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Long Term Evolution

Long Term Evolution

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Long Term Evolution

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  1. Long Term Evolution Technology training (Part 1)

  2. Outline LTE and SAE overview LTE radio interface architecture LTE radio access architecture LTE multiple antenna techniques

  3. Part 1 LTE/SAE overview

  4. Mobile broadband (3GPP) • 3G continues to evolve • Standardized through 3GPP • 3G gracefully evolves into 4G – starting from R7 and R8 • Date rates • R99: 0.4Mbps UL, 0.4Mbps DL • R5: 0.4Mbps UL, 14Mbps DL • R6: 5.7Mbps UL, 14Mbps DL • R7: 11Mbps UL, 28Mbps DL • R8: 50Mbps UL on LTE, 160 Mbps DL on LTE, 42Mbps DL on HSPA • Two branches of the standards • HSPA : Gradual performance improvements at lower incremental costs • LTE: revolutionary changes with significant performance improvements (higher cost, first step towards IMT advanced)

  5. LTE Releases Note: This presentation focuses on R8 features LTE – has an “evolution path” of its own Evolution is towards IMT-Advanced (LTE advanced) LTE advanced – spectral efficiency 30bps/Hz (DL), 15bps/Hz (UL)

  6. LTE requirements • Outlined in 3GPP TR 29.913 • Seven different areas • Capabilities • System performance • Deployment related aspects • Architecture and migration • Radio resource management • Complexity, and • General aspects • Capabilities • DL data rate > 100 Mbps in 20 MHz • UL data rate > 50 Mbps in 20MHz • Rate scales linearly with spectrum • Latency user plane: 5ms (transmission of small packet from UE to edge of RAN) • Latency control plane: transmission time from camped state – 100ms, transmission time from dormant state 50 ms • Support for 200 mobiles in 5MHz, 400 mobiles in more than 5MHz • System performance • Baseline is HSPA Rel. 6 • Throughput specified at 5% and 50% • Maximum performance for low mobility users (0-15km/h) • High performance up to 120 km/h • Maximum supported speed 500km/h • Cell range up to 100km • Spectral efficiency for broadcast 1 b/s/Hz Throughput requirements relative to baseline

  7. LTE requirements (2) • Deployment related aspects • LTE may be deployed as standalone or together with WCDMA/HSPA and/or GSM/GPRS • Full mobility between different RANs • Handover interruption time targets specified • Spectrum flexibility • Both paired and unpaired bands • IMT 2000 bands (co-existence with WCDMA and GSM) • Channel bandwidth from 1.4-20MHz Handover interruption time LTE duplexing options

  8. LTE requirements (3) • Architecture and migration • Single RAN architecture • RAN is fully packet based with support for real time conversational class • RAN architecture should minimize “single points” of failure • RAN should simplify and reduce number of interfaces • Radio Network Layer and Transport Network Layer interaction should not be precluded in interest of performance • QoS support should be provided for various types of traffic • Radio resource management • Support for enhanced end to end QoS • Support for load sharing between different radio access technologies (RATs) • Complexity • LTE should be less complex than WCDMA/HSPA

  9. SAE design targets • SAE – Service Architecture Evolution • SAE = core network • Requirements placed into seven categories • High level and operational aspects • Basic capabilities • Multi-access and seamless mobility • Man-machine interface aspects • Performance requirements for Evolved 3GPP system • Security and privacy • Charging aspects • SAE requirements mainly non access related (highlighted ones have impact on RAN)

  10. Basic principles – Air interface • Downlink OFDM • OFDM = Orthogonal Frequency Division Multiplexing • OFDM = Parallel transmission on multiple carriers • Advantages of OFDM • Avoid intra-cell interference • Robust with respect to multi-path propagation and channel dispersion • Disadvantage of OFDM • High PAPR and lower power amplifier efficiency • Uplink DFTS-OFDM (SC-FDMA) • DFTS = DFT spread OFDM • SC-FDMA = Single carrier FDMA • Advantages (all critical for UL) • Signal has single carrier properties • Low PAPR • Similar hardware as OFDM • Reduced PA cost • Efficient power consumption • Disadvantage • Equalizer needed (not critical from UL) UL modulation DL modulation

