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: 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

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 – 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

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

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

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

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

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

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

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

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 More detailed system parameters

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, ”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

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

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

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