LTE PHY Freescale

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LTE PHY Overviewcompanion white paper:

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TMFreescale Semiconductor Confidential and Proprietary Information. Freescale™ and the Freescale logo are trademarksof Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. © Freescale Semiconductor, Inc. 2006. 1

Agenda

• LTE PHY ObjectivesStatus of the Standards Effort

• LTE Basic Concepts• Multipath• Limitations of Single Carrier Systems for High Data Rates• OFDM / OFDMA• MIMO / MRC• SC-FDMA

• LTE PHYGeneric Frame Structure

• DownlinkModulation ParametersMultiplexingPhysical Channels & Physical SignalsTransport ChannelsMapping Transport Channels to Physical Channels

• UplinkModulation ParametersMultiplexingPhysical Channels & Physical SignalsTransport ChannelsMapping Transport Channels to Physical Channels


LTE Design Objectives

Support scalable bandwidths of 1.25, 2.5, 5.0, 10.0, and 20.0 MHzPeak data rate that scales with system bandwidth

Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20 MHz channelUplink (Single Ch Tx) peak rate of 50 Mbps in 20 MHz channel

Supported antenna configurationsDownlink: 4x2, 2x2, 1x2, 1x1Uplink: 1x2, 1x1

Spectrum EfficiencyDownlink: 3 to 4 X HSDPA Rel. 6Uplink: 2 to 3 X HSUPA Rel. 6

LatencyC-plane: <50 – 100 msec to establish U-planeU-plane: <10 msec from UE to server

MobilityOptimized for low speeds (<15 km/hr)High performance at speeds up to 120 km/hrMaintain link at speeds up to 350 km/hr

CoverageFull performance up to 5 kmSlight degradation 5 km – 30 kmOperation up to 100 km should not be precluded by standard

Status of Standards Effort• Standards are not expected to be completed before the middle of 2008• Many details remain TBD

NOTE: Standard defines both FDD & TDD versions of LTE. This presentation focuses on FDD ONLY


Multipath

Multipath induced by reflection of radio signal off of obstacles• Buildings, vehicles, even people

Signal arrives at receiver staggered in time due to different path lengths• Amount of added relay referred to as “delay spread”• Results in distortion of signal at receiver

Problem becomes more difficult to deal with as data rates increase• LTE data rates represent a major increase of earlier technology• New approaches required to provide high system reliability WHILE minimizing complexity

“New-to-cellular” techniques employed • OFDM / OFDMA• MRC / MIMO• SC-FDMA


ISI ISI ISI

Signal received by direct path

Delayed signal rec’dvia longer path

Inter-Symbol Interference (ISI)

Signal arrives at receiver staggered in time due to different path lengths• Amount of added relay referred to as “delay spread”• Results in distortion of signal at receiver

Multipath results in ISI• Higher data rates mean sorter symbols periods in single carrier systems• Severe ISI can span several symbol periods in high rate systems (e.g. LTE)


frequency

Signal Bandwidth deep fades

Received Signal Distorted byFrequency Selective Fading

frequency

Signal Bandwidth

Transmitted Single Carrier Signal

MultipathDistortion

Frequency Selective Fading

Different Signal Paths Have Different Physical Lengths• Some frequencies combine constructively• Other frequencies combine destructively

Frequency Selective Fading• FSS becomes more severe at higher data rates and/or longer delay spreads• Combination of constructive and destructive interference within signal passband

Channel Equalization• Channel equalization can compensate for frequency selective fading• Basic limitations with single carrier modulation


τ τ τ τ τ

Σ

AdaptiveAlgorithm

Channel Equalization

Single Carrier Systems Employ Channel Equalization to Compensate for Freq Selective Fading• a.k.a time-domain equalization

FIR Transversal Filter is a Common Equalizer Topology• Utilizes delay taps, variable coefficients, and summing amp

But at higher data rates• Sample clock is higher --- more delay taps required • Adaptive algorithm becomes increasingly complex

Single Carrier Modulation / Time Domain Equalization has fundamental limitations• System complexity drastically increased at high data rates• Fundamentally unsuitable for LTE, so….


