Software Defined Radios (SDR) for Aerospace and Defence

With the development of new wireless technologies, the aerospace industry is constantly evolving and becoming more complex, especially with regards to military and defence applications. In this context, the need for reliable and efficient communication systems has never been more critical, as more battles are being fought in the RF domain and radio systems are becoming increasingly tightly integrated with software. Traditional hardware-based systems can no longer address current challenges in the aerospace industry, and software-defined radios (SDRs) are becoming the norm in radio defence applications. 

SDRs have revolutionized the wireless world by providing unparalleled flexibility and performance for a wide range of applications by implementing most of the radio functions at the software domain, leaving only the essential analog hardware in the radio front-end (RFE). From satellite and ground stations to electronic warfare and signal intelligence, SDRs have proven to be the perfect solution to address the most critical challenges in modern RF communications. 

In this article, we provide a comprehensive analysis of SDR technology in the aerospace and defence markets, covering the basic concepts of SDRs, how they work and why they are so advantageous in comparison with traditional systems, and their various applications and use cases in this industry. By the end of this article, you will have a solid understanding of the role of SDRs in the aerospace and defence industry, as well as the benefits they bring to these vital applications. In future articles, we will go deeper into each application and provide a broader view on the role of SDRs in solving their challenges.

What is SDR?

SDRs are advanced RF transceivers that implement most of the radio functions at the software side instead of taking the traditional analog radio approach. A typical SDR consists of two main stages: the radio front end (RFE) and the digital backend. The RFE handles the receive (Rx) and transmit (Tx) functions, implementing the essential hardware functions, including low-noise amplification, anti-aliasing and anti-imaging filtering, and antenna coupling. The highest bandwidth SDRs in the market implement RFEs capable of covering a wide tuning range from 0 to 18 GHz (which can be upgraded to 40 GHz) and reaching instantaneous bandwidth of 3 GHz per channel, which allows these transceivers to be applied in several applications regardless of the operation frequency. Furthermore, high-end RFEs provide several parallel Rx/Tx channels with independent ADCs/DACs, enabling the implementation of multiple-input multiple-output (MIMO) systems capable of handling several radio chains simultaneously. The digital backend, on the other hand, performs all of the software functions of the SDR. 

It consists of an FPGA with on-board DSP capabilities, enabling radio functions ranging from basic operations, such as modulation, demodulation, up/down-conversion, and data packaging, to more complex and dedicated algorithms, including communication protocols, detection of power thresholds, automated sequences, and even machine-learning/artificial intelligence programs. The digital backend provides ready-to-use host interface through high-speed optical links, allowing for easy configuration and fast integration with existing architectures. The FPGA-based approach also ensures that the transceiver is completely reconfigurable and upgradeable, enabling the implementation of the latest radio protocols and algorithms over-the-air, on-the-fly, and without any hardware modification. Figure below shows the basic diagram of the SDR boards, including the clock and power boards which provide a consistent time base and properly distribute power inside the device.

High Level Overview of SDR Boards

The radio front-end (RFE) of an SDR is one of the most critical components in terms of performance: if it is not well designed, the range of applications is significantly limited. The RFE channels are developed to work with signals in a very wide tuning-range, ranging from sub-GHz to tens of GHz, which ensures that a general purpose SDR can reach any desired center frequency. At the receiver channel, multistage signal chains are designed to address particular characteristics of each operation band, including a low/baseband chain and a high band chain. Each chain contains several radio components, including low noise amplifiers (LNAs), attenuators, IQ downconverters, anti-aliasing filters, and analog-to-digital converters (ADCs). The signal path then leads to the FPGA, which serializes information using JESD204B protocol. 

The Tx path follows a similar logic, but the other way around: the signal starts at the FPGA and progresses through the low/baseband stage or the high band stage, which includes a digital-to-analog converters (DACs), anti-imaging filters, frequency synthesizers/local oscillators (LOs), IQ upconverters, and RF gain blocks. High-performance SDRs typically have completely independent Tx and Rx chains, enabling parallel operation. Additionally, the RFE relies on other boards for timing and power. The timing board is responsible for providing clock signals for the ADCs, DACs, and local oscillators/mixers, while the power board ensure that the SDR is operating within the required power budget.