  11. Basic principles – Air interface • Sharedchannel transmission • Only PS support • No CS services • Fast channel dependent scheduling • Adaptation in time • Adaptation in frequency • Adaptation in code • Hybrid ARQ with soft combining • Chain combining • Incremental redundancy One shared channel simplifies the overall signaling Scheduler takes the advantage of time-frequency variations of the channel ARQ reduces required Eb/No

  12. Basic principles – air interface • MIMO support • MIMO = Multiple Input Multiple Output • Use of multiple TX / RX antennas • Three ways of utilizing MIMO • RX diversity/TX diversity • Beam forming • Spatial multiplexing (MIMO with space time coding) • MIMO transmission in Rayleigh fading environment increases theoretical capacity by a factor equal to number of independent TX RX paths • As a minimum LTE mobiles have two antennas (possibly four) Outline of spatial multiplexing idea Note: Rayleigh fading de-correlates the paths and provides multiple uncorrelated channels

  13. Basic principles – air interface • ICIC – Inter-cell interference coordination • LTE affected by inter-cell interference (more than HSDPA) • In LTE interference avoidance becomes scheduling problem • By managing resources across multiple cells inter-cell interference may be reduced • Standard supports exchange of interference indicators between the cells One possible implementation of ICIC. Cell edge implements N=3. Cell interior implements N=1.

  14. SAE-Architecture • SAE – flat architecture • Core network, • RAN • RAN consist of single elements: eNode B • Single element simplifies RAN • No single point of failure • Core network provides two planes • User plane (through SGSN) • Control plane (through MME) • Interfaces • S1-UP (eNode B to SGSN) • S1-CP (eNode B to MME) • X2 between two eNode Bs (required for handover) • Uu (UE to eNode B) LTE Network layout UE – user equipment (i.e. mobile) eNode B – base station SGSN – Support GPRS Serving Node GGSN – Gateway GPRS Serving Node MME – Mobility Management Entity PCRF - Policy and Charging Rules function SAE = System Architecture Evaluation

  15. LTE protocol-control plane NAS – Non Access Stratum RRC – Radio Resource Control PDCP – Packet Data Convergence Protocol RLC – Radio Link Control MAC – Medium Access Control S1-AP – S1 Application SCTP – Stream Control Transmission Prot. IP – Internet Protocol Note: LTE control plane is almost the same as WCDMA (PDCP did not exist in WCDMA control plane)

  16. LTE protocol- user plane PDCP – Packet Data Convergence Protocol RLC – Radio Link Control MAC – Medium Access Control GTP-U - GPRS Tunneling Protocol Note: LTE user plane is identical to UMTS PS side. There is no CS in LTE – user plane is simplified.

  17. LTE protocol – X2 Control plane GTP-U: GPRS tunneling protocol STCP: Stream Transmission Control Protocol User plane Connects all eNodeB’s that are supporting end user active mobility (handover) Supports both user plane and control plane Control plane – signaling required for handover execution User plane – packet forwarding during handover

  18. Channel structure Uu interface Note: LTE defines same types of channels as WCDMA/HSPA • Channels – defined on Uu • Logical channels • Formed by RLC • Characterized by typeof information • Transport channels • Formed by MAC • Characterized by how the data are organized • Physical channels • Formed by PHY • Consist of a group of assignable radio resource elements

  19. LTE - channel mapping

  20. Logical channels Red – common, green – shared, blue - dedicated LTE Channels • BCCH – Broadcast Control CH • System information sent to all UEs • PCCH – Paging Control CH • Paging information when addressing UE • CCCH – Common Control CH • Access information during call establishment • DCCH – Dedicated Control CH • User specific signaling and control • DTCH – Dedicated Traffic CH • User data • MCCH – Multicast Control CH • Signaling for multi-cast • MTCH – Multicast Traffic CH • Multicast data

