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Monday, 16 January 2012

Cell search and cell selection in LTE

Cell search and cell selection in UMTS LTE                           Reference: 3GPP TS 36.331

LTE a terminal must perform certain steps before it can receive or transmit data.

These steps can be categorized in:

1)       Cell search and cell selection,
2)      Reception of system information, and
3)      Random access.

1           Cell Search and Cell Selection:


Successful completion of the cell search and selection procedure as well as acquiring
initial system information is required for the UE before it can communicate with the network.

LTE uses a hierarchical cell-search procedure in which an LTE cell is identified by a cell identity,
504 (0 - 503) physical layer cell identity has been defined for this purpose.

With a hierarchical cell search scheme, these identities are divided into 168 unique cell layer identity groups in the physical layer,

 in which each group consists of three physical layer identities
.





In order to know the cell search and selection first we have to know the physical signals and Physical channels in downlink for cell search and selection.

Step 1 – Primary Synchronization Signal
·         Slot Timing Detection
·         Physical Layer ID (0,1,2)
Step 2 – Secondary Synchronization Signal (SSS)
·         Radio Frame Timing detection
·         Find Physical Layer Cell ID                 
·         Cyclic Prefix Length detection
·         FDD or TDD Detection
Step 3 – Reference Signal Detection / PBCH Detection (Receiving System Information)



  • Dowling Signals:
    • Primary Synchronization signal
    • Primary Synchronization signal
    • Cell Specific reference signal
  • LTE Downlink Broadcast Channel:
    • Physical Broadcast Channel(PBCH): MIB

Physical Cell ID = Cell Group ID * 3 + Cell ID

It may happen that UE is at the interference area of multiple cells; in that case UE may get multiple cell ids. In order to camp on a particular cell it has to decode cell specific reference signals.

2           Reference signal contains:

  • Downlink channel estimation.
  • Channel Quality Indicator (CQI= QPSK/16 QAM/64 QAM depending on the signal strength).
Based on these parameters UE camps on a particular cell, and proceed to the next step “Receiving system information”

3           Reception of system information:


After the successful execution of the cell-search procedure described in the previous
section, the device is able to decode the Physical Broadcast Channel (PBCH) and read
out the Master Information Block (MIB).


Master Information Block (MIB):

 As MIB is the most important information block
  •        It is transmitted with a fixed scheduling.
  •        The MIB uses a periodicity of 40ms.
  •        SFNmod4 = 0, and repetitions are scheduled in subframe #0 of all other radio frames.
MIB Contains:
·         Dl Bandwidth: Transmission bandwidth configuration, nRB in downlink.
·         PHICH Related Information: This IE is used to specify the PHICH configuration
·         System Frame Number: Defines the 8 most significant bits of the SFN 

Relationship Between Transmission Bandwidth and nRB
After receiving MIB UE configure the BCCH-DL-SCH channel and map it on PDSCH to receive System Information Block Type1 (SIB1).




System Information Block1 (SIB1):

System Information BlockType1 (SIB1) contains information relevant when evaluating if a UE is allowed to access a cell.  
    • It provides the UE with the scheduling of other system information. 
    • SIB1 uses a fixed schedule with a periodicity of 80ms 
    • The first transmission scheduled in subframe #5 of radio frames for which the SFNmod8 = 0
    • Repetitions are scheduled in subframe #5 of all other radio frames for which SFNmod2 = 0
 System Information Block1 Contains: 

  • Cell access related information:
    • PLMN identity list: List of PLMN identities  
    • Tracking area code: A trackingAreaCode that is common for all the PLMNs listed.
    • Cell identity: Identity of the cell.
    • Cell Barred: 'barred’ means the cell is barred.
  • Information for cell selection: 
    • Minimum required Rx level in the cell and offset. 
    • p-Max: Maximum power value applicable for the cell. 
    • Frequency band indicator: Operating frequency band. The UE shall consider the cell as barred if the frequency band indicated in the freqBandIndicator is not part of the frequency bands supported by the UE. 
    • Scheduling information: Common for all SIBs other than MIB, SIB1, SIB10, SIB11 and SIB12. 
    • TDD configuration 
    • SI-window length 
    • System information value tag


System Information Block2 (SIB2):

SystemInformationBlockType2 (SIB2) contains radio resource configuration information that is common for all UE.
  • SIB2 is not specifically included in the scheduling information in SIB1.
  • First entry in the list of SI messages in schedulingInfoList in SIB1.
    System Information Block Type2 Contains:
    • UL-CarrierFreq: If this IE is absent (for FDD), the UL-Carrier Frequency value should be determined from the default TX-RX frequency separation.
    • UL-Bandwidth: Transmission bandwidth configuration, NRB, in uplink. 
    • Default Paging Cycle: Default paging cycle. 
    • Modification Period Coeff: Actual modification period.
    • p-Max: Maximum power to be used in the target cell. 
    • UL-Cyclic Prefix Length: The value len1 corresponds to normal cyclic prefix and len2 corresponds to extended cyclic prefix .
    • Radio Resource Config Common SIB: The IE RadioResourceConfigCommonSIB is used to specify common radio resource configurations
          • Common Channel Information
          • Random Access Channel Information
          • Random Access Preamble Info
          • HARQ info 
     
  • After SIB2 reception UE configures the Random Access Channel and Common shared channels.  
  • Start up-link synchronization using Random Access Procedure.
     














2 comments:

  1. Informative. Thanks a lot.

    ReplyDelete
  2. 3GPP TS 36.331 is for RRC, and does not contain the content shown here. What is the right reference?

    ReplyDelete