Lecture 13 UMTS Long Term Evolution. I. Tinnirello

Transcription

1 Lecture 13 UMTS Long Term Evolution

2 Beyond 3G International Mobile Telecommunications (IMT)-2000 introduced global standard for 3G Systems beyond IMT-2000 (IMT-Advanced) are set to introduce evolutionary path beyond 3G Mobile class targets 100 Mbps with high mobility and nomadic/local area class targets 1 Gbps with low mobility 3GPP and 3GPP2 are currently developing evolutionary/revolutionary systems beyond 3G 3GPP Long Term Evolution (LTE) 3GPP2 Ultra Mobile Broadband (UMB) IEEE based WiMax is also evolving towards 4G through m

3 3GPP Evolution Release 99 (Mar 2000): UMTS/WCDMA Release-5 (Mar 2002): HSDPA Release-6 (Mar 2005): HSUPA Release-7 (2007): DL MIMO, IMS (IP Multimedia Subsystem), optimized realt time services (VoIP, gaming, push-to-talk) Long Term Evolution (LTE) 3GPP work started in Nov 2004 Standardized in the form of Release 8 Spec finalized and approved in Jan 2008 Target deployment starting from 2010 LTE-Advanced study in progress

4 IEEE Evolution (2002): Line-of-isght fixed operation in 10 to 66 GHz a (2003): Air interface support fro 2 to 11 GHz d (2004): Minor improvements e (2006): Support for vehicular mobility and asymmetrical links m (2011): Higher data rate, reduced latency, efficient security mechanisms

5 Requirements of LTE Peak data rate 100 Mbps DL/50 Mbps UP within 20 MHz bandwidth Up to 200 active users in a cell (5 MHz) Less than 5 ms user-plane latency Mobility Optimized for 0-15 Km/h Km/h supported with high performance Supported up to 350 Km/h or even up to 500 Km/h Enhanced multimedia broadcast multicast service (E-MBMS) Spectrum flexibility: MHz Enhanced support for end-to-end QoS

6 LTE Enabling Technologies OFDM (Orthogonal Frequency Division Multiplexing) SC-FDMA (Single Carrier FDMA) MIMO (Multi-input Multi-output) Multicarrier channel-dependent resource scheduling Fractional frequency reuse

7 LTE Enabling Technologies Single Carrier FDMA (SC-FDMA) A new multiple access technique which has similar structure and performance to OFDMA Linearly pre-coded OFDMA Utilizes single carrier modulation and orthogonal frequency multiplexing using DFT-spreading in the transmitter and frequency domain equalization in the receiver Low PAPR respect to OFDMA H.G: Myung et al, Single Carrier FDMA for Uplink wireless transmission ; IEEE Vehic. Tech.

8 Key Features of LTE Multiple access scheme DL: OFDMA; UP: SC-FDMA Adaptive modulation and coding DL/UP modulations: QPSK, 16QAM and 64QAM Convolutional codes and Rel-6 turbo codes Advanced MIMO spational multiplexing techniques (2 or 4)x(2 or 4) downlink and uplink supported Multi-user MIMO TDD/FDD H-ARQ, mobility support, rate control, security, etc.

9 Key Features of LTE

10 Mobility Intra and Inter Technology

11 LTE Standard Specifications Downloadable from 3GPP

12 Network and Protocol Architecture

13 Protocol Architecture

14 LTE Network Architecture E-UTRAN: Evolved Universal Terrestrial Radio Access Network

15 LTE Network Architecture enb: enhanced Node B All radio interface-related functions MME: Management Mobility Entity Manages Mobility, UE identity and security parameters S-GW: Serving Gateway Node that terminates the interface towards E-UTRAN P-GW: Packet Data Network Gateway Node that terminates the interface towards PDN

16 Novel Components The overall system architecture was revisited: New Radio-Access Network (RAN) New Core Network (referred as EPC) The RAN is responsible of: Scheduling and radio-resource handling Retransmission protocols Coding and antenna schemes The EPC is responsible of: Authentication Charging functionality Connections setup Evolved Packet System (EPS)

17 Core Network The EPC is a radical evolution from the other core network (GSM/GPRS, WCDMA/HSPA) Supports access to the packetswitched domain only (no access to the circuit-switched domain) Contains several nodes: Mobility Management Entity (MME) The Serving Gateway (S-GW) The Packet Data Network Gateway (PDN/P-GW) Other nodes: Policy and Charging Rules Function Home Subscriber Service Multimedia Broadcast Multicast Services All this nodes are logical nodes!

