SDRs and designing for flexibility

History

October 06, 2017

The design of radio equipment for the military market has evolved significantly from the use of transistor circuits to specialized and powerful integrated circuits (ICs). Projects can target rigid, single-purpose platforms or flexible, application-agnostic systems with dedicated digital signal processors (DSPs), such as software-defined radio (SDR). The one thing all these approaches have in common: the difficulty in evaluating and understanding the cost/performance tradeoffs of new designs.

As the complexity of wireless systems grows, so does the challenge of determining the most efficient design to meet current and potential future needs while remaining cost-effective. See the article : Software Defined Radio: Enhancing Communication Capabilities … – Centre for Air Power Studies. Almost all new designs—including radar, electronic warfare, communications, and signals intelligence (SIGINT)—face this dilemma, where a small increase (or savings) in cost can have a disproportionate impact on system performance.

These trade-offs are sometimes evaluated against commercially available solutions; however, this approach typically results in over-engineering the system and incurring significant associated costs. It can also often lead to performance degradation through suboptimal solutions based on the only available platforms.

SDR aims to address this trade-off problem, but even in these designs many trade-offs are required based on performance and cost. Although it seems easy to design a product that exceeds performance requirements in all areas, this approach often results in unnecessary and extreme cost overruns.

When designing SDR platforms, there are typically six key elements that designers must consider that can both drive the architecture and limit the utility of the platform (Figure 1). These elements include transmit and/or receive function, operating frequency, number of radio circuits, instantaneous radio frequency [radio frequency] bandwidth, FPGA/DSP [field-programmable gate array/digital signal processor] resources and digital transmission. In addition to these standard items, some applications may have other requirements to consider, including RF performance, latency, timing, and the like.

Figure 1: SDR platforms typically include six key elements: transmit and/or receive functionality, operating frequency, number of radio circuits, instantaneous RF bandwidth, FPGA/DSP resources, and digital transmission. Diagram courtesy of Vices.






Figure 1

Not all SDR platforms provide receive and transmit functionality; the decision to proceed with one or both is dictated by the final application. These different options can have a large or small impact on the total cost of the system, depending on the architecture. For example, a platform that is modular in design will typically offer both transmit and receive functionality, as the system will already have the necessary resources for both (ie, system clock, FPGA/DSP resources, digital carry, etc.). n.).

In the past, radios were designed for a specific purpose in a specific frequency band (or bands). With SDR, this process is not so simple, and designers must define the upper and lower limits of supported frequencies. Many times this frequency decision is driven by a specific application, such as VHF/UHF radios, while other times it is decided based on available integrated transceiver chips, such as the LMS7002M, which operates from 100 kHz to 3.8 GHz. Finally, there are some that aim to extend the utility of SDR by extending the operating frequency as much as possible without exceeding the cost threshold.

Depending on the application, the designer strives for another key element to consider is the number of radio circuits. This metric allows the user to not only receive/transmit on different frequency bands simultaneously, but may also be needed for multiple input/multiple output (MIMO), radar and communications applications. These applications typically require phase coherence and/or additional radio capacity that a single channel cannot offer due to bandwidth limitations.

High instantaneous bandwidth is critical for some users and not so useful for others. Fortunately, this specification is dictated by the onboard converters (ie A/D and D/A converters). These converters provide different sample rate options, resulting in the maximum instantaneous RF bandwidth along with overall system cost.

In radio design, once the signal reaches the digital domain, one of the most important aspects to consider is the DSP resources available. Many SDRs use FPGA ICs in their design to offer flexibility for different design requirements and usually a migration path to upgrade the FPGA when more resources are needed.

The other important feature to consider when designing a digital system is digital transmission; how data will be sent and received to and from a host system. In some designs, this feature is tied to the instantaneous RF bandwidth, as the received data is sometimes not processed onboard the device and needs very high data transfer rates to be sent to a host system. Typical digital transport links include PCIe, 1G Ethernet, and 10G Ethernet.

To address the problem of evaluating cost/performance tradeoffs in the defense radio space, one approach provides designers with access to real-time cost estimates associated with various platform parameters. The Create Your Own SDR tool by Per Vices uses different algorithms to meet different customer requirements. The categories, parameters and available range of the tool allow the selection of the most productive system if desired, or the selection of only the basic criteria required for an application.

Wireless systems designed and used by the defense market vary dramatically, whether in communications and networking, radar, (counter)electronic warfare or signals intelligence. Each of these systems requires different specifications based on the application. For example, communications and networking equipment typically require high bandwidth and encryption, while radar requires more emphasis on system RF performance, including noise figure, sensitivity, isolation, and dynamic range. These decisions driving the design of wireless systems are challenging. Such complexity in the military radio market will only increase as new electronic components become available, users demand higher performance, and new technologies come online.

Brandon Malatest graduated from the physics program with honors at the University of Waterloo, where he spent most of his time in experimental physics. After graduation, he began a career as an analyst at one of Canada’s largest market research firms. He is now one of the co-founders and COO of Per Vices Corporation, a Canadian company based in Toronto, Ontario, developing high-performance software-defined radio (SDR) platforms that are designed to meet and exceed the requirements of multiple markets. Readers may contact the author at [email protected].

According to Vices Corporation www.pervices.com

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