1. 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

2. Introduction Problem Statement: Proposed Solution
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

3. 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

4. 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

5. 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

6. Visualization and control
MIMO DMT System Model
Visualization and control
Allow user to select system parameters
Display channel measurements and communication performance metrics
No real-time requirements
Transceiver
Embedded PC running MIMO DMT communication system
Real-time requirements
D/A and A/D converters
Transmission Medium
1000ft CAT-5e cable used as communication medium
Pairs of wires in the cable inter-twisted for maximum crosstalk
Measured FEXT ~ 10dB below signal level
Front-End
Hybrid circuits used for echo cancellation and as line drivers
Low-Pass filters to remove aliasing effects

7. Modem Implementation- Hardware
PXI Backplane - PXI-1045
Embedded PC
PXI-8186
TX0
TX1
RX0
RX1
TCP Link
PXI-5421 A/D
PXI-5122 D/A
LPF
LPF
LPF
LPF H H H H
LPF : Low Pass Filter
H: Hybrid

8. 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
May 5, 2007

9. 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
Unifom Gain Bit Allocation
Equal Power is allocated across all tones
Sub-optimal for frequency selective channels
Hughes Hartog Algorithm
Greedy algorithm
One bit is added to the tone which requires least amount of energy

10. 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
Complexity vs performance issues

11. 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
QHY
Successive
Interference
Cancellation
Slicer y1
May 5, 2007

12. 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

13. Experimental Results
More detailed system parameters

14. 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

16. 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 , Jun. 2002

17. 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
Embedded pc – 2.4G , scratch price
$25,000
Total
$5,500
PXI-5122 1
14-Bit 100 MS/s Digitizer
PXI-5421 2
16-Bit 100 MS/s AWG
$4,000
PXI-8186
Embedded Controller
$5,000
PXI-1045
PXI Chassis
Unit Price
Part #
Qty.
Description
Equipment required for a uni-directional system:
May 5, 2007

18. Analog Front-End Hybrid circuits from Texas Instruments
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
May 5, 2007

19. Modem Implementation PXI Chassis ARB ARB Digitizer Embedded PC TX 0
RX 0
RX 1
TCP
LPF
LPF
LPF
LPF
Analog/Digital/D-A DIVIDE .. D-A in parentheses below tX/rX
ARB: Arbitrary Waveform Generator [Digital to Analog Converter]
Digitizer: [Analog to Digital Converter]
LPF: Low pass filter (anti-aliasing filter)‏ H H H H