LTE Channels RA41232EN03GLA2 1 LTE Channels RA41232EN03GLA2 2 LTE Channels RA41232EN03GLA2 3 LTE Channels RA41232EN03GLA2 4 LTE Channels For the Uu (air) interface, LTE divides the Data Link Layer into the following sublayers: • Radio Resource Control (RRC) • Packet Data Convergence Protocol (PDCP) • Radio Link Control (RLC) • Medium Access Control (MAC) As illustrated in the graphic, control traffic and bearer traffic use different protocol stacks. RA41232EN03GLA2 5 LTE Channels The LTE Channel Architecture defines E-RAB channels, Radio Bearer (RB) channels, Signaling Radio Bearer (SRB) channels, Logical Channels, Transport Channels, and Physical Channels. In general, each category behaves as a service access point between adjacent protocol layers. 3GPP TS 36.211 Physical Channel and Modulation 3GPP TS 36.321 Medium Access Control (MAC) Protocol Specification 3GPP TS 36.322 Radio Link Control (RLC) Protocol Specification RA41232EN03GLA2 6 LTE Channels E-RAB - An E-RAB channel carries one or more service data flows between a UE and the EPC. Radio Bearer - A Radio Bearer channel transports the data packets of an E-RAB from the eNodeB toward the UE. Each E-RAB has a one-to-one mapping with a radio bearer. Signaling Radio Bearer - A Signaling Radio Bearer (SRB) channel transports signaling packets between the RRC Sublayer and the PDCP Sublayer. Logical Channel - A Logical Channel transports control or data traffic between the RLC Sublayer and the MAC Sublayer. Logical control channels are mapped to signaling radio bearer channels, while logical traffic channels are mapped to radio bearer channels. Logical Channels describe transmission reliability (RLC Acknowledged Mode, etc.). Transport Channel - A Transport Channel forwards control or data traffic between the MAC Sublayer and the Physical Layer. Each Logical Channel is mapped to a transport channel. Transport Channels describe how the information will be formatted before being transmitted (coding, transport block size, etc.). Physical Channel - A Physical Channel provides the transmission media (resource elements) through which the information is actually transmitted. Each Transport Channel is mapped to a physical channel. RA41232EN03GLA2 7 LTE Channels RA41232EN03GLA2 8 LTE Channels Logical Channels provide control and data transport between the RLC and MAC Sublayers. Signaling traffic is carried by control channels (xCCH), and data traffic is carried by traffic channels (xTCH). Control channels are mapped to SRB channels, and traffic channels are mapped to user plane radio bearer channels. RA41232EN03GLA2 9 LTE Channels Control Channels Broadcast Control Channel (BCCH) – DL channel used to broadcast system information. Paging Control Channel (PCCH) – DL channel used to carry paging information when the network does not know the location of the UE. Common Control Channel (CCCH) – Carries RRC signaling when no RRC connection currently exists for the UE. Dedicated Control Channel (DCCH) – A bidirectional control channel used to carry signaling information when an RRC connection exists for the UE. Multicast Control Channel (MCCH) – Carries multicast signaling information; it controls the operation of the MTCH channel Traffic Channels Dedicated Traffic Channel (DTCH) – A point-to-point channel dedicated to one UE for transmission of user data. The DTCH may be uplink, downlink, or both. Multicast Traffic Channel (MTCH) – A DL channel used to carry multicast data traffic. RA41232EN03GLA2 10 LTE Channels RA41232EN03GLA2 11 LTE Channels Downlink Transport Channels Broadcast Channel (BCH) – Forwards broadcast information to the entire cell. The BCH maps to the BCCH Logical Channel. Paging Channel (PCH) – Forwards UE paging information to the entire cell. The PCH maps to the PCCH Logical Channel. Downlink Shared Channel (DL-SCH) – Carries DL data and some control traffic. For data traffic, DL-SCH supports HARQ and dynamic link adaptation. The DL-SCH maps to the DCCH, CCCH, and DTCH