3GPP Interview QuestionsQ What are the functions of RRC Layer?A Functions of RRC:

Broadcast of System Information

Transport for NAS layer messages

Paging

RRC Connection establishment and release procedure

Signaling Connection Establishment and release procedure

Security Mode Control

Counter Mode Control

Q What are the functions of RLC Layer?A Functios of RLC:

In sequence delivery of PDUs to higher layer

request retransmission of lost RLC PDUs

Segmentation and re assembly of RLC PDUs

Q What are the functions of MAC Layer?A  Functions of MAC:

Mapping logical channels to transport channels

TBS Selection

TFRI selection

Allocate resources for E-DCH UL transmission

Q What is the difference between a SDU and PDU?A The SDU(Service Data Unit) is the input unit to a Layer. The layer does processing using this unit and can attach a header or remove a header depending upon the direction of flow. The output unit coming out of the layer is the Protocol Data Unit(PDU).

Q How is RRC layer involved in the RRC Connection Setup Procedure?A RRC layer is responsible for configuring the lower layers in RNC, i.e. RLC, MAC, PHY. Upon receiving RRC Connection Setup Request, the RRC performs these steps:

Configure PHY layer

Make RLC entities for the Radio Bearers

Configure UE contest at MAC

Configure FP

Send Radio Link Setup Request to NodeB via NBAP

After the Radio Link Setup with NodeB, send the RRC Connection Setup to UE and wait for COMPLETE

UE sends RRC CONNECTION SETUP COMPLETE on RB ID = 2, indicating the successful setup of RRC Connection.

Q What specification would you refer for RLC/MAC?A 25.321 for MAC; 25.322 for RLC

Q What are RB ID=0 to RB ID=4 used for?A RB ID 0 = RACHRB ID = 1 RRC MessagesRB ID = 2 RRC MessagesRB ID = 3 NAS MessagesRB ID = 4 NAS Messages

Q What is loop back test mode?A Test mode provided by 3GPP for testing. The UL data coming from the UE is looped back from RNC to the UE.

How is data rate on HS-DSCH channel calculated?

HS-DSCH used channelisation codes with SF=16

3.84 Mcps = 3.84/100 x 10 ^ 6 chips per 10ms

38400 chips with SF=16 is  2400 symbols

with 16-QAM, each symbol represents 4 bits, therefore 2400x4=9600 bits per frame

Therefore 9600x100 bits per second = .96 mbps

with 15 codes allocated to a UE the data rate will be 15x0.96 = 14.4 mbps

This is the theoretical limit of HSDPA

This has been enhanced in HSPA+ with MIMO and 64-QAM

Following is the theoritical max data rates for HSPA+ (assuming 15 codes allocated to one UE)

Modulation MIMO Data Rate mbps

QPSK none 7.2

16-QAM none 14.2

16-QAM 2X2 24.8

64-QAM none 21.6

64-QAM 2x2 42

64-QAM 4X4 84

Why is CELL_FACH and CELL_PCH not required in LTE?

Before answering that question, lets look at the RRC states in UMTS and what was accomplished by having these.

CELL_DCH state

A dedicated physical channel is allocated to the UE in uplink and downlink.

High data rate can be supported because of the dedicated resources

CELL_FACH state No dedicated physical channel is allocated to the UE.

UE is on shared channel in downlink and uplink.

UE keeps monitoring the FACH channel for transmission destined to its C-RNTI.

In the Uplink it can access the RACH channel to send small amounts of data.

UE was put in CELL_FACH to save some dedicated channel resources in network.

CELL_PCH state No dedicated or shared physical channel is allocated to the UE.

The UE selects a PCH with the algorithm, and uses DRX for monitoring the selected PCH via an associated PICH.

No uplink activity is possible.

The position of the UE is known by UTRAN on cell level according to the cell where the UE last made a cell update in CELL_FACH state.

UE was put in CELL_PCH when there is no activity for a period of time. This saved battery in UE.

URA_PCH: No dedicated channel is allocated to the UE.

The UE selects a PCH with the algorithm, and uses DRX for monitoring the selected PCH via an associated PICH.

No uplink activity is possible.

The location of the UE is known on UTRAN Registration area level according to the URA assigned to the UE during the last URA update in CELL_FACH state.

