3G/4G Mobile Communications Systems. Dr. Stefan Brück Qualcomm Corporate R&D Center Germany

Transcription

1 3G/4G Mobile Communications Systems Dr. Stefan Brück Qualcomm Corporate R&D Center Germany

2 Chapter VI: Physical Layer of LTE 2 Slide 2

3 Physical Layer of LTE OFDM and SC-FDMA Basics DL/UL Resource Grid Downlink Operation Downlink Physical Channels Uplink Operation Uplink Physical Channels UE Categories 3 Slide 3

4 LTE Key Radio Features (Release 8) Multiple access scheme DL: OFDMA with CP UL: Single Carrier FDMA (SC-FDMA) with CP Adaptive modulation and coding DL modulations: QPSK, 16QAM, and 64QAM UL modulations: QPSK, 16QAM, and 64QAM (optional for UE) Rel. 6 Turbo code: Coding rate of 1/3, two 8-state constituent encoders and a contention-free internal interleaver ARQ within RLC sublayer and Hybrid ARQ within MAC sublayer Advanced MIMO spatial multiplexing techniques (2 or 4)x(2 or 4) downlink and 1x(2 or 4) uplink supported Multi-layer transmission with up to four streams in DL Multi-user MIMO also supported in UL and DL Implicit support for interference coordination Support for both FDD and TDD 4 Slide 4

5 LTE Frequency Bands LTE will support all band classes currently specified for UMTS as well as additional bands 5 Slide 5

6 LTE Duplexing Modes LTE supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) to provide flexible operation in a variety of spectrum allocations around the world. Unlike UMTS TDD there is a high commonality between LTE TDD & LTE FDD Slot length (0.5 ms) and subframe length (1 ms) is the same as LTE FDD with the same numerology (OFDM symbol times, CP length, FFT sizes, sample rates, etc.) UL/ DL switching points designed to allow coexistence with UMTS-TDD (TD-CDMA, TD-SCDMA) 6 Slide 6

7 LTE Half-Duplex FDD In addition to FDD & TDD, LTE supports also Half-Duplex FDD (HD-FDD) HD-FDD is like FDD, only the UE cannot transmit and receive at the same time Note, that the enodeb can still transmit and receive at the same time to different UEs; half-duplex is enforced by the enodeb scheduler Reasons for HD-FDD Handsets are cheaper, as no duplexer is required More commonality between TDD and HD-FDD than compared to full duplex FDD Certain FDD spectrum allocations have small duplex space; HD-FDD leads to duplex desense in UE 7 Slide 7

8 OFDM Basics Overlapping Orthogonal OFDM: Orthogonal Frequency Division Multiplexing OFDMA: Orthogonal Frequency Division Multiple-Access FDM/FDMA is nothing new: carriers are separated sufficiently in frequency so that there is minimal overlap to prevent cross-talk. conventional FDM frequency OFDM: still FDM but carriers can actually be orthogonal (no cross-talk) while actually overlapping, if specially designed saved bandwidth! saved bandwidth OFDM frequency 8 Slide 8

9 OFDM Basics Waveforms f = 1/T 1 Frequency domain: overlapping sinc functions Referred to as subcarriers Typically quite narrow, e.g. 15 khz Time domain: simple gated sinusoid functions For orthogonality: each symbol has an integer number of cycles over the symbol time fundamental frequency f 0 = 1/T T = symbol time freq x 10 5 Other sinusoids with f k = k f time Slide 9

10 OFDM Basics The Full OFDM Transceiver 10 Slide 10

11 OFDM Basics Cyclic Prefix ISI (between OFDM symbols) eliminated almost completely by inserting a guard time T G OFDM Symbol OFDM Symbol OFDM Symbol T G Within an OFDM symbol, the data symbols modulated onto the subcarriers are only orthogonal if there is an integer number of sinusoidal cycles within the receiver window Filling the guard time with a cyclic prefix (CP) ensures orthogonality of subcarriers even in the presence of multipath elimination of same cell interference CPUseful OFDM symbol time CPUseful OFDM symbol time CPUseful OFDM symbol time OFDM symbol OFDM symbol OFDM symbol 11 Slide 11

