Multiband military communications challenges overcome by software-defined radio
The ability to access accurate, real-time information across secure data links is a cornerstone of any successful military campaign. This ability is especially needed today given the requirement of the command-and-control center to communicate directly and instantly with soldiers in the field via both voice and data. What should be a strategic and tactical advantage, however, is only as good as the ability of advanced wireless technology to navigate a web of complex radio frequency (RF) and signal-processing technologies and to render a seamless, reliable, and secure communications network that delivers the right information at the right time in an ever-more-challenged RF spectrum.
Historically, the military has relied on a profusion of different, incompatible radios and waveforms serving needs as varied as tactical soldier radios, airborne links, satellite communications, base-relay stations, and emergency transmitters, as well as emerging application-specific functions such as unmanned aerial vehicle (UAV) operation needing both control and in some instances, relaying of video surveillance images. Each of these radios establishes a vital communications link, and leaving one out of the mix would place the operational team at a disadvantage. Yet each radio carries a cost in size, weight, and – in the case of soldier systems – spare battery needs. The problem is further complicated and becomes unmanageable as new requirements and waveforms are added to the list.
To compound matters further, future operational requirements are demanding that systems be multifunctional. In addition to the need for radios to support multiple voice and data waveforms across wider RF frequencies, future soldier systems are also looking to incorporate features such as electronic surveillance and warfare functionality, thereby further increasing the system design challenges.
The solution to all of these challenges is a universal full-duplex radio module that can be used across all platforms and can be dynamically real-time reconfigured in the field to provide flexibility, versatility, efficiency, and longer operating life from a single set of batteries, all while providing significant size, weight, and power (SWaP) advantages.
Making the “universal” radio concept into a reality has proven harder than envisioned: Providing the suitable analog front end (AFE) has been particularly difficult. To complicate matters further, a next-generation “universal” radio would have to combat the challenges posed by increased spectral congestion, driven by the explosion of commercial radio and cellular systems. This not only leads to increased out-of-band signal filtering or dynamic range needs to manage the potential signals of interference, but it also requires the system to provision for operation in other frequency bands where congestion may be less of an issue.
Until recently, an AFE for this type of versatile radio potentially required an array of overlapping parallel channels, each designed to cover a particular segment of the RF spectrum, with bandwidth matched to the intended signal format and potentially with specific filtering to meet the out-of-band signal-rejection requirements. At best, this approach is costly in terms of final printed-circuit-board footprint, weight, and power; in the worst case, it requires multiple complete radios to be developed.
Adopting a software-defined approach
A software-defined radio (SDR) is the ultimate solution to this problem. By increasing the emphasis from analog to digital and software-based signal processing, it can accommodate various physical-layer formats and protocols, encrypt data, and convert analog-to-digital signals with software running on a processor or FPGA, which makes it perfect for military requirements. Users can dynamically control the frequency, modulation, bandwidth, encryption functions, and waveform requirements.
SDR also has the flexibility to incorporate new waveforms and functionality into the system, without having to upgrade or replace hardware components. These SDRs would be used not only by the soldier for data access and communicating back to the command center but also to create wide-area sensor/mesh networks for position/detection and communication among combat soldiers.
The most important component in SDR is the transceiver. To enable all of the flexibility and versatility outlined, the transceiver needs to have an exceptionally wide RF range and be capable of rapidly tuning and configuring the channel bandwidth via software; it must support both frequency-division multiplexing (FDM) and time-division multiplexing (TDM). For advanced waveform support, it must also have high performance in terms of dynamic range and reliability, even under noisy conditions and in environments with intentional and unintentional interference. At the same time, it should operate under reduced power to minimize drain on the soldier’s battery pack.
A highly integrated, mixed-signal RF integrated circuit (RFIC) makes broadband SDR designs smaller, lighter, and less power-hungry. The real challenge comes in the extremely broadband nature of the AFE in the SDR, which in a classical design would need many spectrum- and possibly waveform-specific front ends, each of which is a challenge to design and evaluate, with the final product likely to fall short in the SWaP ranking.
A new generation of wideband, programmable front-end transceivers that support dual independent transceiver channels has the potential to meet these SDR challenges. The system processor can dynamically reconfigure key parameters (such as bandwidth and RF frequency) to match the application needs and deliver optimum results and also has the potential to serve the fast-growing multiple-input, multiple-output (MIMO) segment as well as non-MIMO needs.
The new generation of wideband integrated RF transceivers are the ideal solution for many of the next generation of military communication and electronic surveillance systems. As they support wide channel bandwidths, the devices match many of the new data-centric waveforms being deployed in next-generation systems. This high level of configurability enables cognitive radio systems and the use of adaptive waveforms. With the ability to rapidly tune across a wide RF bandwidth, the devices can be a solution for electronic surveillance, particularly in platforms where power may be limited.
One such transceiver is the Analog Devices AD9361 RF Agile Transceiver (see Figure 1), a 10 mm x 10 mm chip-scale device with a user-tunable bandwidth from 200 kHz to 56 MHz; it also includes other features and performance attributes needed to build a signal chain spanning 70 MHz to 6 GHz. This 2 x 2 direct-conversion component reduces the entire AFE into a single, relatively simple circuit and interfaces with the host processor via an LVDS or CMOS port. Within the IC are 12-bit A/D and D/A converters, fractional-N synthesizers, digital and analog filters, automatic gain control, transmit power monitoring, quadrature correction, and other critical functions.
The receiver noise figure is also less than 2.5 dB, while transmitter error vector magnitude (EVM) is better than -40 dB and transmitter noise floor is below -157 dBm/Hz. For both transmit and receive paths, the local oscillator step size is just 2.5 Hz for precise tuning. Despite the many functions within the IC, power consumption is still low, generally around 1 W.
Integrated system design
A flexible, wideband SDR platform involves a major circuitry-design effort, algorithm development, and consideration of tradeoffs. Although at first glance this may seem a daunting development, a new generation of transceivers must include an available reference design to simplify the system development. In addition to being fully customizable in software without any hardware changes, the reference platform must be optimized for use with FPGAs and provide a guide for both power supply and board layout to ensure optimum performance. Furthermore, to support the increasing prevalence of MIMO configurations, supporting options for beamforming and beamsteering reference designs should illustrate multi-device synchronization (see Figure 2).
By adhering to the design guidelines outlined in the reference designs, engineers can quickly move from evaluation to prototyping through to successfully implementing a full-duplex, lightweight, long-lasting platform that can be dynamically reconfigured in the field and finally bring the “universal” radio from drawing-board concept to real-world applications.
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