Maximizing LTE MIMO
Throughput Using
Drive Test
Measurements
PCTEL RF Solutions
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James Zik, Senior Product
Marketing Manager, PCTEL, Inc.
Bruce Hoefler, Vice President
Product Management, PCTEL, Inc.
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Mobile Bandwidth Need
18X growth from 2011 to 2016
3
Spectrum Crunch
300
Spectrum Surplus (MHz)
200
100
0
-100
-200
-300
2011
2012
2013
2014
FCC Licensed Spectrum Needs*
Need to get maximum throughput on available spectrum
4
*FCC, “Mobile Broadband: The Benefits of Additional Spectrum” (October 2010).
How do we Get There?
• Air Interface Bottleneck Solutions
More Spectrum
•
•
Limited licensed spectrum available
Expensive
WiFi Offload
•
•
Carrier grade WiFi and backhaul required
22% of mobile traffic by 2016 (Cisco VNI Mobile 2012)
Increased Cell
Density
•
•
Small Cells and DAS (expensive)
Backhaul required to each cell/DAS
Spectrum
Efficiency
•
•
Migration
LTE
Migration to to
LTE
and LTE Advanced
LTE MIMO
LTE
MIMO
Must employ all of these solutions to solve the spectrum crunch
5
Why MIMO?
• Low to medium cost method to improve transmission performance
(already built-in on many LTE base stations)
• Increases physical layer capacity (w/ spatial multiplexing MIMO)
– Throughput gain dependent on number of Tx and Rx antennas
i.e. 2x2, 4x4, etc.
LTE Peak Spectral Efficiency per 3GPP
35
LTE Peak Throughput of 4x
30
25
20
Peak Physical Layer
Spectral Efficiency (b/s/Hz)
2x1
2x2
15
4x4
10
8x8
5
LTE Peak Throughput of 2x
0
-10
-5
0
-5
6
5
10
SNR (dB)
15
20
25
30
What is MIMO?
• MIMO is a smart antenna
technoIogy that employs
multiple antennas at the
Tx and Rx ends
• MIMO is NxM (i.e. 2x2,
4x2, 4x4, 8x8, etc.) where
N>1 and M>1
– 2x2 (deployed), 4x4 and
4x2 (emerging),
8x8 (LTE Advanced)
• Radiated signals traveling
on different paths provide
the possibility of
performance
improvements
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How Does MIMO Work?
• Spatial Multiplexing:
Transmits multiple data
streams simultaneously in
the same frequency and
time, taking advantage of
different paths
– Requires separate paths
– Requires high SNR to
improve throughput
8
Transmission shown one way (eNB to UE) for simplicity
Transmission Modes
Single User MIMO Modes
Currently deployed
transmission modes
9
Why Test MIMO?
What are we trying to accomplish by testing MIMO?
• Determine the air interface Maximum Throughput capacity
for different MIMO Modes (MIMO Gain)
– Provide throughput gain of the physical layer for each
transmission mode (using standards number)
• Optimize the RAN physical layer for Maximum Throughput
– Characterize Link efficiency
• Troubleshoot the RAN physical layer
– Isolate path issues
– Test for channel independence
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What Parameters are
Necessary for MIMO Testing
Premise: Operators need to understand MIMO transmission
characteristics of the physical layer in RAN




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Path measurements
Channel Quality Indicator (CQI as defined by 3GPP)
Throughput (maximum air interface throughput capability)
Channel Condition Number (CN)
RF Path Measurements
• Determines if there is a problem with base station port or
a particular antenna with regard to MIMO paths
•
• Antenna, cabling or TX port issues
Measurements are provided for each Tx/Rx antenna pair
– 4 paths for 2x2 MIMO
• RSRP, RSRQ, RS CINR for each path
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CQI
CQI Index
Modulation
0
Consistent indicator of theoretical
transmission physical layer
efficiency
• CQI measurement is an essential
tool for MIMO
– With SISO, CINR translates
directly to CQI
– With MIMO, CINR does NOT
translate to CQI i.e. throughput
Code Rate x
1024
Efficiency
(b/s/Hz)
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
Source: 3GPP TS 36.213 Ver. 10.7.0 (Sept. 2012)
Table 7.2.3-1: 4-bit CQI
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Throughput
How and where is throughput measured?
