiple Res to reduce power differences between Res (PHICH can be dynamically power adjusted to reduce error rate)

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PDCCHPDCCH

PDCCH is used to carry DCI (downlink control information) including

DL scheduling assignmentsUL scheduling grantsPower control commands

( & )LTE allows distributed RB allocation (type 0 & 1) as well as localized RB allocation (type 2)

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PDCCH ProcessingPDCCH Processing

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CCE Aggregation & PDCCH MultiplexingCCE Aggregation & PDCCH Multiplexing

CCE is a convenient name for a set of 36 Res to allow simple and efficient processing in UEs

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Example of PCFICH, PHICH & PDCCH MappingMapping

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Physical Layer Processing of UL SCHPhysical-Layer Processing of UL-SCH

DFT precoding K blocks of M modulation symbols.

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Physical-layer processing for UL-SCH.

Uplink Physical Resource MappingUplink Physical Resource Mapping

Frequency contiguous mapping is preferred to retain good PAPR properties

In R10, two frequency-separated clusters are introduced to increase scheduler flexibility

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PUSCH Frequency HoppingPUSCH Frequency Hopping

A consecutive sub-bands are defined to support frequency hopping

Pre-defined (explicit) frequency hoppingCell-specific frequency hopping

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Hopping according to predefined hopping pattern

H i / i i di t d fi d h i / i i tt

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Hopping/mirroring according to predefined hopping/mirroring patterns. Same hopping pattern as in upper

UL L1/L2 Control Signal on PUCCHUL L1/L2 Control Signal on PUCCH

UL L1/L2 control signaling consists of HARQ acknowledgement, channel state report to assist DL scheduling, and scheduling requestWhen the terminal doesn’t have a valid scheduling grant, PUCCH is used for control signal transmissionPUCCH l d t th d f ll tPUCCH are placed at the edge of overall spectrum

Maximum frequency diversityNot to fragment the UL spectrum for data transmissionNot to fragment the UL spectrum for data transmission

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UL L1/L2 Control Signal on PUSCHUL L1/L2 Control Signal on PUSCH

Channel state report and HARQ ack are transmitted by time multiplexing with PUSCHHARQ ack is placed close to reference signal for better quality channel estimatesAlso RI is closer to reference signal for more protection

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