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LTE

MIB (Master Information Block) And SIB (System Information Block)

MIB (Master Information Block)MIB – Master Information BlockPeriodicity – 40 ms at RRC (10 ms at Physical Layer)Location – 1st Subframe 2nd  Time Slot First 4rth  OFDM SymbolChannel – PBCHInformation Elements (I.E)- SFNNumber of Tx AntennaBandwidthPHICH infoPCID (Physical Cell ID)SIB (System Information Block)Types of SIB:SIB 1 : Cell Selection, Cell Access, SI Scheduling.SIB 2 : RACH, Access Barring, UL frequency Information, MBSFN Config.SIB 3 : Intra Frequency Cell Reselection.SIB 4 : Intra Frequency Neighbour Cell.SIB 5 : Inter Frequency Neighbour Cell.SIB 6 : UTRAN Neighbour Cell.SIB 7 : GERAN Neighbour Cell.Note : SIBs other than SIB1 are carried in SI messages and mapping of SIBs to SI is done by “schedulingInfoList” List include in SIB1.System Information Block Type1 (SIB 1)SIB1 : It contains information relevant when evaluating if a UE is allowed to access a cell and defines the scheduling of other system information.Its periodicity is of 80 ms and repetitions made within 80 ms.SIB1 related information:plmn-IdentityList : List of PLMN identities.sib-MappingInfo : List of the SIBs mapped to this SI message. There is no mapping information of SIB2, it is always present in the first SI message listed in schedulingInfoList list.si-Periodicity : Periodicity of the SI-message in radio frames.Si-WindowLength: Common SI scheduling window for all SI’s.trackingAreaCode : A trackingAreaCode that is common for all the PLMNs listedsystemInfoValueTag : indicates if a change has occurred in the SI messages.schedulingInfoList : The transmission cycles for other SIBs are determinedSystem Information Block Type2 (SIB 2)There is no mapping information of SIB2It is always present in the first System Information message listed in the schedulingInfoList list.ParameterDescriptionac-BarringInfoAccess Class Barring configurationradioResourceConfigused to specify common radio resource configurations in the system information and in the mobility control informationnumberOfRA_PreamblesNumber of non-dedicated random access preamblesPreambleintialreceivedtargetpower PowerRampingstep preamblesGroupAConfig_existProvides the configuration for preamble grouping. If the field is not signalled, the size of the random access preambles group A is equal to numberOfRA-PreamblespreambleTransMaxMaximum number of preamble transmissionra_ResponseWindowSizeDuration of the RA response window. Value in subframes. Value sf2 corresponds to 2 subframesmac_ContentionResolutionTimerTimer for contention resolution. Value in subframes. Value sf8 corresponds to 8 subframesmaxHARQ_Msg3Tx 4Maximum number of Msg3 HARQ transmissions, used for contention based random accessprach_Configused to specify the PRACH configuration in the system information and in the mobility control informationprach_ConfigIndexMentions Preamble format, SFN,subframe numberSystem Information Block Type3 (SIB 3)Direction: E-UTRAN => UERLC Mode: TMLogical Channel: BCCHTransport Channel: DL-SCHThe SIB3 contains cell re-selection information common for intra-frequency, inter-frequency and/or inter-RAT cell re-selection.SIB3 also contains cell reselection priority information for the concerned carrier frequency or a set of frequencies

Admin

August 08, 2021

20 Min Read

LTE

LTE CSFB (Circuit Switch Fall Back)

CSFBTo make a voice call there are mainly two solutions:CSFBIMS based Volte CallThere are pre conditions for CSFB:When UE is in LTE RAT it should be registered with MSC .UE is not directly connected to MSC .MME is connected to MSC via SGs interface.SGs Interface:SGs interface is between MME and MSC.LA Update is transferred by MME to MSC over SGsThere must be MME-MSC/VLR SGs association which is created at below occasion:During Combined EPS and IMSI attach.During Combined TA and LA Update.Combined EPS and IMSI attach procedure:In Attach request message UE send IE Type : combined EPS and IMSI attach.Note : EPS –PS Domain.          IMSI-CS Domain. Pre Conditions:Message in UE Log:CSFB Call flowType of CSFB implementation done in the network.1.Blind CSFB2.Measurement Based CSFB3.RIM or Flash CSFB.Note : When in legacy network (3G/2G) the UE can have PS data sessions. PS data is supported in 2G network only when DTM (Dual Transfer Mode) is supported. When UE supports DTM it can have both voice and data simultaneously.What happens after VOICE Call get over:If RAT is 2GUE will remain in 2G until data transfer completed or resume data flow after call completion.UE go to LTE without resuming data flow.If RAT is 3GUE can wait for data transfer to complete before cell reselection to LTE.UE go to LTE cell during ongoing data flow.

