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Application of Software Defined Radios in Test & Measurement

Parent Category: 2021 HFE

By Ahmed Hussain and Brendon McHugh

Section I: SDRs, Modular Test Equipment, and SDRs as Modular Test Equipment

What is SDR?

SDR stands for Software Defined Radio, which implies that all important parameters of the radio, e.g. transmit gain, receive gain, modulation scheme, packet size, operating frequency, and bandwidth, can be defined using software. A SDR’s radio front-end (RFE) contains the transmit (Tx) and the receive (Rx) functions to transmit and receive signals over a wide operating/tuning frequency band. Modern and advanced SDRs, such as those developed by Per Vices, are wideband and can provide a frequency tuning range from DC up to 18 GHz for each transmit and receive chain as default, (or further upgraded to 40 GHz). To date, this is one of the highest bandwidth SDRs available in the market.

Today, the most efficient and capable SDRs have a digital backend containing one or more Field Programmable Gate Arrays (FPGAs), with on-board Digital Signal Processing (DSP) capabilities for modulation, demodulation, upconverting, downconverting, etc. The flexible design of SDRs allows the backend to have configurable and upgradeable capabilities for the latest radio protocols, DSP algorithms, etc. Advanced SDRs usually have multiple independent Tx and Rx channels with dedicated Digital to Analog Converters (DACs) and Analog to Digital Converters (ADCs), like in the overview in Figure 1.

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Figure 1 • A high level overview of SDR is shown.

 

What is modular test equipment and why do test engineers need it?

Modular test equipment makes it possible to perform many different test and measurement (T&M) scenarios using a single device with many different module configurations - thanks to a modular design principle. Modular RF test equipment allows for various RF T&M functions and capabilities to be incorporated into one piece of equipment due to its ability to be replaced or upgraded via software and hardware modules.

Currently, with the rapid pace of technology development, the static infrastructure in test and measurement labs becomes an enormous issue for delivering test results in time. This is a challenge for many test engineers since devices, protocols, frequency bands, antenna ports, and several other features and design parameters are being constantly updated and require extensive testing. Another challenge for test engineers is due to most standalone instruments lacking measurement capabilities defined by the latest measurement standards. Some of these devices have a fixed user-interface and firmware that would require significant development time to adapt to a new measurement setup. The massive increase in wireless devices means that testing equipment that can be reconfigured is needed more than ever. PCI eXtensions for Instrumentation (PXI) is a popular modular electronic instrumentation platform used in building electronic test equipment and modular laboratory instruments. PXI is an example of an older and widely used software-defined instrumentation standard for modular and reconfigurable automated test systems. System-level tools for FPGAs is another area of modular test equipment where we are seeing profound growth. Several manufacturers are now including FPGAs on modular instruments and giving engineers access to software, generally through licensing of IP cores, to re-program them according to their needs and requirements. We see a similar trend now in the SDR market.

How can SDRs act as modular testing equipment?

SDRs can be paired with external adapters, attenuators, filters, couplers, coaxial cables, and antennas for various RF testing applications. Several SDRs are now implemented on modular hardware platforms where the RFE hardware can be swapped or customized to enable operation at other frequency bands, wider bandwidths, or other RF engineering parameters, e.g. higher Tx and Rx gains, improved on-board filters and LNAs, or higher output RF power. Having physical layer functions defined in an SDR is extremely useful in test and measurement settings where it is often beneficial to test components, devices, and assemblies in nearly real-world signal conditions either in test bed or laboratory simulation settings. This is often true with modern and extremely complex communication protocols, where actual system performance may be poorly represented by testing in simulated real conditions before realizing that this was a poor test to represent system performance in real-world conditions. Systems requiring real-world testing include modern radars, signal intelligence (SIGINT), 5G mmWave technology, IoT, electronic warfare (EW), and other critical applications where a precise understanding of a specific component, device hardware, software or overall system performance under specific conditions is necessary.

SDRs allow the possibility of running multiple software applications over a single hardware device which can be expanded and customized by the test engineers. A single SDR device can act as scanner, spectrum sweeper, modulator/demodulator, Continuous Wave (CW) transmitter, spectrum analyzer, User Equipment (UE) tester and Base Station (BS) tester simply by using the appropriate software application or by changing FPGA bitstreams. SDRs can be customized to a different form factor for various testing missions such as those limited by size, weight and power (SWaP). The customization is often required to provide mobility and portability to quickly move the test and measurement equipment from one T&M facility to another. The latest SDRs usually have very high data throughput interfaces for testing new capabilities of high data networks such as 5G and cloud computing. Several other benefits of SDRs, with regards to meeting T&M needs, include integration with other devices, reducing timelines, and reducing development costs associated with developing a product from scratch.