TFFT = 66.667 μsecCP = 4.6875 μsec

Signal delay < CP

Baseband processorstrips off CP --- resulting

in RECT functions in time domain

Signals with varyingdelays combine

at receiver front end

Orthogonal Frequency Division Multiplexing

LTE PHY Employs OFDM for Downlink• Fundamentally different approach to high data rates and channel compensation

OFDM uses many closely spaced subcarriers rather than a single carrierData transmitted in parallel streams using VERY LONG symbolsOFDM symbols preceded by CP

At the Receiver…• Delay spread is less than CP duration

CP is removed by receiver --- eliminating ISIResulting signal CAN by amplitude and phase distorted, but…The distortion is CONSTANT over the FFT Period (TFFT) within each subcarrierThe result is a RECT function in the time domain


freq

ampl

itude

timeTFFT

fcfreq

ampl

itude

TFFTtime

ampl

itude

Baseband FFT resolvesindividual subcarriers

OFDM symbol islinear summation

of sub-carriers

Orthogonal Frequency Division Multiplexing

OFDM Symbol is a linear summation of individual subcarriers• Individual subcarriers are constant within TFFT, but composite symbol amplitude is NOT

FFT Converts OFDM symbol into Frequency Domain• RECT functions in time domain result in SINC functions in frequency• Resulting subcarriers are very closely spaced (15 kHz) but are non-interfering (i.e. orthogonal)


-0.5

0

0.5

1

1.5

2

15 30 45 60 75 90 105

Frequency (kHz)

Norm

aliz

ed V

olta

ge

FFT points

zero ICI

-0.5

0

0.5

1

1.5

2

15 30 45 60 75 90 105

Frequency (kHz)

Nor

mai

lized

Vol

tage

freq. errorICI induced by

freq. error

Demodulated Signal w/o Freq Offset (Zero ICI) Demodulated Signal w/Freq Offset Causing ICI

OFDM Drawbacks

OFDM Has Two Major Drawbacks• Sensitivity to Carrier Offset and High Peak-to-Average Power Ratio (PAPR)

Sensitivity to Carrier Offset• Subcarrier orthogonality depends on sampling precisely at SINC function zero-crossings• Carrier offset shifts same points to locations exhibiting Inter Carrier Interference (ICI)

High PAPR• Individual subcarriers have constant amplitude in FFT window, but composite symbol does not• Composite OFDM symbol amplitude varies about mean value much more than a single carrier signal

at roughly similar data rates• High PAPR reduces RFPA efficiency AND drives A/D D/A converter requirements.


PHY preamble PHYheader data

RATE4 bits

Reserved1 bit

LENGTH12 bits

Parity1 bit

Tail6 bits

1 OFDM Symbol (4 usec)

Orthogonal Frequency Division Multiple Access Differs from Packet Orient Networks• OFDMA is more efficient, and generally has lower latency• OFDMA is more complex to implement

Packet Oriented Networks• Transmit data packets to users one at a time

Channel estimates performed at start of each headerPHY preambles can become significant contributor to overhead in some circumstances

• Each user consumes all network resources during transmission / receptionLatency can be problematic in high user density situations

OFDMA: Comparison w/Packet Oriented Networks

IEEE 802.11a PHY Packet Structure


0 1 2 3 10 11 19

1 Sub-Frame (1.0 msec)

1 Frame (10 msec)

50 1 2 3 4 6 50 1 2 3 4 6

7 OFDM Symbols(short cyclic prefix)

cyclic prefixes

1 Slot (0.5 msec)

Generic Frame Format

Generic Frame Format• Basic building block for Uplink and Downlink Multiplexing Scheme• Generic frame format used for FDD. Alternative frame formats defined for TDD schemes

Generic Frame Format Parameters• Frame duration: 10 msec• Frame consists of 10 subframes, 1 msec each• Each subframe contains two slots, 0.5 msec each

Slot Parameters• Short or Long CP used depending on propagating conditions within cell• Short CP: Each slot contains 7 symbols• Long CP: Each slot contains 6 symbols


downlink slotTslot

NBW

subc

arrie

rs

Resource Block:

7 sym X 12 sub (short CP)

6 sym X 12 sub (long CP)

Resource Element

12 s

ubca

rrie

rsOFDMA Resource Grid

Two Dimensional Resource Grid• Downlink resources can be visualized via resource grid• Two dimensions

Time (symbols, slots, etc.)Frequency (subcarriers)

• In MIMO apps, there is one resource grid per antenna

Physical Resource Block (PRB)• Basic unit of resource allocated by eNodeB• Consists of 12 contiguous subcarriers• Duration in time is 1 slot• # available PRBs determined by system bandwidth

Bandwidth (MHz) 1.25 2.5 5.0 10.0 15.0 20.0

Subcarrier Bandwidth (kHz) 15

PRB Bandwidth (kHz) 180

#Available PRBs 6 12 25 50 75 100

Number of Available PRBs Depends on System Bandwidth


R

R

R

R

R

R

R

R

12 S

ubca

rrier

s

Subframe

Slot Slot

OFDMA Reference Signals

Reference Signals Interspersed with data-bearing resource elements• Channel Impulse Response (CIR) and network timing updated continuously by UE

Reference Signals transmitted on specified subcarriers on specified symbols• 1st reference signal in slot

Transmitted on every sixth subcarrier– Subcarrier #1, #7, #13….

