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Architecture Overview

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System Architecture Evolution (SAE) • Packet core networks are also evolving to the flat

System Architecture Evolution (SAE) architecture. – This new architecture optimizes network performance,

improve cost-efficiency and facilitate the uptake of mass-market IP based services.

• There are only two nodes in the SAE architecture user plane:

– LTE base station (eNodeB) – SAE Gateway.

Policy and Charging Rules Function

Home Subscriber Server

Mobility Management Entity

Serving GPRS Support Node Packet Data

Serving Node

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High Level Architecture Policy and Charging Rules Function

Home Subscriber Server

Mobility Management Entity

Serving GPRS Support Node

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High Level Architecture • Functional Elements

– Evolved Radio Access Network (RAN) – Serving Gateway (SGW) – Mobility Management Entity (MME) – Packet Data Network Gateway (PDN GW)

• Key Features – EPC to EPS

» Evolved Packet Core -> Evolved Packet System » Separation of the network entity that performs control-plane

functionality (MME) from the network entity that performs bearer-plane functionality (SGW) with a well defined open interface between them (S11).

– S1-flex Mechanism » provides support for network redundancy and load sharing of

traffic across network elements in the CN (core network), the MME and the SGW,

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Radio-Interface Architecture • In 3GPP

– the overall system architecture of both the Radio-Access Network (RAN) and the Core Network (CN) was revisited, including the split of functionality between the two network parts.

» The RAN is responsible scheduling, radio-resource handling, retransmission protocols, coding and various multi-antenna schemes.

• For LTE – it was known as the System Architecture Evolution (SAE)

and resulted in a flat RAN architecture, as well as a new core network architecture referred to as the Evolved Packet Core (EPC).

– The RAN and the EPC can be referred to as the Evolved Packet System (EPS).

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Radio-Interface Architecture • The EPC is responsible for providing a complete

mobile-broadband network. – This includes authentication, charging functionality, and

setup of end-to-end connections.

• Handling these functions separately, instead of

integrating them into the RAN, is beneficial as it allows for several radio-access technologies to be served by the same core network.

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Core Network • The EPC is a radical evolution from the GSM/GPRS

core network used for GSM and WCDMA/HSPA. • EPC supports access to the packet-switched domain

only, with no access to the circuit switched domain.

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Core Network • The Mobility Management Entity (MME) is the control-

plane node of the EPC. – include connection/release of bearers to a terminal, handling

of IDLE to ACTIVE transitions, and handling of security keys.

• The Serving Gateway (S-GW) is the user-plane node connecting the EPC to the LTE RAN.

– The S-GW acts as a mobility anchor when terminals move between eNodeBs, as well as a mobility anchor for other 3GPP technologies (GSM/GPRS and HSPA).

• The Packet Data Network Gateway (PDN Gateway, P-GW) connects the EPC to the internet.

– allocates the IP address for a specific terminal, as well as quality-of service enforcement according to the policy controlled by the Policy and Charging Rules Function (PCRF).

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Core Network • The EPC also contains other types of nodes

– such as Policy and Charging Rules Function (PCRF) responsible for quality-of-service (QoS) handling and charging.

• Home Subscriber Service (HSS) node, a database containing subscriber information.

• In an actual physical implementation, several of them may very well be combined.

– For example, the MME, P-GW, and S-GW could very well be combined into a single physical node.

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Radio-Access Network • LTE radio-access network uses a flat architecture

with a single type of node – eNodeB (eNB). – The eNodeB is responsible for all radio-related functions.

• eNB is connected to the EPC by means of the S1 interface

– to the S-GW by S1 user-plane part, S1-u. – to the MME by S1 control-plane part, S1-c.

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Radio-Access Network • One eNB can be connected to multiple MMEs/S-GWs

for the purpose of load sharing and redundancy.

• The X2 interface, connecting eNodeBs to each other, is mainly used to support active-mode mobility.

– is used to support lossless mobility between neighboring cells by packet forwarding.

– is used for multi-cell Radio Resource Management (RRM) functions such as Inter-Cell Interference Coordination (ICIC).

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Summary

• The E-UTRAN (Evolved Universal Terrestrial Radio Access) consists of eNodeBs, providing the

– user plane (PDCP/RLC/MAC/PHY) and – control plane (RRC) protocol terminations.

• The eNodeBs are interconnected with each other by means of the X2 interface.

• The eNodeBs are also connected – to the EPC (Evolved Packet Core) by the S1 interface, – to the MME (Mobility Management Entity) by the S1-MME – to the Serving Gateway (S-GW) by means of the S1-U.

• The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs.

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Overall Architecture

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Functional Split between E-UTRAN and EPC

internet

eNB

RB Control

Connection Mobility Cont.

eNB MeasurementConfiguration & Provision

Dynamic Resource Allocation (Scheduler)

PDCP

PHY

MME

S-GW

S1MAC

Inter Cell RRM

Radio Admission Control

RLC

E-UTRAN EPC

RRC

Mobility Anchoring

EPS Bearer Control

Idle State Mobility Handling

NAS Security

P-GW

UE IP address allocation

Packet Filtering

User Plane

Control Plane

Non Access Stratum

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User Plane

eNB

PHY

UE

PHY

MAC

RLC

MAC

PDCPPDCP

RLC

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Control Plane

eNB

PHY

UE

PHY

MAC

RLC

MAC

MME

RLC

NAS NAS

RRC RRC

PDCP PDCP

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Non Access Stratum • The non-access stratum (NAS) is highest stratum of the control

plane between UE and MME at the radio interface. • Main functions of the protocols that are part of the NAS are the

support of mobility of the user equipment (UE) and the support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).

