Beyond HSPA+: An Introduction to LTE Fundamentals

LTE tutorial
- Looking forward beyond HSPA+

sppe12083@gmail.com
RAN System Engineer

Outline
• Beyond HSPA+
• LTE: motivation and expectations
• E-UTRAN overview & initial performance evaluation
• OFDMA and SC-FDMA fundamentals
• LTE physical layer
• LTE transmission procedures

All rights reserved @ 2009

Beyond HSPA evolution – 3GPP path

DL: 14.4 Mbps DL: 28 Mbps DL: 42 Mbps DL: 84 Mbps DL: 100+ Mbps
UL: 5.76Mbps UL: 11 Mbps UL: 11 Mbps UL: 23 Mbps UL: 23+ Mbps

UTRAN Rel-99
WCDMA HSDPA/HSUPA HSPA+ (HSPA Evolution)

Rel-5 Rel-6 Rel-7 Rel-8 Rel-9 Beyond Rel-9

E-UTRAN deployment
LTE specification & service LTE-A
process ~ 2007Q4 enhancement

DL:300 Mbps DL: 1 Gbps
UL: 75 Mbps UL: 100 Mbps


LTE - background
• Motivation:
– Based on HSPA success story(274*
commercial HSPA networks worldwide)

– Uptake of mobile data traffic upon
cellular networks enforces:
• Reduced latency
• Higher user data rate
• Improved system capacity and coverage
• Cost-reduction per bit

• Expectation:
– Detailed requirements captured
in 3GPP TR 25.913
– NGMN formally released requirements
on next generation RAN in late 2006**

*source: www.gsacom.com
“ mobile broadband evolution: roadmap from HSPA to LTE” UMTS forum White paper
**http://www.ngmn.org/nc/de/downloads/techdownloads.html

LTE feature overview
• Flexible and expandable spectrum bandwidth

• Simplified network architecture

• High data throughput (Macro eNodeB & Home eNodeB)

• Support for multi-antenna scheme (up to 4x4 MIMO in Rel-8)

• Time-frequency scheduling on shared-channel

• Soft(fractional) frequency reuse

• Self-Organizing Network (SON)


LTE spectrum flexibility
• Operating bands
– Flexible carriers: from 700MHz to
2600MHz
– Extensible bandwidth: from 5MHz to
20MHz

FDD Pair
uplink downlink

5 MHz
20 MHz

Channel bandwidth (MHz)
Transmission bandwidth configuration(RBs)

active RBs


LTE basic parameters

Frequency range UMTS FDD bands and TDD bands defined in 36.101(v860) Table 5.5.1

channel bandwidth (MHz)
1.4 3 5 10 15 20

Transmission bandwidth NRB:
(1 resource block = 180kHz 6 15 25 50 75 100
in 1ms TTI)

Downlink: QPSK, 16QAM, 64QAM
Modulation Schemes:
Uplink: QPSK, 16QAM, 64QAM(optional)

downlink: OFDMA (Orthogonal Frequency Division Multiple Access)
Multiple Access:
uplink: SC-FDMA (Single Carrier Frequency Division Multiple Access)

downlink: TxAA, spatial multiplexing, CDD ,max 4x4 array
Multi-Antenna Technology
Uplink: Multi-user collaborative MIMO

Downlink: 150Mbps(UE Category 4, 2x2 MIMO, 20MHz bandwidth)
Peak data rate 300Mbps(UE category 5, 4x4 MIMO, 20MHz bandwidth)

Uplink: 75Mbps(20MHz bandwidth)


LTE Peak throughput w.r.t UE categories
Table 4.1-1: Downlink physical layer parameter values set by the field ue-Category

UE Category Maximum number of DL-SCH Maximum number of bits of Total number of Maximum number of
transport block bits received a DL-SCH transport soft supported layers for
within a TTI block received within channel spatial multiplexing
a TTI bits in DL

Category 1 10296 Peak rate 10296 250368 1
150Mbps with
Category 2 51024 51024 1237248 2
2x2 MIMO
Category 3 102048 75376 1237248 2
Category 4 150752 75376 1827072 2
Category 5 299552 149776 3667200 4
Peak rate 300Mbps
with 4x4 MIMO

Table 4.1-2: Uplink physical layer parameter values set by the field ue-Category
UE Maximum number of bits of an Support for
Cate UL-SCH transport block 64QAM in
gory transmitted within a TTI UL
Category 1 5160 No
Category 2 25456 No
Peak rate
Category 3 51024 75Mbps No
Category 4 51024 No
Category 5 75376 Yes