  21. Transport channels Red – common, green – shared LTE Channels • BCH – Broadcast CH • Transport for BCCH • PCH – Paging CH • Transport for PCH • DL-SCH – Downlink Shared CH • Transport of user data and signaling. Used by many logical channels • MCH – Multicast channel • Used for multicast transmission • UL-SCH – Uplink Shared CH • Transport for user data and signaling • RACH – Random Access CH • Used for UE’s accessing the network

  22. PHY Channels LTE Channels Red – common, green – shared • PDSCH – Physical DL Shared CH • Uni-cast transmission and paging • PBCH – Physical Broadcast CH • Broadcast information necessary for accessing the network • PMCH – Physical Multicast Channel • Data and signaling for multicast • PDCCH – Physical Downlink Control CH • Carries mainly scheduling information • PHICH – Physical Hybrid ARQ Indicator • Reports status of Hybrid ARQ • PCIFIC – Physical Control Format Indicator • Information required by UE so that PDSCH can be demodulated (format of PDSCH) • PUSCH – Physical Uplink Shared Channel • Uplink user data and signaling • PUCCH – Physical Uplink Control Channel • Reports Hybrid ARQ acknowledgements • PRACH – Physical Random Access Channel • Used for random access

  23. Time domain structure Radio frame : Type 1 Radio frame : Type 2 • Two time domain structures • Type 1: used for FDD transmission (may be full duplex or half duplex) • Type 2: used for TDD transmission • Both Type 1 and Type 2 are based on 10ms radio frame

  24. TDD frame configurations Note: TDD frame structure allows co-existence between LTE TDD and TD-SCDMA Different configurations allow balancing between DL and UL capacity Allocation is semi-static Adjacent cells have same allocation Transition DL->UL happens in the second subframe of each half-frame

  25. Allocatable resources Resource Block (RB) = 12 carriers in one TS (12*15KHz x 0.5ms) • Time domain • 1 frame = 10 sub-frames • 1 subframe = 2 slots • 1 slot = 7 (or 6) OFDM symbols • Frequency domain • 1 OFDM carrier = 15KHz Note: In LTE resource management is along three dimensions: Time, Frequency, Code LTE – radio resource = “time-frequency chunk”

  26. Bandwidth flexibility LTE supports deployment from 6RBs to 110 RBs in 1 RB increments 6RBs = 6 x 12 x 15KHz = 1080KHz -> 1.4MHz (with guard band) 110RBs = 110 X 12 X 15KHz = 19800KHz -> 20MHz (with guard band) Typical deployment channel bandwidths: 1.4, 3, 5, 10, 15, 20 MHz Straight forward to support other channel bandwidths (due to OFDM) UE needs to support up to the largest bandwidth (i.e. 20MHz)

  27. UE States Note: Both the UE states and UE tracking are simpler than in UMTS • UE may be in three states • Detached: not connected to the network • Idle: attached to the network but not active • Connected: attached and active • UE tracking • Detached state: UE position unknown • Idle state: UE position know with the Tracking Area (TA) resolution • Connected: UE location known to the eNodeB resolution

  28. 3GPP Specifications Example specs organization • All 3GPP specs are available at • RAN 1 36.2xx series PHY layer • RAN2 36.3xx series Layers 2 and 3 • RAN3 36.4xx series S1 and X2 interfaces • RAN4 36.1xx series Core performance requirements • RAN5 36. 5xx series Terminal conformance testing

  29. Section review • What are 3GPP broadband cellular technologies? • What releases of 3GPP standard contains LTE? • What were target DL and UL throughputs for LTE? • What does SAE stand for? • What are components of the CS part of the LTE core network? • What is the access scheme used on the DL? • What is the role of fast scheduler on LTE DL? • What is the smallest allocateable resource in LTE DL? • What is Radio Block (RB)? • What are spectrum bandwidth deployment options for LTE? • How many radio blocks are in 20MHz deployment? • Does LTE support TDD deployment? • What are three UE States supported by LTE?

