3gpp interview questions
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3GPP Interview QuestionsTRANSCRIPT
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