Editor's Note: Microwave Journal recently reached out to some of the leading test and measurement companies to learn more about the innovative measurement equipment being deployed to meet the industry's upcoming challenges in the areas of 5G/Massive MIMO, automotive radar/sensors, signal integrity and millimeter wave communications.

Enabling Future T&M with Nonlinear Transmission Line Technology
Walter Strickler, Anritsu Corp., Morgan Hill, Calif

The innovative application of nonlinear transmission line (NLTL) technology in vector network analysis and other test instrumentation has delivered high performance, robust, frequency scalable and cost-effective test solutions. Today the value of that testing innovation has never been greater. The future growth engine for test and measurement is the move toward mass market applications at microwave and millimeter wave (mmWave) frequencies—whether it is for high data rate 5G wireless communications, multi-gigabit per second speed WiGig technology, higher frequency radars for advanced driver assistance systems or even signal integrity applications at wireline data centers. NLTL technology redefines the level of performance and size of instrumentation while reducing the higher costs usually associated with high frequency test and measurement equipment. Anritsu’s patented application of this technology enables the next wave of microwave and mmWave instruments—accelerating next generation product development and lowering production costs with the added portability to be able to install and maintain next generation radio systems.

SRD Harmonic Sampler-Based VNAs

To understand the advantages of NLTL technology, its application in a VNA is explored. Microwave and mmWave VNAs are based on the use of either harmonic mixers or samplers. In traditional sampling VNAs (see Figure 1), short pulse waveforms are used in the VNA receivers to sample stimulus and response signals. These waveforms are also used as harmonic generators to multiply the frequency of internal VNA signals that are used for both the stimulus source and receiver local oscillator. Traditionally, the short pulses are generated with a step-recovery diode (SRD) circuit. However, VNAs utilizing SRD-based harmonic samplers face a number of challenges. SRD-based harmonic samplers have bandwidth limitations. In addition, there is often leakage between test channels, limiting the VNA dynamic range. When extending beyond the fundamental bandwidth of the SRD-based harmonic samplers, short- and long-term stability and quality of broadband VNA measurements may also be challenged due to the following:

• Physically large and inhomogeneous measurement structures utilizing discrete components such as reflectometers, receivers, signal conditioning devices, interconnect cables or waveguides
• High-frequency multiplexing schemes
• Complex receiver structures such as harmonic frequency converters and complex LO (local oscillator) distribution networks
• NLTLs can be used in place of the SRD to overcome the limitations and challenges faced by SRD-based systems.

NLTL Technology

In their most basic form, NLTLs consist of high-impedance transmission lines loaded with varactor diodes that form a propagation medium whose phase velocity, and thus time delay are a function of the instantaneous voltage across the diodes. The lower the voltage, the lower the phase velocity and the longer the time delay of a waveform propagating along the nonlinear transmission line. Conversely, the higher the voltage, the greater the phase velocity and the shorter the time delay. When acting on a section of a trapezoidal voltage waveform applied to its input, an NLTL compresses the waveform’s front, resulting in a step-like voltage that is highly rich in harmonics (see Figure 2).

Non-uniform NLTLs further enhance fall time compression, and result in a train of step-like waveforms when driven by a CW signal. By leveraging the fall-time compression characteristics of an NLTL, a train of very narrow gating pulses can be generated at microwave and mmWave frequencies for sampling receivers starting from a CW signal (see Figure 3); the fall times are about 10 times faster than a pulse generated with an SRD. In addition, broadband distributed harmonic generation is achieved by leveraging the “harmonic growth” characteristics of NLTLs. Since two primary functions of any VNA are generating signals and sampling them, NLTL technology is especially well suited for use in such instruments.

Commercialization of VNAs using NLTL

In 2009, Anritsu introduced the VectorStar series of VNAs signifying the first commercial introduction to NLTL technology into a VNA and has continued to apply its semiconductor, component and miniaturization capabilities to include NLTL technology in a complete line, ranging from VNAs with highest levels of performance to solutions for cost-sensitive applications including testing of microwave systems in field applications. Table 1 shows the advantages and benefits of NLTL technology.