12 Comparison with CDMA Principle OFDM: Particular modulation symbol is carried over a relatively long symbol time and narrow bandwidth LTE: 66.6 µsec symbol time and 15 khz bandwidth For higher data rates send more symbols by using more sub-carriers increases bandwidth occupancy CDMA: Particular modulation symbol is carried over a relatively short symbol time and a wide bandwidth UMTS HSPA: 4.17 µsec symbol time and 3.84 Mhz bandwidth To get higher data rates use more spreading codes time time CDMA symbol 0 symbol 1 symbol 2 symbol 3 OFDM symbol 0 symbol 1 symbol 2 symbol 3 frequency frequency 12 Slide 12

13 Comparison with CDMA Time Domain Short symbol times in CDMA lead to ISI in the presence of multipath CDMA symbols Multipath reflections from one symbol significantly overlap subsequent symbols ISI Long symbol times in OFDM together with CP prevent ISI from multipath CP 1 CP 2 CP CP 1 CP 2 1 CP 2 Little to no overlap in symbols from multipath 13 Slide 13

14 Comparison with CDMA Frequency Domain In CDMA each symbol is spread over a large bandwidth, hence it will experience both good and bad parts of the channel response in frequency domain In OFDM each symbol is carried by a subcarrier over a narrow part of the band can avoid send symbols where channel frequency response is poor based on frequency selective channel knowledge frequency selective scheduling gain in OFDM systems 14 Slide 14

15 OFDM Basics Choosing the Symbol Time for LTE Two competing factors in determining the right OFDM symbol time: CP length should be longer than worst case multipath delay spread, and the OFDM symbol time should be much larger than CP length to avoid significant overhead from the CP On the other hand, the OFDM symbol time should be much smaller than the shortest expected coherence time of the channel to avoid channel variability within the symbol time LTE is designed to operate in delay spreads up to ~5µs and for speeds up to 350km/h (1.2ms coherence 2.6GHz). As such, the following was decided: CP length = 4.7 µs OFDM symbol time = 66.6 µs(= 1/20 the worst case coherence time) f = 15 khz ~4.7 µs ~66.7 µs CP 15 Slide 15

16 Scalable OFDM for Different Operating Bandwidths 20 MHz bandwidth 10 MHz bandwidth With Scalable OFDM, the subcarrier spacing stays fixed at 15 khz regardless of the operating bandwidth 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz Symbol time is fixed to 66.6 µs The total number of subcarriers is varied in order to operate in different bandwidths This is done by specifying different FFT sizes (i.e. 512 point FFT for 5 MHz, 2048 point FFT for 20 MHz) Influence of delay spread, Doppler due to user mobility, timing accuracy, etc. remain the same as the system bandwidth is changed robust design 5 MHz bandwidth 3 MHz bandwidth 1.4 MHz bandwidth common channels centre frequency 16 Slide 16

17 LTE Downlink Frame Format Radio frame = 10ms subframe = 1.0ms slot = 0.5ms slot = 0.5ms OFDM symbol Subframe length is 1ms Consists of two 0.5ms slots 7 OFDM symbols per 0.5ms slot 14 OFDM symbols per 1ms subframe 17 In UL center SC-FDMA symbol used for the data demodulation reference signal (DM-RS) Slide 17

18 LTE Downlink Frame Structure Subframe length relevant to the latency requirement Spectrum allocation Slot duration Sub-frame duration Sub-carrier spacing Sampling 1.92 MHz frequency (1/2 3.84) 1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz 3.84 MHz 0.5 ms 1.0 ms ( = 2 slots) 15 khz (7.5 khz for MBMS) 7.68 MHz MHz MHz MHz (2 3.84) (4 3.84) (6 3.84) (8 3.84) FFT size Sampling rates are multiples of UMTS chip rate, to ease implementation of dual mode UMTS/LTE terminals Number of sub-carriers OFDM symbols per slot CP length Short (short CP), 6 (long CP) 4.69 µs x µs x 1 Long µs FFT size scales to support larger bandwidth Scalable OFDM 18 Slide 18

19 Downlink/Uplink Resource Grid Resource Element (RE) Fundamental element in time/frequency grid (1 subcarrier x 1 symbol) Resource Block (PRB) Minimum resource set for DL/UL data channel assignment (12 subcarriers x 1 slot) Physical Resource Block (180 khz x 1 ms) Virtual Resource Block (same size as PRB) Resource Block Group (RBG) Group of Resource Blocks 19 Slide 19

