LTE tutorial - Looking forward beyond HSPA+
[email protected] 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 UL: 5.76Mbps
UTRAN
Rel-99 WCDMA HSDPA/HSUPA
Rel-5
E-UTRAN
DL: 28 Mbps UL: 11 Mbps
Rel-6
DL: 42 Mbps UL: 11 Mbps
DL: 84 Mbps UL: 23 Mbps
DL: 100+ Mbps UL: 23+ Mbps
HSPA+ (HSPA Evolution)
Rel-7
Rel-8
Rel-9 deployment & service enhancement
LTE specification process ~ 2007Q4 DL:300 Mbps UL: 75 Mbps
All rights reserved @ 2009
Beyond Rel-9
LTE-A
DL: 1 Gbps 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 All rights reserved @ 2009
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 All rights reserved @ 2009
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)
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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 All rights reserved @ 2009
LTE basic parameters Frequency range
UMTS FDD bands and TDD bands defined in 36.101(v860) Table 5.5.1
channel bandwidth (MHz)
Transmission bandwidth NRB: (1 resource block = 180kHz in 1ms TTI)
1.4
3
5
10
15
20
6
15
25
50
75
100
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
Peak data rate
Downlink: 150Mbps(UE Category 4, 2x2 MIMO, 20MHz bandwidth) 300Mbps(UE category 5, 4x4 MIMO, 20MHz bandwidth) Uplink: 75Mbps(20MHz bandwidth)
All rights reserved @ 2009
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 transport block bits received within a TTI
Category 1
10296
Category 2
51024
Category 3
Maximum number of bits of a DL-SCH transport block received within a TTI
Peak rate 150Mbps with 2x2 MIMO
Total number of soft channel bits
Maximum number of supported layers for spatial multiplexing in DL
10296
250368
1
51024
1237248
2
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 Cate gory
Maximum number of bits of an UL-SCH transport block transmitted within a TTI
Support for 64QAM in UL
Category 1
5160
No
Category 2
25456
Category 3
51024
Category 4
51024
No
Category 5
75376
Yes
Peak rate 75Mbps
No No
3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“ All rights reserved @ 2009
LTE UE category UE Category Peak rate (Mbps)
1
2
3
4
5
DL
10
50
100
150
300
UL
5
25
50
50
75
RF bandwidth
20 MHz
DL
QPSK, 16QAM, 64QAM QPSK, 16QAM, 64QAM
Modulation UL
QPSK, 16QAM
2 Rx Diversity 2x2 MIMO 4x4 MIMO
Assumed in performance requirements Optional
Mandatory Not supported
Mandatory
3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“ All rights reserved @ 2009
Channel dependent scheduling •
Time-frequency scheduling
UE #1
UE #2
All rights reserved @ 2009
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;
b- s Su rier r ca
po w
BS 2
er de ns
subcarr ier
ity MS 21
nsity
MS 31 MS 11
MS 32
s ca ubrri er
MS 12
y sit
Pow er d e
n de er w Po
BS 1
MS 22
3GPP R1-050841 “Further Analysis of Soft Frequency Reuse Scheme “ BS 3
All rights reserved @ 2009
E-UTRAN overview
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E-UTRAN architecture
S1
S1
X2
X2
S1
S1 All rights reserved @ 2009
E-UTRAN architecture
All rights reserved @ 2009
E-UTRAN radio protocol notifications
