Experimental Evaluation of MIMO Channels: Multi-User Characteristics and Temporal Variability

1
Experimental Evaluation of MIMO Channels
Multi-User Characteristics and Temporal
Variability

• Michael A. Jensen, Nicholas Bikhazi, Jon W.
  Wallace
• Department of Electrical and Computer Engineering
• Brigham Young University

2
Summary of Main Results

• Experimental Channel Characterization
• Developed new channel sounder
• Assessed MIMO channel time variation
• Includes new channel metrics
• Investigated multi-user MIMO channels
• Antennas and Propagation Assessment
• Developed models for varying channels
• Assessed time-variation of relevant array designs
• Gained understanding of superdirectivity in
  MIMO systems

3
Publications

• J. W. Wallace, A. Gummalla, and M. A. Jensen,
  Characterization of the temporal variation of
  2.45 GHz MIMO wireless channels in an outdoor
  campus environment, submitted to IEEE Trans.
  Vehicular Technology, July 2005.
• M. A. Jensen, M. D. Rice, and A. L. Anderson,
  Aeronautical telemetry using multiple antenna
  transmitters, submitted to IEEE Trans. Aerospace
  and Electronic Systems, July. 2005.
• N. W. Bikhazi and M. A. Jensen, The relationship
  between antenna loss and superdirectivity in MIMO
  systems, submitted to IEEE Trans. Wireless
  Communications, June 2005.
• M. L. Morris, M. A. Jensen, and J. W. Wallace,
  Superdirectivity in MIMO systems, to appear in
  IEEE Trans. Antennas Propag., Sept. 2005.
• J. W. Wallace, B. T. Maharaj, and M. A. Jensen,
  Experimental evaluation of the MIMO wideband
  channel temporal variation, to appear in
  Proceedings of the 27th General Assembly of
  International Union of Radio Science, New Delhi,
  India, Oct. 23-29, 2005.
• M. A. Jensen and J. W. Wallace, Recent advances
  in antennas and propagation for MIMO systems
  multi-user networks and channel temporal
  variation, to appear in Proceedings of the 2005
  International Conference on Electromagnetics in
  Advanced Applications, Torino, Italy, Sep. 12-16,
  2005.
• J. W. Wallace and M. A. Jensen, Measurement and
  characterization of the time variation of indoor
  and outdoor MIMO channels, to appear in 62nd
  IEEE Vehicular Technology Conference Digest (VTC
  Fall 2005), Dallas, TX, Sep. 25-28, 2005.
• M. A. Jensen and J. W. Wallace,
  Antenna-independent capacity bound of
  electromagnetic channels, 2005 IEEE AP-S
  International Symposium Digest, Washington, DC,
  July 3-8, 2005.
• M. L. Morris and M. A. Jensen, Impact of
  supergain in multi-antenna systems, 2005 IEEE
  AP-S International Symposium Digest, Washington,
  DC, July 3-8, 2005.
• A. L. Anderson, M. A. Jensen, and J. R. Zeidler,
  Differential space-time coding with offset
  quadrature phase-shift keying, 6th IEEE Workshop
  on Signal Processing Advances in Wireless
  Communications (SPAWC 2005), New York, NY, June
  5-8, 2005.

4
New Channel SounderBlock Diagram
SP8T Switch
SP8T Switch
LPF
LO
PC
PA
LNA

500 MS/s A/D
AWG

LO
Trigger
Control
Control
500 MHz Clock
SYNC Unit
SYNC Unit
Rubidium Clock
Reset
Rubidium Clock
Trigger

• Features
• 8 x 8 switched architecture
• 3.2 ms channel capture time
• 100 MHz instantaneous bandwidth
• 2-8 GHz center frequency

5
New Channel SounderHardware
Antenna Arrays
PC A/D
RF/SYNC Modules
Rubidium Clocks
AWG
Sample Clock
Microwave Sources
Transmit System
Receive System
6
Channel MeasurementsIndoor Campaign

• Location
• Indoor NLOS
• Tx Hallway
• Rx Offices/Labs
• Parameters
• 8-Element UCAs
• 2.55/5.2 GHz
• 80 MHz BW

