Digital relay with multiple virtual bent-pipe relay channels for TSVCIS

Digital Relay with Multiple Virtual Bent-Pipe Relay Channels for TSVCIS and FM Voice on SDR Dr. Aaron E. Cohen U.S. Naval Research Laboratory, 4555 Overlook Ave. SW, Washington DC 20375 Abstract— This paper presents the results of our experiment with virtual relay channels for FM Narrowband voice communications. This work was performed in both the VHF frequencies and the UHF frequencies. The following military radios were used: RT-1439a, AN/PRC-152a, and AN/PRC-117G radios. The following software defined radios were used during testing Ettus USRP b205mini connected to a desktop computer system capable of handling the processing rates for 5 Million samples per second (Msps) relay channels. This enabled receive frequencies to be placed up to 2.5 MHz apart from one another. The transmit frequencies were also placed within 2.5 MHz apart from one another due to the maximum sample rate of 5 Msps. The following waveforms were tested VHF/UHF Line of sight in plaintext and VULOS in ciphertext mode with the Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS). Keywords—Relay, SDR, TSVCIS, FM, Bent-Pipe, SATCOM, Nanosat, Cubesat, Demonstration, VULOS

I. INTRODUCTION Given recent advances and developments with regards to relays for Military communications, one thing has become clear, more relay channels are needed over longer distances. To meet this need, NRL investigated using virtual relay channels on software defined radio platforms to reduce hardware complexity. This enables multiple narrowband relay channels to be supported by wideband analog to digital converters and wideband digital to analog converters found in modern software defined radios. As will be described in this paper, frequency offsets are required to 1) specify the receive channel to listen to and 2) to shift the relay channel (frequency shifted) output signal to the appropriate location prior to relay transmission. With modern threats to satellite communication systems increasing, there is an urgent need to develop alternatives. Current threats range from anti-satellite missiles, active jamming, and more recently cyber threats. Military communications beyond line of sight (BLOS) rely on satellite communication systems. The U.S. Military has a need for an easily deployable alternative in the event that the satellite communication systems become unavailable. The U.S. Naval Research Laboratory Transmission Technology Branch has experience in developing low cost relays that can meet the documented needs of the U.S. Navy for low cost communication solutions and SATCOM denial mitigation [1, 2, 3, 4]. There is a long history of developing beyond line of sight (BLOS) communication relay platforms for the U.S. Military. BLOS relays became necessary during the Vietnam War due

to the mountainous region where operating in valleys was routine. In 1965, the first airborne relay platforms were flown in a C-7 Caribous [5]. By 1968, secure relay platforms were being flown to prevent adversaries from eavesdropping on communications [5]. Several experimental relay platforms were considered ranging from early unmanned aerial systems (UAS) to tethered balloon platforms [6]. A great many of the tethered balloon relay platforms failed [7]. What was probably needed at the time was a way to station-keep a balloon borne relay without a tether. Advances such as GPS would not become available until 1995. GPS is used for measuring more accurate weather pattern measurements for wind currents at specific altitudes. Advanced knowledge of wind currents from newer weather models combined with GPS coordinates of the balloon enables moving the balloon borne relay into a desired wind current pattern for station keeping. Long before Google Loon project existed, the U.S. Naval Research Laboratory Transmission Technology Branch developed the High Altitude Relay and Router (HARR) payloads [2, 3]. The original relay payloads relied on an Air Force Research Laboratory (AFRL) telemetry system. This telemetry system relayed GPS coordinates to the ground node. Launch and recovery of the payloads was performed by AFRL personnel support during HARR field tests. The HARR field tests successfully verified the performance of the IEEE 802.11b payloads operating in the 1.8 GHz frequency spectrum and the UHF bent pipe payloads operating in an authorized subset of the U.S. Military UHF frequencies. The HARR effort was followed by the Deep Lightning Bolt program effort. In 2008, the payloads were successfully demonstrated out of Patuxent River Naval Air Station. These payloads relayed signals over 100 nmi from ground locations and from an airborne platform. In the summer of 2009, a hardened relay design was installed on an unmanned aerial system (UAS) for the U.S. Army’s C4ISR On-the-Move exercise. Utilizing the low-altitude UAS, both data and voice were relayed between locations at Ft. Dix, NJ as part of a cooperative effort between U.S. Army CommunicationsElectronics Research, Development and Engineering Center (CERDEC) and the U.S. Navy’s Office of Naval Research (ONR). Afterwards, a new effort began to develop Surrogate TACSAT for VHF frequencies. This effort was a balloon borne bent-pipe relay with automatic gain control. In March 2014, the Surrogate TACSAT effort was successfully tested in Yuma, AZ in a tethered state of operation [4]. In 2016, prototype software defined radio relays were developed [8]. More recently, Space Data’s Combat SkySat has successfully completed demonstrations for the U.S. Army and