Test Innovations Enabled by NLTL Technology

Broad Frequency Range

NLTL-based VNA mmWave systems combined with external mmWave modules now enable measurement of frequencies from 70 kHz to 110/125/145 GHz in single coaxial connection. These systems are excellent for device characterization and high data rate signal integrity applications. In addition, the small size and lightweight, high-frequency reflectometers enhance maneuverability and probe positioning in applications such as on-wafer measurements and near field scanning of antennas and circuits. This enables direct connection to the DUT, broadband dynamic range of 107 dB to 110 GHz and calibration stability of 0.1 dB and 0.5° for S₂₁ over 24 hours.

Cost-Effective E-Band VNA

A low cost NLTL-based VNA family consists of a base chassis and small tethered source/receiver modules capable of measuring from 55 to 92 GHz. It is an integrated system ready to operate right out of the box providing high value and enabling mass market production of E-Band components.

Handheld Cable and Antenna Analyzer up to 40 GHz

Microwave handheld analyzers offer field technicians, engineers and wireless network installers industry-leading dynamic range, directivity and durability so that they can conduct highly accurate measurements during the installation, maintenance and troubleshooting of microwave communication systems operating to 40 GHz.

Ultraportable Instrumentation

An ultraportable spectrum analyzer sets a new standard for cost, size and performance associated with traditionally large form-factor instruments to more efficiently advance technology development. A pocket sized VNA with good performance including industry leading dynamic range, sweep speed and amplitude accuracy has been developed with this technology. Its ultraportable size enables direct connection to almost any DUT, eliminating the need for cables or antennas. The Anritsu MS2760A is the first handheld mmWave spectrum analyzer to provide continuous coverage from 9 kHz up to 110 GHz in this form factor (see Figure 4). It is poised for use in the growing 5G network development market, as well as other fast growing mmWave applications, like 802.11ad/WiGig, E-Band microwave wireless communications, satellite communications, electronic warfare and automotive radar.

A new power meter is the first frequency selectable mmWave power analyzer. It is an ultraportable USB-powered instrument that measures the RF power of signals up to 70 GHz and as low as -90 dBm. Unlike spectrum analyzers that are bulky, expensive and complex, or power meters that are not frequency dependent selectable and have limited dynamic range, this unit enables simple, numeric, frequency-based amplitude measurements of up to six signals from 9 kHz to 70 GHz in a package slightly larger than a cell phone and at an extremely affordable price.

CONCLUSION

Nonlinear transmission line technology redefines the level of performance and size of instrumentation while breaking down the cost barriers usually associated with high frequency test and measurement equipment. Anritsu’s patented application of this technology enables the next wave of microwave/mmWave instruments by accelerating next generation product development and lowering production costs — with the added portability to easily install and maintain next generation radio systems.

5G Coexistence in a Satellite World
Greg Jue, Keysight Technologies, Santa Rosa., Calif.

Today’s sub-6 GHz spectrum is crowded, complex and congested with limited available spectrum. In contrast, centimeter and millimeter wave frequency bands offer the potential for larger swaths of contiguous spectrum for 5G high data throughput applications. For this reason and more, regulators are opening up more spectrum. In July 2016, the U.S. Federal Communication Commission (FCC) allocated 11 GHz of spectrum for wireless broadband in the high-band spectrum to enable rapid development and deployment of next generation 5G technologies and services.¹ This includes 3.85 GHz of licensed spectrum and 7 GHz of unlicensed airwaves: Upper Microwave Flexible Use service in the 28 GHz (27.5 to 28.35 GHz), 37 GHz (37 to 38.6 GHz) and 39 GHz (38.6 to 40 GHz); and a new unlicensed band at 64 to 71 GHz.

Sharing licensed spectrum is a key element of future policy, as evidenced by the FCC announcement. The 28 GHz band is an existing Fixed-Satellite Services (FSS) licensed band for Earth-to-space applications.²

The potential sharing of spectrum between satellite and 5G applications poses questions on how they might peacefully coexist. This may become an increasingly important area of research in uncovering potential issues. A flexible testbed to explore many different coexistence signal scenarios in the R&D lab environment could be beneficial, before actual field testing and deployment of hardware systems.