Transmitted during first OFDM symbol• 2nd reference signal in slot

transmitter on every sixth subcarrier, but offset from 1st reference symbol– Subcarrier #4, #10, #16…

Transmitted on third-to-last OFDM symbol• CIR for intervening subcarriers estimated via interpolation• Frequency hopping of Reference Signals under consideration


XCVRBasebandXCVR-A

BasebandXCVR-B

Conventional Single ChannelReceiver w/Antenna Diversity

MRC/MIMO ReceiverConfiguration (2-ch)

MIMO / MRC Require Receiver Diversity

Maximal Ratio Combining (MRC) & Multiple Input / Multiple Output (MIMO) Require Receiver Diversity

Conventional Receivers Often have Antenna Diversity• Not to be confused with Receiver Diversity

MRC Exploits Receiver Diversity to Increase System RELIABILITY by:• Capturing multiple copies of the signal with different multipath characteristics and noise signatures• Uses baseband combining to eliminate frequency selective fading and improve SNR

MIMO Exploits Receiver Diversity to Increase System DATA RATE by:• Capturing multiple copies of multiple transmitted signals

Each with different multipath characteristics and noise signatures• Using techniques in baseband to resolve transmitted signals into separate channels

Each resolvable channel capable of carrying high data rate traffic


freq freq

deep fades deep fades

freq

Signal from Transceiver A Signal from Transceiver B

SNR for All Subcarriers Enhancedin Composite MRC Signal

MRCCombining

MRC

MRC Exploits Receiver Diversity to Increase System RELIABILITY by:• Capturing multiple copies of the signal with different multipath characteristics and noise signatures• Uses baseband combining to eliminate frequency selective fading and improve SNR

MRC Improves SNR• Coherent combination of signals with uncorrelated noise improves SNR by 3 dB

MRC Enhances System Robustness in Presence of Severe Frequency Selective Fading• Coherent combination improves SNR for all subcarriers• Statistically unlikely to get deep fades on same subcarriers if antennas are adequately separated


XCVR-ABaseband

XCVR-B

XCVR-CBaseband

XCVR-D

ChAC

ChAD

SC = REFA x ChAC

XCVR-ABaseband

XCVR-B

XCVR-CBaseband

XCVR-D

ChBC

SD = REFA x ChAD

ChBD

SC = REFB x ChBC

SD = REFB x ChBD

XCVR-ABaseband

XCVR-B

XCVR-CBaseband

XCVR-DSC = [DATAA x ChAC] + [DATAB x ChBC]

SD = [DATAA x ChAD] + [DATAB x ChBD]

Reference Signal Transmitted from Antenna A

Reference Signal Transmitted from Antenna B

Data Transmitted Simultaneously from BOTH Antennas

MIMO


MIMO

MIMO Increases System DATA RATE by:• Capturing multiple copies of multiple transmitted signals

Each with different multipath characteristics and noise signatures• Resolve transmitted signals into separate channels

Each resolvable channel capable of carrying high data rate traffic

MIMO Process:• Determine CIR from Each TX at Each RX

Transmit Reference Signal from one antenna while other antennas are idle• Transmit independent data streams from TX antennas simultaneously

With CIRs for transmitters know, RX baseband can resolve separate TX datastreamsPrecoding can be used to aid in reolving separate channels

– LTE employs CDD


R0

R0

R0

R0

R0

R0

R0

R0

R1

R1

R1

R1

R1

R1

R1

R1

Denotes UnusedResource Element

R0Reference Signalfrom Antenna 0

R1

Antenna 1

Subc

arrie

rs(fr

eque

ncy)

Reference Signalfrom Antenna 1

Antenna 0

OFDM Symbols(time)

LTE Reference Signals For Two Channel MIMO

LTE Reference Signals Transmitted Sequentially in Multi-Antenna Applications• When Reference Signal is sent on Antenna 0, Antenna 1 is idle• When Reference Signal is sent on Antenna 1, Antenna 0 is idle