Admin

August 08, 2021

20 Min Read

LTE

LTE Synchronization Signals (PSS And SSS)

Initial AcquisitionIn frequency Acquisition we will getBand Information (Supported Band Info)EARFCN Information (Defined EARFCN)PLMN InformationNeed to knowPLMN and EARFCN information will be provided by SIM.Band Information will be provided by Phone (system file).Now work of UE is to obtain time and frequency synchronization.So the first signal to be detected is PSS then SSS in Time domain and frequency domain.From PSS and SSS the PCI ID decoding will be done.PSS (Primary Synchronization Signals)PSS is used for Slot Synchronization.Position of PSS in time domain (1st Subframe,1st Timeslot,7th OFDM Symbol).Periodicity -5 ms.In frequency domain across the 7th OFDM symbol depending of variable bandwidth.PSS is decoded on Centre frequency, where minimum 6RB is required i.e 62 subcarrier.If it will 1.4 Mhz it will be across the bandwidth.Centre frequency is required because it is having higher gain.PSS Information ElementsSubframe Number.Cell number, PSS records.PSS Indices. (0,1,2)Peak Position.SSS (Secondary Synchronization Signals)SSS is used for Frame Synchronization.Position of SSS in time domain (1st Subframe,1st Timeslot,6th OFDM Symbol).Periodicity -5 ms.In frequency domain SSS is decoded on Centre frequency, where minimum 6RB is required i.e 62 subcarrier.If it will 1.4 Mhz it will be across the bandwidth.SSS Information ElementsEARFCN.Cell ID number.Frame Boundary Information.Cyclic PrefixNumber of Barred/detected cell.

Admin

August 08, 2021

20 Min Read

LTE

LTE Protocol Stack

LTE Protocol stack is divided into Three Layers.Layer 1 (Physical Layer)Layer 2 (PDCP,MAC,RLC Layers)Layer 3 (NAS and RRC Layers)Protocol StackFunctions of each Layer.NAS: 3GPP TS 24.301EPS bearer managementAuthenticationPaging originationEPC connection management-IDLE mobility handlingSecurity mode controlTracking area updatePDN connectivity and dysconnectivity procedure.RRC:  3GPP TS 36.331RRC protocol layer exists in UE & eNodeb.It is part of LTE air interface control plane.Broadcast of System Information related to the non-access stratum (NAS) and access stratum (AS)PagingEstablishment, maintenance and release of an RRC connection between the UE and E-UTRANMobility functionsQoS management functions.UE measurement reporting and control of the reporting.Recovery from radio link failurePDCP: 3GPP TS  36.323PDCP protocol layer exists in UE & eNodebHeader compression and decompression of IP data packet.Transfer of user data.Maintenance the PDCP sequence number.Security of data.Ciphering and Integrity Protection.Transfer of control plane data.RLC: 3GPP TS 36.322Concatenation, Segmentation and reassembly of RLC SDUs.Retransmission of RLC PDUs (Only for AM data transfer)Reordering of RLC data PDUs (Only for UM and AM data transfer.Protocol error detection and recovery.MAC: 3GPP TS  36.323Prioritization among various data streams for a given UE.Error Correction trough HARQ.Mapping between logical channels and transport channels.Transport format selection.Padding.Physical layerEncode raw data before modulation.Measure the air interface to know the channel quality.Link adaptation.Power control.

Admin

August 08, 2021

20 Min Read

LTE

LTE Architecture And Its Network Elements

LTE Architecture and its Network ElementsBelow is the LTE architecture2G-3G-LTEFunction of LTE Network ElementsThe Function of the eNodeBRadio resource management (RRM)radio bearer controlmobility managementadmission controldynamic resource allocationRouting of user plane packets towards the S-GWMME selectionPacket compression and cipheringPaging Message scheduling and transmissionThe Function of the MMEAuthenticationNAS SignalingMobility managementPaging procedureTracking area UpdateBearer activation and deactivation processS-GW selection for a UE at the initial attachThe Function of the S-GWThe main function of the Serving Gateway is routing and forwarding of user data packets.It is also responsible for inter-eNB handovers in the U-plane and provides mobility between LTE and other types of networks, such as between 2G/3GThe Function of the P-GWUE IP allocationIP Routing and forwardingPCEF Function-responsible for dictating QoS and BW parameters for subscriber’s session.The Function of the HSSPermanent and central subscriber databaseContains center Authentication functionality The Function of the PCRFPCRF a combination of the Charging Rules Function (CRF) and the Policy Decision Function (PDF), and ensures the service policy and sends Quality of Service (QoS) information for each session begin and accounting rule information.

