LTE Interview Questions and Answers

12 LTE interview questions and answers LTE and LTE advanced technology is fast evolving in cellular arena and demand in the industries have been increased for LTE skilled en gineers. These top 12 LTE interview questions and answers help engineers seeking LTE technology job to crack the interview with ease. One can refer page links mentioned on o n left side panel to learn more about abo ut LTE. These questions are very useful as viva questions also. Question-1: What is the difference between LTE FDD and LTE TDD? Answer-1: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. Read more. more. Question-2: What is resource block in LTE? Answer-2:LTE frame is divided based on time slots on time ax is and frequency subcarrier on frequency axis. Resource block is the smallest unit of resource allocation in LTE system. It is of about 0.5ms duration and composed co mposed of 12 subcarriers in 1 OFDM symbol. One time slot is equal to 7 OFDM symbols in normal cyclic c yclic prefix and 6 OFDM symbols in extended cyclic prefix. One full resource block is equal to 12 subcarriers by 7 symbols in normal CP. Hence it consists of total 84 time/frequency elements referred as resource elements in LTE network. Refer LTE Refer LTE Terminology . Question-3: What are the LTE logical, transport and ph ysical channels? Answer-3:All these channels help LTE UE establish the connection with the eNodeB, maintain the connection and terminate the same. Logical channels are characterized cha racterized by the information that is transferred. Transport channels are characterized by how the data are transferred over the radio interface. Physical channel corresponds to a set of resource elements used by the physical ph ysical layer. Channels are further divided into control channel and traffic channel at logical channel stage. Read more. more. Question-4: Explain the difference between Reference signal (RS) and synchronization signal (SS) in the LTE? Also mention types of RS and SS. Answer-4:Reference signal (RS) is used as pilot subcarrier in LTE similar to other broadband wireless technologies such as WLAN, WIMAX etc. Synchronization signal is used as preamble sequence in LTE for synchronization purpose. RS is used for channel estimation and tracking. SS are of two types viz. P -SS and S-SS. P-SS is used for initial synchronization. S -SS is used for frame boundary determination. RS are of two types viz. Demodulation RS (DRS) and Sounding RS (SRS). DRS is used for sync and channel estimation purpose. SRS is used for ch annel quality estimation purpose. DRS is used in both the uplink and downlink, while SRS is used only in the uplink. Refer LTE PSS SSS and LTE RS DMRS SRS pages to know insight concepts of synchronization signal and reference signal.

Question-5: Explain LTE cell search procedure followed by UE. Answer-5:LTE cell search procedure is used by UE to camp onto the LTE cell i.e. eNodeB. Refer LTE UE cell search procedure and network entry procedure. procedure.

Question-6: What is the function of LTE physical broadcast channel i.e. PBCH? Answer-6:After initial cell synchronization is completed, UE reads MIB (Master information block) on PBCH (Physical channel). Broadcast channel is referred as BCH at transport level and BCCH at logical level. MIB composed of downlink d ownlink channel bandwidth in units of RBs, PHICH duration, PHICH resource and system frame number. Read more. more. Question-7: What is the advantage of using SC -FDMA in the LTE uplink? Answer-7:The main advantage of SC-FDMA is low PAP R compare to OFDMA used in LTE downlink. This increases the efficiency of power a mplifier and hence increases the battery life. Read more. more. Question-8: What is RSSI? Answer-8:RSSI stands for Received Signal Strength Indication. It is used almost in all the RATs to identify power received from the cell in idle as well as connected/dedicated modes. This helps UE always camped on to the best cell all the time. In case of drop in power measured using RSSI, either UE or network initiates the handover or cell re-selection is carried out. Read more. more. Question-9: Explain Circuit Switch Fall Back i.e. CSFB with respect to LTE and GSM. Answer-9:Framework allowing the provisioning of voice services by reuse of legacy GSM served CS infrastructure when the UE is served b y E-UTRAN (LTE).To provide voice call

support, Circuit Switch Fall Back is carried out to GSM RAT from LTE RAT to facilitate the voice over LTE (VoLTE) feature. Read more.