Section II: Types of Test & Measurement Needed when Developing RF Systems

Types of compliance and proof of concept testing

Telecommunication certification organizations around the world provide standards for wireless products operating in the licensed and unlicensed spectrum. The equipment requiring approval may include ISM devices, smart phones, base stations, broadcasting equipment, digital transmission systems, marine and aviation communication systems, Land Mobile Radio System (LMRS), point-to-point communication devices, Emergency Position Indicating Radio Beacon (EPIRB), and radars. Evaluating Maximum Permissible Exposure (MPE) and Specific Absorption Rate (SAR) are required for most certifications. Head SAR and Body SAR compliance tests are required for all devices used in mobile communications, for example, smartphones, as shown in figure 2. The regulatory approval tests must show compliance with SAR, TRP and RF field strength limits as determined by the FCC, 3GPP, ETSI or country’s national communication regulatory authority.

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Figure 2 • This Head SAR measurement and experimental setup can be used to calculate electric field within the tissue due to RF radiation from a smartphone. The electric field probe connected to a test and measurement instrument measures the electric field strength to calculate SAR.

 

For proof of concept testing, developers have the option to use programming languages like C, HDL, Python, LabVIEW and MATLAB to quickly test the proof-of-concept designs and then optimize the architecture for cost, SWaP, or various performance metrics like throughput or beam gain. Test engineers can embed a custom algorithm into the SDR to perform in-line processing inside the FPGA or emulate part of the system that requires a real-time response.

Communication protocol and carrier frequency testing

SDRs are used as a tool for performing urban propagation studies in different frequency bands, while also providing a means to process and visualize the results faster and in a cost-efficient manner. Such propagation studies enable engineers to understand and compare pathloss in different frequency bands, terrains, antenna configurations, protocols and algorithms for frequency planning, device location optimization, safety and security challenges. Using SDRs, we can transmit and receive data using a wide range of spectrally efficient modulation schemes, such as Quadrature Amplitude Modulation (QAM), Frequency Shift Keying (FSK), or Gaussian Minimum Shift Keying (GMSK). SDRs can simultaneously measure over-the-air (OTA) performance using several different parameters, for example, Block Error Rate (BLER), throughput, signal-to-noise radio (SNR), noise and interference level, and Error Vector Magnitude (EVM), to assess the quality of a modulation scheme. Advanced testing of relatively newer technologies and techniques such as Massive MIMO, Carrier Aggregation (CA), and Orthogonal Frequency Division Multiple Access (OFDMA), which are used in 4G LTE, 5G, WiFi6 and upcoming wireless communication standards, can be easily done using SDRs.

Modulation schemes

There are various modulation methods for digital signals. We can divide them into the following types based on the signal property used in the modulation:

  1. Amplitude Modulation: Pulse Amplitude Modulation (PAM), Quadrature Amplitude Modulation (QAM)
  2. Phase Modulation: Phase Shift Keying (PSK), Differential PSK (DPSK), Offset PSK (OPSK)
  3. Frequency Modulation: Frequency Shift Keying (FSK)
  4. Gaussian Minimum Shift Keying (GMSK), Minimum Shift Keying (MSK), Continuous Phase Frequency Shift Keying (CPFSK)

An ideal modulation scheme for a wireless communication system is the one that:

  1. Minimizes the noise interference
  2. Simplifies the receiver design to demodulate the signal
  3. Provides the highest spectral efficiency, power efficiency and bit rate
  4. Shows less sensitivity to multipath environment

SDR is one of the most efficient and robust tools to compare various modulation schemes and optimize the given requirements: for instance, to get the highest spectral efficiency and bit rate.

Section III: Benefits of using SDRs as Modular Test Equipment

Remote testing functionality - Control and monitor SDR from anywhere!