3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“

LTE UE category

UE Category 1 2 3 4 5

Peak rate DL 10 50 100 150 300
(Mbps)
UL 5 25 50 50 75

RF bandwidth 20 MHz

DL QPSK, 16QAM, 64QAM

Modulation QPSK,
UL QPSK, 16QAM 16QAM,
64QAM

2 Rx Diversity Assumed in performance requirements

2x2 MIMO Optional Mandatory

4x4 MIMO Not supported Mandatory

3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“

Channel dependent scheduling

• Time-frequency scheduling

UE #1

UE #2


Soft (fractional) frequency reuse
• Soft Frequency Reuse(SFR):
– inner part of cell uses all subbands with less power;
– Outer part of cell uses pre-served subbands with higher power;

Su rier
ca

b- s
r
po
BS 2

w
erd
en
ity s
sub-
ier

MS 22
carr

Pow MS 21
er d
BS 1 e nsity
MS 31
MS 12 MS 11
y
sit n
de
wer
Po

MS 32
ca ub-
er
s
rri

3GPP R1-050841 “Further Analysis of Soft Frequency Reuse Scheme “
BS 3

E-UTRAN overview


E-UTRAN architecture

S1

S1
S1

S1
X2

X2


E-UTRAN architecture


E-UTRAN radio protocol
notifications common dedicated

System Dedicated Control
RRC Paging
information and information transfer

radio
SRB0 SRB1 SRB2 DRB1 DRB2
bearers

Integrity and Integrity and ciphering and ciphering and
PDCP ciphering ciphering ROHC ROHC

RLC ARQ ARQ ARQ ARQ

logical
channels PCCH BCCH CCCH DCCH 1 DCCH 2 DTCH 1 DTCH 2

MAC Multiplexing and HARQ control

transport
PCH BCH RACH DL-SCH UL-SCH
channels

PHY layer functions

physical
PBCH PRACH PDSCH PUSCH
channels


E-UTRAN radio channels
downlink uplink
Logical
PCCH BCCH CCCH DCCH DTCH MCCH MTCH CCCH DCCH DTCH
channels

DL-SCH MCH Transport channels RACH UL-SCH
PCH BCH

PRACH PUCCH PUSCH
PDCCH PBCH PDSCH PMCH Physical channels

•Logical Channels
Define what type of information is transmitted over the air, e.g. traffic channels,
control channels, system broadcast, etc.
•Transport Channels – no per-user dedicated channels!
Define how is something transmitted over the air, e.g. what are encoding, interleaving
options used to transmit data
•Physical Channels
Define where is something transmitted over the air, e.g. first N symbols in the DL
frame


E-UTRAN bearers
SRB: internal E-UTRAN signalings such as RRC signalings, RB management signalings
NAS signalings: such as tracking area update and mobility management messages
data traffic: E-UTRAN radio bearer + S1 bearer +S5/S8 bearer
L1/L2 control channel
P
TC TT
IP DP P H
P
RT

IP
U

u
-u -u P-
TP GTP
S

GT
NA

-u G P
TP P DP
UD
C
G UD U
C

RR
RR

P
UD
CP
CP

IP IP IP
AP
PD
PD

S1 S L2 L2 L2
TP NA
C
Y
C

Y HY
RL

PH
RL

SC IP PH P
AP
S1
AC
AC

r2 TP
ye P-GW
M
M

La SC S-GW
L1
L1

IP
E
E

Y
LT
LT

PH
L2
Y
PH
eNodeB
UE MME
S5/S8
E-UTRAN radio bearer S1 bearer bearer

EPS bearer


E-UTRAN – Control plane stack

MME/
UE eNodeB
NAS 24.301 NAS
eNodeB
36.331
RRC RRC S1AP 36.413 S1AP
X2AP 36.423 X2AP
36.323
PDCP PDCP
SCTP 36.412 SCTP
36.322 36.422
RLC RLC
IP IP
36.321
MAC MAC L2 L2
36.211~36.214
PHY PHY L1 L1

LTE-Uu S1-MME/X2-C


E-UTRAN – User Plane Stack

UE PDN/S-GW
eNodeB
eNodeB
Application

IP IP
36.323 29.274
PDCP PDCP GTP-u GTP-u
36.322
RLC RLC UDP UDP

36.321 IP IP
MAC MAC
L2 L2
36.211~36.214
PHY PHY L1 L1

LTE-Uu S1-U/X2-u


Radio resource management

QoS management Interference
L3 RRC management mobility
Admission
Load control management
Semi-persistent
control scheduling

PDCP

Hybrid ARQ Dynamic
L2 RLC Link adaptation
manager scheduling

MAC

PDCCH
L1 PHY CQI manager
adaptation

“An overview of downlink radio resource management for LTE”, Klaus Ingemann Pedersen, et al, IEEE communication magazine, 2009 July


E-UTRAN mobility
RRC-idle RRC-connected
• Simplified RRC states
• Idle-mode mobility (similar as HSPA) • Cell reselection decided by UE • Network controlled handovers
• Connected-mode mobility • Based on UE measurements • Based on UE measurements
• Controlled by broadcasted parameters
– handover controlled by network • Different priorities assigned to frequency
layers

