LTE Basics - II

1Marius Pesavento, Willem Mulder, Femto Forum Plenary, June 2010, Reading, UK © mimoOn
LTE Tutorial part 2
Advanced topics in LTE
Marius Pesavento - marius.pesavento@mimoOn.de
Willem Mulder - willem.mulder@mimoOn.de

Outline
Advanced topics in LTE
 The LTE MIMO modes
 Codebook-based precoding
 Closed loop operation
 CQI reporting modes
 Using antenna port 5 (SDMA) techniques
 Simulation results
 Outlook LTE Advanced

MIMO Channel
MIMO
detector

MIMO Precoding
optimum „Eigen“ precoding
requires perfect channel
knowledge (CSI)
at the Transmitter

Parallel AWGN channels
Equivalent SISO channels

Tx Beam 1
Tx Beam 2
Rx Beam 1
Rx Beam 2Transmit
Beamformer(s)
Receive
Beamformer(s)
Transmit/Receive beamforming
interpretation

Codebook based
Spatial Multiplexing (SM)
 Precoding matrix is selected from codebook
 Reduced signaling at cost of quantization error (lose
rate optimality)
 Equivalent MIMO channels no longer parallel
(decoupled), reduction in rate
 Receiver matrix can be designed arbitrarily.
In practice interference among the streams not
completely removed:
 receive SINR for the k-th stream

MIMO Tx processing blocks in LTE
(spatial MUX)
 maximum 4 spatial streams (layers)
 maximum 2 TBs (codewords), each with corresponding MCS.
 2Tx: Code-book with 2 precoding matrices (closed-loop)
 is selected from set of 16 precoding matrices.
 code contains matrices of type: (and column permutated versions)
Turbo
encoder
Rate Match
TB 1
TB 2
CR 1
CR 2
modulator
modulator
MS 1
MS 2
2-MUX
or
1-MUX
2-MUX
or
1-MUX
layer mapping
layer 0
layer 1
layer 2
layer 3
IFFT
frame mapper
frequency first, then
OFDM symbol index
Tx 0
Tx 1
Tx 2
Tx 3
IFFT
frame mapper
OFDM symbol index
IFFT
frame mapper
OFDM symbol index
IFFT
frame mapper
OFDM symbol index
Precoding
; ; ;
Turbo
encoder
Rate Match

MIMO schemes
 Transmit diversity
 increase the reliability of the link,
migrate fading
 diversity order / diversity gain: number
of inpendent replica (fades) of the signal
 Spatial multiplexing
 increase spectral efficiency
 multiplexing gain: number of spatial
streams transmitted on a time-
frequency resource
 upper-bounded by min(Mt,Mr)
 requires rich multipath environment ⇒
full channel rank
 Beamforming (rank 1)
 Tx and Rx beamforming
 array gain through coherent combining
increases signal-to-noise-and-
interference-ratio (SINR)
 requires correlated antennas (e.g. in
Line-Of-Sight transmission)
fading
multipath
fading constructive or destuctive
superposition
LOS
⇒
rank 1
Single stream
only!

DL-MIMO modes in LTE
 Single antenna port (no MIMO)
 Transmit Diversity (TD), space-frequency Alamouti
code
 Open-loop Spatial Multiplexing (SM)
 Closed-loop SM
 Multi-User (MU) MIMO
 Rank 1 closed-loop SM (compressed control
signaling)
 Antenna port 5 beamforming, UE specific reference
signals

Cyclic (Large) Delay Diversity
(CDD)
x0
small delay
spread
frequency
“flat”
delay spread frequency
selective
τ0
τ3
τ1
τ2
τ3
„artificial“
multipath
x0
τ1
τ2

Open-loop SM
 Inferference „randomization“.
 In single layer transmission (TRI = 1) TD mode (Alamouti) is
used.
 is matrix formed from permutation of vectors:
 column permutation changing every k subcarriers in a pre-defined manner.
TB 1
TB 2
CR 1
CR 2
modulator
modulator
MS 1
MS 2
2-MUX
or
1-MUX
2-MUX
or
1-MUX
layer
mapping
L0
L1
L2
L3
IFFT
frame mapper
OFDM symbol index
Tx0
Tx1
Tx2
Tx3
IFFT
frame mapper
OFDM symbol index
IFFT
frame mapper
OFDM symbol index
IFFT
frame mapper
OFDM symbol index
Turbo
encoder
Turbo
encoder
Pre-
coding
DFT
Matrix
Cyclic
Delay
Matrix

Spatial Diversity
Space-Time-Coding: Alamouti
Symbol
modulator
encoded bit
stream
…,b3, b2, b1, b0,...
… s1, s0,...
… s*0, -s*1,...
… y1, y0,...
h0
h1
… s1, s0,...
space-time
encoder
space-time
decoder
y*1
y0
equivalent „MIMO“ channel
„MIMO“ equalizer/detector
decoder
s1
s0
No CSI at
the transmitter
required!!!

