LTE Network Architecture

LTE Network Architecture:

eNode Bs:

The Single E-UTRAN Node

The E-UTRAN OFDM-based structure is quite simple.

It is only composed of one network element: the eNodeB (for evolved Node B.). The 3G RNC (Radio Network Controller) inherited from the 2G BSC (Base Station Controller) has disappeared from E-UTRAN and the eNodeB is directly connected to the Core Network using the S1 interface. As a consequence, the features supported by the RNC have been distributed between the eNodeB or the Core Network MME or Serving Gateway entities.

The X2 Interface

A new interface (X2) has been defined between eNodeB, working in a meshed way (meaning that all Node Bs may possibly be linked together). The main purpose of this interface is to minimize packet loss due to user mobility. As the terminal moves across the access network, unsent or unacknowledged packets stored in the old eNodeB queues can be forwarded or tunnelled to the new eNodeB thanks to the X2 interface.  From a high-level perspective, the new E-UTRAN architecture is actually moving towards WLAN network structures and Wifi or WiMAX Base Stations.

eNode B Functionalities

Functional definition eNodeB as WLAN access points – support all Layer 1 and Layer 2 features associated to the E-UTRAN OFDM physical interface, and they are directly connected to network routers. There is no more intermediate controlling node (as the 2G/BSC or 3G/ RNC was). This has the advantage of a simpler network architecture (fewer nodes of different types, which means simplified network operation) and allows better performance over the radio interface. As described in Chapter 4, the termination of Layer 2 protocols in eNodeB rather than in the RNC helps to decrease data-transmission latency by saving the delay incurred by the transmission of packet repetitions over the Iub interface. From a functional perspective, the eNodeB supports a set of legacy features, all related to physical layer procedures for transmission and reception over the radio interface:

    · Modulation and de-modulation.

    · Channel coding and de-coding.

Besides, the eNodeB includes additional features, coming from the fact that there are no more Base Station controllers in the E-UTRAN architecture. Those features, which are further described in Chapter 4, include the following:

    ·  Radio Resource Control: this relates to the allocation, modification and release of resources for the transmission 

       over  the radio interface between the user terminal and the eNodeB.

    ·  Radio Mobility management: this refers to a measurement processing and handover decision.

  · ·   Radio interface full Layer 2 protocol: in the OSI ‘Data Link’ way, the layer 2 purpose is to ensure transfer of data  

        between network entities. This implies detection and possibly correction of errors that may occur in the physical layer.

Evolved Packet Core (EPC) and its Components

The EPC (Evolved Packet Core) is composed of several functional entities:

              · The MME (Mobility Management Entity)

              · The HSS (Home Subscriber Server)

              · The Serving Gateway.

              · The PDN Gateway (Packet Data Network).

              · The PCRF (Policy and Charging Rules Function) Server.

The following sub-sections discuss each of these in detail:

MME (Mobility Management Entity)

The MME is in charge of all the Control plane functions related to subscriber and session management. From that perspective, the MME supports the following:

· Security procedures – this relates to end-user authentication as well as initiation and negotiation of ciphering and integrity protection algorithms.

· Terminal-to-network session handling – this relates to all the signalling procedures used to set up Packet Data context and negotiate associated parameters like the Quality of Service.

· Idle terminal location management – this relates to the tracking area update process used in order for the network to be able to join terminals in case of incoming sessions.

The MME is linked through the S6 interface to the HSS which supports the database containing all the user subscription information.

HSS (Home Subscriber Server)

The HSS (Home Subscriber Server) is the concatenation of the HLR (Home Location Register) and the AuC (Authentication Center) – two functions being already present in pre-IMS 2G/GSM and 3G/UMTS networks. The HLR part of the HSS is in charge of storing and updating when necessary the database containing all the user subscription information, including (list is non exhaustive):

· User identification and addressing – this corresponds to the IMSI (International Mobile Subscriber Identity) and MSISDN (Mobile Subscriber ISDN Number) or mobile telephone number.

· User profile information – this includes service subscription states and user-subscribed Quality of Service information (such as maximum allowed bit rate or allowed traffic class).

The AuC part of the HSS is in charge of generating security information from user identity keys. This security information is provided to the HLR and further communicated to other entities in the network. Security information is mainly used for:

· Mutual network-terminal authentication.