18 LTE Network Interfaces

19 Nodes Functionalities

20 Protocol Stacks

21 Circuit-Switched Fall Back (CSFB) LTE is a pure PS-switched network! natively supports VoIP using IMS services CSFB mechanism allows to handle voice calls by means of 3G/2G networks

22 Message Sequence for CSFB

23 Background on OFDMA

24 OFDM Basic Concept OFDM is a special case of Frequency Division Multiplexing (FDM) For FDM No special relationship between the carrier frequencies Guard bands have to be inserted to avoid Adjacent Channel Interference (ACI) For OFDM Strict relation between carriers: f k = k f where f = 1/T U (T U - symbol period) Carriers are orthogonal to each other and can be packed tight

25 How does OFDM work? In principle similar to CDMA, with continous orthogonal codes!

26 OFDM Transmission model Channel, h(t) Modulator and transmitter Wireless channel Receiver and demodulator

27 Orthogonality the essential property Example: Receiver branch k Ideal channel: No noise and no multipath 1 T N 1 T 1 U Nc 1 c U a j2π( q k ) t j2πq ft j2πk ft q TU a e dt = e dt k q U = q= 0 T 0 q 0 U 0 T e a, = 0, k k = q q Received signal, r(t) T u = 1/ f gives subcarrier orthogonality over one T u => possible to separate subcarriers in receiver

28 Example of OFDM Lets we have following information bits 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, Just converts the serials bits to parallel bits C1 C2 C3 C

29 Example of OFDM cont.. Modulate each column with corresponding sub-carrier using BPSK Modulated signal for C1 Modulated signal for C2 Modulated signal for C3 Modulated signal for C4

30 Example of OFDM cont.. Final OFDM Signal = Sum of all signal V ( t) N 1 = n= 0 I n ( t)sin(2πnt ) V(t) Generated OFDM signal, V(t)

31 Multipath channel Diffracted and Scattered Paths [ α k, τ k ] LOS Path [ 0 α 0, τ ] [ 1 α 1, τ ] Reflected Path

32 Multipath channel (cyclic prefix) Multipath introduces inter-symbol-interference (ISI) T U Amplitude [α] Example multipath profile T CP Prefix is added to avoid ISI T U τ Time 0 τ 1 τ 2 The prefix is made cyclic to avoid inter-carrier-interference (ICI) [τ] (maintain orthogonality)

33 Multipath channel (cyclic prefix) T cp should cover the maximum length of the time dispersion Increasing T cp implies increased overhead in power and bandwidth (T cp / T S ) For large transmission distances there is a tradeoff between power loss and time dispersion T S CP Useful symbol CP Useful symbol CP Useful symbol T cp T U

34 Multipath channel (frequency diversity) The OFDM symbol can be exposed to a frequency selective channel The attenuation for each subcarrier can be viewed as flat Due to the cyclic prefix there is no need for a complex equalizer Possible transmission techniques Forward error correction (FEC) over the frequency band Adaptive coding and modulation per carrier =

35 Multipath channel (pilot symbols) The channel parameters can be estimated based on known symbols (pilot symbols) The pilot symbols should have sufficient density to provide estimates with good quality (tradeoff with efficiency) Different estimation methods exist Averaging combined with interpolation Minimum-mean square error (MMSE) Pilot carriers /reference signals Data carriers Time Frequency/subcarrier Pilot symbol Frequency

36 The Peak to Average Power Problem A OFDM signal consists of a number of independently modulated symbols The sum of independently modulated subcarriers can have large amplitude variations x(t) = Nc 1 k= 0 j2πk f t Results in a large peak-to-average-power ratio (PAPR) a k e PA

37 The Peak to Average Power Problem Example with 8 carriers and BPSK modulation x(t) plotted It can be shown that the PAPR becomes equal to N c