20 DL Logical, Transport and Physical Channels PCCH: paging control channel BCCH: broadcast control channel CCCH: common control channel DCCH: dedicated control channel DTCH: dedicated traffic channel LTE makes heavy use of shared channels common control, paging, and part of broadcast information carried on PDSCH PCCH BCCH CCCH DCCH DTCH MCCH MTCH Downlink Logical channels PCH: paging channel BCH: broadcast channel DL-SCH: DL shared channel PCH BCH DL-SCH MCH Downlink Transport channels DL-RS SCH PCFICH PBCH PHICH PDSCH PDCCH PMCH Downlink Physical Channels 20 Slide 20

21 Physical Channels to Support LTE Downlink Allows mobile to get timing and frequency sync with the cell Carries basic system broadcast information Carries DL traffic DL resource allocation enode-b Time span of PDCCH HARQ feedback for DL CQI reporting MIMO reporting 21 Slide 21

22 Downlink Operation UE reports CQI (channel quality indicator), PMI (precoding matrix indicator), and RI (rank indicator) in PUCCH or PUSCH Scheduler at enb dynamically allocates DL resources to UE enb sends user data in PDSCH UE reads PCFICH every subframe and determines the number of OFDM symbols occupied by PDCCH UE reads PDCCH to determine the assigned DL resources (PRB and MCS) for a specific Tx mode UE attempts to decode the received packet and sends ACK/NACK using PUCCH or PUSCH 22 Slide 22

23 Downlink Physical Channels in LTE 23 Slide 23

24 Physical Channel Resource Allocation P-SCH and S-SCH Resource Allocation Subframe 0 and 5 of every 10 th frames Middle of bandwidth (6 PRBs) PBCH Resource Allocation Subframe 0 every 10 th frames Middle of bandwidth (6 PRBs) PDCCH, PCFICH, PHICH allocated to the (at most) three OFDM symbols in each subframe The remaining time frequency resources can be allocated for data transmission in the PDSCH 24 Slide 24

25 Downlink Reference Signals Three Types of DL Reference Signals (DL-RS) Cell-specific Reference Signals Associated with PDSCH multiple antenna port transmission Used by UE for coherent demodulation and channel estimation UE-specific Reference Signals UE-specific RS are supported for single antenna port transmission (Rel. 8) In Rel. 10 UE-specific RS are also introduced for multiple antenna port transmission MBSFN Reference Signals Associated with MBSFN transmission 25 Slide 25

26 DL cell-specific RS for multiple Tx Antennas RS are allocated on a per antenna port basis Antenna 0 and 1 Transmitted on 2 OFDM symbols every slot 6 subcarrier spacing and 2x staggering (45 khz frequency sampling) Antenna 2 and 3 Transmitted on 1 OFDM symbols every slot 6 subcarrier spacing with 2x staggering across slots 26 Same frequency spacing for normal and extended CP Slide 26

27 Spatial Multiplexing Code Word Transport block format : CRC encoded data Transmission layer Sub stream resulting from a mapping of modulated code word symbols Number of layers number of antenna ports Code book Quantized set of spatial combination vectors for precoding of symbols layer for transmission on antenna ports Rank of MIMO channel Number of independents TX/RX channels offered by MIMO for spatial multiplexing Rank min(n Tx, N Rx ) UE indicates channel quality (CQI), pre-coding matrix (PMI) and rank (RI) precoding M Tx N Rx Select # code words Modulation + coding Modulation + coding Layer mapping V H U H Demod + decode Demod + decode RI CQI PMI H = UΛV H 27 Slide 27

28 PDSCH Transmission Scheme Codewords (maximum of 2) One codeword for rank 1 transmission Two codewords for rank 2/3/4 transmission Layer mapping Number of layers depend on the number of Tx antennas and the channel rank Fixed mapping schemes of codewords to layers Tx antennas (maximum of 4) Potentially up to 4 layers Precoding Used to support spatial multiplexing Code book based precoding Seven different transmission modes are supported in Rel Slide 28

