Open Loop versus Closed Loop
Performance in LTE
Lutz Schönerstedt (Presenter), Andreas Weber, Michael Ohm, Thorsten Wild
Bell Labs Germany, Stuttgart
12.2.2009 (VDE/ITG-Fachgruppe 5.2.4 at ComNets RWTH Aachen)
Agenda
1. Open And Closed Loop Transmission in LTE
2. Physical Layer Results
3. System Level Performance
4. Conclusion
Open And Closed Loop Transmission in LTE
Pilot Feedback: PMI, CQI, Rank1
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Pilot Feedback: PMI, CQI, Rank
SFBC and PARC, TX-Diversity (Closed Loop) and PSRC1
Pilot Feedback
Comment Freq. Available in Available in
eNodeB UE
Uplink feedback: PMI, CQI, Rank
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Comment Freq.
Resolution
Available in
open loop
Available in
closed loop
PMI Prefered
precoding
matrix indicator
Index of best Tx
weight
Subband - X
CQI Channel quality
indicator
Supported
transport format
Subband X X
Rank No. of spatial
streams supported
Full band X X
Downlink MIMO Modes
LTE-MIMO
2 x 2 and 4 x 2 (TX x RX)
Open loop Closed loop
Single stream
Rank 1
Multi stream
Rank 2-4 Single & multi stream
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Rank 1 Rank 2-4 Single & multi stream
OL Beamforming OL Tx Diversity
SFBC
Space Frequency Block Coding
CDD
Cyclic Delay Diversity
PARC
(Per Antenna Rate Control)
Codebook-basedlinear precoding
Rank 1: CL Tx Diversity
Rank 2-4: PSRC (Per Stream Rate Control)
Comparing Downlink MIMO Modes
Single Stream
� OL SFBC
Uses diversity by orthogonally transmitting one data stream over two transmit
antennas, to reduce dynamic of the received power.
� CL Tx Diversity
Tries to maximize the received power at the mobile by applying precoding (based on
PMI feedback). Sends correlated symbols on transmit antennas.
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PMI feedback). Sends correlated symbols on transmit antennas.
Dual Stream
� OL PARC
Uses diversity to transmit two data streams over two transmit antennas.
� CL PSRC
Uses diversity to transmit two data streams over two transmit antennas.
Tries to maximize the received signal quality of the data streams at the mobile by
applying precoding (based on PMI feedback).
6 | Open Loop versus Closed Loop Performance in LTE| February 2009
Linear Precoding (for PARC, PSRC and CL Tx Diversity)
� Complex linear transmit antenna weights
� Distributes data streams over the antennas
� 2 Tx with 2 layers example for OFDM:
Codebook index
Number of layers υ
1 2
0
1
1
2
1
10
01
2
1
1
−1
1
2
1
−11
11
2
1
2
j
1
2
1
− jj
11
2
1
3
11
-
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R8 codebook for 2 Tx
(3GPP TS 36.211)
3
− j2
1 -
Physical Layer Results
Open Loop Single Layer2
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Open Loop Single Layer2
SFBC 2x1, 2x2 versus Single Antenna 1x1, 1x2
Legend:
� SFBC, 1RX: SFBC 2x1
� SFBC, 2Rx: SFBC 2x2
� SISO: 1x1
� SIMO: 1x2
10-1
100
BLE
R
VehA, 120km/h, ρtx=0.1, ρ
rx=0.1, QPSK, R=1/3
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Gain of SFBC at BLER = 0.1:
� SFBC 2x2 is 1.1dB better than 1x2.
� SFBC 2x1 is 1.8dB better than 1x1.
9 | Open Loop versus Closed Loop Performance in LTE| February 2009
-10 -5 0 510
-2
SNR [dB]
SFBC, 2Rx
SFBC, 1Rx
SISO
SIMO, MRC
10
System Level Performance
Open Loop and Closed Loop Single/Dual Layer3
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Open Loop and Closed Loop Single/Dual Layer3
Cell border throughput over spectral efficiency 3km/h
Simulation Assumptions:
� 210 UEs in 21 sectors
� ISD 500m
� 46dBm per Antenna
� 10MHz bandwidth
220
240
260
280
300
Ce
ll B
ord
er
Th
rou
gh
pu
t [k
bit
s/s
]
S ing le Antenna 3 km/h
S F B C 3 km/h
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� Round Robin
Scheduler
� Single Antenna 1x2
� SFBC 2x2
� CL Tx Diversity 2x2
11 | Open Loop versus Closed Loop Performance in LTE| February 2009
180
200
220
0.95 1.00 1.05 1.10 1.15 1.20 1.25
S pec tral E ffic ienc y [bits /s /Hz ]
Ce
ll B
ord
er
Th
rou
gh
pu
t [k
bit
s/s
]S F B C 3 km/h
S F B C _P AR C 3 km/h
T x Div 3 km/h
T x Div_P S R C 3 km/h
Cumulative Probability of SINR
0.5
0.6
0.7
0.8
0.9
1.0C
um
ula
tiv
e P
rob
ab
ilit
y
S in g le A nten n a
S F B C
C L T X Div
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0.0
0.1
0.2
0.3
0.4
0.5
-20 -10 0 10 20 30
P er S u bc arrier S INR meas u red at Dec oder In pu t [dB ]
Cu
mu
lati
ve
Pro
ba
bil
ity
Cell border throughput over spectral efficiency 250km/h
145
150C
ell
Bo
rde
r T
hro
ug
hp
ut
[kb
its
/s] S ing le Antenna 250 km/h
S F B C 250 km/h
T x Div 250 km/h
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135
140
0.5 0.55 0.6 0.65 0.7
S pec tral E ffic ienc y [bits /s /Hz ]
Ce
ll B
ord
er
Th
rou
gh
pu
t [k
bit
s/s
]
SFBC versus CL Tx Diversity
170
180
190
200
210
220
Ce
ll B
ord
er
Th
rou
gh
pu
t [k
bit
s/s
]S F B C 30 km/h
S F B C 60 km/h
S F B C 90 km/h
S F B C 120 km/h
S F B C 250 km/h
T x Div 30 km/h
T x Div 60 km/h
T x Div 90 km/h
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120
130
140
150
160
170
0.5 0.6 0.7 0.8
S pec tral E ffic ienc y [bits /s /Hz ]
Ce
ll B
ord
er
Th
rou
gh
pu
t [k
bit
s/s
]
T x Div 90 km/h
T x Div 120 km/h
T x Div 250 km/h
Conclusion
4
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4
Conclusion
� SFBC gains from physical layer simulation couldn’t be retrieved in system level
simulation.
� Especially at low speed, a transmission technique with higher channel quality
variance (with the same mean quality) handles more throughput. This is due to
the non linear mapping of channel quality and throughput.
� Frequency selective schedulers, which can take advantage of situations with
high SINR, promise to improve system performance even more.
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high SINR, promise to improve system performance even more.
� With a round robin scheduler and velocities higher than 30 km/h, SFBC
becomes attractive as a fallback mode for closed loop transmit diversity.
16 | Open Loop versus Closed Loop Performance in LTE| February 2009
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