Post on 18-Mar-2020
Dr. Stefan BrückQualcomm Corporate R&D Center Germany
3G/4G Mobile Communications Systems
Chapter VI: Physical Layer of LTE
2
Chapter VI: Physical Layer of LTE
Slide 2
Physical Layer of LTE
� OFDM and SC-FDMA Basics
� DL/UL Resource Grid
� Downlink Operation
� Downlink Physical Channels
� Uplink Operation
� Uplink Physical Channels
3
� Uplink Physical Channels
� UE Categories
Slide 3
LTE Key Radio Features (Release 8)� Multiple access scheme
� DL: OFDMA with CP
� UL: Single Carrier FDMA (SC-FDMA) with CP
� Adaptive modulation and coding� DL modulations: QPSK, 16QAM, and 64QAM
� UL modulations: QPSK, 16QAM, and 64QAM (optional for UE)
� Rel. 6 Turbo code: Coding rate of 1/3, two 8-state constituent encoders and a contention-free internal interleaver
4
and a contention-free internal interleaver
� ARQ within RLC sublayer and Hybrid ARQ within MAC sublayer
� Advanced MIMO spatial multiplexing techniques� (2 or 4)x(2 or 4) downlink and 1x(2 or 4) uplink supported
� Multi-layer transmission with up to four streams in DL
� Multi-user MIMO also supported in UL and DL
� Implicit support for interference coordination
� Support for both FDD and TDD
Slide 4
LTE Frequency Bands
� LTE will support all band classes currently specified for UMTS as well as additional bands
5 Slide 5
LTE Duplexing Modes
� LTE supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) to provide flexible operation in a variety of spectrum allocations around the world.
� Unlike UMTS TDD there is a high commonality between LTE TDD & LTE FDD
6
� Slot length (0.5 ms) and subframe length (1 ms) is the same as LTE FDD with the same numerology (OFDM symbol times, CP length, FFT sizes, sample rates, etc.)
� UL/ DL switching pointsdesigned to allow co-existence with UMTS-TDD(TD-CDMA, TD-SCDMA)
Slide 6
LTE Half-Duplex FDD
� In addition to FDD & TDD, LTE supports also Half-Duplex FDD (HD-FDD)
� HD-FDD is like FDD, only the UE cannot transmit and receive at the same time
7
� Note, that the eNodeB can still transmit and receive at the same time to different UEs; half-duplex is enforced by the eNodeB scheduler
� Reasons for HD-FDD� Handsets are cheaper, as no duplexer is required
� More commonality between TDD and HD-FDD than compared to full duplex FDD
� Certain FDD spectrum allocations have small duplex space; HD-FDD leads to duplex desense in UE
Slide 7
OFDM Basics – Overlapping Orthogonal� OFDM: Orthogonal Frequency Division Multiplexing
� OFDMA: Orthogonal Frequency Division Multiple-Access
� FDM/FDMA is nothing new: carriers are separated sufficiently in frequency so that there is minimal overlap to prevent cross-talk.
conventional FDM
8
� OFDM: still FDM but carriers can actually be orthogonal (no cross-talk) while actually overlapping, if specially designed → saved bandwidth !