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U.S. Marine Corps in the UHF frequency range on balloon borne platforms [9]. Space data provides a commercial off the shelf (COTS) payload, custom COTS high altitude balloons, and ground station that provides a station keeping ability for commercial applications as well as military applications. Recent voice improvements for U.S. Military tactical radios have been developed that have the potential to improve voice quality and enable interoperability [11, 12, 13, 14, 15, 16]. These methods are able to correct errors that occur on the uplink for the relay payload without encryption. The remainder of this paper will discuss the virtual relay channel technology that was developed and tested with VHF/UHF Line of Sight (VULOS) waveform in plaintext and modern ciphertext voice using Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS) [11]. This paper is organized as follows. Section II provides an overview of the multiple simultaneous receiver, an overview of virtual relay channels as implemented in a digital relay for compress and forward relaying with traditional narrowband FM output, and an overview of the narrowband relay for VHF/UHF Line of Sight waveform in plaintext and ciphertext mode. Performance analysis and demonstration results are presented in Section III. Finally, a conclusion and future directions are presented in Section IV. II. VIRTUAL RELAY CHANNELS The multiple channel compress-and-forward relay with virtual narrowband relay channels implemented digitally is composed of the following Wideband broadcast FM reception with adjustable offsets, Narrowband FM modulation with adjustable offset, audio debugging, and sample rate modifications. A multiple channel amplify-and-forward relay with virtual narrowband relay channels was implemented digitally. It is composed of receiver frequency offsets and transmitter frequency offsets but without audio debugging since it supports ciphertext or plaintext voice along with different modulations beyond FM. A. Wideband FM reception of multiple virtual 200 kHz FM channels The first tests were performed with wideband reception of FM radio using the configuration shown in Fig. 1.

Figure 1 Basic configuration for broadcast FM reception. Connected to a computer via a USB 3.0 port not shown. Fig. 8, located after the reference section, illustrates the multiple FM radio station receiver model implemented in GNURadio that simultaneously recorded 4 separate FM radio stations. The radio receiver sample rate was set to 5 million samples per second (Msps) to maximize the number of This work was sponsored by the U.S. Naval Research Laboratory Base Research Program. Distribution A: Approved for Public Release.

available FM stations in the 2.5MHz range determined by Nyquist sampling theorem (5Msps/2). Broadcast FM Radio in North America occupies 200 kHz bandwidth per channel. This can be calculated based on Carson’s bandwidth rule. For reference: CBR = 2 (Δf + fm) with peak frequency deviation (Δf) and highest frequency content in message (fm). Each channel was sampled at 48 KHz and recorded to a wave file. The recording lasted from the time the GNURadio model was launched until the time it was closed from the main display. A minor issue arose when stopping the model through the stop command in the GNURadio companion. This killed the execution without letting the wave routines cleanly exit which prevented writing the proper header to the wave files. B. Audio Debugging Audio debugging was extremely important during testing with unencrypted audio. The first model required the audio sink block to ensure that the reception was working while the issues with wave file writing were being investigated. An audio sink module was used along with several blocks that implemented a selector to allow switching between monitoring different received FM broadcasts. This is shown in Fig. 2, Fig. 8, and Fig. 9 for monitoring different relay channels.

Figure 2 GNURadio selector implementation based on multiply and add blocks for selecting the audio to listen to. An additional chooser block, not shown, generated the comparison values. Multiply constant value is determined by a conditional statement that checks the comparison value to what was chosen by the chooser block. C. Narrowband FM transmission of multiple relay channels with frequency shift The second tests were performed on the complete virtual compress-and-forward digital relay model with the configuration in Fig. 3. This testing consisted of compressing the 200 kHz FM broadcast signals into 25 kHz narrowband FM channels for reception on the military RT-1439a radio. The updated GNURadio model, in Fig. 9 located after the reference section, consists of two receive channels that are tuned to broadcast FM radio. Each channel is converted to narrowband FM transmission. One channel is tuned slightly higher so that both relay channels can be simultaneously transmitted with one software defined radio offset by a specified bandwidth spacing from one another. Again, the radio receiver sample rate was set to 5 million samples per

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second (Msps) to maximize the number of available FM stations in the 2.5MHz range determined by Nyquist sampling theorem (5Msps/2). The transmitter side was also set to 5 Msps to give a large range of frequencies to transmit on simultaneously. A base receiver frequency of 100 MHz was chosen given available radio stations in the 100 MHz to 102.5 MHz bandwidth. A base transmit frequency was set to 70 MHz for the older RT-1439A radios which support 30 to 88 MHz.

