02 ra41232en05gla0 lte channels ppt

8/12/2019 02 Ra41232en05gla0 Lte Channels Ppt

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For the Uu (air) interface, LTE divides the Data Link Layer into the following sublayers:

Radio Resource Control (RRC)

Packet Data Convergence Protocol (PDCP)

Radio Link Control (RLC)

Medium Access Control (MAC)

As illustrated in the graphic, control traffic and bearer traffic use different protocol

stacks.


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The LTE Channel Architecture defines E-RAB channels, Radio Bearer (RB) channels,

Signaling Radio Bearer (SRB) channels, Logical Channels, Transport Channels, and

Physical Channels. In general, each category behaves as a service access point

between adjacent protocol layers.

3GPP TS 36.211 Physical Channel and Modulation

3GPP TS 36.321 Medium Access Control (MAC) Protocol Specification

3GPP TS 36.322 Radio Link Control (RLC) Protocol Specification


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E-RAB - An E-RAB channel carries one or more service data flows between a UE and

the EPC.

Radio Bearer - A Radio Bearer channel transports the data packets of an E-RAB from

the eNodeB toward the UE. Each E-RAB has a one-to-one mapping with a radiobearer.

Signaling Radio Bearer - A Signaling Radio Bearer (SRB) channel transports signaling

packets between the RRC Sublayer and the PDCP Sublayer.

Logical Channel - A Logical Channel transports control or data traffic between the RLC

Sublayer and the MAC Sublayer. Logical control channels are mapped to signaling

radio bearer channels, while logical traffic channels are mapped to radio bearer

channels. Logical Channels describe transmission reliability (RLC Acknowledged

Mode, etc.).

Transport Channel - A Transport Channel forwards control or data traffic between the

MAC Sublayer and the Physical Layer. Each Logical Channel is mapped to a transport

channel. Transport Channels describe how the information will be formatted before

being transmitted (coding, transport block size, etc.).

Physical Channel - A Physical Channel provides the transmission media (resource

elements) through which the information is actually transmitted. Each Transport

Channel is mapped to a physical channel.


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Logical Channels provide control and data transport between the RLC and MAC

Sublayers. Signaling traffic is carried by control channels (xCCH), and data traffic is

carried by traffic channels (xTCH). Control channels are mapped to SRB channels, and

traffic channels are mapped to user plane radio bearer channels.


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Control Channels

Broadcast Control Channel (BCCH)DL channel used to broadcast system

information.

Paging Control Channel (PCCH)DL channel used to carry paging information whenthe network does not know the location of the UE.

Common Control Channel (CCCH)Carries RRC signaling when no RRC connection

currently exists for the UE.

Dedicated Control Channel (DCCH)A bidirectional control channel used to carry

signaling information when an RRC connection exists for the UE.

Multicast Control Channel (MCCH)Carries multicast signaling information; it

controls the operation of the MTCH channel

Traffic Channels

Dedicated Traffic Channel (DTCH)A point-to-point channel dedicated to one UE for

transmission of user data. The DTCH may be uplink, downlink, or both.

Multicast Traffic Channel (MTCH)A DL channel used to carry multicast data traffic.


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Downlink Transport Channels

Broadcast Channel (BCH)Forwards broadcast information to the entire cell. The

BCH maps to the BCCH Logical Channel.

Paging Channel (PCH)Forwards UE paging information to the entire cell. The PCHmaps to the PCCH Logical Channel.

Downlink Shared Channel (DL-SCH)Carries DL data and some control traffic. For

data traffic, DL-SCH supports HARQ and dynamic link adaptation. The DL-SCH maps to

the DCCH, CCCH, and DTCH Logical Channels.

Multicast Channel (MCH)Carries multicast traffic for the entire cell. The MCH maps

to the MCCH and MTCH Logical Channels

Uplink Transport ChannelsUplink Shared Channel (UL-SCH)Carries UL data and some control traffic. The UL-

SCH maps to the DCCH, CCCH, and DTCH Logical Channels.

Random Access Channel (RACH)Used for initial access to the cell or when a UE

needs to transmit on the PUSCH or PUCCH and does not have a valid uplink grant.

The RACH is not mapped to a Logical Channel.


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The graphic shows the mapping between the LTE Logical Channels and Transport

Channels.


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Physical Broadcast Channel (PBCH)DL channel that carries system information(broadcast) traffic. The PBCH uses QPSK encoding.

Physical Hybrid ARQ Indicator Channel (PHICH)Carries Hybrid ARQ (HARQ) ACKs or

NACKs for the UL transmissions on the PUSCH. The PHICH uses BPSK encoding.

Physical Control Format Indicator Channel (PCFICH)Transmitted every subframe toinform the UE about the number of OFDM symbols used for the PDCCH channel. ThePCFICH uses QPSK encoding.

