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8/11/2019 ALU LTE Workshop
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LTE
Uplink
Power cont ro l
Open-loop
power control:
To constrain the dynamic
range
between signals received from different UEs
Unlike
CDMA
there is no intra-cell interference -> exploit fading by means of link
adaptation and scheduling
Classical
PC :
all users achieve the same target
SINR
Interior
users transmit at
reduced
power spectral density
Fractional
PC
more flexible :
Trade-off between spectral
efficiency
and
cell
edge
rates
Target
SINR
increases
with
decreasing
path
loss
Others, e.g.
aperiodic fas t power control
Frac t iona l PC
Interference over Thermal noise loT is a key performance criterion: open-loop
PC
params
can
be adjusted to
reach
a target loT
crucial in reuse-1 deployment to guarantee coverage and stability
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Target
SINR
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loT Control Mechanism Inter-cell
Power
Control
Setting of Target_SINR_dB
determines
the loT
operating point
Especially in a reuse-1
deployment
it is critical
to
manage
the
uplink
in te r fe rence level
In LTE e-NBs
can send
uplink overload indications
to
neighbor e-NBs via the
X2 in te r f ace
Power control parameters i.e. Target SINR can be adapted based on
ove rload ind i ca to r s
Allows
control
of
the
loT level to ensure coverage
and
system
stability
Measu re
Interference
emit
over load ind ica to r
Based on overload
indicator from
neighbor cell
adapt
PC
params
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c
Fractional
Power Control
While using
the
same
target
SINR
for
each
user results in very
good fairness as
far
as
power
allocation is concerned
it
also
results in poor
spectral
efficiency
An improved
power
control
scheme
called
Fractional Power
Control adjusts the target
SINR
in relation to the UE s path loss to
its serving sector
UE_TxPSD_dBm =
PL_dB Nominal_Target_SINR_dB
ULJnterference_dBm
1a is called the fractional
compensation
factor, and is sent via cell broadcast; 0 can use radix-2.3.5 FFTfor DFT-precoding i.e.. cannot assign 7. 11. 13. 17.. .. PRBs
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DL
Scheduling
mechanism
rr ies
DL
r e sou r ce
assignment on L1/L2
control channel PDCCH)
Reported
on
PUCCH or
PUSCH: provides
channel
st te
info
nd
info to
select
MIMO mode
Channel dependent scheduling is supported in both time and frequency domain
enables two dimensional flexibility
CQI feedback can provide both wideband and frequency selective feedback
P/v .l
an d
Rl
feedback allow
fo r MIMO
mode selec t ion
Scheduler chooses bandwidth allocation modulation MIMO mode and power allocation
H RQ operation is asynchronous and adaptive
Assigned
PRBs
need not be contiguous for a given user in the downlink
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Channel Quality Indicator, Pre-coding Matrix Indicator, Rank
Indicator
ess etai led
More
et iled
Periodic Reporting
Aperiodic Reporting
Physical
Channel for
Report
PUCCH
PUSCH
Trigger
for
Report
None
Indication in scheduling
grant
PMI f eedback
for closed-loop
S
Single PMI
Single PMI
and
Multiple PMI
CQI feedback
computed
assuming
calculated PMI
1) Wideband
2) UE-selected subband
coarse
subband
sizes,
one QI
report pe r codev/ord)
1) Wideband
2) UE-selected
best-M subband
granular subband sizes, one
QI
report
pe r
codeword)
3) Higher-layer
configured
subband
one
CQI report per subband pe r
codev/ord)
Rl f eedback
Sent
in
separate
subframe
from
CQI/PMI
Sent together with CQI/PMI
PUS H
is
used here
only
when UL data
is
sent,
in
order to
maintain
single
carrier
transmission
