Approaches for Inaudible Acoustic Communication
|Zhang, B., Zhan, Q., Chen, S., Li, M., Ren, K., Wang, C., & Ma, D. (2014). PriWhisper: Enabling Keyless Secure Acoustic Communication for Smartphones. IEEE Internet of Things Journal, 1(1), 33-45.||Short-range wireless communication technologies have been used in many security-sensitive smartphone applications and services such as contactless micro payment and device pairing. Typically, the data confidentiality of the existing short-range communication systems relies on so-called “key-exchange then encryption” mechanism. Namely, both parties need to spend extra communication to establish a common key before transmitting their actual messages, which is inefficient, especially for short communication sessions. In this work, we present PriWhisper — a keyless secure acoustic short-range communication system for smartphones. It is designed to provide a purely software-based solution to secure smartphone short-range communication without the key agreement phase. PriWhisper adopts the emerging friendly jamming technique from radio communication for data confidentiality. The system prototype is implemented and eval-uated on several Android smartphone platforms for efficiency and usability. We theoretically and experimentally analyze the security of our proposed acoustic communication system against various passive and active adversaries. In particular, we also study the (in)separability of the data signal and jamming sig-nal against Blind Signal Segmentation (BSS) attacks such as Independent Component Analysis (ICA). The result shows that PriWhisper provides sufficient security guarantees for commercial smartphone applications and yet strong compatibilities with most legacy smartphone platforms||The Pre-Whisper project realizes novel NFC approaches for smartphones (among others with audio)||http://www.cse.msu.edu/~glxing/CSR-NFC.htm|
|Edwards, Alexander Orosz, "Ultrasonic Data Steganography" (2016). Honors College Capstones and Theses. 5. http://digitalcommons.kennesaw.edu/honors_etd/5|| What started off as a question on the possibly of data transmission via sound above the level of human hearing evolved into a project exploring the possibility of ultrasonic data infiltration and exfiltration in an information security context. It is well known that sound can be used to transmit data as this can be seen in many old technologies, most notably and simply DTMF tones for phone networks. But what if the sound used to transmit signals was in in the ultrasonic range? It would go generally unnoticed to anyone not looking for it with tools such as a spectrum analyzer. This could provide an unnoticed means of transmitting overhead data without the use of radio signals or physical connections, or, more clandestinely, a means to inject or retrieve data virtually undetect ed for espionage, control, or other malicious activity. As expected, there would obviously be issues with signal quality as the open air is heavy with environmental interference, but in specific cases as seen in the following research, a discrete sonic means of data transmission may not only be practical, but necessary for the task at hand.
This project is an exploration of the practicality of ultrasonic data transmission between computers. It will include research into the topic in general from scientific, technological, and security perspectives. There will be inclusions from other research projects as well as practical applications already in existence. Interestingly, there are already some suspected, but unconfirmed planned systems as well security incidents using this technology. Finally, a short seriesof semi-formal (in a scientific sense) experiments conducted to provide firsthand accounts and results of the ultrasonic data transmission concept
|A hands-on report on the development of an ultrasound communication approach. Related work is presented and problems of the technology are pointed out.||http://digitalcommons.kennesaw.edu/honors_etd/5/|
|Edmund Noval||PhD Thesis: "Security and Privacy for Ubiquitous Mobile Devices".||The author has developed an ultrasonic communication protocol (based on binary phase/frequency shift keying), see Chapter 4 for details. It has a theoretical transmission rate of 4.6kbps and operates between 18 and 22kHz. It uses phase and frequency modulation.||http://publish.wm.edu/cgi/viewcontent.cgi?article=1079&context=etd|
|Lee, H., Kim, T. H., Choi, J. W., & Choi, S. (2015, April). Chirp signal-based aerial acoustic communication for smart devices. In 2015 IEEE Conference on Computer Communications (INFOCOM) (pp. 2407-2415). IEEE.