RRC
Paging
common
dedicated
System information
Dedicated Control and information transfer
radio bearers
logical channels
SRB0
SRB1
SRB2
Integrity and ciphering
Integrity and ciphering
ciphering and ROHC
ciphering and ROHC
RLC
ARQ
ARQ
ARQ
ARQ
DCCH 1
DCCH 2
DTCH 1
PCCH
PCH
DRB2
PDCP
BCCH
CCCH
Multiplexing and HARQ control
MAC transport channels
DRB1
BCH
RACH
DL-SCH
UL-SCH
PHY layer functions physical channels
PBCH
PRACH
All rights reserved @ 2009
PDSCH
PUSCH
DTCH 2
E-UTRAN radio channels uplink
downlink PCCH
BCCH
PCH
BCH
PDCCH
PBCH
CCCH
DCCH
DTCH
DL-SCH
MCH
PDSCH
PMCH
MCCH
Logical channels
CCCH
Transport channels
RACH
MTCH
Physical channels
PRACH
DCCH
DTCH
UL-SCH
PUCCH
PUSCH
•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 All rights reserved @ 2009
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
RR C PD CP
IP
S1
LT E
L1
L1
M AC
RL C M AC LT E
u -u P- TP T G G P DP UD U
u PT G P UD
AP
TP SC IP
RL C
PD CP
RR C
NA S
RT U IP DP P H TC TT P P
data traffic: E-UTRAN radio bearer + S1 bearer +S5/S8 bearer L1/L2 control channel
ye La
r2
Y PH
S NA AP S1 TP SC
IP L2
Y HY PH P
S-GW
IP L2
Y PH
eNodeB
MME
UE E-UTRAN radio bearer
S1 bearer EPS bearer All rights reserved @ 2009
IP L2
S5/S8 bearer
u PGT P UD IP L2
Y PH
P-GW
E-UTRAN – Control plane stack MME/ eNodeB
UE 24.301 eNodeB
NAS RRC PDCP RLC MAC PHY
36.331 36.323 36.322 36.321 36.211~36.214
RRC
NAS S1AP X2AP
36.413 36.423
S1AP X2AP
SCTP
36.412 36.422
SCTP
PDCP RLC IP
IP
MAC
L2
L2
PHY
L1
L1
LTE-Uu
S1-MME/X2-C
All rights reserved @ 2009
E-UTRAN – User Plane Stack PDN/S-GW eNodeB
UE eNodeB
Application IP PDCP RLC MAC PHY
IP 36.323 36.322 36.321 36.211~36.214
29.274
PDCP
GTP-u
RLC
UDP
UDP
IP
IP
L2
L2
L1
L1
GTP-u
MAC PHY
LTE-Uu
S1-U/X2-u
All rights reserved @ 2009
Radio resource management Interference management
QoS management L3
RRC Load control
Admission control
Semi-persistent scheduling
mobility management
PDCP L2
RLC
Hybrid ARQ manager
Dynamic scheduling
Link adaptation
MAC
L1
PHY
PDCCH adaptation
CQI manager
“An overview of downlink radio resource management for LTE”, Klaus Ingemann Pedersen, et al, IEEE communication magazine, 2009 July All rights reserved @ 2009
E-UTRAN mobility • • •
Simplified RRC states Idle-mode mobility (similar as HSPA) Connected-mode mobility –
handover controlled by network
Source eNodeB
Target cell signal quality meets reporting threshold
• • • •
RRC-connected
Cell reselection decided by UE • Network controlled handovers Based on UE measurements • Based on UE measurements Controlled by broadcasted parameters Different priorities assigned to frequency layers
Mobility difference between UTRAN and E-UTRAN
MME/SGW HO decision
RRC-idle
Call Admission
UTRAN
E-UTRAN
Location area (CS core)
Not relevant since no CS connections
Routing area
Tracking area
SHO
No SHO
Cell_FACH, Cell_PCH,URA_PCH
No similar RRC states
RNC hides most of mobility
Core network sees every handover
Neighbour cell list required
No need to provide cell-specific information, only carrier-frequency is required.