Question-10: Explain LTE network architecture and various interfaces. Answer-10:There are various entities forming the LTE network architecture, the main interfaces are Uu between UE and eNB, X2 interface between eNBs and S1 interface between eNB and EPC(Evolved Packet Core). Read more. Question-11: What is SRVCC? Answer-11:SRVCC is the short form of Single-Radio Voice Call Continuity. SRVCC handover is supported from E-UTRAN (i.e. LTE) to UTRAN/GERAN (WCDMA/GSM). SRVCC procedure is used for transferring an on-going PS voice call (IMS) in LTE to a CS voice call via Handover from LTE to GERAN/UTRAN. Read more. Question-12:What is the difference between LTE and LTE Advanced? Answer-12: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,Read more.

LTE Tutorial-Page1 This tutorial section on LTE basics covers following sub topics: Main page features terminologies Frame TDD FDD Channel types PHY stack throughput VoLTE CA cell search network entry Timers PSS vs SSS Security LTE Bands EARFCN Hotspot router

This LTE tutorial covers LTE system overview, LTE air interface,LTE SAE and provide link for LTE Frame structure, LTE physical layer,LTE p rotocol stack,LTE terminologies,LTE advanced,LTE vendors etc. This tutorial is ideal for begineer to learn basic knowledge on LTE and LTE advanced technologies.

LTE Overview LTE standard has been published by 3GPP as an extension of UMTS(based on 3GPP standard) and 1xEV-DO(base on 3GPP2 standard) technolo gies. LTE is mainly designed for high speed data applications both in the uplink and downlink. LTE network offers about 300Mbps data rate in the downlink and about 75 Mbps in the uplink. There is possibility of supporting voice over LTE(VoLTE) in the future. There are various methods un der progress to support VoLTE some of them includes VOIP, legacy fallback to previous existing wireless networks. This tutorial on LTE covers following in addition to LTE air interface and LTE system architecture:

LTE Air interface The Air interface between LTE network and UE supports high data rate owing to OFDM and Multiple antenna techniques employed. OFDMA is used from network to UE air interface and SC-FDMA is used from UE to network air interface. Refer following links to know OFDMA basics. OFDMA Types OFDM versus OFDMA OFDMA Physical layer

LTE System Architecture Evolution

As shown in the figure LTE SAE(System Architecture Evolution) consists UE,eNodeB and EPC(evolved packet core). Various interfaces are designed between these entities which include Uu between UE and eNodeB, X2 between two eNodeB, S1 between EPC and eNodeB. eNodeB has functionalities of both RNC and NodeB as per previous UMTS architecture.LTE is completely IP based network. The basic architecture contains the following network elements. 1. LTE EUTRAN (Evolved Universal Terrestrial Radio) 2. LTE Evolved Packet Core.

LTE EUTRAN It is a radio access network standard meant to be a replacement of the UMTS, HSDPA and HSUPA . Unlike HSPA, LTE's E-UTRA is an entirely new air interface system. It provides higher data rates, lower latency and is optimized for packet data. EUTRAN (Evolved Universal Terrestrial Radio) consists of eNB (Base station). EUTRAN is responsible for complete radio management in LTE. When UE powered is on, eNB is responsible for Radio Resource Management, i.e. it shall do the radio bearer control, radio admission control, allocation of uplink and downlink to UE etc. When a packet from UE arrives to eNB, eNB shall compress the IP header and encrypt the data stream. It is also responsible for adding a GTP -U header to the payload and sending it to the SGW. Before the data is actually transmitted the control plane has to be established. eNB is responsible for choosing a MME using MME selection function. The QoS is taken care by eNB as the eNB is only entity on radio. Other functionalities include scheduling and transmission of paging messages, broadcast messages, and bearer level rate enforcements also done by eNB.

LTE Evolved Packet Core (EPC) The LTE EPC consists of MME, SGW, PGW, HSS and PCRF.

Mobility Management Entity (MME): The MME is a control entity. It is responsible for all the control plane operations. All the NAS signaling originates at UE and terminates in MME. MME is also responsible for tracking area list management, selection of PGW/SGW and also selection of other MME during handovers. MME is also responsible for SGSN (Serving GPRS Support Node) selection during LTE to 2G/3G handovers. The UE is also authenticated by MME.MME is also responsible for bearer management functions including establishment of dedicated bearers for all signaling traffic flow.