MME/SGW
Mobility difference between UTRAN and E-UTRAN
UTRAN E-UTRAN
HO decision Location area (CS core) Not relevant since no CS connections
Call
Admission Routing area Tracking area
Source
target SHO No SHO
eNodeB
eNodeB
Cell_FACH, Cell_PCH,URA_PCH No similar RRC states

RNC hides most of mobility Core network sees every handover
No need to provide cell-specific
Target cell signal Neighbour cell list required information, only carrier-frequency is
quality meets required.
reporting threshold


Overview of a PS call – control plane
• UE activities after power-on
Power up
Initial Derive system Random
Data Tx/Rx
cell search information Access

UE E-UTRAN
paging

S Radom Access procedure
/SS
P SS H
Connection
BC RRC Connection Request establishment
ICH
H/PH H
FIC CC RRC Connection Setup
PC PD ss
A cce RRC Connection Setup Complete
m
ado Radio bearer
Rn SC
H
H Security procedures establishment
PD CC
/PU
CH
P US RRC Connection Reconfiguration
RRC Connection Reconfiguration Complete


Overview of a PS call – control plane
• UE activities after power-on
Power up
Initial Derive system Random
Data Tx/Rx
cell search information Access

UE E-UTRAN
ion gr ant paging
ss ing
nsmi l
ed u
tra k sch Radom Access procedure
ta in
da upl Connection
DL K & rt RRC Connection Request establishment
AC r epo
tus RRC Connection Setup
el sta
nn sio
n RRC Connection Setup Complete
cha mis Radio bearer
& a ns
A CK a tr Security procedures establishment
at
U Ld RRC Connection Reconfiguration
RRC Connection Reconfiguration Complete


Overview of a PS call – user plane

PS data
Tx
via S1 interface

1 resource block: eNodeB
180 kHz = 12 subcarriers to RF
PDCP
OFDM Signal
Generation (Ciphering
Header Compression,)
1 resource block pair
1 TTI = 1ms = 2 slots
resource RLC
mapping (Segmentation, ARQ)

scheduling
data modulator

coding

UE HARQ Multiplexing
per user


Overview of a PS call – user plane

PS data
Tx
via S1 interface

1 resource block: eNodeB
180 kHz = 12 subcarriers to RF
PDCP
OFDM Signal
Generation (Ciphering
Header Compression,)
1 resource block pair
1 TTI = 1ms = 2 slots
resource RLC
mapping (Segmentation, ARQ)

scheduling
data modulator

coding

UE HARQ Multiplexing
Occupying different radio per user
resources across TTIs
adapts to time-varying
radio channel condition!


LTE initial deployment scenario
• Similar coverage as 3G HSPA on existing 3G frequency bands
– LTE radio transmission technology itself does not provide coverage boost.
– Lower frequency (e.g, 900MHz) provides better coverage but demands large-
size antennas.

• “Over-layed” initial deployment on hot-spot area
– Spectrum availability
– Backhaul capacity
– Handset maturity (multi-mode)

urban sub-urban Rural
(0.6 ~ 1.2km) (1.5 ~ 3.4km) (26 ~ 50 km)


LTE initial trial performance
• LTE data rates
– Peak rate measured in lab and trial align with
3GPP performance targets
– In reality, user throughputs are impacted by
• RF conditions & UE speed
• Inter-cell interference & multiple users sharing the capacity
• Application overhead

Peak rate measured with a single user in
unloaded, optimal radio condition

Average: 10 active
users with 3Mbps
Top 5%, loaded throughput per user

Average

Cell edge 1Mpbs throughput
at cell edge

Active users per cell Active users per cell

Source: www.lstiforum.org

Macro Cellular network: peak rate Vs average rate
• Unlike circuit-switched network design, live network throughput
is not fixed any more, being dependent on many environmental
factors such as CQI,Tx buffer status,etc.
• In macro cellular network, network average throughput falls
behind peak rate by 10x.
• Cellular booster for Mobile broadband
HSPA cell throughput Tput (Mbps) G-factor (dB)
– Ubiquitous coverage
– High capacity & data rate 8 25
– Low cost 15
4
>> “FemtoCell” – Home eNodeB!
10
2
2

0 -3

3GPP TS 25.101 Table 9.8D3, 9.8D4, 9.8F3 for PA3

LTE initial trial performance
• User plane latency
– 3GPP RTT target is 10ms for short IP packet air interface RTT
– Field trial results: End-to-End Ping
• 10~13ms with pre-scheduled uplink
• <25ms with on-demand uplink

EPC
App Server

• Control plane latency
– Short latency helps to keep “always on” user experience
– Field trial results
• Measured idle to active latency: 70~ 100ms Less than 50msec
Active Dormant
(Cell_DCH) (Cell_PCH)

Less than 100msec

Camped-state
(idle)

* Measurement taken with one UE in unloaded case
* Source: www.lstiforum.org

OFDMA and SC-FDMA rationale


OFDM fundamentals – frequency spectrum
FDM OFDM

…
f f

π⋅f
sin( )
Δf
No Inter-Carrier
Interference!