Logical Channels. Multicast Channel (MCH) – Carries multicast traffic for the entire cell. The MCH maps to the MCCH and MTCH Logical Channels Uplink Transport Channels Uplink Shared Channel (UL-SCH) – Carries UL data and some control traffic. The ULSCH maps to the DCCH, CCCH, and DTCH Logical Channels. Random Access Channel (RACH) – Used for initial access to the cell or when a UE needs to transmit on the PUSCH or PUCCH and does not have a valid uplink grant. The RACH is not mapped to a Logical Channel. RA41232EN03GLA2 12 LTE Channels The graphic shows the mapping between the LTE Logical Channels and Transport Channels. RA41232EN03GLA2 13 LTE Channels RA41232EN03GLA2 14 LTE Channels Physical Broadcast Channel (PBCH) – DL channel that carries system information (broadcast) traffic. The PBCH uses QPSK encoding. Physical Hybrid ARQ Indicator Channel (PHICH) – Carries Hybrid ARQ (HARQ) ACKs or NACKs for the UL transmissions on the PUSCH. The PHICH uses BPSK encoding. Physical Control Format Indicator Channel (PCFICH) – Transmitted every subframe to inform the UE about the number of OFDM symbols used for the PDCCH channel. The PCFICH uses QPSK encoding. Physical Downlink Control Channel (PDCCH) – Informs the UE about the resource allocation for PCH and DL-SCH, plus the HARQ information relating to the DL-SCH. It also controls the UL-SCH scheduling grants and indicates the UE identity. The PDCCH has four formats, hence the need for the PCFICH. The PDCCH signaling is located in the first 1–3 OFDM symbols in each subframe. The PDCCH uses QPSK encoding. Physical Downlink Shared Channel (PDSCH) – Carries downlink data and higher layer signaling. The PDSCH is allocated to different UEs periodically, usually every 1 ms. PDSCH channel coding, modulation, and subcarrier allocation is dynamically controlled by the PDCCH. The PDSCH may use QPSK, 16QAM, or 64QAM encoding. Physical Multicast Channel (PMCH) – Carries the MBMS data and control if the cell supports MBMS functionality. The PMCH may use QPSK, 16QAM, or 64QAM encoding. RA41232EN03GLA2 15 LTE Channels Physical Random Access Channel (PRACH) – Carries random access preambles used when the UE makes initial contact with the network, etc. Physical Uplink Shared Channel (PUSCH) – Carries uplink data and higher layer signaling. PUSCH is a shared channel allocated to different UEs periodically, usually every 1 ms. The channel coding, modulation, and subcarrier allocation is dynamically controlled by the PDCCH. The PUSCH may use QPSK, 16QAM, or 64QAM encoding. Physical Uplink Control Channel (PUCCH) – Carries uplink control information for a UE, including CQI, HARQ ACKs and NACKs, and UL scheduling requests. Depending on format, the PUCCH may use BPSK or QPSK encoding RA41232EN03GLA2 16 LTE Channels The graphic shows the mapping between LTE Transport Channels and Physical Channels. The PDCCH, PCFICH, PHICH, and PUCCH Physical Channels are not mapped to Transport Channels. RA41232EN03GLA2 17 LTE Channels RA41232EN03GLA2 18 LTE Channels The PDSCH carries DL data packets and some control packets. DL traffic is mapped into resource blocks; each DL allocation is described in the PDCCH. The PDSCH may use QPSK, 16QAM or 64QAM modulation. The MAC Sublayer in the eNodeB is responsible for completely filling the DL allocation. If necessary, padding is added to completely fill the allocated resource blocks. During a subframe, several different UEs may share the PDSCH. Using the PDCCH control channel, the eNodeB will assign one or more resource blocks to each UE. RA41232EN03GLA2 19 LTE Channels DL control channels are carried in the first (even) slot of each subframe of a Transmission Time Interval (TTI). DL control channels consist of some number of Resource Element Groups (REGs); a REG consists of 4 Resource Elements. The four resource elements in a REG are consecutive when not used for other purposes, such as Reference Signals. Nine REGs form a Control Channel Element (CCE). RA41232EN03GLA2 20 LTE Channels