Serving Gateway (SGW): Serving gateway terminates the interface towards EUTRAN. For each UE there is a single Serving GW associated with EPS at a given point of time. SGW acts as a local mobility entity for inter eNB handovers. It also acts a mobility ancho r for inter 3GPP mobility. SGW is responsible for packet routing and forwarding, buffering the do wnlink packets. As eNB is responsible for uplink packet marking, SGW is responsible for downlink packet marking.

PDN Gateway (PGW): PGW terminates SGi interface towards the PDN. PGW is responsible for all the IP packet based operations such as deep packet inspection, UE IP address allocation, Transport level pack et marking in uplink and downlink, accounting etc. PGW contacts PCRF to determine the QoS for bearers. It is also responsible for UL and DL rate enforcement.

Home Subscriber Server (HSS): The HSS is a central database that contains user-related and subscription-related information. The functions of the HSS include functionalities such as mobility management, call and session establishment support, user authentication and access authorization. It also holds information about the PDNs to which the user can connect. In addition the HSS holds dynamic information such as the identity of the MME to which the user is currently attached or registered. The HSS may also integrate the authentication center (AUC), which generates the vectors for authentication and security keys.

Policy Control and Charging Rules Function (PCRF): The PCRF is responsible for policy control decision-making as well as for controlling the flow based charging functionalities in the Policy Control Enforcement Function (PCEF), which resides in the P-GW. The PCRF provides the QoS authorization (QoS class identifier [QCI] and bit rates) that decides how a certain data flow will be treated in the PCEF and ensures that this is in accordance with the user's subscription profile. Refer LTE PCRF vs PCEF➤.

LTE Advanced Architecture

LTE Advanced architecture for E-UTRAN consists of P -GW, S-GW, MME, S1-MME, eNB, HeNB, HeNB-GW, Relay Node etc. LTE Advanced protocol stack consists of user plane and control plane for AS and NAS. Refer LTE Advanced Architecture and Stack ➤.

LTE tutorial-Page2 This tutorial on LTE covers following topics.

Introduction: LTE is the next generation of technology which is backword compatible with cellular technologies such as HSPA,GSM,CDMA etc. LTE means Long Term Evolution.LTE which is known as 4G technology is being specified in Release 8 and 9 of the 3GPP standard. Release 10 is referred as LTE-Advanced. The LTE radio transmission and reception specifications are documented in TS 36.101 for the UE ( User Equipment) and TS 36.104 for the eNB (Evolved Node B). Downlink and uplink transmission in LTE are based on the use of multiple access technologies: specifically, orthogonal frequency division multiple access (OFDMA) for the downlink, and single-carrier frequency division multiple access (SC-FDMA) for the uplink. The work on the specifications is ongoing, and many of the technical documents are updated quarterly. The latest versions of the 36-series documents can be found at http://www.3gpp.org/ftp/specs/archive/36_series/ LTE Physical layer is described in TS36.211 and TS36.212 releases. 36.211 mentions physical channels and modulation while 36.212 mentions multiplexing and channel coding.

LTE system basic parameters and LTE Frame structure: Frame Size=10ms No. of slots=20. No of Slots per Sub frame =2. Slot duration=0.5 ms Sub frame duration=1 ms Basic time unit Ts for BW of 20MHz, (1/15000)*2048 seconds equal to 32.55ns. There are two types of frames in LTE;FDD and TDD. Type 1, applicable to FDD- Here there are total 20 slots, each is 0.5ms. 2 slots constitute 1 sub frame. Total Frame duration is 10ms. Type 2, applicable to TDD- Here there are 10 sub frames, each is 1 ms,sub frame 0 and 5 are dedicated for downlink always while sub frames 1 and 6 are dedicated for control frame.Sub frames 2, 3, 4 and 7, 8, 9 depend on UL/DL configuration table defined in the standard.Frame has switch point periodicity of 5 ms.