• NAS control protocol performs followings: – Evolved Packet System (EPS) bearer management; – Authentication; – ECM-IDLE mobility handling; – Paging origination in ECM-IDLE; – Security control.

3GPP TS 24.301 - Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3

EPS Connection Management (ECM)

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Control Plane • PDCP sublayer (terminated in eNB on the network side)

performs the functions – ciphering and integrity protection;

• RLC and MAC sublayers (terminated in eNB on the network side) perform the same functions as for the user plane;

– RRC (terminated in eNB on the network side) performs the functions listed in subclause 7, e.g.:

– Broadcast; – Paging; – RRC connection management; – RB control; – Mobility functions; – UE measurement reporting and control.

• NAS control protocol (terminated in MME on the network side) performs among other things:

– EPS bearer management; – Authentication; – ECM-IDLE mobility handling; – Paging origination in ECM-IDLE; – Security control.

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eNB Functions

• eNB hosts the following functions – Radio Resource Management:

» Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);

– IP header compression and encryption of user data stream; – Selection of an MME at UE attachment when no routing to

an MME can be determined from the information provided by the UE;

– Routing of User Plane data towards S-GW; – Scheduling and transmission of paging messages (originated

from the MME); – Scheduling and transmission of broadcast information

(originated from the MME or operation&maintain (O&M)); – Measurement and measurement reporting configuration for

mobility and scheduling.

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MME Functions • MME hosts the following functions

– NAS (Non Access Stratum) signalling; – NAS signalling security; – Access Stratum (AS) security control; – Inter CN node signalling for mobility between 3GPP access

networks; – Idle mode UE reachability (including control and execution of

paging retransmission); – Tracking area list management (for UE in idle and active

mode); – P-GW and S-GW selection; – MME selection for handovers with MME change; – SGSN (Serving GPRS Support Node) selection for

handovers to 2G or 3G 3GPP access networks; – Roaming; – Authentication; – Bearer management functions including dedicated bearer

establishment.

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Serving Gateway (S-GW) Functions

• Serving Gateway (S-GW) hosts the following functions

– The local mobility anchor point for inter-eNB handover; – Mobility anchoring for inter-3GPP mobility; – E-UTRAN idle mode downlink packet buffering and initiation

of network triggered service request procedure; – Lawful Interception; – Packet routing and forwarding; – Transport level packet marking in the uplink and the

downlink; – Accounting on user and QCI (QoS class identifier) granularity

for inter-operator charging; – UL and DL charging per UE, PDN (Packet Data Network),

and QCI

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PDN Gateway (P-GW) Functions

• PDN Gateway (P-GW) hosts the following functions

– Per-user based packet filtering (by e.g. deep packet inspection);

– Lawful Interception; – UE IP address allocation; – Transport level packet marking in the downlink; – UL and DL service level charging, gating and rate

enforcement; – DL rate enforcement based on aggregate maximum bit rate

(AMBR);

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Radio Protocol Architecture • RAN protocol architecture is designed for the user as

well as the control planes.

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1

2

3

4

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Radio Protocol Architecture 1. Packet Data Convergence Protocol (PDCP)

performs IP header compression. – Robust Header Compression (ROHC). – Ciphering, the control plane, integrity protection of the

transmitted data, as well as in-sequence delivery and duplicate removal for handover.

2. Radio-Link Control (RLC) is responsible for segmentation/concatenation, retransmission handling, duplicate detection, and in-sequence delivery to higher layers. – provides services to the PDCP in the form of radio bearers.

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Radio Protocol Architecture 3. Medium-Access Control (MAC) handles multiplexing

of logical channels, hybrid-ARQ retransmissions, and uplink and downlink scheduling. – The scheduling functionality is located in the eNodeB for

both uplink and downlink. – provides services to the RLC in the form of logical channels.

4. Physical Layer (PHY) handles coding/decoding, modulation/demodulation, multi-antenna mapping, and other typical physical-layer functions. – offers services to the MAC layer in the form of transport

channels.

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LTE Data Flow 1. The PDCP performs (optional) IP-header

compression, followed by ciphering. – A PDCP header is added, carrying information required for

deciphering in the terminal.

2. The RLC protocol performs concatenation and/or segmentation of the PDCP SDUs and adds an RLC header. – The RLC PDUs are forwarded to the MAC layer, which

multiplexes a number of RLC PDUs and attaches a MAC header to form a transport block.

3. The physical layer attaches a CRC to the transport block, performs coding and modulation, and transmits the resulting signal, possibly using multiple transmit antennas.

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LTE Data Flow

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Radio-Link Control • It is responsible for segmentation/concatenation of

(header-compressed) IP packets, and RLC SDUs are from the PDCP into suitably sized RLC PDUs.

– retransmission of erroneously received PDUs, as well as removal of duplicated PDUs.

– ensures in-sequence delivery of SDUs to upper layers.

• For LTE, the RLC PDU size varies dynamically.

