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An overview of LTE including OFDMA and SC-FDMA basic principles

An overview of LTE including OFDMA and SC-FDMA basic principles

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    Lte Tutorial Lte Tutorial Presentation Transcript

    • LTE tutorial - Looking forward beyond HSPA+ sppe12083@gmail.com RAN System Engineer
    • Outline • Beyond HSPA+ • LTE: motivation and expectations • E-UTRAN overview & initial performance evaluation • OFDMA and SC-FDMA fundamentals • LTE physical layer • LTE transmission procedures All rights reserved @ 2009
    • Beyond HSPA evolution – 3GPP path DL: 14.4 Mbps DL: 28 Mbps DL: 42 Mbps DL: 84 Mbps DL: 100+ Mbps UL: 5.76Mbps UL: 11 Mbps UL: 11 Mbps UL: 23 Mbps UL: 23+ Mbps UTRAN Rel-99 WCDMA HSDPA/HSUPA HSPA+ (HSPA Evolution) Rel-5 Rel-6 Rel-7 Rel-8 Rel-9 Beyond Rel-9 E-UTRAN deployment LTE specification & service LTE-A process ~ 2007Q4 enhancement DL:300 Mbps DL: 1 Gbps UL: 75 Mbps UL: 100 Mbps All rights reserved @ 2009
    • LTE - background • Motivation: – Based on HSPA success story(274* commercial HSPA networks worldwide) – Uptake of mobile data traffic upon cellular networks enforces: • Reduced latency • Higher user data rate • Improved system capacity and coverage • Cost-reduction per bit • Expectation: – Detailed requirements captured in 3GPP TR 25.913 – NGMN formally released requirements on next generation RAN in late 2006** *source: www.gsacom.com “ mobile broadband evolution: roadmap from HSPA to LTE” UMTS forum White paper **http://www.ngmn.org/nc/de/downloads/techdownloads.html All rights reserved @ 2009
    • LTE - background • Motivation: – Based on HSPA success story(274* commercial HSPA networks worldwide) – Uptake of mobile data traffic upon cellular networks enforces: • Reduced latency • Higher user data rate • Improved system capacity and coverage • Cost-reduction per bit • Expectation: – Detailed requirements captured in 3GPP TR 25.913 – NGMN formally released requirements on next generation RAN in late 2006** *source: www.gsacom.com “ mobile broadband evolution: roadmap from HSPA to LTE” UMTS forum White paper **http://www.ngmn.org/nc/de/downloads/techdownloads.html All rights reserved @ 2009
    • LTE feature overview • Flexible and expandable spectrum bandwidth • Simplified network architecture • High data throughput (Macro eNodeB & Home eNodeB) • Support for multi-antenna scheme (up to 4x4 MIMO in Rel-8) • Time-frequency scheduling on shared-channel • Soft(fractional) frequency reuse • Self-Organizing Network (SON) All rights reserved @ 2009
    • LTE spectrum flexibility • Operating bands – Flexible carriers: from 700MHz to 2600MHz – Extensible bandwidth: from 5MHz to 20MHz FDD Pair uplink downlink 5 MHz 20 MHz Channel bandwidth (MHz) Transmission bandwidth configuration(RBs) active RBs All rights reserved @ 2009
    • LTE basic parameters Frequency range UMTS FDD bands and TDD bands defined in 36.101(v860) Table 5.5.1 channel bandwidth (MHz) 1.4 3 5 10 15 20 Transmission bandwidth NRB: (1 resource block = 180kHz 6 15 25 50 75 100 in 1ms TTI) Downlink: QPSK, 16QAM, 64QAM Modulation Schemes: Uplink: QPSK, 16QAM, 64QAM(optional) downlink: OFDMA (Orthogonal Frequency Division Multiple Access) Multiple Access: uplink: SC-FDMA (Single Carrier Frequency Division Multiple Access) downlink: TxAA, spatial multiplexing, CDD ,max 4x4 array Multi-Antenna Technology Uplink: Multi-user collaborative MIMO Downlink: 150Mbps(UE Category 4, 2x2 MIMO, 20MHz bandwidth) Peak data rate 300Mbps(UE category 5, 4x4 MIMO, 20MHz bandwidth) Uplink: 75Mbps(20MHz bandwidth) All rights reserved @ 2009
    • LTE Peak throughput w.r.t UE categories Table 4.1-1: Downlink physical layer parameter values set by the field ue-Category UE Category Maximum number of DL-SCH Maximum number of bits of Total number of Maximum number of transport block bits received a DL-SCH transport soft supported layers for within a TTI block received within channel spatial multiplexing a TTI bits in DL Category 1 10296 Peak rate 10296 250368 1 150Mbps with Category 2 51024 51024 1237248 2 2x2 MIMO Category 3 102048 75376 1237248 2 Category 4 150752 75376 1827072 2 Category 5 299552 149776 3667200 4 Peak rate 300Mbps with 4x4 MIMO Table 4.1-2: Uplink physical layer parameter values set by the field ue-Category UE Maximum number of bits of an Support for Cate UL-SCH transport block 64QAM in gory transmitted within a TTI UL Category 1 5160 No Category 2 25456 No Peak rate Category 3 51024 75Mbps No Category 4 51024 No Category 5 75376 Yes 3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“ All rights reserved @ 2009
    • LTE UE category UE Category 1 2 3 4 5 Peak rate DL 10 50 100 150 300 (Mbps) UL 5 25 50 50 75 RF bandwidth 20 MHz DL QPSK, 16QAM, 64QAM Modulation QPSK, UL QPSK, 16QAM 16QAM, 64QAM 2 Rx Diversity Assumed in performance requirements 2x2 MIMO Optional Mandatory 4x4 MIMO Not supported Mandatory 3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“ All rights reserved @ 2009
    • Channel dependent scheduling • Time-frequency scheduling UE #1 UE #2 All rights reserved @ 2009