LTE Features The key features of LTE physical layer are mentioned below. Channel Bandwidth: 1.4/3/5/10/15/20 MHz FFT size : 128/256/512/1024/1536/2048 Cyclic Prefix : Normal, Extended DL multiple access: OFDMA UL multiple access: SC-FDMA Duplexing :FDD & TDD Subcarrier mapping: Localized Subcarrier hopping: Yes Data Modulation : QPSK/16QAM/64QAM Subcarrier spacing: 15KHz Channel Coding : convolutional coding and turbo coding MIMO :2 or 4 at transmit and 2 or 4 at receive side HARQ :incremental redundancy 3GPP released documents of LTE and LTE-advanced are available at 3GPP web sitehttp://www.3gpp.org

REFERENCES 1. TS 36.201- Evolved Universal Terrestrial Radio Access (E-UTRA); LTE physical layer; General description 2. TS 36.211- Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation

3. TS 36.212- Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding 4. LTE Terminology

LTE tutorial-Page3 This page on LTE terminology covers LTE and LTE advanced technology related terms. It include terms eNB,eNodeB,UE,OFDMA,SC-FDMA,LTE frame,Resource block(RB), Resource Element(RE),Slot,sub frame,reference signal, synchronization signal,S-GW,MME,X2 interface, S1 interface, Uu interface, Control channel, data channel,LTE channel types,logical channel, transport channel, physical channel, P-SS,SSS,PBCH,PDSCH,PDCCH,PCFICH,PCH,RS,SRS,DMRS,PRACH, PUSCH,PUCCH, carrier aggregation,voice over LTE etc. eNB or eNodeB It is similar to Base station which is used in GSM networks. Also called as eNodeB. UE: It is similar to mobile subscriber. OFDMA: Orthogonal Frequency Division Multiple Access, used in physical layer of LTE Downlink. SC-FDMA: Single Carrier Frequency Division Multiple Access, used in physical layer of LTE Uplink. LTE Frame: LTE frame are of 2 types TDD and FDD. In both the cases, frame is composed of 10 sub frames and each sub frame is made of 2 slots. Frame size is 10ms. Resource Block (RB): It is the smallest block of resource that can be allocated to UE by eNB; it is 12 subcarriers for 7 symbols. Resource Element (RE): The smallest unit of radio resources, one subc arrier for one symbol. Slot: 7 consecutive symbols for short Cyclic Prefix, 6 symbols for long cyclic prefix. Sub frame: 2 consecutive timeslots. Reference Signal: Similar to pilot carrier and is used for channel estimation at the receiver. Synchronization signal: There are two synchronization signals, Primary and secondary. Both are transmitted in slot 0 and slot 10 in all the frames. It is same as preamble used in earlier systems and used for time, frequency synchronization purpose. S-GW: Serving Gateway MME: Mobility Management Entity

X2 interface: Interface used between eNodeB and eNodeB. S1 interface: Interface used between eNodeB and core network interface (MME/S-GW). Uu interface: This is the air interface used between eNodeB and UE. Control channel: This channel carry control information used to make, maintain and terminate the connection. Used for the transfer of control plane information in LTE. Data channel: This channel carry traffic information. Used for the transfer of u ser plane information. Channel structure in LTE: LTE adopts a hierarchical channel structure. LTE defined three channel types i.e. logical,transport and physical channels. Each associats with a service access point (SAP). (SAP) between different layers. These channels are used by lower layers to provide services to the upper layers. Logical Channels: What to Transmit. They are used by MAC layer to provide services to RLC layer. Each logical channel is defined as per type of information it carries. In LTE, there are two categories of logical channels depending on the service they provide: control channels and traffic channels. Transport Channels: How to Transmit. PHY uses transport channel to offer services to the MAC layer. It is characterized by how and with what characteristics data is transferred over the air.

