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.
•        SFN_mod4 = 0, and repetitions are scheduled in subframe #0 of all other radio frames.

MIB Contains:

·         Dl Bandwidth: Transmission bandwidth configuration, n_RB 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 n_RB

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 SFN_mod8 = 0
• Repetitions are scheduled in subframe #5 of all other radio frames for which SFN_mod2 = 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, N_RB, 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.