Addressing Multi-Channel
Synchronization for MIMO and
Beamforming Test
Sheri DeTomasi
Keysight Technologies
© Keysight
Technologies 2015
Agenda
© Keysight
Technologies 2015
Overview of Multi-Antenna Techniques
Multi-Antenna Test Challenges
Calibration of a Multi-Antenna System
Measurement Examples
© Keysight
Technologies 2015
Page 2
The Data Challenge
 Internet
 Email
 Navigation
 Texting
 Music
 Safety alerts
 Youtube
 TV
 Social networking
 Video/Photo Sharing
 Games
• More connected devices
• Expanded coverage
 Cloud
 Imaging
 Banking
• Explosion of mobile data
© Keysight
Technologies 2015
Page 3
How do wireless service
providers keep up with the
Data Demand?
 Make better use of spectrum
 Combine carriers with carrier
aggregation
 Wider channel bandwidths
 Denser modulation techniques
 Multi-antenna techniques
 Heterogeneous networks
© Keysight
Technologies 2015
Page 4
Comparing Wireless Standards
Cellular
Peak
Data Rate
(DL/UL)
Modulation Types
Beamforming
LTE-Advanced
300 Mbps/
75 Mbps
Up to 20 MHz
Tx Bandwidth
MIMO
Technology
LTE
Wireless LAN
802.11n
802.11ac
1 Gbps/
500 Mbps
600 Mbps
(40 MHz, 4 streams)
6.9 Gbps
(160 MHz, 8 streams,
256 QAM)
Up to 100 MHz
20 MHz mandatory, 40
MHz optional
20 MHz, 40 MHz, 80
MHz to 160 MHz,
80+80 MHz
BPSK, QPSK,
16QAM, 64QAM
BPSK, QPSK,
16QAM, 64QAM,
256 QAM (opt)
QPSK, 16QAM, 64QAM
(256 QAM in small cell)
5G
Being Defined
• Higher freq
• Higher mod
• Wider BW
• Massive MIMO
DL
up to 4x4
Up to 8x8
UL
N/A
Up to 4x4
1 mandatory
1 mandatory, 2 to 8
layers opt
Up to dual layer (DL)
Up to 8 layer (DL)
Up to 4 Tx
beamforming
Up to 8 layers Tx
beamforming
Multi-Antenna techniques are a key technology used to get
higher spectral efficiency and increase peak data rates
© Keysight
Technologies 2015
Page 5
Enhanced Multi-Antenna Techniques
– Use of multiple antennas to improve spectral efficiency - increase
overall capacity and the data rate through multiple antennas
© Keysight
Technologies 2015
Page 6
Enhanced Multi-Antenna Techniques
Transmit Diversity
Receive Diversity
Space-time block
coding (STBC)
Path diversity
X1, X2
– Improves robustness
y1, y2
-X2, X1*
Spatial multiplexing
– Improves spectral efficiency
and throughput
Spatial division
multiplexing
MIMO
y1
X1
Spatial multiplexing with
Beamsteering
4 streams, 3 users
y2
X2
– Increases signal robustness
w/the added advantage of the
improved throughput through
spatial multiplexing MIMO
Multi-user MIMO
Transmit Beamforming
MIMO (4x2)
Matrix
© Keysight
Technologies 2015
Page 7
How does Beamsteering Work?
