Ultrawideband radar system design

Ultra wideband (UWB) radar has become increasingly popular in both commercial and defense industries. UWB radars (whether impulse, LFM, noise, or OFDM-based) are defined as having a bandwidth of greater than 0.5 GHz, or more than 20% of their center frequency, and are regulated by FCC rules that allow UWB technology to coexist with existing radio services without causing interference. They offer several advantages, including high accuracy for target detection, good precision for penetrating radars, and low cost for combining radar and communication systems. UWB Radars can pass through walls and other obstacles for geolocation/positioning, and can support multipath immunity and frequency diversity with minimal hardware modifications.

Modern UWB Radar systems often operate in environments that are unpredictable, with interference, jamming and other “real world” performance limitations. Therefore, during system development, it is critical for engineers to understand how their actual hardware will perform in these environments.

Problem
Effective radar system design requires comprehensive system validation, a time-consuming and expensive process often necessitating costly facilities and complex measurement systems. Radar algorithms, such as target recognition and countermeasures, need to be validated early enough to change the signal processing hardware design. Hardware receivers must also be tested with realistic threats and jamming scenarios. Together, these often require outdoor ranges, chambers and real-time hardware simulators costing tens of thousands of dollars per hour.

Unlike communication system designers, UWB Radar system designers face a number of unique challenges, beyond sheer bandwidth. Impulse radar signals, for example, can change shape during propagation (e.g., non-sinusoidal waveforms), while for noise-like radars, just figuring out how to model the noise in the waveform can be challenging. With linear FM systems, generating UWB signals with Doppler frequency offsets, target echoes and clutter to perform receiver verification can be challenging. As a result, designing and testing UWB Radar systems requires a variety of signal sources, target environment setups and measurements. Carefully designed and optimized waveforms are absolutely essential to ensure excellent real-world performance.

Solution
To successfully develop UWB radar systems, today’s system engineers require a more flexible, lower cost means of validation with stimulus/response equipment that is specifically geared toward UWB. That R&D test bed starts with electronic design automation (EDA) software to model a working reference design. The reference design is used to generate test vectors, as well as process received signals that are captured from live measurements, and organize a “model-based design flow.” The test bed also includes a UWB signal generator with a wideband arbitrary waveform generator (AWG) to render simulated signals, including realistic threats and jamming scenarios, for testing UWB hardware receivers. Finally, the UWB test bed should include a wide-bandwidth oscilloscope for waveform capture.

An additional role of the EDA software is to surround the raw radar design and test equipment with the environmental, baseband and RF modeling required to close a round-trip signal processing loop in order to perform early simulation-based verification. As hardware becomes available, the software continues to connect directly into the physical hardware measurement. By leveraging the design tools into verification, a consistent approach is maintained throughout the research and development process, saving time, promoting re-use, and making optimum use of the capital equipment assets.

Agilent Technologies provides a prime example of just such a test system. As shown in Figure 1, the system starts with the SystemVue simulation and modeling environment, which is used with Agilent’s 81180A wideband AWG (upper left), the N8267D PSG vector signal generator (lower right) and the 32 GHz Infiniium 90000 X-Series oscilloscope (lower left). Together, these components allow engineers to carefully design and optimize the UWB signals that are so critical to the design, verification and test of UWB Radar systems.




The test bed shown in Figure 1 supports investigations of UWB architectures, as well as direct connection to test equipment for verification. It can be used to model, encode and download UWB test signals and also post-process received signals. The wide-bandwidth 90000 X-Series oscilloscope allows RF engineers to measure and analyze UWB Radar transmitter outputs using up to 32 GHz of true analog bandwidth, without the need for external down-conversion. This direct approach reduces hardware calibration, system impairments, and measurement system complexity and uncertainty.

Agilent’s E8267D PSG microwave vector signal generator features wideband baseband IQ inputs. When combined with a wideband AWG, such as the 81180A, M9330A or the new M8190A, the PSG provides the flexibility necessary to create microwave and millimeter-wave signals for UWB Radar scenarios, as well as component validation.

With this test system, SystemVue generates and downloads different UWB radar test vectors to the wideband AWG to create the necessary baseband signals. The output differential IQ signals of the AWG are then modulated by the PSG to create an X, Ku or Ka band test signal, to be used directly as an input to a device under test (DUT) for Radar component test. Next, the output of the DUT is captured using the Infiniium 90000 X-Series oscilloscope where radar measurements can be made (Figure 2). Signals can be analyzed inside the Infiniium oscilloscope using the Oscilloscope Signal Analyzer (OSA) or Vector Signal Analysis (VSA) software. For further analysis and signal processing, measured signals up to 32 GHz wide can be brought back to SystemVue with the help of the 89600B VSA software, to close a unique “round trip” signal processing loop (Figure 2). Because of the versatility of this UWB test bed, it can be used for both validation and troubleshooting of UWB transmitters and UWB receivers.



The above is an excerpt from Agilent's application note: Solutions for UWB Radar System Design. The rest of the article covers UWB waveform creation, example of LFM UWB transmitter/receiver signals, and a summary of results. To download the entire application note, head to www.agilent.com/find/powerofx.