Andy Pye spoke to David Hall of National Instruments about test engineering requirements for meeting the future speed goals of wireless technology.
Within a decade, the number of connected devices is predicted to outnumber connected people by 10 to 1. As a result, future wireless standards are evolving to address connecting things instead of merely people.
The International Telecommunication Union’s (ITU’s) vision for International Mobile Telecommunications in 2020 (IMT-2020) outlines one of the clearest requirements for future wireless standards. Designed as a framework to communicate the technical requirements of 5G, it outlines three distinct use cases (Fig 1), which reflect the changing requirements for technologies such as 802.11ad, 802.11ax, Bluetooth 5.0 and NFC=.
The first wireless use case, Enhanced Mobile Broadband (eMBB), defines the evolution in network capacity and peak data rates expected from a future wireless technology. eMBB technologies will drive higher peak data rates through a combination of wider bandwidths, higher-order modulation schemes and MIMO/beamforming technologies. Specifically for 5G, the eMBB use case is designed to deliver up to 10 Gbps of downlink throughput, which is 100x that of single-carrier LTE.
The second use case, Massive Machine-Type Communication (mMTC), is designed to deliver wireless access to more devices in more locations at a lower cost. By linking together more devices in more locations, mMTC technology will connect traffic lights, vehicles and even roads in a smart city. In the short term, the need to cost-effectively connect more devices in more Industrial IoT applications is driving new mobile technologies such as LTE for M2M Communications and narrowband IoT (NB-IoT).
The third and final use case is Ultra-reliable Machine-Type Communication (uMTC). In this scenario, two key requirements of the wireless connection are latency and packet error rates. Examples would be a doctor performing remote surgery using a robot connected via wireless; or an car avoiding a massive pileup by communicating the existence of an accident ahead. In both of these applications, the reliability of the wireless communications link is not just a convenience, but a lifesaver.
Vector Signal Transceiver
In 2012, NI announced the PXI Vector Signal Transceiver (VST). It was unique in that it combined a 6GHz RF signal generator, 6GHz RF signal analyser and a user-programmable FPGA into a single PXI module. Not only could the instrument’s RF performance allow it to serve applications from R&D to manufacturing test, but its user-programmable FPGA enabled applications ranging from measurement acceleration to channel emulation.
However, the evolution of wireless technology now demands more: NI has this month introduced a second-generation VST that offers wider bandwidth, extended frequency range and a larger FPGA in a smaller form factor. The NI PXIe-5840 module is claimed to be the world’s first 1GHz bandwidth VST and is designed to solve the most challenging RF design and test applications.
The NI PXIe-5840 combines a single two-slot PXI Express module, a 6.5GHz RF vector signal generator, 6.5GHz vector signal analyser, high-performance user-programmable FPGA and high-speed serial and parallel digital interfaces. With 1GHz of bandwidth, the latest VST is ideally suited for a wide range of testing applications, including 802.11ac/ax devices, mobile/Internet of Things devices, 5G designs, radio frequency integrated circuits (RFICs) and radar prototyping.
Over the past decade, wireless standards have evolved to use significantly wider bandwidth channels to achieve higher peak data rates. For example, since 2003, Wi-Fi has evolved from 20, to 40, to 160MHz. Mobile communication channels have evolved from 200kHz to 100MHz. In the future, technologies like LTE-Advanced Pro and 5G will drive this trend even further.
Especially when testing semiconductor devices, the bandwidth requirements of the instrument often exceed the bandwidth of the signal. For example, when testing RF power amplifiers (PAs) under digital pre-distortion (DPD) conditions (Fig 2), the test equipment itself must extract a PA model, correct for non-linear behaviour, and then generate a corrected waveform. Advanced DPD algorithms often require 3X to 5X the RF signal bandwidth. As a result, instrument bandwidth requirements can be up to 500MHz for LTE-Advanced (100MHz signal) and 800MHz for 802.11ac/ax (160MHz signal).
One of the most significant enhancements of the second-generation VST is its wider instantaneous bandwidth: 1GHz. Because of this wider bandwidth, engineers can use the second-generation VST to solve application challenges that currently can’t be met using existing instrumentation.
EVM measurement performance
A second critical requirement of next-generation wireless test instrumentation is better RF performance. With higher order modulation schemes and wideband multicarrier signal configurations, the RF front ends of today’s wireless devices must have better linearity and phase noise.
Therefore, when assessing EVM performance on a wireless device, the RF signal analyser should typically deliver EVM performance that is at least 10dB better than that of the device it is testing. Contemporary 802.11ac devices require EVM performance of -32dB when generating the 256-QAM modulation scheme, which requires the instrument to have -42dB or better EVM performance. In the future, the 802.11ax 1024-QAM modulation scheme will likely push device EVM limits to -35dB and instrument EVM requirements to -45dB (Fig 3).
