LTE 개요 및 주요특징 작성일 : 2015. 3. 2. 소속 : ETRI/ 무선전송연구1실 작성자 : 고영조 연락처 : 2015. 3. 2. 소속 : ETRI/ 무선전송연구1실 작성자 : 고영조 연락처 : koyj@etri.re.kr

Outline LTE radio access Basic transmission and multiple access schemes OFDM and OFDMA SC-FDMA (DFT-S-OFDM) Physical layer (PHY) Uplink Uplink physical resources Uplink reference signals Uplink L1/L2 control signaling Downlink Downlink physical resources Downlink reference signals Downlink L1/L2 control signaling Medium Access Control layer (MAC) Downlink scheduling Uplink scheduling

OFDM Characteristics of OFDM (Orthogonal Frequency Division Multiplexing) Parallel transmission of data over multiple carriers A high data rate stream is divided into multiple streams with lower data rates and carried by multiple sub-carriers. The subcarriers are orthogonal to each other Different subcarriers modulated with different amplitudes and phases are orthogonal over the time period of symbol duration The OFDM signals can be generated by applying IFFT operation Guard time and cyclic extension is used to remove inter-symbol interference (ISI) and inter-carrier interference (ICI)

OFDM OFDM signal T T: Symbol duration fc: Carrier frequency , OFDM signal T: Symbol duration fc: Carrier frequency Ns: Number of subcarriers di: QAM symbol T Baseband OFDMA signal

OFDM Characteristics of OFDM The subcarriers are orthogonal to each other Different subcarriers modulated with different amplitudes and phases are orthogonal over the time period of symbol duration time frequency

OFDM Characteristics of OFDM OFDM signals can be generated by applying IFFT operation Equivalent baseband signal In discrete time, OFDM transmitter OFDM receiver

OFDM Characteristics of OFDM Transmission on multiple frequencies A high data rate stream is divided into multiple streams with lower data rates and carried by multiple sub-carriers. The subcarriers are orthogonal to each other Different subcarriers modulated with different amplitudes and phases are orthogonal over the time period of symbol duration OFDM signals can be generated by applying IFFT operation Guard time and cyclic extension is used to remove inter-symbol interference (ISI) and inter-carrier interference (ICI)

OFDM Characteristics of OFDM Guard time and cyclic extension are used to remove inter-symbol interference (ISI) and inter-carrier interference (ICI) ISI/ICI: Interference between an original signal and a delayed version of it Multipath Channel Tx Rx ISI & ICI without guard time f Tsub CP Insertion

OFDM Characteristics of OFDM Simple frequency-domain equalization Long OFDM symbol time with a cyclic prefix  Fading caused by multipath propagation can be considered as constant (flat) over an OFDM sub-carrier if the sub-carrier is sufficiently narrow-banded In OFDM, the equalizer only has to multiply each detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant complex number. Disadvantages Sensitive to frequency-offset Cause of frequency offset  Mismatch between Tx and Rx oscillator frequencies, and Doppler shift Frequency-offset causes inter-carrier interference Potentially high PAPR (Peak-to-Average Power Ratio) High PAPR reduces power utilization efficiency

OFDMA Application of OFDM to multiple access OFDMA: Orthogonal Frequency Division Multiple Access Different UE are served with different resources: orthogonal separation in the time-frequency domain

SC-FDMA (Single-Carrier FDMA) Serial transmission of data over single carrier. Cf. OFDM: Parallel transmission of data over multiple carriers DFT precoding applies before the sub-carrier mapping In comparison with OFDM, SC-FDMA is also called DFT-Spread OFDM. Low PAPR because of the single carrier transmitter structure. Attractive alternative to OFDMA, especially in the uplink, where UE can be benefited from low PAPR in terms of transmit power efficiency SC-FDMA transmitter

SC-FDMA (Single-Carrier FDMA) Localized transmission Distributed transmission SC-FDMA subcarrier mapping