||Smart devices such as smartphones and tablet/wearable PCs are equipped with voice user interface, i.e., speaker and microphone. Accordingly, various aerial acoustic communication techniques have been introduced to utilize the voice user interface as a communication interface. In this paper, we propose an aerial acoustic communication system using inaudible audio signal for low-rate communication in indoor environments. By adopting chirp signal, which is widely used for radar applications due to its capability of resolving multi-path propagation, the proposed acoustic modem supports long-range communication independent of device characteristics over severely frequency-selective acoustic channel. We also design a backend server architecture to compensate for the low data rate of chirp signal-based acoustic modem. Via extensive experiments, we evaluate various characteristics of the proposed modem including multi-path resolution and multiple chirp signal detection. We also verify that the proposed chirp signal can deliver data at 16 bps in typical indoor environments, where its maximum transmission range is drastically extended up to 25 m compared to the few meters of the previous research.||Basis for encoding the informationa are chirp sounds. Frequency range: 19.5-22kHz, range: up to 25 meters||http://ieeexplore.ieee.org/abstract/document/7218629/|
|T. Hosman, M. Yeary, J. K. Antonio, and B. Hobbs, ―Multitone FSK for ultrasonic communication,‖ in Proc. Instrumentation and Measurement Technology Conference IEEE, 2010, pp. 1424–1429.||Traditional radio frequency communication schemes are not capable of transmitting signals through metal enclosures. However, in some applications it is necessary to transmit information to/from devices located inside metal enclosures, e.g., a closed shipping container in transit. A conformal ultrasonic communication system based on multi-tone FSK (MFSK) has been developed and evaluated using steel corner posts from shipping containers as the communication medium. The communication system is configurable, consisting of two or more modules. A module is mounted to a metal surface and utilizes an inexpensive ultrasonic transducer to send and receive modulated signals through the metal channel. A module also makes use of an inexpensive DSP chip for modulating and demodulating MFSK signals. For the shipping container application, experiments were conducted that achieve data rates of approximately 800 bps. Experiments related to two scenarios for the shipping container application were investigated: (1) communicating through one container and (2) communication between stacked containers. For the second case, experiments were conducted with modules on two separate corner posts that are under compressive load.||Application: communication inside shipping containers||http://ieeexplore.ieee.org/abstract/document/5488066/|
|C. Li, D. Hutchins, and R. Green, ―Short-range ultrasonic digital communications in air,‖ IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 55, no. 4, pp. 908–918, 2008.||The use of ultrasound in air as a means of communicating digital signals is demonstrated. The work uses capacitive transducers with a useful bandwidth to transmit digitally coded signals across an air gap in the laboratory, using three of the common methods used in digital communications. These are on-off keying (OOK), binary frequency-shift keying (BFSK), and binary phase shift keying (BPSK). All three methods are simulated numerically using the available bandwidth of the transducer systems and are compared to results obtained experimentally. It is demonstrated that BPSK can be used to transmit signals with a low bit error rate.||3 types of modulations are evaluated: on-off keying, binary frequency shift keying and binary phase keying||http://ieeexplore.ieee.org/abstract/document/4494786/|
|Santagati, G. E., & Melodia, T. (2015). U-Wear: Software-Defined Ultrasonic Networking for Wearable Devices (pp. 241–256). ACM Press. https://doi.org/10.1145/2742647.2742655|| Wearable medical sensing devices with wireless capabilities have become the cornerstone of many revolutionary digital health applications that promise to predict and treat major diseases by acquiring and processing health information. Existing wireless wearable devices are connected through radio frequency (RF) electromagnetic wave carriers based on standards such as Bluetooth or WiFi. However, these solutions tend to almost-blindly scale down traditional wireless technologies to the body environment, with little or no attention to the peculiar characteristics of the human body and the severe privacy and security requirements of patients. We contend that this is not the only possible approach, and we present U-Wear, the first networking framework for wearable medical devices based on ultrasonic communications.
U-Wear encloses a set of physical, data link and network layer functionalities that can flexibly adapt to application and system requirements to efficiently distribute information between ultrasonic wearable devices. U-Wear also offers reconfiguration functionalities at the application layer to provide a flexible platform to develop medical applications. We design two prototypes that implement U-Wear and operate in the near-ultrasonic frequency range using commercial-off-the-shelf (COTS) speakers and microphones. Despite the limited bandwidth, i.e., about 2 kHz, and COTS audio hardware components not optimized for operating at high frequency, our prototypes (i) achieve data rates up to 2.76 kbit/s with bit-error-rate lower than 10-5 using a transmission power of 20 mW; (ii) enable multiple nodes to share the medium; and (iii) implement reconfigurable data processing to extract medical parameters from sensors with high accuracy.
