LTE CHANNEL STRUCTURE AND MAPPING

LTE CHANNEL STRUCTURE

(3GPP TS 36.211 and 36.212)
The physical layer provides transport channels to the L2. These transport channels differ in their characteristics how data is transmitted and are mapped to different logical channels provided by the MAC layer. Logical channels describe which type of data is conveyed.

LOGICAL CHANNELS

The logical channels can be divided into control channels and traffic channels. The control channels are used for transfer of control plane information and the traffic channels are used for the transfer of user plane information. The following logical channels are supported for LTE:

Control Channels

• Broadcast Control Channel (BCCH): A downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH): A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE.
• Common Control Channel (CCCH): This channel is used by the UEs having no RRC connection with the network. CCCH would be used by the UEs when accessing a new cell or after cell reselection.
• Multicast Control Channel (MCCH): A point-to-multipoint downlink channel used for transmitting MBMS scheduling and control information from the network to the UE, for one or several MTCHs. After establishing an RRC connection this channel is only used by UEs that receive MBMS.
• Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection

Traffic Channels

• Dedicated Traffic Channel (DTCH): A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
• Multicast Traffic Channel (MTCH): A point-to-multipoint downlink channel for transmitting traffic data from the network to the UEs using MBMS.

TRANSPORT CHANNELS

An effort has been made to keep a low number of transport channels in order to avoid unnecessary switches between different channel types, which are found to be time consuming in UMTS. In fact there is currently only one transport channel in downlink and one in uplink carrying user data, i.e., channel switching is not
needed. For LTE, the following transport channels are provided by the physical layer:

Downlink:

• Broadcast Channel (BCH): A low fixed bit rate channel broadcast in the entire coverage area of the cell. Beam-forming is not applied.
• Downlink Shared Channel (DL-SCH): A channel with possibility to use HARQ and link adaptation by varying the modulation, coding and transmit power. The channel is possible to broadcast in the entire cell and beamforming may be applied. UE power saving (DRX) is supported to reduce the UE power consumption. MBMS transmission is also supported.
• Paging Channel (PCH): A channel that is broadcasted in the entire cell. DRX is supported to enable power saving.
• Multicast channel (MCH): A separate transport channel for multicast (MBMS). This channel is broadcast in the entire coverage area of the cell. Combining of MBMS transmissions from multiple cells (MBSFN) is supported.

Uplink:

• Uplink Shared channel (UL-SCH): A channel with possibility to use HARQ and link adaptation by varying the transmit power, modulation and coding. Beamforming may be applied.

• Random Access Channel (RACH): A channel used to obtain timing synchronization (asynchronous random access) and to transmit information needed to obtain scheduling grants (synchronous random access). The transmission is typically contention based. For UEs having an RRC connection there is some limited support for contention free access.

LTE CHANNEL STRUCTURE AND MAPPING

PHYSICAL CHANNELS

The physical layer offers services to the MAC layer in the form of transport channels. User data to be  transmitted is delivered to the physical layer from the MAC layer in the form of transport blocks. 

The MAC  layer at the transmitter side also provides the physical layer with control information necessary for  transmission and/or reception of the user data. The physical layer defines physical channels and physical signals. A physical channel corresponds to a set of physical resources used for transmission of data and/or control information from the MAC layer. A physical signal, which also corresponds to a set of physical

resources, is used to support physical-layer functionality but do not carry any information from the MAC layer.

Physical channels

•  Physical Downlink Shared Channel (PDSCH) – transmission of the DL-SCH transport channel

•  Physical Uplink Shared Channel (PUSCH) – transmission of the UL-SCH transport channel

•  Physical Control Format Indicator Channel (PCFICH) – indicates the PDCCH format in DL

•  Physical Downlink Control Channel (PDCCH) – DL L1/L2 control signaling

•  Physical Uplink Control Channel (PUCCH) – UL L1/L2 control signaling

•  Physical Hybrid ARQ Indicator Channel (PHICH) – DL HARQ info

•  Physical Broadcast Channel (PBCH) – DL transmission of the BCH transport channel.

•  Physical Multicast Channel (PMCH) – DL transmission of the MCH transport channel.

•  Physical Random Access Channel (PRACH) – UL transmission of the random access preamble as given by the RACH transport channel.

Physical signals

•  Reference Signals (RS) – support measurements and coherent demodulation in uplink and downlink.

