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  • LTE MAC Layer Interview Questions And Answers (31-40)

    31.       Which entities and situations and when do they initiate MAC RA Procedure? List them.
    Ans: UE MAC and eNB MAC are the entities who are responsible for generating the RA procedure.The situation are as follows:
    (a)        At the time of When UE need to change its state to Idle to Connected state.
    (b)        At the time of Handover.
    (c)         At the Time of UE first Power ON procedure.
    (d)        At UE attach Procedure
    (e)        At the time of RRC Reconfiguration established.

    32.       When is your Temporary C-RNTI gets converted to C-RNTI? How many bits is it?
    Ans: When UE get a RRC connection Setup msg form eNB, than UE convert its state from IDLE to Connected than its RNTI also converted from T-CRNTI to C-RNTI and next msg which is RRC connection setup complete will be using C-RNTI.
    Both T-CRNTI and C-RNTI are of 16 Bits.
    33.       How many RA Preambles are there? Why some of them are reserved by eNB?
    Ans There are 64(0-63) RA preambles which is 6 bits. Some of them are reserve for Non-Contention Based RA Procedure which is used by eNB when UE is already in  registered mode.
    34.       What is the RA-Preamble-Group A and B? What is the difference?
    Ans: RA Preamble Group A and Group B comes in a Category of Contention Based Rap and remaining preambles comes in a NON Contention Based RA Preamble.

    For Example:- Contention Based Group A (0-30)

                         Contention Based Group B (31-50)

    And remaining comes in Non-Contention Based (51-63), 

    **Note: An operator above uses the preambles in groups A, B and NCB according to their radio conditions.

    => Difference between them are as follows:
    Contention Based
    Non Contention Based
    It is Generated by UE
    It is generated by eNB
    Having 4 msgs
    Having only 3 msgs
    Possibility of Collision
    No Possibility of collision

    35.       While UE trying RA, if paging & Reserved-preamble comes from eNB, will UE abandon the current RA and restart?
    Ans: No
    36.       RA-RNTI = 1+t_id+10*f_id. Explain what is t_id and f_id and their possible value range?
    Ans: Here t_id is the index of the first the subframe of the PRACH which range is from (0 <≤ t_id <10), and f_id is the index of the PRACH within that subframe in ascending order of frequency domain which range is (0≤ f_id< 6).
    37.       What are the types of RAR?
    Ans: RAR are of two types (i) Contention Based and (ii) NON contention Based. Refer to Ans No. 34.
    38.       How do you know how many RAPIDs are there and length of the RAR?
    39.       What is Back-off indicator? What is the UE action on receiving this?
    Ans: Back-off Indicator is indirectly way to telling to UE by eNB that currently I don’t have Radio Resource to you due to certain reason So UE have to wait for certain amount of time as mentioned is a RAP header and it is of 4 bits in size. After receiving this US have to wait for RR and redo RAP.
    40.       What is RAR window?
    Ans: RAR window size is a timer used by UE at the time when UE send the preamble to eNB and getting back RAR from eNB, till this time RAR must be received otherwise RAP consider as unsuccessful.


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