|A networking solution for wearable medical devices based on ultrasonic communication is presented
The prototype uses a Teensy 3.1 board, a small MEMS microphone and a COTS speaker
|Santagati, G. E., & Melodia, T. (2016). A Software-Defined Ultrasonic Networking Framework for Wearable Devices. IEEE/ACM Transactions on Networking||Wearable medical devices with wireless capabilities have become the cornerstone of many revolutionary digital health applications that promise to predict and treat major diseases by acquiring and processing physiological information. Existing wireless wearable devices are connected through radio frequency electromagnetic wave carriers based on standards, such as Bluetooth or Wi-Fi. However, these solutions tend to almost blindly scale down traditional wireless technologies to the body environment, with little or no attention to the peculiar characteristics of the human body and the severe privacy and security requirements of patients. We contend that this is not the only possible approach, and we introduce U-Wear, the first networking framework for wearable medical devices based on ultrasonic communications. U-Wear encloses a set of physical, data link, and network layer functionalities that can flexibly adapt to application and system requirements to efficiently distribute information between ultrasonic wearable devices. U-Wear also offers reconfiguration functionalities at the application layer to provide a flexible platform to develop medical applications. We design two prototypes that implement U-Wear and operate in the near-ultrasonic frequency range using commercial-off-the-shelf (COTS) speakers and microphones. Despite the limited bandwidth, i.e., about 2 kHz, and COTS hardware components not optimized for operating at high frequency, our prototypes: 1) achieve data rates up to 2.76 kbit/s with bit-error-rate lower than 10⁻⁵ using a transmission power of 13 dBm (20 mW); 2) enable multiple nodes to share the medium; and 3) implement reconfigurable processing to extract medical parameters from sensors with high accuracy.||Ultrasonic communication for communication between medical devices; 2.7kbit/s with error rate 10^-5||http://ieeexplore.ieee.org/abstract/document/7725492/|
|Wang, Q., Ren, K., Zhou, M., Lei, T., Koutsonikolas, D., & Su, L. (2016, October). Messages behind the sound: real-time hidden acoustic signal capture with smartphones. In Proceedings of the 22nd Annual International Conference on Mobile Computing and Networking (pp. 29-41). ACM.||With the ever-increasing use of smart devices, recent research endeavors have led to unobtrusive screen-camera communication channel designs, which allow simultaneous screen viewing and hidden screen-camera communication. Such practices, albeit innovative and effective, require well-controlled alignment of camera and screen and obstacle-free access. In this paper, we develop Dolphin, a novel form of real-time acoustics-based dual-channel communication, which uses a speaker and the microphones on off-the-shelf smartphones to achieve concurrent audible and hidden communication. By leveraging masking effects of the human auditory system and readily available audio signals in our daily lives, Dolphin ensures real-time unobtrusive speaker-microphone data communication without affecting the primary audio-hearing experience for human users, while, at the same time, it overcomes the main limitations of existing screen-camera links. Our Dolphin prototype, built using off-the-shelf smartphones, realizes real-time hidden communication, supports up to 8-meter signal capture distance and ±90° listening angle and achieves decoding rate above 80% without error correction. Further, it achieves average data rates of up to 500bps while keeping the decoding rate above 95% within a distance of 1m.||The dolphin framework is presented for ultrasonic communication with smartphones technology behind: orthogonal frequency division multiplexin (OFDM)||http://dl.acm.org/citation.cfm?id=2973765|
|Ka, S., Kim, T. H., Ha, J. Y., Lim, S. H., Shin, S. C., Choi, J. W., ... & Choi, S. (2016, October). Near-ultrasound communication for TV's 2nd screen services. In Proceedings of the 22nd Annual International Conference on Mobile Computing and Networking (pp. 42-54). ACM.