4G/LTE - Basic Procedures  




Frequency and Bandwidth Detection

LTE support various different options of channel bandwidth, which are 1.4, 3, 5, 10, 15, 20 Mhz. How can a UE figure out which bandwidth to use when it try to attach to the network ? This is the procedure in which UE figures out the bandwidth.

    i)Search the center frequency

      Note 1: This algorithm would be the most complicated and is up to each chipset manufacturer for implementation

      Note 2: Depending on UE PHY protocol stack implementation, UE may measure RSSI and determine whether it goes to next step or not.

    ii) Decode PSS (Primary Sync Signal)


    iii) Decode SSS (Secondary Sync Signal)

      Note 1 : If UE combined the result of PSS, SSS, UE can figure out Cell ID. (So if you see Cell ID in UE trace log and it matches the one eNodeB is configured, it means UE successfully detected PSS, SSS

    iv) with the result of step ii) and iii), UE can detect cell specific Reference Signal


    v) Decode BCH which occupies 72 subcarriers (6 RBs) at the center frequency.


    vi)BCH (MIB) tells the frequency information of the system (eg. System Frequency Bandwidth)


      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))