4G/LTE - Basic Procedures  




Overal LTE Sequence

Most of us would be specialized in one or a few specific area/layer as an engineer. Technically it is understandable because not a single person can master everything, but I strongly recommend you to try to have some big picture of the whole process. Whenever you have some issues or something for you to work, try to ask your self "Where is the current issue located in the whole picture ?".

You don't have to try rote memorization for these whole process, but if you keep trying to map your current issues to specific parts within the whole picture, these whole sequence would automatically pops up in your memory. I say so often to my kid or my juniors "Don't try to memorize it, just repeat it until you automatically recall" (They are looking at me seeminly saying "What is the difference ?" -:) I just smile -:).

Following is my version of the whole LTE procedures.. but if you are trying to describe it, you would have a little bit different version.. but I think overall logic would be similar. Every now and then, just try to recall these sequence in your mind and ask your self "how in detail I can explain about each of these steps ?". Actually each of these steps can be described in a volum of a thick book.

    1) UE is Off

    2) Power On UE

    3) < Frequency Search >

    4) < Timing Sync >

    5) < Cell Search> : This includes MIB decoding and essential SIB decoding etc.

                               Normally a UE would find multiple cells in this process

    6) < Cell Selection >

    7) < Initial RACH Process >

    8) < Registration/Authentication/Attach>

    9) < Default EPS Bearer Setup >

    10) Now UE is in IDLE Mode

    11) <(If the current cell become weak or UE moves to another cell regisn) Cell Reselection>

    12) <(When Paging message comes or User make a call) RACH Process>

    13) < Setup Dedicated EPS Bearer >

    14) Receive data

    15) Transmit data

    16)  (If UE power is percieved too weak by the network) Network send TPC command to increase UE Tx Power

    17) (If UE power is percieved too strong by the network) Network send TPC command to decrease UE Tx Power

    18) < (If UE moves to another cell region) Network and UE perform Handover procedure >

    19) User stop call and UE gets into IDLE mode

NOTE 1 : The steps from 7) and onwards would be same or similar to every UE and every situation, but there can be many variations in procedure 3) ~ 6).  Usually 3)~4) is determined by the internal algorithm implemented in Modem chipset. Even though overall logic is known to DSP engineers working on this area, there are a lot of variations in each modem manufacturer.  It can be interplay of multiple factors - 3GPP requirement, Stored Information on UE and Modem specific angorithm. Step 6) is pretty complicated and shows various different behaviors depending on situation, but a good thing is that this step is relatively clearly defined in 3GPP.

NOTE 2 : When I was writing this note, I was thinking of the procedure of the cases of blind search assuming that there is almost no specific information stored on UE side and Test USIM with test PLMN (e.g, 001-01) and the device has never been attached to any network/test equipment. Just brand new and powered on for the first time. I took this assumption because that would common to most of the devices.

NOTE 3  : When you want to talk about the detailed variation of these procedure, you may need to a specific cases. Otherwise each person in the discussion may talk about different things based on their own assumption in their mind. Some of the specific cases you may set would be something like this (these are just some examples. There are huge set of different cases that you can think of and you may have over hundreds of test cases trying to check the detailed behavior if you are the person in charge of verification of cell selection / reselection / roaming).

    Case 1 : Just bought a new device for testing purpose and put test USIM with PLMN 001-01 and I am using a protocol test equiment as Network.

    Case 2 : I am using the same device as Case 1 and same test USIM, but this UE has been tested before to the same equipment and same band and frequency.

    Case 3 : I bought a new commercial device from this vendor (e.g, SamSung, Apple etc) for this operator (e.g, AT&T, Verizon etc) and put the USIM provided by the network operator. I powered on the device for the first time within the area covered by the operator that the UE subscribes (e.g, within US)

    Case 4  : Same device / USIM as in Case 3, but I just bought it in US and travelled to other country and powered on the UE.