frequency
OFDM
frequency
saved bandwidth
Slide 8
OFDM Basics – Waveforms
� Frequency domain: overlapping sinc functions� Referred to as subcarriers
� Typically quite narrow, e.g. 15 kHz
� Time domain: simple gated sinusoid functions
4 5 6 7 8 9
x 105
-0.2
0
0.2
0.4
0.6
0.8
1
freq
∆∆∆∆f = 1/T
T = symbol time
9
� Time domain: simple gated sinusoid functions� For orthogonality: each symbol has an
integer number of cycles over the symbol time
� fundamental frequency f0 = 1/T
� Other sinusoids with fk = k • f00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
time
T = symbol time
Slide 9
OFDM Basics – The Full OFDM Transceiver
10 Slide 10
OFDM Basics – Cyclic Prefix� ISI (between OFDM symbols) eliminated almost completely by inserting a
guard time
� Within an OFDM symbol, the data symbols modulated onto the subcarriers are only orthogonal if there is an integer number of sinusoidal cycles within the receiver window
OFDM Symbol OFDM Symbol OFDM Symbol
TG TG
11
� Filling the guard time with a cyclic prefix (CP) ensures orthogonality of subcarriers even in the presence of multipath � elimination of same cell interference
CPUseful OFDM symbol time
OFDM symbol
CPUseful OFDM symbol time
OFDM symbol
CPUseful OFDM symbol time
OFDM symbol
Slide 11
Comparison with CDMA – Principle� OFDM: Particular modulation symbol is carried over a relatively long symbol
time and narrow bandwidth� LTE: 66.6 µsec symbol time and 15 kHz bandwidth
� For higher data rates send more symbols by using more sub-carriers →increases bandwidth occupancy
� CDMA: Particular modulation symbol is carried over a relatively short symbol time and a wide bandwidth� UMTS HSPA: 4.17 µsec symbol time and 3.84 Mhz bandwidth
� To get higher data rates use more spreading codes
12
� To get higher data rates use more spreading codes
frequency
time
symbol 0
symbol 1
symbol 2
symbol 3
frequency
time
symbol 0
symbol 1
symbol 2
symbol 3
CDMA OFDM
Slide 12
Comparison with CDMA – Time Domain� Short symbol times in CDMA lead to ISI in the presence of multipath
� Long symbol times in OFDM together with CP prevent ISI from multipath
1 2 3 4
1 2 3 4
1 2 3 4
CDMA symbols
Multipath reflections from one symbol significantly overlap subsequent symbols → ISI
13
� Long symbol times in OFDM together with CP prevent ISI from multipath
1 2CPCP
1 2CPCP
1 2CPCP
Little to no overlap in symbols from multipath
Slide 13
Comparison with CDMA – Frequency Domain
14
� In CDMA each symbol is spread over a large bandwidth, hence it will experience both good and bad parts of the channel response in frequency domain
� In OFDM each symbol is carried by a subcarrier over a narrow part of the band → can avoid send symbols where channel frequency response is poor based on frequency selective channel knowledge → frequency selective scheduling gain in OFDM systems
Slide 14
OFDM Basics – Choosing the Symbol Time for LTE� Two competing factors in determining the right OFDM symbol time:
� CP length should be longer than worst case multipath delay spread, and the OFDM symbol time should be much larger than CP length to avoid significant overhead from the CP
� On the other hand, the OFDM symbol time should be much smaller than the shortest expected coherence time of the channel to avoid channel variability within the symbol time
� LTE is designed to operate in delay spreads up to ~5µs and for speeds up to
15
� LTE is designed to operate in delay spreads up to ~5µs and for speeds up to 350km/h (1.2ms coherence time @ 2.6GHz). As such, the following was decided:� CP length = 4.7 µs
� OFDM symbol time = 66.6 µs(= 1/20 the worst case coherence time)
∆f = 15 kHz
CP
~4.7 µs ~66.7 µs