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Radio-Link Control • Since the RLC, scheduler, and rate adaptation

mechanisms are all located in the eNodeB, dynamic PDU sizes are easily supported for LTE.

• By monitoring the sequence numbers of the incoming PDUs, the receiving RLC can identify missing PDUs.

• Status reports are then fed back to the transmitting RLC entity, requesting retransmission of missing PDUs.

– MAC-based hybrid-ARQ protocol.

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Medium-Access Control • The MAC layer handles logical-channel multiplexing,

hybrid-ARQ retransmissions, and uplink and downlink scheduling.

– Is also responsible for multiplexing/demultiplexing data across multiple component carriers when carrier aggregation is used.

• The MAC provides services to the RLC in the form of logical channels.

• A logical channel is defined by the type of information it carries and is generally classified

– as a control channel, used for transmission of control and configuration information necessary for operating.

– as a traffic channel, used for the user data.

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Logical Channels • The Broadcast Control Channel (BCCH), used for

transmission of system information from the network to all terminals in a cell.

– A terminal uses the system information to find out how the system is configured.

• The Paging Control Channel (PCCH), used for paging of terminals whose location on a cell level is not known to the network.

• The Common Control Channel (CCCH), used for transmission of control information in conjunction with random access.

• The Dedicated Control Channel (DCCH), used for transmission of control information to/from a terminal.

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Logical Channels • The Multicast Control Channel (MCCH), used for

transmission of control information required for reception of the MTCH (see below).

• The Dedicated Traffic Channel (DTCH), used for transmission of user data to/from a terminal.

– This is the logical channel type used for transmission of all uplink and non-MBSFN (Multicast-Broadcast Single Frequency Network) downlink user data.

• The Multicast Traffic Channel (MTCH), used for downlink transmission of MBMS services.

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Logical Channels • Part of the MAC functionality is multiplexing of

different logical channels and mapping of the logical channels to the appropriate transport channels.

RLC

MAC

PHY

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Mapping between logical channels and transport channels

BCCHPCCH DCCH DTCH MCCH MTCH

BCHPCH SCHRACH MCH

Logical channels

Transport channels

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DL Transport Channels • Broadcast Channel (BCH)

– Fixed format – Must be broadcast over entire coverage area of cell

• Downlink Shared Channel (DL-SCH) – Supports Hybrid ARQ (HARQ) – Supports dynamic link adaption by varying modulation, coding

and transmit power – Suitable for transmission over entire cell coverage area – Suitable for use with beamforming – Support for dynamic and semi-static resource allocation – Support for discontinuous receive (DRX) for power save – support for MBMS transmission.

• Paging Channel (PCH) – Support for UE DRX – Requirement for broadcast over entire cell coverage area – Mapped to dynamically allocated physical resources

• Multicast Channel (MCH) – Requirement for broadcast over entire cell coverage area – Support for MB-SFN (Single-Frequency-Network) – Support for semi-static resource allocation

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Transport Channels • For the physical layer, the MAC layer uses services in

the form of transport channels. • A transport channel is defined by how and with what

characteristics the information is transmitted over the radio interface.

• The following transport-channel types are defined for LTE:

• The Broadcast Channel (BCH) has a fixed transport format, provided by the specifications.

– for transmission of parts of the BCCH system information, more specifically the so-called Master Information Block (MIB)

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Transport Channels • The Paging Channel (PCH) is used for transmission

of paging information from the PCCH logical channel. – Supports discontinuous reception (DRX) to allow the terminal

to save battery power by waking up to receive the PCH only at predefined time instants

• The Downlink Shared Channel (DL-SCH) is the main transport channel used for transmission of downlink data in LTE.

– Supports dynamic rate adaptation and channel dependent scheduling in the time and frequency domains, hybrid ARQ with soft combining, and spatial multiplexing.

– Also uses for transmission of the parts of the BCCH system information not mapped to the BCH.

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Transport Channels • The Multicast Channel (MCH) is used to support

MBMS. – the scheduling and transport format configuration are

coordinated among the transmission points involved in the MBSFN transmission.

• The Uplink Shared Channel (UL-SCH) is the uplink counterpart to the DL-SCH.

– Uplink transport channel used for transmission of uplink data.

• The Random-Access Channel (RACH) is also defined as a transport channel, although it does not carry transport blocks.

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Logical Channels • DL-SCH and UL-SCH are the main downlink and

uplink transport.

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Uplink Transport Channels

• Uplink – Shared Channel (UL-SCH) – Support possible use of beamforming – Support dynamic link adaptation (varying modulation,

coding and/or Tx power) – Support for HARQ – Support for dynamic and semi-static resource allocation

• Random Access Channel (RACH) – Supports transmission of limited control information – Possible risk of collision

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Uplink Transport Channels

• Mapping Uplink Physical Channels to Transport Channels

UplinkPhysical channels

UplinkTransport channels

UL-SCH

PUSCH

RACH

PUCCHPRACH

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RLC Format • To each RLC PDU, there is an associated sub-

header in the MAC header. – contains the identity of the logical channel (LCID) from

which the RLC PDU originated and the length of the PDU in bytes.

• There is also a flag indicating whether this is the last sub-header or not.

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MAC Format • The MAC layer inserts the MAC control elements into

the transport blocks to be transmitted over the transport channels.

• A MAC control element is used for inband control signaling – timing-advance commands and random access response.

