Military software-defined radio benefits from commercial and consumer trends
The pace of technology innovation, together with improvements in component technologies and amplifier design techniques used in the commercial wireless industry, have strong leverage into the requirements for military software-defined radio (SDR).
With commercial cellular technology accounting for more than two billion cellphones a year and millions of high-power infrastructure-access points (base stations, repeaters, microcells, picocells), the cellphone industry not only addresses our personal connectivity needs, but also connects our homes, automobiles, and devices. Three major trends in the cellular industry are directly applicable to the SDR needs of defense electronics: the increase in the spectrum used by the device, the increase in the waveform bandwidth, and (in consideration of these first two innovations) the ever-increasing need to improve radio-frequency (RF) power efficiency.
Spectrum range increasing
The commercial wireless technology follows reports and specifications created by the telecommunications organizational partnership known as the 3rd Generation Partnership Project (3GPP). The 3GPP standards called Long-Term Evolution (LTE) and LTE-Advanced have become the most rapidly deployed and adopted commercial wireless technologies in history. The 3GPP group plans advancements of the LTE technology and reports guidelines for the use of spectrum bands through staged releases on a near-annual basis. Looking at the progress, 3GPP Release 12 demonstrates an increase from 11 operating bands (3GPP Release 8) to 44 bands in the last four years. These spectrum bands are coordinated between national spectrum authorities, international guidelines, and wireless service providers to meet the growing demand of the commercial industry. Some fielded designs of popular smartphones address more than 24 bands. LTE technology is used between 450 MHz to 3.8 GHz today, and with the study of LTE-U (Unlicensed) technology, we could see the implementation of LTE technology reaching 6 GHz in the near future.
The impact of the wide spectrum range within the small form factor of a cell phone or a smartphone is that the RF front-end electronics need to become scalable and function over a much wider bandwidth. Designs from five years ago on a six band phone may have included separate RF paths for transmit and receive bands that contained RF filters and amplifier combinations. These separate RF paths were relatively narrow bands, which use a relatively small fractional bandwidth. For example, a frequency duplex band of 60 MHz at 2 GHz covers requires an RF design to cover only 3 percent of the fractional bandwidth of operation. At the component level, narrowband filters and amplifiers could be designed for optimum performance within these very specific bands.
With smartphone designs now operating across dozens of frequency bands, the scalability of narrow band RF designs is no longer practical. Consider an extreme design example, one that could operate across all 44 frequency bands of the Release 12 spectrum range. This would require the RF design to cover over 88 percent of the fractional bandwidth of operation versus the 3 percent fractional bandwidth of the banded method. While digital tunable filter technology has yet to meet the mainstream price and performance requirements to match this trend in commercial designs, the RF amplifier technologies have already adapted to the wide fractional bandwidth of use for many commercial designs. Filter technologies like Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) have had tremendous technical advances in size and price to remain cost effective for relatively narrow fractional bandwidth and low power filter requirements.
The cellular infrastructure technology has undergone a similar challenge. To keep pace with a multiband cell phone the design drivers for cellular base stations are creating innovations in higher power technologies. For the higher power infrastructure applications covering broad frequency ranges, the advancements of gallium nitride (GaN) have now lead to commercial power transistors that have been designed for wideband performance with high power density.
Related to the defense electronics industry, producing a cost effective RF front end for tactical radios has always been a challenge. While no longer a program, the traditional design goal of the Joint Tactical Radio System (JTRS) was to focus on the radio waveforms in the frequency spectrum between 2 MHz and 2 GHz. The operating frequency spectrum, power, and performance requirements of the different JTRS radio waveforms were a major technical barrier to the program. Newer radio programs are looking for more modest integration requirements.
