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SDR in Military and Aerospace - Beyond Tactical Radios

September 10, 2020

In the telecommunications and electronics domain, Software Defined Radio (SDR) has been a major focus of military organizations for years. It is recognized as an optimal solution capable of shaping more flexible and powerful tactical radios that traditionally relying on single channel radio communications technology with limited performance.  Tactical radios are inherently designed military to enable radio communications in areas not covered by telecommunications infrastructure and provide robust connectivity between all units in the battlefield and in all types of extreme environmental conditions. They are typically designed to transfer any kind of information, whether it is voice, picture, video or other kinds of data.

The flexibility of the SDR platform shows promising opportunities in the technological landscape as it enables an upgradable and reconfigurable platform to take on various signal processing and communication tasks. It’s uniqueness is influenced by modifications of obsolete radio front-end architectures and its transition to digital implementation of programmable integrated circuits and applicability of appropriate software utilization. SDR applications enable size, weight and power (SWaP) reductions, jamming or interception resistance, TRANSEC and COMSEC features, multi-channel Tx or Rx radio functionality and improvements of other capabilities important for the most demanding communications networks. After more than 30 years of technological deployment and myriad applications, it can be quite reliably said that SDR is no longer exclusively related to tactical radios, but has expanded to aerospace, radar, medical imaging, GNSS, and other diverse communications systems.

Wide SDR applicability is based on its capability to change radio parameters (such as power levels, radio frequency bands, radio channel bandwidths, waveforms, etc.).  As a proven technology today, it enables provisioning of new services in wireless cellular communications networks, such as: 5G, support for radio systems/receivers with Single Antenna Interference Cancellation (SAIC) technique, various monitoring systems, IoT (Internet of Things) health technology solutions, Wireless Body Area Networks (WBAN), generation of different waveforms, medical applications such as Nuclear Magnetic Resonance Spectroscopy (NMRS) or Magnetic Resonance Imaging (MRI) and deployment of aerospace and satellite communications systems in higher, not so crowded frequency bands. 

Therefore, there is a range of SDRs, from basic to highly complex, that serve a multitude of purposes for a range of applications, that are still being uncovered. In the future, experts predict that SDR platforms will become indispensable for digital radio and TV broadcasting, video streaming transmissions, multimedia services availability, monitoring systems and all other novel radio applications.

Military and Aerospace radio needs beyond tactical radios
As new product families of unmanned aerial vehicles, aircraft, spacecraft, drones, satellites, communications and navigation systems are presented to the market every year, the SDR solutions continue to evolve along with the customers’ needs. The SDR market is generally fragmented into military and commercial clusters, with the commercial markets typically including aerospace, civil and medical applications, telecommunications, satellite and radar systems, etc. Nowadays, the most important SDR features are offering performance capabilities beyond that of a tactical radios, and include a high probability of intercept (POI), high data throughput, and radio signal rebroadcasting (conversion from one to the another radio channel). With the help of dynamic range and background noise frameworks, SDR systems are able to fulfill these radio needs of the various markets. These frameworks have to be capable of handling the implementation of next-generation data converters for high-volume data analysis and digital signal processing (DSP), supporting multi-channel AD (analog-to-digital) conversions,  being embedded with optimal weight, volume and power consumption solutions, electromagnetic interference (EMI) management, and specific high quality connectors that can enable high data throughput and handle challenging signal levels.

The latest SDR platforms provide diverse functionalities and are configurable for multiple use cases. Modern SDRs are recognized as an all encompassing solution, allowing real-time communication through various branches of defense departments, whether it be army, navy, or air force operations. High performance radio receivers, radio exciters or digital signal processors embedded in radar systems are of the most importance for establishing the efficient SDRadar (Software Defined Radar) network that is necessary for land, sea or air surveillance, including the latest counter-drone airborne solutions.

Radio signal rebroadcasting represents the SDR capability to transpose signals - to receive a signal in one frequency and transmit it on another. This useful feature can increase the communications network coverage by changing the frequency range. Therefore propagation conditions and radio signal rebroadcasting can aid to overcome the physical barriers such as hills or mountains. Furthermore, it offers flexibility that enables radios to change operating frequencies and protocols in order to subsequently join desired specific radio networks and is a feature unique to SDRs, as traditional radios do not have these capabilities.