• Control elements are identified with reserved values in the LCID field, where the LCID value indicates the type of control information.

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Scheduling • The basic operation of the scheduler is dynamic

scheduling, – each 1 ms interval takes a scheduling decision and sends

scheduling information to the selected set of terminals. – across multiple cells residing in different eNodeBs is

supported using signaling over the X2 interface.

• The downlink scheduler is responsible which terminal(s) to transmit and uses the set of resource blocks in the terminal’s DL-SCH

• Similarly, terminals are to transmit on their respective UL-SCH and on which uplink time–frequency resources

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Scheduling • The uplink scheduling decision is taken per terminal and not

per radio bearer. • Although the eNodeB scheduler controls the payload of a

scheduled terminal, the terminal is still responsible for selecting from which radio bearer(s) the data is taken.

TF: Transport Format

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Scheduling • Downlink channel-dependent scheduling is supported

through channel-state reports, – reflecting the instantaneous channel quality, as well as

information necessary to determine the appropriate antenna processing in the case of spatial multiplexing.

• In the uplink, the channel-state information

necessary for uplink channel-dependent scheduling can be based on a sounding reference signal transmitted from each terminal for which the eNodeB wants to estimate the uplink channel quality.

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Physical Layer • The physical layer is responsible for

– coding – physical-layer hybrid-ARQ processing – modulation – multiantenna processing – mapping of the signal to the appropriate physical time–

frequency resources

• The physical layer provides services to the MAC layer

in the form of transport channels. • Data transmission in downlink and uplink use the DL-

SCH and UL-SCH transport-channel types respectively.

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Physical Layer • These channels, known as L1/L2 control channels,

are used for – downlink control information (DCI), providing the terminal

with the necessary information for proper reception and decoding of the downlink data transmission,

– uplink control information (UCI) used for providing the scheduler and the hybrid-ARQ protocol with information about the situation at the terminal.

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Mapping Downlink Physical Channels to Transport Channels

BCH PCH DL-SCHMCH

DownlinkPhysical channels

DownlinkTransport channels

PBCH PDSCHPMCH PDCCH

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Physical Channels • LTE DL physical channels are:

– Physical Downlink Shared Channel (PDSCH) – Physical Downlink Control Channel (PDCCH) – Common Control Physical Channel (CCPCH)

» PBCH Physical Broadcast Channel » PCFICH Physical Control Format Indicator Channel » PHICH Physical Hybrid ARQ Indicator Channel » PMCH Physical Multicast Channel

• UL – PRACH Physical Random Access Channel – PUCCH Physical Uplink Control Channel – PUSCH Physical Uplink Shared Channel

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General Structure for DL Physical Channels

• The baseband signal representing a DL physical channel is defined in terms of the following steps:

– scrambling of coded bits in each of the code words to be transmitted on a physical channel

– modulation of scrambled bits to generate complex-valued modulation symbols

– mapping of the complex-valued modulation symbols onto one or several transmission layers

– precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports

– mapping of complex-valued modulation symbols for each antenna port to resource elements

– generation of complex-valued time-domain OFDM signal for each antenna port

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General Structure for DL Physical Channels

Scrambling Modulation mapper

Layermapper Precoding

Resource element mapper

OFDM signal generation

Resource element mapper

OFDM signal generationScrambling Modulation

mapper

layers antenna portscode words

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Physical Channels (DL) • The PDSCH is the main physical channel used for

– unicast data transmission, – also for transmission of paging information.

• The PBCH carries part of the system information, required by the terminal in order to access the network.

• The PMCH is used for MBSFN operation. • The PDCCH is used for downlink control information,

mainly scheduling decisions, required for reception of PDSCH, and for scheduling grants enabling transmission on the PUSCH.

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Physical Channels (DL) • The PHICH carries the hybrid-ARQ

acknowledgement to indicate to the terminal whether a transport block should be retransmitted or not.

• The PCFICH is a channel providing the terminals with information necessary to decode the set of PDCCHs.

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Physical Channels (UL) • The PUSCH is the uplink counterpart to the PDSCH.

– at most one PUSCH per uplink component carrier per terminal.

• The PUCCH is used by the terminal – to send hybrid-ARQ acknowledgements, indicating to the

eNodeB whether the downlink transport block(s) was successfully received or not.

– to send channel-state reports aiding downlink channel-dependent scheduling, and for requesting resources to transmit uplink data upon.

– at most one PUCCH per terminal.

• The PRACH is used for random access.

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Physical Downlink Shared Channel (PDSCH)

• The PDSCH is utilized basically for data and multimedia transport.

• Therefore it is designed for very high data rates. • Modulation options therefore include QPSK, 16QAM

and 64QAM. – Spatial multiplexing is exclusive to the PDSCH. It is not used

on either the PDCCH or the CCPCH.

• CRC insertion: 24 bit CRC is the baseline for PDSCH

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Physical Downlink Control Channel (PDCCH)

• The PDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands.

1.Scheduling decisions – for Downlink, including PDSCH resource indication, transport

format, hybrid- ARQ information, and control information related to spatial multiplexing (if applicable).

– for Uplink, including PUSCH resource indication, transport format, and hybrid-ARQ-related information.

2.Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.

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PDCCH

PDCCH

format

Number of

CCEs

Number of

PDCCH bits

0 1

1 2

2 4

3 8

• A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs)

– a control channel element corresponds to a set of resource elements.