SC-FDMA (Single-Carrier FDMA) SC-FDMA vs. OFDM OFDMA: Subcarrier-by-subcarrier detection SC-FDMA: Detection after IDFT  SINR of a modulation symbol is averaged over the whole transmission band OFDM SC-FDMA FFT

SC-FDMA vs. OFDM PAPR/CM (Cubic Metric) x[n]: time domain signal after IFT SC-FDMA has a lower PAPR than OFDM OFDM SC-FDMA vs. OFDM

LTE radio access

References “3G Evolution: HSPA and LTE for Mobile Broadband”, E. Dahlman, S. Parkvall, J. Skold, and P. Beming, Academic Press (2nd Ed, 2008). “LTE-The UMTS Long Term Evolution: from theory to practice”, S. Sesia, I. Toufik, and M. Baker, Jonh Wiley & Sons Ltd (2009) Physical layer specifications 3GPP TS 36.201: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer – General Description". 3GPP TS 36.211: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation". 3GPP TS 36.212: "Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding". 3GPP TS 36.213: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures". 3GPP TS 36.214: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer – Measurements“ 3GPP TS 36.300: “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2” MAC layer specification 3GPP TR 36.321, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”

Overview of LTE radio access Long Term Evolution (LTE) Evolution of 3G WCDMA Significantly improved performance in a wide range of spectrum allocations Data rates up to ~ 300 Mbps in 20 MHz bandwidth First step toward 4G (IMT-Advanced) LTE standardization activity started since RAN long term evolution workshop, Nov. 2~3, 2004, Toronto. First release of technical specifications: Release 8 (2008)

Basic transmission schemes Downlink OFDM Robust against multi-path channels Long OFDM symbol time with a cyclic prefix Simple frequency-domain equalization Frequency-domain scheduling: additional degree of freedom cf. HSPA: only time-domain scheduling Flexible transmission bandwidth Spectrum allocation with different sizes Broadcast/multicast transmission Uplink DFT-spread OFDM (DFT-S-OFDM, SC-FDMA) Single-carrier transmission Motivated by low PAPR (peak-to-average power ratio) Allows for higher average transmission power (Note) Obsession with the single-carrier property -> Leading to very complicated spec!!

Key features of LTE Rate control Channel-dependent scheduling in the time-frequency domain Scheduler can take into account channel variation in both time and frequency domains Downlink scheduling Based on channel-status report by UE (FDD) Uplink scheduling Based on UE sounding Inter-cell interference coordination Restricting the transmission power of certain parts of the spectrum in a cell Fractional frequency reuse Rate control Adaptive modulation and coding (rather than power control) Hybrid ARQ with soft-combining Multiple parallel H-ARQ processes Soft-combining Retransmission with incremental redundancy Multiple antennas 1, 2 and 4 Tx antennas in downlink (extended to include 8 Tx in LTE-A) 1 Tx antenna in uplink (extended to include 2 and 4 Tx in LTE-A) Multicast/broadcast support MBSFN

Key features of LTE Channel-dependent scheduling

Key features of LTE Rate control Rate control is more efficient than power control Full power transmission -> relatively efficient power utilization Link adaptation through AMC (Adaptive Modulation and Coding) High order modulation (16, 64 QAM) and high code rate for high SINR Low order modulation (QPSK) and low code rate for low SINR

Key features of LTE Hybrid ARQ with soft-combining

Key features of LTE Spectrum flexibility Flexibility in duplex arrangement Support FDD (paired spectrum) and TDD (unpaired spectrum) Commonality between FDD and TDD Support half-duplex FDD No simultaneous reception and transmission (Reception and transmission separated in frequency and time) Scalable transmission bandwidths 1 ~ 20 MHz Transmission bandwidths 1.4, 3, 5, 10, 15, 20 MHz

LTE-Advanced LTE-Advanced 3GPP RAT for IMT-Advanced (4G) LTE (3.9G) + alpha Including new features such as carrier aggregation, relaying, coordinated multipoint transmission, uplink MIMO etc. 3GPP standardization schedule