– Use multiple antennas to electronically steer a single data stream
– Used in radar, sonar, seismology, electronic warfare, wireless
communications, radio astronomy, acoustics
– Takes advantage of multi-antenna interference to control directionality
– Multi-antenna signals are weighted relatively in both magnitude and phase to
produce spatial selectivity
• Constructive (in-phase) interference power gain in desired directions
• Destructive (out-of-phase) interference power nulling in other directions
Direction of
destructive
interference
nulling
0 deg
Direction of
constructive
interference
gain
+30
deg
-30 deg
-60 deg
Antenna array geometry along with
relative per antenna element
mag/phase weightings determine
constructive/destructive interference pattern
+60
deg
-90 deg
||||
N Antenna Elements
Correlated co-polarized 0.5 wavelength
separation
+90
deg
© Keysight
Technologies 2015
Page 8
Agenda
© Keysight
Technologies 2015
Overview of Multi-Antenna Techniques
Multi-Antenna Test Challenges
Calibration of a Multi-Antenna System
Measurement Examples
Page 9
Advantages and Challenges with Beamforming
Common Issues
Advantages
– Improved user experience through better
signal quality
• Beamforming selectivity
• Increases the signal to noise ratio
Single User MIMO
• Coherent signal gain
– Increase cell capacity with MU-MIMO
Challenges
– Requires more antennas
– Phase alignment of RF carrier is critical in
beamforming: < 1 deg at RF frequency
– Often requires multiple, highly synchronized
instruments in test
Multi-User MIMO
© Keysight
Technologies 2015
Page 10
Enhanced Multi-Antenna Designs
Test Challenges
Complicated test setup for higher order MIMO
including beamforming
•Complexity of connecting multiple synchronized instruments
•Control of relative phase, time and frequency of multiple channels
•Calibration of the measurement system at the device under test
Visualizing and validating the MIMO signal at the RF
antenna
• Analyzing multi-antenna beamforming signals with up to 8x8 MIMO
• Simultaneous UL & DL measurements
Cost and footprint
• Test equipment cost & size
• Time consuming tests
• Development while standards evolving
© Keysight
Technologies 2015
Page 11
Complicated Test Set-up
Need for multiple test channels
– Carrier aggregation with
carriers in different frequency
bands
– Multi-standard radios
– Diversity where antennas are
in different frequency bands
– Spatial multiplexing MIMO up
to 8x8
– Beamsteering
Considerations for Test
Equipment
• Is channel testing done
sequentially or in parallel?
• What level of time
synchronization is needed?
• Do you need phase coherent
channels?
• Calibration of the measurement
system at the device under test
© Keysight
Technologies 2015
Page 12
Critical Multi-Channel Synchronization Parameters
Time Alignment
– Use clock synchronization & triggering to
ensure waveform playback and waveform
captures start synchronously on all channels
– MIMO, carrier aggregation
Phase Coherence
– Two signals are coherent if they if they have a
constant relative phase at all instances in time
– Coherence is then a statistical property
between signals
– Beamforming
© Keysight
Technologies 2015
Page 13
Multi-Channel Time Synchronization
Example using PXI 10 MHz CLK
Master Trigger Module
Q
D
Slave Module
Slave Module
Ref10
– One unit designated master,
synchronizes with multiple slaves
Sync State
Machine
...
D1
D2
D3
…
Dn
– Master uses one PXI trigger line to send
to all slave units
Q
Ref10
D
Sync State
Machine
Ref10
Q
D
Q
D
Q
PXI_TRIGGER
(Slave 1)
PXI_TRIGGER
(Slave N)
1
2
First Module
Ready
3
Last Module
Ready
4
5
6
Rdy seen at Sync
State Machine
7
8
Trg seen at
modules
9
All modules assert
not ready
PXI_CLK10
~
~
~
~
PXIe_DSTARCm
~
~
PXI_CLK 10
~
~
~
~
~
~
~
~
~
~
~
~
PXIe_DSTARCp
~
~
PXI Trig 2
~
~
PXIe_DSTARCn
~
~
PXI Trig 1
PXI Trig 3
– A common clock reference provides
time alignment but not phase coherence
Q
Sync State
Machine
PXI_TRIGGER
(Master)
– Slaves each use one PXI trigger line to
send to master unit
– When all channels are armed, master
can signal a trigger event and all units
start on next 10MHz clock edge after
trigger event
D
All modules ready
PXIe_DSTARB[m..p]