target eNodeB
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Overview of a PS call – control plane • UE activities after power-on Power up
Initial cell search
Derive system information
Random Access
Data Tx/Rx
UE
E-UTRAN paging
SS S /S S P
Radom Access procedure H BC
H HIC P / H H CC FIC PD PC
m ado Rn
A
RRC Connection Request
Connection establishment
RRC Connection Setup
ss cce
RRC Connection Setup Complete
H SC PD
H CC U P H/ SC U P
Security procedures RRC Connection Reconfiguration RRC Connection Reconfiguration Complete
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Radio bearer establishment
Overview of a PS call – control plane • UE activities after power-on Power up
Initial cell search
Derive system information
Random Access
Data Tx/Rx
UE ss mi
ion
s ran t sch a k t n da pli u L D K& AC
A
& CK
gr g n i l ed u
nn cha
U
at Ld
ant
s atu t s el
a a tr
E-UTRAN paging
Radom Access procedure
ort p e r
o issi m ns
RRC Connection Request
Connection establishment
RRC Connection Setup
n
RRC Connection Setup Complete
Security procedures RRC Connection Reconfiguration RRC Connection Reconfiguration Complete
All rights reserved @ 2009
Radio bearer establishment
Overview of a PS call – user plane PS data via S1 interface
Tx
eNodeB
1 resource block: 180 kHz = 12 subcarriers
to RF
OFDM Signal Generation
1 resource block pair 1 TTI = 1ms = 2 slots resource mapping
PDCP (Ciphering Header Compression,)
RLC (Segmentation, ARQ) scheduling
data modulator
coding
UE
HARQ
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Multiplexing per user
Overview of a PS call – user plane PS data via S1 interface
Tx
eNodeB
1 resource block: 180 kHz = 12 subcarriers
to RF
OFDM Signal Generation
1 resource block pair 1 TTI = 1ms = 2 slots resource mapping
PDCP (Ciphering Header Compression,)
RLC (Segmentation, ARQ) scheduling
data modulator
coding
UE
HARQ
Occupying different radio resources across TTIs adapts to time-varying radio channel condition!
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Multiplexing per user
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 largesize 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)
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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 throughput per user
Top 5%, loaded Average Cell edge
1Mpbs throughput at cell edge
Active users per cell Source: www.lstiforum.org All rights reserved @ 2009
Active users per cell
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
Ubiquitous coverage High capacity & data rate Low cost
Tput (Mbps)
G-factor (dB)
8
25
4
15
>> “FemtoCell” – Home eNodeB!
10 2 2 0
3GPP TS 25.101 Table 9.8D3, 9.8D4, 9.8F3 for PA3 All rights reserved @ 2009
-3
LTE initial trial performance •
User plane latency – –
3GPP RTT target is 10ms for short IP packet Field trial results: • 10~13ms with pre-scheduled uplink • τ
ak +1
OFDM fundamentals- Cyclic Prefix Tu
ak −1
directed path:
ak
ak +1
reflected path:
τ
τ
Integration interval of direct path
directed path: reflected path:
τ
Tcp >τ
Guard time: Cyclic Prefix Vs Padding Zeroes a0 a1
…
a Nc −1 guard time
FFT integration time=1/Carrier spacing OFDM symbol time
All rights reserved @ 2009
IFFT
P/S Tu
add Cyclic Prefix
an OFDM symbol Tu+Tcp
OFDM fundamentals – general link level chains Binary input data
Coding
Interleaving
QAM mapping
Pilot Insertion
S/P
IFFT
P/S
add CP
5 MHz Bandwidth
FFT
Sub-carriers
Guard Intervals
RF Tx
…
Symbols
DAC
Pulse shaping
Frequency
… Time
Binary output data
de-coding
deinterleaving
QAM de-mapping
RF Rx
ADC
Equalizer
P/S
Timing and frequency Sync
FFT
S/P
CP 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 All rights reserved @ 2009
OFDM fundamentals – frequency domain equalizer * w(τ ) = h (−τ ) h(τ ) ⊗ w(τ ) = 1 2
MRC filter: Zero Forcing: MMSE:
ε = E{ sˆ(t ) − s(t ) }
Channel model
transmitter
receiver
n(t ) S (t )
h(τ )
+
r (t )
w(τ )
~ s (t )
W0
rn
D
W0
D
R0
D
W1
r (t )
WL-1
Time domain
WN −1
DFT
sˆn
+
⊗
RN −1
⊗
Sˆ0
Sˆ N −1
IDFT
sˆ(t )
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 All rights reserved @ 2009
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. – 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
All rights reserved @ 2009
f
OFDM fundamentals – downlink OFDMA 1 resource block: 180 kHz = 12 subcarriers
f PDCCH
1 slot = 0.5 ms
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.
