LTE Channels

LTE Channels - 1 / 29

company.nokia.com Nokia 2015

LTE Channels



This material, including documentation and any related computer programs, is protected by

copyright controlled by Nokia. All rights are reserved. Copying, including reproducing, storing,

adapting or translating, any or all of this material requires the prior written consent of Nokia.

This material also contains confidential information, which may not be disclosed to others

without the prior written consent of Nokia.



Contents

1 Introduction .......................................................................................................................... 4

1.1 Air Interface Protocols ......................................................................................................... 4

1.2 LTE Channel Architecture .................................................................................................... 7

1.3 Exercise 1 ............................................................................................................................... 8

2 Logical Channels ................................................................................................................... 9

3 Transport Channels .............................................................................................................. 11

4 Physical Channels ................................................................................................................. 13

4.1 Channel Mapping .................................................................................................................. 13

4.2 Physical Broadcast Channel (PBCH) ................................................................................... 15

4.3 Physical Downlink Shared Channel (PDSCH) ..................................................................... 16

4.4 Physical Downlink Control Channel (PDCCH) .................................................................... 18

4.5 Enhanced Physical Downlink Control Channel (E-PDCCH) .............................................. 19

4.6 Physical Control Format Indication Channel (PCFICH) .................................................... 20

4.7 Physical Hybrid ARQ Indicator Channel (PHICH) ............................................................... 21

4.8 Physical Random Access Channel (PRACH) ....................................................................... 22

4.9 Physical Uplink Control Channel (PUCCH) ......................................................................... 23

4.10 Physical Uplink Shared Channel (PUSCH) .......................................................................... 25

4.11 Mapping DL/UL TDD Physical Channels ............................................................................ 26

4.12 Exercise 2 ............................................................................................................................... 28

4.13 Exercise 3 ............................................................................................................................... 28

4.14 Complete The Course .......................................................................................................... 29



1 Introduction

1.1 Air Interface Protocols

The TCP/IP Model has five protocol stack layers: the Application Layer, the Transport Layer, the

Network Layer (or Internet Protocol Layer), the Data Link Layer and the Physical Layer.

LTE divides the Data Link Layer into four Sub-layers:

the Radio Resource Control or the RRC Layer,

the Packet Data Convergence Protocol or the PDCP Layer,

the Radio Link Control or the RLC Layer, and

the Medium Access Control or the MAC Layer.

The main function of the Medium Access Control protocol is the management of uplink and

downlink transport resources for the terminals connected to the eNodeB. The MAC Layer

manages the Physical Layer scheduling and performs the HARQ functionality.

The Radio Link Control or RLC protocol performs segmentation and reassembly of PDCP

packets into smaller blocks that can be handled by the MAC Layer. The RLC Layer also offers

ARQ retransmission, if this is required, in addition to the lower-layer HARQ retransmission.

One of the main tasks of the Packet Data Convergence Protocol or PDCP is to compress and

decompress IP packet headers using the Robust Header Compression or RoHC protocol

defined in RFC 4995.

In the user plane, the IP user traffic is carried over PDCP, whereas ciphering and deciphering of

user data, along with signaling data, is also performed by the PDCP Layer.



It can be seen that control traffic and user traffic use different protocol stacks. In the control

plane the Non Access Stratum or NAS protocols and the Radio Resource Control, which is the

access stratum specific control protocol of E-UTRAN, handle the signaling traffic between the

terminal and the network.

Move your mouse pointer over the protocols in the figure to see a short description of each

protocol.



1.2 LTE Channel Architecture

The LTE Channel Architecture defines E-UTRAN Radio Access Bearer or E-RAB, Radio Bearer,

Signaling Radio Bearer, Logical Channels, Transport Channels, and Physical Channels.

An E-RAB carries one or more service data flows between a UE and the EPC. A Radio Bearer

transports the data packets of an E-RAB from the eNodeB to the UE. Each E-RAB has a one-to-

one mapping with a radio bearer. A Signalling Radio Bearer transports signalling packets

between the RRC Sub-layer and the PDCP Sub-layer.