All modules
ready
Module
n is Trigger
Module
Trg Module
Ready
All Ready seen
at Trg Module
Trg Module
asserts BP Trg
© Keysight
Technologies 2015
Trg seen at all
Sync SMs
Page 14
Considerations for Phase Coherence
Phase Matching Issues
Two signals are coherent
if they if they have a
constant relative phase
at all instances in time
Solution
– Use a single, shared LO for all channels to achieve
True Phase Coherence
– All channels will have a constant, relative phase error
– Calibration is critical to remove any fixed phase offset
between channels
However, each signal
generation and
measurement channel
has its own phase noise
© Keysight
Technologies 2015
Page 15
Source Simplified Block Diagram
Example of 2 phase coherent channels
Modulator 1
Source output 1
Level control
RA
M
Output
signal
1
Σ
90°
Modulator 2
Download &
definition
Source output 2
Level control
Controller
RA
M
90°
4 LO
outputs
Output
signal
2
Σ
Frequency reference
Reference
oscillator
Local oscillator
Synthesizer
© Keysight
Technologies 2015
Page 16
Analyzer Simplified Block Diagram
Example of 2 phase coherent channels
Downconverter 1
Digitizer 1
Mixer
Analog-digital
converter
Input
signal
1
RF input
attenuator
Pre-selector, or
low-pass filter
IF gain
IF filter
Downconverter 2
Digitizer 2
Input
signal
2
Results & display
Controller
Local
oscillator
Synthesizer
Reference
oscillator
Frequency reference
© Keysight
Technologies 2015
Page 17
Synchronization across Multiple Chassis
Extending PXI 10 MHz CLK to a second chassis
Example Two Chassis Receiver Configuration, two receivers per Chassis
– Front panel connections
between master in each chassis
Chassis 1, Master
10 MHz OCXO Out
10 MHz OCXO
100 MHz
10 MHz Ref In
Reference
Module
PLL
/10
100 MHz In System
Sync
Master
10 MHz
Sync over TRIG
10 MHz TTL TRIGs
100 MHz Out
– Common 10 MHz reference
shared via front panel
10 MHz
100 MHz In Slave
Sync
Digitizer Sync over TRIG
– Alignment of 10 MHz reference
required
Chassis 2, Slave
– Within each chassis, backplaneExternal SMB
Splitter
triggers used
10 MHz OCXO
10 MHz Ref In
Equal Length SMB
Cables
– Provides synchronous
frequency and timing but need
more for phase coherency
Reference
Module
PLL
/10
100 MHz In Group
Sync
Master
10 MHz
Sync over TRIG
10 MHz TTL TRIGs
100 MHz Out
10 MHz
100 MHz In Slave
Sync
Digitizer Sync over TRIG
© Keysight
Technologies 2015
Page 18
Agenda
© Keysight
Technologies 2015
Overview of Multi-Antenna Techniques
Multi-Antenna Test Challenges
Calibration of a Multi-Antenna System
Measurement Examples
Page 19
Calibration of Phase Coherent Measurement Systems
Why is this important?
– Even when using a shared LO, there
will be some static time and phase
skew between instrument channels in
the measurement system
– Time and phase variations due to
cables, connectors, and signal
conditioning
Correcting out these offsets will ensure
that any measured differences is due to
the signal/device under test and not the
test equipment
Uncorrected
Corrected
using Keysight LTE-A
Multi-Channel Reference
Solution Tools
© Keysight
Technologies 2015
Page 20
Calibration of Phase Coherent
Signal Generation System
The End Goal – Perfectly Aligned in Time
and Phase
– Determine channel to channel time and
phase differences and apply corrections
– Apply known signal to all channels and
measure each channel relative to master
source
– Collect BB time and phase differences
between channels
– Repeat for each frequency, amplitude, and
sample rate
– Apply time and phase corrections to multichannel phase coherent measurements
– Keysight has helpful tools to automate this
process and allows for integration of deltas
for design verification
© Keysight
Technologies 2015
Page 21
Multi-Channel Calibration Methods
Traditional Source
Calibration
VSG 1
VSG 2
VSG 3
New Source
Calibration
VSG 1
4chan
Scope
VSG 4
• New Patent-Pending MultiChannel Calibration Method
VSG 2
4 Way
combiner
VSA
VSG 3
VSG 4
Analyzer
Calibration
VSA 1
– Automated for faster tests
– Less HW & SW lowers the cost
–
VSG
4 Way
Splitter
VSA 2
VSA 3
For best results minimize cable length, keep cable
lengths the same, and select high quality splitter,
connectors, and cables.