Slide 15
Scalable OFDM for Different Operating Bandwidths
� With Scalable OFDM, the subcarrier spacing stays fixed at 15 kHz regardless of the operating bandwidth � 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz,
20 MHz
� Symbol time is fixed to 66.6 µs
common channels
10 MHz bandwidth
20 MHz bandwidth
5 MHz bandwidth
1.4 MHz bandwidth
3 MHz bandwidth
16
� Symbol time is fixed to 66.6 µs
� The total number of subcarriers is varied in order to operate in different bandwidths� This is done by specifying different FFT
sizes (i.e. 512 point FFT for 5 MHz, 2048 point FFT for 20 MHz)
� Influence of delay spread, Doppler due to user mobility, timing accuracy, etc. remain the same as the system bandwidth is changed � robust design
centre frequency
Slide 16
LTE Downlink Frame Format
slot = 0.5ms slot = 0.5ms
subframe = 1.0ms
Radio frame = 10ms
17
� Subframe length is 1ms
� Consists of two 0.5ms slots
� 7 OFDM symbols per 0.5ms slot � 14 OFDM symbols per 1ms subframe
� In UL center SC-FDMA symbol used for the data demodulation reference signal (DM-RS)
OFDM symbol
Slide 17
LTE Downlink Frame Structure
Spectrum allocation 1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz
Slot duration 0.5 ms
Sub-frame duration 1.0 ms ( = 2 slots)
Sub-carrier spacing15 kHz
(7.5 kHz for MBMS)
Sampling frequency
1.92 MHz(1/2 × 3.84) 3.84 MHz 7.68 MHz
(2 × 3.84)15.36 MHz(4 × 3.84)
23.04 MHz(6 × 3.84)
30.72 MHz(8 × 3.84)
Sampling rates are
multiples of UMTS chip
rate, to ease
implementation of
dual mode UMTS/LTE
terminals
Subframe length relevant to the latency requirement
18
frequency (1/2 × 3.84) 3.84 MHz (2 × 3.84) (4 × 3.84) (6 × 3.84) (8 × 3.84)
FFT size 128 256 512 1024 1536 2048
Number of sub-carriers 75 150 300 600 900 1200
OFDM symbols per slot 7 (short CP), 6 (long CP)
CP length
Short4.69 µµµµs x 6
5.21 µµµµs x 1
Long 16.67 µµµµs
FFT size scales to
support larger
bandwidth �
Scalable OFDM
Slide 18
Downlink/Uplink Resource Grid
� Resource Element (RE)� Fundamental element in time/frequency
grid (1 subcarrier x 1 symbol)
� Resource Block (PRB)� Minimum resource set for DL/UL data
channel assignment (12 subcarriers x 1 slot)
� Physical Resource Block (180 kHz x 1
19
� Physical Resource Block (180 kHz x 1 ms)
� Virtual Resource Block (same size as PRB)
� Resource Block Group (RBG)� Group of Resource Blocks
Slide 19
DL Logical, Transport and Physical ChannelsLTE makes heavy use of shared
channels � common control,
paging, and part of broadcast
information carried on PDSCHPCCH: paging control channel
BCCH: broadcast control channel
CCCH: common control channel
DCCH: dedicated control channel
DTCH: dedicated traffic channel
BCCHPCCH CCCH DCCH DTCH MCCH MTCHDownlinkLogical channels
20
PCH: paging channel
BCH: broadcast channel
DL-SCH: DL shared channel
BCHPCH DL-SCH MCH
DownlinkTransport channels
DownlinkPhysical Channels
PDSCH PDCCHPBCH PHICHPCFICHSCHDL-RS PMCH
Slide 20
Carries DL traffic
Carries basic system broadcast information
Allows mobile to get timing and frequency sync with the cell
Physical Channels to Support LTE Downlink
21
eNode-BDL resource allocation
HARQ feedback for DL
CQI reporting
MIMO reporting
Time span of PDCCH
Slide 21
Downlink Operation
� UE reports CQI (channel quality indicator), PMI (precoding matrix indicator), and RI (rank indicator) in PUCCH or PUSCH
� Scheduler at eNB dynamically allocates DL resources to UE
� eNB sends user data in PDSCH
22
� UE reads PCFICH every subframe and determines the number of OFDM symbols occupied by PDCCH
� UE reads PDCCH to determine the assigned DL resources (PRB and MCS) for a specific Tx mode
� UE attempts to decode the received packet and sends ACK/NACK using PUCCH or PUSCH