29 PDSCH Transmission Modes (Rel. 8) Mode 1: Uses a single Tx antenna at enb together with cell-specific RS Mode 2: Uses transmit diversity based on Alamouti scheme Mode 3: Open loop spatial multiplexing No UE feedback Exploits cyclic delay diversity (CDD) Mode 4: closed loop spatial multiplexing Up to 2 codewords and 4 layers Rank (RI) and precoding (PMI) feedback Mode 5: Multi-user MIMO Single codeword and single layer per UE UE reports PMI but no RI Mode 6: closed loop Rank = 1 precoding (restricted Mode 4) No RI reports are needed Mode 7: same as Mode 1 with UE-specific RS 29 Slide 29

30 Multiple Antenna Techniques Supported in LTE SU-MIMO Multiple data streams sent to the same user (max. 2 codewords) Significant throughput gains for UEs in high SINR conditions MU-MIMO or Beamforming Spatial Division Multiple Access (SDMA) Different data streams sent to different users using the same time-frequency resources Improves throughput even in low SINR conditions (cell-edge) Works even for single antenna mobiles Transmit diversity (TxDiv) Improves reliability on a single data stream Fall back scheme if channel conditions do not allow spatial multiplexing Useful to improve reliability on common control channels 30 Slide 30

31 Precoding in Transmission Mode 4 For two Tx antennas 4 single layer precoding vectors and 2 dual layer precoding matrices are supported See table below (TS ) For four Tx antennas 16 precoding vectors/matrices are supported per layer The construction is based on Householder transformation For details, see TS Slide 31

32 PDSCH Transmission Mode Configuration 32 Slide 32

33 CQI/PMI/RI Reporting CQI/PMI/RI reporting is either on PUSCH or PUCCH, periodic or aperiodic Aperiodic CQI/PMI/RI reporting is defined by the following characteristics: The report is scheduled by the enb via the PDCCH Transmitted together with uplink data on PUSCH From the frequency span perspective these reports can be: Frequency selective: UE Selected Subband CQI and Higher Layer Configured Subband CQI Frequency non-selective: Wideband CQI reports When a CQI report is transmitted together with Uplink data on PUSCH, it is multiplexed with the transport block by L1 The CQI report is not part of the uplink transport block Periodic CQI/PMI/RI reporting is defined by the following characteristics: Periodic CQI reports are sent on PUCCH From the Frequency span perspective these reports can be: Frequency selective: UE Selected Subband CQI Frequency non-selective: Wideband CQI reports 33 Slide 33

34 CQI Definition 34 Slide 34

35 Downlink Peak Rates bandwidth # of parallel streams supported MHz 5.4 Mbps 10.4 Mbps 19.6 Mbps 3 MHz 13.5 Mbps 25.9 Mbps 50 Mbps 5 MHz 22.5 Mbps 43.2 Mbps 81.6 Mbps 10 MHz 45 Mbps 86.4 Mbps Mbps 15 MHz 67.5 Mbps Mbps Mbps 20 MHz 90 Mbps Mbps Mbps Assumptions: 64QAM, code rate =1, 1OFDM symbol for L1/L2, ignores subframes with P-BCH, SCH 35 Slide 35

36 LTE Uplink Transmission Scheme (1/2) To facilitate efficient power amplifier design in the UE, 3GPP chose single carrier frequency domain multiple access (SC-FDMA) in favor of OFDMA for uplink multiple access. SC-FDMA results in better PAPR Reduced PA back-off improved coverage SC-FDMA is still an orthogonal multiple access scheme UEs are orthogonal in frequency Synchronous in the time domain through the use of timing advance (TA) signaling Node B UE C UE B UE A Only need to be synchronous within a fraction of the CP length 0.52 µs timing advance resolution α β UE A Transmit Timing UE B Transmit Timing γ UE C Transmit Timing 36 Slide 36

37 LTE Uplink Transmission Scheme (2/2) SC-FDMA implemented using an OFDMA front-end and a DFT pre-coder, this is referred to as either DFT-pre-coded OFDMA or DFT-spread OFDMA (DFTS-OFDMA) Advantage is that numerology (subcarrier spacing, symbol times, FFT sizes, etc.) can be shared between uplink and downlink Can still allocate variable bandwidth in units of 12 sub-carriers Each modulation symbol sees a wider bandwidth DFT precoding 37 Slide 37