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Scheduler proportional fair principles
The
SINR
per
PRB
on
the
UL
or per resource block group
RBG
on
the DL
for
the
traffic
channel is estimated from the
SRS
for
the UL
and
the CQI
report for
the DL
Note that
the RBG
size is bandwidth
dependent;
for 10MHz it is 3 PRBs resulting in 17 RBGs
The prioritymetric per user is formed bymapping the SINR to an achievable rate per
PRB or
RBG
using a look up
table
and dividing by the average user rate and
multiplying by the QoS and GoS weight
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| Mont h 2008
This i s RBG
index fo r
th e
downlink
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Priority
t t r i e
2
LE I
1
Q LE2
0
LE 3
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Scheduler proportional fair principles
For each
PRB UL
or
RBG
DL
we find the user with the highest priority metric
In
the
downlink we are
pretty
much done here as
the
OFDM
nature
of the downlink lets
us simply assign
each
RBG to the user with th e highest priority metric which maximizes
the original sum rate metric a user is allowed to be assigned discontiguous
PRBs
- There are additional points to account for such as CCE search space constraints on the
PDCCH
^
This
is RBG
i n de x f o r
t downlink
w
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Scheduler proportional fair principles
For the uplinkv/e have to account for the SC-FDMA constraint of contiguous
PRBs,
as well as the fact
that
there
are
UE
power
headroom
PH)
constraints
The PH limits the max number of
PRBs
which can be assigned while the maintains its current
transmit power spectral density Tx PSD as se t by power control
axToral ower
NumPR
Uax m
TxPS
Finding
th e
optimal solution which maximizes
the
desired sum rate metric is not feasible with
the
SC-FDMA constraint, and hence
the
Maximum Priority Envelope MPE algorithm has been developed
for uplinkuser scheduling and resource allocation which accounts for
power headroom
constraints
an d th e contiguous PRB
restriction
UE
UE2
UE3
In this oxamplo
UE2has
umPRB^xPwr=3
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Scheduler proportional fair principles
We
then
go
through an iterative
procoss
of
assigning
contiguous sets of
winning
user
PRBs
called
envelope groups), oach time taking into account
CCE
soarch space constraints on
the PDCCH
m L
Example:
Assume S has
9 w highest priority
metric
make
assignment for
UE2 update PRBs not f
allocated
to
UE
in
which i t was th e winne r
CS
t i t
>
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S,
gets
assigned
to
UE3 and
~
gets
assigned to
UE1
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Frequency Non Selective Scheme
esourc e
Unit
ndex
Single
priority
metric formed and used in
th e first
stage
of th e MPE algorithm
Then MPE algorithm continues as in FSS
s cheme
The SRS
SYNC SINR
is a scalar quantity per user
that
is formed by averaging the SRS
SINR
across
PRBs
and then filtered in time; used to form a single priority metric, which is replicated and used
for all
PRBs
Tosupport a large number of UEs the SRS period needs to be reduced given the multiplexingcapabilities
max of 8
UEs
per
SRS
transmission per frequency comb
The regular
MPE
algorithm as in
the FSS
algorithm is
then
utilized, which minimizes
testing verification
to
just the new
code
introduced
Currently also investigating an intermediate solution where the resolution of the frequency
selective scheduler is reduced bya certain factor in order to retain some frequency selectivenessin
the
scheduling while reducing complexity study in progress
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DL MCS
t ab l e
Along with the
of
PRBs allocated an MCS