PCFICH Channel The PCFICH channel is 4 REGs (16 resource elements) long and is located in the 1st symbol. The PCFICH channel indicates how many (1-3) symbols are used by the DL control channels in this subframe. The PCFICH REGs are distributed evenly across the system bandwidth (occupied subcarriers). The exact location of the PCFICH REGs is calculated by the UE based on the physical cell identity. PHICH Channel The PHICH channel is 3 REGs (12 resource elements) long and is located in the 1st or 3rd symbol. This channel carries the HARQ ACKs and NACKs for packets sent by a specific UE on the uplink. The PHICH REGs are distributed evenly across the system bandwidth (occupied subcarriers). The resources used for the PHICH are configured on a semi-static basis. For FDD, the PHICH ACK/NACK is located 4 subframes after the UL subframe which contains the MAC PDU. RA41232EN03GLA2 21 LTE Channels The PDCCH channel occupies the remaining resource elements in the 1st-3rd symbols (as signaled by the PCFICH channel). The PDCCH channel describes DL traffic allocations for this subframe and future UL bandwidth grants. Multiple PDCCH control channels are supported and a UE monitors a set (aggregation) of control channels. The PDCCH can be transmitted with 4 different formats. PDCCH describes resource allocation characteristics such as resource block group size and localized or distributed RBs using a Downlink Control Information (DCI) format. PDCCH DCI main formats are numbered 0-3, with minor variations labelled 1A, 1B, 1C, etc. 3GPP TS 36.211 Physical Channel and Modulation RA41232EN03GLA2 22 LTE Channels For each DL allocation, the PDCCH contains the UE C-RNTI, Transport Format, DL resource allocation in resource blocks, and a Modulation and Coding Scheme (MCS) index. In addition it contains UL grants, Transport Format and Transmit Power Commands (TPC) for UL transmissions on the PUSCH or PUCCH. C-RNTI – uniquely identifies the UE within the cell. Transport Format –service-specific specifies the physical layer processing, such as channel coding and interleaving, and any rate matching. Starting PRB – identifies the starting physical Resource Block number for the DL or UL allocation. Number of PRBs – identifies the total number of physical Resource Blocks in the allocation. MCS Index – identifies the modulation and coding scheme used for the allocation. Transmit Power Command – instructs the UE to adjust its power level for the UL transmission. Depending on the PDCCH format, the UE power may be adjusted from –1 dB to +3 dB. 3GPP TS 36.213 Physical Layer Procedures RA41232EN03GLA2 23 LTE Channels 3GPP TS 36.213 Physical Layer Procedures RA41232EN03GLA2 24 LTE Channels Using the TBS index (left column) and the number of Physical Resource Blocks allocated, the UE calculates the size of the Transport Block. This table shows only a few columns of the actual values. The full table in TS 36.213 has 110 NPRB columns. RA41232EN03GLA2 25 LTE Channels Discontinuous Reception (DRX) LTE supports DRX to enable UE power savings by turning off some or all of its radio circuitry, thereby increasing the battery lifetime of the UE. The DRX function is configured and controlled by the network. The UE behaviour is based on a set of rules that define when the UE must monitor the PDCCH for scheduling assignments. When the UE does not have an established RRC connection, that is, no radio bearers configured for data transmission, it wakes up and monitors the paging channel every DRX cycle. When the UE has an RRC connection, the DRX function is characterized by a DRX cycle(s), an on-duration period(s), and an inactivity timer. Using RRC signaling, the eNodeB may configure the UE with a Discontinuous Reception (DRX) cycle that allows it to monitor the PDCCH in a semi-periodic manner (as opposed to every 1ms TTI). The DRX operation is governed by a Long DRX cycle, a DRX Inactivity Timer, a DRX Retransmission Timer and, optionally, a Short DRX cycle and a DRX Short