Admin

August 08, 2021

20 Min Read

LTE

LTE Subcarriers, Modulation And Frame Structure

LTE Subcarriers, modulation and Frame structureLTE subcarrier is the smallest independent part of an LTE signal.The smallest defined piece of an LTE signal is a single subcarrier is RE (Resource Element).Modulation scheme used in LTE.QPSK16QAM64QAMLTE Frame Structure in FDDEach LTE downlink subcarrier operates in 10 ms long frames.Each frame is made up of 10 subframes, each 1 ms.Each subframe contains 2 slots, each 0.5 ms.Each slot carries seven modulated symbols, which could be QPSK, 16QAM, or 64QAM.Generic Frame Sequences:Resource BlockIn the frequency domain, 12 subcarriers form one physical resource block (180kHz * 0.5ms). The resource block size is the same for all bandwidths.2 RB(Resource Block) = 1 Scheduling Block(SB)1 Scheduling Block(SB)=1 TTI =1 SubframeScalable Bandwidth:LTE channel bandwidth can be 1.4, 3, 5, 10, 15, 20 MHz.The wider bandwidth the higher throughput.

Admin

August 08, 2021

20 Min Read

LTE

LTE Q & A (14/11/2020)

1.    How Many S1AP ID Is Created In Initial Attach Procedure Of LTE? A.   OneB.   TwoC.   ThreeD.   Depends On Resource Availability  2.    Volte Supported UE At The Time Of Emergency Call Will Create. A.    Will Not Use Any PDNB.   A New PDN ConnectionC.   Will Use PDN That Network AllowD.   Will Use Old PDN 3.    In HARQ What The Max Number Of MAC PDU Retransmission Is A.   8B.   2C.   4D.   Depend On Data Length 4.    If UE Is In Idle State And Need To Attach And Move Into Connected State. In That Scenario While Sending Attach Request UE Will Get Attach Through A.   STMSIB.   MTMSIC.   IMSID.   MSISDN  5.    Logical Channel Prioritization In MAC Is Done Where And In Which Direction A.   UE-UplinkB.   UE-DownlinkC.   E-NB UplinkD.   E-NB Downlink

Admin

August 08, 2021

20 Min Read

LTE

LTE Q & A (21/10/2020)

1.Components of TAITAC,MNCTAC,MCCMNC,TAC,MCCTAC,MNCAns. MNC,TAC,MCC2. ERAB Consist ofS1 and S5/S8 BearerRadio and S5/S8 BearerRadio and S1 BearerS1, Radio and S5/S8 BearerAns. Radio and S1 Bearer3. Default bearer usesNon-GBRGBRBothAns. Non-GBR4. Which one is not a SIB2 parameterPrach-configPowerRampingStepRach-ConfigCommonq_RxLevMinOffsetAns. q_RxLevMinOffset5. TRACKING AREA UPDATING (TAU) procedure is always initiated byUEPGWMMEEnbUE or MMEAns. MME

Admin

August 07, 2021

20 Min Read

LTE

RACH In LTE (Random Access Procedure In LTE)