Slide 22
Downlink Physical Channels in LTE
23 Slide 23
Physical Channel Resource Allocation
� P-SCH and S-SCH Resource Allocation� Subframe 0 and 5 of every 10th frames
� Middle of bandwidth (6 PRBs)
� PBCH Resource Allocation� Subframe 0 every 10th frames
� Middle of bandwidth (6 PRBs)
� PDCCH, PCFICH, PHICH allocated to the
24
� PDCCH, PCFICH, PHICH allocated to the (at most) three OFDM symbols in each subframe
� The remaining time – frequency resources can be allocated for data transmission in the PDSCH
Slide 24
Downlink Reference Signals
� Three Types of DL Reference Signals (DL-RS)
� Cell-specific Reference Signals� Associated with PDSCH multiple antenna port transmission
� Used by UE for coherent demodulation and channel estimation
� UE-specific Reference Signals� UE-specific RS are supported for single antenna port transmission (Rel. 8)
25
� UE-specific RS are supported for single antenna port transmission (Rel. 8)
� In Rel. 10 UE-specific RS are also introduced for multiple antenna port transmission
� MBSFN Reference Signals � Associated with MBSFN transmission
Slide 25
DL cell-specific RS for multiple Tx Antennas
26
� RS are allocated on a per antenna port basis
� Antenna 0 and 1� Transmitted on 2 OFDM symbols every slot
� 6 subcarrier spacing and 2x staggering (45 kHz frequency sampling)
� Antenna 2 and 3� Transmitted on 1 OFDM symbols every slot
� 6 subcarrier spacing with 2x staggering across slots
� Same frequency spacing for normal and extended CP Slide 26
Spatial Multiplexing� Code Word
� Transport block format : CRC encoded data
� Transmission layer� Sub stream resulting from a mapping of modulated code word symbols
� Number of layers ≤ number of antenna ports
� Code book� Quantized set of spatial combination vectors for precoding of symbols layer for transmission on
antenna ports
� Rank of MIMO channel� Number of independents TX/RX channels offered by MIMO for spatial multiplexing
27
� Rank ≤ min(NTx, NRx)
� UE indicates channel quality (CQI), pre-coding matrix (PMI) and rank (RI)
M Tx N Rx
V
RIHVUH Λ=
UHSelect
# code
words
Modulation
+ coding
PMICQI
Modulation
+ coding
Demod +
decode
Demod +
decode
precoding
Layer
mappingH
Slide 27
PDSCH Transmission Scheme
� Codewords (maximum of 2)� One codeword for rank 1 transmission
� Two codewords for rank 2/3/4 transmission
28
� Layer mapping� Number of layers depend on the number of Tx antennas and the channel rank
� Fixed mapping schemes of codewords to layers
� Tx antennas (maximum of 4)� Potentially up to 4 layers
� Precoding� Used to support spatial multiplexing
� Code book based precoding
� Seven different transmission modes are supported in Rel. 8Slide 28
PDSCH Transmission Modes (Rel. 8)
� Mode 1: Uses a single Tx antenna at eNB together with cell-specific RS
� Mode 2: Uses transmit diversity based on Alamouti scheme
� Mode 3: Open loop spatial multiplexing� No UE feedback
� Exploits cyclic delay diversity (CDD)
� Mode 4: closed loop spatial multiplexing� Up to 2 codewords and 4 layers
29
� Up to 2 codewords and 4 layers
� Rank (RI) and precoding (PMI) feedback
� Mode 5: Multi-user MIMO� Single codeword and single layer per UE
� UE reports PMI but no RI
� Mode 6: closed loop Rank = 1 precoding (restricted Mode 4)� No RI reports are needed
� Mode 7: same as Mode 1 with UE-specific RS
Slide 29
Multiple Antenna Techniques Supported in LTE
� SU-MIMO� Multiple data streams sent to the same user
(max. 2 codewords)
� Significant throughput gains for UEs in high SINR conditions
� MU-MIMO or Beamforming� Spatial Division Multiple Access (SDMA) � Different data streams sent to different users