Maximum Throughput
Capacity of the Air Interface
(Physical Layer)
Physical Layer Throughput
(for RAN optimization)
Throughput measurement includes RAN, Backhaul,
Network Loading, Server, etc.
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Throughput (Mbps)
CQI
Index
5 MHz
10 MHz
20 MHz
1
0.55
1.10
2.19
2
0.84
1.69
3.38
3
1.36
2.71
5.43
4
2.17
4.33
8.66
5
3.16
6.31
12.63
6
4.23
8.47
16.93
7
5.32
10.63
21.26
8
6.89
13.78
27.56
9
8.66
17.33
34.65
10
9.83
19.66
39.32
11
11.96
23.92
47.84
12
14.05
28.10
56.19
13
16.28
32.57
65.14
14
18.41
36.83
73.66
15
20.00
39.99
79.99
User layer throughput
reduced up to 10X due
to control overhead
-
Handshaking
Synchronization
Retransmission
Condition Number (CN)
• CN is a measure of the independence (or correlation) of the
channels (paths)
– Measured from 0 to 50 dB; lower values are better indicating low
correlation
– CN helps analyze potential causes for throughput issues but is not used
to calculate throughput
– Studies show MIMO can still be effective with high CN if CINR is high
Industry Norm
CN (dB)
0
~<13*
~13 to 19*
~>19*
Indication
Two totally independent channels, an ideal condition that
can enable maximum throughput
Favorable condition that can enable much better throughput
than SISO/MISO based transmission systems
Medium correlation that can provide marginal throughput
improvement
High correlation where MIMO generally would not induce a
condition that would increase throughput
*The CNs indicating the level of correlation are based on industry published approximations and can vary by several dB depending on conditions
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Interpreting MIMO
Measurements
• Separate CN, ECQI and transmission mode
measurements allow operators to diagnose the
causes of low throughput
– Low CN and low CQI means there is an interference
or a power issue
– High CN and low CQI means high channel correlation
and probable low SINR
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MIMO Testing Benefits
• Characterize RF propagation for MIMO
– Consistent, repeatable RF data independent of the backhaul, network layer overhead &
server loading
– Higher dynamic range to determine noise floor and potential interference effects
– High data density to locate fading issues and reduced MIMO throughput
• Determine channel independence
– Analyze network problems related to multipath conditions with condition number
– Understand how antenna tilt or relocation can affect throughput
• Provides result for various transmission modes
– Understand how different transmission modes affect RAN performance
• Troubleshoot antenna/cabling issues and base station Tx port issues
– Path measurements
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MIMO oDAS Case Study
• FDD-LTE 2x2 MIMO outdoor DAS
• When: April 2012
• Outdoor DAS deployed due to cell tower restrictions
• Area characterized by high foliage and low antenna height
Antenna Locations
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How we Tested
• Simultaneous oDAS Testing with Scanner and UE Data Card
‒ PCTEL SeeGull® EX Scanner
‒ Test data card on another system
• Orientation Test
• Walk Test vs Drive Test
• MIMO Transmission Modes (on the scanner)
• RSRP, CN, CINR and Throughput
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Why Use a Scanner
• 3GPP TR 37.976:
“3GPP already defined conducted tests for MIMO and
multiple antenna receivers …. but it is clear that the ability to
duplicate these gains in the field is highly dependent on the
performance of the receive-antenna system……..”
“The MIMO OTA throughput is measured at the top of
physical layer of HSPA and LTE system”
• Scanner use omni-directional antennas
• Scanners measure throughput at the physical layer
• Scanners provide throughput for multiple MIMO transmission
modes
• UEs only provide throughput for the MIMO mode the UE is
locked onto
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Orientation Analysis
UE Data Card
Frequency
Position: 0°
18
16
14
12
10
8
6
4
2
0
kb/s
Position A Average: 2.820 Mbps
Frequency
Position: 90°
80
70
60
50
40
30
20
10
0
kb/s
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Position C Average: 1.232 Mbps
2.3X difference depending on
orientation due to directionality
of UE antennas
(stationary test)
MIMO antennas in UEs
are typically very
directional
Speed Analysis
UE Drive Test (~20 mph)
UE Walk Test
((
4
2
)
4
)
(24)
45
80
40
70
35
60
30
25
20
15
10
# of Data Points
# of Data Points
(42)
50
40
30
20
5
10
0
0
1
Throughput (kb/s)
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500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Throughput (kb/s)
UE Throughput drops by 2X w/ drive test, scanner is not affected by speed
Drive Test Data
(Throughput)
UE
Lost Connection
Data
UE Data
Significant differences
between scanner & UE
600
Note: Throughput is very low.