Physical Channels: Actual Transmission Each physical channel maps to a set of resource elements in the time frequency grid that carry information from upper layers. The basic entities that make a physical channel are REs and RBs. A resource element is one subcarrier by one OFDM symbol and typically this could carry one (or two with spatial multiplexing) modulated symbol(s). A resource block is a collection of resource elements and in the frequency domain this represents the smallest quanta of resources that can be allocated. P-SS: Primary synchronization signal S-SS: secondary synchronization signal PBCH: Physical Broadcast Channel PDSCH: Physical Downlink Shared Channel PDCCH: Physical Downlink Control Channel PCFICH: Physical Control Format Indicator Channel PHICH: Physical Hybrid ARQ Indication Channel PCH: Paging channel RS: Reference Signal, used both in uplink and downlink

SRS: Sounding reference signal, used in uplink DMRS: Demodulation Reference Signal PRACH: Physical Random Access Channel used in uplink PUSCH: Physical Uplink Shared Channel PUCCH: Physical Uplink Control Channel

What is LTE | What does LTE mean

This tutorial section on LTE basics covers following sub topics: Main page features terminologies Frame TDD FDD Channel types PHY stack throughput VoLTE CA cell search network entry Timers PSS vs SSS Security LTE Bands EARFCN Hotspot router

LTE stands for Long Term Evolution. The technology designed and developed by 3GPP as air interface for cellular mobile communication systems. It is used to increase the capa city and data transfer speed of mobile telephone networks used mainly for data communication.LTE is marketed as 4G technology. For current release of LTE specifications, one can visit 3GP P site. One can refer list of 3GPP reference documents. LTE Advanced is the latest enhancement to LTE technology to further increase the data rate and coverage limit. Refer LTE vs LTE Advanced to know features of both the technologies. Though LTE was designed for high speed broadband data communication, voice also can be supported using VOIP(Voice over IP) protocols or using other legac y system fall-back.

source of figure: wikimedia commons As mentioned in the diagram above LTE system composed of two main parts. User devic e is referred as UE i.e. User Equipment and Base station is known as eNB. eNB is conn ected to internet back bone via SGW(serving gateway) or MME(Mobility Management Entity). Refer LTE tutorial which explains system architecture mentioning connectivity between LTE system components. Following are the silent parameters of the LTE s ystem: LTE uses OFDMA in the downlink and SC-FDMA in the uplink, refer SC-FDMA vs OFDMA. It supports two types of frame structures as per FDD and TDD topologies, refer FDD LTE vs TDD LTE. The resource allocation in LTE is as based on resource block concept defined. Refer LTE Terminology. LTE supports various frequency bands in both TDD(Band 33 to 43) and FDD(Band 1 to 25). • •

•

•

Similarities and difference of LTE with other wireless standards LTE is somewhat similar to HSPA and its downlink access technique is similar to mobile wimax i.e. OFDMA. Refer following links LTE vs WiMAX, LTE vs GSM and LTE vs UMTS which compares LTE with WiMAX, GSM and UMTS technologies respectivel y. This page just covers what does lte stand for or what is LTE technology is all about. For further and detailed study one can refer lte related links provided on this page.

LTE UE categories-LTE ue cat.1,2,3,4,5,6,7,8

This tutorial section on LTE basics covers following sub topics: Main page features terminologies Frame TDD FDD Channel types PHY cell search network entry EARFCN Hotspot router

This page covers LTE UE category features such as downlink and uplink PHY layer parameters, buffer sizes, max.MCH bits etc. It covers LTE ue category-1, cat.2, cat.3, cat.4, category-5. Feature

LTE ue Cat.1 Downlink(DL)

LTE ue Cat.2

LTE ue Cat.3

LTE ue Cat.4

LTE ue Cat.5

DL 50,

DL 100,

DL 150,

DL 300,

UL 25

UL 50

UL 50

UL 75

DL: QPSK,

DL: QPSK,

DL: QPSK,

DL: QPSK,

Modulation type

16QAM,

16QAM,

16QAM,

16QAM,

supported

64QAM,

64QAM,

64QAM,

64QAM,

UL: QPSK, 16QAM

UL: QPSK, 16QAM

UL: QPSK, 16QAM

UL: QPSK, 16QAM

2x2 MIMO

Not supported

Mandatory

Mandatory

Mandatory

4x4 MIMO

Not supported

Not

Not

Not

supported

supported

supported

Total Layer-2 Buffer Size(Bytes)