38 SC-FDMA Signal SC-FDMA uses DFT precoding of user data Individual bits mapped across multiple frequencies DFT size (N) defines number of subcarriers allocated to user data Time domain signal more resembles a single carrier signal Peak-to-average power ratio (PAPR) is reduced 38 Slide 38

39 Localized and Distributed SC-FDMA Localized Assignment Uses consecutive subcarriers Simpler to implement Used in LTE Distributed Assignment Distributes subcarriers across frequency bands Increases frequency diversity Not applied in LTE 39 Slide 39

40 Comparison of OFDMA and SC-FDMA Figure taken from 40 Slide 40

41 Downlink/Uplink Resource Grid Resource Element (RE) Fundamental element in time/frequency grid (1 subcarrier x 1 symbol) Resource Block (PRB) Minimum resource set for DL/UL data channel assignment (12 subcarriers x 1 slot) Physical Resource Block (180 khz x 1 ms) Virtual Resource Block (same size as PRB) Resource Block Group (RBG) Group of Resource Blocks 41 Slide 41

42 UL Logical, Transport and Physical Channels CCCH: common control channel DCCH: dedicated control channel DTCH: dedicated traffic channel RACH: random access channel UL-SCH: UL shared channel PUSCH: physical UL shared channel PUCCH: physical UL control channel PRACH: physical random access channel 42 Slide 42

43 Physical Channels to Support LTE Uplink Random access for initial access and UL timing alignment Carries UL Traffic UL scheduling request for time synchronized IEs enode-b UL scheduling grant HARQ feedback for UL 43 Slide 43

44 Uplink Operation If UE does not have UL-SCH resources, UE send SR (scheduling request) on PUCCH Scheduler at enb allocates resources to UE in terms of UL grant on PDCCH Assigned resources: PRB and MCS UE sends user data on PUSCH If enb decodes the UL data successfully, it sends ACK on PHICH 44 Slide 44

45 Uplink Physical Channels in LTE 45 Slide 45

46 PUSCH PUSCH may carry UL data ACK/NACK for DL data CQI/PMI/RI The allocation of PRB resources is continuous Frequency hopping is supported to obtain frequency diversity Intra- and inter-subframe hopping are supported Demodulation-RS (DM-RS ) are embedded in the SC-FDMA symbols 46 Slide 46

47 PUCCH PUCCH may carry ACK/NACK for DL data CQI/PMI/RI Scheduling request PUCCH and PUSCH are never transmitted simultaneously Reason: Reduction of PAPR PUCCH uses one PRB in each of the two slots in a subframe Multiple UEs may be assigned the same PRB resource for PUCCH transmission Assignment of different cyclic shifts of scrambling sequence and orthogonal spreading sequences (DM-RS are embedded in the SC- FDMA symbols 47 Slide 47

48 MIMO Support is Different in Downlink and Uplink Downlink Supports SU-MIMO, MU-MIMO, TxDiv Uplink Initial release of LTE does only support MU-MIMO with a single transmit antenna at the UE Desire to avoid multiple power amplifiers at UE 48 Slide 48

49 Uplink Peak Rates bandwidth Highest Modulation 16 QAM 64QAM 1.4 MHz 2.9 Mbps 4.3 Mbps 3 MHz 6.9 Mbps 10.4 Mbps 5 MHz 11.5 Mbps 17.3 Mbps 10 MHz 27.6 Mbps 41.5 Mbps 15 MHz 41.5 Mbps 62.2 Mbps 20 MHz 55.3 Mbps 82.9 Mbps Assumptions: code rate =1, 2PRBs reserved for PUCCH (1 for 1.4MHz), no SRS, ignores subframes with PRACH, takes into account highest prime-factor restriction 49 Slide 49

50 LTE-Release 8 User Equipment Categories Category Peak rate Mbps DL UL Capability for physical functionalities RF bandwidth 20MHz Modulation DL QPSK, 16QAM, 64QAM UL QPSK, 16QAM QPSK, 16QAM, 64QAM Multi-antenna 2 Rx diversity Assumed in performance requirements. 2x2 MIMO Not supported Mandatory 4x4 MIMO Not supported Mandatory 50 Slide 50