7
Channel Measurements

• Data Records
• Transmitter off
• Stationary (15 s)
• Moving (30 cm/s)
• 2 s _at_ 3 ms (fast)
• 15 s _at_ 25 ms (slow)
• General Results
• Interference negligible
• Low stationary Doppler (lt 1 Hz)
• Variation captured by slow rate

8
Time-Variant MIMO Channel Metrics

• Purpose of Metrics
• Channel classification
• Assess model accuracy
• Link to higher layers

9
Eigenvalue MetricsLevel Crossing Rate/Avg. Fade
Duration
Typical Results ELCR Mean 0.15/l
Std 0.11/l EAFD Mean 0.08 Std 0.08

• ELCR/EAFD indicate
• How quickly MAC/PHY must adapt mod/rate
• Level of precoding required for static mod/rate

10
Eigenvector MetricsAngular Deviation
EAD
Indicates rate Rx (Tx) PHY must update beamformer
11
Capacity with CSI DegradationDegraded Tx CSI

• Observations
• Transmit CSI can be useful for many wavelengths
• Physical distance appears dominant (large-scale
  fading)
• 5.2 GHz benefits more from Tx CSI

12
Capacity with CSI DegradationDegraded Tx and Rx
CSI
Assumption Interference due to imperfect CSI is
iid Gaussian

• Observations
• Electrical distance appears dominant (small-scale
  fading)
• Distance at which drops below 50 max 0.1l

13
Time-variant MIMO Modeling

• Multi-variate Complex Normal (MVCN)

• Time-varying Cluster (TVC)
• Double-directional spatial spectra
• Decompose spectra into clusters
• Track power fluctuations of clusters

14
TVC ModelExample

• Parameters
• Location 1
• 2.55 GHz
• 12 clusters
• Observation
• Bartlett spectra match

15
Model ComparisonsTX/RX CSI Degradation

• Separable MVCN Overpredicts capacity loss
• Captures long-term variation well
• TVC Model Better fit of short term capacity
  loss
• Capacity bias

16
ApplicationAntennas for Dismounted Troops
Helmet/Wearable Antennas
Model

• Elements are
• Omnidirectional
• Directive (cos f pattern)

Omnidirectional
Directive
17
Vest-Worn AntennaCapacity Degradation

• Observations
• Directive elements achieve same perfect CSI
  capacity
• Directive elements offer lower temporal
  variability

18
Channel CharacterizationMulti-User Channels

• Goals
• Obtain real channel information
• Explore channel structure
• Characterize interference environment
• Assess required CSI estimation/distribution
  frequency
• Develop multi-user MIMO channel models

Multi-User Downlink Channel
H1
Rx 1
H2
Tx
Rx 2
19
Multi-User ChannelsIndoor Environment
20
Multi-User ChannelsOutdoor Environment
21
MIMO AntennasCompact Arrays
Portable Device Antennas
Helmet/Wearable Antennas

• Issues for Compact Arrays
• Mutual coupling
• Superdirectivity

22
Example ComputationImpact of Radiated Power
Constraint
y
x
D
NT 8
23
Limiting SuperdirectivityIncluding Antenna Loss
Transmit Array
24
Example ComputationCapacity versus Tx Efficiency
L l/2 NT 16
25
Summary of Achievements

• Experimental Channel Characterization
• Developed new channel sounder
• Assessed MIMO channel time variation
• Includes new channel metrics
• Investigated multi-user MIMO channels
• Antennas and Propagation Assessment
• Developed models for varying channels
• Assessed time-variation of relevant array designs
• Gained understanding of superdirectivity in
  MIMO systems

26
Plans for Upcoming Year

• Time-Varying MIMO Channels
• Apply understanding to assess array designs for
  partial CSI scenarios
• Implement PHY/channel models (and measured
  channel data) into QualNet for network
  simulations
• Analyze additional (outdoor) scenarios
• Use real-time platform to quantify performance
  degradations for mobile channels
• Multi-User MIMO Channels
• Assess overlap of channel null spaces

27
Plans for Upcoming Year

• Array Beamforming
• Use experimental assessment of channel spatial
  characteristics to explore tradeoff between
  beamforming and STC in network context
• Antenna Design
• Measure performance of vest antenna
  (collaboration with Windermere)

28
Plans for Upcoming YearNetwork Simulations
Implement MIMO/PHY models in QualNet Joint
UCSD/BYU activity