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PDCCH • The PDCCH conveys UE-specific control information. • Robustness rather than maximum data rate is the

consideration. • QPSK is the only available modulation format. • The PDCCH is mapped onto resource elements in up

to the first three OFDM symbols in the first slot of a subframe.

sync sync

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PDCCH

Subframe Number of OFDM symbols for PDCCH

Subframe 1 and 6 for frame structure type 2 1, 2

MBSFN subframes on a carrier supporting both PMCH and PDSCH 1, 2

MBSFN subframes on a carrier not supporting PDSCH 0

All other subframes 1, 2, 3

Maximum number of OFDM symbols used for PDCCH

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PDCCH Processing

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PDCCH Processing • A CRC is attached to each DCI message payload. • The identity of the terminal (or terminals) addressed –

the Radio-Network Temporary Identifier (RNTI) – is included in the CRC calculation.

• After CRC attachment, the bits are coded with a rate-1/3 tail-biting convolutional code and rate matched to fit the amount of resources used for PDCCH transmission.

– Tail-biting convolutional coding is similar to conventional convolutional coding with the exception that no tail bits are used.

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PDCCH Processing • The sequence of bits corresponding to all the PDCCH

resource elements to be transmitted in the subframe – including the unused resource elements, is scrambled by a

cell- and subframe-specific scrambling sequence to randomize inter-cell interference, followed by QPSK modulation and mapping to resource elements.

• This structure is based on Control-Channel Elements (CCEs), which in essence is a convenient name for a set of 36 useful resource elements (nine resource-element groups).

• The number of CCEs, one, two, four, or eight, required depends on the payload size of the control information (DCI payload) and the channel-coding rate.

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PDCCH Processing • The number of CCEs depends on

– the size of the control region – the cell bandwidth – the number of downlink antenna ports – the amount of resources occupied by PHICH

• The size of the control region can vary dynamically from subframe to subframe as indicated by the PCFICH.

• A specific PDCCH can be identified by the numbers of the corresponding CCEs in the control region.

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PDCCH Processing • As the number of CCEs may vary and is not signaled, the

terminal has to blindly determine the number of CCEs used for the PDCCH it is addressed upon.

• The sequence of CCEs should match the amount of resources available in a given subframe - the number of CCEs varies according to the value transmitted on the PCFICH

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Blind Decoding of PDCCHs • Each PDCCH supports multiple formats and the

format used is a priori unknown to the terminal. – The terminal needs to blindly detect the format of the

PDCCHs.

• In each subframe, the terminals will attempt to decode all the PDCCHs that can be formed from the CCEs in each of its search spaces.

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Blind Decoding of PDCCHs • It is required to have mechanisms to limit the CCE

aggregations that the terminal is supposed to monitor.

• From a scheduling point of view: CCE aggregations are undesirable as they may influence the scheduling flexibility and require additional processing at the transmitter side.

• From a terminal-complexity point of view: the terminal to monitor all possible CCE aggregations, also for the larger cell bandwidths, is not attractive.

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Blind Decoding of PDCCHs • To impose as few restrictions as possible on the

scheduler while at the same time limit the maximum number of blind decoding attempts in the terminal.

• LTE defines search spaces, which describe the set of CCEs the terminal is supposed to monitor for scheduling assignments/ grants relating to a certain component carrier.

• A search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to decode.

– For multiple aggregation levels, corresponding to one, two, four, and eight CCEs, a terminal has multiple search spaces.

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Blind Decoding of PDCCHs • The terminals will attempt to decode all the PDCCHs

that can be formed from the CCEs in each of its search spaces.

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Example 1. Terminal A cannot be addressed on a PDCCH

starting at CCE number 20, whereas terminal B can. 2. If terminal A is using CCEs 16–23, terminal B

cannot be addressed on aggregation level 4 as all CCEs in its level-4 search space are blocked by the use for the other terminals. – each terminal in the system has a terminal-specific search

space at each aggregation level.

3. The terminal-specific search spaces partially overlap between the two terminals in this subframe (CCEs 24–31 on aggregation level 8) but, as the terminal-specific search space varies between subframes, the overlap in the next subframe is most likely different.

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Blind Decoding of PDCCHs • LTE has also defined common search spaces.

– all terminals in the cell monitor the CCEs for control information.

• It is primarily transmission of various system

messages, it can be used to schedule individual terminals as well.

• Also, it can be used to resolve situations where scheduling of one terminal is blocked due to lack of available resources in the terminal-specific search space.

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Blind Decoding of PDCCHs • As the main function of the common search space is

to handle scheduling of system information intended for multiple terminals, and such information must be receivable by all terminals in the cell.

– scheduling is used the common search space.

• The common search spaces are only defined for

aggregation levels of four and eight CCEs and only for the smallest DCI formats, 0/1A/3/3A and 1C. (Table 10-4)

– Without support for DCI formats with spatial multiplexing in the common search space.

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Blind Decoding of PDCCHs • The downlink DCI formats to decode in the terminal-

specific search space depend on the transmission mode configured for the terminal.

• The DCI monitoring is described for the case of a single component carrier.