38 The Peak to Average Power Problem High efficiency power amplifiers are desirable For the handset, long battery life For the base station, reduced operating costs A large PAPR is negative for the power amplifier efficiency Non-linearity results in intermodulation Degrades BER performance Out-of-band radiation P OUT AM/AM characteristic IBO OBO PA Average Peak P IN 38

39 The Peak to Average Power Problem Different tools to deal with large PAPR Signal distortion techniques Clipping and windowing introduces distortion and out-of-band radiation, tradeoff with respect to reduced backoff Coding techniques FEC codes excludes OFDM symbols with a large PAPR (decreasing the PAPR decreases code space). Tone reservation, and pre-coding are other examples of coding techniques. Scrambling techniques Different scrambling sequences are applied, and the one resulting in the smallest PAPR is chosen

40 Summary Advantages Splitting the channel into narrowband channels enables significant simplification of equalizer design Effective implementation possible by applying FFT Flexible bandwidths enabled through scalable number of sub-channels Possible to exploit both time and frequency domain variations (time domain adaptation/coding + freq. domain adaptation/coding) Challenges Large peak to average power ratio

41 Radio Interface

42 Frame Structure Two radio frame structures Type 1: FS1 FDD Type 2: FS2 TDD A radio frame has a duration of 10 ms A resource block (RB) spans 12 subcarriers over a slot duration of 0.5ms One subcarrier has a 15 KHz bandwidth, thus 180 KHz per RB

43 Frame Structures FDD TDD

44 Resource Grid

45 LTE Bandwidth/Resource Configuration

46 Bandwidth Configuration Example

47 LTE Physical Channels A set of subcarriers lasting some symbols DL: Physical Broadcast Channel (PBCH) Physical Control Format Indicator Channel (PCFICH) Physical Downlink Control Channel (PDCCH) Physical Hybrid ARQ Indicator Channel (PHICH) Physical Downlink Shared Channel (PDSCH) Physical Multicast Channel (PMCH) UP: Physical Uplink Control Channel (PUCCH) Physical Uplink Shared Channel (PUSCH) Physical Random Access Channel (PRACH)

48 LTE Transport Channels DL: Broadcast Channel (BCH) Downlink Shared Channel (DL-SCH) Paging Channel (PCH) Multicast Channel (MCH) UP: Uplink Shared Channel (UL-SCH) Random Access Channel (RACH)

49 LTE Logical Channels Control channels for control-plane info Broadcast Control Channel (BCCH) Paging Control Channel (PCCH) Common Control Channel (CCCH) Multicast Control Channel (MCCH) Dedicated Control Channel (DCCH) Traffic channels for user-plane info Dedicated Traffic Channel (DTCH) Multicast Traffic Channel (MTCH)

50 Channel Mappings

51 LTE Layer 2

52 RRC Layer Terminated in enb on the network side Broadcast, Paging RRC connection management Radio Bearer management Mobility Functions UE measurement reporting and control RRC states: RRC_IDLE, RRC_CONNECTED

53 Resource Scheduling of Shared Channels Dynamic resource scheduler resides in enb on MAC layer Radio resource assignment based on radio condition, traffic volume and QoS requirements Radio resource assignment consists of: Physical Resource Block (PRB) Modulation and Coding Scheme (MCS)

54 Radio Resource Management Radio Bearer Control (RBC) Radio Admission Control (RAC) Connection mobility Control (CMC) Dynamic resource allocation (DRA) or packet scheduling Inter-cell interference coordination (ICIC) Load Balancing (LB) Other: ARQ, H-ARQ, Rate Control, DRX, QoS, Scurity

55 LTE Uplink

56 UL Resource BLock

57 Uplink Channels Transport Channels (TrCH) UL-SCH Uplink Shared Channel RACH Random Access Channel Control Information UCI Uplink Control Information Mapping to Physical Channels

58 Uplink Control Signalling Conveys L1 and L2 control information HARQ acknowledhements for DL-SCH blocks Channel quality reports: CQI, RI and PMI Scheduling requests Transmitted on PUCCH if no resources are allocated to UL-SCH Multiplexed with UL-SCH on the PUSCH (before SC_FDMA) if there is a valid schedule grant

