By Brendon McHugh, field application engineer and technical writer, Per Vices
Software defined radios (SDRs) have been making the news in a variety of markets and for different applications, ranging from use in military communications and radars to integration into satellites and ground stations. By providing the versatility and performance required for RF applications and advanced signal processing, the use of SDRs is taking off and is predicted to grow to USD $14.69B in 2030. From small, inexpensive SDRs for amateur projects to ultra-high performance, high spec SDRs for government and defense use, it is important to understand what product you need, how to use it, and how to integrate it. Here, we will focus on higher end SDRs, but the same principles apply across the board.
What Is An SDR?
A software defined radio (SDR) is a signal processing device where the components that were traditionally defined in hardware (such as mixers, oscillators, filters, amplifiers, etc.) are now software controlled. They are comprised of a radio frontend (RFE) and a digital backend. The RFE is comprised of multiple receive (Rx) and transmit (Tx) channels, each independently able to tune to a wide range of frequencies, with the range depending on the product. For example, some SDRs operate from near DC to 6 GHz while others extend from near DC all the way to 18 GHz and beyond. One of the other key performance requirements for many applications comes in the form of instantaneous bandwidth, where high end SDRs offer 1 GHz or 3 GHz of instantaneous bandwidth per channel. The digital backend is the other element of an SDR, and it is where the digital signal processing (DSP) takes place, typically on field-programmable gate arrays (FPGAs). The FPGA is where basic radio DSP functions are performed, such as modulation, demodulation, filtering, up/down converting, CORDIC mixing, and data packetization. The type of FPGA will impact the number of complex algorithms that can be executed in parallel along with the resources available for additional IP to be incorporated on the FPGA for very low latency DSP operations. The FPGA, as the name suggests, is re-programmable, enabling a high level of flexibility for different DSP operations to be loaded depending on the application requirements at any given time.
Applications And Markets For SDRs
SDRs are one of the most useful and versatile systems for a variety of applications due to their flexibility. For example, the same SDR can be used for spectrum monitoring/electronic warfare (EW) and then be reprogrammed to be used for radar at a later date. The main markets in which high end SDRs are being successfully deployed include: aerospace, test and measurement, radar, IoT/5G, GNSS/GPS, EW/signals intelligence (SIGINT), medical/MRI, military/defense, satellite/ground stations, and spectrum monitoring. Any application that requires the capture (or transmission) of RF data along with the added flexibility of converting the RF data into a digital format and processing the data is a candidate for an SDR. While each of these markets and applications have unique requirements and specifications, there are common core elements that integrators should be aware of and adhere to.
Common Mistakes To Avoid When Integrating SDRs
1.) Not considering backhaul performance
An often-overlooked element is the mechanism by which the data is sent to or from the device. Ideally, integrators require a high-capacity link to ensure that they are able to stream large volumes of data with the lowest possible latency. This is particularly important for high data capture and low latency applications such as spectrum monitoring, EW, SIGINT, and high frequency trading. For these applications, data must be captured, analyzed, and relayed as close to real time as possible. The highest performing SDRs have 40/100G Ethernet interfaces enabling up to 400 Gbps through 4 qSFP+ ports.
2.) Overlooking FPGA resources
Often, during the development stage, all that's necessary is to pass data through to another system, which can be useful for debugging applications. As development progresses, it's often substantially more valuable to offload functionality to the FPGA, which requires ensuring the FPGA has sufficient resources to implement different DSP operations and IP cores. For some applications, it is more effective to use FPGAs to perform many DSP operations due to the parallel architecture when compared to CPUs. FPGAs also have the added benefit of being flexible and able to be reprogrammed, enabling updates and upgrades to be completed via software/firmware updates that can be pushed remotely, even to SDRs located in satellites orbiting space. These powerful FPGAs allow for the processing of complex algorithms, such as those required in ground stations to track multiple satellite constellations and can keep up with the expanding demands of the satellite industry. It is vital to ensure that the FPGA selected for production systems has the resources to meet your project requirements as they relate to DSP on the FPGA to reduce latency, compute resources, and overall size, weight, and power (SWaP).
3.) Dismissing GPIO considerations
Integrating new technology into legacy applications often requires an abundance of additional general-purpose input/output (GPIO) pins to be made available to help control external equipment. Ensuring sufficient GPIO, and that it can be controlled in a synchronous manner, is of great use. As there are a number of peripherals needed for receive and transmit functionality, such as antennas, sequencers, pre-amplifiers, power amplifiers, UART interfaces and more, having sufficient GPIO can ensure proper control.
4.) Not asking about level of integration
Integrating a new product or technology can be very challenging and can lead to many hurdles that can be costly, can slow down development, and can cause operational problems. Seek out a manufacturer that will help work with you to help ensure the success of your integration, including the ability to modify hardware, firmware, and software. Things to look for when integrating SDRs are:
- Key performance specifications (tuning range, bandwidth, number of channels, backhaul) that suit the needs of the project. Nothing is worse than investing money into a unit that doesn’t meet your needs.
- SWaP requirements. SDRs come in different form factors, which allow them to be placed in everything from a server room to a compact remote location outdoors, provided they are properly enclosed and ruggedized.
- Ability to run a test trial to sort out bugs and assess performance. Before committing to a large volume order of units, some manufacturers provide a lower cost system that can be used for a proof of concept (POC) stage of a project. The ability to work with a company and products prior to entering into a larger commitment is a great way of ensuring success and confidence, as opposed to rolling out a new multi-unit installation with little to no familiarity.
5.) Custom RFE
Your own application may include demanding RF specifications; a good vendor will help ensure your success with custom radio front end designs that meet your specific needs or applications. They will conduct a preliminary interview/give you a questionnaire to thoroughly assess the project’s scope and technical requirements. They will review what specifications and configurations are the best for the successful and long-term operation of the unit for your application. Keep in mind as technology progresses quickly, it is easier to already have the hardware necessary then have to go back and upgrade. As SDRs are software defined — a lot of the updates and upgrades can be done via software updates, so having a robust FPGA to handle this is a must. Doing the preliminary work — and looking years down the line to see how the SDRs use can evolve — saves money as it can be a one-time investment that can be amortized over a longer period of time and can withstand technological obsolescence more than other devices.
Overall, SDRs are an excellent long-term investment that will pay dividends over their useful life. As one unit performs the operations of many, it eliminates the need of having a complex and costly ecosystem of single purpose systems from various vendors. Having the majority of operations performed in software ensures there are fewer hardware failures, which can be difficult to diagnose and repair, especially if the unit is in a remote location. Maintenance, training costs, and human error will also decrease as a single multi-purpose unit provides less likelihood of failing. Purchasing and integrating new hardware into your application can be a daunting task, but if the process is approached methodically and carefully, it can be done smoothly and efficiently using SDRs.