Consider a current tactical multiband radio that currently operates between 30 MHz and 400 MHz. Support for tactical VHF would use the 30 MHz to 88 MHz spectrum, Air Traffic Control VHF between 118 MHz to 137 MHz, maritime VHF between 156 MHz to 174 MHz, and military UHF aeronautical radio and between 225 MHz to 400 MHz. A new requirement might take this existing tactical radio and add an integration requirement for the civil First Responder Network Authority (FirstNet) Band 14 in the upper 700 MHz band. Since the 10 Watt output power requirement of the Band 14 radio is similar to the power of some of the other VHF and UHF band radios, an integrated design might consider an architecture similar to one widely used in the commercial cellular infrastructure industry based on GaN with a shared RF amplifier component design to reduce size and weight of the radio.
In the GaN industry, there are fielded components from major suppliers that now support greater than 10 watts of power between 30 MHz and 2.5 GHz for tactical radio applications. This is over a 98 percent fractional bandwidth from a single RF amplifier device.
While LTE technology has a maximum signal bandwidth of 20 MHz, the LTE-Advanced technology from 3GPP Release 10 saw the introduction of carrier aggregation and carrier aggregation in noncontiguous spectrum increase the data rates delivered through the network. While the individual channel carrier still is limited to 20 MHz, if you consider the carrier aggregation scenarios shown in Figure 1, the bandwidth of the RF signal increases dramatically. A two-carrier contiguous intraband scenario shown in Figure 1a might have as much as 40 MHz output, while an intraband noncontiguous configuration in Figure 1b might have a signal bandwidth of an entire spectrum band (up to 75 MHz). Further, the interband configuration seen in Figure 1c between two separate bands could have several hundred MHz or even over 1 GHz of band separation.
The interband, noncontiguous scenario leads to the requirement for both the low-power cellphone and higher-power base station, creating essentially a single multisignal waveform across the entire operation bandwidth. The filtering and power amplifier requirements to create this signal continue to be the major drivers of improvements to the component technology and amplifier design.
In the tactical radio environment, wideband waveforms are commonplace and have been used in the field for many years. Take, for example, the Link-16 radio that operates across the Aeronautical Radio Navigation Services (ARNS) band from 960 MHz to 1215 MHz: This radio covers 255 MHz of spectrum using frequency hopping and covers over 20 percent of the fractional band of operation.
Improving RF efficiency
As cellphone technology improves in functionality, boosts data rates, increases RF bandwidth, and improves the size and quality of the graphical displays, one of the major drivers in the cellular industry is the continued focus on improving the RF efficiency of the devices. In the cellphone, the improved efficiency enables advanced functionality and longer battery life, but in the cellular infrastructure industry, the improved efficiency impacts the bottom line for a wireless service provider by reducing the operating expense of the network. In both cases, the cellular industry has driven improvements in the efficiency of RF devices.
When you consider the efficiency of the RF power within the context of increasing fractional operating bandwidth and signal bandwidth, mentioned previously, both the designs of the cellphone and the cellular infrastructure have driven toward using envelope tracking (ET) bias-modulated designs. (Figure 2.)
In an ET design, the envelope of the modulated signal is matched to the RF through the power amplifier by using an envelope detector and DC bias modulator controlling the supply voltage. With precise timing and matching of the amplifier nonlinear performance, the power amplifier can approach optimum efficiency when operated near the saturation points of the devices themselves.
Operating a component near saturation requires an analysis of the nonlinear device behavior that can vary due to the RF waveform and power level. Digital predistortion is typically used to compensate for the nonlinear effects and improve the spectral purity.
As the power amplifier might be used over a very large fractional bandwidth, the power device industry has improved the ability of amplifier components to operate over these larger bandwidths. As the signal of interest uses more bandwidth from the amplifier, whether a frequency hopping signal or multiband signal, it is important to choose the envelope detector and DC modulator that has enough bandwidth to match the wideband signal. The timing and shaping of the bias modulation needs to be carefully characterized. Envelope tracking designs are common now in the cellular industry and some amplifier designs have over 50 percent efficiency.
The Rohde & Schwarz FSW Signal and Spectrum Analyzer and SMW Signal Generator (Figure 3) is aimed at designers and developers of newer envelope-tracking designs directly applicable to use in SDR.