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Real-Time MIMO Discrete Multitone Transceiver Testbed

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
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
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
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 00User 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
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
MIMO DMT System Model
Modem Implementation- Hardware
TX0
TX1
RX0
RX1
Embedded PC PXI-8186
PXI Backplane - PXI-1045
LPF
H
LPF
H
LPF
H
LPF
H
PXI-5421 A/D
PXI-5122 D/A
TCP Link
LPF : Low Pass Filter H: Hybrid
Modem Implementation- Software
Real-Time Target Baseband processing - C++ Dynamic Link Library (DLL)00/font> Generates/processes samples sent/received to/from NI hardware LabVIEW VIs Accesses hardware and calls DLL functions Real Time OS 00ETOS 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
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
Slicer
Vectored DMT
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QHY
Successive Interference Cancellation
Estimate channel matrix H
H = Q R
Training (per-tone)00/b>
Uses channel estimate and
both received signals to
effectively cancel crosstalk
y0
y1
For each tone,
0000 0000and 0000are 2x2 matrices
Q
R
Symbol decoding (per-tone)00/b>
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
Experimental Results
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 D. Hughes-Hartog, 00nsemble modem structure for imperfect transmission media.00U.S. Patents Nos. 4,679,227 (July 1987), 4,731,816 (March 1988), and 4,833,706 (May 1989) G. Ginis and J. Cioffi, 00ectored transmission for digital subscriber line systems,00IEEE J. Select. Areas Commun., vol. 20, no. 5, pp. 1085-1104, Jun. 2002
Hardware
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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
$25,000
Total
$5,500
PXI-5122
1
14-Bit 100 MS/s Digitizer
$5,500
PXI-5421
2
16-Bit 100 MS/s AWG
$4,000
PXI-8186
1
Embedded Controller
$5,000
PXI-1045
1
PXI Chassis
Unit Price
Part #
Qty.
Description
Equipment required for a uni-directional system:
Analog Front-End
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Hybrid circuits from
Texas Instruments 4 x $50 Line Driver / 00-wire to 4-wire00Interface
Custom passive analog filters from TTE 4x $275 Serve as anti-aliasing filters for TX and RX
Modem Implementation
Embedded
PC
TX 0
TX 1
RX 0
RX 1
Digitizer
ARB
ARB
LPF
H
LPF
H
LPF
H
LPF
H
ARB: Arbitrary Waveform Generator [Digital to Analog Converter] Digitizer: [Analog to Digital Converter] LPF: Low pass filter (anti-aliasing filter)00/font>
TCP
PXI Chassis
1
Complexity vs performance issues
2
More detailed system parameters
3
Embedded pc 002.4G , scratch price
4
Analog/Digital/D-A DIVIDE .. D-A in parentheses below tX/rX
5

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