Logical channels are located at the horizontal interface between the MAC Sub-layer and RLC

Sub-layer. Logical Channels are used to transfer data between the MAC Sub-layer and the RLC

Sub-layer. Logical control channels are mapped to signalling radio bearer channels, while

logical traffic channels are mapped to the radio bearer.

In addition to the logical channels, there are so-called transport channels - located at the

horizontal interface between the Physical Layer and the MAC Sub-layer. The transport channel

transfers the data between the MAC Sub-layer and Physical Layer. Each Logical Channel is

mapped to a transport channel. Transport Channels describe how the information will be

formatted before being transmitted i.e. channel coding, rate matching, attachment of Cyclic

Redundancy Check, transport block size, etc.

Physical channels transfer the data across the air interface. Each Transport Channel is mapped

to a physical channel.



1.3 Exercise 1



2 Logical Channels

Logical Channels are used to distinguish the type of information transmitted within the

attached radio bearer. The two major groups of logical channel types are Control Channels and

Traffic Channels. Control Channels are for signaling and Traffic Channels are for IP user data.

Logical channel types defined for E-UTRAN signaling are the:

Broadcast Control Channel or BCCH

Paging Control Channel or PCCH

Common Control Channel or CCCH

Dedicated Control Channel or DCCH and

Multicast Control Channel or MCCH

Logical Channel types defined for user data are

Dedicated Traffic Channel or DTCH and

Multicast Traffic Channel or MTCH

The Broadcast Control Channel is used to transmit system information regarding access and

non-access stratum in the downlink direction only. It allows the UE to retrieve cell and network

configuration parameters, required for normal operation within E-UTRAN.

The Paging Control Channel is used to transmit the paging messages. It is a downlink point-to-

multipoint channel. UE listens PCCH when it is in LTE IDLE mode.

The Common Control Channel is an uplink and downlink channel. UEs in RRC IDLE state use

Common Control Channel for initial access signaling, rest of the communication takes place on

Dedicated Control Channel.



The Dedicated Control Channel is a bidirectional RRC signaling channel, used for point-to-point

RRC and non-access stratum signaling procedures. It is the main signaling channel used by the

UEs in RRC CONNECTED state.

The Multicast Control Channel is associated with Multimedia Broadcast Multicast Services or

MBMS and is transmitted in downlink direction only. It allows the eNodeB to inform UEs, that

want to listen to broadcast or multicast service traffic, about availability of such services.

The Dedicated Traffic Channel is used for user radio bearers carrying IP traffic. The eNodeB

connects DTCHs with their associated S1-U tunnel to the SAE Gateway. DTCH can be

bidirectional, uplink only or downlink only. DTCH is of course a point-to-point traffic channel.

The Multicast Traffic Channel is a point-to-multipoint traffic channel for Multimedia Broadcast

Multicast Services. It carries IP traffic for broadcast or multicast services driven by the MBMS

feature.



3 Transport Channels

In contrast to logical channel types, depending on the type of information transmitted, the

transport channel types are used to indicate transport characteristics. A certain transport

channel type is associated with certain bit rates such as transport block sizes or number of

blocks, support for HARQ, support for beam-forming, support for discontinuous reception and

transmission, coding, and so on. Transport channels are present in both downlink and uplink

direction.

In the downlink we have the Broadcast Channel or BCH. This is used in the downlink direction

to transfer the master information block or MIB using the transparent mode of the RLC layer

and the BCCH logical channel. It is important to mention here that the BCCH does not carry

system information blocks or SIB.

The Paging Channel or PCH is used during the paging procedure and it transfers the paging

information to the entire cell using the transparent mode of RLC and PCCH logical channels.

The Downlink Shared Channel or DL-SCH is used to carry user data along with the system

information block and RRC signaling. For data traffic, DL-SCH supports HARQ and dynamic link

adaptation. The DL-SCH maps to the DCCH, CCCH, and DTCH logical channels.

