FPGAs Improve Throughput in Communications
PrintThe traditional arbitrary waveform generator works much like an audio device -- signals are downloaded into the onboard memory and generated like songs are downloaded and played. However, the complexities of the modern communications signal necessitate more than the standard "download and play" architecture and have led to an arbitrary waveform generation enhancement known as onboard signal processing (OSP).
A signal generator with OSP uses a processing element such as a field-programmable gate array (FPGA) to move the burden of computing communications waveforms from the host computer’s processor to the hardware module. By efficiently creating and processing communications waveforms in hardware, OSP significantly extends waveform playback time and even can save you valuable time by adjusting several output waveform characteristics during generation.
Faster Waveform Downloads and Longer Waveform Playback
A modulated signal for communications applications typically undergoes three stages
of creation -- modulation, interpolation, and upconversion. Modulation represents a message by varying the amplitude, phase, and frequency of a reference signal. The National Instruments Modulation Toolkit for LabVIEW provides several functions and tools for standard analog and digital modulation formats, including AM, FM, FSK, QPSK, and QAM, as well as custom modulation formats. Interpolation, or upsampling, translates a low sample rate modulated signal to a much higher sample rate signal that is ready for upconversion. This step, usually performed in software, even can multiply the overall waveform size by up to 2,048 times. For example, you could interpolate a 16 kB modulated waveform to 32 MB with an interpolation factor of 2,048. Finally, the modulated, interpolated data mixes with a carrier and, thus, upconverts to the required carrier frequency.
Figure 1. The NI PXI-5441 and PXI-5671 signal generators use FPGA technology to dramatically decrease download time as well as increase waveform playback time.
Traditional arbitrary waveform generators need to download the entire upsampled, upconverted signal to onboard memory (see Figure 1). OSP, however, performs the interpolation and upconversion stages in the hardware instead of the software, resulting in dramatically faster waveform computation and smaller waveform sizes. Expeditious processing and smaller waveform downloads save time and enable longer playback times, which improve the statistical significance of many communications measurements and visualizations such as bit error rate, trellis plots, and constellation plots.
On-the-Fly Impairments
For channel effects modeling and receiver robustness testing, the OSP can add several impairments to the signal on the fly during waveform generation (see Figure 2). Modulated data is most often represented by in-phase (I) and quadrature-phase (Q) signals. You can adjust the gain and offset for each of these on the fly. Additionally, you can tune the I and Q carrier phases and the carrier frequency of the output waveform on the fly. With a standard signal generator, changes to the gain, offset, phase (quadrature skew), or frequency of the output signals require stopping generation, recalculating the new waveform, and downloading the new waveform again. With OSP, you instantaneously can make these changes without interrupting the waveform generation, saving valuable test time.
Figure 2. Save test and download time by adding several impairments such as gain, offset, I and Q carrier phase (quadrature skew), and carrier frequency offset in hardware.
Wide-Ranging Applications
OSP enhances the functionality of both the NI PXI-5441 100 MS/s arbitrary waveform generator and the NI PXI-5671 2.7 GHz RF vector signal generator beyond the standard "download and play" architecture. Through OSP, these products provide performance improvements when generating many types of signals, including:
- Quadrature digital upconversion -- Filter, upsample, upconvert, and generate complex I and Q waveform data to a programmable carrier frequency.
- Baseband -- Filter, upsample, and generate baseband signals, such as I and Q, without upconversion.
- Single tone -- Generate sine, square, triangle, ramp, and other waveforms with a 355 nHz frequency resolution.
- Frequency hop -- Take advantage of phase continuous frequency sweeps and hops.
OSP combined with the NI Modulation Toolkit for LabVIEW fosters rapid development of custom applications for research, design, characterization, validation, and test of communications systems and components.
Read an onboard signal processing white paper.
This article first ran in the Q4 2005 LabVIEW Special Edition issue of Instrumentation Newsletter.
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