This article shows a new testbed that can be used in the 28 GHz frequency band to explore potential 5G and satellite coexistence scenarios. Flexibility in generating signal scenarios is enabled by using simulation software, combined with wideband high-frequency test equipment. A case study will be explored with candidate 5G waveforms and satellite waveforms in the 28 GHz frequency band to evaluate their coexistence for several different scenarios. The testbed is scalable, and can also be used for sub- 6 GHz coexistence case studies as well as mmWave applications.

Coexistence Testbed

To generate the wideband 28 GHz satellite and 5G candidate test signals, a vector PSG with wideband IQ inputs is used in combination with a wideband precision arbitrary waveform generator (AWG). Figure 5 shows the testbed that will be used for the 28 GHz case study. The AWG generates I and Q, which is modulated on to the approximate 28 GHz carrier frequencies using the vector PSG. This combination of the AWG and the vector PSG can generate test signals up to 44 GHz with up to 2 GHz of modulation bandwidth. The test signals are analyzed using either a 50 GHz signal analyzer with 1 GHz of bandwidth, or with a 33 GHz oscilloscope. Design simulation software is installed on the embedded controller for the AWG. This will be used to generate the coexistence scenario which will be examined next.

28 GHz Coexistence Case Study

The design simulation schematic in Figure 6 was used to generate the 28 GHz coexistence signal scenario. For this coexistence scenario, a wideband amplitude and phase-shift keying (APSK) waveform was used as an example satellite waveform. The APSK simulation signal source is shown in the upper-left of Figure 6. For the 5G candidate waveform, a wideband custom orthogonal frequency-division multiplexing (OFDM) waveform was used, and is shown on the lower-left of the figure. For both of these simulation sources, there are a number of parameters that can be set to configure the waveform characteristics. Both of these waveform types were chosen to illustrate a coexistence scenario concept, but the user can replace them with other types of waveforms for the actual application of interest.

A signal combiner element in simulation design software is used to resample and combine the satellite waveform and 5G candidate waveform. This element is an enabler in combining multiple input waveforms with different center frequencies, bandwidths and sample rates to create a single composite output waveform that can be downloaded to test equipment to generate coexistence test signals.

The AWG downloader element is used on the right of the simulation schematic. The I and Q of the composite complex waveform is automatically downloaded to the AWG upon completion of the simulation. The I and Q outputs of the AWG are routed to the wideband rear-panel IQ inputs on the vector PSG signal generator to modulate them onto an approximate 28 GHz carrier frequency.

The resulting test signal is shown in Figure 7. The custom OFDM waveform is shown on the left of the spectrum display, and the satellite APSK signal is shown on the right. For this scenario, the center frequencies and bandwidths were set such that there is sufficient guard band between the two signals.

The coexistence for this scenario is demonstrated by using vector signal analyzer software on the test equipment to demodulate the custom OFDM waveform. For this scenario, the OFDM constellation looks relatively clean, indicating good coexistence between the two waveforms. The signal scenario is then modified by changing the frequency separation between the two signals. Figure 8 shows that satellite signal is now encroaching on the custom OFDM waveform and there is insufficient guard band between the two signals.

The coexistence performance impact of this scenario can be observed in the VSA measurement. The constellation shows significant dispersion as a result of the satellite signal interfering with it. A closer examination of this interference can be observed by measuring the EVM vs. subcarrier, as shown in Figure 9.

On the left VSA display, the EVM vs. subcarrier on the upper right shows the impact of the satellite signal, especially on the subcarriers near the upper band edge. The EVM result on the lower right corner of the left VSA display shows a relatively high EVM, indicating that there is poor coexistence between the candidate 5G custom OFDM waveform and satellite waveform for this scenario. This EVM number is an average across the entire acquisition time and bandwidth of the signal, but the VSA software can provide a breakdown of errors versus frequency (or subcarrier) or errors vs. time (or symbols).