level is sent in th e scheduling grant
corresponding
to 29 different possibilities
The MCS index corresponds
to
a modulation
format and transport
block size
index
Transport block size table
gives
th e block size to
use
b as ed o n
th e
of
PRBs
a l loca ted
The code rates given in
th e
table are for 50
PRBs assuming
th e RS pattern
for 1
antenna
port
and
for 3 OFDM symbols
used
for L /L2
cont rol
Precise code rate
will depend
on th e
exact
of
PR s allocated
as
well
as of Tx antenna ports and
of OFDM
symbols
reserved for
L /L2
control
Note: 3GPP allows
th e
UE
to
skip decoding if
the
code
rate
on
the
ini t ial
transmission
exceeds
0.93
M CS
Index M odula t ion
TBS
I nde x
TBS r 5
PRBs
Approx
C o d e Ra t e
0
QPSK
0
1384
11
1
QPSK 1
1800
14
2 QPSK
2
2216
18
3 QPSK
3 2856
0 23
4
QPSK
4
3624 29
5 QPSK
5 4 3 9 2
35
6
QPSK
6 5 1 6
41
7
QPSK 7
62 49
8 QPSK
8
6 9 6 8
55
9 QPSK 9
7992 6 4
1
16QAM
9 7992
32
11
16QAM 10 876 3 5
12
16QAM 11
9 9 1 2
39
13 16QAM
12
1 1 4 4 8
46
14 1 6 Q A M
13
1 2 9 6
52
15 1 6 Q A M
14 1 4 1 1 2
56
16
16QAM
15 1 5 2 6 4
0 61
17
64QAM
15
15264 4
18
64QAM
16
16416
43
19
64QAM
17
18336 4 9
2 64QAM
18 19848 53
21
64QAM
19
21384
57
22 64QAM
2
2 2 9 2 61
23 64QAM
21 2 5 4 5 6
67
24 64QAM
22
27376
72
25
64QAM
23
28336
75
26
B4QAM
24
3 576
0 81
27
2 5
26
31704
0 84
6 4 Q A M
6696^
97
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UL MCS t ab l e
Along
with
the of PRBs
allocated an
M S
level is
sent
in
th e
scheduling grant
corresponding to 29 different possibilities
The
M S
index corresponds
to
a modulation
format
and
transport
block
size index
Transport block s ize table givos the block size to
u se b as ed o n
th e
of
PRBs
al located
The
code
rates given in
the table
are for 50
PRBs assuming no SRS allocation
Precise code
rate
will depend on
SRS allocation
U I
puncturing
as
well
as precise of PRBs
allocated and the
U category
Mote: UEs
which
are not capable of 64-QAM
ca n
continue to
use
16-QAM
for
MCS
Indices
21
and higher
in which c ase th e co de rate
will
be
higher
than
that
shown
in t he t ab le
MCS I ndex
Modu la t ion
TBS
Index
T BS f or 5
P R B s
Approx
C o d e R a te
0
QPSK
0
384 1
1
QPSK
1 1 8
1 3
2
QPSK
2
2 2 1 6
1 6
3
QPSK
3
2 8 5 6
2
4
QPSK
4
3624 2 5
5
QPSK
5 4 3 9 2
31
6
QPSK
6 5 1 6
36
7
QPSK
7
62 43
8
QPSK
8
6 9 6 8
49
9
QPSK
9
7992 56
10
QPSK
8 7 6
61
11 16QAM
8 7 6
31
2
16QAM
11
9 9 1 2
35
3 16QAM
2
1 1 4 4 8
4
4
6QAM
3 1 2 9 6
45
5
6QAM
4
1 4 1 1 2
49
6
6QAM
5 15264
53
7
6QAM 6
1 6 4 1 6
57
8 16QAM
7
8336 6 4
9
16QAM 8 9848
69
2
16QAM 9
21384
74
21
64QAM 9 2 1 3 8 4
5
22
64Q A M
2
2 2 9 2
53
23
64 QAM 21
2 5 45 6 59
24
64QAM
22 2 7 3 7 6
63
25 64QAM
23
2833 6 66
26 64QAM
24 3 5 7 6
71
27 64QAM
25 317 4
1 3
28
64 QA M 26 3 6 6 9 6
85
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Q
4
0
1
2
Q
Q
3
Q
L
A
0
A
L
A
0
L
A
0
2
A
l
R
g
s
R
v
O
A
l
c
e
L
2
2
Q
1
Q
Q
L
A
0
A
c
e
L
n
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MIMO
in
LTE
RF Hardware
To support
MIMO
2x2
the
RF
hardware
products
must
have 2
RF
transmit
paths
the
product
name
should end
with
2x such as
RRH2y
TRDU2x
T aseband Unit RRH x
PRI
M RRH
is
MIMO
ready
with
a singl
however 2
M TRX
are
required
to s
MC RRH
supporting
LTE
2x2 MIMO
LTE BBU
module
LT
MIMO
i
-
2
Antennas
X
F
e
P o w e r
Supply
Powe r
Feeding
Optical
interface
Fibre
CPRI I
outi t i
or
Ethe rne t
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MIMO in LTE
Antenna
Design
Xpol
2
uncorrelated outputs:
good diversity gains
algorithms
supported
-
TxDiv/SFBC
- CL OL SM
up to
2 streams
- UL
MU-MIMO with
2 u s e r s
poi
closely spaced
correlation between elements with equal polarisation: array and diversity gains
algorithms
supported:
TxDiv/sfbc.
