Customer Solutions
Generating Noise and Interference for DSL Design Validation and Test
Author(s):
John Meade, Telebyte Inc.
Industry:
Telecommunications
Product:
LabVIEW, Modular Instruments, PXI/CompactPCI
The Challenge:
Generating real-world xDSL interference for design validation, troubleshooting, and quality assurance for xDSL modems and digital subscriber line access multiplexers (DSLAMs).
The Solution:
Developing a PXI-based interference simulator that combines the superior analog performance of the NI PXI-5421 arbitrary waveform generator and the flexibility of NI industry-standard instrument drivers and LabVIEW.
Testing DSL in Real-World Conditions
A very popular method for achieving high-speed access to the Internet over conventional telephone lines is using digital subscriber line (DSL) technology. We can implement DSL technology using signaling devices, such as modems and digital subscriber line access multiplexers (DSLAMs). Real-world noise and interference can impact the performance of these signal devices significantly, driving manufacturers to test them during the design and verification phases of product development. Tests involve sending data through a simulated telephone line represented by a local loop simulator and then corrupting the data with noise/interference prior to demodulation. A measurement is then made of the xDSL modem speed that you can support at a given simulated local-loop length. These tests are typically referred to as “reach tests." Common interference noise types include additive white Gaussian noise, near-end cross talk, and far-end cross talk.
Noise/interference generator solutions provided by traditional instrument vendors make it difficult to accurately represent real-world noise conditions, and do not provide a versatile software and hardware platform to meet continually changing demands. For example, different standards of DSL such as ADSL, ADSL+, and VDSL are used in different regions of the world and the international standards that govern the test requirements frequently change.
World-Class Hardware Performance
Reliable simulation of transmission line interference requires hardware with a low noise floor, so we can discern the simulated noise conditions from the noise inherent to the generation system. Using the new NI PXI-5421 100 MS/s 16-bit arbitrary waveform generator, we developed an interference simulator that goes below the -140 dBm/Hz noise floor level required by several International Telecommunication Union (ITU) standards. Crest factor, the ratio of peak voltage to rms voltage of the output signal, typically describes how well the noise/interference generated by the loop simulator approximates a Gaussian amplitude distribution. Using the Gaussian noise algorithms in LabVIEW, we also created a system that exceeded the standard crest factor requirements of five, delivering crest factors in excess of six.
Increasing Flexibility with LabVIEW and the NI PXI-5421
Another challenge with most existing test systems is simulating noise variations that occur in real-world transmission lines. Typically, a short segment of live noise is recorded and later repeated in a loop to obtain longer durations of noise. The longer noise segment is then used to approximate the noise conditions over a transmission line for a bit error rate test. However, the periodic nature of looping these short recordings does not provide an accurate picture of the random noise variations that occur on a real transmission line. To overcome this challenge, we developed patent-pending software algorithms using LabVIEW to generate nonperiodic noise patterns using a finite amount of memory and disk space. When used with the flexible memory architecture of the new PXI-5421 waveform generator, the noise/interference simulator can generate nonperiodic noise from 100 seconds to more than 16 hours.
Our unique approach to testing xDSL signaling devices based on a precision hardware and versatile software platform gives us the ability to provide additional features not available in today’s standard test solutions. Noise types in xDSL networks, such as far-end cross talk, are dependent on the actual loop length, which can vary from a few hundred meters to several kilometers. In comparison, traditional ADSL+ test systems used in Japan deliver the ability to test just a few loop lengths. We combined software developed with NI LabVIEW and the flexible memory and high resolution of the PXI-5421 waveform generator to develop an interference simulator to test loop lengths from 0 to 8,100 m in five-meter increments. Also, the interference simulator can upload custom noise files, such as those recorded from a customer’s real-world field test where nonstandard noise sources are present.
For more information, contact:
John Meade, Vice President of Engineering
Telebyte Incorporated
270 Pulaski Road
Greenlawn, NY, 11740
Tel: (800) 835-3298
E-Mail: jmeade@telebytebroadband.com.
