{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,17]],"date-time":"2026-04-17T06:27:18Z","timestamp":1776407238038,"version":"3.51.2"},"reference-count":35,"publisher":"MDPI AG","issue":"20","license":[{"start":{"date-parts":[[2021,10,9]],"date-time":"2021-10-09T00:00:00Z","timestamp":1633737600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100006360","name":"Federal Ministry for Economic Affairs and Energy","doi-asserted-by":"publisher","award":["19A20012D"],"award-info":[{"award-number":["19A20012D"]}],"id":[{"id":"10.13039\/501100006360","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Regional Government of Andalusia","award":["P20-00265"],"award-info":[{"award-number":["P20-00265"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>The interfaces between users and systems are evolving into a more natural communication, including user gestures as part of the interaction, where air-writing is an emerging application for this purpose. The aim of this work is to propose a new air-writing system based on only one array of ultrasonic transceivers. This track will be obtained based on the pairwise distance of the hand marker with each transceiver. After acquiring the track, different deep learning algorithms, such as long short-term memory (LSTM), convolutional neural networks (CNN), convolutional autoencoder (ConvAutoencoder), and convolutional LSTM have been evaluated for character recognition. It has been shown how these algorithms provide high accuracy, where the best result is extracted from the ConvLSTM, with 99.51% accuracy and 71.01 milliseconds of latency. Real data were used in this work to evaluate the proposed system in a real scenario to demonstrate its high performance regarding data acquisition and classification.<\/jats:p>","DOI":"10.3390\/s21206700","type":"journal-article","created":{"date-parts":[[2021,10,10]],"date-time":"2021-10-10T21:37:49Z","timestamp":1633901869000},"page":"6700","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":9,"title":["Air-Writing Character Recognition with Ultrasonic Transceivers"],"prefix":"10.3390","volume":"21","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5014-6428","authenticated-orcid":false,"given":"Borja","family":"Saez-Mingorance","sequence":"first","affiliation":[{"name":"Infineon Technologies AG, Am Campeon 1-15, 85579 Neubiberg, Germany"},{"name":"Department of Electronic and Computer Technology, University of Granada, Avenida de Fuente Nueva s\/n, 18071 Granada, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5981-4135","authenticated-orcid":false,"given":"Javier","family":"Mendez-Gomez","sequence":"additional","affiliation":[{"name":"Infineon Technologies AG, Am Campeon 1-15, 85579 Neubiberg, Germany"},{"name":"Department of Electronic and Computer Technology, University of Granada, Avenida de Fuente Nueva s\/n, 18071 Granada, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3204-1555","authenticated-orcid":false,"given":"Gianfranco","family":"Mauro","sequence":"additional","affiliation":[{"name":"Infineon Technologies AG, Am Campeon 1-15, 85579 Neubiberg, Germany"},{"name":"Department of Electronic and Computer Technology, University of Granada, Avenida de Fuente Nueva s\/n, 18071 Granada, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6476-8105","authenticated-orcid":false,"given":"Encarnacion","family":"Castillo-Morales","sequence":"additional","affiliation":[{"name":"Department of Electronic and Computer Technology, University of Granada, Avenida de Fuente Nueva s\/n, 18071 Granada, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9736-1608","authenticated-orcid":false,"given":"Manuel","family":"Pegalajar-Cuellar","sequence":"additional","affiliation":[{"name":"Department of Computer Science and Artificial Intelligence, University of Granada, Calle Periodista Daniel Saucedo Aranda s\/n, 18071 Granada, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3294-8934","authenticated-orcid":false,"given":"Diego P.","family":"Morales-Santos","sequence":"additional","affiliation":[{"name":"Department of Electronic and Computer Technology, University of Granada, Avenida de Fuente Nueva s\/n, 18071 Granada, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2021,10,9]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"403","DOI":"10.1109\/THMS.2015.2492598","article-title":"Air-writing recognition\u2014Part I: Modeling and recognition of characters, words, and connecting motions","volume":"46","author":"Chen","year":"2015","journal-title":"IEEE Trans. Hum.-Mach. Syst."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"436","DOI":"10.1109\/THMS.2015.2492599","article-title":"Air-writing recognition\u2014Part II: Detection and recognition of writing activity in continuous stream of motion data","volume":"46","author":"Chen","year":"2015","journal-title":"IEEE Trans. Hum.-Mach. Syst."