Space-Frequency Transmit Diversity
“Alamouti-zation”
 Single CW transmission, i.e single MCS.
 Simple receiver structure, no matrix inversion required
XS0S10
S2S3
XS50
port #0
0
00
00 0X0X
00
port #1
subcarrier index
port #2
port #3
IFFT
IFFT
IFFT
IFFT
S4
0
0000
00
000
S6S7
0
S4
*
-S3
*S2
*
-S1
*S0
* 0000
-S7
*S6
*
-S5
*
00
0
„zeros“ as reference
signal place holder
0
„zeros“ from
orthogonal SF code
X reference signal
S1
data symbol
equivalent channel
received
vector
equalized
symbol rather feed un-scaled
„equalized symbol“ and
scaling factor to soft
demodulator than perform
division at this point

Diversity order
(Symbol) error rate:
Diversity gain:
Coding gain:

Multiplexing – Diversity tradeoff
LTE spatial diversity techniques achieve:
 diversity order (Mt£Mr):
 full diversity for 2Tx
 half diversity for 4Tx
 rate (min(Mt , Mr)):
 full rate only for single antenna receiver
 half rate for 2Tx and 2Rx
 ¼ rate for 4Tx and 4Rx
 In LTE orthogonal space frequency block codes (OSTBC) are
used that allow simple receiver structures
⇒Symbol by symbol detection rather than vector detection.

Cell-Edge Beamforming
 to improve coverage for cell-edge user.
 to reduce inter-and intra-cell interference.
 rank-1 assumption (LOS).
reduced signaling overhead
 eNB aquires statistical information, e.g.
DoDs of co-channel users at cell-edge
 eNB computes optimum beamformer weights
for each user and applies them in the DL
transmission, no codebook and subband
restriction.
 multiple users are served on overlapping
resources (MU-MIMO)
 beamformer weights are explicitly signaled
using user specific RS.
 UE “sees” equivalent SI channel.
 dedicated RS of all users in a cell are
transmitted on the same RE (interference),
UE correlates received signal with dedicated
(RNTI-based) pseudo random sequence.
 LTE-feature that is expected to not be
supported at initial network rollout.
LOS
LOS
LOS
LOS
cell-specific
frequency shift

Femtocell basestation
FU1
FU2
MU1
MU2
MU3
Macro
basestation
Antenna port 5 downlink beamforming
Space Division Multiple Access (SDMA)

Beamformer design and signaling
 Beamformers can be implicitly signaled to the users of
the cell using antenna port 5 reference signals.
Problem: Uncertainties in the DL channels
 UL-DL reciprocity (e.g. in TDD)
 channel feedback (requires cooperation of base
stations)
 Paramterer estimation, Line-Of-Sight
Robust designs wrt. channel mismatch can be used.
Subject to
QoS constraint for
femtocell user
Maximum interference
constrainst for macrocell user

CQI report and 4-bit CQI table
CQI
index
modulation code
rate x
1024
efficiency
0 out of range
1 QPSK 78 0.1523
2 QPSK 120 0.2344
3 QPSK 193 0.3770
4 QPSK 308 0.6016
5 QPSK 449 0.8770
6 QPSK 602 1.1758
7 16QAM 378 1.4766
8 16QAM 490 1.9141
9 16QAM 616 2.4063
10 64QAM 466 2.7305
11 64QAM 567 3.3223
12 64QAM 666 3.9023
13 64QAM 772 4.5234
14 64QAM 873 5.1152
15 64QAM 948 5.5547
subband CQI index =differential CQI + wideband CQI index
Differential CQI value Offset level
0 ≤1
1 2
2 3
3 ≥4
Spatial differential CQI
value
Offset level
0 0
1 1
2 2
3 ≥3
4 ≤-4
5 -3
6 -2
7 -1
3-bit subband/wideband
spatial differential CQI
2-bit subband differential CQI