The digital backend is essentially the brain of the SDR, performing any computation needed to handle the acquired data from the RFE. In this stage, high-end SDRs implement powerful FPGAs with on-board DSP functions that are suitable for various applications, including modulation/demodulation schemes, up/down converting, CORDIC mixing of digital signals, data packetization for VITA49 protocol, and FIFO queues. Application-specific and customized functions can also be implemented on the FPGA, including channelization, security schemes, complex communication protocols, frequency hopping, and even artificial intelligence/ML. Because virtually any digital function can be implemented in the FPGA, the digital backend provides a huge level of flexibility to the SDR, allowing a general-purpose device to instantaneously becoming a specialized transceiver. 

Furthermore, because the signal flow through the FPGA chains instead of being sequentially processed by a CPU, several channels can be computed in parallel with minimal latency, enabling very high-throughput operations. The backend also handles communication with the host or network, packetizing data into Ethernet and transporting over high-speed SFP+/qSFP+ links with data rates of up to 10-400Gbps. The host system is connected to the Ethernet ports and MGMT ports of the transceiver, enabling separate links for configuration/control of the SDR and exchange of raw IQ data.

A Closer Look at the Components of SDRs and Differentiating Features

As with any other RF device, the SDR must be properly tuned in order to operate with a certain frequency. However, the software-based configuration provides a much easier tuning process, minimizing the labor involved in the task. In an SDR, tuning involves multiple stages of frequency conversion and processing to prepare signals for transmission or reception, effectively connecting the SMA output/input from or to the FPGA. In modern SDRs, the transceiver architecture uses a superheterodyne design, which means that it implements a complex baseband stage that is mixed with an IQ converter to a real-valued IF stage, which is subsequently converted to a final RF stage. 

The highband Tx and Rx chains are automatically enabled by configuring the SDR frequencies within the range from 6GHz to 18GHz. To understand the SDR tuning process, it is important to discuss the transmit and receive mechanics of the superheterodyne architecture. At the Tx path, the user-defined samples are generated from the data port, with the sampling rate defining the user bandwidth UBW (which covers the frequency limits of the generated sample, from -UBW/2 to +UBW/2). Interpolation is then performed in the SDR to obtain a virtually larger bandwidth, which depends on the sampling rate of the device itself. A CORDIC mixer is then used to upconvert the samples through a local oscillator set to the numerically controlled oscillator (NCO) in the FPGA. 

The signal is then converted to the analog domain using a DAC, and finally passes through an anti-imaging filter (AIF) to remove image signals. Now we are in the analog circuit, and a complex (IQ) upconverter/mixer is used to further increase the frequency of the signals, using a midband oscillator (IF). The signal passes through a band-pass filter before reaching the real upconverter, that uses a highband mixer and a highpass filter to convert the signal to the final frequency. A RF amplifier is then used to provide enough gain to transmit the signal through the SMA port.

The Rx tuning sequence is similar but the other way around, with the process starting at the RF input and gain stage where the signals are picked up by the antenna and amplified. The signals then pass through a high-pass filter which rejects any frequency below 6 GHz. Next, the real downconverter/mixer reduces the frequency to an intermediary level, and a band-pass eliminates any frequency outside the range between 1.4 - 2.3 GHz. The signals are then further downconverted by a complex (IQ) mixer using the IF LO, which separates the signal into IQ pairs for separate processing of the IQ signals. 

The IQ data then go through an anti-aliasing filter (AAF) which attenuates signals outside the AAF bandwidth. The analog signals are converted to digital using an ADC before proceeding to the FPGA, for digital CORDIC mixing. The signals are further down sampled using decimation before being sent over Ethernet for the host. The overall tuning range is largely determined by the real mixers with LOs, numerically controlled oscillators (NCOs), and other components down to silicon limitations.