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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 Pulse signal CP Shaping generation N samples insertion N+Ncp samples
x(t)
“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 All rights reserved @ 2009 “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
Coding
Interleaving
QAM mapping
DFT (size M)
IFFT (size N)
Subcarrier mapping
RF Tx
FDMA: user multiplexing in frequency domain
P/S
DAC
add CP
Pulse shaping
Single Carrier: sequential transmission of the symbols over a single frequency carrier
RF Rx
ADC
Freq Domain Equalizer
P/S
Timing and frequency Sync
Binary output data
de-coding
deinterleaving
QAM de-mapping
IDFT (Size M)
FFT (size N)
“Introduction to Single Carrier FDMA”, Hyung G Myung, 2007 EURASIP All rights reserved @ 2009
S/P
CP removal
SC-FDMA – multiple access with SC-FDE •
Multiple access in LTE uplink Terminal A data stream
DFT
OFDM
Pulse Shaping
f
Pulse Shaping
f
0
Terminal B 0
data stream
DFT
OFDM
Orthogonal uplink design in frequency domain!
All rights reserved @ 2009
SC-FDMA – multiple access with SC-FDE •
Multiple access in LTE uplink Terminal A data stream
DFT
OFDM
Pulse Shaping
f
Pulse Shaping
f
0
Terminal B 0
data stream
DFT
OFDM
Orthogonal uplink design in frequency domain!
All rights reserved @ 2009
SC-FDMA – multiple access with FDE block-wise signals
DFT (M)
IFFT (N)
CP insertion
Adopted by LTE uplink!
Also called DFTSpread OFDM!
A B C D
Distributed FDMA: DFT (M)
…
IFFT (N)
A B C D
DFT (M)
… … … …
Localized FDMA:
time domain:
D/A conversion /pulse shaping
IFFT (N)
OverSampling in freq domain results in interpolation at time domain output
Upsampling in freq domain makes repeated sequence at time domain output
A* * * B * * * C * * * D* * *
ABCDABCDABCDABCD
frequency domain:
All rights reserved @ 2009
RF
OFDMA Vs SC-FDMA •Frequency domain
•Time domain:
- OFDM modulates each subcarrier with one data symbol - OFDM symbol is a sum of all data symbols by IFFT - 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 *
time domain
t
* Assuming bandwidth expansion factor Q=4 in distributed FDMA. All rights reserved @ 2009
f frequency domain
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.
All rights reserved @ 2009
LTE Physical layer and transmission procedures
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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 • or user data PDCP RLC
•
How is it transmitted? – – –
Downlink OFDMA and uplink SC-FDMA Channel dependent scheduling, HARQ,etc multiple antenna support
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 All rights reserved @ 2009
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 • PUCCH: carries control information Synchronization signal (PSS,SSS) • 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
All rights reserved @ 2009
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
Tcp
Tcp-e
66.7 us
66.7 us
⎧5.2 μs, Tcp = ⎨ ⎩4.7 μs,
for first OFDM symbol for remaining symbols
Tcp _ e = 16.7 μs
*An alternative slot structure for MBMS is 6 OFDM symbols per slot where extended CP is in use. All rights reserved @ 2009
System information broadcast •
System information – MIB: transmitted on PBCH (40msTTI) • information about downlink bandwidth • PHICH configuration • SFN
– SIB: transmitted on PDSCH(DL-SCH) • • • •
SIB1: operator infor & access restriction infor SIB2: uplink cell bandwidth, random access parameters SIB3: cell-reselection SIB4~SIB8: neighbor cell infor
1/3 conv. coding scrambling modulation
De-multiplexing
1.08 MHz
Synchronization signal
CRC insertion
antenna mapping
PBCH: the first 4 OFDM symbol in 2nd Slot per 10ms frame 10MHz 600 subcarriers
One BCH transportation block
10ms frame
10ms frame
All rights reserved @ 2009
Downlink control channels – PCFICH,PHICH •
PCFICH: – tells about the size of the control region. – Locates in the first OFDM symbol for each sub-frame. 2 bits
•
1/16 block code
16 symbols
32 bits
Scrambling
QPSK mod
PCFICH-to-resource-element mapping depends on cell identity so as to avoid inter-cell interference.