Adaptive coding and modulation
in DL grant
Tdoc R1-07CQI_NNSN01
MCS
Index
Modul
ation
Order
TBS
Index
TBS for
1 RB
1layer
…
TBS for
110 RBs
1layer
0 2 0 16 … 3112
1 2 1 24 … 4008
2 2 2 32 … 4968
3 2 3 40 … 6456
4 2 4 56 … 7992
5 2 5 72 … 9528
6 2 6 328 … 11448
7 2 7 104 … 13536
8 2 8 120 … 15264
9 2 9 136 … 17568
10 4 9 136 … 17568
11 4 10 144 … 19080
12 4 11 176 … 22152
13 4 12 208 … 25456
14 4 13 224 … 28336
15 4 14 256 … 31704
16 4 15 280 … 34008
17 6 15 280 … 34008
18 6 16 328 … 35160
19 6 17 336 … 39232
20 6 18 376 … 43816
21 6 19 408 … 46888
22 6 20 440 … 51024
23 6 21 488 … 55056
24 6 22 520 … 59256
25 6 23 552 … 63776
26 6 24 584 … 66592
27 6 25 616 … 71112
28 6 26 712 … 75376
29 2
reserve
d
reserved … reserved30 4
31 6
for code rate approx 1
LTE target!!!

SINR to CQI conversion
for MMSE detector
4 x 4 MIMO, full-rank
for subcarrier k , precoding index i,
and precoding matrix Pi
MMSE estimate of SINR corresponding to layer p and PMI i
subcarrier
layer
(column in PM)
PMI

Average (effective) SINR
Estimate for CW 0 (corresponds to averaging over layer 1 and 2)
Weighting function for average SINR computation (based on rate)
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-2)
SINI(N-1)
SINR(N)
SINR(N-3)
General: averaging over layer, subcarriers,…

MIMO transmission modes
Transmission mode Transmission scheme of PDSCH
1 Single-antenna port, port 0
2 Transmit diversity
3 Transmit diversity if the associated
rank indicator is 1, otherwise large
delay CDD
4 Closed-loop spatial multiplexing
5 Multi-user MIMO
6 Closed-loop spatial multiplexing with a
single transmission layer
7 If the number of PBCH antenna ports
is one, Single-antenna port, port 0;
otherwise Transmit diversity
open-loop,
no-PMI
feedback
closed-loop,
with PMI
feedback

CQI reporting modes
No PMI Single PMI Multiple PMI
Wideband
(wideband CQI) Mode 1-2
UE Selected
(subband CQI) Mode 2-0 Mode 2-2
Higher Layer-
configured
(suband CQI) Mode 3-0 Mode 3-1
Transmission mode 4 : Modes 1-2, 2-2, 3-1
Transmission mode 5 : Mode 3-1
Transmission mode 6 : Modes 1-2, 2-2, 3-1
Transmission mode 7 : Modes 2-0, 3-0
PMI Feedback Type
No PMI Single PMI
Wideband Mode 1-0 Mode 1-1
(wideband CQI)
UE Selected Mode 2-0 Mode 2-1
(subband CQI)
PUSCH CQI: aperiodic
Feedback Type
PUCCH CQI: periodic
Feedback Type

Higher layer configured reporting
modes: aperidodic reporting
#bits 2N 4
∆CQI(2)
∆CQI(1)
∆CQI(3)
∆CQI(4)
∆CQI(N-3)
∆CQI(N-2)
∆CQI(N-1)
∆CQI(N)
CQI(wideband)
Frequency(subbands)
∆CQI(2)
∆CQI(1)
∆CQI(3)
∆CQI(4)
∆CQI(N-3)
∆CQI(N-2)
∆CQI(N-1)
∆CQI(N)
CQI(wideband)
Frequency(subbands)
mode 3-0
single antenna, port 5
TD and open-loop SM
mode 3-1
closed-loop SM
PMI(wideband)
∆CQI(2)
∆CQI(1)
∆CQI(3)
∆CQI(4)
∆CQI(N-3)
∆CQI(N-2)
∆CQI(N-1)
∆CQI(N)
CQI(wideband)
CW 0 CW 1
#bits 2N 4 2N 4 2|1|4
+Rank Indicator (RI)
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-2)
SINI(N-1)
SINR(N)
SINR
SINR(N-3)