    • Soft (fractional) frequency reuse • Soft Frequency Reuse(SFR): – inner part of cell uses all subbands with less power; – Outer part of cell uses pre-served subbands with higher power; Su rier ca b- s r po BS 2 w erd en ity s sub- ier MS 22 carr Pow MS 21 er d BS 1 e nsity MS 31 MS 12 MS 11 y sit n de wer Po MS 32 ca ub- er s rri 3GPP R1-050841 “Further Analysis of Soft Frequency Reuse Scheme “ BS 3 All rights reserved @ 2009
    • E-UTRAN overview All rights reserved @ 2009
    • E-UTRAN architecture S1 S1 S1 S1 X2 X2 All rights reserved @ 2009
    • E-UTRAN architecture All rights reserved @ 2009
    • E-UTRAN radio protocol notifications common dedicated System Dedicated Control RRC Paging information and information transfer radio SRB0 SRB1 SRB2 DRB1 DRB2 bearers Integrity and Integrity and ciphering and ciphering and PDCP ciphering ciphering ROHC ROHC RLC ARQ ARQ ARQ ARQ logical channels PCCH BCCH CCCH DCCH 1 DCCH 2 DTCH 1 DTCH 2 MAC Multiplexing and HARQ control transport PCH BCH RACH DL-SCH UL-SCH channels PHY layer functions physical PBCH PRACH PDSCH PUSCH channels All rights reserved @ 2009
    • E-UTRAN radio channels downlink uplink Logical PCCH BCCH CCCH DCCH DTCH MCCH MTCH CCCH DCCH DTCH channels DL-SCH MCH Transport channels RACH UL-SCH PCH BCH PRACH PUCCH PUSCH PDCCH PBCH PDSCH PMCH Physical channels •Logical Channels Define what type of information is transmitted over the air, e.g. traffic channels, control channels, system broadcast, etc. •Transport Channels – no per-user dedicated channels! Define how is something transmitted over the air, e.g. what are encoding, interleaving options used to transmit data •Physical Channels Define where is something transmitted over the air, e.g. first N symbols in the DL frame All rights reserved @ 2009
    • E-UTRAN bearers SRB: internal E-UTRAN signalings such as RRC signalings, RB management signalings NAS signalings: such as tracking area update and mobility management messages data traffic: E-UTRAN radio bearer + S1 bearer +S5/S8 bearer L1/L2 control channel P TC TT IP DP P H P RT IP U u -u -u P- TP GTP S GT NA -u G P TP P DP UD C G UD U C RR RR P UD CP CP IP IP IP AP PD PD S1 S L2 L2 L2 TP NA C Y C Y HY RL PH RL SC IP PH P AP S1 AC AC r2 TP ye P-GW M M La SC S-GW L1 L1 IP E E Y LT LT PH L2 Y PH eNodeB UE MME S5/S8 E-UTRAN radio bearer S1 bearer bearer EPS bearer All rights reserved @ 2009
    • E-UTRAN – Control plane stack MME/ UE eNodeB NAS 24.301 NAS eNodeB 36.331 RRC RRC S1AP 36.413 S1AP X2AP 36.423 X2AP 36.323 PDCP PDCP SCTP 36.412 SCTP 36.322 36.422 RLC RLC IP IP 36.321 MAC MAC L2 L2 36.211~36.214 PHY PHY L1 L1 LTE-Uu S1-MME/X2-C All rights reserved @ 2009
    • E-UTRAN – User Plane Stack UE PDN/S-GW eNodeB eNodeB Application IP IP 36.323 29.274 PDCP PDCP GTP-u GTP-u 36.322 RLC RLC UDP UDP 36.321 IP IP MAC MAC L2 L2 36.211~36.214 PHY PHY L1 L1 LTE-Uu S1-U/X2-u All rights reserved @ 2009
    • Radio resource management QoS management Interference L3 RRC management mobility Admission Load control management Semi-persistent control scheduling PDCP Hybrid ARQ Dynamic L2 RLC Link adaptation manager scheduling MAC PDCCH L1 PHY CQI manager adaptation “An overview of downlink radio resource management for LTE”, Klaus Ingemann Pedersen, et al, IEEE communication magazine, 2009 July All rights reserved @ 2009
    • E-UTRAN mobility RRC-idle RRC-connected • Simplified RRC states • Idle-mode mobility (similar as HSPA) • Cell reselection decided by UE • Network controlled handovers • Connected-mode mobility • Based on UE measurements • Based on UE measurements • Controlled by broadcasted parameters – handover controlled by network • Different priorities assigned to frequency layers MME/SGW Mobility difference between UTRAN and E-UTRAN UTRAN E-UTRAN HO decision Location area (CS core) Not relevant since no CS connections Call Admission Routing area Tracking area Source target SHO No SHO eNodeB eNodeB Cell_FACH, Cell_PCH,URA_PCH No similar RRC states RNC hides most of mobility Core network sees every handover No need to provide cell-specific Target cell signal Neighbour cell list required information, only carrier-frequency is quality meets required. reporting threshold All rights reserved @ 2009
    • Overview of a PS call – control plane • UE activities after power-on Power up Initial Derive system Random Data Tx/Rx cell search information Access UE E-UTRAN paging S Radom Access procedure /SS P SS H Connection BC RRC Connection Request establishment ICH H/PH H FIC CC RRC Connection Setup PC PD ss A cce RRC Connection Setup Complete m ado Radio bearer Rn SC H H Security procedures establishment PD CC /PU CH P US RRC Connection Reconfiguration RRC Connection Reconfiguration Complete All rights reserved @ 2009
    • Overview of a PS call – control plane • UE activities after power-on Power up Initial Derive system Random Data Tx/Rx cell search information Access UE E-UTRAN ion gr ant paging ss ing nsmi l ed u tra k sch Radom Access procedure ta in da upl Connection DL K & rt RRC Connection Request establishment AC r epo tus RRC Connection Setup el sta nn sio n RRC Connection Setup Complete cha mis Radio bearer & a ns A CK a tr Security procedures establishment at U Ld RRC Connection Reconfiguration RRC Connection Reconfiguration Complete All rights reserved @ 2009
