A Deeper Look at LTE

Iyappan RamachandranAgilent Technologies

May 20, 2010

Cellular Comms Evolution

• 3GPP – collaboration for 3G based on GSM

• 3GPP2 – collaboration for 3G based on IS-95 LTE

LTE PHY Basics

• Six bandwidths– 1.4, 3, 5, 10, 15, and 20 MHz

• Two modes – FDD and TDD

• 100 Mbps DL (SISO) and 50 Mbps UL• Transmission technology

– OFDM for multipath resistance– DL OFDMA for multiple access in frequency/time– UL SC-FDMA to deal with PAPR ratio problem

Frame StructureFrame Structure Type 1 (FDD)

Frame Structure Type 2 (TDD)

Resource grid

• 6 or 7 OFDM symbols in 1 slot

• Subcarrier spacing = 15 kHz

• Block of 12 SCs in 1 slot = 1 RB– 0.5 ms x 180 kHz– Smallest unit of allocation

6 or 7 OFDM symbols

One downlink slot, Tslot

:

:

Transmission BW

Resource block

Resource element

l=0 l=6

12 subcarriers

2-D time and frequency grid

Time

Frequency

1 radio fr

ame = 10 mse

c (307200 x

Ts)

#0#1

#2#3

#4#5

#19#18

#17#16

Sub-fr

ame

NBWDL subcarriers

NscRB subcarriers (=12)

Pow

er

1 slo

t =

0.5 m

sec

DL PHY Channels and Signals

• Signals: generated in PHY layers– P-SS: used for initial sync– S-SS: frame boundary determination– RS: pilots for channel estimation and tracking

• Channels: carry data from higher layers– PBCH: broadcast cell-specific info– PDCCH: channel allocation and control info– PCFICH: info on size of PDCCH– PHICH: Ack/Nack for UL blocks– PDSCH: Dynamically allocated user data

DL Channel Mapping

•

64QAM16QAM QPSK

Frequency

Time

P-SCH - Primary Synchronization Signal

S-SCH - Secondary Synchronization Signal

PBCH - Physical Broadcast Channel

PDCCH -Physical Downlink Control Channel

PDSCH - Physical Downlink Shared Channel

Reference Signal – (Pilot)

DL signal demo (1)DL 5 MHz FDD signal in AWGN channel

DL signal demo (2)DL 5 MHz FDD signal in frequency-selective channel

UL PHY Signals and Channels

• Signals: generated in the PHY layer– Demodulation RS : sync and channel estimation– SRS: Channel quality estimation

• Channels: carry data from higher layers– PUSCH: Uplink data– PUCCH: UL control info– PRACH: Random access for connection

establishment

UL Channel MappingPUSCHDemodulation Reference Signal(for PUSCH)

PUCCH

Demodulation Reference Signal(for PUCCH format 0 or 1, Normal CP)

64QAM16QAMQPSK

QPSKBPSK

Frequency

Time

UL signal demo UL 5 MHz FDD signal in AWGN channel

MIMO in LTE

• Rel 8 defines MIMO only for DL• 1, 2 and 4 transmit antennas defined for

– Transmit Diversity (TxDiv)– Spatial Multiplexing (SpMux)

• Control channels undergo TxDiv based on SFBC• Data channels may undergo TxDiv or SpMux

Orthogonal RS locations

• channel matrix needs to be known in advance for equalization

DL MIMOsignal demo DL 5 MHz TDD 4x2 MIMO signal in frequency-selective channel

Thank you!

Backup slides

SC-FDMA