On the VSA display to the right, the x-axis has been scaled to zoom into the subcarriers impacted by the satellite interferer. The white trace is the average EVM versus subcarrier. It can be observed that it increases significantly at the upper band edge where the satellite signal is interfering with the candidate 5G OFDM signal. The blue and green vertical lines represent the distribution of EVM results at each subcarrier versus symbol.

CONCLUSION

Coexistence may become an increasingly important topic of interest as 5G continues to evolve, as well as spectrum policy. This article discussed a flexible testbed which can be used to investigate potential coexistence issues under various signal scenarios. Although simulation software was combined with wideband test equipment, scenarios could be evaluated in simulation stand-alone (e.g., coexistence impact to simulated bit error rate). Although not shown in this article, this testbed has also been used to generate coexistence signal scenarios in the 39 GHz frequency band. A demonstration video can be viewed from the video link at www.keysight.com/find/5G.

1. Press Release: “FCC Takes Steps to Facilitate Mobile Broadband and Next-Generation Wireless Technologies in Spectrum Above 24 GHz,” https://apps.fcc.gov/edocs_public/attachmatch/DOC-340301A1.pdf.
2. FCC-16-89A1.docx – Federal Communications Commission.

RF Hardware-in-the-Loop Technologies Drive Embedded System Test
David A. Hall, National Instruments Austin, Texas

As wireless technology continues to become more embedded in “mission critical” applications like automotive radar and machine-to-machine communication, new test practices are emerging to ensure system reliability. One emerging test technique for RF-enabled systems is hardware-in-the-loop (HIL) testing—a category of “real-time” testing. The principle of HIL test, the idea that test engineers build test systems that can emulate physical systems typically present in the real world, is not new. However, the growing prevalence of RF technology in mission-critical systems like automotive radar has made HIL testing an increasingly common test technique for wireless applications.

Architecture of an HIL Test System

The idea of HIL testing first emerged as a mechanism for testing complex control systems in both the aerospace and automotive industries. In a typical application, an engineer might use a HIL system to test an engine’s electronic control unit (ECU) by using the HIL test system to model the electrical signals and behavior of an engine. In this application, the test system combines analog and digital analog interfaces for input and output (often referred to as “I/O”) along with deterministic processing elements to effectively emulate the system (see Figure 10).

In recent years, the use of RF technology as both a critical sensing and a communications technology has driven many RF system engineers to use a similar test approach. In a typical RF HIL test system, relatively low-speed analog-to-digital converters (ADC) and digital-to-analog-converters (DAC) have been replaced with a vector signal generator and analyzer. In fact, the HIL test systems used in these applications often architecturally resembles a software-defined radio (SDR). However, many of the embedded processing elements remain the same. Common applications for RF HIL systems include channel emulation channel sounding, real-time GNSS simulation, radar target generation and cognitive test.

One of the most exciting real-time test applications is in the testing of automotive radars—both as a stand-alone product and as part of a more complex Advanced Driver Assistance System (ADAS). Modern ADAS technology uses a combination of radar (historically at 24 GHz and increasingly at 77 and 79 GHz), cameras, ultrasound and LIDAR technology to sense the world around them. ADAS designs range in complexity from the more simplistic blind-spot detection warning indicator to fully autonomous driving systems. Although many of today’s critical automotive sensing technologies are relatively old (in fact, the first prototype automotive radars go back to the late 1950s)—improvements in sensor size and cost along with advances in signal processing technology have made autonomous driving systems much more practical.

HIL Testing for Automotive Radar Applications

Testing a radar sensor requires engineers to validate both the electromagnetic and functional characteristics of the device. For example, typical RF measurements might include output power and pulse linearity (measured by demodulating the FMCW output). Equally important are functional characteristics that include range accuracy, range resolution, and the ability to detect an object in the presence of interference.