-ecommended for
balanced performances
- CL OL SM up to 2
streams
- UL MU-MIMO with 4 use r s
UL
performance
DL performance
Suitable radio
environments: large/outdoor cell/cell border
LOS
environments
pol widely spaced
4 uncorrelated outputs: good diversity gains
algorithms
supported:
- TxDiv/SFBC
- CL OL SM
up to
4 streams
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MU-MIMO wi th
4
use r s
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1 6 cm
X
X
X
X
X
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X
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X
T
3 ^cm
X
>1 5m
-
1 6
cm
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MIMO
in LT E
eminder
Reminder
on some defini t ions
i i
U user 2
U user
SISO
ch nnel
SIMO channel i.e.
RxDiv
MIMO ch nnel
MU MIMO
ch nnel
SU-MIMO Single UserMIMO)
Spatial Multiplexing SM): increase peak
rate
by 2 in MIMO 2x2
Transmit Diversity
TxDiv):
improve reliability of a single
data
stream
MU-MIMO Multiuser
MIMO)
Multiple data
streams
from /to
different
users sent on the
same resource
Works
even
with s ingle
antenna/PA
mobile
^ iTTthe sc^OTquent
slides,
the
fdC^efsetrrtcth^ntdr3wfllink
as
uplink
doesAITrb Lu< ent@
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Generali t ies
Terminology
The relationship between codewords rank and layers is not unique and depends on the
MIMO
scheme
to
e c ons id e re d
A few important definitions:
Codeword an independently encoded data block corresponding to a single transport
block
with one
CRC
a codeword is directly
related to
the
channel
coding
operation
Codewords
< layers
ank number of non-redundant data streams
that can
be
transmitted
coded data streams may be split into different layers and how the data stream is split depends on
the
an tenna s cheme
and th e rank
of
th e channel:
- if
rank
= 1 only
one codeword
can be transmitted - if multiple
coded
data
s treams, they
carry
the
same
information
- if rank = 2 either one or two codewords
can
be transmitted while offering a spatial
multiplexing gain
of
2 -> 2 unique
coded
data streams
- if
rank
= k up to codewords while
offering
a SM gain of ->
unique coded data
streams
Layer number of streams including redundant
ones
to be transmitted
- tayers-
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ener li t ies
Examples
Spatial multiplexing can be achieved with either 1 or multiple codewords
t r nsm i s s ion
SU MIMO 2x2 offers 2 possibil ities: 1 or 2
codewords
for rank 2
transmission
3GPP
LTE
V
2
codeword s
WiMAX
o ewo r
Advantage: permit Successive Interference
Cance ll a tion decod ing t the receiver
Advantage: save
signaling
overhead
as th e
H RQ
associated
signaling is
rather
expensive
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Genera l i t i es
Examples
Spatial multiplexing can be achieved with
either
1 or multiple codewords
t ransmiss ion
SU MIMO 2x2 offers 2 p os si bi li tie s: 1 or 2 codewords for rank-2 transmission
2 layers
1 codeword
2 c o d e w o r d s
\
x 2x
Y
2 layers
x 2x i
X2 X
Layer
mapping
i i
i i
i i
/ >
precoder
Advantage:
permit
Successive
Interference
Cancellation
decoding
at
th e
receiver
=>
SIC
allows significant gains
x4 x3 x2 x1
>
Layer
mapping
tV-N
n5
precoder
x 4x 3
A dv ant ag e: save
signaling
overhead as the
HARQ
associated
signaling is
rather
expensive
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Downlink Signal Structure
The
LTE
downlink signal
structure
is
general and applicable to both transmit
diversity and spatial multiplexing. LTE Rel 8 can
support:
up
to
4 layers with 4
transmit antennas
both
open loop and
closed-loop
spatial
multiplexing for 2
and
4
transmit
n t e n n s
up
to
rank 4 transmission
with 4x4 MIMO
Rank Indicator Rl and Precoding Matrix Indicator PMI
are
used
to permit
closed-loop
-
the
precoding codebook is
defined
in TS36.211
odewords
Layers
eNB
antenna
ports
Channel
coding
u w
Srrnmhling
Modulation
m ppe r
Layer
m ppe r
X
Drecoding
RE
m ppe r
CFDM signal
generation
V\
I
i
i
r
l
i t
Channel
coding
|
Scrambling
Modula t ion
m ppe r
RE
m ppe r
CFDM signal
generation
7^
\
v.*
~ y
Rl, CQI
PMI
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Codeword to
layer
mapping
2
Transm i t n t enna s
With 2
transmit antennas,
TxDiv may
be
used.
