LTE INITIAL NETWORK CAMPING

Initial Network Camping:
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What will happen when a mobile is switched on? The first thing the mobile has to do is to find a suitable network and camp to it.
For LTE mobiles, camping happens through  following steps:
 1.Mobile scans the available frequency band for finding a suitable cell.First thing, mobile should know is  the presence of LTE cell. It is done using some predefined sequence known as Primary Synchronisation Signal (PSS). PSS  is transmitted at  a specific time during specific frequencies and at specific position  (To be mroe technical, PSS is transmitted at the last OFDM symbol in first and 11th slots for FDD LTE and third symbol of 3rd and 13th slots for TDD). PSS is a Zadoff-Chu sequence, There are three such sequences defined and the sequence transmitted corresposnds to cells Physial cell ID group.

2. Once the mobile has got the PSS, next level is SSS-Secondary Synchronisation Signal. SSS is transmitted in the same slots as PSS. For FDD, it is transmitted one symbol before PSS. For TDD it is transmitted 3 symbols before. 168 values for SSS are defined  each corresponding to a Physical Cell ID.Once the mobile decodes SSS, it comes to know
       ===>Physical Cell ID (From the pattern transmitted)
       ===>Frame boundaries (Different patterns for slot 0/slot11, hence can know the slot in which it is transmitted,from the slot boundaries already known by PSS, Frame boundaries can be detected)
       ===>FDD/TDD -Since the slots in which PSS/SSS is transmitted is different, once we know the slot boundary, Duplex tech can be known
       ===> CP  length (By blind decoding, if SSS is in symbol 5 or 6 is understood. If it is in 5, extended CP. If it is in 6 normal CP.

If you want a very much deep understanding on synchronisation sequences ,please refer  the below link
http://www.steepestascent.com/content/mediaassets/html/LTE/Help/SynchSignals.html

3.Decoding of PBCH : PBCH corresponds to MIB; It gives the most important parameters like SFN,Bandwidth and CFICH format. MIB is transmitted every 40 ms,fixed scheduling

4.Decoding SIBs: SIBs contain all the required information for initial access to the cell. It gives information about the cells capabilities, common and shared channels,access restrictions etc. Once the SIBs are decoded, mobile can access the network using common channels like RACH.

5.Reference Signals: Once the UE has identified the synchronisation signals-PSS and SSS, it will use the reference signals for RSRP/RSRQ measurements and reporting.


Further Reading

1. 3gpp spec 36.101
2.LTE, the UMTS Long term evolution:From theory to Practice
http://www.amazon.com/LTE-UMTS-Long-Term-Evolution/dp/0470697164

LTE System Information

System Information provides information to the UEs about various parameters  of both the Access Stratum and Non Access Stratum.These are the parameters that are common for all the UEs in the same  cell and are  traditionally broadcasted in all the wireless technologies. UEs use this information for mutliple purposes, mainly system access and other idle mode procedures like cell selection,reselection etc.

In LTE, main system information consists of the following:
1.Master Information Block (MIB) : MIB gives information about the most important parameters like SFN,system bandwidth and HICH configuration. It is transmitted every 40 ms and the scheduling information is standardised.
2. System Information Block1: SIB1 is scheduled every 80 ms. SIB1 broadcasts cell access related parameters -cell identifier (plmn id,cell id), qrxlevmin,cell specific timers and the scheduling information for all other SIBs.
3.System Information Block2:  SIB2  provides information about common and shared channels-rach,prach,bcch,dlsch,ulsch etc.
4.System Information Block 3 to 8  provides information required for cell reselection.
SI3 provides information on intrafrequency cell reselection except neighbour cell information.SIB4 provides info on neighbour cell informations. SIB5 is for interfrequency cell reselection,SIB6 is for UTRAN,SIB7 for GERAN and SIB8 for CDMA related cell reselection.

UE reads the system  information  during the initial attach process and whenever it is camped to a new cell, it reads the relevent sysinfos.If some System Information value is changed, the network pages the UE telling the UEs that system information is changed and UEs will read the system info during the next modification cycle.Modification cycle is a cell specific parameter that is  signalled in the sib2.Whenever the UE receives a paging indicating sib modification, it will invalidate all the sysinfos and retrieve all the sysinfos again.