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)

LTE Cell Selection

Cell Selection Criterion

Cell Selection procedure allows the UE to camp on to a cell. There are three types of Cell selection in LTE:

1. Initial cell selection: UE scans all frequencies
2. Stored information cell selection: Stored list available with UE (earlier camed on cell). This can be also vendor specific implementation; like how each one of them would like to optimise the stored information cell selection
3. "Redirected carrier information" in RRC_Connection_Release message

Working:

• RRC asks PHY to scan frequencies and report cells.
• Once when UE PHY scans the UE supported frequencies and reports the found out cells during the scan, the results are reported to UE RRC along with their Cell IDs and the Cell specific Power values (RSRP, RSRQ).
• UE RRC will select the strongest Cell ID from the list and then will go for Cell validation procedure
• When the selected Cell ID passes the cell validation procedure, the UE camps onto it and proceeds to decode the broadcast information (MIB, SIBs). It is during this procedure, that the below mentioned calculations take place:

The cell selection criterion S is fulfilled when:

Srxlev > 0  AND  Squal > 0 

where:
Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation
Squal = Qqualmeas – (Qqualmin + Qqualminoffset)

where:

Srxlev	Cell selection RX level value (dB)
Squal	Cell selection quality value (dB)
Qrxlevmeas	Measured cell RX level value (RSRP)
Qqualmeas	Measured cell quality value (RSRQ)
Qrxlevmin	Minimum required RX level in the cell (dBm)
Qqualmin	Minimum required quality level in the cell (dB)
Qrxlevminoffset	Offset to the signalled Qrxlevmin taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN
Qqualminoffset	Offset to the signalled Qqualmin taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN
Pcompensation	max(PEMAX –PPowerClass, 0) (dB)
PEMAX	Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm)
PPowerClass	Maximum RF output power of the UE (dBm) according to the UE power class as defined in

The signalled values Q_{rxlevminoffset} and Q_{qualminoffset} are only applied when a cell is evaluated for cell selection as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN. During this periodic search for higher priority PLMN the UE may check the S criteria of a cell using parameter values stored from a different cell of this higher priority PLMN.

LTE Initial Attach with Call flow and messages

Here comes the big one: The LTE Initial Attach 

messages messages everywhere...

LTE initial Attach

Message Box in Blue Color: NAS messages
Message Box in Red Color: P-GW and PCRF interaction

Looks like the image is not clear enough. The solution is to click on the image and then zoom in. thought of mentioning the workaround :-)

The call flow is self explanatory. This procedure also gives the default bearer setup during the Initial Attach procedure. Bearers will be covered in a separate post.