30bps/Hz in DL 15bps/Hz in UL LTE-Advanced Requirements ITU Requirement 3GPP Requirement Peak data rates 1Gbps 1Gbps in DL, 500Mbps in UL Bandwidth 40MHz (max. scalable BW) Multi-carrier allowed Up to 100MHz User plane latency 10ms Improved compared to LTE Control plane latency 100ms Active  Active dormant(<10ms) Camped  Active (<50ms) Peak spectrum efficiency 15bps/Hz in DL 6.75bps/Hz in UL 30bps/Hz in DL 15bps/Hz in UL Average spectrum efficiency Set for four scenarios and several antenna configurations Cell edge spectrum efficiency VoIP capacity Up to 200 UEs per 5MHz

LTE-Advanced Requirements ITU system performance requirement Environment Indoor Micro-cell Base coverage Urban Rural/High speed Spectrum Efficiency DL (4x2 MIMO) 3 2.6 2.2 1.1 UL (2x4 MIMO) 2.25 1.8 1.4 0.7 Cell Edge Spectrum Efficiency 0.1 0.075 0.06 0.04 0.07 0.05 0.03 0.015 Case-1 Config LTE Cell Avg. SE [bps/Hz/cell] LTE-A Cell Avg. SE [bps/Hz/cell] LTE Cell Edge SE [bps/Hz/user] LTE-A Cell Edge SE [bps/Hz/user] UL 1x2 0.735 1.2 0.024 0.04 2x4 - 2.0 0.07 DL 2x2 1.69 2.4 0.05 4x2 1.87 2.6 0.06 0.09 4x4 2.67 3.7 0.08 0.12

Frame structure [TS36.211 Sec4] Basic time unit, Ts = 1/(15000 x 2048) second Frame structure type 1 FDD Slot = 0.5 ms, subframe = 1 ms (TTI) One radio frame consists of 10 subframes Supports full-and half-duplex operations at the terminal Full duplex: simultaneous transmission/reception Half duplex: only transmission or reception at a time

Frame structure [TS36.211 Sec4] Frame structure type 2 TDD Special subframe to provide guard time for downlink-to-uplink switching DwPTS: downlink part UpPTS: uplink part GP: No transmissions to avoid interference between uplink and downlink transmissions

Frame structure [TS36.211 Sec4] Frame structure type 2 (continued) Seven configurations with different UL/DL ratios

LTE - Uplink

Uplink transmission scheme [TS36.211 Sec5.3] Basic transmission scheme For both FDD and TDD, the uplink transmission scheme is based on single-carrier FDMA, more specifically DFT S-OFDM Low PAPR (Peak-to-Average Power Ratio) Sub-carrier spacing f = 15 kHz. Two cyclic-prefix lengths Normal cyclic prefix and extended cyclic prefix corresponding to seven and six SC-FDMA symbol per slot, respectively. Normal cyclic prefix: TCP = 160Ts (SC-FDMA symbol #0) , TCP = 144Ts (SC-FDMA symbol #1 to #6) Extended cyclic prefix: TCP-e = 512Ts (SC-FDMA symbol #0 to SC-FDMA symbol #5)

Uplink physical resources [TS36.211 Sec5.2] Resource element (RE) Basic resource unit Resource block (RB) Basic allocation unit Normal CP: 12 subcarriers x 7 SC-FDMA symbols Extended CP: 12 subcarriers x 6 SC-FDMA symbols # of resource blocks can range from NRB-min = 6 to NRB-max = 110. (LTE) Always consecutive allocation A set of frequency consecutive RBs To maintain low PAPR

Overview of UL Physical Channel Processing LTE Rel-8/9 UL physical channel processing PAPR (Peak-to-Average Power Ratio), CM (Cubic Metric) LTE-A UL physical channel processing

Uplink reference signals Uplink reference signals [TS 36.211 Sec 5.5 ] Two types of reference signals Demodulation reference signals (DMRS) Transmitted within data transmission RBs Used for channel estimation -> coherent demodulation of uplink transmissions Sounding reference signals (SRS) Transmitted over a range of frequency bands Used for estimation of uplink channel quality -> frequency-domain scheduling Structure of DMRS Time-division multiplexed with other uplink transmissions

