2 by 2 MIMO Discrete Multitone Transceiver Testbed

Author(s):
Aditya Chopra - University of Texas at Austin
Alex Olson - University of Texas at Austin
Ian Wong - University of Texas at Austin
Brian Evans - University of Texas at Austin
Yousof Mortazavi - University of Texas at Austin

Industry:
Industrial Controls/ Devices/ Systems, University/Education, Research, RF/Communications

Products:
Real-Time Module, LabVIEW, Data Acquisition, PXI/CompactPCI, Modular Instruments

The Challenge:
Prototyping the physical layer of a communications system to rapidly evaluate system design tradeoffs.

The Solution:
Using NI PXI signal generation and digitizer hardware and the NI LabVIEW Real-Time Module to provide hardware and software platforms for the physical layer of a real-time communications system.

The design and development process of a communications often includes tests using a prototype that mimics the operation of the final product. Because development time on the final product is sometimes lengthened due to resource constraints, we sought to create a flexible prototype that facilitated rapid exploration of various design tradeoffs in a discrete multitone transceiver (DMT) modem.

 

The physical layer of a communications system converts data to an analog waveform that can be sent over a wire or other medium. To further our research, our goal was to construct a prototype of the physical layer of a 2x2 (two transmitters, two receivers) high-speed DMT. With this prototype, we could quickly evaluate system design tradeoffs in different environments, while avoiding the time-consuming process of acquiring channel measurements or using rough theoretical models. With NI software and hardware, we circumvented the time-consuming and expensive process of designing and programming custom digital signal processing (DSP) boards.

 

System Design

DMTs process samples in blocks and make use of various signal processing algorithms, such as the fast Fourier transform (FFT). In terms of computation, each block of samples requires multiple FFTs and other significantly more complex operations – all using floating point arithmetic. Thus, the LabVIEW Real-Time Module proved an ideal solution, because it provides high-speed, deterministic execution while natively supporting the NI analog I/O hardware.

 

Signal generation and high-speed digitizer hardware from NI also proved an excellent match. Our application requires the ability to generate and digitize analog waveforms at up to 2 million samples/second and 16-bits/sample simultaneously on multiple channels. Furthermore, it requires that all channels maintain sample clock synchronization as well as all initiate sampling simultaneously.

The NI PXI chassis provided the necessary high-speed backplane to link the analog I/O boards with the high-speed real-time controller. In addition, the NI-TClk synchronization provided the means for all I/O boards to begin sampling at the same time. We used two NI PXI-5421 arbitrary waveform generators, two NI PXI-5122 digitizers, and an NI PXI-8186 real-time controller to accomplish our goal.

 

Although the NI PXI hardware and LabVIEW Real-Time supported our modem, we also needed to visualize its performance during runtime. LabVIEW was an excellent match for our user-interface development needs. With LabVIEW, we quickly constructed a GUI for visualizing key performance data related to the operation of the modem. We dynamically configured the real-time system remotely from the GUI using the shared variable feature in LabVIEW.

In short, we built a functional high-speed modem that transmits more than 4 MB/s using a PXI system for real-time processing and a desktop computer for visualization. By using LabVIEW Real-Time and the PXI hardware, we avoided time-consuming embedded programming and analog circuit design. The only other hardware needed was an analog filter and a hybrid (two-wire-to-four-wire interface) to complete the modem.

 

To read additional material on this application, click here.

 

For more information, contact:

 

Aditya Chopra

 

University of Texas at Austin

 

Electrical & Computer Engineering

 

1 University Station CO803

 

Austin, TX 78712-0240

 

Tel: (512) 775-4506

 

E-mail: adityachopra@gmail.com

 

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