Hardware considerations for software-defined tactical radio
A trend in the design of battery-powered tactical radios is that of a software-defined product, where a variety of legacy and future waveforms are installed into a small, lightweight enclosure.
By Richard Buckley
A trend in the design of battery-powered tactical radios is that of a software-defined product, where a variety of legacy and future waveforms are installed into a small, lightweight enclosure. To provide such a system, the radio hardware needs to support additional frequency coverage, higher data rates, faster frequency transitions, and simultaneous operation of multiple channels. These features need to be accommodated in an ever-shrinking space, while maintaining or exceeding performance established in legacy platforms. Above all, the hardware needs to provide the ruggedness, high reliability, and long battery life that our end users require in hostile environments.
These design challenges present opportunities for novel use of present and future technologies at the disposal of the radio design engineer. This article presents the challenges faced with designing such equipment, and recommends areas where radio manufacturers and their component suppliers may make meaningful contributions to the next generation of military radio equipment.
HF, VHF and UHF: unique in their own ways
Present multiband tactical radios operate over the 2-to-512 MHz frequency range. Next-generation radios aim to cover 2-to-2000 MHz and beyond, to accommodate high data rate waveforms in less crowded frequency bands. These frequencies have all been in use for decades, but combining these bands into one small, battery-powered package is a relatively new technology area. Each frequency band has unique characteristics.
High frequency (HF) operation typically uses the 1.6-to-30 MHz frequency range, and provides a means to communicate over long distances with a minimum of infrastructure required. Despite its maturity, HF continues to be a widely used communications medium.
This long distance path typically requires large power amplifiers, antennas, and associated antenna matching units, which become challenging problems in a mobile, tactical environment where big antennas present a target, and large power amplifiers are impractical for battery-powered equipment. One solution is to design a 'jerk-and-run' capability at the system level, where a lower power HF asset can plug into a vehicular system, offering the best of both worlds: a medium-range battery-powered solution and a long-range, high-power solution. This is an example where interoperability between portable and vehicular radios is a significant benefit to the end user.
The long-range nature of HF means that undesired signals from sources such as nearby high-power transmitters accompany the desired receive signal. These co-site problems are typically mitigated by designing sharp filters to attenuate out-of-band signals, and high intercept point front-end circuits that can tolerate large in-band interferers. Trading off several decibels of receive noise to achieve better co-site performance is wise for an HF radio, given that the spectrum contains a relatively high Quasi-Minimum Noise (QMN) environment that sets the low-end sensitivity threshold, in contrast to higher bands with lower environmental noise levels.
In HF transmit mode, governing documents such as MIL-STD-188-141B4 put tight constraints on spectral performance. The use of a narrow (often 3 kHz wide) information bandwidth dictates the use of narrow intermediate frequency (I.F.) filters and synthesizer loop bandwidths. This ensures that the transmit noise, spurious, and harmonic specifications of a high-power transmitter can co-exist with sensitive receivers in the HF, VHF, and UHF bands.
Very high frequency (VHF) radios typically operate in the 30-to-225 MHz frequency band, suitable for medium-propagation distances that require smaller antennas, matching circuits, and power amplifiers (relative to HF operation). VHF uses wider bandwidths than HF (up to 25 kHz) to support faster data rates. The faster data rates and frequency agility associated with VHF waveforms drive the designer toward somewhat wider filter and synthesizer bandwidths than the HF designer. However, designers must contend with the co-site challenges in this notoriously crowded frequency band.
External devices often provide the necessary filtering of undesired signals. Of course, users of battery-powered equipment rarely have the means to carry external accessories, and require that the radio itself provide an optimum balance of out-of-band filtering, in-band intercept point, and low-noise figure to support the mission-at-hand. As such, designing a separate VHF front end that is robust for small and large signals is a good choice, where a slight tradeoff of power consumption is restricted to only the VHF waveforms.
Support of VHF Air Traffic Control (ATC) requires a highly linear transmission and tight spectral control to support an understandably critical mission. Such requirements historically require Class-A amplifiers, which provide linearity at the expense of efficiency. Such a scheme represents a major shortening of battery life in all but the lowest duty cycle transmissions. An optimum design would use one of several switched mode techniques for providing an efficient amplifier, controlled in a fashion that yields a linear result. Often, these schemes perform very well at a single frequency, but less so over a wide frequency range. This is one of many areas where clever use of digital signal processing (DSP) can be employed to optimize hardware circuits as they function in a variety of conditions such as frequency, voltage, load, and temperature variations.
Ultra High Frequency (UHF) provides still different challenges over the 225-to-512 MHz frequency range, including short-haul Line-Of-Sight (LOS) communications and satellite communications (SATCOM). The propagation characteristics of this band through weather conditions, foliage, and other obstacles make UHF SATCOM an indispensable communications medium.6
In the case of satellite communications, directional antennas are used to improve antenna gain, and consequently improve data rates on both transmit and receive links. MIL-STD-188-181B provides useful calculations7 that point to the need for higher transmit power and lower receive noise figure than other waveforms, in order to have a small antenna size appropriate for a tactical mission. One solution is to have antennas equipped with receive LNAs, to provide the extra sensitivity required.