VSA 4
© Keysight
Technologies 2015
Page 22
Tips for True Phase Coherent Measurement Systems
Minimize Phase Variations and Calibrate
1. Use common LO Source
2. Use phase stable LO
3. Use low phase noise LO
4. Use reference clock synchronization to ensure
waveforms and acquisitions start at the same time
5. Calibrate out any time and phase skew between
generator channels or between analyzer channels
6. Calibrate out any losses and mismatches due to cables
and connectors
© Keysight
Technologies 2015
Page 23
Agenda
© Keysight
Technologies 2015
Overview of Multi-Antenna Techniques
Multi-Antenna Test Challenges
Calibration of a Multi-Antenna System
Measurement Examples
Page 24
Test Challenge: Complicated Test Setup
LTE-Advanced Multi-Channel Reference Solution
Software
Accelerate LTE-A designs and gain deeper insight faster
Multi-Channel
Configuration &
Correction Utilities
Signal Studio
N7624B & N7625B for
LTE/LTE-A FDD & TDD
89600 VSA & WLA Software
(Optional X-Series measurement
applications for SISO analysis)
Hardware
Available Configurations
• Analyzers, sources or both
• 2, 4, or 8 channels
• Time synchronized or
phase coherent
• Chassis, controller, cables
• V2802A LO distribution
Test stimulus
from VSGs
Device
under test
Signal to be
analyzed by VSAs
© Keysight
Technologies 2015
Page 25
Test Challenge: Visualization & Validation of Signals
Test Challenge: Characterizing the full set of
LTE/LTE-A tests, including 8x8 MIMO, cross
channel measurements and multi-antenna
beamforming, which typically includes 4 or 8
antenna elements
Solution
89600 VSA Software
Spectrum measurements like SEM, OBW, power, spurs,
ACPR
Isolation/cross-channel skew crosstalk, time alignment error
(TAE)
MIMO and beamforming measurements with beamforming
weightings per antenna
eNB
Modulation quality with EVM per layer
LTE/LTE-A FDD/TDD UL & DL, WLAN including 802.11ac
M9391A VSAs provide up to 8
LO locked phase coherent, time
aligned analyzers, 160 MHz BW
© Keysight
Technologies 2015
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8x2 BF modulation analysis example
MIMO/BF – Downlink
89600 VSA SW
© Keysight
Technologies 2015
Page 27
Testing the Receiver Side
Test Challenge: Ability to generate complex MIMO and
beamforming signals that have relative phase, time and
frequency characteristics to adhere that the latest wireless
standards
LO Distribution
Solution
or
or
Signal Studio
– Support for the latest WLAN and cellular
multi-channel signals including 802.11ac and
LTE-Advanced TDD and FDD
Up to 8x8 LO locked phase coherent,
time aligned generators, 160 MHz BW
– MIMO signal generation solutions that can be used
with RF fading simulators
– Select transmission mode including diversity,
spatial multiplexing, beamforming up to 8x8
MIMO and carrier aggregation
– Define, generate and export waveform files
– Can be combined with carrier aggregation
© Keysight
Technologies 2015
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From Simulation to Manufacturing
LTE design flow
– Model new LTE/LTE-A and
WLAN 802.11ac MIMO designs
– Verify interpretations of the
standard
– Rapid proto-typing and
integrated measurements
IP Reference Libraries
– Use in design simulation,
verification and testing
4G LTE-Advanced, LTE
3G HSPA+, WCDMA, EDGE, GSM
WLAN 802.11ac/n/a/b/g
WPAN 802.11ad, 802.15.3c