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Blind Decoding of PDCCHs • For multiple component carriers, the above

procedures are applied in principle to each of the activated downlink component carriers. (Ch 12)

• There is one UE-specific search space per aggregation level and per (activated) component carrier upon which PDSCH can be received (or PUSCH transmitted), although there are some carrier-aggregation-specific modifications.

• UE-specific search space per aggregation level and component carrier used for the PDSCH/PUSCH.

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Blind Decoding of PDCCHs • Where PDSCH/PUSCH transmissions on component

carrier 1 are scheduled using PDCCHs transmitted on component carrier 1.

• For component carrier 2, a carrier indicator is assumed in the UE-specific search space as component carrier 2 is cross-carrier scheduled from PDCCHs transmitted on component carrier 1.

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Blind Decoding of PDCCHs • For component carriers 3 and 4, the terminal will

handle the two search spaces independently, assuming (in this example) a carrier indicator for component carrier 4 but not for component carrier 3.

• If the UE-specific and common search spaces relating to different component carriers happen to overlap for some aggregation level when cross-carrier scheduling is configured, the terminal only needs to monitor the common search space.

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Physical Hybrid ARQ Indicator Channel (PHICH)

• The PHICH is used to transmit the hybrid-ARQ acknowledgements in response to UL-SCH transmission.

• The hybrid-ARQ acknowledgement (one single bit of information per transport block) is repeated three times, followed by BPSK modulation on either the I or the Q branch and spreading with a length-four orthogonal sequence.

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PHICH

• Multiple PHICHs mapped to the same set of resource elements constitute a PHICH group,

– where PHICHs within the same PHICH group are separated through different orthogonal sequences.

Sequence index Orthogonal sequence seqPHICHn Normal cyclic prefix

4PHICHSF =N

Extended cyclic prefix

2PHICHSF =N

0 [ ]1111 ++++ [ ]11 ++

1 [ ]1111 −+−+ [ ]11 −+

2 [ ]1111 −−++ [ ]jj ++

3 [ ]1111 +−−+ [ ]jj −+

4 [ ]jjjj ++++ -

5 [ ]jjjj −+−+ -

6 [ ]jjjj −−++ -

7 [ ]jjjj +−−+ -

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PHICH • PHICH duration in MBSFN and non-MBSFN

subframes

Non-MBSFN subframes MBSFN subframes

PHICH duration Subframes 1 and 6 in case of

frame structure type 2

All other cases On a carrier supporting

both PDSCH and PMCH

Normal 1 1 1

Extended 2 3 2

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PHICH • After forming the composite signal representing the PHICHs in a

group, cell-specific scrambling is applied and the 12 scrambled symbols are mapped to three resource-element groups.

– separated by approximately one-third of the downlink cell bandwidth.

• In the first OFDM symbol in the control region, resources are first allocated to the PCFICH, the PHICHs are mapped to resource elements not used by the PCFICH

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PHICH • Each downlink subframe is normally divided into a

control region, and a data region. – control region consisting of the first few OFDM symbols, – data region consisting of the remaining part of the subframe.

• The control region carries L1/L2 signaling necessary to control uplink and downlink data transmissions.

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PCFICH • The PCFICH indicates the size of the control region

in terms of the number of OFDM symbols – that is, indirectly where in the subframe the data region starts.

• If PCFICH is incorrectly decoded, – neither know how to process the control channels nor where

the data region starts for the corresponding subframe. – The terminal could blindly try to decode all possible control

channel formats and, from which format that was correctly decoded, deduce the starting position of the data region, but this can be a very complex procedure.

• Two bits of information (32-bit codeword), corresponds to the three control-region sizes of one, two, or three OFDM symbols.

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PCFICH • The coded bits are scrambled with a cell- and subframe-

specific scrambling code to randomize inter-cell interference, QPSK modulated, and mapped to 16 resource elements.

• As the size of the control region is unknown until the PCFICH is decoded, the PCFICH is always mapped to the first OFDM symbol of each subframe.

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PCFICH • The transmission power of the PCFICH is under

control of the eNodeB.

• For coverage in a certain cell, the power of the PCFICH can be set higher than for other channels by “borrowing” power from, for example, simultaneously transmitted PDCCHs.

– increasing the power of the PCFICH to improve the performance in an interference-limited system.

• As the PCFICH-to-resource-element mapping depends on the cell identity, the probability of (partial) collisions with PCFICH in neighboring cells in synchronized networks is reduced, thereby improving the performance of PCFICH power boosting as a tool to control the error rate.

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PCFICH • The mapping of resource elements in the first OFDM

symbol in the subframe is done in groups of four resource elements (resource-element groups) and separated in different frequencies.

• a symbol quadruplet consisting of four (QPSK) symbols is mapped. • instead of simply mapping the symbols one by one, is the support

of transmit diversity. • transmit diversity with up to four antenna ports is specified for

L1/L2

• Transmit diversity for four antenna ports is specified in terms of groups of four symbols (resource elements).

• To avoid collisions in neighboring cells, the location of the four groups in the frequency domain depends on the physical-layer cell identity.

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PCFICH • In first OFDM symbol; there are two resource

element groups per resource block, as every third resource element is reserved for reference signals (or non-used resource elements corresponding to reference symbols on the other antenna port).

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PCFICH • In second OFDM symbol; (if part of the control

region) there are two or three resource-element groups depending on the number of antenna ports configured.

• In third OFDM symbol (if part of the control region) there are always three resource-element groups per resource block.