· Radio path ciphering and integrity protection, to ensure data and signalling transmitted between the network and the terminal is neither eavesdropped nor altered.

The Serving GW (Serving Gateway)

From a functional perspective, the Serving GW is the termination point of the packet data interface towards E-UTRAN. When terminals move across eNodeB in E-UTRAN, the Serving GW serves as a local mobility anchor, meaning that packets are routed through this point for intra E-UTRAN mobility and mobility with other 3GPP technologies, such as 2G/GSM and 3G/UMTS.

The PDN GW (Packet Data Network Gateway)

Similarly to the Serving GW, the PDN gateway is the termination point of the packet data interface towards the Packet Data Network. As an anchor point for sessions towards the external Packet Data Networks, the PDN GW also supports Policy Enforcement features (which apply operator-defined rules for resource allocation and usage) as well as packet filtering (like deep packet inspection for virus signature detection) and evolved charging support (like per URL charging).

The PCRF (Policy and Charging Rules Function) Server

 The PCRF server manages the service policy and sends QoS setting information for each user session and accounting rule information. The PCRF Server combines functionalities for the following two UMTS nodes:

· The Policy Decision Function (PDF)

· The Charging Rules Function (CRF)

The PDF is the network entity where the policy decisions are made. As the IMS session is being set up, SIP signalling containing media requirements are exchanged between the terminal and the P-CSCF. At some time in the session establishment process, the PDF receives those requirements from the P-CSCF and makes decisions based on network operator rules, such as:

· Allowing or rejecting the media request.

· Using new or existing PDP context for an incoming media request.

· Checking the allocation of new resources against the maximum authorized

The CRFs role is to provide operator-defined charging rules applicable to each service data flow. The CRF selects the relevant charging rules based on information provided by the P-CSCF, such as Application Identifier, Type of Stream (audio, video, etc.), Application Data Rate, etc.

LTE Interview Questions?

1)     What happens when a LTE UE is powered on? From PHY Layer Point of view & NAS Point of view?

2)     Explain attach procedure in LTE?

3)     Why there is two types of security in LTE?

4)     What are the measurement events in LTE?

Ans: 

Intra/Inter Frequency Events:

Event A1 (Serving becomes better than threshold) 

Event A2 (Serving becomes worse than threshold) 

Event A3 (Neighbour becomes offset better than PCell) 

Event A4 (Neighbour becomes better than threshold) 

Event A5 (PCell becomes worse than threshold1 and neighbour becomes better than threshold2) 

Event A6 (Neighbour becomes offset better than SCell) 

Inter RAT Events:

Event B1 (Inter RAT neighbour becomes better than threshold) 

Event B2 (PCell becomes worse than threshold1 and inter RAT neighbour becomes better than

threshold2)

5)     What is DCI?

6)     What are the contents of DCI?

7)     What are the main difference between DCI0 and DCI1a?

8)     What is contention resolution?

9)     When Radio Link Failure is detected?

Ans:

 Radio link failure to be detected:

  1) upon T310 expiry
  2) upon random access problem indication from MAC while neither T300, T301, T304 nor T311 is running
  3) upon indication from RLC that the maximum number of re-transmissions has been reached

10)   What is SRS used for?
Ans: UL reference signal used to measure the channel quality over a section of the bandwidth.
Node B use this information for frequency selective scheduling and link adaptation decisions.
11)   What is DMRS/DRS?

Ans: DMRS/DRS is uplink reference signal.
     Used for : 1)Channel Estimation and synchronization in UL
                2)EnodeB can use DMRS for calculating TA command for each UE.
     Two Types: 1) PUSCH DMRS.
                2) PUCCH DMRS.
     PUSCH DMRS: 
                1) Included in every resource block allocated to UE for PUSCH transmission.
                2) Distributed only in Frequency domain to preserve the PAPR characteristic of SC-FDMA.
                3) 12 Resource element per resource block allocated to PUSCH DMRS.
     PUCCH DMRS: 
                1) Included in every resource block allocated to UE for PUCCH transmission(if transmitted).PUCCH occupies 2 resource block per 1 ms subframe when transmitted.
                2) No of REs used for PUCCH DMRS depends on a) PUCCH format to be transmitted and whether b) normal or extended cyclic prefix used.
                3) PUCCH DRMS used more no of bits in case of format 1,1a,1b and less no of bits in caseof format 2, 2a, 2b.