150 000

700 000

1 400 000

1 900 000

3 500 000

10296

51024

75376

75376

75376

10296

51024

75376

75376

149776

Data Rate(Mbps)

10, Uplink(UL) 5

DL: QPSK, 16QAM, 64QAM, UL: QPSK, 16QAM, 64QAM Mandatory Mandatory

Max. no. of bits of a MCH transport block received within a TTI Max. no. of bits of a DL-SCH transport block received within a TTI

Max. no. of supported layers for spatial

1

2

2

2

4

5160

25456

51024

51024

75376

multiplexing in DL Max. no. of bits of an UL-SCH transport block transmitted within a TTI

LTE Tutorial-Page6


This tutorial on LTE covers various LTE frequency band s.

LTE,E-UTRA Frequency Bands

LTE,E-Utra frequency band

Uplink(UL)opearating band BS receive UE transmit (FUL(low) -FUL(High)) ,MHz

Downlink(DL)opearating band BS transmit UE receive (FDL(low) -FDL(High)) ,MHz

Duplex mode

1

1920 MHz-1980 MHz

2110 MHz-2170 MHz

FDD

2

1850-1910

1930 -1990

FDD

3

1710-1785

1805 -1880

FDD

4

1710 -1755

2110 -2155

FDD

5

824-849

869-894

FDD

6

830-840

875-885

FDD

7

2500-2570

2620-2690

FDD

8

880-915

925-960

FDD

9

1749.9-1784.9

1844.9-1879.9

FDD

10

1710-1770

2110-2170

FDD

11

1427.9-1447.9

1475.9-1495.9

FDD

12

699-716

729-746

FDD

13

777-787

746-756

FDD

14

788-798

758-768

FDD

15

Reserved

Reserved

FDD

16

Reserved

Reserved

FDD

17

704-716

734-746

FDD

18

815-830

860-875

FDD

19

830-845

875-890

FDD

20

832-862

791-821

FDD

21

1447.9-1462.9

1495.9-1510.9

FDD

23

2000-2020

2180-2200

FDD

24

1626.5-1660.5

1525-1559

FDD

25

1850-1915

1930-1995

FDD

33

1900-1920

1900-1920

TDD

34

2010-2025

2010-2025

TDD

35

1850-1910

1850-1910

TDD

...

....

36

1930-1990

1930-1990

TDD

37

1910-1930

1910-1930

TDD

38

2570-2620

2570-2620

TDD

39

1880-1920

1880-1920

TDD

40

2300-2400

2300-2400

TDD

41

2496-2690

2496-2690

TDD

42

3400-3600

3400-3600

TDD

43

3600-3800

3600-3800

TDD

Carrier Aggregation Intra frequency bands

E-UTRA CA Band

E-UTRA Band

Uplink(UL)opearating band BS Receive/UE Transmit (FUL(low) -FUL(High)) ,MHz

Downlink(DL)opearating band BS Transmit/UE Receive (FDL(low) -FDL(High)) ,MHz

Duplex Mode

CA_1

1

1920 MHz-1980 MHz

2110 MHz-2170 MHz

FDD

CA_40

40

2300-2400 MHz

2300-2400 MHz

TDD

Carrier Aggregation Inter frequency bands

E-UTRA CA Band

E-UTRA Band

Uplink(UL)opearating band BS Receive/UE Transmit (FUL(low) -FUL(High)) ,MHz

Downlink(DL)opearating band BS Transmit/UE Receive (FDL(low) -FDL(High)) ,MHz

Duplex Mode

CA_1-5

1

1920 MHz-1980 MHz

2110 MHz-2170 MHz

FDD

CA_1-5

5

824-849 MHz

869-894 MHz

FDD

LTE Frame Structure of LTE Tutorial-Page7 This page of LTE tutorial covers LTE Frame structure. It includes TDD and FDD frame structure as per LTE standard.


LTE Frame structure The LTE frame structure are of two types based on topology either FDD or TDD. Total Frame duration is about 10ms. There are total 10 subframes in a frame. Each subframe composed of 2 time slots. Type 1, LTE frame structure is applicable to FDD system. As shown in the figure below, an LTE TDD frame is made of total 20 slots, each of 0.5ms. Two consecutive time slots will form one subframe. 10 such subframes form one radio frame. One subframe duration is about 1 ms. Hence LTE radio frame will have duration of about 10ms. Each radio frame will have 307200 Ts. Where in one Ts equals 1/(15000 x 2048) seconds.