Uplink reference signals SRS symbol structure [TS36.211 Sec 5.5.3]

Uplink L1/L2 control signaling Uplink L1/L2 control signaling [TS36.211 Sec 5.4] Signaling to support downlink and uplink transport channels H-ARQ acknowledgement in response to downlink data Downlink channel quality feedback Scheduling request for uplink resource allocation Two different transmission methods If no simultaneous transmission of UL-SCH L1/L2 control signaling on PUCCH If simultaneous transmission of UL-SCH L1/L2 control signaling on PUSCH

Uplink L1/L2 control signaling L1/L2 control signaling on PUCCH [TS36.211 Sec 5.4] PUCCH structure Frequency hopping for frequency diversity Positioned at the edge of the band to avoid fragmentation of PDSCH transmission, making it possible to allocate a wide spectrum without breaking the single-carrier property FDM/CDM (Code Division Multiplexing)

Uplink L1/L2 control signaling Physical uplink control channel (PUCCH) [TS36.211 Sec 5.4] PUCCH formats Cyclic shift of a sequence in each symbol Format 1: Scheduling Request (SR) Information is carried by the presence/absence of transmission Format 1a and 1b: HARQ-ACK 1 and 2 bits, respectively Format 2: CQI Format 2a and 2b: CQI + HARQ-ACK

Uplink L1/L2 control signaling PUCCH formats 1, 1a, and 1b [TS36.211 Sec 5.4.1] SR, ACK/NAK Two-dimensional CDM Spreading along the frequency axis Block-wise spreading along the time axis Resources Resource index -> (Orthogonal sequence index, Cyclic shift)

Uplink L1/L2 control signaling PUCCH formats 1, 1a, and 1b (continued) [TS36.211 Sec 5.4.1] Normal PUCCH formats 1 and 1a/1b -> Length-4 Walsh sequences Shortened PUCCH formats 1 and 1a/1b ->Length-3 DFT sequences

Uplink L1/L2 control signaling PUCCH formats 2, 2a, 2b [TS36.211 Sec 5.4.2] CQI or CQI + ACK/NAK One-dimensional CDM Resources Resource index -> Cyclic shift Symbol-level cell-specific CS hopping Slot-level CS remapping d(10) for format 2a/2b

Uplink L1/L2 control signaling L1/L2 control signaling on PUSCH [TS36.212 Sec 5.2.2.6, 5.2.27] If control signaling is simultaneous with UL-SCH, control signaling is multiplexed with data on PUSCH Simultaneous transmission of PUCCH and PUSCH is not allowed to keep the single-carrier property H-ARQ ACK/NAK Placed right next to RS Channel quality status report Rank Indicator (RI) (3, 2) block code, encoded separately from CQI/PMI Placed right next to ACK/NAK Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) Tail-biting convolution code or block code Mapped across the full subframe duration

Uplink L1/L2 control signaling Multiplexing of control and data on PUSCH [TS36.212 Sec 5.2.2.6, 5.2.2.7] HARQ ACK, RI, CQI A/N resources puncture into data, placed next to RS RI bits placed next to the A/N bits in PUSCH, irrespective whether or not A/N is present CQI resources placed at the beginning of the data resources with time-first mapping

Uplink transmission PUSCH frequency hopping [TS36.211 Sec 5.3.7] [TS36.213 Sec 8.4.1] Cell-specific hopping/mirroring Slot level hopping according to predefined hopping/mirroring patterns Different cells have different patterns Period is one frame Hopping according to explicit information The uplink scheduling grant indicates the offset of the resource to use for the second slot. Dynamic selection between the two modes Indication bits in the scheduling grant

LTE - Downlink

Downlink transmission Basic transmission scheme OFDM numerology

Downlink physical resources [TS36.211 Sec 6.2] Resource element Basic resource unit Resource block (RB) Basic allocation unit Normal CP: 12 subcarriers x 7 SC-FDMA symbols Extended CP: 12 subcarriers x 6 SC-FDMA symbols Non-consecutive allocation allowed