The Multicast Channel or MCH carries multicast traffic for the entire cell. The MCH maps to the

MCCH and MTCH logical channels.

In the uplink we have the Uplink Shared Channel or UL-SCH this is used to carry user data,

along with control information, in the uplink direction.

The Random Access Channel or RACH is used for initial access to the cell or when a UE needs to

transmit on the Physical Uplink Shared Channel or Physical Uplink Control Channel and does

not have a valid uplink grant. It carries random access preamble control information between

the MAC and the physical layer.



4 Physical Channels

4.1 Channel Mapping

When compared to previous technologies such as WCDMA or HSPA, the number of transport

channels has been reduced. Instead of several dedicated channels, there is a single shared

channel in downlink and a single shared channel in uplink.

As far as the channel mapping is concerned, dedicated traffic channels and dedicated control

channels are mapped into the downlink shared channel in the downlink direction or uplink

shared channel in the uplink direction, which in turn is mapped into the Physical Downlink

Shared Channel or Physical Uplink Shared Channel, respectively.

Logical Common Control Channels or CCCH are also mapped into transport shared channels.

In the downlink direction, the logical Paging Common Control Channel is mapped to the Paging

Channel or PCH at the transport Layer, which in turn is mapped to PDSCH. Similarly, the

Broadcast Control Channel is mapped to the Broadcast Channel or BCH, which in turn is

mapped to the Physical Broadcast Channel.

In uplink, the transport Random Access Channel or RACH is mapped to the Physical Random

Access Channel.

The Logical Multicast Control Channel and Multicast Traffic Channel are mapped to the

Transport Multicast Channel or MCH, which in turn is mapped to the Physical Multicast Channel

at the Physical Layer.

Note that some of the physical channels do not carry any higher-layer information, so there is

no channel mapping for these channels.

Use your mouse pointer to get more details in the channel mapping figure.



4.2 Physical Broadcast Channel (PBCH)

During cell search, the UE must decode important information carried over the Physical

Broadcast Channel or PBCH before it can start communicating with the eNodeB.

The PBCH transfers the Master Information Block which carries system information such as

system bandwidth, number of transmit antennas, and the system frame number.

The PBCH structure is independent of the system bandwidth and it always occupies 72 central

subcarriers belonging to the first four OFDM symbols of the second slot of every 10ms radio

frame.

The PBCH is transmitted over a 40ms transmission time interval.

Out of a total of 288 resource elements, 240 resource elements are occupied by PBCH and

the other 48 resource elements are reserved for reference signals.

The PBCH uses QPSK modulation, therefore 240 resource elements give 480 bits of

information.



4.3 Physical Downlink Shared Channel (PDSCH)

The Physical Downlink Shared Channel or PDSCH is the main data bearing downlink channel.

The user data, paging and other RRC signaling messages along with actual system information

in the downlink are carried over the Physical Downlink Shared Channel. The set of System

Information Blocks are broadcast using PDSCH, in contrast to the Master Information Block

which is broadcast using PBCH.

The PDSCH is allocated to different UEs periodically at the transmission time interval of 1ms.

PDSCH is a shared channel so it means that during a sub-frame, several different UEs can

share the PDSCH. In contrast to uplink, resource blocks allocated to a certain UE during a time

slot need not necessarily be located adjacently in the frequency domain.

Using the Physical Downlink Control Channel (PDCCH) control channel, the eNodeB will assign

one or more resource blocks to each UE. The Physical Downlink Shared Channel is the only

channel which can be modulated using a QPSK, 16QAM, or 64QAM modulation scheme. The

eNodeB selects the appropriate modulation and coding scheme according to its link

adaptation algorithm.

There are several transmission modes available for PDSCH. These Transmission Modes

configures the multi-antenna techniques.

Use your mouse pointer for more detail about Transmission Modes and System Information

Blocks.



4.4 Physical Downlink Control Channel (PDCCH)

The Physical Downlink Control Channel or PDCCH is used to transfer Downlink Control

Information and contains important downlink and uplink scheduling information for all mobile

terminals in the cell.