||In this paper, we propose a near-ultrasound chirp signal-based communication for the TV's 2nd screen services. While near-ultrasound (with under 20 kHz frequency) communication has been developed for various applications recently, none of the previous work provides a perfect solution for 2nd screen services between TVs and smart devices. This is due mainly to the following real world challenges. The embedded signal in TV contents should be successfully received in a typical TV-watching environment by (i) delivering information at least at 15 bps with significantly low volume to avoid human perception, (ii) despite the presence of ambient noise, e.g., a tick, a snap, or a knock. To fulfill (i), we design chirp quaternary orthogonal keying (QOK) symbols. Especially, we aim to minimize inter-symbol interference (ISI) effects by symbol design in order to completely eliminate guard intervals. To resolve (ii), we propose the novel J-shape detection algorithms for both frame synchronization and carrier sensing. The proposed modem achieves almost zero frame error rate on a smartphone 2.7 m away from the TV even with minimal receive sound pressure level of 35 dBSPL, i.e., the noise level in a very quiet room. Moreover, throughout experiments and log analysis of 2nd screen service deployed in a nation-wide TV broadcasting system, J-shape detection algorithms are proven to achieve highly resilient performance for both frame synchronization and carrier sensing compared to previous schemes.||Near-ultrasound signals; modulation: chirp quaternary orthogonal coding (QOK); Application: sync TV and smartphone (for second screen applications)||http://dl.acm.org/citation.cfm?id=2973774|
|Lin, M. C., Huang, F. Y., & Chiueh, T. D. (2015, July). A-NFC: Two-way near-field communications (NFC) via inaudible acoustics. In Information, Intelligence, Systems and Applications (IISA), 2015 6th International Conference on (pp. 1-6). IEEE.||Near Field Communication (NFC) is a technology for wireless short-range communication. The main attribute of NFC is proximity and that data transmission will take place only within a very small area. However, NFC-based applications are still not prevalent due to slow progress of NFC hardware penetration. In this paper, we present A-NFC, an NFC-like communication system, which operates on existing mobile phones. It is an inaudible acoustics-based system utilizing the microphones and speakers on mobile phones. In addition, A-NFC is purely software based and can run on different operating systems. A-NFC implements full-duplex two-way communication by designing two independent channels in the inaudible frequency band. To the best of our knowledge, A-NFC is the first acoustics communication system capable of inaudible and two-way communications on various popular platforms (Android and iOS included). Extensive experiments demonstrate that A-NFC can reliably provide NFC capability with a data rate up to 2.2 Kbps on various devices.||Inaudible two-way communication for Android and iPhone Frequency band: 15.8-20.6 KHz, Modulation: Frequency-Division Duplexing (FDD) Send and receive on different frequency channels||http://ieeexplore.ieee.org/abstrac/document/7388085/|
|Arentz, W. A., & Bandara, U. (2011). Near ultrasonic directional data transfer for modern smartphones (p. 481). ACM Press.||The rapid advancements in the smartphone domain has made WiFi, Bluetooth and 3G commonplace in most devices, as well as accelerometers, gyros, fast CPUs and large amounts of memory. During these advancements the IR transmitter was eventually dropped, starting perhaps most importantly with the release of the Apple iPhone. It has since been observed a need for directional transmission functionality. The system described herein propose a way to to achieve short range directional data-communication on a smartphone without adding external hardware. The outcome of this project is intended to be open-sourced, enabling any application developer or phone-manufacturer to include the technology into their products.||A feasibility study on ultrasonic communication between smartphones
Low technical complexity
Frequency band: 20-23kHz
Modulation: pulsed signal (short pulse=0, long pulse=1)
They indended to make their software open source (in 2011)
|Lazik, P., Rajagopal, N., Shih, O., Sinopoli, B., & Rowe, A. (2015). ALPS: A Bluetooth and Ultrasound Platform for Mapping and Localization (pp. 73–84). ACM Press.|| The proliferation of Bluetooth Low-Energy (BLE) chipsets on mobile devices has lead to a wide variety of user-installable tags and beacons designed for location-aware applications. In this paper, we present the Acoustic Location Processing System (ALPS), a platform that augments BLE transmitters with ultrasound in a manner that improves ranging accuracy and can help users configure indoor localization systems with minimal effort. A user places three or more beacons in an environment and then walks through a calibration sequence with their mobile device where they touch key points in the environment like the floor and the corners of the room. This process automatically computes the room geometry as well as the precise beacon locations without needing auxiliary measurements. Once configured, the system can track a user's location referenced to a map.