Downlink physical channels and signals [TS36.211 Sec 6.2] Physical downlink shared channel (PDSCH) carries the downlink shared channel (DL-SCH) and paging channel (PCH) Physical downlink control channel (PDCCH) informs the UE about the resource allocation of PCH and DL-SCH, and Hybrid ARQ information related to DL-SCH carries the uplink scheduling grant Physical control format indicator channel (PCFICH) informs the UE about the number of OFDM symbols used for the PDCCHs; transmitted in every subframe. Physical broadcast channel (PBCH) The coded broadcast channel (BCH) transport block is mapped to four subframes within a 40 ms interval Each subframe is self-decodable Physical Hybrid ARQ Indicator Channel (PHICH) carries Hybrid ARQ ACK/NAKsin response to uplink transmissions. Physical multicast channel (PMCH) carries the multicast channel (MCH) transport channel Physical signals Reference signal Synchronization signal

Downlink physical resources Downlink frame structure [frame type 1 (FDD)]

Synchronization and cell search Two cell search procedures [TS36.211 Sec 6.11] Initial synchronization Synchronization -> PBCH decoding -> SI acquisition New cell identification Synchronization -> RS detection -> RSRP/RSRQ measurement

Downlink Physical Channel Overview of downlink physical channel processing Note (number of codewords)  2 (number of layers)  (channel rank) Resource element mapper Time-frequency resource OFDM signal generation IFFT

DL L1/L2 control signaling [TS36.211 Sec 6.7, 6.8, 6.9] Downlink L1/L2 control signaling Transmitted in the control region, which is the first part of the subframe Control region size Variable, 1 ~ 3 (4) OFDM symbols Three types of physical channels PCFICH Informs the UE about the size of the control region PDCCH/E-PDCCH Carries downlink scheduling assignment, uplink scheduling grants Each (E-)PDCCH carries a signal for a single UE PHICH Carries H-ARQ ACK/NAK in response to uplink UL-SCH transmission Multiple PHICHs in each cell

Downlink reference signal Cell-specific reference signal (CRS) Introduced in Rel-8 for channel estimation and demodulation Cell-specific RS sequences and frequency shifts High overheads Tx antennas 0 and 1: twice in the slot Tx antennas 2 and 3: once in the slot

Downlink reference signal Channel State Information Reference Signal (CSI RS) Introduced in Rel-10 for CSI report Sparse in frequency and time Low overhead: around 1/(10*14*6) = 1/840 = 0.12% per antenna port (8 antenna ports = 0.96%) Periodicity of CSI RS is configurable as an integer multiple of subframe

Downlink reference signal Demodulation RS (DM RS) Introduced in Rel-10 for data demodulation UE-specific PDSCH and the DM RS are subject to the same precoding operation Present only in resource blocks and layers scheduled for transmission Lower overhead compared to CRS

Downlink MIMO LTE LTE-Advanced 1, 2, 4 Tx antennas Spatial multiplexing up to 4 layers Closed-loop codebook based precoding Open-loop Multi codewords Spectral efficiency up to 15 bps/Hz Tx diversity SFBC for 2 Tx antennas SFBC + FSTD for 4 Tx antennas Semi-static SU- and MU MIMO switching LTE-Advanced 1, 2, 4, 8 Tx antennas Spatial multiplexing up to 8 layers Closed-loop precoding Spectral efficienty up to 30 bps/Hz Dynamic SU- and MU-MIMO switching

LTE – Scheduling

Downlink assignment eNB UE Scheduling decision, DCI (Downlink control information) and Data preparation (E-)PDCCH /PDSCH transmission UE (E-)PDCCH monitoring If downlink assignment, DCI acquisition and PDSCH detection/decoding PUCCH (ACK/NACK) transmission

Uplink grant eNB Scheduling decision, DCI (Downlink control information) preparation (E-)PDCCH transmission UE (E-)PDCCH monitoring and/or PHICH detection If uplink grant, DCI acquisition and PUSCH preparation PUSCH transmission PUSCH detection/decoding