The PDCCH may occupy the first 1, 2, 3, or 4 OFDM symbols of each sub-frame. The number

of OFDM symbols occupied by the PDCCH is indicated by the Physical Control Format

Indication Channel or PCFICH.

Using the PDCCH, each UE is able to identify the resource blocks allocated to it on the PDSCH

during each sub-frame. It also includes the information about the selected modulation scheme

and coding rate in the downlink, as well as downlink hybrid ARQ-related information.

Note that the PDCCH also informs each UE which resource blocks it can use for transmission

on the physical shared channel in the uplink, as well as the uplink modulation scheme, and a

coding rate to be used in each sub-frame. In addition to this, the Transmit Power Commands

or TPC for uplink transmission are also carried by the PDCCH.



4.5 Enhanced Physical Downlink Control Channel (E-PDCCH)

The Enhanced - Physical Downlink Control Channel (E-PDCCH), is a downlink control channel

that has been introduced in 3GPP Release 11, to support LTE-Advanced features, like

Coordinated Multipoint, extended downlink MIMO and enhanced ICIC.

In 3GPP R8, the Downlink Control Information (DCI) uses the first 1 to 4 symbols of a subframe

spreading over the entire bandwidth. This channel architecture proved to be limited for LTE-

Advnaced features.

The E-PDCCH uses the same resources as the PDSCH for expanding the control information.

The E-PDCCH is configured per UE by a dedicated RRC signal. The UE can be configured with

two sets of E-PDCCH sets, where each set contains 2, 4, or 8 PRB pairs.

Each RB pair consists of a number of Enhanced Control Channel Elements (ECCE). In addition,

each ECCE consist out of 4 or 8 Enhanced Resource Element Groups (EREG). The EREG is

formed by 9 Resource Elements (REs).

Finally, the transmission could be localized or distributed.



4.6 Physical Control Format Indication Channel (PCFICH)

The Physical Control Format Indication Channel or PCFICH is used for informing the UE about

the number of OFDM symbols allocated to the Physical Downlink Control Channel or PDCCH.

The number of OFDM symbols allocated for the PDCCH in each sub-frame is dynamic and can

possibly occupy 1, 2, 3 or 4 symbols.

The PDCCH is transferred once in a sub-frame, which is why the PCFICH is also transmitted in

every sub-frame and is carried in the first OFDM symbol of each sub-frame.

The PCFICH occupies 4 Resource Element Groups or REGs which correspond to 16 resource

elements. To get frequency diversity gain, resource element groups of PCFICH are distributed

evenly across the system bandwidth.

The PCFICH uses Quadrature Phase Shift Keying or QPSK encoding.



4.7 Physical Hybrid ARQ Indicator Channel (PHICH)

As the name implies, the Physical Hybrid ARQ Indicator Channel or PHICH conveys positive

acknowledgement or negative acknowledgement for the data blocks received in the uplink

direction.

Each PHICH acknowledgement occupies 12 resource elements when using the normal cyclic

prefix and 6 resource elements when using the extended cyclic prefix.

The resources used for the PHICH are configured on a semi-static basis and can be

transmitted using the first, the first two or the first three OFDM symbols.

Unlike other control channels, PHICH is modulated by using Binary Phase Shift Keying or BPSK.



4.8 Physical Random Access Channel (PRACH)

The Physical Random Access Channel or PRACH is an uplink contention-based channel, which

allows any UE to request network entry, access a target cell after handover, access a cell to

send a Scheduling Request, and so on.

Every time the UE wishes to initiate communication with the network, a procedure called

random access has to be performed. A so-called random access preamble is sent to the

eNodeB over the Physical Random Access Channel. Open loop power control together with

optional power ramp-up is used during the random access process at the beginning of the

connection. The response from the network is sent over the PDSCH. Uplink PRACH data is

orthogonal with the data in Physical Uplink Control Channel and Physical Uplink Shared

Channel.