12)   What is SPS? Explain SPS?
Ans: http://howltestuffworks.blogspot.in/2013/10/semi-persistent-scheduling.html

13)   What is DRX?

14)   Explain Connected mode DRX and Idle mode DRX?

15)   Why PHICH configuration is mentioned in MIB?

16)   In what are the scenario RACH is triggered?

17)   What is RACH Procedure?

18)   How UE come to know which RACH Preamble to USE?

19)   Why there is no SOFT HO in LTE?

20)   What PLMN Selection Order UE follows during  Automatic PLMN selection and Manual PLMN Selection?

21)   What is Timing Advance? What happens if Timing Advance Timer Expires?
Ans: The timing of UL radio frame is relative to DL radio frame. EnB provides timing advance command to each UE such that all UL transmissions arrive at the eNodeB in synchronous manner.

   If TA timer expires UE goes of reestablishment procedure or move to idle. 

22)   What is SR? What is the use of SR? 

23)   What is MAC CE? 

24)   What is BackOff Indicator? What is the use of Backoff indicator?

Ans : 

     Backoff Indicator is a special MAC subheader that carries the parameter indicating the time delay between a PRACH and the next PRACH.

if the Random Access Response contains a Backoff Indicator subheader

   set the backoff parameter value in the UE as indicated by the BI field of the Backoff Indicator subheader

else, 

   set the backoff parameter value in the UE to 0 ms.

25)   What is BSR?

      Ans: The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE.

26)   At what scenario UE triggers BSR?

Ans: 

• UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR";
• UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR"
•  retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"
• periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR".

27)   When different types of BSR are Triggered?

Ans:

For Regular and Periodic BSR:

 if more than one LCG has data available for transmission in the TTI where the BSR is transmitted

      report Long BSR

 else,

      report Short BSR.

For Padding BSR:

if the number of padding bits is equal to or larger than the size of the Short BSR plus its subheader but smaller than the size of the Long BSR plus its subheader:

       if more than one LCG has data available for transmission in the TTI where the BSR is transmitted: report Truncated BSR of the LCG with the highest priority logical channel with data available for transmission;

      else

      report Short BSR.

 else if the number of padding bits is equal to or larger than the size of the Long BSR plus its subheader,         
      report Long BSR.

28)   What is the use of system info modification period?

29)   What is the content of RAR?

Ans: 

A MAC RAR consists of the four fields

•          R
•          Timing Advance Command
•          UL Grant
•          Temporary C-RNTI

30) What is the USE of UE specific Reference signal?

31) What is Cell Specific Reference Signal?

32) In what are the scenario UE Triggers RRC Connection Reestablishment?

Ans: 

      UE Triggers RRC Connection Reestablishment procedure on following condition: 

• Upon detecting Radio Link Failure
• Handover Failure
• Mobility From E-UTRA Failure 
• Integrity Failure Indication Received From Lower Layers
• RRC Connection Reconfiguration Failure

      33) What is GUTI?

      34) What is the significance of S-TMSI? 

      35) What is the content of Paging Message?

      36) When UE activates integrity and ciphering?

Ans:

• The SECURITY MODE COMMAND message is used to command the UE for the activation of AS security. E-UTRAN always initiates this procedure prior to the establishment of Signalling Radio Bearer2 (SRB2) and Data Radio Bearers (DRBs).
• AS security comprises of the integrity protection of RRC signalling (SRBs) as well as the ciphering of RRC signalling (SRBs) and user plane data (DRBs). The integrity protection algorithm is common for signalling radio bearers SRB1 and SRB2. The ciphering algorithm is common for all radio bearers (i.e. SRB1, SRB2 andDRBs). Neither integrity protection nor ciphering applies for SRB0.
• The eNodeB sends integrity protected SECURITY MODE COMMAND message to the UE. The UE shall derive K_eNB and K_RRCint which is associated with integrity protection algorithm indicated in the SECURITY MODE COMMAND. Then, UE verifies the Integrity of the received SECURITY MODE COMMAND by checking the Message Authentication Code (MAC) in the SECURITY MODE COMMAND message. If the SECURITY MODE COMMANDmessage fails the integrity protection check, then the UE sends SECURITY MODE FAILURE to the eNodeB.
• If the SECURITY MODE COMMAND passes the integrity protection check, then the UE shall derive the encryption keys K_RRCenc key and the K_UPenc keys associated with the ciphering algorithm indicated in theSECURITY MODE COMMAND.
• The UE shall apply integrity protection using the indicated algorithm (EIA) and the integrity key, K_RRCintimmediately, i.e. integrity protection shall be applied to all subsequent messages received and sent by the UE, including the SECURITY MODE COMPLETE message.
•  The UE shall apply ciphering using the indicated algorithm (EEA), K_RRCenc key and the K_UPenc key after completing the procedure, i.e. ciphering shall be applied to all subsequent messages received and sent by the UE, except for the SECURITY MODE COMPLETE message which is sent un-ciphered.

