High-Speed Data Streaming for IF and Baseband Signals

As wireless technology becomes more popular, there is an increasing need to monitor the spectrum. Military surveillance, satellite communications, spectrum monitoring, and other signal intelligence applications are a few areas where this need is most apparent. However, monitoring the spectrum frequently involves the difficult task of taking data at high sampling rates for extended periods of time. This paper discusses the technology of high-speed data streaming, how it is used for IF and baseband data streaming, and the hardware and software that enable acquisitions or generations at high sampling rates for hours at a time.

Introduction to Streaming

Numerous applications cannot sustain a sufficient sampling rate for lengthy acquisitions or generations. In these situations, you must compromise by using a slow enough sampling rate to transfer data over the bus or by sampling at the necessary high speeds for the short periods of time that onboard instrument memory can accommodate. Neither sacrifice is desirable. You can address this need through high-speed streaming, which transfers data to or from an instrument at a rate high enough to sustain continuous acquisition or generation. This is accomplished by having a bus with sufficient bandwidth for overall data throughput and a system that stores the entire acquisition or generation waveform. High-speed data streaming of noise mapping signals is a common application because data collection is often desired at high rates for an extended period of time.

Intermediate Frequency and Nyquist Sampling Theory

Intermediate frequency (IF) is a term used often when describing communications or RF applications and signals that require upconversion or downconversion. The IF is the transitional frequency that a signal is shifted to before analog upconversion or after analog downconversion. This signal results from combining the initial signal with a local oscillator in a mixer. Intermediate frequencies are used for data-intensive processing of RF signals in hardware. For the purposes of data streaming, it is important that you treat an IF signal the same as a baseband signal, even though you sample it at a much higher frequency. This high-speed data streaming is made possible by merging high-bandwidth buses such as PXI or PXI Express with large storage systems such as RAID arrays.

When sampling IF or baseband signals, Nyquist Theory still applies. The theory states that a signal must be sampled at least twice as fast as the highest frequency component that must be analyzed. This means the Nyquist frequency is the sampling rate divided by two (f_s/2), but how does the theory translate to practical application? To acquire a signal without aliasing, the signal frequency must be below the Nyquist frequency. Aliasing is not always bad, and you can actually use it to bend the rules of Nyquist, but this is a discussion outside the scope of this paper. Find more information on this topic in the tutorial titled Aliasing and Sampling at Frequencies Above the Nyquist Frequency.

There are two main takeaways for IF signal streaming. First, from an acquisition perspective, you can treat IF streaming just like digitizing a lower-frequency baseband signal; the necessary sampling rate is just much higher. Also, as with any signal acquisition, the sampling rate of the acquisition hardware must be at least twice the frequency of the desired signal.

Signal Intelligence and Streaming Applications

With the growing adoption of wireless communication and increased traffic competing for limited bandwidth, there is a mounting need to monitor channels, locate transmissions, and scan spectrums. A few areas that emphasize this need include military surveillance, satellite communications, and spectrum monitoring. These signal intelligence applications often require the ability to acquire a portion of the spectrum (IF signal) at high sampling rates while streaming the data to hard disk for minutes or even hours at a time. To capture the RF signals, a high-speed digitizer can acquire the IF data from a downconverter. The downconverter operates at the RF frequency and uses one or more mixers to translate RF signals to a frequency range that you can capture with a high-speed digital-to-analog converter. Using two channels of a National Instruments PXIe-5122 high-speed digitizer sampling data at 100 MS/s, it is possible to acquire two IF signals with 50 MHz of bandwidth on each channel. This helps you acquire a total RF bandwidth of 100 MHz.

Once saved, you can postprocess this data in software with a power spectrum or time-frequency spectrogram. You also can stream captured spectrum from disk with an arbitrary waveform generator to simulate a real-world environment.

Historically, the limiting factor for high-speed data streaming has been the bandwidth of the bus between the instrument and the disk storage. In other words, continuous high-rate acquisition or generation is limited by expensive onboard device memory. By combining the high bandwidth of PXI and PXI Express with RAID technology, you can augment device memory with the storage capacity of the RAID system for continuous high-sampling-rate acquisitions and generations.

PXI Express and RAID Array Solution

Streaming high-frequency IF signals to disk requires significant bandwidth. Because it is based on the high-bandwidth PCI and PCI Express buses, the PXI platform enables instruments to stream data to or from sources other than onboard device memory. A PXI/PXI Express digitizer/oscilloscope is able to continuously acquire at a high sampling rate because the high bandwidth of the bus allows real-time data transfer to PC memory or disk at rates up to 1 GB/s, which means you can fetch data before it is overwritten in device memory. The dedicated per-slot bandwidth available on PXI Express instrumentation enables multimodule systems to achieve even higher aggregate data rates.

In the past, only custom hardware that was expensive to build and maintain could serve these applications. However, you now can stream waveforms to disk for signal intelligence applications with commercial off-the-shelf (COTS) PXI and PXI Express instrumentation.

The bottleneck for an acquisition or generation is no longer the bus but actually the reading or writing of the data to the system storage – a hard drive or even a RAID array. This means you can acquire or generate data for long periods of time at the high sampling rates you need instead of compromising your sampling rate or test time. For example, you can combine an NI PXIe-5122 digitizer with 14 bits of resolution with a 12-drive RAID array with 4 TB storage capacity. In this configuration, you can capture data at the maximum sampling rate of 100 MS/s on both simultaneously sampled channels (400 MB/s total data throughput) for more than 2.5 hours. You also can stream to the onboard memory of a PC or embedded PXI controller instead of writing it to disk. In this case, the amount of data storage is limited by the amount of available PC memory. View stream-to-disk and stream-to-memory benchmarks in High-Speed Data Streaming: Programming and Benchmarks.

NI LabVIEW Multithreaded Programming

Multithreaded applications provide the greatest benefits for parallel test and stream-to-disk applications, and using proper programming in streaming applications achieves the maximum performance of PXI Express instruments. You can accomplish this by parallelizing the code. In a streaming application, the two main bus- and processor-intensive tasks are acquiring data from the digitizer and writing data to a file. Knowing this ahead of time, you can divide processes into multiple loops. Data is shared between each loop with the use of a National Instruments LabVIEW queue structure. This is commonly referred to as a producer-consumer algorithm structure.

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Producer/Consumer Loop Architecture with Queue Structure

In the figure, the top loop (producer) acquires data from a high-speed digitizer and passes it to a queue. The bottom loop (consumer) reads data from the queue and writes it to hard disk. In the background, LabVIEW handles the queue as a block of allocated PC memory. This memory block is used as a temporary storage FIFO (first in, first out) for data passing between two loops. In most programming languages, sharing memory among multiple processes requires significant overhead programming. However, LabVIEW handles all of the memory access to ensure that read-write race conditions do not occur. LabVIEW automatically creates independent execution threads for the two while loops, and stream-to-disk applications benefit from this parallel execution because the completion of one task does not hold up execution of the entire program. Find more information on streaming application design in High-Speed Data Streaming: Programming and Benchmarks.

System Configurations

You can use several different configurations for IF and baseband streaming applications. See how to configure your own system.

Links

High-Speed Data Streaming: Programming and Benchmarks
Benefits of PXI Express for Mixed-Signal Test
Example Code, Products, and Benchmarks for IF and Baseband Streaming
IF and Baseband Recording and Playback System Bundles

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