Software Defined Radio: Past, Present, and Future

Contents

The Past—30 Years of Software Defined Radios

It's hard to believe that the term “software defined radio” (SDR) has been around for about 30 years. That’s a long time in the technology world. SDR, still a common topic of discussion, carries more than its share of misconceptions. SDR is “a radio in which some or all of the physical-layer functions are software defined,” per the Wireless Innovation Forum (formerly the SDR Forum). The term focuses on the physical (PHY) layer processing of the waveform and is not related to the radio frequency (RF) front end, which is a common misconception.

Thirty years later, SDR is now a dominant industry standard—from military tactical radios to cellular handsets—it’s almost a given that a radio is an SDR. There will continue to be innovations in semiconductor and software technology that will drive higher development productivity and more cost-effective products, so there is no end in sight for SDRs. These factors mean that SDR is really a solved problem- and radios are now evolving to become frequency-agile intelligent communication systems.

 

The Present—  Software Defined Radio Becomes the De Facto Industry Standard

In markets such as signals intelligence (SIGINT), electronic warfare, test and measurement, public-safety communications, spectrum monitoring, and military communications (MILCOM), Software Defined Radios have become the de facto industry standard. Some of these markets were using hardwired application-specific integrated circuits (ASICs), while others were already using programmable digital signal processors (DSPs). Figure 1 shows the progress of SDR adoption through the last 30 years. Closest to the center, the dark blue section is representative of the first set of markets to move from hardware radio architectures to SDR architectures, regardless of whether they used the term SDR.

The technology that drove the move to SDR in these markets was the advent of RF integrated circuits (RFICs) from companies like Analog Devices and cost-effective DSP-intensive FPGAs from companies like Xilinx. These two technologies came together to meet a multibillion dollar need in the military tactical radio market, creating something of a “market ripple,” where the market had a huge impact on the evolution of SDR technology far beyond just the MILCOM market. The Joint Tactical Radio System (JTRS) program funded the development and productization of SDR for military radios, which created a strong ecosystem of vendors including semiconductors, tools, and software companies. On the tools front, SDR required waveforms to be as portable as possible between different hardware platforms, which resulted in tools like the Software Communications Architecture (SCA) Core Framework, as well as better programming tools from electronic design automation (EDA) and semiconductor companies.

Figure 1. Successive generations of SDRs have come to dominate the radio industry and will continue to evolve.

 

The advancement of RFICs, FPGAs, and EDA tools was a significant factor in enabling the second generation of SDRs being driven by 4G LTE infrastructure. Virtually all LTE base stations were developed with RFICs and FPGAs. Some of the larger infrastructure vendors would eventually go to ASICs, but even then, the baseband ASICs were largely programmable, as they used processors coupled to hardened blocks called hardware accelerators for compute-intensive functions, such as turbo decoding, that would typically exceed the performance or power limitations of the processors.

The next market ripple, shown in the third generation, occurred when 4G LTE handsets moved consistently to SDR architectures. This shift was enabled by low-power, high-performance DSP cores optimized for handsets offered by companies such as Ceva, Tensilica, and Qualcomm. Like baseband ASICs for infrastructure, these cores would be integrated into application-specific standard products (ASSPs) or ASICs for much of the PHY processing, coupled with hardware accelerators. Once this changeover occurred, SDRs increased orders of magnitude in volume and reach to become the de facto industry standard for radios.

 

The Future—Next Generation of Software Defined Radios

What’s next for SDR? As the ubiquity of 4G handsets has propelled SDRs, the prospects of emerging technologies such as 5G, the Internet of Things (IoT), and sensor networks promise to again increase the volume of SDRs by another order of magnitude. What will be the technology driver lifting SDR to these lofty heights? As with previous leaps in SDR adoption, it will likely be a combination of both hardware and software technologies.

One of the next technology drivers in hardware looks to be the combination of analog and digital technology onto a single monolithic chip to reduce cost and size, weight, and power (SWaP). For infrastructure, this driver could be FPGAs with integrated analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). For handsets and sensors, this could be application processors, also with integrated ADCs and DACs.

New innovations in hardware won’t be very useful, however, if the software and tools don’t follow. That is the whole point of SDR, after all. To enable the development of these chips, as well as the waveforms and application software running on them, there will be a requirement for better system-level tools that can be used to design and debug across the analog and digital domains. As SDRs become used for increasingly complex tasks, they are being designed with more powerful FPGAs designed for intensive DSP (Figure 2). As a result, there is an inevitable growing need for FPGA tools that can handle rapidly increasing amounts of data and complexity.

 

Figure 2. The number of DSP slices in each subsequent FPGA generation continues to grow rapidly.

 

While general-purpose processors (GPPs) have served the SDR community well in the past, they are struggling to meet the performance required for areas like 5G and MILCOM. Software tools such as the LabVIEW FPGA Module and RF Network on Chip (RFNoC) offer a streamlined user experience that makes FPGA programming vastly more efficient.

Ultimately, integration will drive the next generation of SDRs. The integration of analog and digital technology into mixed-signal chips will be key, but SDRs have fundamentally reached a point where the primary limitation on growth is in software, not hardware. Without software development environments that can seamlessly program both GPPs and FPGAs, the additional hardware features of next-generation SDRs will be underused and development will stall. The ability of tools like LabVIEW FPGA to enable wireless engineers who are not HDL experts to develop and rapidly iterate on sophisticated designs offer the best opportunity moving forward to unlock the next generation of SDRs.

 

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