For LOS communications, UHF frequencies allow wide information bandwidths, which support higher data rates than do relatively old narrowband waveforms. The hardware paths that accommodate these bandwidths are several megahertz wide, as opposed to the narrowband IF sections described earlier. Equipments dedicated to wideband formats have been in the field for several years; putting narrowband and wideband waveforms in a single, small, battery-powered radio is a new challenge.
To make the most of data throughput, the hardware must support fast transmit-to-receive times and high frequency-hop rates for agile applications, driving the need for fast switches and high-performance synthesizer designs. Designers must weigh the classic tradeoff of synthesizer phase noise versus switching speed differently for wideband waveforms. In anticipation of these needs, several component suppliers have introduced fast, tightly integrated synthesizer designs for consideration by the radio designer.
The addition of 512-to-2000 MHz capability is not great in terms of number of octaves, but represents a move from lumped element designs (often used at lower frequencies) to distributed designs. This band has seen extensive use of radio frequency integrated circuit (RFIC) designs that provide custom solutions in small packages. Many components have served commercial markets in narrow portions of the 512-to-2000 MHz band, but large programs such as the Joint Tactical Radio System (JTRS) have enticed suppliers to offer oscillators, mixers, switches, splitters, combiners and power amplifier devices that cover these wider frequency ranges.
Multiband hardware: one signal path or several?
Taken as a whole, radio hardware that accommodates all frequencies between 2 and 2000 MHz must be extremely flexible.
One design view would accommodate bandwidth flexibility in the form of direct modulation and demodulation. This topology would make use of DSP techniques to filter, modulate, and demodulate the waveform. Notionally, the receive path would pass through a preselector to provide a first level of coarse filtering, then directly demodulate down to baseband I and Q signals, where adjustable bandwidth DSP-based filtering would remove undesired interferers and mixing products. The downside of such an approach is that all signals within the preselector passband reach the DSP hardware, meaning that all circuitry in the signal path need to handle potentially large interfering signals; this can reduce battery life profoundly in some circumstances. Further, DSP filtering to the extent required to meet legacy requirements can require substantial processing power, which in turn requires higher clock speeds, itself a power consumption driver. Finally, the prospect of discriminating a very small desired signal from very large interferers may require A-D converter bit lengths that cannot presently support the speed requirements. Designers must give similar considerations to transmit mode, although a prior knowledge of the expected signal level simplifies matters to some extent.
Ultimately, future technologies may simplify these problems. Higher-bit A-D converters would reduce the burden on the hardware to provide receive AGC and input filtering. Processors with less power-per-instruction cycle ease the battery life demands. In transmit, faster direct digital synthesizers (DDS) are becoming available, which someday may make low-noise direct conversion a possibility. Micro-electromechanical Systems (MEMS) are beginning to become commercially available, with the prospect of high intercept point, low insertion loss front-end components.
A more conservative approach would break the design into two or more bands, where each receive signal path has its own optimization of pre-selection, hardware filter bandwidth choices and synthesizer design. In some cases, one path could use a conventional design with IF filtering to ensure satisfaction of legacy requirements, and another path could support newer wideband waveforms with a different set of tradeoffs. Solutions for transmit mode can be viewed similarly. Many variations exist, all with various merits and weaknesses that need to be balanced by the radio hardware designer.
A residual advantage of banded approaches is that many components do not presently support full 2-to-2000 MHz operation, at least not without some performance compromises. Notable examples include power amplifiers and antennas. Future technological advances in these areas will help to make the most of performance per unit volume that our customers demand.
A related consideration of banded vs. one-path-fits-all design is the multi-channel aspect of new radio requirements. For instance, a transmit channel running as high as 20 watts would require significant isolation not to interfere with a desired receive sensitivity of -120 dBm. Antenna separation on a portable radio platform may provide only a tiny fraction of the required isolation. A respectable wideband transmit noise floor of -150 dBc/Hz many megahertz away from the transmit frequency, combined with a worst-case antenna isolation, may still not be enough to leave the receive signal unaffected. Transmitter post-selection would likely be required to provide acceptable results when transmitting and receiving in the same band. Likewise, robust pre-selection is required to prevent overload conditions in the receiver front end.
The situation improves with frequency separation between transmit and receive channels, leaving the prospect of transmitting in one frequency band, and receiving in another band, a more readily achieved goal. Again, software can help to provide a practical solution, providing interference cancellation when the transmit properties of a waveform are known.
From an electromechanical standpoint, a multi-channel radio would benefit from independent power supplies, localized regulation of key voltages, improved battery technology, new cooling techniques, and substantial shielding between circuits, mechanically and on printed circuit layouts. These methods influence the size and weight of the finished product, and require careful consideration when making packaging trade studies.
A suite of solutions?
Size and power consumption are key factors when making the design tradeoffs outlined herein. Ultimately, the best solution may be an interoperable suite of wearable, handheld, manpack, vehicular, and fixed-station products, with software truly portable between the platforms. Software design would be the key to such interoperability, with current U.S. military programs driving radio manufacturers in this area. Success for the end user will likely come in the form of capabilities sized to the package required for each mission, seamlessly integrated with its larger and smaller counterparts.
Richard Buckley is senior member of the technical staff at the Harris Corp. RF Communications Division in Rochester, N.Y.