•  Primary and Secondary Synchronization signals (P-SCH and S-SCH) – DL only and used in the cell search procedure.

•  Sounding Reference Signal (SRS) – supports UL scheduling measurements

LTE AIR INTERFACE - PHYSICAL LAYER

The LTE physical layer is based on Orthogonal Frequency Division Multiplexing scheme OFDM to meet the targets of high data rate and improved spectral efficiency. The spectral resources are allocated/used as a combination of both time (aka slot) and frequency units (aka subcarrier). MIMO options with 2 or 4 Antennas is supported. Multi-user MIMO is supported in both UL and DL. It also depends upon UE capabilites.The modulation schemes supported in the downlink and uplink are QPSK, 16QAM and 64QAM.

Downlink (DL) Physical Channel

The downlink transmission uses the OFDM with cyclic prefix . Some of the reasons for using OFDM are given below:

• Multiple carrier modulation (MCM) helps in countering the frequency selective fading as the channel appears to have nearly flat frequency response for the narrow band subcarrier.
• The frequency range of the resource block and the number of resource blocks can be changed (or adapted to the channel condition) allowing flexible spectrum allocation.
• Higher peak data rates can be achieved by using multiple resource blocks and not by reducing the symbol duration or using still higher order modulation thereby reducing the receiver complexity.
• The multiple orthogonal subcarriers inherently provides higher spectral efficiency.
• The cyclic prefix (CP) is the partial repetition of the bit/symbol sequence from the end to the beginning. This makes the time domain input sequence to appear periodic over a duration so that the DFT representation is possible for any frequency domain processing. Also the duration if chosen larger than the channel delay spread, will help in reducing the inter-symbol interference.

The following pilot signals are defined for the downlink physical layer:

• Reference signal: The reference signal consists of known symbols transmitted at a well defined OFDM symbol position in the slot. This assists the receiver at the user terminal in estimating the channel impulse response so that channel distortion in the received signal can be compensated for. There is one reference signal transmitted per downlink antenna port and an exclusive symbol position is assigned for an antenna port (when one antenna port transmits a reference signal other ports are silent).
• Synchronization signal: Primary and secondary synchronization signals are transmitted at a fixed subframes (first and sixth) position in a frame and assists in the cell search and synchronization process at the user terminal. Each cell is assigned unique Primary sync signal.

Uplink (UL) Physical Channel

The uplink transmission uses the SC-FDMA (Single Carrrier FDMA) scheme. The SC-FDMA scheme is realized as a two stage process where the first stage transforms the input signal to frequency domain (represented by DFT coefficients) and the second stage converts these DFT coefficients to an OFDM signal using the OFDM scheme. Because of this association with OFDM, the SC-FDMA is also called as DFT-Spread OFDM. The reasons (in addition to those applicable for OFDM for downlink) for this choice are given below:

• The two stage process allows selection of appropriate frequency range for the subcarriers while mapping the set of DFT coefficients to the Resource Blocks. Unique frequency can be allocated to different users at any given time so that there is no co-channel interference between users in the same cell. Also channels with significant co-channel interference can be avoided.
• The transformation is equivalent to shift in the center frequency of the single carrier input signal. The subcarriers do not combine in random phases to cause large variation in the instantaneous power of the modulated signal. This means lower PAPR (Peak to Average Power Ratio).
• The PAPR (Peak to Average Power Ratio) of SC-FDMA is lesser than that of the conventional OFDMA, so the RF power amplifier (PA) can be operated at a point nearer to recommended operating point. This increases the efficiency of a PA thereby reducing the power consumption at the user terminal.

The following pilot signals are defined for the uplink physical layer:

• Demodulation Reference signal: This signal send by the user terminal along with the uplink transmission, assists the network in estimating the channel impulse response for the uplink bursts so as to effectively demodulate the uplink channel.
• Sounding Reference Signal: This signal is sent by the user terminal assists the network in estimating the overall channel conditions and to allocate appropriate frequency resources for uplink transmission.

UE IDENTIFIERS IN LTE

Below are some important UE identifiers used in LTE.

 

Cell Radio Network Temporary Identifier (C-RNTI)

The Cell Radio Network Temporary Identifier (C-RNTI) is used over the LTE air interface.

