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Beyond-Line-Of-Sight Troposcatter Communications Primer

Hardware solutions that mitigate the design challenges and meet requirements of the latest tropospheric scatter applications.

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Though tropospheric scatter (troposcatter, or tropo) communications technology has existed since the 1950s and was used by the U.S. military from 1960 to 2002, this legacy technology is being revitalized in the wake of concerns around the reliability of tactical satellite communications (Satcom). For several decades, satellites were a reliable and secure method of communications that provided strategic services for every branch of the military. However, as the technology of non-allied countries has improved, satellites and Satcom systems are becoming more vulnerable. Many are looking again to troposcatter technology to help ensure critical voice and data communication are transmitted securely around the globe.

The following provides insights into the challenges and requirements of tactical troposcatter communications and further discusses COTS hardware solutions that mitigate the design challenges and meet requirements of the latest troposcatter applications.

Troposcatter communications design requirements and challenges
Troposcatter communications rely on transmitted signals scattering in the troposphere, with a significant-enough signal power scattered in the direction of a waiting receiver. Much like with Satcom signals, only a very small portion of the signal energy ever reaches the receiver, with the majority of the signal energy reflecting throughout the atmosphere or being sent to space. Using a highly directional antenna with substantial gain can help mitigate some of the signal energy losses. However, this also means that both the transmitting and receiving antenna would both require precise alignment pointing to the same region of the troposphere in order to transmit and receive signals to intersect the area known as the common scattering volume, where the forward scattering phenomenon occurs.

Typical troposcatter communication frequencies range from tens of megahertz to over 10 GHz, depending on the technology used. Higher-frequency systems allow for smaller antennas and interconnects for comparable gain and performance to lower-frequency troposcatter systems. However, higher-frequency signals tend to suffer from greater loss within the RF circuitry and through the atmosphere. Hence, there is a tradeoff between the size and weight of troposcatter communication systems with cost and portability.

Portability is a key factor for troposcatter communication systems, as the main applications for this technology are deployable systems mounted on land mobile vehicles or systems that are readily transportable by mobile units and deployed in temporary bases. These systems may also need to be deployed rapidly as a replacement or redundant system in case of satellite communications or other line-of-sight (LOS) communication failure. Beyond-line-of-sight (BLOS) communications, such as satellite and troposcatter systems, are essential in operating theatres where mountainous or hilly landscapes prevent the reliable use of LOS communications.

Therefore, ruggedness and reliability are also critical requirements for new troposcatter systems, which will likely be included in addition to Satcom and other currently employed LOS communications systems. Light weight and compact size are other factors that are highly valued for the latest troposcatter communications, as these do come as added equipment that must be stored, shipped, and/or mounted on mobile platforms.

Another factor to consider is the throughput and bandwidth demand of the latest tactical communication systems. Where legacy tactical communication systems required only a few megahertz of bandwidth and throughput that only reached several megabits per second (mbps), the latest tactical communication systems that are part of the overall Department of Defense’s modernization efforts require hundreds of megahertz of bandwidth and throughput that reaches hundreds of megabits per second in order to be comparable to military satellite systems.

To reach high levels of throughput, a troposcatter communication system also requires high-performance RF devices with low added noise and phase noise to minimize the degradation of modulation signals sent through the communication link. Moreover, to minimize the error-vector magnitude (EVM) and provide higher dynamic range, RF devices for troposcatter systems must also be highly linear.

C-band troposcatter solutions
The latest troposcatter communication system requirements place substantial challenges on RF designers and suppliers, with simultaneous requisites for low weight, ruggedness, reliability, compact size, high bandwidth, excellent electrical performance, and efficiency. Fortunately, Wolfspeed has a well-established line of gallium nitride (GaN) high-electron-mobility transistor (HEMT) power amplifier (PA) solutions that include the CGHV1F025S. This high-efficiency and high-gain transistor can be utilized for ground-based Satcom applications and is well suited to use in a range of BLOS, aerospace, and Satcom applications, including troposcatter.

The CGHV1F025S has an operating frequency range of DC to 15 GHz, presenting a good balance of size and weight for troposcatter antenna and interconnect while offering frequency capability to serve even the latest tactical communication systems capable of high throughput performance and frequency agile operation powered by software-defined radio (SDR) with direct-digital synthesis (DDS) technology. Moreover, this transistor makes the most of potentially limited power sources with nearly 50% typical power efficiency at 9 GHz. With 25-W typical power output and 11 dB of typical small signal gain, this transistor is extremely power-efficient, as it is offered in a 3 × 4-mm dual-flat-no-lead plastic package.

GaN
The CGHV1F025S’s viability as a troposcatter communications amplifier is demonstrated by the application fixture CGHV1F025S-AMP4, which was originally designed for C-band Satcom applications. The CGHV1F025S-AMP4 is built with two CGHV1F025S amplifiers with a Wilkinson combiner and operates from 4.4 GHz to 5 GHz with an overall saturated output power of 50 W. The design enables over 50% drain efficiency and over 13 dB of small signal gain with less than 3% EVM demonstrated, with an under y-6.-dB peak-to-average power ratio (PAPR) offset quadrature phase-shift keying (OQPSK) signal.

Conclusion
The result of the effort of Wolfspeed’s engineers is an amplifier design that enables a tactical radio system to be operated under a constant amplitude signal with performance characteristics that are desirable for troposcatter communication applications. The size, weight, and ruggedness of the GaN HEMT technology is industry-leading, with other benefits coming from the COTS plastic packaging style used with the CGHV1F025S transistors.

References

  1. Network Modernization: Tropo, https://peoc3t.army.mil/tn/tropo.php
  2. Application Fixture (CGHV1F025S-AMP4) for C-Band Satellite Communications,
    https://www.wolfspeed.com/downloads/dl/file/id/1350/product/0/troposcatter_tactical_radio_reference_design.pdf
  3. CGHV50200 200-W 4400-5000-MHz GaN HEMT, https://www.wolfspeed.com/cghv50200f
  4. CGHV1F025S, https://www.wolfspeed.com/media/downloads/424/CGHV1F025S.pdf

 



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