3GPP Long Term Evolution LTE

Transcription

1 Chapter 27 3GPP Long Term Evolution LTE Slides for Wireless Communications Edfors, Molisch, Tufvesson 630

2 Goals of IMT-Advanced Category peak data rate DL / Mbit/s max DL modulation 64 QAM 64 QAM 64 QAM 64 QAM 64 QAM peak data rate UL / Mbit/s max UL modulation 16 QAM 16 QAM 16 QAM 16 QAM 64 QAM max no. layers for DL MIMO Slides for Wireless Communications Edfors, Molisch, Tufvesson 631

3 Bands for FDD Operation Operating band UL DL ba nd w i d th Europe, Asia America Europe, Asia America America Japan Europe, Asia Europe, Asia Japan Americas Japan Americas Americas Americas Americas Slides for Wireless Communications Edfors, Molisch, Tufvesson 632

4 Bands for TDD Operation Operating band band ba nd w i d th Europe, Asia Europe, Asia Europe China Europe, Asia Slides for Wireless Communications Edfors, Molisch, Tufvesson 633

5 Network structure Slides for Wireless Communications Edfors, Molisch, Tufvesson 634

6 Key Physical Layer Aspects Downlink: OFDMA Uplink: DFT spread OFDMA or SC-FDMA MIMO: DL: 2-4 Tx antennas, 2-4 Rx antennas UL: 1 Tx antenna (or antenna selection), 2-4 Rx antennas Both SU-MIMO and MU-MIMO supported Modulation schemes: QPSK, 16QAM, and 64QAM In both downlink and uplink spread spectrum Slides for Wireless Communications Edfors, Molisch, Tufvesson 635

7 Frames, slots, and symbols Slot: 7 consecutive OFDMA symbols (0.5ms) 1 OFDMA symbol = 71.4 μs Sub-frame: 2 slots (1 ms) Frame: 20 slots (10 ms) Slot = 0.5 ms Most basic time unit T s = 1/ (2048*15) ms (30.72 MHz sample rate for 20 MHz, 2048 point FFT) CP LB#0 CP LB #1 CP LB #2 CP LB #3 CP LB #4 CP LB #5 CP LB#6 T CP = 160 T S 144 T S 66.7 μs = 2048 T S Normal cyclic prefix mode CP LB#0 CP LB#5 T CP = 512 T Extended cyclic prefix mode (e.g., MBSFN operation) S Slides for Wireless Communications Edfors, Molisch, Tufvesson 636

8 Uplink/Downlink Resource Grid N subcarriers RB sc UL RB N RB N sc subcarriers N UL symb One uplink slot Tslot SC-FDMA symbols k N Resource block N UL RB N UL symb RB sc N 1 RB sc resource elements Resource element ( k, l) Same numerology for both downlink and uplink One resource block = 6*12 = 84 resource elements (RE) 7 REs used for DM RS per slot Assignment: Resource blocks (contiguous or distributed subcarriers) UL l 0 l N symb 1 k 0 Slides for Wireless Communications Edfors, Molisch, Tufvesson 637

9 UL SC-FDMA Transmit Chain 0 d S/P M-point DFT Subcarrier mapping N-point IFFT P / S Add CP To DAC & RF 0 0 Can view M-point DFT also as spreading across subcarriers DFT-Spread FDMA Slides for Wireless Communications Edfors, Molisch, Tufvesson 638

10 Mapping to physical resources in DL Two-step procedure Map symbols onto virtual resource blocks (VRB) Map VRB onto physical resource blocks (PRBs) Mapping of PRB to VRB can be Continuous (for obtaining best channel) Distributed (for frequency diversity) Slides for Wireless Communications Edfors, Molisch, Tufvesson 639

11 LTE Uplink Reference Signals: 2 Types Data Mod Resource: One per slot for accurate estimates for demodulation Sounding RS: Once every 2-10ms. Occupies a larger bandwidth than data to enable BS to estimate frequency response over a large portion of system bandwidth 1. Data demodulation RS 2. Sounding RS Subcarrier Data BW System BW Time Slides for Wireless Communications Edfors, Molisch, Tufvesson 640

12 Channel coding procedure Slides for Wireless Communications Edfors, Molisch, Tufvesson 641

13 Turbo Codes Generates long codewords by encoding data with two different convolutional encoders for each of the encoders, data are interleaved with different interleavers Slides for Wireless Communications Edfors, Molisch, Tufvesson 642