HARQ process Hybrid ARQ New transmission Retransmission If NDI (New Data Indicator) is toggled, it indicates a new transmission Scheduled through (E)PDCCH Retransmission Non-adaptive retransmissions (uplink): triggered by a NACK on PHICH Adaptive retransmissions are scheduled through PDCCH Downlink HARQ Uplink HARQ Mode Adaptive Non-adaptive/Adaptive Sync Asynchronous Synchronous (HARQ RTT = 8 ms) ACK/NACK Using PUCCH/PUSCH PHICH (+ PDCCH) 사용 Process No Indicated in PDCCH Fixed (determined by the subframe) Note Retransmissions are always scheduled through PDCCH If non-adaptive, retransmissions on the same uplink resource as previously used by the same HARQ process

HARQ process Downlink and uplink Hybrid ARQ

Downlink scheduling DL Scheduling eNB receives CSI from UE: Rank/PMI, CQI, HARQ ACK/NAK Rank/PMI  the number of layers to transmit CQI  MCS for each codeword eNB performs scheduling for every subframe UE selection, resource allocation, MCS selection etc

Uplink scheduling UL Scheduling eNB receives SRS (Sounding Reference Signal) from UE eNB estimates Rank/PMI and CQI from the SRS eNB performs scheduling for every subframe UE selection, power control, resource allocation, MCS selection etc

Semi-persistent scheduling Assignment/grant recurs with a configured interval between the assignments/grants SPS interval: 10, 20, 32, 40, 64, 80, 128, 160, 320, 640 ms SPS configuration by RRC signaling Activation/Re-activation and Release by PDCCH In DL configuration only Number of configured HARQ processes In UL configuration only Implicit release after a number of empty transmissions

Scheduling request Scheduling Request If a regular BSR triggered but no UL resources An SR is triggered If an SR is pending If no valid PUCCH resource for SR configured in any TTI Initiate a random access procedure Else if valid PUCCH resource for SR Transmit SR on PUCCH

Buffer status reporting BSR on a logical-channel group basis A BSR Indicates the amount of data across all logical channels in a logical channel group or all four logical channel groups Regular BSR Triggered if Arrival of UL data with higher priority than the one currently being transmitted Arrival of UL data with no already existing data retxBSR-Timer expires and the UE has data available for transmission retxBSR-Timer restarts upon the indication of an uplink grant for new transmission If there are UL resources for new transmission, transmitted on the allocated resources If no UL resources, triggers a Scheduling Request

Buffer status reporting Periodic BSR Timer controlled periodic reporting Triggered if periodicBSR-Timer expires (can be disabled) Transmitted on UL resources allocated for new transmission Padding BSR Transmitted, instead of padding, on UL resources allocated for new transmission if there are enough padding bits At most one Regular/Periodic BSR is reported in a TTI

Buffer status reporting BSR New Data New Data No Data retxBSR - Timer Regular BSR Regular BSR Periodic BSR Periodic BSR Periodic BSR periodicBSR - Timer UL Transmission UL Transmission Scheduling Request Scheduling Request Regular BSR Periodic BSR New Transmission Timer starts Timer restarts Timer expires

UL scheduling logical channel prioritization Applies to a new transmission RRC controls scheduling by signaling for each logical channel Priority: 1, 2, ..16 Prioritized Bit Rate (PBR): 0, 8, …, 2048 kbps Bucket Size Duration (BSD): 50, 100, …., 1000 ms UE maintains Bj for each logical channel j Bj = 0 when the logical channel is established Incremented by PBR X TTI duration for each TTI Not exceeding the bucket size of the logical channel j Bj <= Bucket size = PBR x BSD Scheduling prioritization Step 1: All the logical channels with Bj > 0 are allocated resources in a decreasing priority order until meeting their Bj Step 2: Bj is decremented by the total size of MAC SDUs served to the logical channel j Step 3: if any resources remain, all the logical channels are served in a strict decreasing priority order

Logical channel prioritization UL scheduling prioritization of logical channels