    • Overview of a PS call – user plane PS data Tx via S1 interface 1 resource block: eNodeB 180 kHz = 12 subcarriers to RF PDCP OFDM Signal Generation (Ciphering Header Compression,) 1 resource block pair 1 TTI = 1ms = 2 slots resource RLC mapping (Segmentation, ARQ) scheduling data modulator coding UE HARQ Multiplexing per user All rights reserved @ 2009
    • Overview of a PS call – user plane PS data Tx via S1 interface 1 resource block: eNodeB 180 kHz = 12 subcarriers to RF PDCP OFDM Signal Generation (Ciphering Header Compression,) 1 resource block pair 1 TTI = 1ms = 2 slots resource RLC mapping (Segmentation, ARQ) scheduling data modulator coding UE HARQ Multiplexing Occupying different radio per user resources across TTIs adapts to time-varying radio channel condition! All rights reserved @ 2009
    • LTE initial deployment scenario • Similar coverage as 3G HSPA on existing 3G frequency bands – LTE radio transmission technology itself does not provide coverage boost. – Lower frequency (e.g, 900MHz) provides better coverage but demands large- size antennas. • “Over-layed” initial deployment on hot-spot area – Spectrum availability – Backhaul capacity – Handset maturity (multi-mode) urban sub-urban Rural (0.6 ~ 1.2km) (1.5 ~ 3.4km) (26 ~ 50 km) All rights reserved @ 2009
    • LTE initial trial performance • LTE data rates – Peak rate measured in lab and trial align with 3GPP performance targets – In reality, user throughputs are impacted by • RF conditions & UE speed • Inter-cell interference & multiple users sharing the capacity • Application overhead Peak rate measured with a single user in unloaded, optimal radio condition Average: 10 active users with 3Mbps Top 5%, loaded throughput per user Average Cell edge 1Mpbs throughput at cell edge Active users per cell Active users per cell Source: www.lstiforum.org All rights reserved @ 2009
    • Macro Cellular network: peak rate Vs average rate • Unlike circuit-switched network design, live network throughput is not fixed any more, being dependent on many environmental factors such as CQI,Tx buffer status,etc. • In macro cellular network, network average throughput falls behind peak rate by 10x. • Cellular booster for Mobile broadband HSPA cell throughput Tput (Mbps) G-factor (dB) – Ubiquitous coverage – High capacity & data rate 8 25 – Low cost 15 4 >> “FemtoCell” – Home eNodeB! 10 2 2 0 -3 3GPP TS 25.101 Table 9.8D3, 9.8D4, 9.8F3 for PA3 All rights reserved @ 2009
    • LTE initial trial performance • User plane latency – 3GPP RTT target is 10ms for short IP packet air interface RTT – Field trial results: End-to-End Ping • 10~13ms with pre-scheduled uplink • <25ms with on-demand uplink EPC App Server • Control plane latency – Short latency helps to keep “always on” user experience – Field trial results • Measured idle to active latency: 70~ 100ms Less than 50msec Active Dormant (Cell_DCH) (Cell_PCH) Less than 100msec Camped-state (idle) * Measurement taken with one UE in unloaded case * Source: www.lstiforum.org All rights reserved @ 2009
    • OFDMA and SC-FDMA rationale All rights reserved @ 2009
    • OFDM fundamentals – frequency spectrum FDM OFDM … f f π⋅f sin( ) Δf No Inter-Carrier Interference! − 2Δf − Δf 0 Δf 2Δf frequency domain 1 Tu = Δf Time domain All rights reserved @ 2009
    • OFDM fundamentals – multicarrier modulation Nc − 1 Nc − 1 “+1” f1 Modulated subcarriers x (t ) = ∑x k =0 k (t ) = ∑a k =0 k e j 2 π k Δ ft Specifying system sampling rate: f s = 1 / Ts = N ⋅ Δf “-1” f2 Nc−1 + xn = x(nTs) = ∑ ak e j 2πkΔfnTs We get: “+1” f3 k =0 Nc−1 j 2πk n N −1 j 2πk n = ∑ ak e N = ∑ ak e ′ N k =0 k =0 e j 2πf0t a0 a0 x0 (t ) a1 X0 a0 , a1 ,..., a N c −1 j 2πf1t X1 e … a0 , a1 ,..., a N c −1 S/P x(t ) … S/P a1 x1 (t ) + a Nc −1 IFFT P/S π e j 2xf Nc−1t (t ) … 0 … a Nc −1 Nc−1 XN-1 0 All rights reserved @ 2009
    • OFDM fundamentals- Cyclic Prefix Tu directed path: ak −1 ak ak +1 reflected path: τ τ Integration interval of direct path directed path: reflected path: τ Guard time: Cyclic Prefix Vs Padding Zeroes Tcp >τ guard time FFT integration time=1/Carrier spacing OFDM symbol time All rights reserved @ 2009
    • OFDM fundamentals- Cyclic Prefix Tu directed path: ak −1 ak ak +1 reflected path: τ τ Integration interval of direct path directed path: reflected path: τ Guard time: Cyclic Prefix Vs Padding Zeroes Tcp >τ a0 a1 add an OFDM symbol … IFFT P/S Cyclic Tu+Tcp a Nc −1 Tu Prefix guard time FFT integration time=1/Carrier spacing OFDM symbol time All rights reserved @ 2009
    • OFDM fundamentals – general link level chains QAM Pilot Binary input data Coding Interleaving S/P IFFT P/S add CP mapping Insertion 5 MHz Bandwidth FFT Sub-carriers Pulse Guard Intervals RF Tx DAC shaping Symbols … Frequency … Time Timing and RF Rx ADC frequency Sync de- QAM CP Binary output data de-coding Equalizer P/S FFT S/P interleaving de-mapping removal “Digital communications: fundamentals and applications” by Bernard Sklar, Prentice Hall, 1998. ISBN: 0-13-212713-x “OFDM for Wireless Multimedia Communications” by Richard van Nee & Ramjee Prasad, Artech house,2000, ISBN: 0-89006-530-6 3GPP TR 25892-600 feasibility study for OFDM in UTRAN All rights reserved @ 2009