Cycle Timer. When DRX is configured, the UE 'wakes up' at the beginning of the DRX cycle and monitors the PDCCH for a configured number of TTIs. This period is called the OnDuration. If no assignment is detected on the PDCCH, the UE goes back to 'sleep' until the next On-Duration. The Long or Short DRX cycle length sets the periodicity of the On-Duration. RA41232EN03GLA2 26 LTE Channels Discontinuous Reception (DRX) cont… If, during the On-Duration, an uplink or downlink assignment is detected on the PDCCH the UE stays awake and starts the DRX Inactivity Timer. Any subsequent PDCCH assignment for an initial transmission (i.e. not for a retransmission) will reset the timer. The UE re-enters DRX when the Inactivity Timer expires. The UE always wakes up to read DL ACK/NACKs on the PHICH for each of its UL transmissions on the PUSCH. DL HARQ operation is also independent of DRX operation, with the exception of the DRX Retransmission Timer. This timer sets how many TTIs the UE stays awake when a retransmission is expected (i.e. the UE has sent an uplink NACK). RA41232EN03GLA2 27 LTE Channels The Physical Broadcast Channel (PBCH) broadcasts RRC System Information Master Information Block (MIB) messages. This information is critical for user devices attempting to enter or re-enter thenetwork. The MIB contains: • DL system bandwidth • Number of eNodeB transmit antennas • Reference Signal transmit power • System frame number For Frame Type 1, the PBCH is located on the 72 subcarriers centred around the DC subcarrier in slot 1, symbols 0 through 3. The PBCH information is spread over four consecutive LTE radio frames (40 ms Transmit Time Interval). 3GPP TS 36.211 Physical Channel and Modulation RA41232EN03GLA2 28 LTE Channels SIB4-SIB8 messages carry cell reselection parameters for EUTRAN and other Radio Access Technology (RAT) neighbours, such as GSM, UTRA, and cdma2000 3GPP TS 36.331 RRC Protocol Specification RA41232EN03GLA2 29 LTE Channels RA41232EN03GLA2 30 LTE Channels Mapping PUSCH to Subframes The graphic illustrates the mapping of the PUSCH channel to subframes. Resources for the PUSCH are allocated on a subframe basis by the eNodeB (in the PDCCH). Subcarriers are allocated in physical resource blocks and may be frequency hopped from subframe to subframe. The PUSCH may use QPSK, 16QAM or 64QAM modulation. The MAC Sublayer in the UE is responsible for completely filling the UL grant. If necessary, padding is added to completely fill the allocated resource blocks. 3GPP TS 36.211 Physical Channels and Modulation RA41232EN03GLA2 31 LTE Channels Mapping PUCCH to Subframes The PUCCH carries uplink control information such as CQI, Scheduling Requests, and ACKs/NACK for a specific UE. It is never transmitted simultaneously with the PUSCH. As shown in the graphic, the PUCCH transmission is frequency hopped at the slot boundary for added reliability. More than one PUCCH channel may be present in a subframe. 3GPP TS 36.211 Physical Channels and Modulation RA41232EN03GLA2 32 LTE Channels RA41232EN03GLA2 33 LTE Channels For PUCCH Format 1, information is carried by the presence or absence of any PUCCH transmission from the UE. For all other formats, the UE explicitly transmits bits. RA41232EN03GLA2 34 LTE Channels The Random Access Channel (RACH/PRACH) is an UL contention-based channel which allows any UE to request network entry, access a target cell after handover, access a cell to send a Scheduling Request, and so on. The UE uses the PRACH channel to send a Random Access Preamble. Mapping UL Physical Channels to Subframes Random Access Preambles are transmitted on blocks of 72 contiguous 1.25 kHz subcarriers allocated for the Physical Random Access Channel (PRACH) by the eNodeB. For burst formats 0-3, the PRACH Configuration Index describes the burst format and subframe location within an LTE radio frame type 3GPP TS 36.211 Physical Channels and Modulation RA41232EN03GLA2 35 LTE Channels RA41232EN03GLA2 36 LTE Channels RA41232EN03GLA2 37