36) How many default and dedicated bearer possible in lte?

37) Can there be multiple default bearer to same PDN?

38) How the position of each SIB is calculated in LTE?

39) How measurement GAP calculation happens in LTE?

      

LTE Protocol Architechture

Type 2 LTE Frame Structure

Type 2 LTE Frame Structure

LTE Protocol Architecture:

3GPP™ Long Term Evolution (LTE) also referred to as E-UTRAN, 

The protocol stack functions consist of the:

• Non Access Statum(NAS)
• Packet Data Convergence Protocol (PDCP)
• Radio Resource Control (RRC)
• Radio Link Control (RLC)
• Medium Access Control(MAC)
• Physical Layer(PHY) 

The difference between control plane and user plan shown in the figure below.

Radio Interface Protocol

CONTROL PLANE PROTOCOLS

RADIO RESOURCE CONTROL (RRC):

The RRC protocol includes the following main functions:

• Broadcast of system information:

1. Including NAS common information
2. Information applicable for UEs in RRC_IDLE, e.g.
• cell (re-)selection parameters, 
• neighbouring cell information and information (also) applicable for UEs in RRC_CONNECTED, e.g. 
• common channel configuration information.
3. Including ETWS notification;

• RRC connection control:

1. Paging;
2. Establishment/ modification/ release of RRC connection, including e.g. assignment/ modification of UE identity (C-RNTI), establishment/ modification/ release of SRB1 and SRB2, access class barring;
3. Initial security activation, i.e. initial configuration of AS integrity protection (SRBs) and AS ciphering (SRBs, DRBs);
4. RRC connection mobility including e.g. infra-frequency and inter-frequency handover, associated security handling, i.e. key/ algorithm change, specification of RRC context information transferred between network nodes;
5. Establishment/ modification/ release of RBs carrying user data (DRBs);
6. Radio configuration control including e.g. assignment/ modification of ARQ configuration, HARQ configuration, DRX configuration;
7. QoS control including assignment/ modification of semi-persistent scheduling (SPS) configuration information for DL and UL, assignment/ modification of parameters for UL rate control in the UE, i.e. allocation of a priority and a prioritized bit rate (PBR) for each RB;
8. Recovery from radio link failure;

• Inter-RAT mobility including e.g. security activation, transfer of RRC context information;

• Measurement configuration and reporting:

1. Establishment/ modification/ release of measurements (e.g. infra-frequency, inter-frequency and inter- RAT measurements);
2. Setup and release of measurement gaps;
3. Measurement reporting;

• Other functions including e.g. transfer of dedicated NAS information and non-3GPP dedicated information, transfer of UE radio access capability information, support for E-UTRAN sharing (multiple PLMN identities);

• Generic protocol error handling;
• Support of self-configuration and self-optimization 

MEDIA ACCESS CONTROL(MAC):

The following functions are supported by MAC sublayer:

• Mapping between logical channels and transport channels;
• Multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels;
• De-multiplexing of MAC SDUs from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels;
• Scheduling information reporting;
• Error correction through HARQ;
• Priority handling between UEs by means of dynamic scheduling;
• Priority handling between logical channels of one UE;
• Logical Channel prioritization;
• Transport format selection.

RADIO LINK CONTROL(RLC): 

The following functions are supported by the RLC sub layer:

• Transfer of upper layer PDUs;
• Error correction through ARQ (only for AM data transfer);
• Concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer);
• Re-segmentation of RLC data PDUs (only for AM data transfer);
• Reordering of RLC data PDUs (only for UM and AM data transfer);
• Duplicate detection (only for UM and AM data transfer);
• RLC SDU discard (only for UM and AM data transfer);
• RLC re-establishment;
• Protocol error detection (only for AM data transfer).