The ever-growing need for smart wireless devices, high-speed/high-throughput telecommunication systems, and overall RF and digital systems is increasing the demand for software-defined radio (SDR) to levels never seen before. These applications vary greatly in terms of performance and cost, as well as size, weight, and power (SWaP). In this scenario, the test and measurement (T&M) industry is crucial in the development process of new devices to ensure that wireless equipment operates properly and meets the required standards and qualifications. Fortunately, SDRs provide a high level of flexibility and programmability, not only for the application but for the T&M process itself, reducing the amount of equipment required and allowing the same device to perform several different functions without hardware modification.
T&M is a well-known process of RF development, required for various design stages of new devices, including proof of concept, pre-simulation, simulation, prototyping, and validation/certification for market release. A major challenge in T&M design is compliance with the large variety of wireless devices that range from robust aerospace and defense systems to mobile health-care and agriculture solutions. In these applications, T&M equipment is required to simulate different RF systems through the testing and measuring of:
- Modulation/demodulation schemes
- Transmit (Tx)/receive (Rx) performance parameters that include path loss, noise, dynamic range, and spurious-free dynamic range (SFDR)
- Antenna performance in terms of near-field/far-field measurements, antenna coupling, range, and radiation pattern
- Electromagnetic compatibility
- Several RF evaluation functions, such as spectrum, signal, and network analysis
In this article, we discuss how SDRs can help the T&M market to keep up with the fast rate of technology development in the wireless industry. We discuss how SDRs can perform several functions found in traditional testbenches, from signal generators to spectrum analyzers, while providing much more flexibility and reconfigurability than conventional RF test equipment. Without any hardware modification, SDRs can be adapted to work with different radio protocols, modulation schemes, frequencies, and bandwidths, all of which are continuously evolving in the RF industry and illustrate the power and long-lasting utility that SDRs provide for T&M engineering.
What is an SDR?
SDRs are essentially transceivers that perform most of the radio and signal-processing functions in the software domain, implementing only the analog hardware necessary for antenna coupling, amplification, and filtering. The analog portion of the SDR is called the radio front end (RFE), which contains all the Rx and Tx channels of the design, operating under a very wide tuning range. The highest-performance SDRs in the market provide RFEs with 3 GHz of instantaneous bandwidth over multiple independent channels, each one with a dedicated digital-to-analog converter/analog-to-digital converter for multiple-input multiple-output (MIMO) operation. The digital back end operates on the digitized signals, performing all on-board digital-signal–processing functions necessary for RF applications, including modulation/demodulation, up-/down-converting, and data packetization over Ethernet optical links. Host connection is extremely important in SDRs for T&M, as it integrates the device with the rest of the system, especially if high-data throughput is critical. The highest-bandwidth SDRs in the market provide backhaul throughput of 4× 100 Gbps over qSFP+ transceivers that can be connected to the host architecture via network interface cards, which is perfect for any high-data–rate testing. Figure 1 shows the general SDR structure being applied in T&M.
The combination between the software-based operation and native host connection in SDRs allows the implementation of open-source and custom software with readily available T&M functions. This significantly increases the range of functionalities of the equipment by taking advantage of built-in signal-processing and RF functions and libraries of the software, without having to develop low-level coding and graphical interfaces. In this context, GNU radio is the best example for RF applications, with built-in functions including frequency spectrum, spectrum waterfall plots, constellation diagrams (important for DC offset and IQ phase imbalance), oscilloscope plots, and waveform generation. It also provides ready-to-use algorithms to calculate important RF parameters, such as SFDR, noise, and dynamic range. By applying only one MIMO SDR with GNU radio, T&M engineers can significantly decrease the equipment count in a test and centralize all RF configuration, which further reduces costs, time, and human error.
SDRs are also capable of working with UHD-based tools and software, which includes custom programs based on Python and C/C++. This tremendously increases the range of possibilities for T&M systems, as both custom and proprietary software can be easily implemented to comply with different applications and protocols. For instance, UHD-powered SDRs can use GPS/GNSS simulation tools, according to different satellite constellations, while addressing interferences and jamming or even implement readily available open-source LTE/5G base stations to test performance and compliance. A major advantage of UHD projects is their tendency toward open-source solutions, which reduces the need for proprietary licenses and promotes an active and resourceful community.
How are SDRs changing the face of T&M in various markets?
Due to their flexibility, programmability, inherent capability of working with multiple channels, and high performance, SDRs are helping engineers to solve many challenges in the T&M industry. For instance, capturing large instantaneous bandwidths is desirable in several T&M applications, but it is very difficult to implement in conventional RF systems, especially when working with high frequencies (e.g., millimeter-wave). By providing software-based signal processing with very low latency, SDRs present a superior performance in bandwidth capturing when compared with conventional systems while also providing better flexibility due to their wide tuning range. State-of-the-art SDRs implement high-speed host communication over optical Ethernet links, enabling the streaming of large amounts of data to a server system or storage solutions, which is fundamental in 5G and IoT networks. The flexibility and programmability of the FPGA not only provides better performance than legacy radio systems but allows the T&M system to be tuned according to the device under test without any hardware modification, providing a versatile and robust solution.
SDRs also reduce the requirement for specialized hardware, as a single device can provide a wide range of functionalities, including future customized functions. Their modular nature also allows device customization to address all performance and SWaP requirements of the application. T&M architectures can pair one or more SDRs to work cohesively, each providing a different function, such as signal generation and spectrum analysis. Each SDR is an RF world of its own, providing total customization in terms of waveform, modulation scheme, and frequency. The highest-throughput SDRs can store and transmit huge amounts of data, providing off-the-shelf solutions for network applications. Standalone RF solutions typically have a fixed number of RF channels, which limits their applicability and increases the equipment count. MIMO SDRs, on the other hand, provide a configurable amount of RF channels that can be used to perform different applications, such as waveform generation and signal reception, or to work coherently in antenna arrays, which is great for beamforming/beam steering. Conventional T&M systems have fixed functionality and cannot be easily updated, whereas SDRs can be programmed to work with the latest protocols and algorithms without any hardware modification.
The software-based nature of SDRs makes them naturally compatible with automated solutions, which requires a certain level of embedded intelligence and host interface. Automated T&M systems provide several advantages over conventional solutions. In the laboratory, tests can be programmed with respect to predefined power thresholds and task sequences, reducing the chances for hardware damage and improving robustness and reliability by eliminating the potential for human error. Also, it allows remote control over the test, which is particularly important for cooperation between laboratories and testing at distant locations with difficult access, such as cell towers in remote areas. SDRs can also be pre-programmed to automatically transmit signals at specific time intervals, allowing the system to operate without human intervention.
In the 5G era, the rapidly growing RF market requires robust and reliable T&M that can keep up with the constant technology innovations in terms of wireless devices and communication protocols. Conventional T&M systems are fixed and single-purpose and cannot provide the flexibility and versatility required to address this ever-growing evolution, which results in SDR-based equipment becoming increasingly desirable in the T&M market. SDRs can perform several functions at the same time through multiple RF channels that can be completely reconfigured to comply with different waveform, frequency, sensitivity, modulation, and latency requirements. They can be easily programmed to work with novel protocols and algorithms, significantly improving the longevity of devices. SDRs are also compatible with open-source software solutions for hosting with built-in T&M functions, including GNU Radio and UHD-based programs, improving their usability and reducing the development time and cost.