Type 2, LTE frame structure is application to TDD system. As shown in the figure, here radio frame composed of two half frames, each of 5ms duration resulting in total frame duration of about 10ms. Each radio frame will have total 10 subframes,each subframe will have 2 time slots. subframe configuration is based on Uplink downlink configuration(0 to 6). Usually in all the cases, subframe #0 and subframe#5 is always used by downlink. The Special subframe carry DwPTS(Downlink Pilot Time Slot),GP(Guard Period) and UpPTS(Uplink Pilot Time Slot). For the 5ms DL to UL switch point periodicity case, SS(Special subframe ) exists in both the half frames. For the 10ms DL to UL switch point p eriodicity case, SS exists only in first half frame.

DL to UL configuration which determines what goes in all the subframes is mentioned below in the table.

LTE Physical Layer of LTE Tutorial-Page8


This tutorial on LTE physical layer for both UE and eNodeB.

LTE Physical Layer Block schematic of PHY layer eNodeB Transmitter: Following diagram depicts LTE eNodeB physical layer modules. LTE eNodeB is similar to Base station of other technologies such as Wimax, GSM etc. eNodeB Physical layer consists of Channel coding,rate matching, scrambler, mapper, layer mapping, pre-coding, resource element mapper, ofdm module. CRC is appended to the data from MAC layer before being passed through the PHY layer.

Let us understand LTE physical layer with examp le of downlink shared channe l(DL-SCH). As shown in the figure for eNodeB Transmitter upper layer d ata in the form of transport block is the input to the physical layer. At first the transport block is passed through a CRC encoder, we will use 24 bit CRC method. If the number of bits is more than 6144 bits then it is broken into smaller blocks. It is then turbo coded. Turbo coding is a form of concatenated coding, consisting of two convolutional encoders

with certain interleaving between them. Rate matching acts as rate coordinator between preceding and succeeding blocks, it uses a buffer. Modulation used is QAM. It is then passed through a OFDM modulator. The same is shown below in the DL-SCH channel processing figure.

CRC A cyclic redundancy check (CRC) is used for error detection in transport blocks. The entire transport block is used to calculate the CRC parity bits. The transport block is divided b y a cyclic generator polynomial to generate 24 parity bits. These parity bits are then appended to the end of transport block. The polynomial is as follows: G(x)= x24 + x23 + x18 + x17 + x14 + x11 + x10 + x7 + x6 + x5 + x4 + x3 + x + 1 Segmentation and 2nd CRC: If the input block size is greater than 6144 bits, it is split in to smaller blocks. Again CRC is performed and redu ndant parity bits are appended to each resulting smaller block. Also, filler bits are added so the co de block sizes match a set of valid block sizes input to turbo code.

Turbo coding The constituent encoders used are convolutional encoders. The input to the first constituent encoder is the input bit stream to the turbo coding block. The input to the second constituent encoder is the output of the QPP interleaver, a permutated version of the input sequence.

Rate Matching and modulation The rate matching block creates an output bit stream with a desired code rate. The rate matching algorithm is capable of producing any arbitrary rate. The bit streams from the turbo encoder are interleaved followed by bit collection to create a circular buffer. Bits are selected and punctured from the buffer to create an output bit stream with the desired code rate.

Physical Channels: Actual Transmission Each physical channel corresponds to a set of resource elements in the time-frequency grid that carry information from higher layers. The basic entities that make a physical channel are resource elements and resource blocks. A resource element is a single subcarrier over one OFDM symbol, and typically this could carry on e (or two with spatial multiplexing) modulated symbol(s). A resource block is a collection of resource elements and in the frequency domain this represents the smallest quanta of resources that can be allocated. The transport channels need to be mapped in to actual physical channels. PDSCH channel carries user data originating from the higher layer. It is associated to DL-SCH. It has various steps involved in it, such as scrambling, modulation mapper, layer mapper, precoding, resource mapping, and OFDM modulation.