    • OFDM fundamentals – frequency domain equalizer MRC filter: * w(τ ) = h (−τ ) Zero Forcing: h(τ ) ⊗ w(τ ) = 1 ε = E{ s(t ) − s(t ) } 2 MMSE: ˆ transmitter Channel model receiver n(t ) S (t ) r (t ) ~ (t ) s h(τ ) + w(τ ) W0 rn R0 ˆ S0 ⊗ D D D r (t ) ˆ s(t ) W0 W1 WL-1 DFT WN −1 IDFT RN −1 ˆ S N −1 + ˆ sn ⊗ Time domain frequency domain Frequency domain equalizer outperforms with much less complexity! “Frequency domain equalization for single carrier broadband wireless systems”, David Falconer , et.al, IEEE Communication magazine, 2002 April All rights reserved @ 2009
    • OFDM fundamentals • Advantages: f – OFDM itself does not provide processing gains, but provides a degree of freedom in frequency domain by partitioning the wideband channel into multiple narrow “flat-fading” sub-channels. f – Channel coding is mandatory for OFDM to combat frequency-selective fading. – Efficiently combating multi-path propagation in term of cyclic prefix – OFDM receiver (frequency domain equalizer) has less complexity than that of Rake receiver on wideband channels. – OFDM characterizes flexible spectrum expansion for cellular systems. • Drawbacks: – high peak-to-average ratio. – Sensitive to frequency offset, hence to Doppler-shift as well All rights reserved @ 2009
    • OFDM fundamentals – downlink OFDMA 1 resource block: 180 kHz = 12 subcarriers f 1 slot = 0.5 ms PDCCH PDSCH • OFDMA provides flexible scheduling in time-frequency domain. • In case of multi-carrier transmission, OFDMA has larger PAPR than traditional single carrier transmission. Fortunately this is less concerned with downlink. • Does OFDMA suits for uplink transmission? – Uplink being sensitive to PAPR due to UE implementation requirements – With wider bandwidth in operation, OFDMA in uplink will have lower power per pilot symbol which in turn leads to deterioration of demodulation performance. All rights reserved @ 2009
    • Wideband single carrier transmission - frequency domain equalizer (SC-FDE) • While time-domain discrete equalizer has effect of “linear convolution” on channel response; frequency domain equalizer actually serves as “cyclic convolution” thereof. • The difference will make first L-1 symbols “incorrect” at the output of FDE. • Solution could be either “overlapped processing” or “cyclic prefix” added in transmitter. transmitter block-wise generation Single carrier x(t) Pulse signal CP Shaping generation N samples insertion N+Ncp samples “Adaptive Frequency-Domain Equalization and Diversity Combining for Broadband Wireless Communications,” M. V. Clark, IEEE J. Sel. Areas Commun., vol. 16, no. 8, Oct. 1998 “Linear Time and Frequency Domain Turbo Equalization,” M. Tüchler et al., Proc. IEEE 53rd Veh. Technol. Conf. (VTC), vol. 2, May 2001 All rights reserved @ 2009 “Block Channel Equalization in the Frequency Domain,” F. Pancaldi et al., IEEE Trans. Commun., vol. 53, no. 3, Mar. 2005
    • SC-FDMA – multiple access with FDE Binary input data QAM DFT Subcarrier IFFT Coding Interleaving mapping P/S add CP mapping (size M) (size N) Pulse RF Tx DAC FDMA: shaping user multiplexing in frequency domain Single Carrier: sequential transmission of the symbols over a single frequency carrier Timing and RF Rx ADC frequency Sync Binary output data de- QAM IDFT Freq Domain FFT CP de-coding (Size M) Equalizer P/S S/P interleaving de-mapping (size N) removal “Introduction to Single Carrier FDMA”, Hyung G Myung, 2007 EURASIP All rights reserved @ 2009
    • SC-FDMA – multiple access with SC-FDE • Multiple access in LTE uplink Terminal A data stream DFT Pulse OFDM Shaping f 0 Terminal B 0 data stream Pulse DFT OFDM f Shaping Orthogonal uplink design in frequency domain! All rights reserved @ 2009
    • SC-FDMA – multiple access with SC-FDE • Multiple access in LTE uplink Terminal A data stream DFT Pulse OFDM Shaping f 0 Terminal B 0 data stream Pulse DFT OFDM f Shaping Orthogonal uplink design in frequency domain! All rights reserved @ 2009
    • SC-FDMA – multiple access with FDE block-wise DFT IFFT CP D/A conversion RF signals (M) (N) insertion /pulse shaping Adopted by LTE uplink! Also called DFT- Spread OFDM! … … … … Localized FDMA: Distributed FDMA: A B C D DFT DFT IFFT (M) A B C D (M) (N) IFFT (N) … OverSampling in freq domain results in Upsampling in freq domain makes interpolation at time domain output repeated sequence at time domain output time domain: A* * * B * * * C * * * D* * * ABCDABCDABCDABCD frequency domain: All rights reserved @ 2009
    • OFDMA Vs SC-FDMA •Time domain: •Frequency domain - OFDM symbol is a sum of all data symbols by IFFT - OFDM modulates each subcarrier with one data symbol - SC-FDMA symbol is repeated sequence of data “chips” - SC-FDMA “distributes” all data symbols on each subcarrier. Input data symbols OFDM symbol SC-FDMA symbol * t f time domain frequency domain * Assuming bandwidth expansion factor Q=4 in distributed FDMA. All rights reserved @ 2009