Another important SDR feature is complex sampling. Also known as IQ sampling, complex sampling is a technique used in modern SDRs to convert high frequency signals to a digital format. Unlike conventional sampling methods, where only the amplitude of the signal is sampled, IQ sampling address both the amplitude and phase of the signal, allowing quadrature modulation and other communication protocols. This is performed by using a dual analog-to-digital converter (ADC) capable of sampling the signal at the same time, but with a 90-degree phase shift between the quadrature oscillators. 

The resulting samples are called in-phase (I) component and the quadrature (Q) component, and they can be combined to obtain the magnitude and phase of the signal. One of the key advantages of complex sampling is its ability to offer a high instantaneous bandwidth (IBW). Since the maximum IBW of an SDR is equal to the sampling rate of the ADCs, using complex sampling allows for higher sampling rates than conventional sampling, which can greatly increase the amount of bandwidth that can be processed in real-time. For example, a top-of-the-line SDR, Cyan, uses 1 and 3GHz converter devices, which enables 1 or 3GHz of IBW. Additionally, the ability to decimate/interpolate allows the actual instantaneous bandwidth to become the "converter bandwidth", enabling further flexibility in system design.

Basic Mechanism of Complex Sampling

Data encapsulation is a crucial aspect of transmitting and receiving digitized messages between RF systems and related equipment. The VITA Radio Transport (VRT) standard, also known as the VITA 49 protocol, defines a standardized format for conveying digitized signal data and metadata between different reference points within a radio receiver. VITA 49 is a packet-based protocol that enables users to transmit and receive data in a reliable and efficient manner. 

To implement VITA 49, SDRs often use a data encapsulation approach that incorporates VITA 49 within UDP (User Datagram Protocol), that in turns is within IP below the Ethernet layer, creating a layered protocol stack where the application sits on top, and the link layer at the bottom. This approach ensures that the data is transmitted in a way that is consistent with the VITA 49 protocol. Additionally, some SDRs offer a PPS (Pulse-per-second) hardware input signal fed into a REF IN port, typically provided by a system GPS receiver, which allows the data samples to be tagged with a specific sample clock count in the VITA 49 header, that is critical for synchronizing multiple radios in a network. 

Data throughput is an essential parameter in SDR selection. The data throughput of software-defined radios (SDRs) is determined by a combination of factors including the network backhaul, the Ethernet protocol implementation, and the number of physical transceiver connections. The maximum backhaul bandwidth is a key constraint on SDR performance and is limited by the line rate of the bus connecting the host machine and the SDR. 

This is determined by the instantaneous bandwidth multiplied by the sample bit width, typically 16 bits for both the I and Q signals. In terms of backhaul, the Crimson SDR model (from Per Vices) features two SFP+ ports implementing 10GBASE-R, with a maximum transfer rate of 10Gbps. In contrast, the 1GSPS Cyan model has four qSFP+ ports, which implements 40GBASE-R each, while the high-bandwidth 3GSPS Cyan model reaches 100Gbps at each port, providing even greater throughput. These specifications highlight the importance of selecting the appropriate SDR based on the required data throughput for a given application. Figure below shows the typical backhaul of the Crimson and Cyan SDR models from Per Vices, highlighting the Ethernet ports available.

Backhaul Diagram of Modern SDRs

Dynamic range and sensitivity are key parameters in determining the overall performance of radio receivers. The noise floor, which is the level of the smallest signal that can be detected, is a critical specification in sensitivity and is usually expressed in dBm/Hz. The third order products and intercept point are also important in dynamic range measurement, as undesired harmonics can significantly degrade  both the Rx and Tx channels. SDRs offer the advantage of implementing adjustable gain, which enables the adjustment of the receiver gain to prevent overloading, distortion, unwanted spurious signals, and addressing a wide variety of signal amplitudes. 

Multiple input multiple output (MIMO) SDRs are possible due to the independent ADC/DACs, signal chains for each Rx/Tx channel, and parallel computing in the FPGA. This allows for greater flexibility in terms of signal processing, data transmission, and enable several application specific algorithms that require multiple radio chains, including spectrum monitoring and beamforming. One of the most critical factors for MIMO operation is phase stability across each of the channels, which is achieved through the robust phase timing board architecture and a consistent distribution of clock signals withing the SDR. MIMO SDRs are particularly important for applications that require multiple antenna arrays. With MIMO SDR, it is possible to achieve higher data throughput and increased network capacity while maintaining high signal quality and reliability.

