LTE MAC Layer Interview Questions And Answers(41-53)

41.  If UE receives the RAR but not contention resolution message, within what time/mechanism it will retransmit?

Ans:There  is a T1 timer.

42.  What are the parameters/info you get from eNB in the RAR?

Ans: RAR info carries:

(a)         Preamble Id

(b)         PUSCH PRB

(c)         TA Value

(d)        T-CRNTI

43.  What is relationship between HARQ Entity and HARQ Process. What is the maximum entity and processes allowed in each UE?

Ans: HARQ Entity is a whole mechanism inside its all the HARQ processes and Multiplexing and De multiplexing occurs. Below is the diagram:

There are only 1 HARQ Entity and min 8 HARQ process are present

44.  Why there is a dedicated MAC process for BCH?

Ans: Refer 3GPP 36.321specification

45.  What is the maximum time a UE will backoff, before retransmitting the RA?

Ans: Its available in MAC header and BI is 4 bits in size shown bel:

E

T

R

R

BI

      Back-off indicator shown below

46.  What is TTI bundling? How many TTI’s can be bundled together?

Ans: This is a method in which UE transmit the PUSCH in multiple subframes in a row (4 subframes according to specification). We can say UE transmit a PUSCH in a 'BUNDLED TTI'. Max 4 TTI’s can be bundled together.

47.  How much is Bj incremented each TTI and what is its maximum value in MAC?

Ans: Bucket size duration is the duration till a logical channel buffer the upper layer data based on Prioritized Bit Rate of logical channel. Total buffer capacity that is calculate on the based on PBR * BSD. Here Variable Bj is used to avoid starvation because of prioritized logical channel.
Each time when UE MAC receives uplink grant to send data, it collects the data based on priorities of each logical channels while considering the value of associated variable Bj. If value of Bj is greater than zero then higher priority LCID can send data and reduce the value of Bj accordingly. If value of Bj is negative for higher priority LCID then data for other lower priority LCIDs would be scheduled.

48.  Rewrite the channels/data in the priority order which MAC considers – CCCH, any other LoCH, BSR, PHR,BSR with padding.

Ans: Refer 3GPP 36.321. MAC specification

49.  What is SPS? Explain SPS?

Ans: SPS - Semi-Persistent Scheduling 

In LTE SPS feature is designed to reduce the control channel overhead for Voice over IP based services. Since VoLTE requires radio resource allocation at regular interval to support large number of VoIP calls there is huge overhead on control signalling.
VoIP periodically generates the small size  packets at short and regular intervals. To avoid lot of DL assignment and UL grant, SPS feature significantly reduces heavy load on PDCCH by doing minimum DL assignment and UL grant. SPS allocates the radio resources for a long period of time.

50.  In what are the scenario RACH is triggered?

Ans: In order to Attach and Synchronised with the network RACH procedure is used, or we can say RACH is indirectly way to getting the UL resources for sending RRC connection request msg.

51.  What is RACH Procedure?

Ans: In order to Attach and Synchronised with the network RACH procedure is used, or we can say RACH is indirectly way to getting the UL resources for sending RRC connection request msg. RACH is a process which is configuration given by EUTRAN to UE so that UE select the Preamble and its Power related configurations.

52.  What is SR? What is the use of SR?

Ans: SR is Scheduling Request used to asking for UL resources when there is insufficient UL resources for sending even Preamble, or we can say if there is less  UL resources  for sending preamble than at that time we used SR. SR is a 1 bit in size.

53.  What is MAC CE? 

Ans:MAC CE is a Mac control Element which is MAC self procedures containing many Control procedure shown below:

                                  MAC DL Control Element                

                                   MAC UL Control Element