4G/LTE - Basic Procedures  




Initialization Sequence : From Power-On to PRACH

Following procedure is to list each steps from Power-On to Initial PRACH. Even though it looks simple, each of these steps can be one of the most complicated procedures happening within the UE. Even more difficult is that a lot of these steps are not specified clearly in 3GPP spec and large portions are up to UE implementation. In addition, if you are a validation/verification engineer of mobile device .. you would have a lot of difficulties since not so many detailed log for this procedure is avaiable, even though it is available it would take long time and practice until you get familiar with those logging.

  •   i) UE is Off
  •   ii) Power On UE
  •   iii) < Frequency Search >
  •   iv) Time and Frame Synchronization: In this process,this process can be broken down to multiple steps as below.     a) detect PSS and get the timing sync for subframe and estimate frequency offset.     b) with using the timing sync, you may construct the resource grid for 6RBs at the center and detect SSS
  •   v) Based on PSS and SSS, you can calculate PCI (Physical Cell ID)
  •   vi) MIB decoding : UE can figure out System Bandwidth in this process. (As you see in Downlink Framestructure, MIB/PBCH is located at the 6 RBs around the center frequency. So the success of MIB decoding does not guarantee that signal quality across the whole band is good)
  •   vii) With the PCI and Bandwidth, detect CSR (Cell Specific Reference Signal) and perform Channel Estimation and Equalization. In this process, UE will detect/measure reference signal across the whole system bandwidth. So RSRP/RSRQ measured at this step can be a good indicator for overall signal quality.
  •   viii) Decode PDCCH and extract DCI information for SIB. PDCCH is spread across the whole bandwidth, so the signal quality across the whole bandwidth should be good enough for this step.
  •   viii) SIB deconding (SIB1 should be decoded first and then SIB2 and then remaining SIBs)
  •   ix) < Cell Selection > : UE may find multiple suitable cells, but it try camp on to HPLM cell with the highest priority
  •   x) < Initial RACH Process >

Note 1 : UE may repeat the step iii)~viii) multiple times before it finds a suitable cell. This sequence is part of Cell Search process.

Note 2 : PCI (step v) will be automatically derived as a result of step iv).