using the same time-frequency resources
30
using the same time-frequency resources
� Improves throughput even in low SINR conditions (cell-edge)
� Works even for single antenna mobiles
� Transmit diversity (TxDiv)� Improves reliability on a single data stream
� Fall back scheme if channel conditions do not allow spatial multiplexing
� Useful to improve reliability on common control channels
Slide 30
Precoding in Transmission Mode 4
� For two Tx antennas 4 single layer precoding vectors and 2 dual layer precoding matrices are supported� See table below (TS 36.211)
� For four Tx antennas 16 precoding vectors/matrices are supported per layer � The construction is based on Householder transformation
� For details, see TS 36.211
31 Slide 31
PDSCH Transmission Mode Configuration
32 Slide 32
CQI/PMI/RI Reporting
� CQI/PMI/RI reporting is either on PUSCH or PUCCH, periodic or aperiodic
� Aperiodic CQI/PMI/RI reporting is defined by the following characteristics:� The report is scheduled by the eNB via the PDCCH
� Transmitted together with uplink data on PUSCH
� From the frequency span perspective these reports can be:� Frequency selective: UE Selected Subband CQI and Higher Layer Configured
Subband CQI
� Frequency non-selective: Wideband CQI reports
33
� When a CQI report is transmitted together with Uplink data on PUSCH, it is multiplexed with the transport block by L1 � The CQI report is not part of the uplink transport block
� Periodic CQI/PMI/RI reporting is defined by the following characteristics:� Periodic CQI reports are sent on PUCCH
� From the Frequency span perspective these reports can be:� Frequency selective: UE Selected Subband CQI
� Frequency non-selective: Wideband CQI reports
Slide 33
CQI Definition
34 Slide 34
Downlink Peak Rates
bandwidth
# of parallel streams supported
1 2 4
1.4 MHz 5.4 Mbps 10.4 Mbps 19.6 Mbps
3 MHz 13.5 Mbps 25.9 Mbps 50 Mbps
5 MHz 22.5 Mbps 43.2 Mbps 81.6 Mbps
35
5 MHz 22.5 Mbps 43.2 Mbps 81.6 Mbps
10 MHz 45 Mbps 86.4 Mbps 163.2 Mbps
15 MHz 67.5 Mbps 129.6 Mbps 244.8 Mbps
20 MHz 90 Mbps 172.8 Mbps 326.4 Mbps
Assumptions: 64QAM, code rate =1, 1OFDM symbol for L1/L2, ignores subframes with P-BCH, SCH
Slide 35
LTE Uplink Transmission Scheme (1/2)
� To facilitate efficient power amplifier design in the UE, 3GPP chose single carrier frequency domain multiple access (SC-FDMA) in favor of OFDMA for uplink multiple access.� SC-FDMA results in better PAPR
� Reduced PA back-off � improved coverage
� SC-FDMA is still an orthogonal
36
� SC-FDMA is still an orthogonal multiple access scheme� UEs are orthogonal in frequency
� Synchronous in the time domain through the use of timing advance(TA) signaling� Only need to be synchronous
within a fraction of the CP length
� 0.52 µs timing advance resolution
Node BUE C
UE B
UE A
UE A Transmit Timing
UE B Transmit Timing
UE C Transmit Timing
αααα
ββββ
γγγγ
Slide 36
LTE Uplink Transmission Scheme (2/2)� SC-FDMA implemented using an OFDMA front-end and a DFT pre-coder,
this is referred to as either DFT-pre-coded OFDMA or DFT-spread OFDMA (DFTS-OFDMA)� Advantage is that numerology (subcarrier spacing, symbol times, FFT
sizes, etc.) can be shared between uplink and downlink
� Can still allocate variable bandwidth in units of 12 sub-carriers
� Each modulation symbol sees a wider bandwidth
37
+1 -1 -1 +
1 -1 -1 -1 +1 +
1 +1 -1
DFT pre-coding
Slide 37
SC-FDMA Signal
� SC-FDMA uses DFT precoding of user data
38
� SC-FDMA uses DFT precoding of user data� Individual bits mapped across multiple frequencies
� DFT size (N) defines number of subcarriers allocated to user data
� Time domain signal more resembles a single carrier signal� Peak-to-average power ratio (PAPR) is reduced
