LTE Interview Questions

1) Used Modulation

O-FDMA - In Downlink

SC-FDMA - In Uplink

2) Used Subcarrier Spacing 15 KHz

3) No. of Bandwidths vs Resource Blocks

(1 Radio Frame = 10 Subframe)

(1 Radio Frame = 10ms)

(1 Subframe = 1ms so, 10 SF = 10ms)

(1Subframe = 2 Time slots)

(10 Subframe = 20 Time slots/ 1 SF = 0.5ms)

1.4 Mhz 6 RBs

3 MHz 15 RBs

5 MHz 25 RBs

10 MHz 50 RBs

15 MHz 75 RBs

20 MHz 100 RBs

4) CQI (Channel Quality Indicator)

SINR(UE)-->CQI-->>MCS(eNodeB)

Range : 0 to 15

Subrange : 0 -> out of Range

1 to 6 : QPSK

7 to 9 : 16QAM

10 to 15 : 64QAM

here: QPSK & QAM are Modulation Schemes.

5) LTE Events

A1, A2, A3, A4, A5, A6, B1, B2.

A1: Serving Coverage/Quality is better than Threshold.

A2: Serving Coverage/Quality is Worse than Threshold.

A3: Neighbor Cell coverage becomes better than Serving cell coverage by an offset.

A4: Neighbor Cell Becomes better than a Threshold.

A5: Serving Cell Becomes worse than Threshold-1, while Neighbor Cell Becomes better than Threshold-2.

A6: Neighbor Cell Becomes better than a secondary cell by an offset (In CA Case)

B1: Inter-System Neighbor(Other RAT- 2G/3G/CDMA)Cell becomes better than Threshold.

B2: Serving Cell becomes worse than a Threshold-1 while Inter System Neighbor Cell becomes better than Threshold-2.

Note: Used Terms Meaning

Threshold: It is used when measurement of serving & Neighbor cells are not compared and while taking decision to trigger measurement report both or one cell measured independently.

for example: A1, A2.

Offset: Offset is the parameter that makes serving cell look better than the actual measurement of it compare to neighbor cell.

Hysteresis: It is the Parameter which is used to make Neighbor Cell look worse than the current measured value.

It helps UE to make sure that neighbor cell is strong enough and if the network triggers a handover after receiving the measurement report from the UE, handover to target cell will successful.

6) MIB Periodicity in LTE

40ms generation Periodicity with 10ms retransmission Periodicity.

7) What are LTE Idle Mode Parameters?

qHyst : Cell Reselection Parameter that defines the hysteresis value in the Intra-Frequency Cell ranking criteria

tReselectionEutra: Cell Reselection Timer value for an Eutra Frequency.

sIntraSearch: Threshold for Intra Frequency Measurement.

Note: qHyst and tReslectionEutra parameters can be tuned together to ensure that UE reselect cells as early as possible without experiencing ping-pong Reselection.

-> By decreasing sIntraSearchP/sNonIntraSearchP from higher to lower value are making aggressive cell selection.

8) RRC Failure Reasons:

-> Bad RF Conditions.

-> Congestion ( SRS, PUCCH, PUSCH, PDCCH, High PRB)

-> Interference (UL RSSI), Overshooting

-> Transmission Issue

-> High BLER

-> Wrong Parameter Configuration

-> Timer T300 Value too small and other.

-> RACH Power & Mobility Setting.

-> Hardware Issue.

more.....

by Youtube: @ankurtomar