RACH is an Uplink Transport Channel which is used for initial Random Access. The major function of RACH is to allow the UEs to get Uplink Synchronization. RACH plays an important role in the transmission of Uplink Scheduling Request.Data transmission happens in two ways from UE, once when the UE has a dedicated RRC (PUSCH resource available) and once when the UE needs to access the network and then begin data transmission.When no dedicated connection is established, a Scheduling request will be transmitted on RACH called as RA_SR (Random Access Scheduling request).The process of accessing the network when no dedicated RRC is established or when the UE Transmits for the first time ( after power-on ) is called Random Access and the Channel that plays the major role in this aspect is called RACH channel.We can sequence the below scenarios when Random Access is requiredInitial Access: When the UE tries to access the network during RRC_IDLE or initial power on mode.During RRC _Reconnection establishment.When the UE losses the Uplink Synchronization from the networkWhen no dedicated PUSCH resources available and during HandoverRACH in LTE Channel Structure:The Fig 1: Shows the position of RACH in the Uplink LTE channel Structure. RACH is responsible for the Random-Access process. To access any network the UE needs to be synchronized both in the downlink and in the uplink. Downlink synchronization happens by cell selection and PLMN selection, but the uplink synchronization is taken care by the Random-Access processRandom Access process takes place in two ways:Contention Based Random AccessContention Free Random Access.Before we discuss about these two processes, we need to understand Preambles. Preambles are basically “Signatures” or specific patterns that is transmitted by UE to eNB.There are in total 64 Preambles signatures that are available in a LTE call and UEs can randomly select any of the preambles. The preambles are orthogonal to each other and are divided into two groups.One group is called contention free preambles which are allocated to UE which undergoes Handover and the other is contention-based preambles which are allocated to UEs which are going through the process of Random Access.We will talk about in details about the Random-Access types below.Contention Based Random Access: As the name suggest this process is based on the scenario where there is a contention to access the network. And this contention happens when multiple UEs try to access the network at the same time.Step 1: The UE transmits the Preamble messages from the 64 Sequence available. The preamble messages are grouped also based on the data payload in L3 Packets. It gives the eNB an idea about the requirement of Scheduling resources.Step 2: The eNB now sends RA response to all the UEs which has sent RA preamble intimating them that resources have been reserved for them. It can only send the response if it has correctly decoded the preamble. At this point the eNB doesn’t have any idea of the identity of the UE. This response is generated by the UE from the MAC layer and is transmitted in the DL-SCH. The Random-Access Response is addressed to an RA-RNTI which identifies the Resource Block on which the preamble was decoded. There is a timer or time slot during which the UE is scheduled to receive the RAR. If the UE doesn’t receive the RAR in that specified time slot, it again sends RA preamble at the next available time with increased power. It is to be noted that if several UE has sent preambles in the same Random Access slot and all the preambles have been decoded by the eNB , all the UEs will receive RAR and will have the information that they have scheduled slot for sending RRC request as we will see in step3.Step3: The UE now utilizing the resources allocated in Step 2, sends RRC connection request on UL-SCH. In the RRC messages the UE sends its identifier to the eNB to make it aware of its identity. This identifier is used to solve any contention resolution that can happen.Step4: As we can see that all the UEs which sent the same preamble at the same time, while sending the RRC request message will collide and this will result in interference. Among those UEs the one which is having the best radio conditions, the message will be successfully decoded. In Contention resolution message the eNB send the UE identifier, so the UEs which can’t decode the message or whose identifier doesn’t match with the one sent, they back off and wait for the next turn to send the RA request. The UE which received the message and can decode its identify then starts transmitting on the Uplink resource block allocated to it.Contention Free Random Access: In contention free Random Access the process is initiated by the network. It happens in case of a Handover of a UE from one EnB to another eNB.Step1: The eNB reserves a set of preambles for this purpose and assigns a preamble from this and sends to UE.Step 2:  Since this entire process is controlled by the eNB there is no collision. The UE now sends back the response of Random-Access Preamble with the UE ID.Step 3: The eNB then sends Random Access response with the message of resources allocated.Summary in Brief:Now let us explain what the significance of below terms is:RA-RNTI: It is Random Access Radio Network Terminal Identifier. It basically contains the UE identifier.T-C-RNTI: Temporary Cell RNTI.  Used for scheduled unicast transmission. It is created by eNB and is used to identify a UE within the scope of the eNB during random access process and set up of RRC connection.Importance of SIB2 in Random Access:SIB2 carries the below information in LTE:RACH related parametersIdle mode paging configurationsPUCCH and PUSCH configurationsUplink power control and Sounding Reference SignalsUplink carrier frequency and Bandwidth and Cell barring informationSo, from the above points we can see that all parameters that are required for transmission of RACH are carried by SIB2. Now let us see what are parameters that are present in RACH, transmitted by SIB2. These are RACH configuration at the MAC level across cellNumber of RA preambles: There are 64 preamble sequence available which can be used for contention and non-contention based Random Access. The parameter has a range from 4-64Size of RA preambles groupA: The 64 Preamble sequence available are divided into two groups , Group A and Group B. GroupA preambles are used when sending small packets and GroupB preambles are used when sending long packets . The Parameter has a range from 4 to 60.Preamble Initial Received Target Power:Initial UE transmit power for the PRACH which the eNB expects. The value starts from -120dBm to -90dBm with a step size of 2dBmPower Ramping Step:Multiple attempts to access the PRACH may fail due to unfavorable radio conditions so the UE increases power for random access preambles by a step specified by this parameter. The step can be 0 2 4 or 6 db.Power Ramping Parameters: Indicates the steps by which the UE should increase the transmit power where random access preambles is increased each time after a RACH access failure.MAC Contention Resolution Timer:The total time when a UE waits for Msg4 during a random-access procedure. This timer starts when a UE initially sends or resends Msg3.Max HARQ Msg3 Tx:Maximum number of Msg3 HARQ transmissions8.Preamble Trans Max: Maximum number of Preamble Transmissions, possible values are 3 4 5 6 7 8 10 20 50 100 200.So we saw the information that are transmitted in SIB2 for RACH on MAC layer , let us know see the parameters which are configured in PRACH at the physical layerPRACH ConfigRoot Sequence Index: Used to determine 64 physical RACH preamble sequences available in the cell. These preambles are generated by Zadoff Chu Sequence which has a series of root sequences. We can cyclic shift each sequence to obtain the preamble sequence. The Range of Rach Root Sequence is from 0 to 837.PRACH Config Index:By this parameter we can define at what time – frequency grid the UE sends the RACH request to eNB.High Speed Flag:This parameter determines what type of scenario the particular cell serves, whether high speed scenarios like catering to railway traffic or other scenariosZero Correlation Zone Config: This guarantees that the preambles generated are orthogonal to each otherPRACH Freq Offset:This parameter actually helps the cell to inform the UE and other neighbor cells about which PRB can be used for Random Access.Source: https://www.slideshare.net/YoungHwanKimPRACH Preamble Format:PRACH is the physical channel that initiates the exchange of information with ENB from UE end in the uplink transmission. The Preambles that is sent through this channel to Enb helps to establish the timing advance required for transmission. The signature or sequence that we mentioned earlier is basically the PRACH preamble.PRACH Preamble = Cyclic Prefix with length TCP  + Signature (Tpre)+GPCyclic Prefix is used to restrict inter symbol interference.Guard Period (GP) is the unused portion of time at the preamble which is used for absorbing propagation delayFormat 0Source : www.researchgate.comTs=1/(15000*2048) which is equal to sampling rate of 30.72MHZPreamble Formats:The different Preamble formats are designed to cater to different cell radius and radio conditionsZadoffChu-Sequence:The requirement for generating RA preamble are good auto- correlation function (ACF) and good cross correlation function (CCF). Zadoff -CHU sequence suffices to both having good ACF and CCF.Resource Block of RACH in LTE Frame Structure :Preamble length is 6RB , 1 RB=180KHZ, so 6RB=1.8MHZCalculation:1 Subcarrier of PRACH preamble =1.25KHZ, whereas UL Subcarrier=15KHz.So, 12 PRACH preamble subcarrier = 1 UL Subcarrier (12*1.25KHZ=15KHZ)Now the Random Access preamble has 839 Subcarrier .So total BW=839*1.25KHZ=1048.75.Now there is a 15KZ guard band on either side, so total BW =(1048.75+30)KHZ=1078.75Khz=1.8MHZ =6RB.Some Brainstorming now, with a cup ofHow the UE decides when and where it can send the RACH request?Ans: By using the parameter PRACH config index and PRACH Freq OffsetWhat if the UE doesn’t receive the RACH response in first trial?Ans: Just retry and send PRACHIf PRACH response is not received, do we transmit with same power again?Ans: No, we need to increase the power by step of 2DBDifference between PRACH of WCDMA and LTE?Ans: IN WCDMA PRACH can be used to transfer RRC messages and application data but in LTE PRACH doesn’t transfer RRC or application data