Slide 38
Localized and Distributed SC-FDMA
� Localized Assignment� Uses consecutive subcarriers
� Simpler to implement
� Used in LTE
� Distributed Assignment� Distributes subcarriers across
frequency bands
� Increases frequency diversity
39
� Increases frequency diversity
� Not applied in LTE
Slide 39
Comparison of OFDMA and SC-FDMA
40 Slide 40
Figure taken from http://cp.literature.agilent.com/litweb/pdf/5989-7898EN.pdf
Downlink/Uplink Resource Grid
� Resource Element (RE)� Fundamental element in time/frequency
grid (1 subcarrier x 1 symbol)
� Resource Block (PRB)� Minimum resource set for DL/UL data
channel assignment (12 subcarriers x 1 slot)
� Physical Resource Block (180 kHz x 1
41
� Physical Resource Block (180 kHz x 1 ms)
� Virtual Resource Block (same size as PRB)
� Resource Block Group (RBG)� Group of Resource Blocks
Slide 41
UL Logical, Transport and Physical Channels
CCCH: common control channel
DCCH: dedicated control channel
DTCH: dedicated traffic channel
42
RACH: random access channel
UL-SCH: UL shared channel
PUSCH: physical UL shared channel
PUCCH: physical UL control channel
PRACH: physical random access channel
Slide 42
Physical Channels to Support LTE Uplink
Random access for initial
access and UL timing
alignment
Carries UL Traffic
UL scheduling request for
time synchronized IEs
43
UL scheduling grant
time synchronized IEs
HARQ feedback for UL
eNode-B
Slide 43
Uplink Operation
� If UE does not have UL-SCH resources, UE send SR (scheduling request) on PUCCH
� Scheduler at eNB allocates resources to UE in terms of UL grant on PDCCH� Assigned resources: PRB and MCS
� UE sends user data on PUSCH
44
� If eNB decodes the UL data successfully, it sends ACK on PHICH
Slide 44
Uplink Physical Channels in LTE
45 Slide 45
PUSCH
� PUSCH may carry� UL data
� ACK/NACK for DL data
� CQI/PMI/RI
� The allocation of PRB resources is continuous
� Frequency hopping is supported to
46
� Frequency hopping is supported to obtain frequency diversity� Intra- and inter-subframe hopping
are supported
� Demodulation-RS (DM-RS ) are embedded in the SC-FDMA symbols
Slide 46
PUCCH
� PUCCH may carry� ACK/NACK for DL data
� CQI/PMI/RI
� Scheduling request
� PUCCH and PUSCH are never transmitted simultaneously� Reason: Reduction of PAPR
47 Slide 47
� PUCCH uses one PRB in each of the two slots in a subframe
� Multiple UEs may be assigned the same PRB resource for PUCCH transmission � Assignment of different cyclic shifts
of scrambling sequence and orthogonal spreading sequences
� (DM-RS are embedded in the SC-FDMA symbols
MIMO Support is Different in Downlink and Uplink
� Downlink� Supports SU-MIMO, MU-MIMO, TxDiv
48
� Uplink� Initial release of LTE does only support MU-MIMO with a single transmit antenna
at the UE � Desire to avoid multiple power amplifiers at UE
Slide 48
Uplink Peak Rates
bandwidthHighest Modulation
16 QAM 64QAM
1.4 MHz 2.9 Mbps 4.3 Mbps
3 MHz 6.9 Mbps 10.4 Mbps
5 MHz 11.5 Mbps 17.3 Mbps
49
10 MHz 27.6 Mbps 41.5 Mbps
15 MHz 41.5 Mbps 62.2 Mbps
20 MHz 55.3 Mbps 82.9 Mbps
Assumptions: code rate =1, 2PRBs reserved for PUCCH (1 for 1.4MHz), no SRS, ignores subframes with PRACH,takes into account highest prime-factor restriction
Slide 49
LTE-Release 8 User Equipment Categories
Category 1 2 3 4 5
Peak rate Mbps
DL 10 50 100 150 300
UL 5 25 50 50 75
Capability for physical functionalities
RF bandwidth 20MHz
Modulation DL QPSK, 16QAM, 64QAM
UL QPSK, 16QAM QPSK,
50
UL QPSK, 16QAM QPSK,16QAM,64QAM
Multi-antenna
2 Rx diversity Assumed in performance requirements.
2x2 MIMO Not supported Mandatory
4x4 MIMO Not supported Mandatory
Slide 50