− 2Δf − Δf 0 Δf 2Δf
frequency domain

1
Tu =
Δf

Time domain


OFDM fundamentals – multicarrier modulation
Nc − 1 Nc − 1

“+1” f1 Modulated
subcarriers
x (t ) = ∑x
k =0
k (t ) = ∑a
k =0
k e j 2 π k Δ ft

Specifying system sampling rate: f s = 1 / Ts = N ⋅ Δf
“-1” f2
Nc−1
+ xn = x(nTs) = ∑ ak e j 2πkΔfnTs
We get:
“+1” f3 k =0
Nc−1 j 2πk
n N −1 j 2πk
n
= ∑ ak e N
= ∑ ak e
′ N

k =0 k =0

e j 2πf0t a0
a0 x0 (t ) a1 X0
a0 , a1 ,..., a N c −1
j 2πf1t X1
e

…
a0 , a1 ,..., a N c −1 S/P
x(t )

…
S/P a1 x1 (t ) + a Nc −1 IFFT P/S
π
e j 2xf Nc−1t (t )
…

0

…
a Nc −1 Nc−1 XN-1
0


OFDM fundamentals- Cyclic Prefix
Tu
directed path: ak −1 ak ak +1
reflected path:
τ
τ Integration interval
of direct path

directed path:

reflected path:
τ

Guard time: Cyclic Prefix Vs Padding Zeroes Tcp >τ

guard time FFT integration time=1/Carrier spacing

OFDM symbol time


OFDM fundamentals- Cyclic Prefix
Tu
directed path: ak −1 ak ak +1
reflected path:
τ
τ Integration interval
of direct path

directed path:

reflected path:
τ

Guard time: Cyclic Prefix Vs Padding Zeroes Tcp >τ

a0
a1 add an OFDM symbol

…
IFFT P/S Cyclic
Tu+Tcp
a Nc −1 Tu Prefix

guard time FFT integration time=1/Carrier spacing

OFDM symbol time


OFDM fundamentals – general link level
chains
QAM Pilot
Binary input data Coding Interleaving S/P IFFT P/S add CP
mapping Insertion

5 MHz Bandwidth
FFT
Sub-carriers
Pulse
Guard Intervals RF Tx DAC
shaping
Symbols
…
Frequency

…
Time Timing and
RF Rx ADC
frequency Sync

de- QAM CP
Binary output data de-coding Equalizer P/S FFT S/P
interleaving de-mapping removal

“Digital communications: fundamentals and applications” by Bernard Sklar, Prentice Hall, 1998. ISBN: 0-13-212713-x
“OFDM for Wireless Multimedia Communications” by Richard van Nee & Ramjee Prasad, Artech house,2000, ISBN: 0-89006-530-6
3GPP TR 25892-600 feasibility study for OFDM in UTRAN

OFDM fundamentals – frequency domain
equalizer
MRC filter:
*
w(τ ) = h (−τ )
Zero Forcing: h(τ ) ⊗ w(τ ) = 1
ε = E{ s(t ) − s(t ) }
2
MMSE: ˆ
transmitter Channel model receiver
n(t )

S (t ) r (t ) ~ (t )
s
h(τ ) + w(τ )

W0
rn R0 ˆ
S0
⊗
D D D
r (t ) ˆ
s(t )
W0 W1 WL-1 DFT WN −1 IDFT
RN −1 ˆ
S N −1
+ ˆ
sn ⊗
Time domain frequency domain

Frequency domain equalizer outperforms with much less complexity!
“Frequency domain equalization for single carrier broadband wireless systems”, David Falconer , et.al,
IEEE Communication magazine, 2002 April

OFDM fundamentals
• Advantages: f
– OFDM itself does not provide processing gains,
but provides a degree of freedom in frequency
domain by partitioning the wideband channel into
multiple narrow “flat-fading” sub-channels.
f
– Channel coding is mandatory for OFDM to combat
frequency-selective fading.
– Efficiently combating multi-path propagation in term of cyclic prefix
– OFDM receiver (frequency domain equalizer) has less complexity than that of
Rake receiver on wideband channels.
– OFDM characterizes flexible spectrum expansion for cellular systems.
• Drawbacks:
– high peak-to-average ratio.
– Sensitive to frequency offset, hence to Doppler-shift as well


OFDM fundamentals – downlink OFDMA

1 resource block:
180 kHz = 12 subcarriers

f
1 slot = 0.5 ms PDCCH

PDSCH

• OFDMA provides flexible scheduling in time-frequency domain.
• In case of multi-carrier transmission, OFDMA has larger PAPR than traditional
single carrier transmission. Fortunately this is less concerned with downlink.
• Does OFDMA suits for uplink transmission?
– Uplink being sensitive to PAPR due to UE implementation requirements
– With wider bandwidth in operation, OFDMA in uplink will have lower power per pilot
symbol which in turn leads to deterioration of demodulation performance.


