COTS Gear Generates Multi-Emitter Test Signals
Sep 27, 2012 1:26 PM Greg Jue
Multiple emitters continue to crowd the airwaves of commercial, industrial, and military applications. Military systems such as radar and electronic-warfare (EW) platforms, for example, must wrestle with signals emanating from a wide range of sources. And checking these systems with test systems that closely resemble the actual operating environment can be difficult and costly. Fortunately, it is possible to use commercial-off-the-shelf (COTS) test equipment to create a practical test laboratory for evaluating the performance of commercial, industrial, and military equipment—even with fairly complex stimuli.
Military electronics systems such as radar and EW systems must constantly operate in environments with many types of signals over a wide range of frequencies. These signals can range from standard cellular communications and satellite-communications (satcom) signals to intentional jammers from unfriendly sources. Evaluating military electronics equipment under a variety of emitter test signal scenarios helps to determine how well the equipment will perform when faced with interference. Co-existence between radar and wireless systems, for example, can be problematic.1 It may prove useful to evaluate potential co-existence issues before field deployment, such as how a radar signal may impact 802.11ac WLAN performance and vice versa.
Capturing actual working waveforms in the field and playing them back in a laboratory environment can create realistic conditions for evaluating a military electronics device under test (DUT) with interference signals.2 Such test signals can also be useful in the laboratory for evaluating the performance of equipment under development. However, the test signal source should provide enough flexibility to create a wide range of test-signal configurations and conditions, which may be difficult to capture. For example, the test signal source should provide control of an emitter’s frequency, power, and channel bandwidth (such as the number of subcarriers), even for a complex signal such as in an orthogonal frequency-division-multiple-access (OFDMA) system.
Of course, it is possible to assemble customized test systems capable of generating a wide range of signal environments. But such test solutions are expensive additions to any test lab. A more cost-effective alternative is the use of COTS test equipment for hardware testing in the lab environment. Using the minimum amount of test-equipment resources is desirable both from cost and laboratory space perspectives.
Recent advances in integrating electronic-system-level (ESL) design simulation with precision wideband arbitrary waveform generators (AWGs) have made possible a new approach to creating and analyzing multi-emitter test signals. For example, design simulation arbitrary resampling techniques enable signals with different sampling rates (e.g., multiple radar, wireless communications, and wireless networking signals) to be combined into a single waveform which is downloaded to a high precision COTS AWG to create the multi-emitter test signal (Fig. 1).
As will be shown with several case studies, it is possible to combine ESL design simulation with wideband COTS test equipment to create and analyze wideband multi-emitter test signals. Radar signals will be combined with GSM, EDGE, and WCDMA cellular signals, along with IEEE 802.11ac wireless-local-area-network (WLAN) signals, to create several multi-emitter test signals. These case studies will examine how radar and WCDMA signals can coexist, and how radar and IEEE 802.11ac signals can coexist, and what happens when radar signals interfere with those wireless systems.
Simulation signal sources for wireless communications, wireless networking, and radar waveforms have traditionally been used in electronic design automation (EDA) or ESL design environments to design RF systems and circuits. Traditionally, they have been used for a single signal format, with the simulation time step or sample rate geared for that signal format. For example, an LTE signal simulation would involve having a sample rate at a multiple of 15.36 MHz, such as 30.72 MHz with 2x’s oversampling or 61.44 MHz with 4x’s oversampling. But combining multiple signal types, with different sample rates, can pose quite the challenge.
For example, combining a 10-MHz-bandwidth LTE signal with a 30.72-MHz sampling rate and an IEEE 802.11ac 160-MHz-bandwidth WLAN signal with 320-MHz sample rate previously required determining up-sample and down-sample factors, so as to achieve a common sample rate between the LTE and WLAN signals prior to summing them together in the design simulation environment. Design simulation arbitrary resampling technology addresses this issue by enabling multiple inputs with different center frequencies, bandwidths, and sample rates to be combined and resampled to produce an output sample rate defined by the user.
With an output sample rate that corresponds to an actual AWG sample rate, a simulated waveform can be downloaded to an AWG to create a multi-emitter test signal in a lab environment. To demonstrate, this capability will be used in several examples. The first will combine two radar emitters with LTE, EDGE, GSM, and WCDMA emitters and evaluate coexistence issues between WCDMA and radar signals. The second will combine radar and IEEE 802.11ac WLAN signals to study coexistence issues for the emerging WLAN standard in the C-band radar frequency range.
The first case study involves creating a multi-emitter test signal comprised of radar signals, LTE, EDGE, GSM, and WCDMA signals. The signals will be created in simulation, and then downloaded to a precision AWG to create the multi-emitter test signal on the test bench. Wideband signal analysis will then be performed with an ultra-wideband real-time oscilloscope with vector signal analysis (VSA) software. Modulation-domain analysis, by means of error-vector- magnitude (EVM) analysis, will be performed with an RF signal analyzer with VSA software.
Figure 2 shows in schematic form some of the different wireless and radar signals that can coexist and that would need to be summed to create a realistic interference signal for testing. Each signal type—radar, GSM and EDGE, LTE, and WCDMA—has its own unique center frequency, bandwidth, and sample rate appearing at the input to the Signal Combiner model prior to being downloaded to the M8190A AWG for generation of interference signals. The output sample rate is set to the AWG’s sample rate of 8 GSamples/s to download from simulation to the precision AWG to create the multi-emitter test signal.
