5G & 4G in WIRELESS: Channels in LTE

Why Channels?

In order to carry information from one layer to other, or, from one entity to other entity. This is applicable to the Protocol stack or between UE and eNB.

The Protocol Stack of UE and the associated channels are mentioned below:

UE Protocol Stack

RLC uses Logical channels to transfer information to the MAC and lower layers; similarly, the MAC layer uses the Logical channels to transfer information to RLC and upper layers.

MAC uses Transport channels to transfer information to the PHY and lower layers; similarly, the PHY layer uses the Transport channels to transfer information to MAC and upper layers.

The PHY layer uses the Physical Channels in order to transfer information on the Air Interface.

All the above channels have Downlink and Unlink purposes; which means that when the information is being transferred from the eNB to UE Downlink Channels are used.

Similarily, Uplink channels are used when information is passed from UE to eNB.

The below table lists the LTE channels for DL and UL:

Physical DL Channels	Physical UL Channels
Physical broadcast channel (PBCH) Physical control format indicator channel (PCFICH) Physical downlink control channel (PDCCH) Physical downlink shared channel (PDSCH) Physical multicast channel (PMCH) Physical Hybrid ARQ Indicator Channel (PHICH)	Physical uplink control channel (PUCCH) Physical uplink shared channel (PUSCH) Physical random access channel (PRACH)

Transport DL Channels	Transport UL Channels
Broadcast Channel (BCH) Downlink Shared Channel (DL-SCH) Paging Channel (PCH) Multicast Channel (MCH)	Uplink Shared Channel (UL-SCH) Random Access Channel(s) (RACH)

Logical Control Channels	Logical Traffic Channels
Broadcast Control Channel (BCCH) Paging Control Channel (PCCH) Common Control Channel (CCCH) Multicast Control Channel (MCCH) Dedicated Control Channel (DCCH)	Dedicated Traffic Channel (DTCH) Multicast Traffic Channel (MTCH)

Downlink Channels

The below diagram depicts the association of LTE Downlink Channels:

LTE Downlink Channels

Each channel has a specific role to play and the functions of each channel are mentioned below:

1. Downlink Logical Channels:

a. PCCH: used for paging the UE

b. BCCH: Used for broadcasting MIBs/SIBs

c. CCCH: Common to multiple UE's

d. DCCH: used to transmit dedicated control information for a particular UE

e. DTCH: Dedicated Traffic for a particular UE

f. MCCH: used for transmit information for Multicast reception

g. MTCH: used to transmit Multicast data

2. Downlink Transport Channels:

a. PCH: used for Paging

b. BCH: used for MIB, get mapped to BCCH

c. DL-SCH: used for SIB, data transfer

d. MCH: used for transmitting MCCH information to set up multicast transmissions

3. Downlink Physical Channels:

a. PBCH: used for transmitting MIB

b. PDSCH: for SIB, data

c. PMCH: Multicast channel

d. PHICH: for HARQ ack/nack status

e. PDCCH: control channel (carries information to UE about the scheduling of PDSCH), UL grant, Indication for paging, carries HARQ ACK/NACK

Uplink Channels

The below diagram depicts the association of LTE Uplink Channels:

LTE Uplink Channels

1. Uplink Transport Channels:

a. RACH: used for the initial access to the network (RANDOM ACCESS Procedure)

b. UL-SCH: used for UL data transfer

2. Uplink Physical Channels:

a. PRACH: transmit RACH

b. PUSCH: used for UL data

c. PUCCH: used for control signaling requirements (SRs, HARQ)

7 comments:

AnonymousMarch 7, 2012 at 4:41 PM
All the above channels have Downlink and Unlink purposes; which means that when the information is being transferred from the Upper layers to the lower layers (RRC-->PDCP-->RLC-->MAC-->PHY) the Downlink Channels are used.

Similarily, Uplink channels are used when information is passed to Upper layers from Lower layers (RRC<--PDCP<--RLC<--MAC<--PHY).

I think the above statements are wrong, as per the standards
Uplink is from UE to Network
Downlink is from Network to UE
It is irrespective of the flow inside the protocol stack
Pain is inevitableJuly 10, 2012 at 10:35 PM
Good Explanation but mapping of LTE physical channels on to actual LTE Resource Grid i.e. resource element and finally to OFDM symbol would be helpful to understand what actually goes in AIR :)
Arijit SatpathyDecember 16, 2012 at 4:15 PM
Agree, corrected the statements. Thanks for pointing it out!
Arijit SatpathyDecember 16, 2012 at 4:22 PM
Hi Pain II, you can use the Visualisation tool to find out the mapping of the PHYSICAL chaneels to Resource grid. The tool can be downloaded from:
http://www.lte-bullets.com/visualisation_tool.html