    • OFDMA Vs SC-FDMA • Similarities – Block-wise data processing and use of Cyclic Prefix – Divides transmission bandwidth into smaller sub-carriers – Channel inversion/equalization is done in frequency domain – SC-FDMA is regarded as DFT-Precoded or DFT-Spread OFDMA • Difference – Signal structure: In OFDMA each sub-carrier only carries information related to only one data symbol while in SC-FDMA, each sub-carrier contains information of all data symbols. – Equalization: Equalization for OFDMA is done on per-subcarrier basis while for SC-FDMA, equalization is done over the group of sub-carriers used by transmitter. – PAPR: SC-FDMA presents much lower PAPR than OFDMA does. – Sensitivity to freq offset: yes for OFDMA but tolerable to SC-FDMA. All rights reserved @ 2009
    • LTE Physical layer and transmission procedures All rights reserved @ 2009
    • LTE physical layer – a vertical view • What kind of information is transmitted? – Upper layer SDUs plus additional L1 control information in transmission, e.g Reference Signals, Sync signals,CQI, HARQ,etc control information • How is it transmitted? or user data – Downlink OFDMA and uplink SC-FDMA – Channel dependent scheduling, HARQ,etc PDCP – multiple antenna support RLC • Related L1 procedures – random access, power control, time alignment, etc MAC Transport blocks coding Scrambling modulation multiplex control information reference signals signals from other channels frequency time All rights reserved @ 2009
    • LTE physical layer - a horizontal view • PBCH: carries system broadcast information • PCFICH: indicates resources used for PDCCH • PHICH: carries ACK/NACK for HARQ operation. • PDCCH: carriers scheduling assignments and other control information • PDSCH: conveys data or control information • PMCH: for MBMS data transmission • Reference signal • Synchronization signal (PSS,SSS) • PUCCH: carries control information • PRACH: to obtain uplink synchronization • PUSCH: for data or control information • Reference Signals (Demod RS & SRS) Feedback C QIs, data transm ission PDCCH n otifies how to demodula te d ata All rights reserved @ 2009
    • Fundamental Downlink transmission scheme 1 radio frame = 10 sub-frames = 10 ms 1 sub-frame = 2 slot = 14 OFDM symbols* 1 sub-frame = 1 ms 1 resource element 1 slot = 0.5 ms = 7 OFDM symbols 1 resourrc block = 12 sub-carriers = 180KHz 1 radio frame = 10 ms ⎧5.2 μs, for first OFDM symbol Tcp = ⎨ ⎩4.7 μs, 66.7 us Tcp for remaining symbols 66.7 us Tcp _ e = 16.7 μs Tcp-e *An alternative slot structure for MBMS is 6 OFDM symbols per slot where extended CP is in use. All rights reserved @ 2009
    • System information broadcast • System information – MIB: transmitted on PBCH (40msTTI) One BCH transportation block • information about downlink bandwidth CRC insertion • PHICH configuration • SFN 1/3 conv. – SIB: transmitted on PDSCH(DL-SCH) coding • SIB1: operator infor & access restriction infor scrambling • SIB2: uplink cell bandwidth, random access parameters • SIB3: cell-reselection modulation • SIB4~SIB8: neighbor cell infor antenna PBCH: the first 4 OFDM mapping symbol in 2nd Slot per 10ms frame De-multiplexing 10MHz 1.08 MHz 600 subcarriers Synchronization signal 10ms frame 10ms frame All rights reserved @ 2009
    • Downlink control channels – PCFICH,PHICH • PCFICH: – tells about the size of the control region. – Locates in the first OFDM symbol for each sub-frame. 16 symbols 2 bits 1/16 32 bits 32 bits Scrambling QPSK mod block code PCFICH-to-resource-element mapping depends on cell identity so as to avoid • PHICH: inter-cell interference. – acknowledges uplink data transfer – Locates in 1st OFDM symbol for each sub-frame One PHICH group inferior to PCFICH allocation contains 8 PHICHs 1 bit 3x 3 bits I BPSK mod repetition 12 symbols … Orthogonal code 1 bit 3x 3 bits Q BPSK mod repetition scrambling Orthogonal code All rights reserved @ 2009
    • Downlink control channels - PDCCH • Downlink control information (DCIs) – Downlink scheduling assignments – Uplink scheduling assignments – Power control commands • Control region size indicated by PCFICH • Blind decoded by UE in its “search space” and common “search space” – allows UE’s micro-sleep even in active state • QPSK always used but channel coding rate is variable control information control region reference signals 1 sub-frame = 1 ms R1-073373 “ Search space definition ofr L1/L2 control channels. “Downlink control channel design for 3GPP LTE”, Robert Love, Amitava Ghosh, et,al. IEEE WCNC 2008. All rights reserved @ 2009
    • Downlink control channels – PDCCH • How to map DCIs to physical resource elements – Control Channel Elements(CCEs), consisting of 36 REs, are used to construct control channels. – CCE aggregated at pre-defined level(1,2,4,8) to ease blind detections. • Usually 5MHz bandwidth system renders 6 UL/DL scheduling assignments within a sub-frame. CCH candidate 10 CCH candidate 1 CCH candidate 3 CCH candidate 4 CCH candidate 5 CCH candidate 6 CCH candidate 7 CCH candidate 9 CCH candidate 2 CCH candidate 8 Control channel candidate set Control Channel Element 0 Or search space Control Channel Element 1 Control Channel Element 2 Control Channel Element 3 Control Channel Element 4 Control Channel Element 5 Control channel candidates on which the UE attempts to decode the information R1-070787 “Downlink L1/L2 CCH design” (10 decoding attempts in this example) All rights reserved @ 2009