  30. Part 2 LTE Radio Access

  31. Overview Overview of OFDM/OFDMA LTE Downlink transmission Overview of DFTS-OFDM LTE Uplink transmission Multi-antenna transmission

  32. Single carrier transmission Transmission of single carrier in mobile terrestrial environment Note: over small portion of the signal spectrum, fading may be seen as flat Data are used to modulate amplitude/phase (frequency) of a single carrier Higher data rate results in wider bandwidth Over larger bandwidths ( > 20KHz), wireless channel is frequency selective As a result of frequency selectivity the received signal is severely distorted Channel equalization needed Complexity of equalizer increases rapidly with the signal bandwidth requirements

  33. Multi-carrier transmission Signal for each stream experiences flat fading Channel fading over smaller frequency bands – flat (no need for equalizer) Divide high rate input data stream into many low rate parallel streams At the receiver – aggregate low data rate streams

  34. FDM versus OFDM Note: orthogonality between carriers in time domain allows closer spacing in frequency domain. FDM versus OFDM OFDMA minimizes separation between carriers Carriers are selected so that they are orthogonal over symbol interval Carrier orthogonality leads to frequency domain spacing Df=1/T, where T is the symbol time In LTE carrier spacing is 15KHz and useful part of the symbol is 66.7 microsec

  35. OFDM transmitter/receiver Practically OFDM TX/RX is implemented using IFFT/FFT Use of the IFFT/FFT at the baseband means that there is no need for separate oscillators for each of the OFDM carriers FFT (IFFT) hardware is readily available – TX/RX implementation is simple

  36. Guard time OFDM symbols with guard time OFDM symbols without guard time Duration of the OFDM symbol is chosen to be much longer than the multi-path delay spread Long symbols imply low rate on individual OFDM carriers In multipath environment long symbol minimizes the effect of channel delay spread To make sure that there is no ISI between OFDM symbols – guard time is inserted

  37. Cyclic prefix Guard time eliminates ISI between OFDM symbols Multipath propagation degrades orthogonality between carriers within an OFDMS symbol To regain the orthogonality between subcarriers – cyclic prefix is used Cyclic prefix fills in the guard time between the OFDM symbols

  38. Block diagram of full OFDM TX/RX LTE supports numerous AMC schemes AMC adds additional level of adaptation to the RF channel Size of CP depends on the amount of dispersion in the channel Two CP are used: normal (4.7 us) and extended (16.7 us)

  39. OFDMA time-frequency scheduling Minimum allocateable resource in LTE is Resource Block pair Resource block pair is 12 carriers wide in frequency domain and lasts for two time slots (1ms) Depending on the length of cyclic prefix RB pair may have 14 or 12 OFDM symbols PHY channels consist of certain number of allocated RB pairs Overhead channels are typically in a predetermined location in time frequency domain Within a RB different AMC scheme may be used Allocation of the radio block is done by scheduler at eNode B

  40. Part 3 LTE Downlink Transmission

  41. LTE OFDM Basic timing unit: Ts = 1/(2048 x 15000) ~ 23.552 ns

  42. Detailed time domain structure Need for two different CP: To accommodate environments with large channel dispersion To accommodate MBSFN (Multi-Cast Broadcast Single Frequency Network) transmission In case of MBSFN it may be beneficial to have mixture of sub-frames with normal CP and extended CP. Extended CP is used for MBSFN sub-frames TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for other six symbols TCP-e: 512 Ts (16.7 us) for all symbols