Data is transmitted simultaneously• A priori CIR knowledge enables receiver to resolve separate data steams


BitStream

SingleCarrier

ConstellationMapping

S/PConvert

M-PointDFT

SubcarrierMapping

N-PointIDFT

CyclicPrefix

&Pulse

Shaping

RFE

Channel

RFEN-Point

DFT

CyclicPrefix

Removal

FreqDomain

Equalizer

SCDetector

BitStream

Functions Common to OFDMA and SC-FDMA SC-FDMA Only

Symbol

Block

P/SConvert

M-PointIDFT

Symbol

Block

Const.De-map

SC-FDMA

Single Carrier – Frequency Division Multiple Access• Basic transmission scheme for LTE Uplink• Extension of Single Carrier – Freq Domain Equalization to accommodate multiple users• Main advantage is lower PAPR, thus improving RFPA efficiency at UE• Also has less sensitivity to carrier offset

High degree of Commonality with OFDM Signal Chain• SC-FDMA sometimes refered to as “DFT-Spread OFDMA”

Basic Operation• Sequence of single carrier symbols S/P converted and fed to DFT• Discrete freq domain representation mapped to subcarriers & transmitted in same manner as OFDM


Localized Subcarriers

freq freq

Distributed Subcarriers

Distributed vs. Localized SC-FDMA Subcarrier Mapping

Two Basic Schemes for SC-FDMA Subcarrier transmission:• Localized and Distributed• Advantages and disadvantages for both

Advantages of Localized Subcarrier Mapping• Supports Channel Dependent Scheduling• CIR not degraded for narrow system bandwidths

Advantages of Distributed Subcarrier Mapping• Somewhat lower PAPR than localized mapping• Inherent frequency diversity

LTE Currently Planning on Using Localized Subcarrier Mapping• Support for Channel Depending Scheduling was deciding factor


LTE Downlink: Basic Modulation Parameters

Cyclic Prefix LengthConfiguration NBW Nsymb

Ts μsec

160 for l = 0 5.21 for l = 0

144 for l = 1, 2…5 4.69 for l = 1, 2…5

Δf = 15 kHz 12 6 512 16.67

Δf = 7.5 kHz 24 3 1024 33.33Extended CP

Normal CP Δf = 15 kHz 12 7


Physical Signals, Physical Channels & Transport Channels

Physical Signals• Resource elements used to support PHY functions only• Physical Signals do not transport information originating in higher layers

Physical Channels• Resource elements used to transport info originating in higher layers• Physical Channels have unique characteristics depending on function• Physical Channels map to Transport Channels

Transport Channels• Service Access Points for higher layers• Unique characteristics depending on function


LTE Downlink: Physical Signals

Two types of DL Physical Signals• Reference Signal: used by UE for CIR estimation• Synchronization Signal: facilitates Cell Search procedure

Reference Signal• Selected from a set of 510 unique cell ID sequences

Product of 1 of 3 orthogonal sequences and 1 of 170 PRN sequences• Mapped to resource elements as shown on following slide

Synchronization Signal• utilize the same type of cell ID sequences as reference signals • primary and secondary synchronization signals

– Used for different purposes within cell search algorithm– transmitted 72 subcarriers centered around the DC subcarrier– Primary: transmitted during the 0th slot of a frame– Primary: transmitted during the 10th slot of a frame


R3

R3

R2

R1R1

Denotes UnusedResource Element



R0

R0

R0

R0

R0

R0

R0

R0

Antenna 0

Subcarriers(frequency)

OFDM Symbols(time)

R0

R0

R0

R0

R0

R0

R0

R0

Antenna 0

R1

R1

R1

R1

R1

R1

Antenna 1

R1R1

R0

R0

R0

R0

R0

R0

R0

R0

Antenna 0

R1

R1

R1

R1

R1

R1

Antenna 1

R2

R2

R2

Antenna 2

R3

R3

Antenna 3



1 An

tenn

a2

Ante

nnas

4 An

tenn

as

Reference Signal Transmission


LTE Downlink: Physical Channels

Downlink Physical Channels• Physical Downlink Shared Channel (PDSCH)• Physical Downlink Control Channel (PDCCH)• Common Control Physical Channel (CCPCH)

Physical Downlink Shared Channel (PDSCH)• Supports multiple functions including transport of high rate data and multimedia content• Capable of supporting highest LTE data rates• Subcarriers modulated with QPSK, 16QAM, or 64 QAM depending on channel conditions