PHICH:
One PHICH group contains 8 PHICHs
32 bits
– acknowledges uplink data transfer – Locates in 1st OFDM symbol for each sub-frame inferior to PCFICH allocation 1 bit
3x repetition
3 bits
BPSK mod
12 symbols
… 1 bit
I
Orthogonal code 3x repetition
Q
3 bits
BPSK mod scrambling Orthogonal code
All rights reserved @ 2009
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. All rights reserved @ 2009
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.
CCH candidate 10
CCH candidate 9
CCH candidate 8
CCH candidate 7
CCH candidate 6
CCH candidate 5
CCH candidate 4
CCH candidate 3
CCH candidate 2
Usually 5MHz bandwidth system renders 6 UL/DL scheduling assignments within a sub-frame. CCH candidate 1
•
Control Channel Element 0 Control Channel Element 1 Control Channel Element 2 Control Channel Element 3 Control Channel Element 4 Control Channel Element 5
R1-070787 “Downlink L1/L2 CCH design”
Control channel candidates on which the UE attempts to decode the information (10 decoding attempts in this example) All rights reserved @ 2009
Control channel candidate set Or search space
Downlink control channels - PDCCH •
Each PDCCH carries one DCI message. Control information
RNTI
CRC attachment
Control information
RNTI
Control information
RNTI
CRC attachment
1/3 Conv Coding
1/3 Conv Coding
Rate mattching
Rate mattching
……
CCE aggragation and PDCCH multiplexing Scrambling
QPSK
Interleaving
Cell specific Cyclic shift All rights reserved @ 2009
CRC attachment
1/3 Conv Coding
Rate mattching
Downlink shared channel: PDSCH • •
Support up to 4 Tx antennas* Resource block allocation: – Localized: with less signaling overheads – Distributed: benefits from frequency diversity
•
Channelization (location): control information reference signals
data region
Transport block from MAC
Transport block from MAC
CRC
CRC
Segmentation
Segmentation
FEC
FEC
RM+HARQ
RM+HARQ
Scrambling
Scrambling
Modulation
Modulation
User A User B User C unused Cell-specific, bit-level scrambling for interference randomization **
Antenna mapping RB mapping
To OFDM modulation for each antenna
1 sub-frame = 1 ms
* For MBSFN, antenna diversity scheme does not apply. ** For MBSFN, it’s MBSFN-area-specific scrambling. All rights reserved @ 2009
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 All rights reserved @ 2009
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
a3
a2
OFDM modulation
a3 …
…
− a0* a1* − a3* a2*
OFDM modulation
UE
… All rights reserved @ 2009
OFDM modulation
…
a1e j 2πΔf ⋅Δt a2 e j 2πΔf ⋅2 Δt a3e j 2πΔf ⋅3Δt
OFDM modulation
UE
a0
LTE Multiple antenna scheme •
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 r UE feedback. γ S Open-loop mode possible for high speed r1 S1 Precoding
S2
H r2
eNodeB
•
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
All rights reserved @ 2009
SIC receiver
UE PMI, RI, CQI
LTE Multiple antenna scheme LTE channels
DL data channel
DL control channel
Multiple Antenna Schemes
comments
open-loop spatial multiplexing
