SDRs and designing for flexibility
The design of radio equipment for the military market has evolved significantly from the use of transistor-based circuits to specialized and powerful integrated circuits (ICs). Designs may be geared towards rigid, single-purpose platforms or towards flexible, application-agnostic systems with dedicated digital signal processors (DSPs), such as software-defined radio (SDR). The one thing all of these approaches have in common: difficulty in estimating and understanding the cost/performance trade-offs of new designs.
As the complexity of wireless systems increases, so does the challenge of determining the most efficient design to meet current and potentially future needs while remaining cost effective. Nearly all new designs – including radar, electronic warfare, communications, and signals intelligence (SIGINT) – face this dilemma, where a marginal increase (or savings) in cost could have a disproportionate impact on the performance of the system.
These trade-offs are sometimes evaluated against commercially available solutions; this approach, however, typically results in overengineering of the system and incurring significant associated costs. It can also often result in a performance hit through suboptimal decisions based on the only available platforms.
SDR aims to address this trade-off issue, but even in these designs many compromises are necessary based on performance and costs. While it seems straightforward to design a product that exceeds performance requirements across all areas, this approach often results in unnecessary and extreme cost overrun.
When designing SDR platforms, there are typically six crucial elements that designers need to consider that can both drive the architecture and limit the utility of the platform (Figure 1). These elements include transmit and/or receive functionality, operating frequency, number of radio chains, instantaneous RF [radio frequency] bandwidth, FPGA/DSP [field-programmable gate array/digital signal processor] resources, and digital backhaul. In addition to these standard elements, some applications may have other requirements as well that need to be considered, including RF performance, latency, synchronization, and the like.
Not all SDR platforms provide receive and transmit functionality; the decision to proceed with either one or both is dictated by the end application. These different options can have a large or trivial impact on the overall cost of the system, depending on the architecture. For example, a platform that is modular in design will typically offer transmit as well as receive functionality, as the system will already have the resources required for both (that is, system clock, FPGA/DSP resources, digital backhaul, etc.).
Specs depending on purpose
In the past, radios were designed for a dedicated purpose on a dedicated frequency band (or bands). With SDR, this process is not as simple, and designers need to determine the upper and lower bounds of frequencies to support. 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 that operates from 100 kHz to 3.8 GHz. Finally, there are some that aim to extend the utility of the SDR by extending the operating frequency as much as possible while not exceeding a cost threshold.
Depending on the application the designer is aiming at another key element to consider is the number of radio chains. This metric enables the user to not only receive/transmit on different frequency bands simultaneously, but it can also be necessary for multiple input/multiple output (MIMO), radar, and communication applications. These applications typically require phase coherency and/or additional radio capacity that a single channel cannot offer due to bandwidth limitations.
High instantaneous RF bandwidth is critical for some users and not as useful for others. Luckily, this specification is dictated by the converters on board (i.e., analog-to-digital and digital-to-analog converters). These converter devices provide different options for sample rates, which drives the maximum instantaneous RF bandwidth along with the overall costs of the system.
In a radio design, once the signal reaches the digital domain, one of the most important aspects to consider is the available DSP resources offered. Many SDRs utilize 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 required.
The other important characteristic to consider for the digital system design is the digital backhaul; 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 since the data received is sometimes not processed onboard the unit and needs very high data-transfer rates to send to a host system. Typical digital backhauls include PCIe, 1G Ethernet, and 10G Ethernet.
To address the problem of estimating cost/performance trade-offs in the defense radio space, one approach provides designers with access to real-time cost estimates associated with different platform parameters. The “Build Your Own SDR” tool from Per Vices uses various algorithms to meet a variety of customer requirements. The tool’s categories, parameters, and available range enable selection of the most performant system, if desired, or selection of only the bare criteria required for an application.
Wireless systems designed and used by the defense market vary drastically, whether in communications and networking, radar, (counter) electronic warfare, or signals intelligence. Each of these systems require different specifications based on the application. For example, communication and networking equipment typically requires high bandwidth and encryption, while radar requires greater emphasis on the RF performance of the system, 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 on line.
Per Vices Corporation www.pervices.com