channels in lte1

7/28/2019 Channels in LTE1

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Channels in LTEWhy 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 theLogical 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 usesthe 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 beingtransferred 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 ChannelsPhysical broadcast channel (PBCH)

Physical control format indicator channel (PCFICH)Physical downlink control channel (PDCCH)Physical downlink shared channel (PDSCH)

Physical uplink control channel (PUCCH)

Physical uplink shared channel (PUSCH)Physical random access channel (PRACH)
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Physical multicast channel (PMCH)Physical Hybrid ARQ Indicator Channel (PHICH)

Transport DL Channels Transport UL ChannelsBroadcast 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 ChannelsBroadcast 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 ChannelsThe 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 UEb. BCCH: Used for broadcasting MIBs/SIBsc. CCCH: Common to multiple UE'sd. DCCH: used to transmit dedicated control information for a particular UE
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e. DTCH: Dedicated Traffic for a particular UEf. MCCH: used for transmit information for Multicast receptiong. MTCH: used to transmit Multicast data

2. Downlink Transport Channels:

a. PCH: used for Pagingb. BCH: used for MIB, get mapped to BCCHc. DL-SCH: used for SIB, data transferd. MCH: used for transmitting MCCH information to set up multicast transmissions

3. Downlink Physical Channels:

a. PBCH: used for transmitting MIBb. PDSCH: for SIB, datac. PMCH: Multicast channeld. PHICH: for HARQ ack/nack statuse. 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 Channels1. 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
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b. PUSCH: used for UL datac. 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 frequencies2. Stored information cell selection: Stored list available with UE (earlier camed on cell). This can be alsovendor specific implementation; like how each one of them would like to optimise the stored information cellselection3. "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 thescan, 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 todecode the broadcast information (MIB, SIBs). It is during this procedure, that the below mentioned calculationstake 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 inthe Srxlev evaluation as a result of a periodic search

for a higher priority PLMN while camped normally ina VPLMN

Qqualminoffset Offset to the signalled Qqualmin taken into account inthe Squalevaluation as a result of a periodic searchfor a higher priority PLMN while camped normally ina VPLMN

Pcompensation max(PEMAXPPowerClass, 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
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The signalled values Qrxlevminoffset and Qqualminoffset are only applied when a cell is evaluated for cell selection as aresult of a periodic search for a higher priority PLMN while camped normally in a VPLMN. During this periodicsearch for higher priority PLMN the UE may check the S criteria of a cell using parameter values stored from adifferent cell of this higher priority PLMN.

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
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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.

channels in lte1

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