59 Data and Control Information on PUSCH

60 Channels and Signals A physical channel is defined as a set of resource elements carrying information originating at a higher layer or in support to the physical layer itself For the uplink, the following PHY channels are defined.. PUSCH: Phy Uplink Shared Channel PUCCH: Phy Uplink Control Channel; PRACH: Phy Random Access Channel..plus the following signals Souding Reference Signal (SRS) Not associated with any other transmission Demodulation Reference Signal (DRS) associated with PUSCH or PUCCH, for channel estimation Desired features: small power variations in time and frequency

61 Sounding Reference Signal enodeb needs channel quality information in order to assign resources From DRS enobeb can only get channel estimates on UE allocated spectrum No information available out of assigned spectrum SRS overcomes this problem!

62 Sounding Reference Signal (2) May cover large frequency span (not assigned to UE) Multiple of 4 resource blocks span Can be transmitted from every 2ms to every 160ms Transmitted on last symbol of subframe (at most every 2 subframes) Multiple UEs can transmit simultaneously thanks to cyclic shifts (orth codes) Wideband: one transmission covers band of interest Frequency hopping: narrowband, location changes with time

63 Physical Uplink Control Channel PUCCH: Conveys uplink control information Used when UE has no valid schedule grant Never transmitted simulanteously with PUSCH Transmitted with frequency hopping on band edges to leave contiguos bandwidth to PUSCH Multiple PUCCH over a RB by means of orthogonal codes

64 Physical Uplink Shared Channel PUSCH: Carries data and control information, by means of the following processing chain Allocated spectrum to a UE can changge every subframe

65 SC-FDMA Modulation in LTE UP

66 Uplink Summary

67 LTE Downlink

68 Downlink Channels Transport Channels (TrCH) DL-SCH Downlink Shared Channel BCH PCH MCH Control Information CFI HI DCI Mapping to Phy Channels Broadcast Channel Paging Channel Multicast Channel Control Format Indicator HARQ Indicator Downlink Control Information

69 Downlink Channels and Signals Phy Channel: set of resource elements carrying information from higher layers Phy DL Shared Ch PDSCH; Phy DL Control CH PDCCH; Phy Multicast Ch PMCH; Phy Broadcast Channel PBCG; Phy Control Format Indicator Channel PCFIC; Phy HARQ Indicator Channel PHICH Phy Signal: set of resource elements used for physical layer information Reference Signals Synchronization Signals

70 Phy Donwlink Channels PCFICH: Specifies how many OFDM symbols are used for PDCCH transmission PDCCH: carries control information including scheduling assignments; PHICH: hybrid ARW and NACK indicators for Ues PDSCH: main downlink channel to transport data blocks to the mobiles PBCH: broadcast information with a coded block of 1920 samples every 40 ms

71 Donwlink Reference Signals Used for channel estimations and to obtain quality measurements at the UE side Cell-specific: Structure depends on the cell ID UE- specific Used for beamforming Arranged across time and frequency

72 DL Reference Signal

73 Synchronization Signals Always transmitted in the same place regardless of the total bandwidth First 72 carriers around DC carrier OFDM symbols 5 and 6 of first slot in subframe 0 and 5

74 Downlink Processing Chain The general structure of downlink physical channels processing is the following one (for PDSCHs)

75 Transmission schemes (1)

76 Transmission Schemes (2)

77 Transmission Schemes (3)

78 Downlink Summary

79 Cell Search UE acquires time and frequency synchronization with a cell and detects the cell ID of that cell Based on BCH and hierarchical Synchronization signals Primary-SCH and Secondary-SCH are transmitted twice per radio frame for FDD Cell search: 5 ms timing identified using P-SCH Radio timing and group ID from S-SCH Full cell ID from DL RS Decode BCH

80 Random Access Open loop power controlled with power ramping RACH signal bandwidth: 1.08 MHz (6RBs)

81 LTE Release 10 and beyond Carrier aggregation to give up to 100MHz bandwidth Downlink transmission with 8 antennas and layers Uplink multi-antenna up to 4 antennas Coordinated Multi Point transmission Relaying from Relay Nodes to enb Latency Improvements Self Optimising Networks enhancements Home enobeb (femtocells)