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 capacity and speed of mobile telephone networks.LTE is marketed as 4G technology. For current release of LTE specifications, one can visit 3GPP site http://www.3gpp.org/ftp/Specs/html-info/SpecReleaseMatrix.htm.

source of figure: wikimedia commons

As mentioned in the diagram above LTE system composed of two main parts. User device is referred as UE i.e. User Equipment and Base station is known as eNodeB. eNodeB is connected to internet back bone via SGW-serving gateway or MME-Mobility Management Entity.LTE is somewhat similar to HSPA and its downlink access technique is similar to mobile wimax i.e. OFDMA.

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,S-SS,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 subcarrier 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 GatewayMME: Mobility Management EntityX2 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 user 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 TransmissionEach 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 signalS-SS: secondary synchronization signalPBCH: Physical Broadcast ChannelPDSCH: Physical Downlink Shared ChannelPDCCH: Physical Downlink Control ChannelPCFICH: Physical Control Format Indicator ChannelPHICH: Physical Hybrid ARQ Indication ChannelPCH: Paging channelRS: Reference Signal, used both in uplink and downlinkSRS: Sounding reference signal, used in uplinkDMRS: Demodulation Reference SignalPRACH: Physical Random Access Channel used in uplinkPUSCH: Physical Uplink Shared ChannelPUCCH: Physical Uplink Control Channel

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 anchor for inter 3GPP mobility. SGW is responsible for packet routing and forwarding, buffering the downlink 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 packet 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

Types of OFDMA

OFDMA stands for Orthogonal Frequency Division Multiple Access. It is the most popular access technique adopted in next generation wireless technologies such as Mobile WiMAX, LTE, LTE Advanced etc. In OFDMA technique, number of frequency bins in a single IFFT is divided and allocated to different mobiles(users). In Mobile WiMAX resource allocation is based on number of slots. Slot definition varies based on zone types, but slot is composed of number of subcarriers and number of OFDMA symbols. One slot in PUSC is made of 1 subchannel and 2 OFDMA symbols. Here 1 subchannel consists of many subcarriers. For example in a 1024 point FFT, there are 30 subchannels and each subchannel is made of 24 data subcarriers. In LTE and LTE Advanced resource allocation is based on number of resource blocks. One Resource block consists of 12 subcarriers and 7 OFDMA symbols. In Mobile WiMAX terminal is referred as Mobile Subscriber Station and in LTE/LTE Advanced terminal is referred as UE.

There are two main types of OFDMA viz. distributed OFDMA and localized OFDMA depends on how the sub carriers are assigned to the terminals/mobile subscribers.

If the subcarriers are contiguously assigned to each terminal then it is calledlocalized OFDMA. Number of subcarriers assigned to the terminal is determined based on data rate requirement as desired by the user. This is because one subcarrier carry one or more than one bits based on modulation technique. For BPSK it is one bit, for QPSK 2 bits,for 16QAM 4 bits and so on. Hence bits carried on total subcarriers in a symbol time gives us data rate.If the distributed subcarriers are assigned to each terminal then it is calleddistributed OFDMA. This distributed subcarriers are obtained using permutation technique.As mentioned and depicted in the figure above, resource allocation out of chunk of sucbcarriers of a IFFT size can be either continuous or permuted per user. Distributed OFDMA has advantage over localized OFDMA as it is more resistant to time varying fading channel. Localized OFDMA is simpler to implement compare to distributed one. Interleaving also does the same job as performed in a distributed OFDMA.

OFDM vs OFDMA-difference between OFDM and OFDMA

This page on OFDM vs OFDMA describes difference between OFDM and OFDMA technologies.