    • Downlink control channels - PDCCH • Each PDCCH carries one DCI message. Control information Control information Control information RNTI CRC attachment RNTI CRC attachment RNTI CRC attachment 1/3 Conv Coding 1/3 Conv Coding …… 1/3 Conv Coding Rate mattching Rate mattching Rate mattching CCE aggragation and PDCCH multiplexing Scrambling QPSK Interleaving Cell specific Cyclic shift All rights reserved @ 2009
    • Downlink shared channel: PDSCH Transport block Transport block • Support up to 4 Tx antennas* from MAC from MAC • Resource block allocation: CRC CRC – Localized: with less signaling overheads – Distributed: benefits from frequency diversity Segmentation Segmentation • Channelization (location): FEC FEC RM+HARQ RM+HARQ control information reference signals Scrambling Scrambling User A data region Modulation Modulation User B User C Antenna mapping unused Cell-specific, bit-level RB mapping scrambling for interference randomization ** 1 sub-frame = 1 ms To OFDM modulation for each antenna * For MBSFN, antenna diversity scheme does not apply. ** For MBSFN, it’s MBSFN-area-specific scrambling. All rights reserved @ 2009
    • Downlink reference signals • Cell-specific reference signals are length-31 Gold sequence, initialized based on cell ID and OFDM symbol location. • Each antenna has a specific reference signal pattern, e.g 2 antennas – frequency domain spacing is 6 sub-carriers – Time domain spacing is 4 OFDM symbols – That is, 4 reference symbols per Resource Block per antenna time frequency Antenna 0 Antenna 1 3GPP TS 36.211 “ physical channels and modulation“ section 6.10.1.1 All rights reserved @ 2009
    • LTE Multiple antenna scheme NodeB transmitter WCDMA STTD scheme: S 0 , S1 , S 2 , S3 S 0 , S1 , S 2 , S3 UE STTD − S * , S * ,− S * , S * 1 0 3 2 LTE SFBC (space frequency block coding): LTE CDD (cyclic delay diversity): eNodeB transmitter eNodeB transmitter a0 a0 a1 a1 a2 OFDM a2 OFDM a3 modulation a3 modulation … … − a0 * UE UE a0 a1e j 2πΔf ⋅Δt * a1 − a3* OFDM a2 e j 2πΔf ⋅2 Δt OFDM modulation modulation a3e j 2πΔf ⋅3Δt * a2 … All rights reserved @ 2009 …
    • LTE Multiple antenna scheme • Downlink SU-MIMO – Transmission of different data streams simultaneously over multiple antennas – Codebook based pre-coding: signal is “pre-coded” at eNodeB before transmission while optimum pre-coding matrix is selected from pre-defined codebook based on r UE feedback. r S γ – Open-loop mode possible for high speed S1 r1 Pre- H SIC coding receiver S2 r2 eNodeB UE PMI, RI, CQI • Uplink MU-MIMO: collaborative MIMO – Simultaneous transmission from 2UEs on same time-frequency resource – Each UE with one Tx antenna – Uplink reference signals are coordinated between UEs All rights reserved @ 2009
    • LTE Multiple antenna scheme LTE channels Multiple Antenna Schemes comments open-loop spatial multiplexing large delay CDD/ SFBC closed-loop spatical multiplexing SU-MIMO DL data channel PDSCH multi-user MIMO MU-MIMO UE specific RS beam-forming Applicable > 4 Antennas PDCCH SFBC PHICH SFBC DL control channel PCFICH open-loop transmit diversity SFBC PBCH SFBC Sync Signals PVS receiver diversity MRC/IRC UL data channel PUSCH multi-user MIMO MU-MIMO PUCCH receiver diversity MRC UL control channel PRACH receiver diversity MRC All rights reserved @ 2009
    • Synchronization and Cell Search • LTE synchronization design considerations: – high PSR (Peak to side-lobe ratio: the ratio between the peak to the side-lobes of its aperiodic autocorrelation function) to ease time-domain processing – low PAPR for coverage – Generalized Chirp Like (GCL) sequences overwhelm Golay and Gold sequences! • Synchronization signals – PSS: length-63 Zadoff-Chu sequences • Auto-correlation/cross-correlation/hybrid correlation based detection – SSS: an interleaved concatenation of two length-31 binary sequences • Alternative transmission (SSS1 and SSS2) in one radio frame 1 radio frame = 10 ms SSS 0 1 2 3 4 5 6 7 8 9 PSS 3GPP TS 36.211 “physical channels and modulation “ “Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE All rights reserved @ 2009
    • Synchronization and Cell Search • LTE synchronization design considerations: – high PSR (Peak to side-lobe ratio: the ratio between the peak to the side-lobes of its aperiodic autocorrelation function) to ease time-domain processing – low PAPR for coverage – Generalized Chirp Like (GCL) sequences overwhelm Golay and Gold sequences! • Synchronization signals – PSS: length-63 Zadoff-Chu sequences • Auto-correlation/cross-correlation/hybrid correlation based detection – SSS: an interleaved concatenation of two length-31 binary sequences • Alternative transmission (SSS1 and SSS2) in one radio frame 1 radio frame = 10 ms SSS 0 1 2 3 4 5 6 7 8 9 PSS 62 Central Sub-carriers 3GPP TS 36.211 “physical channels and modulation “ “Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE All rights reserved @ 2009