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Summary for PCFICH • The four resource-element groups are separated by

one-fourth of the downlink cell bandwidth in the frequency domain, with the starting position given by physical-layer cell identity.

– 16 QPSK symbols are used for the transmission of four resource-element groups

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Common Control Physical Channel (CCPCH)

• CCPCH carries cell-wide control information. • Like the PDCCH, robustness rather than maximum

data rate is the chief consideration. • QPSK is therefore the only available modulation

format. • CCPCH is transmitted as close to the center

frequency as possible. CCPCH is transmitted exclusively on the 72 active subcarriers (=6 PRBs) centered on the DC subcarrier.

• Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol (0..5/6) within the slot and l refers to the subcarrier.

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Downlink L1/L2 Control Signaling • Downlink control signaling supports the transmission

of downlink and uplink transport channels. – the corresponding information partly originates from the

physical layer (Layer 1) and partly from Layer 2 MAC.

• Downlink L1/L2 control signaling consists of downlink scheduling assignments, including

– information required for the terminal to be able to properly receive,

– demodulate, – decode the DL-SCH on a component carrier, uplink

scheduling grants informing the terminal about the resources – transport format to use for uplink (UL-SCH) transmission – hybrid-ARQ acknowledgements in response to UL-SCH

transmissions.

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Downlink L1/L2 Control Signaling • The downlink L1/L2 control signaling is transmitted

within the first part of each subframe.

• Each subframe can be divided into a control region followed by a data region, where the control region corresponds to the part of the subframe in which the L1/L2control signaling is transmitted.

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Downlink L1/L2 Control Signaling • The control signaling can be dynamically adjusted

radio resources to match the instantaneous traffic situation.

– For example, a small number of users being scheduled in a subframe, the required amount of control signaling is small and a larger part of the subframe can be used for data transmission.

• The downlink L1/L2 control signaling consists of four different physical-channel types:

• PCFICH, informing the terminal about the size of the control region (one, two, or three OFDM symbols).

– There is one and only one PCFICH on each component carrier or, equivalently, in each cell.

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Downlink L1/L2 Control Signaling • PDCCH carries signal downlink scheduling

assignments and uplink scheduling grants. – Each PDCCH carries signaling for a single terminal, but can

also be used to address a group of terminals. – Multiple PDCCHs can exist in each cell.

• PHICH, used to signal hybrid-ARQ acknowledgements in response to uplink UL-SCH transmissions.

– Multiple PHICHs can exist in each cell.

• R-PDCCH (Relay Physical Downlink Control Channel), used for relaying.

– the R-PDCCH is not transmitted in the control region.

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Downlink Scheduling Assignment • Downlink scheduling assignments are valid for the same

subframe in which they are transmitted. • The scheduling assignments use one of the DCI formats 1, 1A,

1B, 1C, 1D, 2, 2A, 2B, or 2C and the DCI formats used depend on the transmission mode configured

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Uplink

• The principle advantage of SC-FDMA over conventional OFDM is a lower PAPR (by approximately 2 dB) than would otherwise be possible using OFDM.

• Data is mapped onto a signal constellation that can be QPSK, 16QAM, or 64QAM depending on channel quality.

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Uplink Physical Channels

• Physical Uplink Shared Channel (PUSCH) – Resources for the PUSCH are allocated on a sub-frame basis by

the UL scheduler. – Subcarriers are allocated in multiples of 12 (PRBs) and may be

hopped from sub-frame to sub-frame. – The PUSCH may employ QPSK, 16QAM or 64QAM modulation. – CRC insertion: 24 bit CRC

• Physical Uplink Control Channel (PUCCH) – It is never transmitted simultaneously with PUSCH data. – For frame structure type 2, the PUCCH is not transmitted in the

UpPTS field. – PUCCH conveys control information including channel quality

indication (CQI), ACK/NACK, HARQ and uplink scheduling requests (SR).

– The PUCCH transmission is frequency hopped at the slot boundary

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PUSCH • The baseband signal representing the physical uplink

shared channel is defined in terms of the following steps:

– scrambling – modulation of scrambled bits to generate complex-valued

symbols – transform precoding to generate complex-valued symbols – mapping of complex-valued symbols to resource elements – generation of complex-valued time-domain SC-FDMA signal

for each antenna port

Scrambling Modulation mapper

Transform precoder

Resource element mapper

SC-FDMA signal gen.

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PUCCH

• The physical uplink control channel supports multiple formats.

• Formats 2a and 2b are supported for normal cyclic prefix only.

PUCCH

format

Modulation

scheme

Number of bits per

subframe, bitM

1 N/A N/A

1a BPSK 1

1b QPSK 2

2 QPSK 20

2a QPSK+BPSK 21

2b QPSK+QPSK 22

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Summary for Physical Channels • Physical broadcast channel (PBCH)

– The coded BCH transport block is mapped to four subframes within a 40 ms interval;

– 40 ms timing is blindly detected, i.e. no explicit signalling indicating 40 ms timing;

– Each subframe is assumed to be self-decodable • Physical control format indicator channel (PCFICH)

– Informs the UE about the number of OFDM symbols used for the PDCCHs;

– Transmitted in every subframe. • Physical downlink control channel (PDCCH)

– Informs the UE about the resource allocation of PCH and DL-SCH, and Hybrid ARQ information related to DL-SCH;

– Carries the uplink scheduling grant. (rf. MAPs) • Physical Hybrid ARQ Indicator Channel (PHICH)

– Carries Hybrid ARQ ACK/NAKs in response to uplink transmissions.