OFDM

In OFDM systems, only a single user can transmit on all of the sub-carriers at any given time. In order to support multiple users time and/or frequency division access techniques are used in OFDM. The major setback to this static multiple access scheme is the fact that the different users see the

wireless channel differently is not being utilized. OFDMA, on the other hand, allows multiple users to transmit simultaneously on the different sub-carriers per OFDM symbol. OFDM is employed in Fixed WiMAX system deployed around the world for broadband internet service. Figure 1 depicts OFDM frame structure employed in fixed WiMAX system. Here Downlink sub frame is transmitted by Base station to subscriber stations and Uplink sub frame is transmitted by multiple subscriber stations to the Base Station. Both the frame is composed of more than one OFDM symbols and each symbol is made up of subcarriers, which fall in data and pilot subcarriers, where data subcarriers carry the user data. There are 192 data sub carriers in Fixed WiMAX System. The point here is Subscriber station has been assigned one or more symbols by BS and all the data carriers of the symbols are occupied by one SS. It is depicted in the figure that entire 256 carriers are allocated to the user at its predetermined time slot in TDD frame.

 

OFDMA

In the case of OFDMA, which is employed in Mobile WiMAX system deployed around the world and also employed in LTE system being deployed, total subcarriers are permuted and assigned to sub channel. Hence many SSs can occupy the same sub channel but use different subcarriers to transmit the information. Figure 2 describes OFDMA frame used in Mobile WiMAX System. It clearly mentions that one symbol is composed of more than one sub channel and each sub channel is composed of distributed subcarriers. The point here is each symbol is used by more number of SSs to transmit and receive the information which is depicted by Burst 1 and Burst 2 in the figure. As mentioned in OFDMA subcarriers are divided among users at the same time instant. Figure mentions 1024 FFT case here. 

The Frame structures mentioned here only for demonstrating the concept and it differs in the actual wimax system.Both OFDM and OFDMA is used to achieve high data rate transmission over the air. With OFDMA system can support more subscribers with sub channelization concept compare to OFDM.

Both OFDM and OFDMA is implemented using IFFT and FFT operation at transmitter and receiver respectively.For OFDM entire input of IFFT is occupied fully by either subscriber sttaion or Base Station. For OFDMA part of input values (consecutively) is occupied by Subscriber station and at rest of the inputs zeros or nulls are inserted. Same is done with other subscribers and so on.

To undertand difference between OFDM and OFDMA one should also refer difference between FDM and OFDM multiplexing techniques. Refer OFDM physical layer and OFDMA physical layer as per fixed wimax and mobile wimax standards.Further OFDM vs OFDMA can be explored by studying compasion between WiMAX and LTE standards. Refer wimax vs lte page in terminology section.

Types of OFDMA

OFDMA stands for Orthogonal Frequency Division Multiple Access. It is the most popular access technique adopted in next generation wireless technologies such as Mobile WiMAX, LTE, LTE Advanced etc. In OFDMA technique, number of frequency bins in a single IFFT is divided and allocated to different mobiles(users). In Mobile WiMAX resource allocation is based on number of slots. Slot definition varies based on zone types, but slot is composed of number of subcarriers and number of OFDMA symbols. One slot in PUSC is made of 1 subchannel and 2 OFDMA symbols. Here 1 subchannel consists of many subcarriers. For example in a 1024 point FFT, there are 30 subchannels and each subchannel is made of 24 data subcarriers. In LTE and LTE Advanced resource allocation is based on number of resource blocks. One Resource block consists of 12 subcarriers and 7 OFDMA symbols. In Mobile WiMAX terminal is referred as Mobile Subscriber Station and in LTE/LTE Advanced terminal is referred as UE.

There are two main types of OFDMA viz. distributed OFDMA and localized OFDMA depends on how the sub carriers are assigned to the terminals/mobile subscribers.

If the subcarriers are contiguously assigned to each terminal then it is calledlocalized OFDMA. Number of subcarriers assigned to the terminal is determined based on data rate requirement as desired by the user. This is because one subcarrier carry one or more than one bits based on modulation technique. For BPSK it is one bit, for QPSK 2 bits,for 16QAM 4 bits and so on. Hence bits carried on total subcarriers in a symbol time gives us data rate.If the distributed subcarriers are assigned to each terminal then it is calleddistributed OFDMA. This distributed subcarriers are obtained using permutation technique.As mentioned and depicted in the figure above, resource allocation out of chunk of sucbcarriers of a IFFT size can be either continuous or permuted per user. Distributed OFDMA has advantage over localized OFDMA as it is more resistant to time varying fading channel. Localized OFDMA is simpler to implement compare to distributed one. Interleaving also does the same job as performed in a distributed OFDMA.