How SDRs Meet Technical Needs
High Probability of Interception (POI) is the length of time that a signal needs to be present in order for there to be a probability of intercepting it and capturing it for analysis. This enhances the detection capabilities and shortens the response timeline. To achieve this in SDRs, the system must be supported by a high number of operating channels and high RF (radio frequency) bandwidths. High POI of SDR systems support radio communications intelligence, and RF spectrum monitoring are capable of not only signal detection, but analytics and performing predictions based on different simulation algorithms. This is also important for different Electronic Warfare (EW) systems in order for them to provide real-time responsive data in ultra-broadband radio eavesdropping and radio goniometry domains by finding the angle to the targeted radio source. Moreover, benefits of high POI aid in overcoming the challenges transitioning from tactical radios to an SDR platform, making them an indispensable part of test and measurement systems, autonomous vehicles and drones, 5G cellular technology, other military applications, aerospace and telecommunications systems, or different regulatory agencies responsible for RF spectral monitoring.

High data throughput is the key performance metric for low latency transmissions and is applicable to different use cases, such as SDRadar systems, surveillance and monitoring systems, distributed networks and automotive industry, civil & defense real-time communications and high frequency trading. High data throughput requires high bandwidth availability. In order to achieve the SDR’s maximum usable bandwidth, it is important to take into account the analog bandwidth, the FPGA processing bandwidth, that is correlated with the FPGA sampling rates and the host bandwidth that is based on the maximum available communications rate between the SDR and the host.

SDR systems, embedded with powerful signal processing algorithms, demand significant computing performance via speed and memory. Managing processing speed is typically achievable with the available computing solutions and supported by significant memory resources. By increasing the receiver sensitivity and analyzing the ultra-broadband windows, SDR platforms transfer the extremely large amounts of data that have to be processed and delivered to the hosts’ destination addresses. Therefore, in order to enable data processing and distribution in a real or near-real time, it is of the most importance that modern SDR solutions have high data throughput, the measure of how much data is successfully processed. High data throughput is the SDR feature directly correlated with high POI, thus, the high POI feature consequently needs the support of high data throughput to transfer and distribute all processed data.

SDR Product Examples
When analyzing the available SDR solutions, some are quite unique and equipped by state of the art technology, such as Crimson TNG and Cyan by Per Vices. The Cyan SDR in Figure 1, is state of the art SDR integrated transceiver, capable of capturing 1 GHz of bandwidth per each of 16 available phase coherent and configurable radio channels, offering the 100% POI within the 16 GHz ultra-broadband window. It operates in up to 18 GHz radio frequency range and is embedded with 50 ohm, high frequency Amphenol SMA connectors.

Figure 1
Figure 1. Cyan - a high POI SDR platform.

Numerous facets influence how well SDRs handle large amounts of data. The ADC (analog-to-digital converter) clock determines the sampling intervals and sampling rates that have to be set properly. Higher sample rates enable more radio frequency spectrum processing, but also involve frequency planning in order to avoid 2nd and 3rd harmonics and increase the dynamic range that is very challenging for multichannel wideband SDR receivers. Input bandwidth determines the frequency range within the ADC operates. Higher input bandwidth enables to capture more of the spectrum at any given time. For example, if a radio signal is transmitted on 10 MHz and then it jumps to 50 MHz, then to 500 MHz at random time intervals, on receiving side 100% POI can be achieved only by the radio receiver with at least 490 MHz input bandwidth - from 10 to 500 MHz. By having such a high bandwidth, system inherently needs high digital throughput support. Moreover, trade offs between ADC sampling rate/bandwidth and resolution is needed due to precise and accurate digitization of an analog signal within the optimal time. The ADC resolution represents the number of bits it uses to convert the input samples in a digital form. Depending on the resolution, all analog signals within the operating bandwidth are sampled by ADC and then converted into a resolution-digit binary number. The higher resolution values enable more accurate analyze of the signal within the bandwidth and generate high volumes of data that have to be processed and transmitted. However, it consequently causes long time data acquisition that is questioning high resolution applicability for some use cases. Furthermore, MIMO (multiple-inputs/multiple-outputs) technology implies the deployment of multiple antennas on both sides as transmitter and receiver. As the transmitted signals are received by multiple antennas, the processed data is correlated in order to minimize errors and improve the data throughput.