• Physical downlink shared channel (PDSCH) – Carries the DL-SCH and PCH.

• Physical multicast channel (PMCH) – Carries the MCH.

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Summary for Physical Channels

• Physical uplink control channel (PUCCH) – Carries Hybrid ARQ ACK/NAKs in response to downlink

transmission; – Carries Scheduling Request (SR); (rf. BW-REQ) – Carries CQI reports.

• Physical uplink shared channel (PUSCH) – Carries the UL-SCH.

• Physical random access channel (PRACH) – Carries the random access preamble.

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Uplink Physical Signals

• Uplink physical signals are used within the PHY and do not convey information from higher layers.

• Two types of UL physical signals are defined: – Demodulation reference signal, associated with

transmission of PUSCH or PUCCH – Sounding reference signal, not associated with

transmission of PUSCH or PUCCH

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Uplink Reference Signals

• The demodulation signal facilitates coherent demodulation.

• It is transmitted in the fourth SC-FDMA symbol of the slot and is the same size as the assigned resource.

• There is also a sounding reference signal used to facilitate frequency dependent scheduling.

• Both variants of the UL reference signal are based on Zadhoff-Chu sequences.

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Downlink Channel Coding

• Different coding algorithms are employed for the DL physical channels.

• The PDSCH uses up to 64-QAM modulation. Rate 1/3 turbo coding has been selected for the PDSCH.

• For the common control channel (CCPCH), modulation is restricted to QPSK.

• For control channels, coverage is the paramount requirement.

• Convolutional coding has been selected for use with the CCPCH, though a final determination regarding code rate has not yet been made.

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Channel Coding and Interleaving

• Channel coding scheme for transport blocks – Turbo Coding with a coding rate of R=1/3, – Two 8-state constituent encoders and – A contention-free quadratic permutation

polynomial (QPP) turbo code internal interleaver. • Maximum information block size of 6144 bits • Error detection is supported by the use of 24-

bit CRC

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Channel Coding

TrCH Coding scheme Coding rate

UL-SCH

DL-SCH

PCH

MCH

Turbo coding 1/3

BCH

Tail biting

convolutional

coding

1/3

Usage of channel coding scheme and coding rate for transport CHs

Control Information Coding scheme Coding rate

DCI

Tail biting

convolutional

coding

1/3

CFI Block code 1/16

HI Repetition code 1/3

Block code variable

UCI Tail biting

convolutional

coding

1/3

Usage of channel coding scheme and coding rate for control information

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Channel Coding • Channel coding for DL-SCH (as well as for PCH and

MCH) is based on Turbo coding. • The encoding consists of two rate-1/2, eight-state

constituent encoders, implying an overall code rate of 1/3, in combination with QPP-based interleaving.

QPP : Quadrature Permutation Polynomial.

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Channel Coding • The QPP interleaver provides a mapping from the input (non-

interleaved) bits to the output (interleaved) bits according to the function:

– i is the index of the bit at the output of the interleaver, – c(i) is the index of the same bit at the input of the interleaver, – K is the code-block/interleaver size.

• The values of the parameters f1 and f2 depend on the code-block size K.

• The range of code-block sizes is from 40 bits to 6144 bits, together with the associated values for the parameters f1 and f2.

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Downlink Physical-Layer Processing (10) • LTE downlink, there are four different types of

transport channels defined, – the Downlink Shared Channel (DL-SCH), – the Multicast Channel (MCH), – the Paging Channel (PCH), – the Broadcast Channel (BCH).

• DL-SCH is mapping to the resource elements of the OFDM time–frequency grid.

– To transmit user data and dedicated control information, as well as part of the downlink system information.

• The DL-SCH physical-layer processing is to a large extent applicable.

– also to MCH and PCH transport channels, although with some additional constraints.

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Processing Steps • Within each transmission time interval (TTI), corresponding to

one subframe of length 1 ms, up to two transport blocks of dynamic size are delivered to the physical layer and transmitted over the radio interface for each component carrier.

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Processing Steps • In the case of no spatial multiplexing there is at most

a single transport block in a TTI. • In the case of spatial multiplexing, with transmission

on multiple layers in parallel to the same terminal, there are two transport blocks within a TTI.

• CRC Insertion Per Transport Block – a 24-bit CRC is calculated for and appended to each

transport block.

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Code-Block Segmentation and Per-Code-Block CRC Insertion • The LTE Turbo-coder internal interleaver is only

defined a maximum block size of 6144 bits. – including the transport-block CRC, exceeds this maximum

code-block size, code-block segmentation, is applied before the Turbo coding.

• Code-block segmentation implies that the transport

block is segmented into smaller code blocks. – matching the set of code-block sizes supported by the Turbo

coder. – the specification includes the possibility to insert “dummy”

filler bits at the head of the first code block.

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Code-Block Segmentation and Per-Code-Block CRC Insertion • Transport block implies that an CRC is calculated for

and appended to rear. – CRC is small for the transport block.

• The code block also adds additional error-detection capabilities (using CRC) and thus further reduces the risk for undetected errors in the code block.