Difference between SC-FDMA and OFDM

This page describes difference between SC-FDMA and OFDM modulation techniques.SC-FDMA means Single Carrier Frequency Division Multiple Access and OFDMmeans Orthogonal Frequency Division Multiplexing.

As shown in the figure in SC-FDMA one extra module DFT is added before IFFT module in the transmitter chain and IDFT is added in the receiver chain. This converts OFDM chain into SC-FDMA chain. Without this two modules the chain is referred as OFDM transmit and receive chain.

SC-FDMA system usually will have low PAPR compare to OFDM system.

SC-FDMA system is less sensitive to frequency offset compare to OFDM system.

SC-FDMA is widely used in LTE subscriber terminals in the transmit path and its variant OFDMA is used in the eNodeB downlink(or receive path of LTE subscribers). While OFDM is used in many broadband technologies such as wimax-16d/16e, WLAN-11a/11n/11ac.

OFDM is referred as multicarrier modulation. It uses multiple rf carrier signals at different frequencies which sends some of the bits on each of the assigned channels. This seems to be similar to FDM but in the case of OFDM, total subcarriers are divided into subchannels and these subchannels are mapped to one single data/traffic source.

Merits of OFDM

SC-FDMA has merits as mentioned above. OFDM also has many advantages compare to SC-FDMA.•  Frequency selective fading will be able to affect few of the subchannels/subcarriers and not entire band.• OFDM overcomes effect of ISI occuring mostly in multipath channel environment.

FDM vs OFDM - Difference between FDM and OFDM

This page describes difference between FDM and OFDM techniques.FDM stands for Frequency Division Multiplexing and OFDM stands for Orthogonal Frequency Division Multiplexing.

As shown in the figure in FDM systems carriers are far apart with respect to each other and in OFDM systems carriers are densely packed and are orthogonal to the other carriers. Orthogonal means peak of one carrier occurs at null of the other. Hence OFDM system is bandwidth efficient compare to FDM system. In FDM system carriers are not orthogonal.

OFDM system usually will have more Peak to Average Power ration i.e. PAPR compare to FDM system. PAPR can be reduced by use of scrambler module and other techniques in OFDM systems.

OFDM system provides higher data rate compare to FDM system in the same bandwidth usage.

FDM systems are used in radio, satellite communications requiring good amount of guard bands between adjacent frequency bands. OFDM systems are used in wimax-16d/16e, wlan-11g/11n and LTE technologies requiring higher data rate and mainly used for broadband internet service.

In OFDM systems multipath interference is more compare to FDM systems but can be avoided/reduced using high end algorithms such as cyclic prefix insertion etc.

In FDM case entire bandwidth is used by user/subscriber, while in OFDM bandwidth is divided into many narrow band channels and each is allocated to user/subscriber. Hence OFDM supports more subscribers/channels compare to FDM.

If one wants to understand difference between FDM and OFDM, it requires wast knowledge on carrier multiplexing techniques for optimal utilization of scarce frequency resoures.

Basics of LTE Technology

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 msSub frame duration=1 msBasic 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 MHzFFT size : 128/256/512/1024/1536/2048Cyclic Prefix : Normal, ExtendedDL multiple access: OFDMAUL multiple access: SC-FDMADuplexing :FDD & TDDSubcarrier mapping: LocalizedSubcarrier hopping: YesData Modulation : QPSK/16QAM/64QAMSubcarrier spacing: 15KHzChannel Coding : convolutional coding and turbo codingMIMO :2 or 4 at transmit and 2 or 4 at receive side HARQ :incremental redundancy LTE EARFCN stands for E-UTRA Absolute Radio Frequency Channel Number. The page describes LTE EARFCN equation and table for calculation of downlink EARFCN(NDL) and uplink EARFCN(NUL). It provides link to LTE EARFCN calculator which is very useful for LTE EARFCN to frequency and vice versa conversion.EARFCN number is within range 0 to 65535 and equation between LTE carrier frequency(MHz) and EARFCN is mentioned below.