SDRs are one of the best types of modular test equipment which allow controlling and monitoring of measurement parameters. This includes determining all available networks and their types, frequency scanning, site validation, coverage footprints, and transmit / receive path information. SDRs can be used to stream data to a host system for remote and long duration testing; for example, ensuring there is no network interference in an area. SDRs can work as automatic test equipment for verifying performance and diagnosing faults in a device under test (DUT), such as 5G beam verification in smartphones. Other examples of tests include radio access network (RAN) testing, continuous wave (CW) transmit and receive testing, pathloss measurements, microwave and millimeter wave measurements, and many other wireless radio measurements. A notable real-world application of SDR is drive testing for measuring and assessing the network coverage, capacity and Quality of Service (QoS) of a mobile radio network on different frequencies and user scenarios. An overview of drive testing is shown in figure 3. This is done to evaluate the quality of a network as well as a DUT’s hardware and protocol configurations. Drive testing becomes a lot cheaper since SDR technology is inexpensive compared to standalone scanners and more flexible to choose several different parameters on the go, even inside the vehicle of the testing engineer.

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Figure 3 • This is a high level overview of drive testing for 4G LTE and 5G network quality survey in microwave and mmwave frequency bands. [icons taken from https://thenounproject.com/]

 

Hardware flexibility - SDR vs. traditional standalone T&M equipment

SDRs reduce the requirement of specialized hardware. A single module may address a wide range of use cases, even those that are currently unknown. SDRs offer flexibility, especially in their capability to be paired with the other SDRs or devices as well as being modular in their design. Wide frequency range coverage starting from DC to 18 GHz (upgradable to 40GHz) makes Per Vices SDRs a very high performance SDR. An advantage of having a large number of Tx and Rx channels on SDRs is that this allows multiple tests simultaneously. For instance, testing multiple protocols on different frequencies / bandwidths simultaneously creates huge savings in test time which is critical in commercial product development. High speed SDRs allow testing of high throughput links: for example, 40 GBps links can be used for testing quality and throughput of wireless channels on a high data rate network. Having the highest throughput SDR in the hands of test engineers will help them easily succeed with testing performance and quality of high speed data networks.

The design and construction of hardware T&M equipment has high overhead but distributing software updates generally has much lower associated costs. Using an SDR does not lock a platform into a specific set of communication standards and functions but rather opens many doors of opportunity. In comparison, most standalone T&M hardware quickly becomes obsolete by the latest innovations in technology. SDRs are flexible in both software and hardware and have the ability to grow with technology - thanks to SDR’s modularity and software-defined technology. This allows for less upfront development costs as well as less requirements for expensive equipment upgrades in the future.

Flexibility of SDRs Digital Boards (FPGAs, Processors, etc.)

SDRs offer flexibility, especially in their capability to be re-programmed to implement any given RF / communications system and wireless standard. Not only can they implement the baseband processing section of the required communication protocol, they can detect impairments, keep performance metrics, and collect data related to the compliance of a waveform transmitted by the DUT. The FPGAs can implement the processing required to decipher the communication protocol, detect impairments, keep performance metrics, and collect data related to the compliance of a waveform transmitted by the DUT. SDRs with built-in FPGAs for T&M lower the total cost of test through better use case coverage and improves the test engineers ability to debug failures through continuous integration (CI) systems.

SDRs can also provide direct access to the FPGA. For example, one can implement Machine Learning (ML) and Artificial Intelligence (AI) technology IP cores into FPGA for various T&M data analysis and optimization. Moreover, SDRs are UHD and GNU Radio compatible right out of the box for easy-to-use software based T&M devices, like signal generators, oscilloscopes, power meters, and spectrum analyzers. For example, an SDR could be used for GNU Radio-based test and measurement functions like that shown in figure 4. Clearly then, a high performance SDR and its digital backend are critical for today’s RF engineering T&M needs.

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Figure 4 • An IQ diagram from a basic GNU Radio based OTA measurement is shown.

 

Conclusion

SDRs have proven to be a very practical and efficient tool for test and development engineers to use in fast, demanding and dynamically changing technical environments such as 5G and 6G prototyping labs and drive tests, vehicular radar design and algorithm testing, and medical and military R&D projects. The software flexibility and hardware modularity in SDRs have played a crucial role to make it as useful as it is today in the daily life of engineers. With the increasing push to higher frequencies and larger bandwidth applications, advanced SDRs with high speed FPGAs, such as Cyan from Per Vices, will be very prominent among the top electronics and wireless industry players in the near future.

About the Authors

Ahmed Hussain has a PhD in Signals & Systems from Chalmers University of Technology in Sweden. Ahmed has hands-on working experience with Software defined radios, Antennas and Test & Measurement Instruments. Ahmed worked 5+ years at Samsung Electronics in South Korea. Ahmed has also worked for Ericsson and Mathworks in Sweden.

Brendon McHugh is the Field Application Engineer and Technical Writer at Per Vices. He possesses a degree in theoretical and mathematical physics from the University of Toronto.

November 2021

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