Wideband single carrier transmission -
frequency domain equalizer (SC-FDE)
• While time-domain discrete equalizer has effect of “linear
convolution” on channel response; frequency domain equalizer
actually serves as “cyclic convolution” thereof.
• The difference will make first L-1 symbols “incorrect” at the output
of FDE.
• Solution could be either “overlapped processing” or “cyclic prefix”
added in transmitter.
transmitter
block-wise generation

Single carrier x(t)
Pulse
signal CP
Shaping
generation N samples insertion N+Ncp samples

“Adaptive Frequency-Domain Equalization and Diversity Combining for Broadband Wireless Communications,” M. V. Clark,
IEEE J. Sel. Areas Commun., vol. 16, no. 8, Oct. 1998
“Linear Time and Frequency Domain Turbo Equalization,” M. Tüchler et al., Proc. IEEE 53rd Veh. Technol. Conf. (VTC), vol. 2,
May 2001
“Block Channel Equalization in the Frequency Domain,” F. Pancaldi et al., IEEE Trans. Commun., vol. 53, no. 3, Mar. 2005

SC-FDMA – multiple access with FDE

Binary input data

QAM DFT Subcarrier IFFT
Coding Interleaving mapping P/S add CP
mapping (size M) (size N)

Pulse
RF Tx DAC
FDMA: shaping
user multiplexing in frequency domain
Single Carrier:
sequential transmission of the symbols
over a single frequency carrier

Timing and
RF Rx ADC
frequency Sync
Binary output data

de- QAM IDFT Freq Domain FFT CP
de-coding (Size M) Equalizer
P/S S/P
interleaving de-mapping (size N) removal

“Introduction to Single Carrier FDMA”, Hyung G Myung, 2007 EURASIP

SC-FDMA – multiple access with SC-FDE

• Multiple access in LTE uplink

Terminal A
data stream
DFT Pulse
OFDM
Shaping f
0

Terminal B
0
data stream Pulse
DFT OFDM f
Shaping

Orthogonal uplink design in frequency domain!


SC-FDMA – multiple access with FDE

block-wise DFT IFFT CP D/A conversion RF
signals (M) (N) insertion /pulse shaping

Adopted by
LTE uplink!
Also called DFT-
Spread OFDM!

… … … …
Localized FDMA: Distributed FDMA:

A B C D DFT DFT IFFT
(M) A B C D (M) (N)
IFFT
(N)
…

OverSampling in freq domain results in Upsampling in freq domain makes
interpolation at time domain output repeated sequence at time domain output

time domain: A* * * B * * * C * * * D* * * ABCDABCDABCDABCD

frequency domain:


OFDMA Vs SC-FDMA
•Time domain: •Frequency domain
- OFDM symbol is a sum of all data symbols by IFFT - OFDM modulates each subcarrier with one data symbol
- SC-FDMA symbol is repeated sequence of data “chips” - SC-FDMA “distributes” all data symbols on each subcarrier.

Input data symbols

OFDM symbol

SC-FDMA symbol *

t f
time domain frequency domain

* Assuming bandwidth expansion factor Q=4 in distributed FDMA.


OFDMA Vs SC-FDMA
• Similarities
– Block-wise data processing and use of Cyclic Prefix
– Divides transmission bandwidth into smaller sub-carriers
– Channel inversion/equalization is done in frequency domain
– SC-FDMA is regarded as DFT-Precoded or DFT-Spread OFDMA
• Difference
– Signal structure: In OFDMA each sub-carrier only carries information related
to only one data symbol while in SC-FDMA, each sub-carrier contains
information of all data symbols.
– Equalization: Equalization for OFDMA is done on per-subcarrier basis while
for SC-FDMA, equalization is done over the group of sub-carriers used by
transmitter.
– PAPR: SC-FDMA presents much lower PAPR than OFDMA does.
– Sensitivity to freq offset: yes for OFDMA but tolerable to SC-FDMA.