-Uniquely identifies the UE within a certain cell.
-Exists only when the UE is in the ECM-CONNECTED state.
-Allocated by eNodeB

Temporary Mobile Subscriber Identity (S-TMSI)

-Uniquely identifies the UE within a certain tracking area.
-Used over air interface when the UE is in the ECM-IDLE state.
-Allocated by MME

Globally Unique Temporary Identity (GUTI)

-Considered an extended version of the S-TMSI
-Uniquely identifies both the UE within a certain tracking area and the MME handling the UE.

International Mobile Subscriber Identity (IMSI)

-Uniquely identifies the UE anywhere in the world
-Used carefully over air interface. Not transmitted unencrypted over the air interface if not absolutely necessary
-Allocated by home Network operator

International Mobile Equipment Identity (IMEI)

Uniquely identifies the teminal equipment hardware globally.
This number can be used by the network to stop a stolen phone from accessing the network.

LTE NETWORK TOPOLOGY AND ARCHITECTURE

 
There is a new approach in the inter-connection between radio access network and core network. The EPS architecture is made up of an EPC (Packet Core Network, also referred as EPC) and an eUTRAN Radio Access Network (also referred as LTE)

There are no circuit switched components in LTE/EPC . 

The CN provides access to external packet IP networks and performs a number of CN related functions (e.g. QoS, security, mobility and terminal context management) for idle (camped) and active terminals. The RAN performs all radio interface related functions.

The LTE/EPC radio access network - Evolved UTRAN (E-UTRAN) - will only contain Node Bs. No RNC is provided anymore. This means, that the evolved Node Bs take over the radio management functionality.

This will make radio management faster and the network architecture simpler. E-UTRAN exclusively uses IP as transport layer. Behind the EPC follow one or more IP networks.
Evolved Packet Core:
- Also known as the System Architecture Evolution (SAE) core;
- Simplified , all-IP network architecture;
- Supports higher throughput and lower latency;
- Supports mobility between legacy 3GPP-based systems, but also non-3GPP systems like WiMAX and CDMA2000.

Mobility Management Entity – MME:
- Controls the signaling between the UE and the core network;
- Establishment, maintenance and release of radio bearer services;
- Responsible for paging and tracking the UE between calls and selecting of proper S-GW upon connection;
- Acts the termination point for ciphering protection, and therefore is the point of lawful interception of signaling.

LTE NETWORK ARCHITECURE

Serving Gateway – SGW:
- Routes data packets, maintains the data connection for inter-eNodeB handovers, as well as, inter-system handovers between LTE and GSM/UMTS;
- Store UE contexts, for example bearer service parameters and routing information;
- Is the main junction between the radio access network and the core network.

Packet Switched Data Network Gateway – PGW:
- Provides connectivity for the UE to exernal packet data networks;
- Allocates IP addresses for UE and QOS enforcement;
- Maintains the mobility connection between LTE/UMTS/GSM systems and non-3GPP systems like WiMAX and CDMA2000.

Enhanced UTRAN – eUTRAN:
- Simply a collection of eNodeBs networked together;
- Responsible for radio resource management, header compression, security, and connectivity to the evolved packet core.

Enhanced NodeB – eNodeB:
- Contains the radio and antenna equipment to link the UE and the LTE core network via the RF air interface;
- Practical equivalent to the BTS in GSM and the NodeB in UMTS, however functionality is more robust in LTE;
- Radio controller functionality now resides in the eNodeB resulting in a more efficient, less latent network. For example, mobility is governed by the eNodeB instead of the BSC or RNC.

Home Subscriber Service – HSS:
- database similar to the HLR in GSM / WCDMA core network that contains subscriber-related information supporting call control and session management;
- Primarily involved in authentication, authorization, security ciphering, and can provide user locations details.

Policy Control and Charging Rules Functions – PCRF:
- Responsible for policy control decision making;
- Provides the QOS authorization to decide how data will be treated with respect to the user’s subscription.

Serving GPRS Support Node – SGSN:
- Interconnects the LTE, UMTS, and GSM networks for increased mobility.

EUTRA CELL RESELECTION

EPS Mobility management (EMM) comprises the functions and procedures that maintain the connectivity between UE and EPS as UE moves between the coverage areas of different base station or access networks.

When registered to the EPS network UE can move from one cell to another cell, from one registered area to other or even leaves the EPS network and return back in other time.

UE performs Cell Reselection in ECM_IDLE state in order to ensure that it is camped on the best available cell.