UE selected reporting: mode 2-0 for
single antenna, port 5, TD and open
loop SM: aperiodic reporting
report # of bits
bitmap of
prefered M
subband
locations
wideband
CQI
4
subband
∆CQI
2
L
CQI(N-1)
CQI(N)
CQI(N-2)
CQI(N-3)
CQI(4)
CQI(3)
CQI(2)
CQI(1)
Frequency (subbands)
SINR(N-1)
SINR(N)
SINR(N-2)
SINR(N-3)
SINR(4)
SINR(3)
SINI(2)
SINR(1)
measurements
CQI(wideband)
∆average CQI(selected subbands)
SINR

closed-loop SM: aperidodic reporting
report location of
preferred
subbands
subband
CQI
subband
CQI
#bits L 4|2|8 4 2 4 2measurements per PMI required !!!
select best PMI and subands
(in terms of combined data rate)
SINR SINR
CQI(2)
CQI(1)
CQI(3)
CQI(4)
CQI(N-3)
CQI(N-2)
CQI(N-1)
CQI(N)
Frequency(subbands)
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-3)
SINR(N-2)
SINI(N-1)
SINR(N)
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-2)
SINI(N-1)
SINR(N)
CW 0 CW 1
SINI(N-3)
CQI(2)
CQI(1)
CQI(3)
CQI(4)
CQI(N-3)
CQI(N-2)
CQI(N-1)
CQI(N)
∆averageCQI(selectedsubbands)
∆averageCQI(selectedsubbands)
CW 0 CW 1
CQI(2)
CQI(1)
CQI(3)
CQI(4)
CQI(N-3)
CQI(N-2)
CQI(N-1)
CQI(N)
PMI
CQI(wideband)
CQI(wideband)

Wideband CQI reporting
modes: aperidodic reporting
#bits 4 4 2N| N|4N
PMI(2)
PMI(1)
PMI(3)
PMI(4)
PMI(N-3)
PMI(N-2)
PMI(N-1)
PMI(N)
CQI(wideband)
Frequency(subbands)
mode 1-2
closed-loop SM
CQI(wideband)
CW0 CW1
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-2)
SINI(N-1)
SINR(N)
SINR
SINR(N-3)

Higher layer configured reporting
modes for perdiodic feedback:
perdiodic reporting
#bits 4 2|1|4
CQI(wideband)
Frequency(subbands)
CQI(wideband)
Frequency(subbands)
mode 1-1 (rank 1)
closed-loop SM, MU-MIMO
mode 1-1 (rank 2)
closed-loop SM, MU-MIMO
PMI(wideband)
differentialspatialCQI(wideband)
CW 0 CW 1
#bits 4 3 2|1|4
PMI(wideband)
CW 0
CQI(wideband)
Frequency(subbands)
mode 1-0
single antenna port,
open-loop SM, TD
#bits 4
CW 0
+Rank
Indicator
(RI)

single antenna, port 5, TD and open
loop SM: periodic reporting
report # of bits
bitmap of prefered M
subband locations for
current bandwidth
part
L
wideband
CQI
4
subband
CQI
4
CQI(N-1)
CQI(N)
CQI(N-2)
CQI(N-3)
CQI(4)
CQI(3)
CQI(2)
CQI(1)
Frequency (subbands)
SINR(N-1)
SINR(N)
SINR(N-2)
SINR(N-3)
SINR(4)
SINR(3)
SINI(2)
SINR(1)
measurements
CQI(wideband)
CQI(selected subbands)
SINR
N1 N2 N
J
  JkNL //log DL
RB2=

UE selected reporting:
mode 2-1for closed-loop SM:
periodic reporting
report location of
preferred
subbands in
Bandwidth part j
sub-
band
CQI
sub-
band
CQI
#bits L 2|1|4 4 4 3 3
  JkNL //log DL
RB2=
CQI(2)
CQI(1)
CQI(3)
CQI(4)
CQI(N-3)
CQI(N-2)
CQI(N-1)
CQI(N)
Frequency(subbands)
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-3)
SINR(N-2)
SINI(N-1)
SINR(N)
SINR(2)
SINR(1)
SINR(3)
SINR(4)
SINR(N-2)
SINI(N-1)
SINR(N)
CW 0 CW 1
SINI(N-3)
CQI(2)
CQI(1)
CQI(3)
CQI(4)
CQI(N-3)
CQI(N-2)
CQI(N-1)
CQI(N)
CQI(selectedsubbands)
differentialCQI(selectedsubbands)
CW 0 CW 1
measurements per PMI !!!
CQI(2)
CQI(1)
CQI(3)
CQI(4)
CQI(N-3)
CQI(N-2)
CQI(N-1)
CQI(N)
PMI
select best PMI and subands
(in terms of combined data rate)
differentialspatialCQI(wideband)
CQI(wideband)
SINR SINR
N1
NJ
k = subband size
J = number of bandwidth parts (see also next slide) +Rank Indicator (RI)