    • Synchronization and Cell Search • Hierarchical cell ID(1 out of 504): – Cell ID = 3* Cell group ID + PHY ID : N ID = 3 ⋅ N ID) + N ID) CELL (1 (2 ⎧ − j πun63 +1) (n μ = 25 N ID) = 0 (2 • PSS structure ⎪ e n = 0,1,...,30 d u (n) = ⎨ πu ( n +1)( n + 2 ) μ = 29 N ID) = 1 (2 −j ⎪e ⎩ 63 n = 31,32,...,61 μ = 34 N ID) = 2 (2 x0 pss 62 sub-carriers excluding DC carrier PSS sequences x1 CP pss … … … IFFT insertion f 62 x pss f The indices (m0, m1) define odd sub-carriers the cell group identity. • SSS structure even sub-carriers + + S 0m ( 0 ) SSC1 S1m (1) SSC1 C0 C0 + + + + S1m (1) SSC2 S 0m ( 0 ) SSC2 slot 0 … slot 10 C1 Z1m ( 0 ) C1 Z1m (1) All rights reserved @ 2009
    • LTE Cell Search Vs WCDMA cell search • PSS detection • P-SCH detection – Slot timing – Slot boundary – Physical layer ID (1 of 3) • S-SCH detection • SSS detection – frame timing – Radio frame timing – code group ID – Cell group ID (1 of 168) • CPICH detection – CP length – Cell-specific scrambling code • PBCH decoding identified – PBCH timing • BCH reading – System information access All rights reserved @ 2009 “cell searching in WCDMA”,Sanat Kamal Bahl, IEEE Potential 2003;
    • LTE uplink • SC-FDMA: fundamental uplink radio parameters are aligned with downlink scheme, e.g frame structure, sub-carrier spacing, RB size.… • Multiplexing of uplink data and control information – Combination of FDM and TDM are adopted in LTE uplink • Uplink transmission are well time-aligned to maintain orthogonality (no intra-cell interference) • PRACH will not convey user data like WCDMA does, but serve to obtain uplink synchronization All rights reserved @ 2009
    • Fundamental uplink transmission scheme 1 sub-frame = 1 ms 1 slot = 0.5 ms = 7 OFDM symbols 1 radio frame = 10 ms under eNodeB scheduling f ⎧5.2μs, for first OFDM symbol Tcp = ⎨ ⎩4.7 μs, 66.7 us Tcp for remaining symbols 66.7 us Tcp _ e = 16.7 μs Tcp-e • Uplink transmission frame aligned with downlink parameterization to ease UE implementation. All rights reserved @ 2009
    • Uplink reference signal • Uplink reference signals – Mostly based on Zadoff-Chu sequences (cyclic extensions) interference – Pre-defined QPSK sequences for small RB allocation randomization • Demodulation Reference Signal (DRS) in a cell across intra-cell and inter-cells – Each cell is assigned 1 out of 30 sequence groups – Each sequence group contains 1(for less than 5 RB case) or 2 (6RB+ case) RS sequence across all possible RB allocations – Sequence-group hopping is configurable in term of broadcasting information where the hopping pattern is decided by Cell ID – Cyclic time shift hopping applies to both control channel and data channel • DRS on PUSCH 0 0 RS sequence block of DFT OFDM add CP … … (size M) modulator data symbols Instantaneous bandwidth 0 (M sub-carriers) 0 One DFTS-OFDM symbol 3GPP TS 36.101 “physical channels and modulation” section 5.5.1 All rights reserved @ 2009
    • Uplink reference signal • DRS on PUCCH – See next slides • Sounding Reference Signal (SRS) – Not regularly but allows eNodeB to estimate uplink channel quality at alternative frequencies – UE’s SRS transmission is subject to network configuration – Location: always on last OFDM symbol of a sub-frame if available one sub-frame wideband, non-frequency hopping SRS narrowband, frequency hopping SRS All rights reserved @ 2009
    • Uplink control channel transmission - PUCCH • Uplink control signaling – Data associated: transport format, new data indicator, MIMO parameters – Non-data associated: ACK/NACK, CQI, MIMO codeword feedback no explicit tranmission • Channelization from UE as it follows eNodeB scheduling! – In the absence of uplink data transmission: in reserved frequency region on band edge – In the presence of uplink data transmission: see multiplexing with data on PUSCH Control region 1 Control region 2 Uplink control TDM with data ….. downlink total uplink data transmission system bandwidth f downlink data transmission 1 ms sub-frame standalone uplink control All rights reserved @ 2009
    • Uplink control channel transmission - PUCCH • To cater for multiple downlink transmission mode, while preserving single-carrier property in uplink, multiple PUCCH formats exist. • PUCCH is thus mainly classified by PUCCH format 1 & 2 – PUCCH format 1/1a/1b: 1 or 2 bits transmitted per 1ms, for ACK/NACK/SR – PUCCH format 2/2a/2b: up to 20 bits transmitted per 1ms, for CQI/PMI/RI reference reference ACK/NACK CQI signal signal ….. ….. 1 ms sub-frame 1 ms sub-frame All rights reserved @ 2009
    • Multiuser transmission on PUCCH • In PUCCH format 1, multiple PUCCHs are distinguished by cyclic shift of ZACAC sequences plus orthogonal cover sequence • In PUCCH format 2, multiple PUCCHs are distinguished by cyclic shift of ZACAC sequences. ACK/NACK bit channel status report BPSK/QPSK Length-12 phase QPSK rotated sequence Length-12 phase rotated sequence IFFT IFFT IFFT IFFT Length-4 IFFT IFFT IFFT IFFT IFFT Walsh sequence RS RS RS RS RS 1 slot = 0.5 ms 1 slot = 0.5 ms All rights reserved @ 2009
    • Uplink data transmission - PUSCH • In case of PUSCH available, control signaling is multiplexed with data on PUSCH. – To cater for radio channel variation, link adaptation applies to data part – Control signaling does not adopt adaptive modulation but the size of REs (resource elements) can change w.r.t varying radio condition DFTS-OFDM CQI/PMI modulation RS ACK/NACK Turbo Rate UL-SCH coding matching RI PUSCH data Conv Rate baseband CQI,/PMI coding matching MUX DFT IFFT modulation Block Rate RI coding matching ACK/NACK Block QPSK coding t All rights reserved @ 2009
