LTE: LTE Cell Search Procedure | What happens in LTE after Mobile switch On

When you first power on the device or your device got into out of coverage and try to detect/search a new cell, UE does not have any idea on which frequency it has to try camp on.

So it may have to do some kind of blind search.

For example, let's assume that your device support LTE Band 1,2,20.

·        UE is off.

·        Power On.

·        Scan all the frequency.

Here two type of frequency scan: a) SLS ii) DBS

SLS: Storage List Search: Its check and Scan those frequency which was stored in mobile before switch on.

DBS: Deviated Band Search: If after SLS search UE didn’t find any cell then UE will start DBS Search where UE will scan all the frequency for whole band.

For example, let's assume that your device support LTE Band 1,2,20. *#0011# or *#2263#

The NodeB around your UE may use any frequency channel from earfcn range (23, 577) FDDBand-1,

Earfcn range (6173, 6427) for FDD Band 20 and Band 2.

There can be lot of possibilities of frequencies that eNB would use. Then how UE can detect/find the cell (NodeB) it would camp on?

Example:

L1CL_HL_WAKE_UP_TIME_CONFIG_CMD

L1CL_UE_CAPABILITIES_CONFIG_CMD Rel.9 Cat: 4, Rel.10 Cat: 7, Rel.11 Cat: 10

L1CL_FREQUENCY_SCAN_REQ   earfcn range (722, 934), FDD Band 2;

L1CL_FREQUNCY_SCAN_IND     startearfcn 722, Best3 earfcn: 724(220), 900(224), 934(225)

L1CL_FREQUENCY_SCAN_REQ   EARFCN RANGE (6173, 6427); FDD BAND 20

L1CL_FREQUNCY_SCAN_IND     startearfcn 6173, earfcn: 16, best 3 earfcn 6181(152), 6180(147), 6400(148)

L1CL_FREQUENCY_SCAN_REQ   earfcn range (23, 577), FDD Band 1;

L1CL_FREQUNCY_SCAN_IND     startearfcn 23, Best3 earfcn: 34(147), 35(146), 58(146);

                                                                                                                        // STEP: 1 is    completed

L1CL_FREQUENCY_SCAN_REQ   earfcn 6400(B20), 934(B2), 900(B2)..Report RSSI // STEP: 2 is completed

L1CL_CELL_SCAN_REQ             earfcn (900)

L1CL_CELL_SCAN_IND              earfcn 900, pci/rsrp/rsrq/:/0/-51/-7.13

                                                                                                 //PSS-SSS and decode ref signal //STEP:3

L1CL_MIB_IND                         earfcn 900 FDD Band: 2 PhycellId: 0, BW: 20MHz, ant port: 2

MasterInformationBlock             4G BCCH: MIB: Cell BW: 20MHz   //STEP:4

L1CL_SIB1_READ_IND              earfcn: 900(FDD Band: 2) cellid: 0: ok        

Systeminformationblock (sib1)   4G BCCH_DL_SCH:SIB1: plmn: 001/01(Test equi) //STEP:5

L1CL_IDLE_REQ                       earfcn: 900, FDD Band: 2 PhycellId: 0, BW: 20 MHz, ant Ports: 2

L1CL_IDLE_CNF                        

L1CL_SI_READ_REQ                  Rsib: 1, win: 40ms, sib(periodicity):1(rf32)

L1CL_SI_READ_CONF

SystemInformation (sib2, sib3)   4GBCCH_DL_SCH:SIB2 (MaxTxHarq: 4 maxRaPreamble: 48 SRS); SIB3 (ReselPrio: 4)

Step-1) UE tune to each and every channel that it support and measure RSSI.

(RSSI is simply a measurement of whatever energy/power it can measure. This measurement does not require any channel coding process. So at this step, UE does not need to know anything about the network. At this step, UE does not try to decode PCPICH (in WCDMA) or Sync/Reference Signal (in LTE) to detect Physical Cell ID. It just measure the power of each channel.)

As UE measure RSSI for each channel, it create a list of each channel numbers with the measured RSSI.

Step-2) Then UE go through the list from Step-1) and figure out all the channels which shows RSSI value greater than the threshold (this threshold is also up to UE/chipset implementation, not determined by 3GPP).

Then the question would be "Any frequency with Passing RSSI value can be the one that UE can camp on?” The answer is "Not Necessarily". >

Step-3) This Step is little Complicated so it will take time to understand complete process. UE do synchronization process followed be PSS and SSS and decode reference signal and detect physical cell ID from the each candidate from Step-2).

