Systems specifically designed for spectrum monitoring applications typically acquire RF signals from a reference antenna or downlink, analyze the shape (or mask) of the acquired signals, and compare the mask of the acquired signals to some user-supplied reference mask or power level. These are the foundational requirements for nearly any spectrum monitoring system. However, since the spectrum monitoring equipment industry is not a high-volume one, many of these systems are built using standard spectrum analyzers originally designed for an entirely different purpose, such as R&D or production testing. The size of the spectrum monitoring marketplace has not supported highly specialized spectrum monitoring functions at a budget-friendly price point. Often, the software or firmware that runs the spectrum analyzer hardware was not designed with off-air spectrum monitoring as a driving use case for the instrument, which results in a clumsy user interface with poor productivity and significant gaps in functionality.

System Architecture

Combining the NI PXIe-5668R 26.5-GHz PXI Express vector signal analyzer (VSA) with low-volume Spectrum Defender software ends this industry paradigm. Users can now enjoy the benefits of a highly specialized and capable spectrum monitoring solution within budget. This powerful, high-volume hardware and low-volume software combination is a natural fit in this application area.

The Solution

Acquired Data Solutions (ADS) of Rockville, MD, is a data acquisition and control company spanning a variety of operational, environmental, and technical domains including RF spectrum, and systems that emulate environments and capture responses for development and operational test. Recently, ADS took advantage of these benefits for a customer engaged in satellite operations for the military/aerospace industry. During the requirements discovery phase, ADS realized that a few additional capabilities would extend the system’s value proposition to real-world users. These additional capabilities fell into two categories: productivity enhancements and signal analysis insights.

The underlying PXI Express hardware and Spectrum Defender software suite could easily manage four channels of simultaneous spectrum monitoring across multiple satellite transponders or polarities, but it was quickly determined that a single operator probably could not carefully monitor all four channels simultaneously. An operator could certainly not recognize if very short-duration/ intermittent anomalous events exhibited time correlation across multiple spectrum monitoring channels.

This limitation led to a newly identified requirement for raw IQ signal recording and synchronized slow motion playback of anomalous events across multiple channels. High-speed disk array hardware and a license for Spectrum Defender’s Recorder option were added to enable slow-motion replay of anomalous events for further investigation, troubleshooting, and interference source identification.

Also during requirements discovery, it was found that users faced a large number of operational tasks in a network operating center (NOC) environment. It seemed unlikely that even one operator could check in on RF mask compliance on more than an occasional basis. This pragmatic use-case scenario indicated that a fully autonomous spectrum monitoring solution was needed with push notifications of anomalous events. Push notifications were implemented through email and SNMP traps (SNMP is a health monitoring technology common in IT-centric environments). A higher-level system health monitoring package would receive and log SNMP traps and OIDs automatically.

Signal Analysis Insights

Unlike traditional spectrum monitoring solutions, the PXI Express VSA and Spectrum Defender software can deliver access to real-time streaming IQ data with no acquisition dead time. In practice, this permits signal analysis not only in the frequency/spectral domain, but also in the time domain, modulation domain, and joint time-frequency domain. A wide range of new analysis capabilities and signal insight become possible when RF signals are captured as a raw IQ series, instead of just conventional frequency domain snapshots.

With access to raw IQ data, users could post-process anomalous event recordings to increase resolution bandwidth in the frequency domain, demodulate signals, add band-limiting filters, and test compliance against a variety of different spectral masks in an iterative fashion.

Finally, adopting the PXI Express platform and Spectrum Defender software gave the user a clear migration path to higher frequency bands (such as the K and Ka band), up to 765 MHz instantaneous capture bandwidths, and advanced analysis features. The VSA delivers a migration path to these features. The figure shows a block diagram of the final system configuration. Note the addition of RF capture capability and push notifications through email and SNMP. With the driver abstraction layers built into Spectrum Defender’s software architecture, the user of this spectrum monitoring system enjoyed a clear system upgrade path, and a long system lifecycle.

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