Cell Search Procedure in LTE

Cell Search Procedure in LTE


Thought of adding a small diagram, that explains the very important cell search procedure in LTE, which is the very first that any LTE UE will perform before initiating network entry procedure,
For full explanation of cell search procedure




Cell Search Procedure in LTE
What is a cell search procedure? Who does this?  Why do we need this? How is this done?
I will try to answer these questions related to the LTE cell search procedure.
A cell search is nothing but a procedure that, any sniffer or a UE shall perform in order to get more details about the nearby eNodeB/cell. So this is the first step that a UE shall do, as soon as it is powered on. The cell search procedure, is the UE’s way of acquiring cell specific information and this, the UE has to perform once or several times based on the condition of the network.

Basically the steps involved in cell search are as follows, after the UE is powered ON
1.     It shall check for power levels of different LTE bands to chose the best one with good power
2.     Try doing a CP correlation, to get following details,
1.     Symbol boundary
3.     After the CP correlation peak has been found, the UE shall look for the downlink synchronization signals such as the PSS/SSS. The PSS and SSS will convey many information to the UE such as the,
1.     N_ID_1 from PSS
2.     N_ID_2 from SSS
3.     Physical layer cell ID using formula Cell_ID = 3*N_ID_2+N_ID_1
4.     Duplexing mode FDD/TDD from the position of PSS & SSS within the radio frame
5.     Slot boundary
6.     Subframe Number from SSS
4.     The next step in the cell search is decoding the MIB information, which is transmitted on the PBCH, from which the UE will get these informations,
1.     System bandwidth
2.     PHICH duration
3.     PHICH resource type
4.     System frame number
5.     Number of Transmit antennas used by eNodeB ( This is got from MIB CRC and not from MIB Payload, unlike above parameters)
5.     The PSS/SSS & PBCH are always mapped to the centre of the bandwidth, hence the UE can decode these, even without knowing the system bandwidth but to decode all channels further the UE needs to know all the above information collected till now, from Step-1
6.     Now that, the UE has these information, it will be able to decode other control and data channels such as the PCFICH, PDCCH, PDSCH, CSRS etc
7.     Next the UE should decode the SIB information, which is transmitted by the eNodeB on PDSCH with corresponding PDCCH scrambled with SI-RNTI value of 0xFFFF or 65535, to decode PDSCH, the UE should decode corresponding PDCCH and to decode PDCCH, the UE should know number of control symbols in the current subframe, which it gets from decoding the PCFICH
8.     Once after decoding PCFICH, the UE shall decode PDCCH for SI_RNTI, from which it gets following informations,
1.     SIB scheduling within the subframe
2.     Modulation scheme used for SIB TB
3.     Resource allocation type for SIB
4.     Redundancy version
5.     New data indicator
6.     HARQ process number etc
9.     The UE shall now perform a channel estimation/equalization on the PDSCH complex symbols
10.  After getting all the information related to SIB, the UE shall actually decode the SIB1 message from PDSCH from which the UE shall get below information,
1.     Scheduling of other SIB messages such as the SIB2, SIB3 etc, where each SIB message carrier unique eNodeB information
11.  From subsequent SIB messages, the UE shall acquire below information,
1.     PRACH opportunity window
2.     PUCCH formats
3.     DL CSRS power
4.     TDD DL-UL config and special subframe type information
5.     PLMN ID etc
12.  Now, the UE has gathered enough information about the eNodeB and its time now for UE to start talking to the eNodeB, which is called as Initial Attach procedure (Which shall be discussed in a seperate article)



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