A random access preamble occupies the bandwidth of 72 subcarriers in frequency domain

which corresponds to 6 resource blocks. The location of those resource blocks is dynamically

defined by two RRC Layer Parameters called PRACH Configuration Index and PRACH Frequency

offset. The UE may learn the configuration from the system information.

There are four different formats of PRACH preambles defined for Frequency Division Duplex or

FDD operation. Each format is defined by the duration of the sequence and its cyclic prefix.



4.9 Physical Uplink Control Channel (PUCCH)

The Physical Uplink Control Channel or PUCCH carries various Uplink Control Information or UCI

in those sub-frames where there is no Physical Uplink Shared Channel.

It carries Channel Quality Indicator or CQI, HARQ positive and negative acknowledgement and

UL Scheduling Request Indicator or SRI.

The size of the resources occupied by the PUCCH in frequency domain depends on resources

reserved for CQI, HARQ acknowledgement, and SRI.

Note that when the UE transmits data on the shared channel, the control information can be

embedded with the user data. In fact, there is relatively little control information to be sent in

the uplink, since the eNodeB is responsible for scheduling uplink resources, and the scheduling

information is sent over the Physical Downlink Control Channel to the UE.

PUCCH resource blocks are located at both edges of the uplink bandwidth. The PUCCH

achieves frequency diversity by using frequency hopping from one edge of the bandwidth to



the other edge. Inter-slot hopping may also be applied for the PUCCH. Depending on the

format, the PUCCH may use BPSK or QPSK encoding.

Use your mouse pointer for more detail about PUCCH formats.



4.10 Physical Uplink Shared Channel (PUSCH)

In the uplink direction, user data is carried over the Physical Uplink Shared Channel or PUSCH.

Due to Single Carrier FDMA in the uplink direction, it is not possible for an individual UE to send

PUSCH and PUCCH at the same time. Therefore, the Physical Uplink Shared Channel also

carries RRC signaling messages along with the control information like acknowledgement for

HARQ transmission, Channel Quality Indicator, Precoding Matrix Indicator or PMI and Rank

Indicator or RI in the presence of uplink user data.

PUSCH is a shared channel, so that the total resources blocks available to the PUSCH are

shared between the active connections. Resource blocks allocated to a certain UE on the

PUSCH must always be located adjacently in the frequency domain. However, frequency

hopping can be applied to achieve frequency diversity for PUSCH.

There is a separate power control mechanism for the PUSCH and PUCCH. The PUSCH may use

QPSK, 16QAM, or 64QAM encoding. It is the eNodeB which selects the appropriate modulation

and coding scheme for PUSCH transmission according to its link adaptation algorithm, and

sends this information to the UE through the PDCCH.



4.11 Mapping DL/UL TDD Physical Channels

TD-LTE uses Frame Type 2 (FT2), unlike FD-LTE which uses Frame Type 1 (FT1) structure.

The length of FT2 is 10 ms and it contains 9 sub-frames. Sub-frame 0 and sub-frame 5 are

always used for the downlink. Meanwhile, sub-frame 2 is always for the uplink. Moreover, the

first sub-frame is always a special sub-frame.

In TD-LTE, the transmission is not continuous, since certain sub-frames can be either downlink

or uplink. This means that each of these sub-frames can be used for downlink or uplink

transmissions based on the demand.

The special sub-frame is introduced to permit efficient switching from downlink to uplink

transmission. The switching between transmission directions adds small delay for both UE and

eNodeB. This delay is compensated by a Guard Period (GP).

The DwPTS (Downlink Pilot Time slot) and the UpPTS (Uplink Pilot Time slot) are taken from TD-

SCDMA technology. DwPTS is used to carry control information as well as ensuring downlink

synchronization. UpPTS is primarily intended for Sounding Reference Signals (SRS)

transmission from the UE.

Use your mouse-pointer to see the FT2 configurations.



4.12 Exercise 2

4.13 Exercise 3



4.14 Complete The Course

Documents

LTE Channels