Report timing configuration for
mode 2-1: periodic reporting
report location of preferred
subbands in
Bandwidth part j
PMI WB
CQI
CW1
sub-
band
CQI
WB
CQI
CW2
sub-
band
CQI
#bits L 2|1|4 4 4 3 3
RI
SB
CQI
j = 0
WB
CQI
SB
CQI
j= 1
SB
CQI
j = 2
WB
CQI
J = 3: SB bitmap-report is split into J bandwidth parts that are reported in consecutive intervals
SB
CQI
j = 0
SB
CQI
j= 1
SB
CQI
j = 2
SB
CQI
j = 0
SB
CQI
j= 1
SB
CQI
j = 2
SB
CQI
j = 0
SB
CQI
j= 1
SB
CQI
j = 2
RI
SB
CQI
j = 0
WB
CQI
SB
CQI
j= 1
SB
CQI
j = 2
WB
CQI
SB
CQI
j = 0
SB
CQI
j= 1
SB
CQI
j = 2
K = 2: SB reporting periodicity (J consecutive SB reports) with respect to WB reporting periodicity
MRI = 2: RI reporting periodicity with respect to WB reporting periodicity
NP = 2: Reporting Periodicity in TTIs
…continue…

Multi-User DL MIMO
 MAC scheduler decides on users
„pairing“
 Maximum 2 users on same resource
due to 2TB restriction
∆CQI(2)
∆CQI(1)
∆CQI(3)
∆CQI(4)
∆CQI(N-3)
∆CQI(N-2)
∆CQI(N-1)
∆CQI(N)
CQI(wideband)
Frequency(subbands)
mode 3-1
closed-loop SM
PMI(wideband)
∆CQI(2)
∆CQI(1)
∆CQI(3)
∆CQI(4)
∆CQI(N-3)
∆CQI(N-2)
∆CQI(N-1)
∆CQI(N)
CQI(wideband)
CW 0 CW 1
#bits 2N 4 2N 4 2|1|4

channel „experienced“ at user 1
channel „experienced“ at user 2
No joint processing at Rx, no cooperation!
4
2
4
2
desired interfering
desiredinterfering
Pre-coding matrix selection (UE1) maximize
minimize
2
2
4
4
4
2
UE 1
UE 2
for
UE 1
for
UE 2
UE 1 UE 2
Multi-User DL MIMO

UL-MIMO
 UL-MIMO is not explicitly supported in LTE,
 single antenna port in UL, no single-
user(SU) SM in UL
 demodulation UL reference signals (DRS)
of different users overlap/interfere
 DRS of different users are separated in TD
according to „cyclic-shift“ of identical sequence
(theoretical max. of 8 user, depending on delay-
spread)
 depends on eNB scheduler‘s flexibility and PHY
support
delay spread max 12 cyclic shift
h1[t] h2[t] h3[t] h12[t]
r*PUSCH(k)
DRS demapperFFT x
h1[t]+ h2[t]+…+ h12[t]
„cyclic-shift“ zero
IFFT

Source: 3GPP self evaluation results, 3GPP TSG-RAN chair, oct 2009
Spectral efficiency
3GPP self-evaluation results

0
2000000
4000000
6000000
8000000
10000000
12000000
14000000
-10 -5 0 5 10 15 20
SNR in dB
1
2
mimoOn
3
4
16QAM 1/2 EVA5 50 RB 2x2 SFBC
LTE DL simulation results reported
by the Top4 leading LTE vendors
Throughput[Bits]

LTE-Advanced: Concepts
 Improved Spectrum Flexibility
 Bandwidth up to 100MHz
 Spectrum and carrier aggregation
 MIMO
 support SM in UL
 Higher order MIMO in DL
 Coordinated Multipoint Transmission
from interference randomization to
interference coordination
 Multihop Relays
 L1 repeaters to improve coverage
 L3 relays for self-backhauling eNB
100 MHz
20 MHz 20 MHz
eNB
relay
node
UE

LTE-Advanced
advanced MIMO receiver structure
SD-SIC for OFDMA
2 codewords used,
each S/P-mapped onto 2 Tx antennas
1,kY
2,kY
LMMSE / Soft IC
LLR calc.
Rate
matching
LLR calc.
Decoder
Decoder
Signal
construction
Signal
construction
Channel estimate
Channel estimate
3,kY
4,kY
S/P
S/P
Rate
matching
Figure: NSN: R1-083732 / 2008-09-23

End of Part 2
Thank you!!!

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