large delay CDD/ SFBC
closed-loop spatical multiplexing
SU-MIMO
multi-user MIMO
MU-MIMO
UE specific RS beam-forming
Applicable > 4 Antennas
PDSCH
PDCCH
SFBC
PHICH
SFBC
PCFICH
open-loop transmit diversity
PBCH
SFBC
Sync Signals UL data channel
SFBC
PVS receiver diversity
MRC/IRC
multi-user MIMO
MU-MIMO
PUCCH
receiver diversity
MRC
PRACH
receiver diversity
MRC
PUSCH
UL control channel
All rights reserved @ 2009
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 0
1
2
1 radio frame = 10 ms 3 4 5
SSS 6
7
8
9
3GPP TS 36.211 “physical channels and modulation “ “Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE All rights reserved @ 2009
PSS
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 0
1
2
1 radio frame = 10 ms 3 4 5
SSS 6
7
8
9
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 All rights reserved @ 2009
PSS
Synchronization and Cell Search •
Hierarchical cell ID(1 out of 504): –
•
Cell ID = 3* Cell group ID + PHY ID : ( n +1) ⎧ − j πun63 ⎪ e d u (n) = ⎨ πu ( n +1)( n + 2 ) −j 63 ⎪⎩e
PSS structure
CELL (1) ( 2) N ID = 3 ⋅ N ID + N ID
n = 0,1,...,30 n = 31,32,...,61
x 0pss
x
62 pss
( 2) N ID =0 ( 2) N ID =1
( 2) N ID =2
62 sub-carriers excluding DC carrier
…
x1pss
PSS sequences
μ = 25 μ = 29 μ = 34
IFFT
CP insertion
…
…
f
f
+
S1m (1)
C1
+
C0
SSC1
Z1m ( 0 )
S1m (1)
C0
SSC2 S 0m ( 0 )
+
+
S 0m ( 0 )
C1 All rights reserved @ 2009
odd sub-carriers even sub-carriers
SSC1
+
SSS structure
+
•
The indices (m0, m1) define the cell group identity.
Z1m (1)
SSC2
slot 0 … slot 10
LTE Cell Search •
Vs
PSS detection
•
– Slot timing – Physical layer ID (1 of 3)
•
P-SCH detection – Slot boundary
•
SSS detection
S-SCH detection – frame timing – code group ID
– Radio frame timing – Cell group ID (1 of 168) – CP length
•
WCDMA cell search
•
CPICH detection – Cell-specific scrambling code identified
PBCH decoding – PBCH timing – System information access
•
BCH reading
All rights reserved @ 2009
“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
All rights reserved @ 2009
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 Tcp
Tcp-e
•
66.7 us
66.7 us
⎧5.2μs, Tcp = ⎨ ⎩4.7 μs,
for first OFDM symbol for remaining symbols
Tcp _ e = 16.7 μs
Uplink transmission frame aligned with downlink parameterization to ease UE implementation. All rights reserved @ 2009
Uplink reference signal •
Uplink reference signals – –
•
Demodulation Reference Signal (DRS) in a cell – – – –
•
Mostly based on Zadoff-Chu sequences (cyclic extensions) Pre-defined QPSK sequences for small RB allocation
interference randomization 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
… …
DFT (size M)
RS sequence
block of data symbols
OFDM modulator
add CP Instantaneous bandwidth (M sub-carriers)
0 0 One DFTS-OFDM symbol
3GPP TS 36.101 “physical channels and modulation” section 5.5.1 All rights reserved @ 2009
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
All rights reserved @ 2009
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 from UE as it follows eNodeB scheduling!
