Real-Time MIMO Discrete Multitone Transceiver Testbed

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Real-Time MIMO Discrete Multitone Transceiver
Testbed

• Alex G. Olson, Aditya Chopra, Yousof Mortazavi,
  Ian C. Wong, and Brian L. Evans

Embedded Signal Processing Laboratory The
University of Texas at Austin
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Introduction

• Problem Statement
• Single-channel wireline communications systems
  may not provide sufficient data rates for future
  telemetry applications
• There is an upper limit to the communication bit
  rates depending on certain parameters
• Modulation and coding schemes
• Transmit Energy
• Receiver noise floor
• Transmission Bandwidth
• Proposed Solution
• Use multiple transceivers operating in parallel
  on different wires

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Discrete Multitone Modulation

• DMT is a commonly used modulation scheme in
  wireline communication systems (eg. DSL)
• The Idea
• Divide frequency selective channel into many
  narrowband subchannels
• Data is transmitted over each frequency flat
  subchannel
• FFT (Fast Fourier Transform) is used for
  modulation/demodulation

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MIMO DMT Testbed

• Design Goal Create a 2x2 DMT hardware testbed
• Enables rapid prototyping/testing of new designs
• Provides user with complete control over system
  parameters
• Relatively unconstrained by resource and
  real-time issues
• Can be connected to different cable designs
• Allow visualization of the channel parameters and
  various communication performance metrics
• Benefits of Hardware Testbed
• Configurable User can choose system parameters
  and signal processing blocks
• Allows evaluation of the communication
  performance and computational complexity
  tradeoffs
• Cable modelling not required
• Design Challenges
• Real-time constraints on the transmitter and
  receiver system
• Analog front-end

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MIMO DMT System Model
NEXT Near End Crosstalk Portion of the
transmitted signal that leaks onto the local
receiver
FEXT Far End Crosstalk Portion of the
transmitted signal that leaks onto the remote
receiver
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MIMO DMT System Model
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Modem Implementation- Hardware
PXI Backplane - PXI-1045
TX0
TX1
RX0
RX1
Embedded PC PXI-8186
TCP Link
PXI-5421 A/D
PXI-5122 D/A
LPF
LPF
LPF
LPF
H
H
H
H
LPF Low Pass Filter H Hybrid
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Modem Implementation- Software

• Real-Time Target
• Baseband processing - C Dynamic Link Library
  (DLL)?
• Generates/processes samples sent/received to/from
  NI hardware
• LabVIEW VIs
• Accesses hardware and calls DLL functions
• Real Time OS ETOS is running on target to
  ensure
• reatime performance
• Desktop PC
• Connects to real-time target via TCP/IP link
• Visualization and control using LabVIEW

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Bit Allocation

• Fixed amount of energy available to transmit per
  DMT symbol
• DMT allows different number of bits transmitted
  on each tone
• Bit Allocation can be adjusted to maximize
  throughput or the SNR margin on each tone
• Hughes Hartog Bit Allocation algorithm D.
  Hughes-Hartog,1987 implemented in the testbed

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FEXT Cancellation

• Far End Crosstalk provides significant amount of
  deterioration to the bit rate
• Using vectored DMT Ginis and Cioffi,2002 the
  multiple receivers are assumed to operate
  together to cancel crosstalk
• Crosstalk can be cancelled through linear and non
  linear receivers
• The Zero Forcing equalizer is an example of a
  linear receiver with crosstalk cancellation
• Successive Interference Cancellation is a non
  linear crosstalk cancellation receiver

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Vectored DMT

• Uses channel estimate and both received signals
  to effectively cancel crosstalk

Estimate channel matrix H
Training (per-tone)?
For each tone, H, Q and R are 2x2 matrices
H Q R
Symbol decoding (per-tone)?
Q
R
y0
Successive Interference Cancellation
Slicer
QHY
y1
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Experimental Results

• System Parameters
• 256 tones per DMT symbol
• Maximum Transmitted Voltage 5.0V
• Receiver noise floor -60dB
• 1000ft CAT-5 cable
• Inter-twisted pairs for maximum FEXT
• FEXT limits SNR to 10dB

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Experimental Results
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Conclusions

• FEXT (crosstalk) can be effectively cancelled
  with Vectored DMT
• Experimental tests show that a 2x2 Vectored DMT
  system achieves 1.99x data rate (4Mbps) over
  single-channel DMT
• Vectored DMT is practical and has low
  implementation cost (for 2x2 MIMO systems)
• Hughes-Hartog bit loading (fine gains) can
  provide 100Kbps data rate improvement over
  uniform gain

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• D. Hughes-Hartog, Ensemble modem structure for
  imperfect transmission media. U.S. Patents Nos.
  4,679,227 (July 1987), 4,731,816 (March 1988),
  and 4,833,706 (May 1989)
• G. Ginis and J. Cioffi, Vectored transmission
  for digital subscriber line systems, IEEE J.
  Select. Areas Commun., vol. 20, no. 5, pp.
  1085-1104, Jun. 2002

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Hardware

• National Instruments
• PXI Chassis
• Embedded Computer
• A/D and D/A boards
• LabVIEW Real-Time
• Hardware LabVIEW Real-Time form a 2x2 MIMO DMT
  modem

Equipment required for a uni-directional system
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Analog Front-End

• Hybrid circuits from Texas Instruments
• 4 x 50
• Line Driver / 2-wire to 4-wire Interface
• Custom passive analog filters from TTE
• 4x 275
• Serve as anti-aliasing filters for TX and RX

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Modem Implementation
Digitizer
PXI Chassis
ARB
ARB
EmbeddedPC
TX 0
TX 1
RX 0
RX 1
TCP
LPF
LPF
LPF
LPF
ARB Arbitrary Waveform Generator
Digital to Analog Converter Digitizer
Analog to Digital Converter LPF Low pass
filter (anti-aliasing filter)?
H
H
H
H