The platform consists of time-synchronized ultrasonic transmitters that utilize the bandwidth just above the human hearing limit, where mobile devices are still sensitive and can detect ranging signals. To aid in the mapping process, the beacons perform inter-beacon ranging during setup. Each beacon includes a BLE radio that can identify and trigger the ultrasonic signals. By using differences in propagation characteristics between ultrasound and radio, the system can classify if beacons are within Line-Of-Sight (LOS) to the mobile phone. In cases where beacons are blocked, we show how the phone's inertial measurement sensors can be used to supplement localization data. We experimentally evaluate that our system can estimate three-dimensional beacon location with a Euclidean distance error of 16.1cm, and can generate maps with room measurements with a two-dimensional Euclidean distance error of 19.8cm. When tested in six different environments, we saw that the system can identify Non-Line-Of-Sight (NLOS) signals with over 80% accuracy and track a user's location to within less than 100cm.
|Extend BT LE Beacons with ultrasound to improve ranging accuracy
Focus: estimate 3D location of a beacon relative from a smartphone (by using BT and ultrasonic sound)
This enables indoor localization and tracking
|Lazik, P., & Rowe, A. (2012). Indoor pseudo-ranging of mobile devices using ultrasonic chirps (p. 99). ACM Press.||In this paper, we present an indoor ultrasonic location tracking system that can utilize off-the-shelf audio speakers (potentially already in place) to provide fine-grained indoor position data to modern mobile devices like smartphones and tablets. We design and evaluate a communication primitive based on rate-adaptive wide-band linear frequency modulated chirp pulses that utilizes the audio bandwidth just above the human hearing frequency range where mobile devices are still sensitive. Typically transmitting data, even outside of this range, introduces broadband human audible noises (clicks) due to the non-ideal impulse response of speakers. Unlike existing audio modulation schemes, our scheme is optimized based on psychoacoustic properties. For example, all tones exhibit slowly changing power-levels and gradual frequency changes so as to minimize human perceivable artifacts. Chirps also bring the benefit of Pulse Compression, which greatly improves ranging resolution and makes them resilient to both Doppler shifts as well as multi-path propagation that typically plague indoor environments. The scheme also supports the decoding of multiple unique identifier packets being transmitted simultaneously. By applying a Time-Difference-of-Arrival (TDOA) pseudo-ranging technique the mobile devices can localize themselves without tight out-of-band synchronization with the broadcasting infrastructure. This design is not only scalable with respect to the number of transmitters and tracked devices, but also improves user privacy since the mobile devices compute their positions locally. We show through user studies and experimentation on smartphones that we are able to provide sub-meter (95% < 10cm) accurate indoor positioning in a manner that is imperceptible to humans.||Acoustic indoor tracking system with normal speakers (in ultrasound band)
Modulation: chirp pulses
Localization by time of flight measurements
|Murata, S., Yara, C., Kaneta, K., Ioroi, S., & Tanaka, H. (2014). Accurate Indoor Positioning System Using Near-Ultrasonic Sound from a Smartphone (pp. 13–18). IEEE.||This paper describes new concepts and techniques for an indoor positioning system that uses near-ultrasonic sound from a smartphone. The indoor positioning system can be used in many practical applications, for example, in detecting the location of moving objects, such as a person or a wheelchair, and navigation within a wide indoor area. Indoor positioning systems seem to require a higher positioning accuracy compared with systems for use in outdoor areas. The authors have previously proposed a solution for indoor positioning using ultrasonic sensors. However, these suffer from a shortcoming in that users have to possess a special ultrasonic transmitter. The system proposed here does not need such a transmitter, because a smartphone is used as the sound source. Smartphones are already widely used, so the proposed system seems to be easy to introduce for practical use. The sound transmitted from the smartphone has been investigated and confirmed, as has the validity of the developed receiving unit which makes use of the timer count values of a microcomputer, which gives an indication of the timing of the detection of sound from the smartphone. Positioning tests for static and moving objects have been carried out in both quiet and noisy environments. It has been verified that the positioning accuracy is sufficient for navigation for visually impaired persons and for other applications.||Indoor tracking system based on ultrasound that is sent from a smartphone
Several receiving units in the room detect the ultrasonic signal from the user