PDSCH Supports Spatial Multiplexing• Up to two code words sent per subframe• Term “layer” in this context corresponds to transmitting antenna

Up to four Tx antennas used for LTE spatial multiplexingCode word-to-antenna mapping shown in table below

Transmission rank Number of code words Codeword-to-layer mapping

1 1 x(0)(i) = d(0)(i)x(0)(i) = d(0)(i)x(1)(i) = d(1)(i)d(0)(i) is mapped to layer 0d(1)(i) is mapped to layers 1 and 2d(0)(i) is mapped to layers 0 and 1d(1)(i) is mapped to layers 2 and 3

2 2

3 2

4 2


LTE Downlink: Physical Channels

Physical Downlink Control Channel• Conveys UE-specific Control information• Robustness is main consideration• QPSK is only modulation format• Mapped onto first three symbols off the 1st slot in a subframe

Common Control Physical Channel• Conveys cell-wide control information• Robustness is main consideration• QPSK is only modulation format• Transmitted as close to center frequency as possible

transmitted exclusively on the 72 active subcarriers centered around the DC subcarrier


LTE Downlink: Transport Channels

Transport Channels are included in PHYAct as SAP’s for Higher layers

Transport ChannelsBroadcast Channel (BCH)Downlink Shared Channel (DL-SCH)Paging Channel (PCH)Multicast Channel (MCH)

Broadcast ChannelFixed formatMust be broadcast over entire coverage area of cell

Downlink Shared ChannelSupports Hybrid ARQ (HARQ)Supports dynamic link adaption by varying modulation, coding and transmit powerSuitable for transmission over entire cell coverage areaSuitable for use with beamformigSupport for dynamic and semi-static resource allocationSupport for discontinuous receive (DRX) for power save


LTE Downlink: Transport Channels

Paging ChannelSupport for UE DRXRequirement for broadcast over entire cell coverage areaMapped to dynamically allocated physical resources

Multicast ChannelRequirement for broadcast over entire cell coverage areaSupport for MB-SFNSupport for semi-static resource allocation


CCPCH PDSCH PDCCH

BCH PCH MCHDL-SCH

DL TransportChannels

DL PhysicalChannels

Under

Consideration

LTE Downlink: Mapping Transport Channels to Physical Channels


LTE Uplink: Physical Signals

LTE Uplink Physical SignalsUplink Reference SignalRandom Access Preamble

Uplink Reference SignalsTwo variants

– Demodulation signal facilitates coherent demod– Channel sounding signal

Both based on Zadoff-Chu sequences

Random Access PreambleTransmission by UE initiates cell search procedureDerived from Zadoff-Chu sequencesTransmitted on contiguous blocks of 72 subcarriersRandom Access Preamble format shown below

– TRA = 1000 μsec– TGT = 102.6 μsec– TPRE = 800 μsec

TRA

CP Preamble

TPRETCP TGT

GT

Random Access Preamble Format


LTE Uplink: Physical Channels

UL Slot StructureUplink Slot structure also same as DL

– 7 SC-FDMA symbols/slot with normal CP– 6 SC-FDMA symbols/slot with extended CP

Physical Uplink Shared Channel (PUSCH)Resource allocation controlled on sub-frame basis by base station schedulerResources allocated in PRB’s

– Same BW (12 subcarriers) as Downlink– May be frequency hopped from sub-frame to sub-frame

QPSK, 16 QAM, 64 QAM modulation used

Physical Uplink Control Channel (PUCCH)Carries Uplink control information

– CQI, ACK/NACK, HARQ, and uplink scheduling requests Never transmitted simultaneously with PUSCH data Freq hopped at slot boundary for added reliability (see below)

resource iresource j

resource i resource j

frequ

ency

subframe

slot

bou

ndar

y

PUCCH is Hopped at Slot Boundary


LTE Uplink: Transport Channels

Uplink Shared Channel (UL-SCH)Support possible use of beam formingSupport dynamic link adaption (varying modulation, coding, and/or Tx power)Support for HARQSupport for dynamic and semi-static resource allocation

Random Access Channel (RACH)Supports transmission of limited control informationPossible risk of collision


PRACH PUCCH PUSCH

BCH UL-SCHUL Transport

Channels

UL PhysicalChannels

LTE Uplink: Mapping Transport Channels to Physical Channels

LTE PHY Freescale

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