Cell Reselection procedure : SIB-3

SIB-3 contains the information related to cell reselection procedure. The parameters directly related to procedure are:
1. Qhyst = hysteresis value for ranking criteria (0 to 24 dbm).
2. Treselection EUTRA = Cell reselection timer value for EUTRAN (0....7s)

While others are related to measurement ans evaluation process.

threshServingLow = The threshold for the serving frequency used in reselection evaluation towards towards lower priority EUTRAN frequencies o
r RAT.

q-RxLevMin = The minimum required Rx level in cell in dBm.
S-IntraSearch = the threshold (in dB) for intra frequency measurements. The UE can omit measurements of intra-frequency neighbour cells if the serving cell is above a certain threshold.


SIB-4 contains neighbour cell information.

Cell Reselection procedure- SIB-4

physCellId - Physical layer cell identity of the neighbour cells.
Qoffset = the offset value used for cell reselection process between serving and neighbour cells (-24..24 dB)


Now,

Rs=Qmeas,s + Qhyst,s          //For serving cell
Rn=Qmeas,n + Qhyst,n         //For neighbour cell

When Rn becomes greater than Rs cell reselection is completed.

Rn > Rs -> Cell Reselection.

LTE ECM/EMM AND RRC STATES

EMM and ECM states

There are two sets of states defined  for each UE based on the information held by the Mobility Management Entity.

EPS MOBILITY MANAGEMENT

The two EPS Mobility Management (EMM) states, EMM-REGISTERED and EMM-DEREGISTERED describe whether or not the UE is registered to MME and can be reached by paging.

In EMM-DEREGISTERED state the MME hold no valid location information for the UE. The UE is not reachable since its location is not known.
UE enters EMM-REGISTERED state either due to LTE attach procedure or due to a tracking area update (TAU) from IRAT.  In this state UE can be reached by paging.

EPS CONNECTION MANAGEMENT

The two EPS Connection Management (ECM) states ECM_IDLE and ECM-CONNECTED, describe the signalling connectivity between UE and Evolved Packet Core.

In ECM-IDLE state, there exist no signalling connection between UE and the MME.
In ECM_CONNECTED state there exist a signalling connection between UE and MME. This signalling connection consist of two parts:
an RRC connection between UE and eNodeB, and an S1 -MME connection between eNodeB and MME.

EMM and ECM states

EMM-DEREGISTERED & ECM-IDLE ⇒ RRC_IDLE:
- Mobility: PLMN selection.
- UE Position: not known by the network.


EMM-REGISTERED & ECM-IDLE ⇒ RRC_IDLE:
- Mobility: cell reselection.
- UE position: known by network at TA level.

EMM-REGISTERED & ECM-CONNECTED with RB established ⇒ RRC_CONNECTED.
- Mobility: handover.
- UE Position: known by the network at cell level

Tune in For more.. :)

LTE TRACKING AREA UPDATE PROCEDURE

Tracking Area Update-LTE

The network knows the location of a UE that is roaming within a network. This makes it possible for the mobile subscriber to receive a call wherever it is. To keep the network up to date with the subscriber’s location, the UE performs location updating.

Tracking Area Update

The location of a UE in LTE_IDLE mode is maintained on a Tracking Area (TA) List level. In LTE there is only one common Intra-LTE TA concept defined both for the RAN and for the CN. When a UE in LTE_IDLE mode moves into a cell that belongs to a TA different from the one(s) it is currently registered with, it performs a TA Update. The cell tracking area is broadcast in System Information.

Traffic Case Tracking Area Update

1. The UE performs cell re-selection (typically based on system information received in the previous cell).

2. The UE reads the System Information, broadcast in the selected cell.

4. If the TA that the cell belongs is not the TA that the UE is registered in, the RRC Connection is established, with the NAS message “Tracking Area Update (TAU) Request”.

5. The Tracking Area Update (TAU) Request message is provided to the MME in the “Initial UE Message”.

6. Conditional step:

If the integrity protection check fails the MME performs authentication and activates the NAS security functions using the Authentication and NAS Security Activation procedure.

7. The MME sends the NAS message “Tracking Area Update (TAU) Accept” to the UE, using the DL NAS Signalling Transfer procedure.

8. If the GUTI has been re-allocated, the NAS message “Tracking Area Update (TAU) Complete” is transferred to the MME, using the UL NAS Signalling Transfer procedure

9. Since the MME did not establish any bearers or user plane connection the MME releases the radio connection (andRAN resources) by the MME-initiated Connection Release.