(Some candidate give successful result but some would not. UE make the list of all the successful tries).

i)            Now from Step-2) list, One by One UE will start synchronisation process for all frequency.

ii)          In above log we can see 1^st preferred earfcn is 900.

iii)         Actually for both FDD and TDD network broadcast synchronisation signal in downlink direction and this synchronisation is primary and secondary synchronisation signal. By using PSS and SSS UE would find multiple cells in this pro

cess.

iv)         For UE camp on, basically as default its use 1.4MHZ where 6RB will utilised means (6*12)72 subcarrier where 10 subcarrier will reserved and will use for DTX purpose and remaining 62 subcarrier will utilised for camp on process. 31 subcarrier for primary synchronisation and remaining 31 subcarrier for secondary synchronisation. But question is from where UE will start to read signal in 62 subcarrier?

v)           Both the FDD and TDD versions of LTE broadcast Synchronisation Signals in the downlink direction. UE use the synchronisation primary and secondary signal in TDD and FDD for the same purpose. In TDD achieve radio frame and slot synchronisation but in FDD UE identify the central of channel bandwidth.

vi)          But question is how UE identify the central frequency? because it is call DC subcarrier and here no power is required to sync with DC subcarrier, Once UE sync with DC subcarrier it start to read PSS and SSS .

vii)        PSS>>In FDD, The Primary Synchronisation Signal (PSS) is broadcast twice during every radio frame of time slots 0 and 10. PSS is broadcast using the central 62 subcarriers belonging to the last symbol of time slots 0 and 10.

In TDD, PSS is broadcast using the central 62 subcarriers belonging to the third symbol of time slot 2 (subframe 1) and the third symbol of time slot 12 (subframe 6).

PSS is used to:

a.     Achieve subframe, slot and symbol synchronisation in the time domain.

b.     Identify the centre of the channel bandwidth in the frequency domain.

c.     Deduce a pointer towards 1 of 3 Physical layer Cell Identities (PCI).

d.     PCI are organised into 168 groups of 3 so the Primary Synchronisation Signal identifies the position of the PCI within the group but does not identify the group itself.

vii)        SSS>> In FDD, The Secondary Synchronisation Signal (SSS) is broadcast twice within every radio frame. The two transmissions of the SSS are different so the UE can detect which is the first and which is the second. SSS is broadcast using the central 62 subcarriers belonging to the second to last symbol of time slots 0 and 10.

In TDD, SSS is broadcast using the central 62 subcarriers belonging to the last symbol of time slot 1 (subframe 0) and the last symbol

Of time slot 11 (subframe 5)

SSS is used to:

a.     achieve radio frame synchronization

b.     deduce a pointer towards 1 of 168 Physical layer Cell Identity (PCI) groups

c.     allows the PCI to be deduced when combined with the pointer from the PSS

Step-4) From the list with successful result from Step-3), UE decode MIBs for each and every candidate. With this procedure, now UE can make a list of frequency, Physical Cell ID (PSC in case of WCDMA) and BW.

MasterInformationBlock ::= SEQUENCE { dl-Bandwidth ENUMERATED { n6, n15, n25, n50, n75, n100}, phich-Config PHICH-Config, systemFrameNumber BIT STRING (SIZE (8)), spare BIT STRING (SIZE (10)) }

When the UE has obtained the slot after decoding PSS, the best case scenario is the UE figured out the Frame boundary(using SSS) in Subframe 0(which happens in slot0), in which case it can immediately(in the slot1 of subframe 0) obtain the MIB info. The worst case (or the only other) scenario is that it figures out in the subframe 5, in which case it has to wait another 5 subframes for the Frame start.
Once it obtained the Frame boundary, it makes sense to finish the MIB decoding in the subframe 0 itself, rather than waiting for another 5ms.

Step-5)

After decoding MIB, UE has to decode PDCCH to read other system information blocks (SIBs).  PDCCH, PHICH and PCFICH share the resources in the control region of a subframe. So to find the available resources for PDCCH, UE has to know the PHICH configuration only, as PCFICH resources are fixed and known.

·                  If CFI->2 it means 2 symbol are used for PDCCH allocation.

·                  PHICH = Location of PCFICH, Where it will decode PCFICH. It will go with MIB.

·                  PCFICH = PCFICH carry CFI -> Control format indicator. CFI indicate how many OFDM symbol will use to carry PDCCH at each subframe.

·                  PDCCH= Its indicate which type of data is going on PDSCH.

·                  PDSCH=Here Read SIB

Based on USIM information(Ex-PLMN) and the candidate table from the Step-4), can figure out which cell is the real candidate cell to camp on and try decoding System Information and proceed to registration process. Here it will verify USIM Stored PLMN with SIB1 PLMN info.