LTE Physical layer and transmission
procedures


LTE physical layer – a vertical view
• What kind of information is transmitted?
– Upper layer SDUs plus additional L1 control information in transmission, e.g
Reference Signals, Sync signals,CQI, HARQ,etc
control information • How is it transmitted?
or user data – Downlink OFDMA and uplink SC-FDMA
– Channel dependent scheduling, HARQ,etc
PDCP
– multiple antenna support
RLC • Related L1 procedures
– random access, power control, time alignment, etc
MAC

Transport blocks coding Scrambling modulation multiplex
control information

reference signals
signals from
other channels

frequency

time


LTE physical layer - a horizontal view

• PBCH: carries system broadcast information
• PCFICH: indicates resources used for PDCCH
• PHICH: carries ACK/NACK for HARQ operation.
• PDCCH: carriers scheduling assignments and other control information
• PDSCH: conveys data or control information
• PMCH: for MBMS data transmission
• Reference signal
• Synchronization signal (PSS,SSS) • PUCCH: carries control information
• PRACH: to obtain uplink synchronization
• PUSCH: for data or control information
• Reference Signals (Demod RS & SRS)
Feedback C
QIs,
data transm
ission
PDCCH n
otifies how
to demodula
te d ata


Fundamental Downlink transmission scheme
1 radio frame = 10 sub-frames = 10 ms
1 sub-frame = 2 slot = 14 OFDM symbols*
1 sub-frame = 1 ms
1 resource
element
1 slot = 0.5 ms =
7 OFDM symbols
1 resourrc block =
12 sub-carriers = 180KHz

1 radio frame = 10 ms

⎧5.2 μs, for first OFDM symbol
Tcp = ⎨
⎩4.7 μs,
66.7 us
Tcp for remaining symbols

66.7 us Tcp _ e = 16.7 μs
Tcp-e

*An alternative slot structure for MBMS is 6 OFDM symbols per slot where extended CP is in use.


System information broadcast
• System information
– MIB: transmitted on PBCH (40msTTI) One BCH transportation block

• information about downlink bandwidth
CRC insertion
• PHICH configuration
• SFN 1/3 conv.
– SIB: transmitted on PDSCH(DL-SCH) coding

• SIB1: operator infor & access restriction infor
scrambling
• SIB2: uplink cell bandwidth, random access parameters
• SIB3: cell-reselection
modulation
• SIB4~SIB8: neighbor cell infor
antenna
PBCH: the first 4 OFDM mapping
symbol in 2nd Slot per
10ms frame
De-multiplexing
10MHz
1.08 MHz
600 subcarriers

Synchronization signal
10ms frame

10ms frame


Downlink control channels – PCFICH,PHICH
• PCFICH:
– tells about the size of the control region.
– Locates in the first OFDM symbol for each sub-frame.
16 symbols
2 bits 1/16 32 bits 32 bits
Scrambling QPSK mod
block code

PCFICH-to-resource-element mapping
depends on cell identity so as to avoid
• PHICH: inter-cell interference.

– acknowledges uplink data transfer
– Locates in 1st OFDM symbol for each sub-frame
One PHICH group
inferior to PCFICH allocation
contains 8 PHICHs
1 bit 3x 3 bits I
BPSK mod
repetition
12 symbols
…

Orthogonal code
1 bit 3x 3 bits Q
BPSK mod
repetition scrambling

Orthogonal code


Downlink control channels - PDCCH
• Downlink control information (DCIs)
– Downlink scheduling assignments
– Uplink scheduling assignments
– Power control commands
• Control region size indicated by PCFICH
• Blind decoded by UE in its “search space” and common “search
space” – allows UE’s micro-sleep even in active state
• QPSK always used but channel coding rate is variable
control information
control region reference signals

1 sub-frame = 1 ms

R1-073373 “ Search space definition ofr L1/L2 control channels.
“Downlink control channel design for 3GPP LTE”, Robert Love, Amitava Ghosh, et,al. IEEE WCNC 2008.


Downlink control channels – PDCCH
• How to map DCIs to physical resource elements
– Control Channel Elements(CCEs), consisting of 36 REs, are used to
construct control channels.
– CCE aggregated at pre-defined level(1,2,4,8) to ease blind detections.
• Usually 5MHz bandwidth system renders 6 UL/DL scheduling
assignments within a sub-frame.

CCH candidate 10
CCH candidate 1

CCH candidate 3

CCH candidate 4

CCH candidate 5

CCH candidate 6

CCH candidate 7

CCH candidate 9
CCH candidate 2

CCH candidate 8
Control channel
candidate set
Control Channel Element 0 Or search space
Control Channel Element 1

Control channel candidates on which the UE attempts to
decode the information
R1-070787 “Downlink L1/L2 CCH design” (10 decoding attempts in this example)


Downlink control channels - PDCCH
• Each PDCCH carries one DCI message.
Control information Control information Control information