– New 5G libraries available
Test Software
Test Equipment
I/O Lib, ComExpert
89600 VSA
Signal Studio
3rd Party
RF Sources & Analyzers
AWG & Digitizers
Scopes, Logic, Modular
© Keysight
Technologies 2015
Page 29
Case Study – Xidian University, China
Application: Research Verification
Need 8x8 system with wide
bandwidth and phase coherent
channels
Key Test Challenges:
– Complexity and size of integrating 8
sources and 8 analyzers
– Phase coherency between channels to test
beamforming
– Characterizing wireless communications
Solution provides:
– Compact size
– Automatic routing of triggers
– Reduced complexity
8x8 implementation
• 8 PXI VSGs, 6 GHz, 160 MHz
• 8 PXI VSAs, 6 GHz, 160 MHz
• MATLAB
• 89601B VSA SW
• 4 PXI Chassis with 1 embedded controller plus LO
distribution unit (not shown)
© Keysight
Technologies 2015
Page 30
Keysight Solution for MIMO Throughput Test
LTE/LTE-A 4x4 MIMO waveform generation, EVM, throughput test
Throughput test
PXIe VSG M9381A
DUT
SystemVue Software to create
LTE-A FDD or TDD channel &
capture measurements via 89600
VSA software
PXIe VSA M9391A
LTE-A Multi-Channel Reference Solution
• PXI chassis w/ embedded controller
• 4 M9381A PXI VSGs
• 4 M9391A PXI VSAs
© Keysight Tools
• Configuration & Correction
Page 31
Technologies 2015
Configuration and Calibration Demo
Software
Accelerate LTE-A designs and gain deeper insight faster
Multi-Channel
Configuration &
Correction Utilities
Signal Studio
N7624B & N7625B for
LTE/LTE-A FDD & TDD
89600 VSA & WLA Software
(Optional X-Series measurement
applications for SISO analysis)
Hardware
Available Configurations
• Analyzers, sources or both
• 2, 4, or 8 channels
• Time synchronized or
phase coherent
• Chassis, controller, cables
• V2802A LO distribution
Test stimulus
from VSGs
Device
under test
Signal to be
analyzed by VSAs
© Keysight
Technologies 2015
Page 32
Summary
Newer carrier aggregation, spatial
multiplexing, beamforming applications require
tightly synchronized, phase-coherent channels
for accurate test
Channel-to-Channel phase and time alignment
and calibration is critical in the multi-antenna
measurement system
Keysight provides in-depth signal generation
& analysis solutions for advanced technology
& research
© Keysight
Technologies 2015
Page 33
Thank You !!!
Questions and Answers
© Keysight
Technologies 2015
Copyright © 2014 Agilent. All rights reserved
Page 34
LTE / LTE-Advanced Design and Test Solutions
89600 VSA/WLA
Signal Studio
Software
For Signal Analyzers, Scopes, LA
SystemVue and ADS
Signal Analyzers
Signal
Generators
3D EM Simulation
SystemVue (BB)
ADS/GG (RF/A)
Baseband
Generator and
Channel Emulator
RF Module Development
RF Proto
Design
Simulation
RF Chip/module
BTS and Mobile
BB Chipset Development
L1/PHY
RF and BB
Design
Integration
L1/PHY
DigRF v4
FPGA and ASIC
Protocol Development
L2/L3
LTE UE Signalling, RF, Protocol and
Functional Test
Scopes and
Logic Analyzers
PXI Modular
Solutions
RF Handheld Analyzers
Battery Drain
Characterization
Pre-Conformance
System
Design
Validation
System Level
RF Testing
Conformance
Manufacturing
BTS
or
Mobile
Power Measurement
Network Deployment
Manufacturing
Test
Systems for RF and
Protocol Conformance
3-year repair warranty coverage on all instruments
Optional Calibration and Repair Service Plans
© Keysight
Technologies 2015
Page 35