Physical Downlink Control Channel (PDCCH)Informs the UE about the resourceallocation for PCH and DL-SCH, plus the HARQ information relating to the DL-SCH. Italso controls the UL-SCH scheduling grants and indicates the UE identity. The PDCCHhas four formats, hence the need for the PCFICH. The PDCCH signaling is located inthe first 13 OFDM symbols in each subframe. The PDCCH uses QPSK encoding.

Physical Downlink Shared Channel (PDSCH)Carries downlink data and higher layersignaling. The PDSCH is allocated to different UEs periodically, usually every 1 ms.PDSCH channel coding, modulation, and subcarrier allocation is dynamicallycontrolled by the PDCCH. The PDSCH may use QPSK, 16QAM, or 64QAM encoding.

Physical Multicast Channel (PMCH)Carries the MBMS data and control if the cellsupports MBMS functionality. The PMCH may use QPSK, 16QAM, or 64QAMencoding.


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Physical Random Access Channel (PRACH)Carries random access preambles used

when the UE makes initial contact with the network, etc.

Physical Uplink Shared Channel (PUSCH)Carries uplink data and higher layersignaling. PUSCH is a shared channel allocated to different UEs periodically, usually

every 1 ms. The channel coding, modulation, and subcarrier allocation is dynamically

controlled by the PDCCH. The PUSCH may use QPSK, 16QAM, or 64QAM encoding.

Physical Uplink Control Channel (PUCCH)Carries uplink control information for a

UE, including CQI, HARQ ACKs and NACKs, and UL scheduling requests. Depending on

format, the PUCCH may use BPSK or QPSK encoding


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The graphic shows the mapping between LTE Transport Channels and Physical

Channels. The PDCCH, PCFICH, PHICH, and PUCCH Physical Channels are not mapped

to Transport Channels.


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The PDSCH carries DL data packets and some control packets. DL traffic is mapped

into resource blocks; each DL allocation is described in the PDCCH. The PDSCH may

use QPSK, 16QAM or 64QAM modulation.

The MAC Sublayer in the eNodeB is responsible for completely filling the DLallocation. If necessary, padding is added to completely fill the allocated resource

blocks.

During a subframe, several different UEs may share the PDSCH. Using the PDCCH

control channel, the eNodeB will assign one or more resource blocks to each UE.


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DL control channels are carried in the first (even) slot of each subframe of a

Transmission Time Interval (TTI). DL control channels consist of some number of

Resource Element Groups (REGs); a REG consists of 4 Resource Elements.

The four resource elements in a REG are consecutive when not used for otherpurposes, such as Reference Signals. Nine REGs form a Control Channel Element

(CCE).


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PCFICH Channel

The PCFICH channel is 4 REGs (16 resource elements) long and is located in the 1st

symbol. The PCFICH channel indicates how many (1-3) symbols are used by the DL

control channels in this subframe. The PCFICH REGs are distributed evenly across thesystem bandwidth (occupied subcarriers). The exact location of the PCFICH REGs is

calculated by the UE based on the physical cell identity.

PHICH Channel

The PHICH channel is 3 REGs (12 resource elements) long and is located in the 1st or

3rd symbol. This channel carries the HARQ ACKs and NACKs for packets sent by a

specific UE on the uplink. The PHICH REGs are distributed evenly across the system

bandwidth (occupied subcarriers). The resources used for the PHICH are configured

on a semi-static basis.

For FDD, the PHICH ACK/NACK is located 4 subframes after the UL subframe which

contains the MAC PDU.


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The PDCCH channel occupies the remaining resource elements in the 1st-3rd symbols

(as signaled by the PCFICH channel). The PDCCH channel describes DL traffic

allocations for this subframe and future UL bandwidth grants.

Multiple PDCCH control channels are supported and a UE monitors a set(aggregation) of control channels. The PDCCH can be transmitted with 4 different

formats.

PDCCH describes resource allocation characteristics such as resource block group size

and localized or distributed RBs using a Downlink Control Information (DCI) format.

PDCCH DCI main formats are numbered 0-3, with minor variations labelled 1A, 1B,

1C, etc.



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For each DL allocation, the PDCCH contains the UE C-RNTI, Transport Format, DL

resource allocation in resource blocks, and a Modulation and Coding Scheme (MCS)

index. In addition it contains UL grants, Transport Format and Transmit Power

Commands (TPC) for UL transmissions on the PUSCH or PUCCH.

C-RNTIuniquely identifies the UE within the cell.

Transport Formatservice-specific specifies the physical layer processing, such as

channel coding and interleaving, and any rate matching.

Starting PRBidentifies the starting physical Resource Block number for the DL or UL

allocation.

Number of PRBsidentifies the total number of physical Resource Blocks in the

allocation.

MCS Indexidentifies the modulation and coding scheme used for the allocation.