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"217","DOI":"10.1016\/j.eswa.2019.06.034","article-title":"Fingertip detection and tracking for recognition of air-writing in videos","volume":"136","author":"Mukherjee","year":"2019","journal-title":"Expert Syst. Appl."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Kumar, P., Saini, R., Behera, S.K., Dogra, D.P., and Roy, P.P. (2017, January 8\u201312). Real-time recognition of sign language gestures and air-writing using leap motion. Proceedings of the 2017 Fifteenth IAPR International Conference on Machine Vision Applications (MVA), Nagoya, Japan.","DOI":"10.23919\/MVA.2017.7986825"},{"key":"ref_5","first-page":"51","article-title":"Use hand gesture to write in air recognize with computer vision","volume":"17","author":"Khan","year":"2017","journal-title":"IJCSNS"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"8855","DOI":"10.1109\/JSEN.2019.2922395","article-title":"Character recognition in air-writing based on network of radars for human-machine interface","volume":"19","author":"Arsalan","year":"2019","journal-title":"IEEE Sens. J."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"152728","DOI":"10.1109\/ACCESS.2020.3017869","article-title":"A Gesture Air-Writing Tracking Method that Uses 24 GHz SIMO Radar SoC","volume":"8","author":"Wang","year":"2020","journal-title":"IEEE Access"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1066","DOI":"10.1109\/TIM.2019.2909249","article-title":"Detecting mid-air gestures for digit writing with radio sensors and a CNN","volume":"69","author":"Leem","year":"2019","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Fang, Y., Xu, Y., Li, H., He, X., and Kang, L. (2020, January 24\u201325). Writing in the air: Recognize Letters Using Deep Learning Through WiFi Signals. Proceedings of the 2020 6th International Conference on Big Data Computing and Communications (BIGCOM), Deqing, China.","DOI":"10.1109\/BigCom51056.2020.00008"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"3227","DOI":"10.1109\/TMC.2020.2997080","article-title":"RF-Pen: Practical Real-Time RFID Tracking in the Air","volume":"20","author":"Wang","year":"2020","journal-title":"IEEE Trans. Mob. Comput."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"85","DOI":"10.1109\/MMUL.2013.50","article-title":"A new writing experience: Finger writing in the air using a kinect sensor","volume":"20","author":"Zhang","year":"2013","journal-title":"IEEE Multimed."},{"key":"ref_12","doi-asserted-by":"crossref","unstructured":"Feng, Z., Xu, S., Zhang, X., Jin, L., Ye, Z., and Yang, W. (2012, January 9\u201311). Real-time fingertip tracking and detection using Kinect depth sensor for a new writing-in-the air system. Proceedings of the 4th International Conference on Internet Multimedia Computing and Service, Wuhan, China.","DOI":"10.1145\/2382336.2382356"},{"key":"ref_13","first-page":"8088","article-title":"Air Writing via Receiver Array-Based Ultrasonic Source Localization","volume":"69","author":"Chen","year":"2020","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1756","DOI":"10.1109\/JSEN.2020.3016292","article-title":"A Three-Dimensional Pen-Like Ultrasonic Positioning System Based on Quasi-Spherical PVDF Ultrasonic Transmitter","volume":"21","author":"Chen","year":"2020","journal-title":"IEEE Sens. J."},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Holm, S. (2012, January 13\u201315). Ultrasound positioning based on time-of-flight and signal strength. Proceedings of the 2012 International Conference on Indoor Positioning and Indoor Navigation (IPIN), Sydney, NSW, Australia.","DOI":"10.1109\/IPIN.2012.6418728"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1946","DOI":"10.1109\/TIM.2017.2681338","article-title":"Wideband acoustic positioning with precision calibration and joint parameter estimation","volume":"66","author":"Kumar","year":"2017","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"1561","DOI":"10.1109\/TIM.2011.2181911","article-title":"High-accuracy reference-free ultrasonic location estimation","volume":"61","author":"Saad","year":"2012","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_18","unstructured":"Ahmed, T.B.K. (2010). Angle-of-Arrival Based Ultrasonic 3-D Location for Ubiquitous Computing. [Ph.D. Thesis, University College Dublin]."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Ruiz, D., Ure\u00f1a, J., Garc\u00eda, J.C., P\u00e9rez, C., Villadangos, J.M., and Garc\u00eda, E. (2013). Efficient Trilateration Algorithm Using Time Differences of Arrival, Elsevier.","DOI":"10.1016\/j.sna.2012.12.021"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Saez-Mingorance, B., Escobar-Molero, A., Mendez-Gomez, J., Castillo-Morales, E., and Morales-Santos, D.P. (2021). Object Positioning Algorithm Based on Multidimensional Scaling and Optimization for Synthetic Gesture Data Generation. Sensors, 21.","DOI":"10.3390\/s21175923"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Anzinger, S., Bretthauer, C., Manz, J., Krumbein, U., and Deh\u00e9, A. (2019, January 23\u201327). Broadband acoustical MEMS transceivers for simultaneous range finding and microphone applications. Proceedings of the 2019 20th International Conference on Solid-State Sensors, Actuators and Microsystems & Eurosensors XXXIII (TRANSDUCERS & EUROSENSORS XXXIII), Berlin, Germany.","DOI":"10.1109\/TRANSDUCERS.2019.8808264"},{"key":"ref_22","unstructured":"Digilent (2021, August 31). Analog Discovery 2. Available online: https:\/\/digilent.com\/reference\/test-and-measurement\/analog-discovery-2\/."