In order to test both physical and functional characteristics, a growing trend in radar test is to utilize test equipment that can be configured both as a measurement instrument and as a radar target generator. In this case, the test system uses its real-time signal processing elements to essentially trick the radar sensor into believing that it sees an object or target when in fact it merely sees a piece of test equipment. This process, called “target emulation” or “target generation,” uses signal processing to recreate the electromagnetic response of a radar stimulus bouncing off an object.

Figure 11 illustrates two approaches to radar target generation. In the first approach, considered “passive target generation,” a delay line is used to simulate the round-trip propagation time of a radar stimulus. The second approach is considered “active target simulation” and uses an HIL test system to digital recreate the radar system’s environment. In a typical active target simulator, a wideband vector signal analyzer receives the stimulus from the radar sensors. Onboard the instrument, an embedded processor applies a delay to simulate distance and Doppler shift to simulate velocity. Finally, a vector signal generator generates this digitally created radar environment as a signal back to the radar sensor.

For a given application, either passive or active radar target simulation might be preferable depending on the type of object one is trying to simulate. For example, passive simulators tend to better simulate short-range targets, while active simulators can simulate more sophisticated driving scenarios like a lane change or an object crossing the road.

Audi Augments Drive Testing Using HIL Test

NI has been working in conjunction with several leading automotive manufacturers to refine next generation radar test technologies. More specifically, NI has recently been working with Audi AG in Ingolstadt, Germany as they pioneer research efforts to develop fully autonomous vehicles.

The team at Audi has recognized safety and reliability are key concerns with next-generation autonomous vehicles. Because of this, Audi uses HIL testing as a mechanism to simulate an ADAS system’s environment in a laboratory environment. By simulating a typical drive test in the lab, Audi was able to improve the reliability of their embedded software much earlier in the design process—even catching bugs that might not otherwise have been caught until a physical drive test.

Audi’s target emulation system, illustrated in Figure 12, combines NI’s second-generation vector signal transceiver (VST) with specialized up-converters and down-converters designed for the 79 GHz radar band. In this scenario, the VST functions like a software-defined radio and its LabVIEW-programmable FPGA executes real-time signal processing routines to emulate a radar target. Dr. Neils Koch, radar component owner at Audi AG, added that “with the PXI VST, the combination of wide bandwidth and low-latency software has allowed us to identify critical bugs in our radar module that we could not detect before.”

Trends in ADAS

Going forward, the trend in ADAS design is to combine multiple sensing technologies to take advantage of the comparative benefits of each technology. For example, radar is often one of the most accurate technologies for measuring range to an object, even at night or in foggy conditions. However, technologies like camera-based image recognition are more accurate at determining the exact position of an object. By combining inputs from multiple sensors as part of a “sensor fusion” architecture, ADAS systems benefit from an improved ability to sense the world around them (see Figure 13).

The use of sensor fusion, combined with the growing reliance on inherently less predictable algorithms like neural networks to detect objects, will continue to drive the importance of embedded software test long term. In the future, engineers will increasingly integrate radar testing with the testing of other embedded sensors technology in highly synchronized and extremely flexible HIL test beds.

Additional Applications for RF HIL Testing

Although radar target emulation is an HIL application that will likely affect us as consumers in the short term, it is one of only several emerging applications for RF HIL test systems. For example, in the defense industry, engineers use similar test strategies to simulate the environment of advanced electronics. Also, in the wireless industry, engineers are using similar HIL techniques for increasingly complex real-time channel sounding.

Today’s RF HIL test systems bear a striking architectural resemblance to the advanced software-defined radio technology that engineers are using to prototype 5G communications systems. Similar to radar target emulation, wideband RF front-ends and intense signal processing elements are necessary to interpret and react to a stimulus signal in real-time. As a result, many of the same technology innovations that are allowing companies like Nokia, Intel and Samsung to prototype 5G systems are the same technologies used by a much broader set of RF HIL test applications.