As shown in figure, Scrambling Produces a block of scrambled bits from the input bits according to the relation given by the equation. b^=b+c mod 2 Where b^ denotes the scrambled bits, b denotes the input bits, c denotes the scrambling sequence. Modulation Maps the bit values of input to complex modulation symbols with the modulation scheme specified. There are three modulation sch emes for the PDSCH: QPSK (Quadrature phase shift keying), 16QAM (Quadrature Amplitude Modulation) and 64QAM (Quadrature Amplitude Modulation). Layer mapper splits the data sequence in to a number of layers. Precoding is used for transmission in multi-antenna wireless communications. In conventional single-stream beam forming, the same signal is emitted from each of the transmit antennas with appropriate weighting (phase and gain) such that the signal power is maximized at the receiver output. The resource-mapping block maps the actual data symbols, reference signal symbols and control information symbols into a certain resource element in the resource grid.

Block schematic of PHY layer User Equipment (UE): Following diagram depicts LTE User Equipment(UE) physical layer modules. LTE UE is similar to subscriber station of other technologies such as Wi max,GSM etc. It consists of channel coding,rate matching,scrambler,mapper,transform precoder, resource element mapper and SC FDMA. CRC is appended to the data before passed to the PHY.

LTE Tutorial-Page9


This LTE tutorial page covers LTE protocol stack .

LTE UE internal Protocol Stack modules Following figure depicts LTE protocol stack with main functions of each layer. NAS is not shown in the figure,it sits above RRC in the control plane(on left side) and above PDCP in the user plane(on right side). Above NAS upper layers exist.

LTE UE user plane User plane in LTE UE consists of upper layers,NAS,PDCP,RLC,MAC,PHY and RF. The functions of each are outlined below: The modules are depicted in the figure on right side.

NAS: In the uplink it does packet filtering.

PDCP: In the uplink it performs sequence number addition, handover data handling,integrity protection, ciphering and header compression. In the downlink it does in sequence delivery,duplicate packet detection,integrity validation, deciphering,header decompression. RLC: In the uplink it provides buffer status report,segmentation and conc atenation,ARQ(for AM mode). In the downlink it does re-ordering,assembly and ARQ(for AM mode). MAC: In the uplink it does channel mapping,multiplexing,handling control elements, random access procedure, logical channel priority,HARQ and sendin g BSRs. In the downlink it does channel mapping, de-multiplexing,DRX,Handling control elements,HARQ. PHY: CRC attachment Coding block scrambling/descrambling modulation/de-modulation measurement Resource element mapping/demapping HARQ MIMO • • • • • • • •

RF: Radio Transmission and Reception •

LTE UE control plane Control plane in LTE UE consists of upper layers,NAS,RRC,PHY an d RF. The functions of each are outlined below: The same is depicted in the figure on left hand side. Upper layer: provide interfacing between upper layer information with lower layers(NAS).

NAS: Mobility Management Session Management Bearer Management Paging Control Security Management • • • • •

RRC: Configuration Management Connection Management Paging control Security Management Broadcast Measurement configuration Measurement Reporting Cell selection and reselection • • • • • • • •

Mobility Management

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PHY and RF: functions are same as mentioned above.

LTE RRC states-IDLE/Connected There are two radio resource control (RRC) states are defined viz . RRC IDLE and RRC CONNECTED. RRC IDLE state, UE is known in EPC and has IP address but not known in E-UTRAN/eNB UE can receive broadcast/multicast data, monitors a paging channel to detect incoming calls, performs neighbor cell measurements and cell selection/reselection and acquires system information In the RRC IDLE state, a UE specific DRX (discontinuous reception) cycle may be configured by upper layers to enable UE power savings • •

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In the RRC CONNECTED state, UE known in EPC and E-UTRAN/eNB UE location is known at the cell level Mobility is UE-assisted, network-controlled UE also monitors control channels associated with the shared data channel to determine if data is scheduled for it, provides channel quality feedb ack information, performs neighbor cell measurements and measurement reporting and acquires system information • • • •

LTE Interview Questions and Answers

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