    • Uplink data transmission - PUSCH • UL-SCH processing chain – No Tx diversity/spatial multiplexing as downlink does Transport block from MAC @UE – PUSCH frequency hopping (on slot basis) • Subband-based hopping according to cell-specific hopping patterns CRC • Hopping based on explicit hopping information in scheduling grant Segmentation FEC RM+HARQ Scrambling Modulation UE-specific, bit-level scrambling To DFTS-OFDM and map to assigned frequency resorurce All rights reserved @ 2009
    • Random Access • LTE random access serves to obtain uplink synchronization, not to carry data. – Contention-based random access: preambles based on ZC sequences – Contention-free random access: faster with reserved preambles (e.g, for handover) • Random access resources UE eNodeB – 64 preambles classified into 3 parts: RA preambles temporary C-RNTI; Preamble set #0 Preamble set #1 reserved timing advance; … … NAS UE ID RRC initial uplink grant Connection RA response (timing – RA area: Request adjustment, UL grant) • 1 in every 1~20 ms(configurable) 1ms 6 RBs random access area UE terminal ID early contention resolution Contention resolution 10 ms frame All rights reserved @ 2009
    • Random Access • PRACH structure – Preamble sequence: cyclic shifted sequences from multiple root ZC sequences – CP: facilitates frequency-domain prcoessing at eNodeB – Guard time: to handle timing uncertainty near user Other users CP Preamble Sequence Guard time Other users far user Other users CP Preamble Sequence Other users timing • PRACH format options uncertainty preamble format RA window (ms) Tcp length (ms) Tseq length (ms) Typical usage 0 1 0.1 0.8 for small~medium cells (up to ~ 14 km) for larget cells(up to ~ 77km) without link 1 2 0.68 0.8 budget problem for medium cells(up to ~ 29km) 2 2 0.2 1.6 supporting low data rates 3 3 0.68 1.6 for very large cells(up to ~ 100km) All rights reserved @ 2009
    • Layer 1 procedures – power control • Uplink power control – WCDMA power control is continuous at 1500Hz; while LTE runs power control slower at 200Hz – Based on open-loop setting while assisted by close-loop adjustment – Independent power control on PUCCH and PUSCH respectively • PUCCH power control To increase uplink data rate, LTE would increase user’s bandwidth PT = min{Pmax , P0 + PLDL + Δ format + δ } rather than increase Tx power! • PUSCH power control – Independent of PUCCH power control – UE Power Headroom in use to indicate the true desired Tx power PT = min{Pmax , P0 + α ⋅ PLDL + 10 ⋅ log10 ( M ) + Δ MCS + δ } All rights reserved @ 2009
    • Layer 1 procedures – Timing Alignment • To maintain uplink intra-cell orthogonality, timing alignment is necessary. – The further away from eNodeB, the earlier the UE transmits. – Configurable by eNodeB at granularity of 0.52us from 0 ~0.67 ms (corresponding to max cell radius of 100km) Tx Rx Tp1 Timing aligned uplink Rx Ta1 reception at eNodeB for Tx different users Tp2 Rx Ta2 Tx All rights reserved @ 2009
    • All rights reserved @ 2009
    • Backup - OFDMA Vs SC-FDMA • Channel equalizer: – OFDMA: divides wideband into multiple narrow “flat-fading” sub- bands hence equalization done on each sub-band is sufficient. – SC-FDMA: frequency domain equalization on the whole group bandwidth of sub-carriers in use. equalizer Detect Sub-carrier equalizer Detect OFDMA: DFT … … … … … de-mapping equalizer Detect Sub-carrier SC-FDMA: DFT equalizer IDFT detect … … … … … de-mapping All rights reserved @ 2009
    • Backup - OFDMA Vs SC-FDMA s (t ) 2 • PAPR: PAPR = E ( s(t ) 2 ) ⎡ (vn ) rms ⎤ 20 log10 ⎢ 3 3 ⎥ ⎢ (vref ) rms ⎥ 20 log10 (vn ) rms − 1.5237 ⎣ ⎦= 3 • CM: a better measure of UE PA back-off CM = F 1.85 SC-FDMA has around 2dB CM gain against OFDMA! “3G evolution, HSPA and LTE for mobile broadband(2nd edition)”, ISBN: 978-0-12-374538-5, page.118, All rights reserved @ 2009
    • Backup - Zadoff-Chu sequence characteristics • Zadoff-Chu sequences ⎧ − j πun ( n +1) ⎪ e 63 n = 0,1,...,30 d u ( n) = ⎨ πu ( n +1)( n + 2) ⎪e − j 63 n = 31,32,...,61 ⎩ • Property of ZC sequences: – Constant amplitude, even after Nzc-point DFT. – Ideal cyclic auto-correlation – Constant cross-correlation[=sqrt(1/Nzc)], assuming Nzc is a prime number “Polyphase codes with good periodic correlation properties”, J.D.C.Chu, IEEE trans on Informaiton theory, ,vol.18, pp.531-532, July 1972 “Phase shift pulse codes with good periodic correlation properties”, R.Frank,S.Zadoff and R.Heimiller, IEEE Trans on Information Theory, Vol 8, pp 381-382, Oct 1962. All rights reserved @ 2009
    • Backup – mobility: intra-MME handover UE Source eNodeB Target eNodeB EPC Measurement reporting Handover decision Handover request Admission control Handover request Ack RRC Connection Reconfiguration Detach from Deliver packets old cell to target eNodeB Data forwarding buffer packets From source eNodeB RRC Connection Reconfiguration complete Path switch procedure UE context release Flush buffer Release resource All rights reserved @ 2009