CONCLUSION

Increasingly complex systems of systems like autonomous vehicles are quickly rewriting the rules for how test and measurement equipment vendors must design instrumentation. In the past, an instrument was merely a mechanism to report on the analog characteristics of a signal. Today, instruments are being used to report on the functional characteristics of a system as well. Going forward, software is the critical technology that allows engineers to construct increasingly complex measurement systems capable of characterizing everything from the simplest RF component to a fully autonomous vehicle. Software remains a key investment area for test equipment vendors—and the ability to differentiate products with software will ultimately define the winners and losers in the industry.

3D Over-the-Air Testing of 5G Massive MIMO Antenna Arrays
Reiner Stuhlfauth and Corbett Rowell, Rohde & Schwarz, Munich, Germany

5G will apply multiple antenna systems and combine them with enhanced spatial multiplexing to provide data for multiple users, known as Massive MIMO. One consequence is that performance evaluation of radiation patterns cannot be done in a conducted way, so connection over-the-air will be essential. This article presents technical aspects on how to measure three-dimensional antenna patterns using an over-the-air testing setup.

The upcoming 5G standard promises more throughput, capacity and implementation flexibility while achieving lower operational expenses (OPEX). Other goals include ultra-reliable low-latency communications (uRLLC) and massive machine type communications (mMTC). Software-defined networks (SDN) and Massive MIMO multiple antenna scenarios are likely technology choices for achieving these goals.

To obtain the wider bandwidths for higher throughput, 5G systems will use frequencies in the centimeter and millimeter wave ranges. One drawback to this approach is higher free-space path loss. Antenna arrays that provide a much higher antenna gain can compensate for free-space path loss. To maintain the same Rx power at a frequency of 28 GHz compared to 900 MHz means increasing the antenna gain by 30 dB. Using a high number of antenna elements and steering the energy pattern, known as beamforming, can achieve this goal.

Beamforming also significantly reduces the energy consumption by targeting individual user equipment (UE) with their assigned signal. In a base station without beamforming, energy not received by the UE can create interference for adjacent UEs, or, is simply lost.

Current standards like LTE or WLAN employ MIMO to obtain a higher capacity through spatial multiplexing. Multi-user MIMO extends MIMO by sending data to different UEs simultaneously using beamforming. The term Massive MIMO describes the combination of beamforming and spatial multiplexing of many antennas in a dynamic manner depending on hardware configuration and channel conditions (see Figure 14).

Challenges for Massive MIMO

While Massive MIMO offers many advantages, there are also several challenges including:

1. High throughput for fronthaul interface connection
2. Antenna array calibration
3. Mutual coupling between antenna elements
4. Irregular antenna arrays
5. Antenna array complexity

Massive MIMO introduces similar challenges for characterizing signals and measuring antenna array power that cannot be met by the traditional conductive interface with a cable. Meaningful characterization can only be accomplished using Over-The-Air (OTA) testing. Major reasons include: cost, high losses and coupling experienced at higher frequencies make cable testing unfeasible; and, Massive MIMO systems integrate the radio transceivers into the antennas, which results in the loss of RF test ports. What are the consequences of this paradigm change?

3D OTA Measurements

In the past, power was measured as a function of time, spectrum or code (CDMA systems). The addition of beamforming adds another dimension: Space or power vs. direction of departure. Figure 15 gives an example of a power measurement. OTA measurement parameters can be divided into two general categories: R&D and certification or conformance testing for more complete investigation of the DUT radiated properties, and production for calibration, verification, and functional testing.

The primary test parameters for antenna designers include gain patterns, radiated power, receiver sensitivity, transceiver/receiver characterization and beam steering/beam tracking. Each has its own implications for OTA measurements. Beam steering/beam tracking is of special interest; however, because of the frequencies employed by Massive MIMO. Although static beam pattern characterization is used for existing cellular technology, mmWave systems will require dynamic beam measurement to characterize beam tracking and beam steering algorithms accurately.

