Tackling Ruggedization Challenges for RF Communications in Software Defined Radios
Software Defined Radios or SDRs are used in a wide variety of design requirements. This includes spectrum monitoring and analysis, control and management of a network of radios, and designing and deploying next-generation wireless communications systems. These capabilities can lend themselves to applications such as drone detection/control and deterrence, controlling the wideband spectrum for electronic warfare, secure communications and networking, massive MIMO testbeds, passive RADAR, signals intelligence, and much more.
There are various solutions for these applications, but one of the most ubiquitous approaches is utilizing NI’s Ettus Research brand of SDRs. We’ll use these enclosures for many of our examples, but a similar approach could apply to all types of SDR or radio frequency (RF) communications devices. All types of engineers and specialists in the RF communications and control arena have done prototyping and analysis using these kinds of lab and controlled-environment commercial grade systems. A key challenge has been adapting the systems to a wider range of environments. This includes outdoor applications; deployments in land, sea, and airborne craft; colder or hotter environments, or other implementations where the units are exposed to shock/vibration or debris ingress (sand, dust, salt-fog, etc.).
Challenge 1: Ruggedizing the Commercial Device for Outdoor Use
The first challenge was to provide an enclosure solution for the elements outside. The SDR might be affixed to a pole and fully exposed to the elements, but more typically enclosed in an outdoor box or under some type of plate or canopy. Therefore, the chassis needed to be IP67 weather-resistant (water immersion for 30 minutes, etc.). A fan was not a desired option for the application, so the first step was to develop a conduction-cooled enclosure. Working with the board provider allowed specific heat-sinks to be developed to optimize the cooling in the system and negate the hot spots of the boards. To ensure proper cooling, thermal simulation studies can be performed to the expected environmental conditions. The enclosure would need to be able to cool the user programmable Kintex-7 FPGA in outdoor conditions to 50 °C as well as survive temps to -10 °C. (We’ll discuss more extreme temperature ranges in Challenge 4). With large FPGA and a dual daughtercards with 10 MHz - 6 GHz of instantaneous bandwidth, the unit reaches the middle to upper range of performance of current spectrum monitoring and high-speed connectivity.
Challenge 2: Making the Commercial Enclosure Ready for Transport Grade Usage
The same type of unit in the first challenge could be designed for different application requirements. For example, an application required the unit to be simply Transport grade for Defense projects. The Transport grade ensures the system will survive at least 7.6G (peak) 11ms road shock among various other requirements to ensure the unit will be functional when transported via aircraft, vehicles, etc. Therefore, the design challenge was simply to provide a more rugged air-cooled solution. This can be achieved with a thicker metal with a more re-enforced structure, securing of loose parts inside the box, and using dampeners and special components as needed. A secondary challenge was to keep the cost and weight relatively low. The thick metal and fins for the conduction-cooled approach in Challenge 1 adds weight and some size. Re-designing the enclosure to a more rugged air-cooled solution met all of the objectives of the application. See Figure 1 for an example of an air-cooled solution. An additional benefit of this design type is the ability to optimize the cooling with a superior fan and cooling approach than the standard lab-based version.
Challenge 3: Minimizing SWaP in a Compact Sized Enclosure
While the RX310 offers high performance, the size and weight are not optimal for some applications where very low SWaP (Size, Weight, and Power) is needed. Another design challenge emanated from a customer’s need to have a man-wearable SDR that was rugged but compact and lightweight. With Analog Devices AD9361 direct-conversion transceiver capable of streaming up to 56 MHz of real-time RF bandwidth, and a lower wattage Spartan6 FPGA, the B210 SDR was a perfect starting point. It enables testing, monitoring, and experimentation with various signals such as FM, TV broadcast, cellular, Wi-Fi, and more. For use in the field where it may be constantly jostled, exposed to elements, or even dropped, the unit would be sturdy. First, a chassis would need to be compact, strong, and versatile for the application. It would be designed with a size limited to approximately 3 inches in height with a weight requirement limited to less than 7 lbs. See Figure 2 for an example. A key challenge for the design was to make it man-wearable. With an antenna placed on top of the unit, soldier or other personnel could carry the system in a backpack for multi-band communication options, signal jamming/control, and a multitude of other options. The IP67 weather-resistant design allows the unit to survive mud, sand, dust, and a significant amount of liquid exposure.
Challenge 4: Maximizing Performance for MIL Rugged Applications
While the compact unit is attractive for many designs, other systems demand the highest level of versatility, wideband performance, processing speeds, configurability, etc. Often, these are higher-end projects, where the unit needs to survive the rigors of more extreme temperatures, higher level of shock/vibration. With a Zynq UltraScale+ RFSoC and 100GbE speeds (via dual QSFP28 ports), 1 MHz to 7.2 GHz frequency range, multi-radio clocking and synchronization, the X410 fits this set of criteria. To provide the higher ruggedization, the unit can employ an IP67 design as shown in Figure 3a, or another version with MIL 38999 circular connectors for the interfaces.
Some applications require that the unit survives more extreme temperatures. For these conditions, a fan and/or heater option could be employed. Figure 3b shows a model of a RX410 with a MIL grade fan. The fan provides airflow over the fins of the enclosure from the conduction-cooled wall, so that no airflow is actually entering the system. This can help the unit survive applications up to 71 °C. For colder temperatures, an internal heater can be used for temperatures below about 15 °C. Depending on the environmental conditions of the application, either a 28VDC (18-36V range) or a 48VDC (36-72V range with recommended max of under 60V) heater can be employed for faster heat-up times.
Other Design Considerations
Customers have also incorporated special devices, batteries, or other equipment inside these enclosures. Depending on the requirements, there is the ability to modify the standard enclosures to meet these needs. Another challenge in these systems is the ability to incorporate different I/O options. A creative way to overcome this issue was to create a modular approach for the I/O plates. Therefore, a modification can be made to the enclosure without a significant impact on cost, lead-time and design approach. Although there is some flexibility in these systems, at times, the customer has used other SDRs before and desire to continue to use them.
One example is an engineer required the networked series of SDRs to network multiple systems together in an application. Therefore, versions like the RN310, which is a medium sized networked software defined radio that provides reliability and fault-tolerance for deployment in large-scale and distributed wireless systems. With a Zynq-7100 SoC FGPA and dual core ARM CPU, the ability for both standalone operation and synchronization can be useful in many applications requiring ruggedization. See Figure 4 for an example of a rugged RN310. While we’ve focused on the example of the NI/Ettus SDRs, there is certainly the capability to ruggedized other SDRs and specialty enclosures for a wide range of rugged applications.
This article was written by Justin Moll, Vice President, Sales and Marketing, Pixus Technologies. For more information, visit here .
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