Overview of SDRs for Aerospace and Defence

SDRs have become an essential component in modern satellite systems, acting on several applications that include earth observation, communication, navigation, and scientific research. One of the key benefits of using SDRs in satellite systems is their ability to adapt to changing requirements, allowing for remote reconfiguration and updates over the air, without any in-situ intervention. Moreover, SDRs enable the use of advanced waveforms and modulation techniques, such as orthogonal frequency-division multiplexing (OFDM), improving the efficiency of data transmission and increasing the overall capacity of the system. With MIMO SDRs, it is possible to support multiple antennas on a single platform, enabling beamforming techniques that enhance the system's performance and increasing its resilience to interference and jamming.

Ground stations on Earth are extremely important for the satellite infrastructure, as they act as gateways between satellites and the land infrastructure. SDRs are becoming an increasingly popular solution for these systems, due to their wide performance range, application flexibility, and several key RF features. With the ability to communicate with multiple satellites across different frequencies and varying data rates, SDRs offer a versatile and cost-effective solution for ground stations, especially considering the ground station as a service (GSaaS) architecture, which becoming more popular with the advent of 5G networks and cloud solutions for satellite communications. Furthermore, the use of SDRs enables the ground station to offload data to storage solutions or other interfacing equipment at very high data rates, up to 400Gbps.

Software-defined radios (SDRs) have become a critical technology in the field of electronic warfare (EW). One of the most important capabilities required in EW is the ability to quickly re-tune the radio for jamming techniques. SDRs are highly adaptable and can support a variety of tuning specifications, including fast re-tuning time, modulation with noise, and wide or narrow-band operation. SDRs are used for both noise and deception jamming, which rely on signal generation using field-programmable gate arrays (FPGAs). Noise jamming is produced by modulating a radio frequency (RF) carrier wave with noise or random amplitude changes, while deception jamming uses complex receiving and transmitting circuits to process and re transmit jamming pulses that appear as a real target to the victim radar. SDRs equipped with FPGAs are also capable of digital beamforming to emit RF energy in a directed way towards a target in order to jam. MIMO antennas can also be used to further enhance the jamming capabilities of SDRs in aerospace electronic warfare.

Finally, SDRs are being increasingly applied in aerospace defence systems, due to the high-levels of flexibility to adapt to different critical situations and reliable RF performance over multiple channels. Radar systems are one example of ideal SDR implementation, as they can easily change frequency, modulation type, and waveform to adapt to changing requirements and address several challenges, including clutter and jamming mitigation. Signal intelligence (SIGINT) is another crucial area where SDRs play a significant role, allowing for the collection and analysis of complex signal data to support critical decision-making. Spectrum monitoring, a critical component in SIGINT, and recording also rely on MIMO SDRs, as they can monitor and record radio signals over a wide frequency range. Additionally, SDRs are widely utilized in military communication systems, providing secure and reliable communication channels in complex environments. Overall, SDRs offer a flexible and adaptable platform for defence applications, providing a significant advantage in mission-critical operations. 

Conclusion

This article has provided an in-depth analysis of SDRs and explored the wide range of applications where software-defined radios (SDRs) are used in the aerospace and defence industries. SDRs are incredibly versatile and can be reprogrammed and adapted to suit a variety of needs, making them ideal for these industries. We highlighted that some of the largest adopters of SDRs in the aerospace industry are satellites, ground stations, and EW equipment, with each application having its own specific requirements and challenges that SDRs can help address. Additionally, SDRs are already being used in other areas of defence, including radars, signals intelligence, spectrum monitoring and recording, and tactical communications. SDRs have vast potential in this industry, and these transceivers will undoubtedly continue to play a significant role in the aerospace and defence industries for years to come, but it is imperative that designers are prepared to select, develop, and operate the right SDR for their application.

Company Info

Per Vices has extensive experience in designing, developing, building, and integrating SDRs for various applications in the aerospace and defense industry. Contact [email protected] today to see how we can help you with your SDR needs.