RNTI CRC attachment RNTI CRC attachment RNTI CRC attachment

1/3 Conv Coding 1/3 Conv Coding
…… 1/3 Conv Coding

Rate mattching Rate mattching Rate mattching

CCE aggragation and PDCCH multiplexing

Scrambling

QPSK

Interleaving

Cell specific
Cyclic shift


Downlink shared channel: PDSCH
Transport block Transport block
• Support up to 4 Tx antennas* from MAC from MAC

• Resource block allocation:
CRC CRC
– Localized: with less signaling overheads
– Distributed: benefits from frequency diversity Segmentation Segmentation

• Channelization (location): FEC FEC

RM+HARQ RM+HARQ
control information

reference signals Scrambling Scrambling
User A
data region Modulation Modulation
User B
User C
Antenna mapping
unused

Cell-specific, bit-level RB mapping
scrambling for interference
randomization **

1 sub-frame = 1 ms To OFDM modulation for each antenna

* For MBSFN, antenna diversity scheme does not apply.
** For MBSFN, it’s MBSFN-area-specific scrambling.

Downlink reference signals
• Cell-specific reference signals are length-31 Gold sequence,
initialized based on cell ID and OFDM symbol location.
• Each antenna has a specific reference signal pattern, e.g 2
antennas
– frequency domain spacing is 6 sub-carriers
– Time domain spacing is 4 OFDM symbols
– That is, 4 reference symbols per Resource Block per antenna
time

frequency
Antenna 0 Antenna 1

3GPP TS 36.211 “ physical channels and modulation“ section 6.10.1.1

LTE Multiple antenna scheme
NodeB transmitter
WCDMA STTD scheme:
S 0 , S1 , S 2 , S3
S 0 , S1 , S 2 , S3
UE
STTD − S * , S * ,− S * , S *
1 0 3 2

LTE SFBC (space frequency block coding): LTE CDD (cyclic delay diversity):
eNodeB transmitter eNodeB transmitter
a0 a0
a1 a1
a2 OFDM
a2 OFDM
a3 modulation a3 modulation
…

…
− a0
* UE UE
a0
a1e j 2πΔf ⋅Δt
*
a1
− a3* OFDM
a2 e j 2πΔf ⋅2 Δt
OFDM
modulation modulation
a3e j 2πΔf ⋅3Δt
*
a2
…

…

• Downlink SU-MIMO
– Transmission of different data streams simultaneously over multiple antennas
– Codebook based pre-coding: signal is “pre-coded” at eNodeB before transmission
while optimum pre-coding matrix is selected from pre-defined codebook based on
r
UE feedback. r
S γ
– Open-loop mode possible for high speed S1 r1

Pre- H SIC
coding receiver
S2 r2

eNodeB UE
PMI, RI, CQI
• Uplink MU-MIMO: collaborative MIMO
– Simultaneous transmission from 2UEs on
same time-frequency resource
– Each UE with one Tx antenna
– Uplink reference signals are coordinated
between UEs


LTE channels Multiple Antenna Schemes comments
open-loop spatial multiplexing large delay CDD/ SFBC
closed-loop spatical multiplexing SU-MIMO
DL data channel PDSCH
multi-user MIMO MU-MIMO
UE specific RS beam-forming Applicable > 4 Antennas
PDCCH SFBC
PHICH SFBC
DL control channel PCFICH open-loop transmit diversity SFBC
PBCH SFBC
Sync Signals PVS
receiver diversity MRC/IRC
UL data channel PUSCH
multi-user MIMO MU-MIMO
PUCCH receiver diversity MRC
UL control channel
PRACH receiver diversity MRC


Synchronization and Cell Search
• LTE synchronization design considerations:
– high PSR (Peak to side-lobe ratio: the ratio between the peak to the side-lobes of its
aperiodic autocorrelation function) to ease time-domain processing
– low PAPR for coverage
– Generalized Chirp Like (GCL) sequences overwhelm Golay and Gold sequences!
• Synchronization signals
– PSS: length-63 Zadoff-Chu sequences
• Auto-correlation/cross-correlation/hybrid correlation based detection
– SSS: an interleaved concatenation of two length-31 binary sequences
• Alternative transmission (SSS1 and SSS2) in one radio frame
1 radio frame = 10 ms SSS
0 1 2 3 4 5 6 7 8 9
PSS

3GPP TS 36.211 “physical channels and modulation “
“Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE


• LTE synchronization design considerations:
– high PSR (Peak to side-lobe ratio: the ratio between the peak to the side-lobes of its
aperiodic autocorrelation function) to ease time-domain processing
– low PAPR for coverage
– Generalized Chirp Like (GCL) sequences overwhelm Golay and Gold sequences!
• Synchronization signals
– PSS: length-63 Zadoff-Chu sequences
• Auto-correlation/cross-correlation/hybrid correlation based detection
– SSS: an interleaved concatenation of two length-31 binary sequences
• Alternative transmission (SSS1 and SSS2) in one radio frame
1 radio frame = 10 ms SSS
0 1 2 3 4 5 6 7 8 9
PSS

