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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 

 

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  

May 5, 2007 

8

 

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 

May 5, 2007 

11 

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 

May 5, 2007 

17 

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 

May 5, 2007 

18 

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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