Channelization
– 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
Uplink control TDM with data
…..
downlink data transmission
total uplink system bandwidth
f downlink data transmission
1 ms sub-frame
standalone uplink control All rights reserved @ 2009
Control region 2
Uplink control channel transmission - PUCCH • •
To cater for multiple downlink transmission mode, while preserving single-carrier property in uplink, multiple PUCCH formats exist. PUCCH is thus mainly classified by PUCCH format 1 & 2 – PUCCH format 1/1a/1b: 1 or 2 bits transmitted per 1ms, for ACK/NACK/SR – PUCCH format 2/2a/2b: up to 20 bits transmitted per 1ms, for CQI/PMI/RI
reference signal
ACK/NACK
reference signal
CQI
…..
….. 1 ms sub-frame
1 ms sub-frame
All rights reserved @ 2009
Multiuser transmission on PUCCH • •
In PUCCH format 1, multiple PUCCHs are distinguished by cyclic shift of ZACAC sequences plus orthogonal cover sequence In PUCCH format 2, multiple PUCCHs are distinguished by cyclic shift of ZACAC sequences. ACK/NACK bit
channel status report BPSK/QPSK Length-12 phase rotated sequence
QPSK Length-12 phase rotated sequence
IFFT
IFFT
IFFT
IFFT
Length-4 Walsh sequence
IFFT
RS
RS
IFFT
IFFT
IFFT
RS
RS
RS
1 slot = 0.5 ms
1 slot = 0.5 ms
All rights reserved @ 2009
IFFT
Uplink data transmission - PUSCH •
In case of PUSCH available, control signaling is multiplexed with data on PUSCH. – To cater for radio channel variation, link adaptation applies to data part – Control signaling does not adopt adaptive modulation but the size of REs (resource elements) can change w.r.t varying radio condition DFTS-OFDM modulation
UL-SCH
Turbo coding
Rate matching
CQI,/PMI
Conv coding
Rate matching
Block coding
Rate matching
RI
ACK/NACK
Block coding
CQI/PMI RS ACK/NACK RI PUSCH data
MUX
baseband modulation
DFT
IFFT
QPSK
t
All rights reserved @ 2009
Uplink data transmission - PUSCH •
UL-SCH processing chain – No Tx diversity/spatial multiplexing as downlink does – PUSCH frequency hopping (on slot basis) • Subband-based hopping according to cell-specific hopping patterns • Hopping based on explicit hopping information in scheduling grant
Transport block from MAC @UE
CRC Segmentation FEC RM+HARQ Scrambling Modulation
UE-specific, bit-level scrambling
All rights reserved @ 2009
To DFTS-OFDM and map to assigned frequency resorurce
Random Access •
LTE random access serves to obtain uplink synchronization, not to carry data. – Contention-based random access: preambles based on ZC sequences – Contention-free random access: faster with reserved preambles (e.g, for handover)
•
Random access resources UE
– 64 preambles classified into 3 parts: Preamble set #0
…
Preamble set #1
NAS UE ID RRC Connection Request
– RA area: •
RA preambles
reserved
…
1 in every 1~20 ms(configurable) 6 RBs
eNodeB temporary C-RNTI; timing advance; initial uplink grant
RA response (timing adjustment, UL grant)
1ms random access area
UE terminal ID early contention resolution Contention resolution
10 ms frame
All rights reserved @ 2009
Random Access •
PRACH structure – – –
•
Preamble sequence: cyclic shifted sequences from multiple root ZC sequences CP: facilitates frequency-domain prcoessing at eNodeB Guard time: to handle timing uncertainty near user
Other users
far user
Other users
CP
Preamble Sequence
CP
Guard time Other users
Preamble Sequence
Other users
timing uncertainty
PRACH format options preamble format
RA window (ms)
Tcp length (ms)
Tseq length (ms)
Typical usage
0
1
0.1
0.8
for small~medium cells (up to ~ 14 km)
1
2
0.68
0.8
for larget cells(up to ~ 77km) without link budget problem
2
2
0.2
1.6
for medium cells(up to ~ 29km) supporting low data rates
3
3
0.68
1.6
for very large cells(up to ~ 100km)
All rights reserved @ 2009
Layer 1 procedures – power control •
Uplink power control – – –
•
WCDMA power control is continuous at 1500Hz; while LTE runs power control slower at 200Hz Based on open-loop setting while assisted by close-loop adjustment Independent power control on PUCCH and PUSCH respectively
PUCCH power control PT = min{Pmax , P0 + PLDL + Δ format + δ }
•
PUSCH power control – –
Independent of PUCCH power control UE Power Headroom in use to indicate the true desired Tx power
PT = min{Pmax , P0 + α ⋅ PLDL + 10 ⋅ log10 ( M ) + Δ MCS + δ }
All rights reserved @ 2009
To increase uplink data rate, LTE would increase user’s bandwidth rather than increase Tx power!
