The radio interface described in this specification covers the interface between the User Equipment (UE) and the network, and sidelink transmissions between UEs. The radio interface is composed of the Layer 1, 2 and 3. The TS 36.200 series describes the Layer 1 (Physical Layer) specifications. Layers 2 and 3 are described in the 36.300 series.

Figure 1 shows the E-UTRA radio interface protocol architecture around the physical layer (Layer 1). The physical layer interfaces the Medium Access Control (MAC) sub-layer of Layer 2 and the Radio Resource Control (RRC) Layer of Layer 3. The circles between different layer/sub-layers indicate Service Access Points (SAPs). The physical layer offers a transport channel to MAC. The transport channel is characterized by how the information is transferred over the radio interface. MAC offers different logical channels to the Radio Link Control (RLC) sub-layer of Layer 2. A logical channel is characterized by the type of information transferred.

The physical layer offers data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service:

• Error detection on the transport channel and indication to higher layers
• FEC encoding/decoding of the transport channel
• Hybrid ARQ soft-combining
• Rate matching of the coded transport channel to physical channels
• Mapping of the coded transport channel onto physical channels
• Power weighting of physical channels
• Modulation and demodulation of physical channels
• Frequency and time synchronisation
• Radio characteristics measurements and indication to higher layers
• Multiple Input Multiple Output (MIMO) antenna processing
• Transmit Diversity (TX diversity)
• Beamforming
• RF processing. (Note: RF processing aspects are specified in the TS 36.100 series)

The multiple access scheme for the LTE physical layer is based on Orthogonal Frequency Division Multiplexing (OFDM) with a cyclic prefix (CP) in the downlink, and on Single-Carrier Frequency Division Multiple Access (SC-FDMA) with a cyclic prefix in the uplink and sidelink. To support transmission in paired and unpaired spectrum, two duplex modes are supported: Frequency Division Duplex (FDD), supporting full duplex and half duplex operation, and Time Division Duplex (TDD). The Layer 1 is defined in a bandwidth agnostic way based on resource blocks, allowing the LTE Layer 1 to adapt to various spectrum allocations. A resource block spans either 12 sub-carriers with a sub-carrier bandwidth of 15kHz or 24 sub-carriers with a sub-carrier bandwidth of 7.5kHz or 72 sub-carriers with a sub-carrier bandwidth of 2.5kHz, each over a slot duration of 0.5ms, or 144 sub-carriers with a sub-carrier bandwidth of 1.25kHz over a slot duration of 1ms, or 486 sub-carriers with a sub-carrier bandwidth of approximately 0.37kHz over a slot duration of 3ms. Narrowband operation is also defined, whereby certain UEs may operate using a maximum transmission and reception bandwidth of 6 contiguous resource blocks within the total system bandwidth; for narrowband operation, sub-resource-block operation may also be used in the uplink, using 2, 3 or 6 sub-carriers. For Narrowband Internet of Things (NB-IoT) operation, a UE operates in the downlink using 12 sub-carriers with a sub-carrier bandwidth of 15kHz, and in the uplink using a single sub-carrier with a sub-carrier bandwidth of either 3.75kHz or 15kHz or alternatively 3, 6 or 12 sub-carriers with a sub-carrier bandwidth of 15kHz. The radio frame structure type 1 is only applicable to FDD (for both full duplex and half duplex operation) and, for sub-carrier bandwidths other than 1.25kHz and approximately 0.37kHz, has a duration of 10ms and consists of 20 slots with a slot duration of 0.5ms. Two adjacent slots form one sub-frame of length 1ms, except when the sub-carrier bandwidth is 1.25kHz or approximately 0.37kHz, in which cases one slot forms one sub-frame or has a time duration of 3ms, respectively. When the sub-carrier bandwidth is 15kHz, a slot can be further subdivided into three subslots of length 2 or 3 OFDM or SC-FDMA symbols for reduced latency operation. The radio frame structure type 2 is only applicable to TDD and consists of two half-frames with a duration of 5ms each and containing each either 10 slots of length 0.5ms, or 8 slots of length 0.5ms and three special fields (DwPTS, GP and UpPTS) which have configurable individual lengths and a total length of 1ms. A subframe consists of two adjacent slots, except for subframes which consist of DwPTS, GP and UpPTS, namely subframe 1 and, in some configurations, subframe 6. Both 5ms and 10ms downlink-to-uplink switch-point periodicity are supported. Further details on the LTE frame structure are specified in [2]. Adaptation of the uplink-downlink subframe configuration via Layer 1 signalling is supported. The radio frame structure type 3 is only applicable to LAA secondary cell operation. It has a duration of 10ms and consists of 20 slots with a slot duration of 0.5ms. Two adjacent slots form one subframe of length 1ms. Any subframe may be available for downlink or uplink transmission. For downlink transmission, the eNB shall perform the channel access procedures as specified in [4] prior to transmitting. A downlink or uplink transmission may start at the subframe boundary or later, and may end at the subframe boundary or earlier. For uplink transmission, the UE shall perform the channel access procedures as specified in [4] prior to transmitting. To support a Multimedia Broadcast and Multicast Service (MBMS), LTE offers the possibility to transmit Multicast/Broadcast over a Single Frequency Network (MBSFN), where a time-synchronized common waveform is transmitted from multiple cells for a given duration. MBSFN transmission enables highly efficient MBMS, allowing for over-the-air combining of multi-cell transmissions in the UE, where the cyclic prefix is utilized to cover the difference in the propagation delays, which makes the MBSFN transmission appear to the UE as a transmission from a single large cell. Transmission on a dedicated carrier for MBSFN is supported, as well as transmission of MBSFN on a mixed carrier with both MBMS transmissions and point-to-point transmissions using time division multiplexing. In addition to the 15kHz sub-carrier bandwidth, the sub-carrier bandwidth of 7.5kHz with a longer CP, the sub-carrier bandwidth of 2.5kHz with a long CP (100μs), the sub-carrier bandwidth of 1.25kHz with a very long CP (200μs), and the sub-carrier bandwidth of approximately 0.37kHz with a very long CP (300μs) are all supported on both dedicated and mixed MBSFN carriers. Transmission of PDSCH also in MBSFN subframes that are not used for MCH is supported on mixed MBSFN carriers. Transmission with multiple input and multiple output antennas (MIMO) are supported with configurations in the downlink with up to 32 transmit antenna ports and eight receive antennas, which allow for multi-layer downlink transmissions with up to eight streams and beamforming in both horizontal and vertical dimensions. Multi-layer uplink transmissions with up to four streams are supported with configurations in the uplink with up to four transmit antenna ports and four receive antennas. Multi-user MIMO, i.e. allocation of different streams to different users is supported in both UL and DL. Coordinated Multi-Point (CoMP) transmission and reception are supported, including the possibility to configure a UE with multiple Channel State Information (CSI) feedback processes. Aggregation of multiple cells is supported in the uplink and downlink with up to 32 serving cells, where each serving cell can use a transmission bandwidth of up to 110 resource blocks and can operate with either frame structure type 1 or frame structure type 2. Dual connectivity to groups of serving cells that belong to two different eNode-Bs is also supported. Sidelink transmissions are defined for ProSe Direct Discovery and ProSe Direct Communication between UEs. The sidelink transmissions use the same frame structure as uplink and downlink when the UEs are in network coverage; however, the sidelink transmissions are restricted to a sub-set of the uplink resources. V2X communication between UEs is supported via sidelink transmissions or via the eNB.