The waveform being downloaded to the AWG is a “real” signal representation of the combined “envelope” signals. The schematic diagram shows the different simulation model blocks and the AWG, but does not indicate the time required for processing the waveform information, which may require some time to simulate and download the test-signal data. It should be noted that this signal-generation capability does not occur in real time.
Figure 3 shows the COTS test setup used to create and analyze multi-emitter test-signal environments. The ESL design simulation tool is installed on the AWG’s embedded controller (upper left). The AWG output is analyzed by a 62-GHz high-performance real-time oscilloscope with VSA software (upper right) and an RF signal analyzer with VSA software (lower right). Wideband radar and multi-emitter spectral analysis is performed with the oscilloscope, while demodulation (EVM) of the wireless emitters is accomplished by the RF signal analyzer and VSA software (lower right).
Figure 4’s upper display shows the multi-emitter test signals as measured by the oscilloscope and VSA software. The L-band radar emitter is on the left, followed by the LTE, EDGE, GSM, and WCDMA emitters. The S-band radar emitter is on the right, with a WCDMA emitter sitting within the S-band radar’s bandwidth. Figure 4’s lower display shows the composite time-domain waveform created by combining multiple emitters.
An RF signal analyzer is used to effectively zoom into each of the wireless emitters and demodulate them with the VSA software. Using the 62-GHz oscilloscope with VSA software enables wideband multiple emitter environments to be measured with multi-GHz analysis bandwidths at L-, S-, X-, Ku-, and Ka-band frequencies. Although the COTS test setup of Fig. 3 shows emitters at L- and S-band frequencies, it has also been used to analyze multiple radar emitters centered at 40 GHz and spanning a bandwidth of 2 GHz. To achieve the 40-GHz test signals, the wideband in-phase/quadrature (I/Q) inputs of a vector signal generator (VSG) were modulated with the output I and Q waveforms from the AWG. As Fig. 5 shows, the GSM, EDGE, LTE, and WCDMA emitters are demodulated with the RF signal analyzer and VSA software.
The second case study uses the same COTS test setup to study potential coexistence issues between radar systems and wireless communications signals. The test setup will be used to examine the interaction between a radar emitter and a WCDMA emitter and how well a radar emitter and an IEEE 802.11ac WLAN emitter can coexist.
The multiple-emitter environment shown in Fig. 4 contains two WCDMA signals: one at 2.1 GHz (unencumbered by interferers within its 5-MHz channel bandwidth), and one at 3.4 GHz (sitting within the S-band radar’s bandwidth). Figure 5 shows a measured EVM of approximately 0.85% for the WCDMA signal at 2.1 GHz. Demodulating the WCDMA signal within the bandwidth of the S-band radar shows the impact of the radar signal on the WCDMA EVM performance. The wireless system’s EVM has degraded to approximately 10% (Fig. 6), while the radar signal is also impacted by the presence of the WCDMA emitter.
It may also be useful to investigate potential interference with emerging wireless signal formats such as IEEE 802.11ac WLAN in the C-band radar frequency band. To examine this, a design simulation schematic was created to combine an IEEE 802.11ac WLAN 5.8-GHz emitter with a C-band radar signal (Fig. 7). The IEEE 802.11ac simulation signal source is configured for a 160-MHz bandwidth, while the radar signal source is configured for a 200-MHz LFM chirp bandwidth.
The same COTS test setup was used to create and analyze the test signal; however, in this case the AWG was used to generate differential I/Q outputs which were then fed to the wideband I/Q inputs ports of a VSG. This configuration is used because the 5.8-GHz carrier frequency for the IEEE 802.11ac WLAN signal exceeds the maximum RF bandwidth of the AWG, so the VSG is used to modulate the AWG I/Q outputs on a 5.8-GHz carrier frequency. The VSG can be used for carrier frequencies to 44 GHz. That said, the external wideband I/Q inputs of the VSG are limited to a 2-GHz modulation bandwidth.
The C-band radar signal was set to several different frequencies to effectively “walk-through” the WLAN OFDMA emitter (centered at 5.8 GHz) so that co-existence effects could be analyzed at different frequencies. Figure 8 shows one scenario where the radar emitter’s center frequency was set to 5.6 GHz, where it only slightly overlaps with the 5.-8-GHz IEEE 802.11ac emitter. The EVM of the WLAN was measured at 3.1% for this case.
Figure 9 shows a more severe case for the offending radar emitter, where its center frequency is set to 5.7 GHz, resulting in more overlap between the radar and the IEEE 802.11ac WLAN OFDMA emitter. In this case, the radar emitter destructively impacts the IEEE 802.11ac WLAN emitter’s EVM. Because the VSA software cannot achieve synchronization with the WLAN emitter due to the radar interferer, it is unable to demodulate the waveform.
This approach—using a combination of COTS test hardware and software—can be used to create multi-emitter test signals using ESL design simulation software and wideband AWGs. Advances in ESL design simulation arbitrary resampling technology, combined with the high signal fidelity possible with recent advances in AWG technology, enable wideband multi-emitter signal generation. The flexibility of this approach enables detailed examination of the coexistence between radar emitters and wireless communications emitters using the same test equipment setup.
The results shown here represent work in progress; some of the potential limitations, such as AWG memory utilization and the number/types of emitters are being explored. This approach has also been used to generate multi-radar emitter waveforms at X-band and Ka-band spanning 2 GHz of bandwidth. Although the examples did not demonstrate real-time evaluations, the flexibility gained by combining design simulation with COTS wideband test equipment may be attractive in addressing emerging multi-emitter signal scenarios for cost-effective research-and-development testing.
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