LTE EARFCN Equation

Fdownlink = FDL_Low + 0.1 (NDL - NDL_Offset) 

Fuplink = FUL_Low + 0.1 (NUL - NUL_Offset) 

Where,NDL is downlink EARFCN NUL is uplink EARFCN NDL_Offset is offset used to calculate downlink EARFCN NUL_Offset is offset used to calculate uplink EARFCN

LTE Tutorial-Page7

This tutorial on LTE covers LTE frame structure.

LTE Frame structure

The LTE frame structure are of two types based on topology either FDD orTDD. 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 periodicity 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 Tutorial-Page10

This tutorial on LTE covers LTE channels.

LTE technology works based on three channel types viz. logical channel,transport channel and physical channels. These channels are used by lower layers to provide services to the upper layers.The access points to the Layer L2/L3 are transport channels. They get mapped to physical channels. These physical channels will have different modulation-code rate as mentioned below and are exclusively used by LTE PHYSICAL Layer to carry upper layer information.

LTE Logical,Transport and Physical channels

Following figure mentions LTE logical channels,transport channels and physical channels and mapping between them.

As shown logical channels are of two types; one carrying control information and the other carrying traffic informtion. These gets mapped to transport channels as depicted in the figure.

Physical Channels

PDSCH - Stands for Physical Downlink Shared Channel, mainly used to carry high speed data/multimedia information. Can be either QPSK/16QAM/64QAM.PDCCH - Stands for Physical Downlink Control Channel, mainly used to carry UE specific control information. It will have QPSK modulation used.It is mapped on resource elements(REs) in first 3 OFDM syms(symbols) in first slot of subframe.CCPCH - Stands for Common Control Physical Channel, carries cell-wide control information. QPSK is used. CCPCH is transmitted exclusively on 72 subcarriers centered around DC carrier.

Physical Signals

Physical signals do not convey L2/L3 layer information, but mainly used for synchronization and channel estimation purpose. RS is used for estimating channel response. P-SS and S-SS synchronization signals used for determining network frame timing information i.e. start of the information.

Transport Channels

• Downlink and Uplink transport channels carry L2/L3 information. • It also configures LTE PHY layer.• It sends status information such as packet error and CQI to upper layers.• Also supports peer-peer signaling between higher layers.Based on broadcast,unicast or multicast concept different transport channels exist. Downlink channels include BCH(broadcast channel),DL-SCH(downlink shared channel, to multiple mobile subscribers or UEs),PCH(paging channel, used for UE DRX and broadcasted over entire cell ),MCH(multicast channel, transmitted over entire cell). Uplink channels include RACH(Random Access Channel), UL-SCH(Uplink Shared Channel).

Uplink PRBs(Physical Resource Blocks) are assigned to UE by eNodeB scheduler. PUSCH is used and shared by multiple UEs to carry upper layer information towards eNodeB. It will employ QPSK/16QAM/64QAM modulation types.

his page covers LTE timers which include T300, T301, T303, T304, T305, T310, T311, T320 and T321.

LTE Timers Function at Start/Stop/Expiry

T300>>Starts at the RRC connection REQ transmit >>Stops at the Receipt of RRC connection setup or reject message OR at the cell reselection time OR upon abortion of connection establishment by Higher layers (L2/L3).>>At the expiry performs the actions 

T301

>>Starts at the RRC Connection Re-establishment REQUEST>>Stops at the Receipt of RRC Connection Re-establishment ORRRC Connection Re-Establishment REJECT message ORWhen selected cell becomes unsuitable to continue further>>At expiry, it Go to RRC_IDLE mode

T303>>Starts when access is barred while performing RRC CONNECTION ESTABLISHMENT for MO(Mobile Originating) calls>>Stops while entering RRC_CONNECTED and upon cell re-selection mode>>At expiry, Informs higher layers about barring alleviation 

T304 >>Starts at the Receipt of RRC CONNECTION RECONFIGURATION message along with Mobility Control Info OR at the receipt of mobility from EUTRA command message including CELL CHANGE ORDER>>Stops at the successful completion of HANDOVER to EUTRA or CELL CHANGE ORDER is met>>At expiry, it performs action based on need.1. In the case of CELL CHANGE ORDER from E-UTRA OR intra E-UTRA handover, initiate the RRC connection re-establishment procedure.2. In case of HANDOVER to E-UTRA, perform the actions defined as per the specifications

applicable for the source RAT.