The physical channels defined in the downlink are:

• the Physical Downlink Shared Channel (PDSCH),
• the Physical Multicast Channel (PMCH),
• the Physical Downlink Control Channel (PDCCH),
• the Enhanced Physical Downlink Control Channel (EPDCCH),
• the MTC Physical Downlink Control Channel (MPDCCH),
• the Relay Physical Downlink Control Channel (R-PDCCH),
• the Short Physical Downlink Control Channel (SPDCCH),
• the Physical Broadcast Channel (PBCH),
• the Physical Control Format Indicator Channel (PCFICH),
• the Physical Hybrid ARQ Indicator Channel (PHICH),
• the Narrowband Physical Broadcast Channel (NPBCH),
• the Narrowband Physical Downlink Control Channel (NPDCCH),
• and the Narrowband Physical Downlink Shared Channel (NPDSCH).

The physical channels defined in the uplink are:

• the Physical Random Access Channel (PRACH),
• the Physical Uplink Shared Channel (PUSCH),
• the Physical Uplink Control Channel (PUCCH),
• the Short Physical Uplink Control Channel (SPUCCH),
• the Narrowband Physical Random Access Channel (NPRACH),
• and the Narrowband Physical Uplink Shared Channel (NPUSCH).

The physical channels defined in the sidelink are:

• the Physical Sidelink Broadcast Channel (PSBCH),
• the Physical Sidelink Control Channel (PSCCH),
• the Physical Sidelink Discovery Channel (PSDCH),
• and the Physical Sidelink Shared Channel (PSSCH).

In addition, signals are defined as reference signals, primary and secondary synchronization signals, resynchronization signals, wake-up signals, and discovery signals. The modulation schemes supported are:

• in the uplink, depending on the type of operation, π/2 BPSK, π/4 QPSK, QPSK, 16QAM, 64QAM and 256QAM,
• in the downlink, QPSK, 16QAM, 64QAM, 256QAM and 1024QAM,
• in the sidelink, QPSK, 16QAM and 64QAM.

The channel coding scheme for transport blocks in LTE is Turbo Coding with a coding rate of R=1/3, two 8-state constituent encoders and a contention-free quadratic permutation polynomial (QPP) turbo code internal interleaver (except for downlink transport blocks in NB-IoT operation). Trellis termination is used for the turbo coding. Before the turbo coding, transport blocks are segmented into byte aligned segments with a maximum information block size of 6144 bits. Error detection is supported by the use of 24 bit CRC. Further channel coding schemes for BCH, control information and downlink transport blocks in NB-IoT operation are specified in [3].

There are several Physical layer procedures involved with LTE operation. Such procedures covered by the physical layer are;

• Cell search,
• Power control,
• Uplink synchronisation and Uplink timing control,
• Random access related procedures,
• HARQ related procedures,
• Relay related procedures,
• Sidelink related procedures,
• Channel Access procedures.

Through the control of physical layer resources in the frequency domain as well as in the time and power domains, implicit support of interference coordination is provided in LTE.

Radio characteristics are measured by the UE and the eNode-B and reported to higher layers in the network. These include, e.g. measurements for intra- and inter-frequency handover, inter RAT handover, timing measurements and measurements for RRM and in support for positioning. Measurements for inter-RAT handover are defined in support of handover to GSM, UTRA FDD, UTRA TDD, NR, CDMA2000 1x RTT, CDMA2000 HRPD and IEEE 802.11.