|Thiel, B., Kloch, K., & Lukowicz, P. (2012). Sound-based proximity detection with mobile phones (pp. 1–4). ACM Press.||We present a method for proximity detection with mobile phones that is based on a combination of Bluetooth communication (for the detection of coarse proximity) and sound beacons in an inaudible spectrum around 18kHz for a finer spatial resolution. The system performs a real-time recognition of personal encounters in two common situations: standing together and walking by each other. We evaluate our approach in a variety of settings ranging from office corridor, through a busy street to a shopping mall.||Combination of BT and ultrasonic information for distance estimation
Modulation: on-off keying
|Yi, W.-J., Gilliland, S., & Saniie, J. (2013). Mobile ultrasonic signal processing system using Android smartphone (pp. 1271–1274). IEEE.||This study introduces a mobile ultrasonic signal processing (MUSP) system using an Android smartphone for remote ultrasonic testing and imaging applications. The Android smartphone has multiple wireless data communication options such as Bluetooth, Wi-Fi and cellular data networks. The smartphone receives the ultrasonic data using the Bluetooth connection from a data acquisition and communication unit (DACU). With the help of Android Native Development Kit (NDK) libraries, we developed two signal processing algorithms in C programming language which are processed by the Android smartphone to explore the smartphone computing capability for ultrasonic testing applications. Split Spectrum Processing (SSP) and Chirplet Signal Decomposition (CSD) algorithms are considered for benchmarking and signal analysis. The analyzed data is displayed in real-time on the smartphone screen and streamed to a central location via Wi-Fi or cellular data networks for storage and further data analysis. A Java programmed server application is implemented to communicate with the Android application over the Internet in order to display and save the retrieved signal data. This system brings the ability to analyze ultrasonic signals remotely and to transfer ultrasound data from one end to the other for extensive signal ultrasonic imaging at a central location. The accessibility of the ultrasonic data at the central location allows experts to review ultrasonic information and make decision about the state of the health of structures and critical components under test.||Two ultrasonic sound descomposition schemes are implemented on an Android platform. The ultrasonic data, however, is sent via BT to the phone. It is then decomposed on the phone.||http://ieeexplore.ieee.org/abstract/document/6674886/|
|Tayyab Javed, (2013). Enabling Indoor Location-based Services Using Ultrasound. Master Thesis. Queen’s University Kingston, Ontario, Canada||In the context of location, large amounts of information are available on the Internet to be accessed by people via different devices. However, at times people have to manually search and access it. If the space where location-based services are available can be identified by hand-held devices, people can be prompted with services available around them. This thesis explores the use of ultrasound as a communication medium to tag such spaces and access location-based services with the related information; and demonstrates the indoor implementation of the prototype of a location-based services enabling system for hand-held devices. This system allows users to search and access the available services in their surroundings through their hand-held devices. A beacon generator placed in the service location broadcasts a service code mappable to the services particular to that location encoded in an ultrasound signal. The hand-held device can identify this signal and prompt the user with available services. System design and architecture is demonstrated and the viability of the system is tested through a variety of environments and scenarios showing that potentially this has both a wide range of applications and can enhance the way people access locationbased services.||Implemented beacon / location-based services. Made some experiment to avoid reverberation: Used a sync frequency, two silence frequencies, and one data frequency. Best result with 1-1-2-4 (length of 1 time sync, 1 time silence, 2 times data, 4 times silence)||http://www.collectionscanada.gc.ca/obj/thesescanada/vol2/OKQ/TC-OKQ-7797.pdf|
|Justin Shumaker (2009). Designing an Ultrasonic Modem for Robotic Communications. U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5066||A modem consists of a modulator and demodulator that transmit and receive data, respectively. The impetus for building such a device stemmed from the 2008 FIRST Robotics League (FRL) competition rules that stated only acoustic and electromagnetic (EM) frequencies ranging from infrared to ultraviolet could be utilized to communicate with the robot. Common inexpensive ultrasonic piezoelectric transducers range in frequency from 24 to 40 KHz. A typical 40-KHz transducer requires 10 or more oscillations to reach peak resonating amplitude.1 This restriction implies a theoretical maximum of 4000 bits/s in the case of a 40-KHz transducer. Unfortunately, additional constraints exist that prohibit data transfer at 40 KHz. Dependent upon the environment, signal attenuation can severely limit the device’s maximum bandwidth. An oscilloscope can provide the necessary insight to develop a rather inexpensive ultrasonic modem for reliably transferring data in an environment saturated with audible acoustic noise up to a distance of 25 m.||Only sending very basic commands, but maybe interesting for the hardware side ?||http://www.dtic.mil/get-tr-doc/pdf?AD=ADA499556|