1. LabVIEW Multicore Real-Time Multi-Input Muli-Output Discrete Multitone Transceiver Testbed
Yousof Mortazavi,
Aditya Chopra, and Prof. Brian L. Evans
Wireless Networking and Communications Group
The University of Texas at Austin

2. Introduction Problem Statement
Single-channel wireline communications systems may not provide sufficient data rates for future telemetry applications
Proposed Solution
Use multiple transceivers operating in parallel on different wires (MIMO)
Develop a hardware testbed to emulate data transmission and reception
Analyze communication performance and algorithmic complexity tradeoffs

3. Discrete Multitone Modulation
DMT modulation is used in wireline communication systems (e.g. DSL)
Divide frequency selective channel into many narrowband subchannels
Transmit data over each frequency flat subchannel
Modulate/demodulate multicarrier signal using Fast Fourier Transform

4. MIMO DMT Testbed Design Goal: Create a 2x2 DMT hardware testbed
Enable rapid prototyping/testing of new designs
Provide user with complete control over system parameters
Connect to different cables
Visualize channel state and communication performance
Benefits of Hardware Testbed
Configure system parameters and signal processing blocks
Evaluate communication performance vs. computational complexity tradeoffs
Support many different cables
Design Challenges
Real-time constraints on transmitter and receiver system
Analog front-ends for signal conditioning

5. Modem Implementation- Hardware
PXI Backplane - PXI-1045
Embedded PC
PXI-8106
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

6. Modem Implementation- Software
Real-Time Target
LabVIEW Real-Time Vis
Accesses hardware
Calls DLL functions
C++ Dynamic Link Library (DLL)
Digital discrete-time baseband processing –
Generates/processes samples sent/received to/from hardware
Real-time operating system
Runs on target to guarantee real-time performance
Desktop PC
TCP/IP link to real-time target
Asynchronous visualization and control using LabVIEW

7. Evolution of the Testbed
Spring/Fall LabVIEW 7.1
Researched ADSL modems, MIMO DMT transmission
Developed unidirectional software simulator for DMT communication
Spring/Fall LabVIEW 8.0
Extended software simulator into a bi-directional communication system (1 TX, 1 RX)‏
Spring LabVIEW 8.2
Implemented Vectored DMT and FEXT cancellation algorithms
Developed working prototype running on National Instruments hardware
Integrated hybrid & analog filters into testbed
Fall LabVIEW 8.5
Implemented decision directed adaptive channel estimation, adaptive bit –swapping and gain adjustment algorithms
Extended ‘channel-shortening’ filter to have user selectable number of taps
Various user-interface enhancements such as display of per tone bit-error rates and noise correlation
Spring LabVIEW 8.5
Separation into two chasses and implementation of (asymmetric) bi-directional testbed
Dealt with Echo/FEXT and implemented feedback channel

8. Bit Allocation
Fixed amount of energy available to transmit per DMT symbol
DMT allows different number of bits transmitted on each tone
Adapt bit allocation to maximize throughput or SNR margin on each tone
Hughes Hartog bit allocation algorithm [1987] implemented
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

9. Far-End Crosstalk Cancellation
Far End Crosstalk provides significant deterioration in bit rate
Using vectored DMT [Ginis &Cioffi, 2002] multiple receivers operate together to cancel crosstalk
Other crosstalk cancellation methods
Linear: zero-forcing equalizer
Non-linear: successive interference cancellation
Complexity vs performance issues

10. 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

11. 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 far-end crosstalk
Far-end crosstalk limits SNR to ~10dB

12. Experimental Results SIC – Successive Interference Cancellation
More detailed system parameters
SIC – Successive Interference Cancellation

13. Target CPU Utilization

15. References
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

16. Backup

17. Analog Front-End Hybrid circuits from Texas Instruments
Line Driver / “2-wire to 4-wire” Interface
Custom passive analog filters from TTE
Serve as anti-aliasing filters for TX and RX