Production Testing

Conformance and production testing have many aspects. Three that are of particular importance include:

• Antenna/relative calibration: To form beams accurately, the phase misalignment between RF signal paths must be less than ±5°. This measurement can be performed using a phase-coherent receiver to measure the relative difference between all antenna elements.
  Five-point beam test: According to 3GPP, the active antenna system (AAS) manufacturer specifies a beam direction, maximum EiRP, and an EiRP threshold for each declared beam. In addition to the maximum EiRP point, four additional points are measured at the declared threshold boundary, i.e., a central point with highest EIRP and the remaining four points declaring the left, right, top and bottom boundary, as Figure 16 illustrates.
• Final functional tests: Performed on the completely assembled unit in production, this can consist of a simple radiated test, a five point beam test and aggregate transceiver functionality, such as an EVM measurement of all transceivers.

Near and Far Field Measurements

OTA measurement systems can be classified according to which part of the radiated field is being sampled. Figure 17 illustrates the near and far fields from a base station antenna array (eight circular microstrip antenna patches at 2.70 GHz with uniform excitation). The near field and far field regions are defined by the Fraunhofer distance R = 2*D2/λ, where D is the maximum antenna aperture or size. In the near field region, at distances less than R, the field consists of both reactive and radiated components; whereas the far field of an antenna has only the radiated component.

Precise phase and magnitude measurements over a three-dimensional surface surrounding the DUT are required for the mathematical transformation to the far-field region, resulting in the antenna 2D and 3D gain patterns. A measurement in the far field region needs only the magnitude to calculate the beam pattern of the antenna and can be measured at a single point in space, if desired.

For small devices (in terms of wavelengths), such as UEs, the required chamber size for far field conditions is dominated by the measurement wavelength. For larger devices, such as base stations or Massive MIMO, the required chamber size may become very large. Chamber sizes can be reduced significantly as long as the measurement system accurately samples the phase and magnitude of the electromagnetic field on the entire enclosing surface.

Measuring in the far field region requires a direct measurement of the magnitude of the plane waves and such chambers are generally quite large where the length is set by a combination of the DUT size and the measurement frequencies.

Although the far field is generally measured at a suitable distance from the DUT, it is possible to manipulate the electromagnetic fields such that a near field chamber can be used to directly measure the plane wave magnitudes. There are two techniques:

• Compact range chambers, which are most often used for large DUTs such as aircraft and satellites; and,
• Plane Wave Converter (PWC): A planar wave is created at the DUT is to replace the measurement antenna with an antenna array. Similar to using lenses in an optics system, the antenna array can generate a planar far field at a targeted zone in the region of the DUT.

Near Field Measurements

Measurements in the near field region require both the field phase and magnitude sampled over an enclosed surface (spherical, linear or cylindrical) in order to calculate the far field magnitude using Fourier spectral transforms.

This measurement is usually performed using a vector network analyzer such as the R&S ZNBT20 with one port at the DUT and the other port at the measurement antenna. For active antennas or Massive MIMO, there are often no dedicated antenna or RF ports, so the OTA measurement system must be able to retrieve the phase in order to complete the transformation into far field. There are two methods of performing phase-retrieval for active antenna systems:

• Interferometric: A second antenna with a known phase is used as a reference. The reference signal is mixed with the DUT signal with unknown phase. Using post-processing, the phase of the DUT signal can be extracted and used for the near-field to far-field transformation.
• Multiple surfaces or probes: A second surface volume is used as the phase reference with at least one wavelength separation between the two measurement radii. Instead of multiple surfaces, two probes with different antenna field characteristics can be used. The two probes need to be separated by at least half-wavelength to minimize mutual coupling.

When selecting a vector network analyzer (VNA), true multiport VNAs, such as the R&S ZNBT20, have an additional advantage for measuring coupling between antenna elements. Having multiple receivers—instead of using switches —to perform tests simultaneously reduces test duration and does a better job of performing complete mutual coupling measurements.

CONCLUSION

Antenna arrays will play an essential role in future wireless communication. But challenges in their development, design and production make thorough testing essential to achieving optimal performance. The elimination of RF test ports and the use of frequencies in the centimeter and millimeter wave length region make OTA an essential tool for characterizing the performance of not just Massive MIMO arrays, but the internal transceivers as well. This will drive a high demand for OTA chambers and measurement equipment to measure the strict radiation properties of antennas and transceiver measurements.