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.