T305>>starts when access is barred while performing RRC CONNECTION ESTABLISHMENT for MO signaling>>Stops when entering RRC_CONNECTED and when UE does cell re-selection>> At expiry, Informs higher layers about barring alleviation 

T310

>>Starts when UE detects PHY layer related problems (when it receives N310 consecutive out-of-sync INDs from lower layers)>>Stops 1. When UE receives N311 consecutive in-sync INDs from lower layers/2. Upon triggering the HANDOVER procedure3. Upon initiating the CONNECTION RE-ESTABLISHMENT procedure>> At expiry, if security is not activated it goes to RRC IDLE else it initiates the CONNECTION RE-ESTABLISHMENT Procedure

T311>>Starts while initiating RRC CONNECTION RE-ESTABLISHMENT procedure>>stops upon selection of suitable E-UTRA cell OR a cell using another RAT>>At expiry it enters RRC IDLE state 

T320

>> Starts upon receipt of t320 or upon cell re- selection to E-UTRA from another RAT with validity time configured for dedicated priorities (in which case the remaining validity time is applied).>>Stops upon entering RRC_CONNECTED state, when PLMN selection is performed on request by NAS OR upon cell re-selection to another RAT>> At expiry, it discards the cell re-selection priority info provided by dedicated signaling 

T321

>>starts upon receipt of measConfig including a reportConfig with the purpose set to reportCGI>> Stops at either of following cases:1. Upon acquiring the information needed to set all fields of globalCellIdfor the requested cell2. upon receipt of measConfig that includes removal of thereportConfig with the purpose set to reportCGI>> At expiry initiates the measurement reporting procedure, stop performing the related measurements and remove the correspondingmeasID

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 example of downlink shared channel(DL-SCH). As shown in the figure for eNodeB Transmitter upper layer data 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 by 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 + 1Segmentation 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 redundant parity bits are appended to each resulting smaller block. Also, filler bits are added so the code 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 pruned 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 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. 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 schemes 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 Wimax,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.

 

VoLTE tutorial-Voice Over LTE basics and types

This VoLTE tutorial covers VoLTE (Voice Over LTE) basics, types of VoLTE which include VoLTE using IMS(VOIP) and VoLTE using CSFB(SRVCC).

LTE is all IP based network and mainly designed for internet/data applications. Hence to support voice over LTE there are two options. The first is using VOIP(voice vover IP) with the help of IMS(IP Multimedia system) and the the second one is using legacy 2G/3G Circuit Switched Fall Back(CSFB) with the concept called SRVCC. We will see below how these works.

VoLTE using IMS(VOIP)

 

As shown in the figure above in LTE called EUTRAN comprised of more than one eNodeB. eNodeB in LTE has functionalities of RNC and NodeB of 3G. MME,S-GW,HSS and P-GW will form EPC(Evolved Packet Core). eNodeBs and EPC form EPS(Evolved Packet System). LTE supports voice call using VOIP using IMS. If it is VOIP to VOIP call, it will remain within LTE network and supported using VOIP protocols. But if it is VOIP to CS call for GSM/WCDMA network, IMS with the help of application servers and legacy MSC transfers the PS call to legacy networks (2G-GERAN/3G-UTRAN). If it is VOIP to PSTN call, IMS directs through PSTN interface to respective exchanges.