UE was put in this state to avoid frequent CELL UPDATES

LTE has only two RRC STATES

RRC_IDLE there is no RRC CONNECTION and no DTCH/DCCH allocated

UE can receive Cell Broadcast and monitors paging for incoming call

UE does cell reselections based on neighbor cell measurements

UE is not known by the eNodeB

RRC_CONNECTED RRC Connection exitsts and UE can receive/transmit data on shared channels

UE monitors the control channels corresponding to the Shared Data channels

UE provides channel quality and feedback information

eNodeB configures DRX based on UE activity.

UE is known at cell level

In LTE, the RRC_CONNECTED state has the benefits of CELL_FACH and to some extent CELL_PCH incorporated in it. Since in the downlink the scarce resource is the resource blocks, the downlink logical channels are always mapped onto shared transport channels(exhibiting the CELL_FACH properties). While in RRC_CONNECTED, the UE is configured with DRX cycles, conserving the battery in the UE. With just two RRC states, the RRC state machine is simplified to a great extent, saving a lot of signalling.

How is TFRI calculated in MAC-hs

TFRI(Transport Format and Resource Indicator) is signalled to the UE in the DL HS-SCCH channel. The UE uses this value to find out the Transport Block size in the upcoming transmission on HS-PDSCH channel.

The MAC-hs Scheduler determines the TBS, number of channelisation codes and the Modulation that is going to be used for the upcoming transmission to the UE on HS-PDSCH. This information has to be communicated to the UE, and is sent on the HS-SCCH channel.

TFRI = TBS index in table A - table B value

TBS index in table A :  This is the value Koi in the TBS table given in 25.321 Annex A(has index from 1 to 254).table B value :  is the index in table B given below corresponding to the number of channelisation codes selected for the transmission and the Modulation to be used.

Table B    [Table 9.2.3.1 in 25.321]

Index

Modulation scheme Number of channelization codes

*Koi

0 QPSK 1 1

1 2 40

2 3 63

3 4 79

4 5 92

5 6 102

6 7 111

7 8 118

8 9 125

9 10 131

10 11 136

11 12 141

12 13 145

13 14 150

14 15 153

15 16QAM 1 40

16 2 79

17 3 102

18 4 118

19 5 131

20 6 141

21 7 150

22 8 157

23 9 164

24 10 169

25 11 175

26 12 180

27 13 184

28 14 188

29 15 192

Example Calculation

If the output of MAC-hs scheduler for the given TTI is TBS = 3704number of channelisation codes = 4Modulation = 16QAM

Then the TBS index from table A is 159

Koi from table B is = 118

Therefore, TFRI = 159-118 = 41

The HS-SCCH will signal a TFRI of 41 to the UE.

The UE at its end will calculate the TBS index by adding the Koi(it knows the modulation and number of codes).

TBS index = signalled TFRI + Koi

And from the TBS table it can determine the TBS.

How does UE become aware of UL frequency in LTEThe UL Tx-Rx frequency separation is dependent on the EUTRA Operating Band in LTE therefore the UE has to be told of the UL frequency by the NW. Unlike in UTRAN, the UE can compute the UL freq since the Tx-Rx separation is fixed.

The SIB2 contains the UL freq information:

SystemInformationBlockType2

freqInfo                         SEQUENCE {

       ul-CarrierFreq                   ARFCN-ValueEUTRA             OPTIONAL,  -- Need OP

       ul-Bandwidth                     ENUMERATED {n6, n15, n25, n50, n75, n100}

The default separation is defined in 36.101

Default UE TX-RX frequency separation (source 3GPP TS 36.101)

E-UTRA Operating Band TX - RXcarrier centre frequency

separation

1 190 MHz

2 80 MHz.

3 95 MHz.

4 400 MHz

5 45 MHz

E-UTRA Operating Band TX - RXcarrier centre frequency

separation

6 45 MHz

7 120 MHz

8 45 MHz

9 95 MHz

10 400 MHz

11 48 MHz

12 30 MHz

13 -31 MHz

14 -30 MHz

17 30 MHz

18 45 MHz

19 45 MHz

20 -41 MHz

21 48 MHz

22 100 MHz

23 180 MHz

24 -101.5 MHz

25 80 MHz