The Trouble With Software Defined Radio (And How To Overcome Them)

By John Oncea, Editor

Software defined radio technology brings the flexibility, cost efficiency, and power to drive communications forward, with wide-reaching benefits realized by service providers and product developers through to end users. But it’s not perfect …

Software defined radio (SDR) has been around as a concept for more than 50 years and the term itself for about 40. It’s used in many markets, including military tactical radios, cellular handsets, signals intelligence, electronic warfare, test and measurement, public safety communications, and spectrum monitoring.

The Wireless Innovation Forum notes there are several ways to define SDR before suggesting that the definition established by the SDR Forum and the Institute of Electrical and Electronic Engineers (IEEE) works best: radio in which some or all of the physical layer functions are software defined.

Wireless Innovation continues, “A radio is any device that wirelessly transmits or receives signals in the radio frequency (RF) part of the electromagnetic spectrum to facilitate the transfer of information.  Traditional hardware based radio devices limit cross-functionality and can only be modified through physical intervention. This results in higher production costs and minimal flexibility in supporting multiple waveform standards. By contrast, software defined radio technology provides an efficient and comparatively inexpensive solution to this problem, allowing multimode, multi-band, and/or multi-functional wireless devices that can be enhanced using software upgrades.”

How SDRs Are Used

SDRs typically use direct conversion receivers and high-performance analog-to-digital converters (ADCs) to sample radio frequency signals. The digital signal processing is then performed by software running on a computer or embedded system.

The evolution of SDR technology has been marked by significant advancements since the early days when they were limited in bandwidth and processing power. Improvements in digital signal processing and hardware design led to wideband SDRs capable of supporting broader frequency ranges and higher data rates and, today, SDRs can support multiple radio chains with sampling rates up to 3 GSPS.

SDRs have found applications in various fields including communications systems and offer flexibility in implementing different modulation schemes and protocols through software updates. In radar systems, SDRs enable advanced signal processing techniques for improved radar performance.

SDRs play a role in spectrum monitoring where they are used to analyze and monitor radio frequency spectrum usage, and signals intelligence where their adaptability makes them valuable for intercepting and analyzing various types of radio signals.

SDRs provide versatile platforms for testing and measuring radio frequency equipment and can be used to implement GPS receivers and other satellite navigation systems. They also have become popular among amateur radio enthusiasts, allowing for experimentation with various modulation methods and signal processing techniques.

The flexibility and adaptability of SDRs have made them invaluable tools in modern radio communication, enabling rapid development and deployment of new radio technologies across a wide range of applications.

Potential Problems With SDRs

SDRs are versatile and cost-effective because they can be upgraded with new software for different applications after purchase and installation. For example, wireless engineers can use SDR hardware for over-the-air lab and field testing and to rapidly prototype custom radio functions. However, they are not infallible.

Like all technologies, SDRs come with security vulnerabilities. According to the International Journal For Research in Applied Science and Engineering Technology (IJRASET), this includes unauthorized access and interception. SDRs can be exploited to intercept and eavesdrop on wireless communications, potentially compromising privacy and sensitive information. For example, SDRs could be used to listen in on walkie-talkie conversations or other radio transmissions not intended for public access.

SDRs also enable replay attacks, where captured radio signals can be recorded and retransmitted later. This is particularly dangerous for systems using static codes, like some car key fobs. An attacker could potentially unlock vehicles by replaying captured key fob signals.

Finally, there’s spoofing and impersonation. The flexibility of SDRs allows malicious actors to impersonate legitimate devices or users on wireless networks. This could lead to unauthorized access or injection of false data into systems.

And, like all technologies, SDRs come with regulatory and legal issues. According to the Federal Communications Commission (FCC), SDRs can be reprogrammed to operate on restricted frequencies, potentially interfering with critical communications or violating regulations. In addition, the ability to modify radio parameters via software makes it challenging to ensure SDRs remain compliant with regulatory certifications after deployment.

Concerning privacy, SDRs can be used to track the locations of wireless devices, according to IJRASET. For example, they could be employed to monitor air traffic or track individuals through their mobile devices. They could be used to jam or interfere with legitimate wireless communications, potentially disrupting critical systems. Finally, the programmable nature of SDRs introduces the risk of malware or malicious software being introduced into radio systems.

Other performance limitations, implementation challenges, and resource requirements include:

• Dynamic Range Issues: Some SDR prototype designs suffer from poor dynamic range compared to traditional hardware radios. This can limit their ability to handle weak signals in the presence of strong interference.
• Processing Speed Constraints: SDRs require significant computational power, which can lead to processing speed limitations. The flexibility and efficiency of SDRs often have to be traded off against processing speed.
• Software Complexity: Writing software to support different target platforms and radio protocols can be difficult. This increases development time and costs.
• Analog-Digital Interfacing: Implementing the interface between analog RF front-end components and digital processing modules is challenging in SDR architectures.
• ADC Limitations: The analog-to-digital converter (ADC) limits the maximum frequency that can be used by the digital part of the SDR system. This constrains the overall frequency range of the radio.
• Power Consumption: SDR systems often require more power for a given function compared to purpose-built hardware radios with optimized analog/digital partitioning.
• Computational Demands: SDRs need substantial processing resources, which can lead to increased size, cost, and power consumption, especially for more complex radio applications.

To address these dangers, several security measures are crucial. According to Cyber Defense Magazine, these include:

• Implementing strong encryption for wireless communications
• Using dynamic rather than static codes for authentication
• Regularly updating and patching SDR software
• Employing robust authentication mechanisms
• Implementing intrusion detection systems for wireless networks
• Ensuring proper regulatory compliance and certification processes

For simple radio system designs, SDR platforms may be prohibitively expensive. Their development requires both software and hardware engineering skills, which can increase project complexity and personnel requirements. Ironically, software reliability issues may become the limiting factor for overall radio reliability, rather than hardware limitations.

While SDRs offer significant benefits, it remains essential to be aware of and mitigate these potential dangers to ensure secure and reliable wireless communications.

The Future Of SDRs

What’s next for SDR?

According to National Instruments, “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.”

The next wave of innovation in software defined radio (SDR) technology is likely to be driven by the integration of analog and digital components onto single chips, reducing cost, size, weight, and power consumption. This trend is expected to manifest in FPGAs with built-in ADCs and DACs for infrastructure applications and in application processors with integrated converters for handsets and sensors.

However, to fully leverage these hardware advancements, corresponding improvements in software and development tools are crucial. As SDRs tackle more complex tasks, there's a growing need for system-level tools that can design and debug across both analog and digital domains. Software solutions like LabVIEW FPGA Module and RF Network on Chip (RFNoC) are emerging to make FPGA programming more efficient, addressing the limitations of general-purpose processors in meeting the performance demands of technologies like 5G and MILCOM. The ability to seamlessly program both GPPs and FPGAs will be key to unlocking the full potential of next-generation SDRs.