Scanner show higher throughput
since it’s measuring at the physical
layer
400
200
Scanner
# of Data Points
# of Data Points
UE
Scanner Data
5000
4000
3000
2000
1000
0
0
23
100
Tput (kb/s)
1000
2500
5000
10000
More
Tput (kb/s)
Scanner Transmission
Modes (MIMO Gain)
1200
1000
800
600
400
200
0
Tput (kb/s)
24
Closed Loop Spatial
Multiplexing (mode 4)
1000
Frequency
# of Data Points
Open Loop Transmit Diversity
(mode 2)
800
600
400
200
0
100
1000
2500
5000 10000 20000 30000 More
Tput (kb/s) 100
1000
2500
5000 10000 20000 30000 More
No throughput gain from multiple data streams
RSRP
# of Data Points
RSRP
dBm
Test results show marginal RSRP
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CINR for MIMO
# of Data Points
CINR
dB
Test results show very LOW CINR
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CN for MIMO
# of Data Points
Condition Number (CN)
dB
Test results show low CN
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MIMO requires high CINR and prefers low CN to maximize throughput
oDAS Case Study
Conclusions
MIMO is ineffective in this network
• Network has a severe interference and/or noise problem
– Marginal RSRP and very poor CINR
Conditions favorable for MIMO to improve throughput
• Low CN
UE Measurements for MIMO must be carefully examined since they
are affected by:
• UE Orientation
• Speed of movement during the test
– Drive test vs walk test for outdoor systems.
• MIMO Transmission mode the UE is operating in
– Another UE may operate in a different transmission mode
A scanner is very effective in characterizing a MIMO network
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MIMO Macro Cell
Case Study (Oct 2011)
PCI-Best Server by RSRP (Baltimore: Urban Environment)
Focus on Best
Server Region
for PCI (best
server)
(PCI, # of data pts)
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LTE RSRP (Best Server)
Excellent RSRP
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MIMO Drive Test
Transmit Diversity and MIMO Throughput
Best Server Region
for PCI of interest
Transmit Diversity
MIMO (mode 4)
Mbps
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MIMO Drive Test
Delta (MIMO-Transmit Diversity)
Throughput
Large MIMO gain for much
of the center region of the cell
Why is (MIMO – Transmit Diversity)
negative at the cell edge (pink)?
For extremely low CINR, transmit diversity
is more efficient. The UE will switch to
transmit diversity in this region.
Mbps
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MIMO Drive Test
Condition Number and CINR
High CN with high
MIMO Gain
Condition Number
Very High CINR
CINR
dB
dB
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Significant MIMO gain exists for LOS condition
if CINR is high even with high CN
Summary
• MIMO was very effective for maximizing throughput in the Baltimore
Macro cell case study
• MIMO may not be effective in sub-optimal designs, deployments or
terrains as shown in the oDAS case study
• Testing Benefits with a Scanner
– Characterize RF propagation for MIMO
• Consistent, repeatable RF data independent of the backhaul, network layer
overhead & server loading
• Higher dynamic range to determine noise floor and potential interference effects
• High data density to locate fading issues and reduced MIMO throughput
– Determine channel independence
• Analyze network problems related to multipath conditions with condition number
• Understand how antenna tilt or relocation can affect throughput
– Provides result for various transmission modes
• Understand how different transmission modes affect RAN performance
– Troubleshoot antenna/cabling issues and base station Tx port issues
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• Path measurements
Questions?
Thank you
For a free LTE MIMO poster, please visit PCTEL RF Solutions website:
http://rfsolutions.pctel.com/content.cgi?id_num=36
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