Valuable digital backhaul support allows SDRs to handle larger volumes of data to meet growing data processing demands. Standard serial and Ethernet interfaces once applicable for backhaul connections in SDR are now obsolete. Generally, copper cables are not an acceptable solution for high data throughput, especially in higher frequencies and over longer distances. Hence, all modern SDR products are supported by optical interfaces for digital backhaul, with fiber optic cable being the most common transmission medium. There are two major categories of fiber optics cables - multimode and single mode (also known as mono-mode). Single mode fiber optics cables are best suited for longer distance transmissions, while multimode fiber optics cables enable higher data rates on shorter distances.

Low latency enabled by fiber optics cables are directly correlated to high data throughputs. If the latency is too high, the message will take more time to reach the destination. At the same time, high latency will slow down devices, services, applications, information processing and overall network performances. Generally, latency is the measure of time that the entire message needs to reach its final destination address and is affected by the propagation and processing delays.

Propagation delay, the length of time it takes for a signal to to travel from sender to receiver, is influenced by transmission media and the distance between the source address (transmitter) and the destination address (receiver). Using fiber optic cable enables minimum propagation delays. Moreover, it has lower weight and mass and smaller size compared to other wired transmission media, provides resistance to electromagnetic interference and signal attenuation, and enables more secured transmissions with early intrusion detection. High data throughput availability, enabled by fiber optics cables, is directly related to low latency transmissions and is extremely important for real-time services provisioning.

The processing of data packets during transmission from source to destination results in another delay. This delay is known as processing delay and is typically overcome by implementation of powerful processing performances and representative memory resources.

To illustrate how this would impact a product we can highlight the low latency capabilities of the Cyan SDR. It is supported by 4 optical - 40 Gbps QSFP+ connectors from Molex. These high quality optical connectors positioned on the rear panel of Cyan, as presented in Figure 2, are the reliable indicator that all high technology features are embedded in the SDR, also providing the support for 40 Gbps backhaul connections, via multimode fiber optics cables to the hosts. This feature is of crucial importance for low latency and real time applications. For example, this allows for Cyan to aid in the fastest transpacific and transatlantic link.

Figure 2
Figure 2. Cyan optical QSFP+ connectors

Some SDRs have the capability to receive the RF signal on one frequency and relay it on another – a special case of radio rebroadcasting. This feature is very useful in some overcrowded RF bands or when the data captured on one band is high frequency and it needs to be relayed to another system on a different band. See Figure 3 for a visual example. In this example, the signal is received on 4 GHz, it is encoded at the FPGA and then the same data is rebroadcasted (transmitted) over another frequency - for example 1 GHz or any other frequency in the SDR operating range.

Figure 3
Figure 3. SDR radio signal rebroadcasting

At the same time, Crimson TNG, presented in Figure 4, is Per Vice’s customizable SDR platform dedicated to customers whose requirements are met within the 1200 MHz RF bandwidth. It operates in up to 6 GHz radio frequency range and is enabled with 20 Gbps data throughput SFP+ backhaul support.

Figure 4
Figure 4. Crimson TNG - a proven and fully customizable SDR platform

Conclusion
Here is a summary of the key benefits:

  • High Probability Of Interception (POI) within the ultra-broadband RF range.
  • Support for implementation of advanced and new signal processing algorithms.
  • High data throughput as an essential feature that enables low latency and real time services.
  • Radio signal rebroadcasting that overcomes the physical obstacles, extends the network coverage and enables the flexibility for late entry into the radio networks operating in different RF bands.
  • System can be used to coordinate real-time communications throughout many operation locations and departments
  • Ability to update software through the FPGA has been shown to lengthen the useful life of the SDR and provide the ability to pivot to various applications

There are many SDR manufacturers today which offer a variety of products which are aligned with the needs of the various SDR market segments. Brand new military technologies, together with commercial cellular communications systems and variety of industrial applications, evolved to SDR as the crucial technology in radio communications domain. As a highly adaptable and flexible technology, SDR enables the deployment of specifically tailored solutions, in compliance with the customers’ needs.

Although it often seem daunting, it is essential for customers to select SDR solution that best fits to their current but also anticipated future requirements. Investing in a long term solution that can grow and adapt with the evolving technological landscape and provide consistent value throughout it’s useful life is important in delivering consistent and reliable performance.