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"38999","DOI":"10.1109\/ACCESS.2021.3064390","article-title":"Gesture Recognition with Ultrasounds and Edge Computing","volume":"9","author":"Saez","year":"2021","journal-title":"IEEE Access"},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Hayward, G., Devaud, F., and Soraghan, J. (2006, January 2\u20136). P1g-3 evaluation of a bio-inspired range finding algorithm (bira). Proceedings of the 2006 IEEE Ultrasonics Symposium, Vancouver, BC, Canada.","DOI":"10.1109\/ULTSYM.2006.332"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"42","DOI":"10.1016\/j.sna.2008.09.014","article-title":"Multiple-frequency ultrasonic distance measurement using direct digital frequency synthesizers","volume":"149","author":"Huang","year":"2009","journal-title":"Sens. Actuators A Phys."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"2324","DOI":"10.1109\/TUFFC.2010.1693","article-title":"Separation of overlapping linear frequency modulated (LFM) signals using the fractional Fourier transform","volume":"57","author":"Cowell","year":"2010","journal-title":"IEEE Trans. Ultrason. Ferroelectr. Freq. Control"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"343","DOI":"10.1109\/TUFFC.2013.2570","article-title":"Time-of-flight measurement techniques for airborne ultrasonic ranging","volume":"60","author":"Jackson","year":"2013","journal-title":"IEEE Trans. Ultrason. Ferroelectr. Freq. Control."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"680","DOI":"10.1121\/1.412989","article-title":"Atmospheric absorption of sound: Further developments","volume":"97","author":"Bass","year":"1995","journal-title":"J. Acoust. Soc. Am."},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Kim, K., and Choi, H. (2021). High-efficiency high-voltage class F amplifier for high-frequency wireless ultrasound systems. PLoS ONE, 16.","DOI":"10.1371\/journal.pone.0249034"},{"key":"ref_30","unstructured":"Gui\u00f1\u00f3n, J.L., Ortega, E., Garc\u00eda-Ant\u00f3n, J., and P\u00e9rez-Herranz, V. (2007, January 3\u20137). Moving average and Savitzki-Golay smoothing filters using Mathcad. Proceedings of the International Conference on Engineering Education (ICEE), Coimbra, Portugal."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Wang, Y., Yao, H., Zhao, S., and Zheng, Y. (2015, January 19\u201321). Dimensionality reduction strategy based on auto-encoder. Proceedings of the 7th International Conference on Internet Multimedia Computing and Service, Zhangjiajie, China.","DOI":"10.1145\/2808492.2808555"},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Perdios, D., Besson, A., Arditi, M., and Thiran, J.P. (2017, January 6\u20139). A deep learning approach to ultrasound image recovery. Proceedings of the 2017 IEEE International Ultrasonics Symposium (IUS), Washington, DC, USA.","DOI":"10.1109\/ULTSYM.2017.8092746"},{"key":"ref_33","doi-asserted-by":"crossref","unstructured":"Karao\u011flu, O., Bilge, H.\u015e., and Uluer, \u0130. (2020, January 5\u20137). Reducing Speckle Noise from Ultrasound Images Using an Autoencoder Network. Proceedings of the 2020 28th Signal Processing and Communications Applications Conference (SIU), Gaziantep, Turkey.","DOI":"10.1109\/SIU49456.2020.9302250"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"150","DOI":"10.1016\/j.jsb.2017.12.015","article-title":"A convolutional autoencoder approach for mining features in cellular electron cryo-tomograms and weakly supervised coarse segmentation","volume":"202","author":"Zeng","year":"2018","journal-title":"J. Struct. Biol."},{"key":"ref_35","unstructured":"Kim, U.H., Hwang, Y., Lee, S.K., and Kim, J.H. (2021). Writing in The Air: Unconstrained Text Recognition from Finger Movement Using Spatio-Temporal Convolution. arXiv."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/21\/20\/6700\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T07:10:57Z","timestamp":1760166657000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/21\/20\/6700"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,10,9]]},"references-count":35,"journal-issue":{"issue":"20","published-online":{"date-parts":[[2021,10]]}},"alternative-id":["s21206700"],"URL":"https:\/\/doi.org\/10.3390\/s21206700","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,10,9]]}}}