VoLTE using 2G/3G CSFB(using SRVCC)

 

When UE moves from LTE network to say legacy networks(GSM/WCDMA), based on measurement report submitted by UE handover is performed and tranfer of Voice over IP call to legacy voice call is performed . This to happens UE should support SRVCC. SRVCC stands for Single Radio Voice Call Continuity. It means UE will be connected with sigle radio at a time and hence one single RAT(either LTE or GSM or WCDMA). Hence battery life becomes longer here with SRVCC capable UE.

Useful Links

Other than this volte tutorial, refer wireless tutorials on various IEEE and 3gpp standards with links mentioned on left side panel.

What is VoLGA?VoLGA stands for "Voice over LTE via Generic Access". The VoLGA service resembles the 3GPP Generic Access Network (GAN). GAN provides a controller node - the GAN controller (GANC) - inserted between the IP access network (i.e., the EPS) and the 3GPP core network. 

The GAN provides an overlay access between the terminal and the CS core without requiring specific enhancements or support in the network it traverses. This provides a terminal with a 'virtual' connection to the core network already deployed by an operator. The terminal and network thus reuse most of the existing mechanisms, deployment and operational aspects.

Automatic Neighbour Relation (ANR)in ANR, wiki, LTE

Manually provisioning and managing neighbor cells in traditional mobile network is challenging task and it becomes more difficult as new mobile technologies are being rolled out while 2G/3G cells already exist. For LTE, task becomes challenging for operators, as in addition of defining intra LTE

neighbour relations for eNBs operator has to provision neighboring 2G, 3G, CDMA2000 cells as well.

According to 3GPP specifications, the purpose of the Automatic Neighbour Relation (ANR) functionality is to relieve the operator from the burden of manually managing Neighbor Relations (NRs). This feature would operators effort to provision 

Figure below shows ANR and its environment as per 3GPP. It shows interaction between eNB and O&M due to ANR.

 

The ANR function resides in the eNB and manages the conceptual Neighbour Relation Table (NRT). Located within ANR, the Neighbour Detection Function finds new neighbours and adds them to the NRT. ANR also contains the Neighbour Removal Function which removes outdated NRs. The Neighbour Detection Function and the Neighbour Removal Function are implementation specific.

An existing Neighbour cell Relation (NR) from a source cell to a target cell means that eNB controlling the source cell  knows the ECGI/CGI and Physical Cell Identifier (PCI) of the target cell and has an entry in the NRT for the source cell identifying the target cell.

For each cell that the eNB has, the eNB keeps a NRT. For each NR, the NRT contains the Target Cell Identifier (TCI), which identifies the target cell. For E-UTRAN, the TCI corresponds to the E-UTAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell. 

The ANR function relies on cells broadcasting their identity on global level, E-UTRAN Cell Global Identifier (ECGI) and allows O&M to manage the NRT. O&M can add and delete NRs. It can also change the attributes of the NRT. The O&M system is informed about changes in the NRT.

Intra-LTE/frequency ANR:  

The eNB serving cell with ANR function, instructs each UE to perform measurements on neighbor cells, as a part of the normal call procedure. The eNB may use different policies for instructing the UE to do measurements, and when to report them to the eNB. 

When UE discovers new cell’s ECGI, the UE reports the detected ECGI to the serving cell eNB. In addition the UE reports the tracking area code and all PLMN IDs that have been detected. The eNB adds this neighbour relation to NRT.

Inter-RAT/Inter-frequency ANR:

The eNB serving cell with ANR function can instruct a UE to perform measurements and detect cells on other RATs/frequencies .during connected mode. The eNB may use different policies for instructing the UE to do measurements, and when to report them to the eNB.

The UE reports the PCI of the detected cells in the target RATs/frequencies. When the eNB receives UE reports containing PCIs of cell(s), eNB may instruct the UE to read the CGI and the RAC of the detected neighbour cell in case of GERAN detected cells and CGI, LAC and, RAC in case of UTRAN detected cells. For the Interfrequency case, the eNB may instruct the UE to read the ECGI, TAC and all available PLMN ID(s) of the inter-frequency detected cell.

The eNB updates its inter-RAT/inter-frequency Neighbour Relation Table after receiving relevant info from UE.