Transmit Power Commandinstructs the UE to adjust its power level for the UL

transmission. Depending on the PDCCH format, the UE power may be adjusted from

1 dB to +3 dB.

3GPP TS 36.213 Physical Layer Procedures


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3GPP TS 36.213 Physical Layer Procedures


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Using the TBS index (left column) and the number of Physical Resource Blocks

allocated, the UE calculates the size of the Transport Block.

This table shows only a few columns of the actual values. The full

table in TS 36.213 has 110 NPRB columns.


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Discontinuous Reception (DRX)

LTE supports DRX to enable UE power savings by turning off some or all of its radio

circuitry, thereby increasing the battery lifetime of the UE. The DRX function is

configured and controlled by the network. The UE behaviour is based on a set of rulesthat define when the UE must monitor the PDCCH for scheduling assignments.

When the UE does not have an established RRC connection, that is, no radio bearers

configured for data transmission, it wakes up and monitors the paging channel every

DRX cycle. When the UE has an RRC connection, the DRX function is characterized by

a DRX cycle(s), an on-duration period(s), and an inactivity timer.

Using RRC signaling, the eNodeB may configure the UE with a Discontinuous

Reception (DRX) cycle that allows it to monitor the PDCCH in a semi-periodic manner

(as opposed to every 1ms TTI). The DRX operation is governed by a Long DRX cycle, a

DRX Inactivity Timer, a DRX Retransmission Timer and, optionally, a Short DRX cycleand a DRX Short Cycle Timer.

When DRX is configured, the UE 'wakes up' at the beginning of the DRX cycle and

monitors the PDCCH for a configured number of TTIs. This period is called the On-

Duration. If no assignment is detected on the PDCCH, the UE goes back to 'sleep' until

the next On-Duration. The Long or Short DRX cycle length sets the periodicity of the

On-Duration.


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Discontinuous Reception (DRX) cont

If, during the On-Duration, an uplink or downlink assignment is detected on the

PDCCH the UE stays awake and starts the DRX Inactivity Timer. Any subsequent

PDCCH assignment for an initial transmission (i.e. not for a retransmission) will resetthe timer. The UE re-enters DRX when the Inactivity Timer expires.

The UE always wakes up to read DL ACK/NACKs on the PHICH for each of its UL

transmissions on the PUSCH. DL HARQ operation is also independent of DRX

operation, with the exception of the DRX Retransmission Timer. This timer sets how

many TTIs the UE stays awake when a retransmission is expected (i.e. the UE has sent

an uplink NACK).


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The Physical Broadcast Channel (PBCH) broadcasts RRC System Information Master

Information Block (MIB) messages. This information is critical for user devices

attempting to enter or re-enter thenetwork. The MIB contains:

DL system bandwidth Number of eNodeB transmit antennas

Reference Signal transmit power

System frame number

For Frame Type 1, the PBCH is located on the 72 subcarriers centred around the DC

subcarrier in slot 1, symbols 0 through 3. The PBCH information is spread over four

consecutive LTE radio frames (40 ms Transmit Time Interval).



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SIB4-SIB8 messages carry cell reselection parameters for EUTRAN and other Radio

Access Technology (RAT) neighbours, such as GSM, UTRA, and cdma2000

3GPP TS 36.331 RRC Protocol Specification


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Mapping PUSCH to Subframes The graphic illustrates the mapping of the PUSCH

channel to subframes. Resources for the PUSCH are allocated on a subframe basis by

the eNodeB (in the PDCCH). Subcarriers are allocated in physical resource blocks and

may be frequency hopped from subframe to subframe. The PUSCH may use QPSK,

16QAM or 64QAM modulation. The MAC Sublayer in the UE is responsible for

completely filling the UL grant. If necessary, padding is added to completely fill the

allocated resource blocks.

3GPP TS 36.211 Physical Channels and Modulation


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Mapping PUCCH to Subframes The PUCCH carries uplink control information such as

CQI, Scheduling Requests, and ACKs/NACK for a specific UE. It is never transmitted

simultaneously with the PUSCH. As shown in the graphic, the PUCCH transmission is

frequency hopped at the slot boundary for added reliability. More than one PUCCH

channel may be present in a subframe.



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For PUCCH Format 1, information is carried by the presence or absence of any PUCCH

transmission from the UE. For all other formats, the UE explicitly transmits bits.


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The Random Access Channel (RACH/PRACH) is an UL 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. The UE uses the PRACH channel to send

a Random Access Preamble.

Mapping UL Physical Channels to Subframes

Random Access Preambles are transmitted on blocks of 72 contiguous 1.25 kHz

subcarriers allocated for the Physical Random Access Channel (PRACH) by the

eNodeB. For burst formats 0-3, the PRACH Configuration Index describes the burst

format and subframe location within an LTE radio frame type



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