LTE Rf Planning Optimization For 4g Interview Questions & Answers

Question 1. What Are The Bandwidths Used For Lte Deployment?

Answer :

This following Bandwidths being used for LTE,

1.4 MHz

3 MHz

5 MHz

10 MHz

15 MHz

20 MHz

Question 2. What Is Subcarrier Bandwidth In Lte?

Answer :

15 kHz

Question 3. What Maximum Lte Throughput Can Be Achieved In The Field?

Answer :

upto 70Mbps on TDD network with 20 MHz bandwidth channel.

Question 4. How Many States A Ue Can Have?

Answer :

There are 2 UE stats i.e. UE Idle and UE Connect.

UE can either be on Connected or on Idle state at a time.

Question 5. What Is Difference Between Ho , Redirection, Cell Selection And Re-selection?

Answer :

Handover (HO): UE moves from one eNB to target eNB while keeping its connected state. LTE Services will be uninterrupted.In handover procedure, target cell would be prepared and UE will latch on target cell based on the configuration sent by source enodeb to UE.

Redirection: UE changes its state from connected to Idle mode during Redirection. LTE Service will be interrupted. Meaning the Source ENB shall release the connection of the UE and will ask the UE to redirect itself onto the target ENB by indicating the carrier frequency or the cell id in the RRC connection release message. For example, During CS Fallback, the UE is redirected from LTE RRC_CONNECTED mode to (2G/3G) idle mode).

Cell Selection: It allows a UE to search and camp on a suitable cell. Cell selection occurs during Initial cell selection (when UE switches ON),  Stored information cell selection (uses stored cell info to identify appropriate cell),  and Cell selection when leaving RRC connected mode (When UE move from RRC CONNECTED to RRC IDLE mode)

Cell Reselection: Its Idle mode procedure and happens from idle mode to idle mode. Reselection can occue on cell within same RAT (Intra-RAT) or different RAT(Inter-RAT).

Question 6. What Is Rrc Reconfiguration?

Answer :

RRC CONNECTION RECONFIGURATION message is the command to modify an RRC connection. Main purposes of RRC Connection Reconfiguration are to,

• Establish/modify/release Radio Bearers
• Perform Handover
• Setup/modify/release Measurements
• Add/modify/release SCells
• Dedicated NAS Information might also be transferred from eNodeB to UE

Question 7. What Are The Handover Types In Lte ?

Answer :

The Handover is the process of transferring an ongoing data session/Call from

one (source) cell connected to the core network to another (target) cell. Handovers are needed when UE moved out of its serving cell’s coverage or for load balancing purposes.

In mobile communication, Handover can either be Network controlled (i.e. HO decision is with network) or Mobile Evaluated (i.e. Mobile terminal makes HO decision and inform Network to arrange resources on target cells)

LTE uses both the approaches in a way that, LTE capable UE sends measurement report to network and based on this report; network directs UE to move to a target cell.

Handover Types in LTE:

Intra-LTE Handover:  Source and target cells are part of the same LTE network.

Handover using X2 Interface

Handover using S1 Interface

Inter-LTE Handover:  Handover happens towards other LTE nodes. (Inter-MME and Inter-SGW

Inter-MME Handover

Inter-MME/SGW Handover

Inter-RAT Handover: Handover between different radio technologies. For example handover from LTE to WCDMA.

Question 8. Difference Between X2 And S1 Hand Over?

Answer :

X2 Hand Over:

HO occurs when source and target eNBs are served within the same MME pool. The procedures relies on the presence of X2 interface between Source and Target eNB,

which is summarized as follows:

• Source eNB makes HO decision and setup a direct tunnel i.e X2 transport bearer between Source and target eNB.
• Detach UE from Source eNB and Forward traffic from source eNB to Target.
• Path switch procedure between Target eNB and MME
• Releases S1 bearer of source eNB
• Release X2 transport bearer for direct packet forwarding.

S1 Hand Over:

S1 handover is when If two eNodeBs are not connected with same MME or the X2 interfaces are not defined between eNB or when X2 procedure fails(due to unreachability/Error response etc).

Summary of S1-HO is as follows:

• Source eNB makes HO decision and setup a indirect tunnel i.e S1 bearer between Source eNB and SGW, and target eNB and SGW.
• S1 bearer for UL setup between target and source eNB
• Detach UE from Source eNB and indirect packet forwarding
• No need for the Path switch procedure between Target eNB and MME, as MME is aware of HO
• Releases S1 bearer of source eNB
• Release S1 transport bearer for indirect packet forwarding.
• If the two eNodeBs are connected with same MME, it is preferred to perform X2 based handover but there is no restriction in using S1 based handover even in this case. If two eNodeBs are not connected with same MME, you have to perform S1 based handover even in this case.

Question 9. What Is The Difference Between Erlang And Gos?

Answer :

Both Erlang and GoS are used in telephone exchange for measurement of calls such as calls dropped, calls passed etc.

Question 10. What Is Bit Error Rate And How It Is Calculated?

Answer :

Bit error rate (BER) is used to measure performance of the wireless or wired system in channel or impairment environment. BER is the ratio of received erroneous bits to the total number of bits transmitted.

Question 11. What Is The Difference Between Cas And Ccs?

Answer :

CAS stands for Channel Associated Signaling and CCS stands for Common Channel Signaling. Both are associated with PCM used in telephony.

Question 12. Explain Multipath Fading.?

Answer :

The variation is received signal strength over time is referred as fading. When the signal traverse from transmit end to receive end, it will have many reflections from buildings and walls till it reaches receive end. This results into multipath fading.

Question 13. What Is Type Of Modulation Used In Gsm?

Answer :

GMSK stands for Gaussian Minimum Shift Keying.

Question 14. What Is The Difference Between Lte And Lte Advanced?

Answer :

• LTE is specified in 3GPP release 8 and release 9.
• LTE advanced is specified in 3GPP release 10.
• The main difference between them is carrier aggregation is introduced in LTE advanced.
• Number of antennas supported by MIMO has been increased to 8 in LTE advanced.

Question 15. What Is The Difference Between Lte Fdd And Lte Tdd?

Answer :

The difference lies in the LTE frame structure in both the FDD and TDD versions of the LTE.

In FDD there will be pair of frequencies assigned in the downlink and uplink directions and hence transmissions from multiple subscribes can happen at the same time but on different frequencies as mentioned.In TDD, one single frequency will be used at different time instants by multiple subscriber terminals (UEs).

Both frame versions of LTE will have 1 ms sub-frame duration and 0.5 ms slot duration.