  43. Exercise – OFDM data rate capability at the PHY Case 1. Normal CP (no MIMO) Resource block: 12 carriers x 14 OFDM symbols = 168 resource elements Each resource element carries one modulation symbol For 64 QAM: 1 symbol = 6 bits Number of bits per subframe = 168 x 6 = 1008 bits/subframe Raw PHY data rate = 1008/1ms = 1,008,000 bits/sec/resource block (180KHz) For 20MHz, Raw PHY data rate = 100 RB x 1,008,000 bits/sec/RB = 100.8Mbps Case 2. Extended CP (no MIMO) Resource block: 12 carriers x 12 OFDM symbols = 144 resource elements Each resource element carries one modulation symbol For 64 QAM: 1 symbol = 6 bits Number of bits per subframe = 144 x 6 = 864 bits/subframe Raw PHY data rate = 864/1ms = 864,000 bits/sec/resource block (180KHz) For 20MHz, Raw PHY data rate = 100 RB x 864,000 bits/sec/RB = 86.4Mbps Note: with the use of MIMO the rates are increased

  44. Downlink reference signals Note: Reference signals are staggered in time and frequency. This allows UE to perform 2-D complex interpolation of channel time-frequency response • For coherent demodulation – terminal needs channel estimate for each subcarrier • Reference signals – used for channel estimation • There are three type of reference signals • Cell specific DL reference signals • Every DL subframe • Across entire DL bandwidth • UE specific DL reference signals • Sent only on DL-SCH • Intended for individual UE’s • MBSFN reference signals • Support multicast/broadcast

  45. Cell specific reference signals Two port TX Four port TX One port TX • DL transmission may use up to four antennas • Each antenna port has its own pattern of reference signals • Reference signals are transmitted at higher power in multi-antenna case • Reference signals introduce overhead • 4.8% for 1 antenna port • 9.5% for 2 antenna ports • 14.3 % for 4 antenna ports • Reference symbols vary from position to position and from cell to cell – cell specific 2 dimensional sequence • Period of the sequence is one frame

  46. Cell specific reference signals (2) Shifts for single port transmission There are 504 different Reference Sequences (RS) They are linked to PHY-layer cell identities The sequence may be shifted in frequency domain – 6 possible shifts Each shift is associated with 84 different cell identities (6 x 84 = 504) Shifts are introduced to avoid collision between RS of adjacent cells In case of multiple antenna ports – only three shifts are useful For a given PHY Cell ID - sequence is the same regardless of the bandwidth used – UE can demodulate middle RBs in the same way for all channel bandwidths

  47. UE Specific RS Note: additional reference signals increase overhead. One of the most beneficial use of beam forming is at the cell edge – improves SNR UE specific RS – used for beam forming Provided in addition to cell specific RS Sent over resource block allocated for DL-SCH (applicable only for data transmission)

  48. PHY channels supporting DL TX Channels required for DL transmission SCH – allows mobile to synchronize to the DL TX during acquisition PBCH – used to broadcast static portion of the BCCH PDSCH – carries user information and signaling from upper layers of protocol stack PDCCH – channel used by MAC scheduler to configure L1/L2 and assign resources (DL scheduling and UL grants) PCFICH – explains to the UE the format of the DL transmission PHICH – support for HARQ on the uplink PUCCH – support for HARQ on the downlink

  49. Summary of PHY DL channels L1/L2 signaling Services to upper layers

  50. Downlink L1/L2 signaling • Three different PHY channel types • PCFIC (PHY Control Format Indicator Channel) • PHICH (PHY – Hybrid ARQ Channel) • PDCCH (PHY Downlink Control Channel) • Signaling that supports DL transmission • Originates at L1/L2 (no higher layer data or messaging) • Consists of • Scheduling assignments and associated information required for demodulation and decoding of DL-SCH • Uplink scheduling grants for UL-SCH • HARQ acknowledgements • Power control commands • L1/L2 signaling is transmitting in first 1-3 symbols of a subframe – control region • Size of control region may vary dynamically – always whole number of OFDM symbols (1,2,3) • Signaling – beginning of the subframe • Reduces delay for scheduled mobiles • Improves power consumption for non-scheduled mobiles