In
LTE,
SFBC
is
implemented
which is a
frequency-domain
version
of
the Alamouti code.
the transmitted diversity streams are orthogonal
SFBC/Alamouti code 2x2 :
1 single possibility:
- transmission relies
on
1 single
codeword
-
the
codeword is
duplicated
on 2 layers
(redundancy)
rank
=
symbols
/
subcarriers
the single
codeword is sent- t-wirp over
1 codeword
=> 2 unique symbols on 2 suucdii ieis = rar
ci ihrarn gjrs SC
2 layers
X2 X,
bK.-iMbi
34 |
Presentation Title
I Month 2008
Modulat ion
+ coding
x2
x,
K
I
Ll t i l lM
Layer
mapping
i
iission
SFBC
precoder
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Codeword
to layer mapping
2 Tr an smi t An t ennas
With 2
transmit antennas,
uses 2
codewords see
Fig. 2)
1 codeword
corresponds
to 1 Transport Block Size TBS
C
Rank-1 transmission is often seen as a special
case
of SU-MIMO
spatial
multiplexing. In
L U V JJ \_
I
LVJULfVUlU
U
UJV U
>
tf ouewor s =
r a r iK
ot tr nsmission
The
codeword to
layer
mapping is trivial: the
codeword
n is mapped to the layer n
^codewords
=
ttl yers
The mapping
between codewords and
layers is shown below:
layer 1
CW 1
Precoding
1x4)
Rank
Fig. 1: Rank-1 transmission
Rank 2
CW 1
CW 2
layer 1
w
layer
2
V
Precoding
2x4)
Fig. 2: Rank-2 transmission
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Codeword to
layer
mapping
4 Tr an sm i t An t enn a s
Transmit
diversity schemes
with
4 transmit
antennas results
in a
combination
of
tw o
SFBC
x
Frequency Shift
Transmit
Diversity
FSTD
tr?n-rm> rwmjp ^Sbf^m
each antenna on
a
different subcarrier
X\
0 0 0
a n t e nna s
r
SFB
x 0 0
0 v. 0
0 0
x
vi
i
V
x
^
SFBC + FSTD
vi
X
U 0
x
xl
0 0
0
0
v.
X
0 0
*4
SFBC + FSTD is a suitable
transmit
diversity as no orthogonal full-rate diversity
code exists beyond 2x2 configuration
Usage in either 4x2
or
4x4
antenna
configuration
?word
36 I Presentation Title I Month 2008 All Rights Reserved 3 Alcatel-Lucent 2008,
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min i IP Symbo l s nn
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u
s
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c
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(
-
8/11/2019 ALU LTE Workshop
35/35
Codeword
to
layer
mapping
4 Tr an sm it An t enna s
Looking
into the
details
of
rank-3
transmission:
codeword
1 is
mapped
on a single layer
1 codeword
per
layer:
the
layer size equal to
the
codeword length
(i.e.
TBS)
codeword 2 is mapped on 2 layers (layers 2 & 3)
1 codeword for two layers:
- if pr ^ 55,
the
codeword is equally split on
the
two layers
- if
A/we
> b5. the 2
layers
have^ttf^same order of magnitude (almost equal); a
* ' ' /~*'*'*:'
^
--
i c I
r> I
j f) n
fr~> t >
I~i -=
^- - -^
~
~i
~ :
r in
3( *pp
c o d ew o r d
2 :
TB S
<
l ayer 2
'
S / P c o n v er t er
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