LTE Interview Questions

1. How UE finds out whether N/W is using FDD or TDD ?

> Once UE finishes slot and frame synchronization (PSS &SSS reading), it figures out below 2 things apart from knowing PCI:
                   a. Whether system is FDD/TDD
                   b. Whether cyclic/extended CP is used

2. How N/W decides whether to use normal or extended CP ?

> Generally for areas having large cell coverage (may be remote area having less users), N/W prefers to use extended CP in order to compensate the large propagation delay. For areas having more UEs, N/W would prefer to use normal CP, so that overall cell Tput could be high due to more symbols.

3. How UE figures out center 62 SCs?

> By reading PSS and SSS

4. How UE figures out the number of eNB Tx Antenna, as it is not part of MIB ?

> eNB uses separate masks for 1/2/4 antenna, which is added as CRC (16bit) in MIB. These masks are pre-defined, and is known both to UE and eNB. So once MIB CRC passes, UE comes to know number of eNB Tx Antennas. This is not the only way for UE to figure out this information. Infact UE figures it out much earlier, and uses this way to cross check it.

5. Why phich-config is needed in MIB ?

> It helps UE in performing PDCCH blind decoding. In order to find the applicable PDCCH REs in control region, UE need to know how many REs will be occupied by PHICH. 1 PHICH group occupies 12 REs. Using Ng and Nprb, no of possible PHICH group can be figured out using predefined table in specification.

6. How UE knows if cell supports IMS emergency call ?

> SIB 1

7. What is the content of MSG1/MSG2?

> MSG1: Preamble Index , Size of L2/L3 message to be sent as MSG3
> MSG2: RAPID (same as preamble index in MSG1), Timing advance, UL Grant, Temp C-RNTI

8. How to debug RACH failure ?

> Case 1: RAR not received:
         > If UE is successfully monitoring PDCCH (masked with RA-RNTI) till ra-ResponseWindowSize SFs (SIB2). Lets say MSG1 was sent at 323/4 and ra-ResponseWindowSize is configured as 10. So UE will be monitoring RAR from 323/8 till 324/4
        > If any CRC error seen (Check radio condition: RSRP/RSRQ/SNR). In poor condition, CRC errors are expected.
 
> Case 2: RAR received:
        > If RAPID matches with preamble index sent in MSG1
        > If RAPID matches, any BackOffInd(BI) received

UMTS Network Architecture

UTRAN (UMTS Terrestrial Radio Access Network) includes RNC(Radio Network Controller),Node B and UE. MSC (for CS services) and SGSN (for PS services) are part of the core network.

Iub interface - Between RNC and Node B

Iu interface - Between RAN and CN (Core Network). There is separate Iu interface for PS and CS CN.

Iur interface - Between 2 RNC

Uu interface - Air interface through which UE and Node B communicates. Connection from UE to Node B is called Uplink and from Node B to UE is called Downlink.

Serving,Controlling and Drift RNC

Each UE has a serving RNC and each Node B has a controlling RNC. Drift RNC comes into picture when UE moves and comes under the coverage area of other RNC and gets connected to it. RNC that provides Iu connection is called serving RNC.

RNC 1 - serving RNC for UE and controlling RNC for Node B 1&2 

RNC 2 - drift RNC for UE and controlling RNC for Node B 3&4

UE identifier in idle mode

UE in idle mode can be identified by IMSI(International Mobile Subscriber Identity), TMSI(Temporary Mobile Subscriber Identity) or P-TMSI(Packet Temporary Mobile Subscriber Identity).

CS core network allocates TMSI and is unique within a location area. MCC, MNC, LAC (Location area code) are specified along with TMSI to ensure that UE identity is unique globally.

PS core network allocates P-TMSI and is unique within a routing area.MCC, MNC, LAC and RAC (Routing area code) are specified along with P-TMSI to ensure that UE identity is unique globally.

IMSI is fixed for a specific USIM and is not allocated by the network during registration.

TMSI and P-TMSI are used in preference to the IMSI to help protect the confidentiality of the UE identity as they are less vulnerable to attacks since they are not fixed. It is usual to reallocate the TMSI as part of the location area update procedure and to reallocate the P-TMSI as part of the routing area update procedure.

3GPP TS 25.331 specifies that when a UE transmits RRC Connection Request message to enter RRC Connected mode then it should include an initial UE identity. The order of preference for which is specified as TMSI, P-TMSI, IMSI and International Mobile Equipment Identity(IMEI). The IMEI is only used if none of the other three identities are available. The IMEI identifies UE rather than USIM. It would be necessary to include the IMEI when a UE without a USIM is used to make an emergency call. The RNC includes the same initial UE identity within the RRC Connection Setup or RRC Connection Reject messages. Other UE identities, e.g. U-RNTI, H-RNTI, E-RNTI, C-RNTI are assigned once the UE is in RRC Connection mode.