The second-generation VST uses advanced, patent-pending IQ calibration techniques to deliver outstanding EVM performance for wideband signals. In addition, the modular design of PXI enables engineers with the most demanding EVM performance requirements to improve further on the VST’s native performance. Using a PXI external local oscillator (LO), systems based on the second-generation VST achieve EVM performance of better than -50dB.
Modular, easily synchronised
Modern communications standards ranging from Wi-Fi to mobile use sophisticated multi-antenna technology. In these systems, MIMO configurations provide a combination of either higher data rates through more spatial streams or more robust communications through “beamforming”. Because of these MIMO benefits, next-generation wireless technologies like 802.11ax, LTE-Advanced Pro, and 5G will use more complex MIMO schemes with up to 128 antennas on a single device (Fig 4).
Not surprisingly, MIMO adds a lot of design and test complexity. It not only increases the number of ports on a device but also introduces multi-channel synchronisation requirements. To test a MIMO device, RF test equipment must be capable of synchronising multiple RF signal generators and analysers. In these configurations, the instrument’s form factor and the synchronisation mechanism are critical.
Fortunately, the second-generation VST is small enough that engineers can synchronise up to eight VSTs in a single 18-slot PXI chassis with one slot dedicated to a PXI controller. In addition, the VST offers a range of technologies to tightly synchronise it with either other VSTs or other PXI modules.
Designed by software
Advanced wireless test applications increasingly require engineers to tailor the behaviour of the instrument’s firmware. In these applications, engineers can experience significant improvements in instrument performance, simply by moving closed-loop control, measurement acceleration, real-time signal processing, or synchronous device under test control onto the instrument itself.
Historically, the only way to customise an instrument’s firmware was to negotiate a non-recurring expense fee with the test vendor, which was generally costly. By contrast, one of the unique attributes of NI’s VST design is the instrument’s FPGA: engineers can easily customise the FPGA with LabVIEW, and many engineers can make modifications directly themselves instead of hiring an expert in VHDL or Verilog for this task.
One application that software-designed instrumentation can uniquely solve is radar prototyping (Fig 5). In this application, customers can use the FPGA as a complete target simulator. In radar applications, a radar system detects a “target,” such as an automobile, aeroplane, or other object, by sending a stimulus signal and then waiting for the response. The combination of the VST’s wide bandwidth and user-programmable FPGA makes it ideal for target emulation.
“The combination of extremely wide bandwidth and low latency software-designed instrument allowed us to discover our automotive radar sensors as never before, and even allowed us to identify problems very early in the design phase that were previously impossible to catch,” says Neils Koch, Component Owner Radar Systems, Audi AG. “With the VST and FPGA programmable by LabVIEW, we were able to rapidly emulate a wide range of diverse scenarios, thus influencing safety and reliability aspects in autonomous driving.”
Another application for which modifying the user-programmable FPGA reaps significant benefits is measurement acceleration. When performing measurements like EVM or ACP on a wireless device, the total measurement execution time is dominated by the measurement algorithm. For these measurements, engineers can reduce measurement time by moving the measurement algorithm onto the FPGA. In addition, many of these measurements are often made under DPD conditions. In these cases, engineers can also use the FPGA to develop their own custom, real-time DPD implementations. The faster FPGA-based DPD implementation not only saves critical test time but also allows engineers to embed highly protected FPGA algorithms. By delivering an FPGA bitfile to prospective customers instead of source code, organisations are better able to protect their DPD IP.
Test systems for smart devices
Smart devices, of which wireless systems are an example, are creating an inflection point in automated test for the test organisations challenged with ensuring the quality of these devices at increasingly lower costs, and the vendors that serve them.
WKS Informatik was established in 1994, to develop test benches for different industry sectors. At that time, the company’s expertise was mainly directed at the use and development of the CAN fieldbus. This important mainstay has been later extended with other fieldbuses, including LIN, FlexRay and Profibus.
In this context, the company’s Gateway Test Bench with 64 CAN interfaces (high speed and low speed), developed for a German DAX company, has received wide attention.
“The NI VST has been great in analysing various signals, which we couldn’t analyse with usual signal analysing devices. The range of measurements, starting with bandwith and finishing with power analysis, that we were able to do with the VST has saved us weeks of development and spared our customer additional hardware costs,” says WKS Informatik CEO Ronald Kaempf.
* 1GHz instantaneous bandwidth for advanced digital pre-distortion (DPD) test and wideband signals such as radar, LTE-Advanced Pro and 5G
* Measurement accuracy that enables systems based on the second-generation VST to measure 802.11ac Error Vector Magnitude (EVM) performance of -50 dB
* Measurement speeds up to 10X faster than traditional instrumentation using FPGA-based measurement acceleration and highly optimised measurement software
* Small size and tight synchronisation allowing for up to 8×8 multiple input, multiple output (MIMO) configuration in a single 18-slot chassis
* User-programmable FPGA that engineers can easily design with LabVIEW