62 Central
Sub-carriers

3GPP TS 36.211 “physical channels and modulation “
“Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE


• Hierarchical cell ID(1 out of 504):
– Cell ID = 3* Cell group ID + PHY ID : N ID = 3 ⋅ N ID) + N ID)
CELL (1 (2

⎧ − j πun63 +1)
(n
μ = 25 N ID) = 0
(2

• PSS structure ⎪ e n = 0,1,...,30
d u (n) = ⎨ πu ( n +1)( n + 2 ) μ = 29 N ID) = 1
(2
−j
⎪e
⎩
63
n = 31,32,...,61 μ = 34 N ID) = 2
(2

x0
pss 62 sub-carriers excluding DC carrier

PSS sequences x1 CP
pss
… …
…

IFFT insertion f
62
x pss
f
The indices (m0, m1) define odd sub-carriers
the cell group identity.
• SSS structure even sub-carriers
+ +
S 0m ( 0 ) SSC1 S1m (1) SSC1

C0 C0
+ + + +
S1m (1) SSC2 S 0m ( 0 ) SSC2 slot 0 … slot 10

C1 Z1m ( 0 ) C1 Z1m (1)

LTE Cell Search Vs WCDMA cell search
• PSS detection • P-SCH detection
– Slot timing – Slot boundary
– Physical layer ID (1 of 3) • S-SCH detection
• SSS detection – frame timing
– Radio frame timing – code group ID
– Cell group ID (1 of 168) • CPICH detection
– CP length – Cell-specific scrambling code
• PBCH decoding identified
– PBCH timing • BCH reading
– System information access

“cell searching in WCDMA”,Sanat Kamal Bahl, IEEE Potential 2003;

LTE uplink
• SC-FDMA: fundamental uplink radio parameters are aligned with
downlink scheme, e.g frame structure, sub-carrier spacing, RB
size.…
• Multiplexing of uplink data and control information
– Combination of FDM and TDM are adopted in LTE uplink
• Uplink transmission are well time-aligned to maintain
orthogonality (no intra-cell interference)
• PRACH will not convey user data like WCDMA does, but serve to
obtain uplink synchronization


Fundamental uplink transmission scheme
1 sub-frame = 1 ms

1 slot = 0.5 ms =
7 OFDM symbols

1 radio frame = 10 ms

under eNodeB scheduling
f
⎧5.2μs, for first OFDM symbol
Tcp = ⎨
⎩4.7 μs,
66.7 us
Tcp for remaining symbols

66.7 us Tcp _ e = 16.7 μs
Tcp-e

• Uplink transmission frame aligned with downlink parameterization
to ease UE implementation.


Uplink reference signal
• Uplink reference signals
– Mostly based on Zadoff-Chu sequences (cyclic extensions)
interference
– Pre-defined QPSK sequences for small RB allocation
randomization
• Demodulation Reference Signal (DRS) in a cell across intra-cell and
inter-cells
– Each cell is assigned 1 out of 30 sequence groups
– Each sequence group contains 1(for less than 5 RB case) or 2 (6RB+ case) RS
sequence across all possible RB allocations
– Sequence-group hopping is configurable in term of broadcasting information where the
hopping pattern is decided by Cell ID
– Cyclic time shift hopping applies to both control channel and data channel
• DRS on PUSCH
0
0
RS sequence

block of DFT OFDM
add CP
…
…

(size M) modulator
data symbols
Instantaneous
bandwidth
0 (M sub-carriers)
0

One DFTS-OFDM symbol

3GPP TS 36.101 “physical channels and modulation” section 5.5.1

Uplink reference signal
• DRS on PUCCH
– See next slides
• Sounding Reference Signal (SRS)
– Not regularly but allows eNodeB to estimate uplink channel quality at alternative
frequencies
– UE’s SRS transmission is subject to network configuration
– Location: always on last OFDM symbol of a sub-frame if available
one sub-frame

wideband, non-frequency hopping SRS narrowband, frequency hopping SRS


Uplink control channel transmission - PUCCH
• Uplink control signaling
– Data associated: transport format, new data indicator, MIMO parameters
– Non-data associated: ACK/NACK, CQI, MIMO codeword feedback
no explicit tranmission
• Channelization from UE as it follows
eNodeB scheduling!
– In the absence of uplink data transmission: in reserved frequency region on
band edge
– In the presence of uplink data transmission: see multiplexing with data on
PUSCH
Control region 1 Control region 2
Uplink
control TDM
with data

…..
downlink
total uplink
data transmission system bandwidth
f
downlink
data transmission
1 ms sub-frame

standalone
uplink control


Beyond HSPA+: An Introduction to LTE Fundamentals

Beyond HSPA+: An Introduction to LTE Fundamentals

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