Upload
voanh
View
308
Download
17
Embed Size (px)
Citation preview
1
Maximizing LTE MIMO
Throughput Using
Drive Test
Measurements
PCTEL RF Solutions
2
James Zik, Senior Product
Marketing Manager, PCTEL, Inc.
Bruce Hoefler, Vice President
Product Management, PCTEL, Inc.
3
Mobile Bandwidth Need
18X growth from 2011 to 2016
4
Spectrum Crunch
-300
-200
-100
0
100
200
300
2011 2012 2013 2014
Spectr
um
Surp
lus
(MH
z)
FCC Licensed Spectrum Needs*
Need to get maximum throughput on available spectrum
*FCC, “Mobile Broadband: The Benefits of Additional Spectrum” (October 2010).
5
• Air Interface Bottleneck Solutions
How do we Get There?
More Spectrum • Limited licensed spectrum available
• Expensive
Migration to LTE
LTE MIMO
• Carrier grade WiFi and backhaul required
• 22% of mobile traffic by 2016 (Cisco VNI Mobile 2012) WiFi Offload
• Small Cells and DAS (expensive)
• Backhaul required to each cell/DAS
Increased Cell
Density
• Migration to LTE and LTE Advanced
• LTE MIMO
Spectrum
Efficiency
Must employ all of these solutions to solve the spectrum crunch
6
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
LTE Peak Throughput of 4x
LTE Peak Throughput of 2x
-5
0
5
10
15
20
25
30
35
-10 -5 0 5 10 15 20 25 30
Peak Physical Layer Spectral Efficiency (b/s/Hz)
SNR (dB)
2x1
2x2
4x4
8x8
7
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
8
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
Transmission shown one way (eNB to UE) for simplicity
9
Transmission Modes
Single User MIMO Modes
Currently deployed
transmission modes
10
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
11
What Parameters are
Necessary for MIMO Testing
Premise: Operators need to understand MIMO transmission
characteristics of the physical layer in RAN
Path measurements
Channel Quality Indicator (CQI as defined by 3GPP)
Throughput (maximum air interface throughput capability)
Channel Condition Number (CN)
12
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
13
CQI
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
CQI Index Modulation Code Rate x
1024
Efficiency
(b/s/Hz)
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
Source: 3GPP TS 36.213 Ver. 10.7.0 (Sept. 2012)
Table 7.2.3-1: 4-bit CQI
14
Throughput
How and where is throughput measured?
Physical Layer Throughput
(for RAN optimization)
Throughput measurement includes RAN, Backhaul,
Network Loading, Server, etc.
CQI
Index
Throughput (Mbps)
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
Maximum Throughput
Capacity of the Air Interface
(Physical Layer)
15
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
CN (dB) Indication
0 Two totally independent channels, an ideal condition that
can enable maximum throughput
~<13* Favorable condition that can enable much better throughput
than SISO/MISO based transmission systems
~13 to 19* Medium correlation that can provide marginal throughput
improvement
~>19* 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
Industry Norm
16
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
17
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
18
• 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
MIMO oDAS Case Study
Antenna Locations
19
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
20
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
21
0
2
4
6
8
10
12
14
16
18
Fre
qu
en
cy
Position: 0°
01020304050607080
Fre
qu
en
cy
Position: 90°
Position A Average: 2.820 Mbps
Position C Average: 1.232 Mbps
Orientation Analysis UE Data Card
2.3X difference depending on
orientation due to directionality
of UE antennas
(stationary test)
kb/s
kb/s
MIMO antennas in UEs
are typically very
directional
22
0
5
10
15
20
25
30
35
40
45
# o
f D
ata
Po
ints
Speed Analysis
UE Drive Test (~20 mph) UE Walk Test
UE Throughput drops by 2X w/ drive test, scanner is not affected by speed
Throughput (kb/s)
((
4
2
)
4
)
(42)
0
10
20
30
40
50
60
70
80
1 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
# o
f D
ata
Po
ints
(24)
Throughput (kb/s)
23
UE Data
Scanner Data
Drive Test Data (Throughput)
0
200
400
600
100 1000 2500 5000 10000 More
# o
f D
ata
Po
ints
UE
0
1000
2000
3000
4000
5000# o
f D
ata
Po
ints
Scanner
UE Data
Tput (kb/s)
Lost Connection
Significant differences
between scanner & UE
Note: Throughput is very low.
Scanner show higher throughput
since it’s measuring at the physical
layer
Tput (kb/s)
24
Scanner Transmission
Modes (MIMO Gain)
0
200
400
600
800
1000
1200
100 1000 2500 5000 10000 20000 30000 More
# o
f D
ata
Po
ints
Open Loop Transmit Diversity (mode 2)
0
200
400
600
800
1000
100 1000 2500 5000 10000 20000 30000 More
Fre
qu
en
cy
Closed Loop Spatial Multiplexing (mode 4)
No throughput gain from multiple data streams
Tput (kb/s) Tput (kb/s)
25
RSRP
Test results show marginal RSRP
RSRP
dBm
# o
f D
ata
Po
ints
26
CINR for MIMO
CINR
Test results show very LOW CINR
dB
# o
f D
ata
Po
ints
27
CN for MIMO
MIMO requires high CINR and prefers low CN to maximize throughput
Condition Number (CN)
Test results show low CN
dB
# o
f D
ata
Po
ints
28
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
29
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)
30
LTE RSRP (Best Server)
Excellent RSRP
31
MIMO Drive Test Transmit Diversity and MIMO Throughput
Best Server Region
for PCI of interest
Transmit Diversity MIMO (mode 4)
Mbps
32
MIMO Drive Test Delta (MIMO-Transmit Diversity)
Throughput
Mbps
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.
Large MIMO gain for much
of the center region of the cell
33
MIMO Drive Test Condition Number and CINR
High CN with high
MIMO Gain
Condition Number CINR
dB
dB
Very High CINR
Significant MIMO gain exists for LOS condition
if CINR is high even with high CN
34
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 • Path measurements
35
Questions?
Thank you
http://rfsolutions.pctel.com/content.cgi?id_num=36
For a free LTE MIMO poster, please visit PCTEL RF Solutions website: