LTE Physical layer Interview Questions and Answers

31. What is CCE? How many CCE’s are required to carry PDCCH? Explain different configurations?

Ans 31: Its a Control Channel Element which consist 9 REG(Resource element Group) and 1 REG is equivalent to 4 Resource Element so it means 1 CCE is equivalent to 36 Resource Element. So its depend on the number of different RNTI’s control information needed to be sent. There are two configuration present:

1.   Common search space-Maximum number of CCE present in this is 16 and carry the information for all UE,s like SI-RNTI,P-RNTI etc.

2.   UE specific search space- Having information of specific particular UE.

32. What is DCI? What is sent in DCI format 0?

Ans32: DCI is a Downlink control channel of PDCCH and its nothing but any information carry by PDCCH channel is DCI. DCI used to carry information about downlink and uplink details whether there is any resource or data schedule to UE. DCI 0 format is used for getting info about the Uplink Resources available or not.

DCI format 0 Information

33. In case of 4×4 MIMO, how many reference signals are there in an RB? How much %age they occupied by Ref signals in this case?

Ans33: There are 4 antennas present and each one have 8 Reference signals but the position of reference signal are different in all the antennas which is shown below:

MIMO 4x4 reference signal

34. What is the valid Cyclic Redundancy Check(CRC) length in bits?

Ans 34: A 15-bit cyclic redundancy check (CRC) value is calculated by the transmitting node in the CRC field

35. Describe briefly that where and how many padding “0” bits are inserted in code segmentation?

36. Which channel case Turbo coding is used? Which channel case Bit Repetition Coding is used?

37. In case of Turbo Encoder, If block size is 80, what will be the number of output bits?

Ans37: Number of output bits is 3x80-240bits

38. In case of Convolutional Encoder- If block size is 80, what will be the number of output bits?

Ans38: Number of output bits will be-160 bits.

39. Define Channel Quality Index(CQI)? Which is the Parameter value that belong to Excellent condition of UE's?

Ans 39: Channel coding bits is report that send by UE to eNb for indicating the experience getting by downlink channel. SNR and SINR are the parameter which show the excellent condition of CQI.

CQI range in 1-15 and table is attached below:

40. Define PMCH, it's position in Frame Structure and Information Carried by PMCH?

Ans40: Its a Physical Multicast Channel which define the physical layer structure to carry multimedia broadcast and multicast services, it occupied the 1,2,3 OFDM symbols in a subframe over the entire  bandwidth.

41. Define PRACH? What Information Carried by PRACH?

Ans41: PRACH is physical random access channel used for carry the preamble toward eNB to request for initial set of Resources. It carried few information:

a.    Prach config index

b.    High speed flag

c.    Zero correlation zone config

d.    Prach freq offset

42. What is RNTI? What is the types of RNTI (with their default values, if any)?

Ans 42: Radio network temporary identities used for provide the identification to any signaling or setup, below are the identities:

a.     C-RNTI- 65527

b.    SI-RNTI-65535

c.    P-RNTI-65534

d.   T-C-RNTI-65523

e.    RA-RNTI- (1+t_id +10f_id) 16 bit and range(1-60)

f.     SPS-RNTI- 65523

43. What is C-RNTI? Difference between C-RNTI and T-C-RNTI?

Ans 43: Cell RNTI is the RNTI allocate to Ue when its com in a RRC connected state whereas T-C-RNTI is allocate when UE is in RRC Idle state. Both are of 16 bits.

44. How will you get the Physical Cell ID from eNB even without reading BCH?

Ans44: It can be calculated at the time of synchronous process occurred and read PSS and SSS because PCI=3xSSS + PSS