Figure 3 Basic configuration for Compress-and-forward relay for Broadcast Wideband FM receiving to Narrowband FM transmission relaying with Narrowband RT-1439A radio tunable to either relay channel. After the successful compress-and-forward relay tests for broadcast FM radio, a complete GNURadio model for relaying only narrowband VHF/UHF Line of Sight (VULOS) transmissions was developed. This model is illustrated in Fig. 10 located after the reference section. It supports plaintext voice and modern ciphertext voice. Tests were performed using the configuration shown in Fig. 4.

simultaneously received on 4 separate radio stations within a 2.5 MHz bandwidth from 100 Mhz to 102.5 MHz. The second configuration was testing with an antenna connected to the RX2 port on the Ettus B205mini and the TX/RX port connected to the RT-1439A radio. Two separate radio stations were received simultaneously and converted to Narrowband FM and transmitted on two separate frequencies in the 70 MHz to 72.5 MHz band. Fig. 5 illustrates the interface for controlling the compress and forward FM relay.

Figure 5 The control interface for the virtual relay channels. Each channel can be moved to receive from one of 4 preprogrammed offsets. The audio source can be set to either relay channel 1 or relay channel 2. The FFT plot shows various FM signals that are easily detectable such as DC 101.1 MHz or WBIG 100.3 MHz. The third configuration was tested with an AN/PRC-117G connected to the RX2 port on the Ettus B205mini and another AN/PRC-117G connected to the TX/TX port. This allowed transmission on one receive frequency in the UHF band of 253 Mhz to 255.5Mhz and reception on the transmit frequency in the UHF band of 293 MHz to 295.5 MHz. Fig. 6 illustrates the interface for controlling the multichannel narrowband TSVCIS relay.

Figure 4 Multiple channel narrowband TSVCIS relay configuration with two AN/PRC-117G radios and the relay implemented with a computer and an Ettus B205mini software defined radio. Note: RF attenuators are not shown. III.

PERFORMANCE ANALYSIS AND DEMONSTRATION

A. Demonstration Description The first configuration was tested with an antenna connected to the Ettus B205mini. Broadcast FM radio was

Figure 6 The control interface for the virtual relay channels. Each channel can be moved to receive from one

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of 4 preprogrammed offsets. This relay was verified with TSVCIS mode and the VHF/UHF Line of Sight (VULOS) waveform. B. Performance of Plaintext Voice Final recordings of received plaintext audio were made to document the experiment and for later analysis/comparisons. There was no discernable difference between relay channels and the final recorded audio. C. Performance of CipherText Voice Final recordings of received ciphertext audio were made to document the experiment, shown in Fig. 7, and for later analysis/comparisons. There was no discernable difference between relay channels and the final recorded audio shown below in Fig. 7. The audio quality was clearly narrowband encoded and synthesized unlike the plaintext voice.

Special thanks to my colleagues Tom Moran, Jeff Kobesky, Lisa Hendricks, Michael Rupar, and Dave Heide for their assistance/encouragement with testing and ability to put up with my enthusiasm while developing this. Thanks to the GNURadio community for developing the open source software that enabled us to verify this concept. REFERENCES [1] [2]

[3] [4] [5]

[6] [7]

Figure 7 Recorded audio waveform of TSVCIS relayed voice. IV. CONCLUSION AND RECOMMENDATIONS As shown in this paper, virtual relay channels is a great way to expand the total number of supported narrowband FM relay channels in the field without requiring additional assets. This method requires an increase in processing complexity with the number of relay channels but each channel can be processed in parallel with field programmable gate arrays (FPGAs), graphics processing units (GPUs), or multicore processors. This work illustrated multiple simultaneous reception of separate FM radio stations, multiple digital relay channels implemented virtually on one software defined radio with one analog demodulator and one analog modulator, and both plaintext and modern ciphertext VHF/UHF Line of Sight (VULOS) waveform relaying with Harris AN/PRC-117G radios and legacy RT-1439a radios. In addition to the basic virtual relay channels for digital relays implemented for this work, future work includes implementing additional decode processing to filter out noise/clean up the received transmissions prior to relaying them. Of course all of these additional processing requirements are dependent on the size, weight, power, and cost (SWAP-c) of the target platform. ACKNOWLEDGEMENTS

[8]

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[10] [11] [12]