Layer 1 procedures – Timing Alignment •
To maintain uplink intra-cell orthogonality, timing alignment is necessary. – The further away from eNodeB, the earlier the UE transmits. – Configurable by eNodeB at granularity of 0.52us from 0 ~0.67 ms (corresponding to max cell radius of 100km) Tx Rx Tp1
Rx Tx
Ta1
Tp2
Rx Tx
Ta2
All rights reserved @ 2009
Timing aligned uplink reception at eNodeB for different users
All rights reserved @ 2009
Backup - OFDMA Vs SC-FDMA • Channel equalizer: – OFDMA: divides wideband into multiple narrow “flat-fading” subbands hence equalization done on each sub-band is sufficient. – SC-FDMA: frequency domain equalization on the whole group bandwidth of sub-carriers in use.
equalizer
Detect
equalizer
All rights reserved @ 2009
IDFT
…
equalizer
Detect
…
Sub-carrier de-mapping
…
DFT
…
…
SC-FDMA:
…
…
Sub-carrier de-mapping
Detect
…
DFT
…
…
OFDMA:
equalizer
detect
Backup - OFDMA Vs SC-FDMA s (t ) 2
• PAPR: PAPR = E ( s(t ) 2 ) • CM: a better measure of UE PA back-off
⎡ (vn3 ) rms ⎤ 20 log10 ⎢ 3 ⎥ ⎢⎣ (vref ) rms ⎥⎦ 20 log10 (vn3 ) rms − 1.5237 CM = = F 1.85
SC-FDMA has around 2dB CM gain against OFDMA! “3G evolution, HSPA and LTE for mobile broadband(2nd edition)”, ISBN: 978-0-12-374538-5, page.118, All rights reserved @ 2009
Backup - Zadoff-Chu sequence characteristics • •
Zadoff-Chu sequences Property of ZC sequences:
⎧ − j πun ( n +1) 63 ⎪ e d u ( n) = ⎨ πu ( n +1)( n + 2) ⎪e − j 63 ⎩
n = 0,1,...,30 n = 31,32,...,61
– Constant amplitude, even after Nzc-point DFT. – Ideal cyclic auto-correlation – Constant cross-correlation[=sqrt(1/Nzc)], assuming Nzc is a prime number
“Polyphase codes with good periodic correlation properties”, J.D.C.Chu, IEEE trans on Informaiton theory, ,vol.18, pp.531-532, July 1972 “Phase shift pulse codes with good periodic correlation properties”, R.Frank,S.Zadoff and R.Heimiller, IEEE Trans on Information Theory, Vol 8, pp 381-382, Oct 1962. All rights reserved @ 2009
Backup – mobility: intra-MME handover UE
Source eNodeB
Target eNodeB
EPC
Measurement reporting Handover decision
Handover request Admission control
Handover request Ack RRC Connection Reconfiguration Detach from old cell
Deliver packets to target eNodeB
Data forwarding buffer packets From source eNodeB
RRC Connection Reconfiguration complete Path switch procedure UE context release Flush buffer Release resource
All rights reserved @ 2009