14 Turbo Codes A brief history of turbo codes: The turbo code concept was first introduced by C. Berrou in 1993, two French engineers (not mathematicians). Today, Turbo Codes are considered as the most efficient coding schemes for FEC Scheme with known components (simple convolutional or block codes, interleaver, soft-decision decoder, etc.) Performance close to the Shannon Limit (E b /N 0 = -1.6 db if R b 0) at modest complexity! With the advent of significant computer power, Turbo Codes have become the standard in wireless communications although LTE uses CRC, convolutional codes (used only for control information) in addition to Turbo Codes used for payload data

15 Turbo Codes: Encoder Data Source X Convolutional Encoder 1 Y 1 X Interleaving Convolutional Encoder 2 Y Y 2 (Y 1, Y2) X: Information Y i : Redundancy Information Copyright 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 34

16 Turbo Codes: Decoder De-interleaving Y 1 Convolutional Decoder 1 Interleaver X Interleaving Convolutional Decoder 2 De-interleaving Y 2 X X : Decoded Information Copyright 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 35

17 Turbo Codes Diagram Copyright 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 36

18 Turbo Code Parameters LTE 1/3 rate Parallel Concatenated Convolutional Code (PCCC) Two 8-state constituent encoders: g 1 (D) = 1 + D + D 3 and g 0 (D) = 1 + D 2 + D 3 Internal interleaver based on Quadratic Permutation Polynomial (QPP) Contention-free property enables decoder parallelism LDPC codes are not used in LTE Slides for Wireless Communications Edfors, Molisch, Tufvesson 643

19 Space-Time Coding in LTE 2 transmit antennas: (0) y (2i) (1) y (2i) (0) y (2i 1) (1) y (2i 1) j 0 0 j 0Re x j Re x jim x 0 Im x (0) (1) (0) (1) ( i) ( i) ( i) ( i) 4 transmit antennas: Similar mapping over 4 symbols real and imaginary parts Slides for Wireless Communications Edfors, Molisch, Tufvesson 644

20 Spatial Multiplexing in LTE Vertical encoding Up to 4 layers (streams) and up to 2 code words Codeword 1 Layer 1 Codeword 1 Layer 1 Layer 2 Codeword 2 Layer 2 Various options Codeword 1 Codeword 2 Layer 1 Layer 2 Layer 3 Layer 4 Codeword 1 Layer 1 Layer 2 Codeword 2 Layer 3 for spatial multiplexing in LTE Slides for Wireless Communications Edfors, Molisch, Tufvesson 645

21 Precoding MIMO in LTE 2 antennas 1 layer: 4 choices 2 layers: 3 choices 4 antennas Codebook not shown Up to 16 choices Codebook index Number of layers j j j j Precoding codebook for 2 antennas (1 and 2 layers) Slides for Wireless Communications Edfors, Molisch, Tufvesson 646

22 Physical channels Traffic channels Dedicated Traffic Channel (DTCH) carries the user data for all uplinks, as well as for those downlink data that are not multicast/broadcast. - Multicast Traffic Channel (MTCH): carries the user data for multicast/broadcast downlink transmission Control channels Broadcast Control Channel (BCCH): carries system information data that are broadcast to the MSs in a cell Paging Control Channel (PCCH): pages MSs in multiple cells Common Control Channel (CCCH): transmits control data for the random access, Dedicated Control Channel (DCCH): is used for the transmission of control information that relates to a specific MS Multicast Control Channel (MCCH) Slides for Wireless Communications Edfors, Molisch, Tufvesson 647

23 Mapping of physical channels to transport channel Slides for Wireless Communications Edfors, Molisch, Tufvesson 648

24 Handover Handover preparation Source BS configures measurements the MS has to do; MS sends results to BS, BS makes handover decision and sends request to target BS, target BS makes admission control and (if positive) sends signal to source BS Handover execution Source BS sends handover command to MS, MS synchronizes itself with target BS, MS sends handover confirm to target BS Handover completion Target BS sends path switch message to Mobility Management Entity; serving gateway switches all routes from source BS to target BS; source BS releases resources Slides for Wireless Communications Edfors, Molisch, Tufvesson 649