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2015 Naval Science and Technology Strategic Plan, http://www.onr.navy.mil/About-ONR/science-technology-strategicplan.aspx J. Doffoh, M. Rupar, R. Mereish, I. Corretjer, and R. Porada, "High Altitude Router and Relay for Over-the-Horizon Networks," in Proceedings of the Military Communications Conference (MILCOM), pp. 1-5, 2006 M. A. Rupar, R. Mereish, I. Corretjer, B. Vorees, and J. Doffoh, “High altitude relay and router (HARR),” NRL Formal Report, NRL/FR/5554-08-10,168, November 20, 2008 A. E. Cohen, E. J. Kennedy, and M. A. Rupar, “VHF Narrowband Relay for LOS Extension,” in Proceedings of the IEEE Military Communications (MILCOM) Conference, pp. 1275 - 1280, 2014 Thomas Matthew Rienzi (1972). "Chapter X: Special Communications Operations and Innovations". Vietnam Studies: CommunicationsElectronics 1962-1970. Washington, D.C.: United States Department of the Army. D. Fales III and Staff, “High Altitude Radio Relay Systems – Final Report”, Technical Report ECOM-0006-F, September 1968, AD0675512 Floyd E. Potter, “Letter Report of Evaluation – Balloon Borne Radio Relay”, 10 Feb. 1969, AD0848980 A. E. Cohen, Y. T. Lee, D. A. Heide, and T. M. Moran, “A Novel Software Defined Radio Relay Method for Power Conservation”, in Proceedings of the IEEE Military Communications (MILCOM) Conference, pp. 1131 – 1136, 2016 Jerry Knoblach, “Combat SkySat: Wide Area Sensing and Communications,” Presentation at Wide Area Sensing Conference (WASC) 2012, March 8, 2011, http://presentations.rmtech.org/ wasc_2012/knoblach.pdf A. E. Cohen, “BENT-PIPE RELAY COMMUNICATION SYSTEM WITH AUTOMATIC GAIN CONTROL AND METHODS OF USING SAME,” Patent Number: US 9461731 B1, Issued October 4, 2016 “Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS) Version 2.1,” July 2, 2012. T. M. Moran, D. A. Heide, and S. S. Shah, “An Overview of the Tactical Secure Voice Cryptographic Interoperability Specification,” in Proceedings of the IEEE Military Communications (MILCOM) Conference, Nov. 2 2010. P.M. Shahan, D.A. Heide, and A.E. Cohen, “Comparison of TSVCIS Voice at 8000 and 12000 BPS versus CVSD at 16000 BPS,” in Proceedings of the IEEE Military Communications (MILCOM) Conference, Nov. 1 2012. D. A. Heide, A. E. Cohen, Y. T. Lee, and T. M. Moran, “Variable Data Rate Vocoder Improvements for Secure Interoperable DoD Voice Communication,” in Proceedings of the IEEE Military Communications (MILCOM) Conference, pp. 702 - 707, 2013 D. A. Heide, A. E. Cohen, Y. T. Lee, and T. M. Moran, “Universal Vocoder using Variable Data Rate Vocoding,” NRL Formal Report NRL/FR/5555—13-10,239, Naval Research Laboratory, pp. 1-34, June 14, 2013 A. E. Cohen and M. A. Rupar, “TSVCIS Performance Analysis over Bent-Pipe Relays with Automatic Gain Control,” in Proceedings of the International Conference on Military Communications and Information Systems (ICMCIS) pp. 1-5, 2017

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Figure 8 Simultaneous FM reception of 4 separate broadcast radio channels using virtual channels through direct digital demodulation of the 200 kHz channels from the wideband 2.5 MHz sampled received signal. A selector method for the audio sink was built using chooser blocks, multiply by constant blocks, and an addition block. All four received radio stations were simultaneously recorded to separate wave files. The receiver was tuned to receive at 100 MHz. Within the 100 MHz to 102.5 MHz band there are several radio stations to choose from. The following 4 broadcast radio stations were chosen for testing 100.3 MHz, 100.7 MHz, 101.1 MHz, and 101.5 MHz.

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Figure 9 Block diagram of a two channel compress-and-forward digital relay. This relay has selectable offsets for receiving 200 KHz wideband FM from a base receive frequency of 100 MHz. Both relay channels compress the 200 KHz signals to 25 KHz signals for transmitting. A combiner combines the output of both relay channels for the software defined radio to transmit at a base frequency of 70 MHz with one relay channel shifted by 300 KHz. For testing purposes, the audio sink is used to eavesdrop on the relayed audio signal. The audio sink can be connected to either relay channel 1 or relay channel 2.

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Figure 10 GNURadio block diagram of the multiple virtual relay channels implemented digitally for tactical secure voice cryptographic interoperability specification (TSVCIS) modern cryptographic voice testing in the VHF/UHF frequency band. Above tests were performed with base receiver frequency set to 253MHz with offsets of (0, 300KHz, 1MHz, and 2MHz). Also output transmit base frequency was set to 293MHz with offsets for channel 2 of (0, 200KHz, 1MHz, and 2MHz).

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