K. J. Ray Liu

Liu's research contributions encompass broad aspects of signal processing and communications, including wireless communications; coding; networking; game theory; multimedia signal processing; information forensics; network security; and signal processing algorithms and architectures. In the past decade, he was credited for pioneering and commercializing ambient sensing and intelligence by leveraging the ambient radio signals ubiquitously surrounding us to decipher human activities.^[2]

Liu applied time-reversal techniques to wireless communications and sensing. Multipaths have long been considered as nuisance and treated as interference and noise. Liu was the first to propose time-reversal division multiple access (TRDMA) in 2011 as a new wireless multiple access scheme for 5G/6G and beyond for broadband communications.^[41][42][43] It was earlier than technology readiness for such a dramatic paradigm shift, but now time-reversal is a promising technology being considered for 6G wireless communications.^[44]

Using the time-reversal principle, he also developed the world’s first centimeter-accuracy indoor positioning and tracking system in 2015 with only a single transmitter and terminal device, both with a single antenna, in a completely non-line-of-site environment.^{[45][46][47][48]} It represented the first non-line-of-sight, non-triangulation technique for accurate position estimation, solving a long-standing conundrum of indoor positioning/tracking for over many decades.^[47]

He further showed that when there is a large enough number of multipath signals (such as indoor environments), the time-reversal focusing spot exhibits a stationary behavior in its energy distribution.^[49] His work enabled accurate tracking of an unlimited number of subjects, indoors, and without costly infrastructure or a priori measurements, it also served as the theoretical foundation for accurate/reliable wireless sensing by simply using ambient radio waves, including Wi-Fi.^[47][48]

With that, Liu and his team at Origin developed an AI platform including many analytic engines with applications such as indoor tracking; gait determination; motion detection for security; sleep monitoring; monitoring small motions inside a car; material sensing; monitoring heart rate and breathing; heart rate variability detection; fall detection; recognizing and counting people in hidden spaces; millimeter-wave imaging; millimeter-wave real-time handwriting tracking and analysis; millimeter-wave keyboard tracking; and sound detection.^[47][48]

He was the first who proposed in 2019 the establishment of an international standard on wireless sensing to the Chair of the IEEE 802 Standard Committee, who facilitated the creation of 802.11bf WLAN Sensing as the world's first wireless sensing standard.^[50]

Remoteeyes was the world's first digital video surveillance system to monitor and secure home/office through the Internet remotely.^[47][51][52]

Liu pioneered cross-layer design using antenna arrays for wireless communications in 1997 by first introducing the concept of duality between uplink and downlink for joint transmit beamforming and power control to increase the number of users in a cellular network by a factor of 100.^[53][54]

When the narrowband space–time codes were exciting the research community, there have been many attempts in the research community to search for the broadband full-diversity space-frequency code or even the full-diversity space-time-frequency code. Liu's group showed for the first time in 2003 that the maximum achievable diversity order with arbitrary channel power delay profile is a product of the number of delay paths, the rank of the channel temporal correlation, and the number of transmit and receive antennas. With that his group developed the world’s first full-diversity, full-rate space-frequency code^[55][56] and the only available systematic space-time-frequency code^[57] that achieves the maximum achievable diversity in space, time, and frequency for broadband wireless communications by exploiting all of the available spatial, temporal and frequency diversities in broadband wireless systems. This work provides a first step toward achieving full-diversity in multiple-input multiple-output (MIMO) broadband OFDM systems that are commonly used in modern wireless communications standards such as 4G/WiFi and beyond.

He published research on cooperative communications, addressing performance, coverage, energy efficiency, and throughput.^[58][47]

Liu published early research in multimedia forensics and security. His 2005 book, Multimedia Fingerprinting Forensics for Traitor Tracing^[59] laid the foundation of this emerging field, in which he contributed new concepts, such as anti-collusion codes,^[60] behavior forensics,^[61] and anti-forensics.^[62] He coined the name “information forensics” when proposing the journal IEEE Transactions on Information Forensics and Security.^[2][47]

Liu was the prime architect and proposer of IEEE Transactions on Information Forensics and Security, IEEE Journal on Selected Topics of Signal Processing, and IEEE Transactions on Multimedia. He also initiated the creation of IEEE Transactions on Computational Imaging and IEEE Transactions on Signal and Information Processing over Networks. As Vice President – Publications, he started the Inside Signal Processing eNewsletter for IEEE Signal Processing Society.^[2]

Liu created IEEE DataPort, a data-sharing platform integrated with IEEE journals and co-led the development of the IEEE App.^[2]

Liu has published over 900 refereed papers, 250 patents, and 10 books, including the following:^[63]

• "Reciprocity, Evolution, and Decision Games in Network and Data Science", Cambridge University Press, 2021^[64]
• "Wireless AI: Wireless Sensing, Positioning, IoT, and Communications", Cambridge University Press, 2019^[48]
• "Behavior Dynamics in Media-Sharing Social Networks", Cambridge University Press, 2011^[65]
• "Cognitive Radio Networking and Security – A Game Theoretic View", Cambridge University Press, 2010^[66]
• "Cooperative Communications and Networking", Cambridge University Press, 2009^[58]
• "Resource Allocation for Wireless Networks: Basics, Techniques, and Applications", Cambridge University Press, 2008^[67]
• "Ultra-Wideband Communication Systems: The Multiband OFDM Approach", Wiley, 2007^[68]
• "Network-Aware Security for Group Communications", Springer, 2007^[69]
• "Multimedia Fingerprinting Forensics for Traitor Tracing", Hindawi, 2005^[59]
• "Design of Digital Video Coding Systems: A Complete Compressed Domain Approach", Marcel Dekker, 2001^[70]
• "Handbook on Array Processing and Sensor Networks", Ed., IEEE-Wiley, 2009^[71]