Admin

August 07, 2021

20 Min Read

LTE

LTE Architecture And Its Network Elements

LTE Architecture and its Network ElementsBelow is the LTE architecture2G-3G-LTEFunction of LTE Network ElementsThe Function of the eNodeBRadio resource management (RRM)radio bearer controlmobility managementadmission controldynamic resource allocationRouting of user plane packets towards the S-GWMME selectionPacket compression and cipheringPaging Message scheduling and transmissionThe Function of the MMEAuthenticationNAS SignalingMobility managementPaging procedureTracking area UpdateBearer activation and deactivation processS-GW selection for a UE at the initial attachThe Function of the S-GWThe main function of the Serving Gateway is routing and forwarding of user data packets.It is also responsible for inter-eNB handovers in the U-plane and provides mobility between LTE and other types of networks, such as between 2G/3GThe Function of the P-GWUE IP allocationIP Routing and forwardingPCEF Function-responsible for dictating QoS and BW parameters for subscriber’s session.The Function of the HSSPermanent and central subscriber databaseContains center Authentication functionality The Function of the PCRFPCRF a combination of the Charging Rules Function (CRF) and the Policy Decision Function (PDF), and ensures the service policy and sends Quality of Service (QoS) information for each session begin and accounting rule information.

Admin

August 07, 2021

20 Min Read