MIMO communications and interference networks
Distributed beamforming is challenging but desired as it scales well with the large numbers of base stations and users in the networks. We propose distributed beamforming schemes via alternating direction method of multipliers (ADMM) for MIMO multi-relay interference networks and MIMO multicell interference networks (heterogeneous cloud radio access network, H-CRAN).
Intelligent reflecting surfaces (IRSs), also known as reconfigurable intelligent surfaces (RISs), passive intelligent mirrors (PIMs), and large intelligent surfaces (LISs), have been proposed as a key enabler for next-generation wireless communication. We explore the coexistence of IRS and relays in a multiuser system. The coexistence system is different from the traditional multi-relay system, since there is no interaction between the relays in a multi-relay system but there is interaction between IRS and relays in the coexistence system. The IRS reflects signals transmitted from both the base station (BS) and relays, and there exist tradeoffs in the design of IRS beamforming to cater for both the end users and the relays.
Figure: A coexisting IRS and relay assisted multiuser system.
The degrees of freedom (DoF) of MIMO interference networks represents the number of interference-free signaling dimensions in the network. In other words, it can be regarded as the maximum number of decodable data streams in noise-free and interference-free environments. Interference alignment (IA) is based on the idea of aligning multiple interfering signals into a subspace of provably smallest dimension such that the dimension of the signal subspace is increased as much as possible for the multiuser environment, i.e., the DoF is maximized.
Figure: DoF allocation in two-cell MIMO networks.
We consider a two-cell uplink cochannel multiple-input multiple-output (MIMO) network with users sequentially arriving to the network. We study the problem of sequential base station (BS) selection for the users, with the selection criterion based on the degrees of freedom (DoF) available for the new arriving user. We find that different sequential BS selections affect individual and network performance in terms of the individual and network sum DoF as well as the number of admissible users in the network. We also study DoF-based mode selection for device-to-device (D2D) communications.
Base station cooperation (BSC), also known as network MIMO, is a multi-antenna signal processing technique that enables several nearby BSs to jointly serve multiple MSs. We proposed a distributed channel selection scheme for realizing network MIMO under time-varying wireless channel. The channel allocation problem is formulated as a noncooperative game and studied in a game-theoretic framework.
The tree representation of the MIMO detection problem is illuminating for the development, interpretation, and classification of various detection methods. We develop enhanced MIMO detection schemes based on modified shortest path algorithms innovatively applied to the detection tree.
Energy harvesting communications
With the advances of energy harvesting wireless communication technology, there is a potential for simultaneous wireless information and energy transfer in multiuser environments. Interference among different users is utilized as a source for wireless energy harvesting. The proposed efficient power recycling and usage scheme provides a promising green solution for future communication.
Figure: A multiuser multichannel wireless system supporting simultaneous wireless information and energy transfer.
We study joint subchannel allocation, power allocation, and beamforming for simultaneous wireless information and power transfer (SWIPT) in multiuser downlink orthogonal frequency-division multiple access (OFDMA) systems. We formulate a multi-objective optimization (MOO) problem where the objectives are to maximize both the information rate and the harvested power for all users in the system.
We present a novel receiver architecture design incorporating an interplay between the information receiver and the energy harvester to enhance the performance of the practical SWIPT receiver. In particular, the energy level of the received signal monitored at the energy harvester is fed back to the information receiver to assist information decoding at the information receiver. Based on this architecture, we propose a new relay protocol called energy-assisted decode-and-forward (EDF), which is an enhancement to the DF relay protocol for SWIPT networks.
Figure: The proposed SWIPT architecture with energy meter readings feedback. (a) Separated receiver architecture. (b) Co-located receiver architecture with power splitting.
Relay-assisted communications, where one or several intermediate relays
help forward the source's information
toward the destination, is a promising
technique to enhance the coverage and reliability of wireless
communication. We analytically quantify the
diversity gain of a noncoherent distributed space-frequency coded (SFC)
wireless two-hop relay system with decode-and-forward relaying.
Relay-assisted communications, where one or several intermediate relays help forward the source's information toward the destination, is a promising technique to enhance the coverage and reliability of wireless communication. We analytically quantify the diversity gain of a noncoherent distributed space-frequency coded (SFC) wireless two-hop relay system with decode-and-forward relaying.
Figure: A two-hop wireless relay system with a single source, R relays, and a single destination.
In a wireless two-way relay network, two users out of direct communication range exchange information through a single relay (e.g., two laptops communicate through the access point). Various protocols have been studied for this scenario. In the conventional decode-and-forward (DF) relaying, each user's message is sent to its destination in two hops and four time slots are needed to complete information exchange. If the network coding (NC) technique is used in conjunction with DF relaying, the relay after decoding the two users' messages broadcasts in one time slot a combined signal (e.g., XOR operation of the two users' messages), and three time slots are needed. The time efficiency and the throughput can be further improved by a protocol called denoise-and-forward (DNF) relaying adopting physical-layer network coding (PNC), introduced in 2006, which requires two time slots. In DNF relaying, the two users transmit concurrently to the relay in one time slot (the multiple access (MA) phase) and the relay receives an interfered signal. Without decoding the two users' messages, which is supposedly infeasible, the relay simply applies a many-to-one mapping technique (denoising) and broadcasts a denoised message in the next time slot (the broadcast (BC) phase).
The two-way relay network can be generalized to a multi-way relay network where each user's message is intended for all other users and full information exchange is desired through a single relay. We generalize the key idea of PNC from two-user to multiuser scenarios by proposing communication protocols that facilitate full information exchange in a multi-way relay network, as well as investigate the decoding strategies and error performances of different communication protocols for the multi-way relay network.
Physical-layer security introduces the concept of establishing secure wireless communications through the inherent randomness and location-specific properties of the wireless channel in the physical layer rather than the conventional cryptographic methods in the application layer. We study the information-theoretic secrecy rates of wireless two-way relay systems.
Machine-to-machine (M2M) communications
The machine-to-machine (M2M) relay implements a data aggregation scheme for M2M communications. The relay (data aggregator) has limited radio resources and thus only a subset of machines are selected for transmission. Missing data from nonselected machines are reconstructed at the data aggregator by exploiting data correlation.
Figure: Data aggregation in M2M communications.
In M2M communication, a massive number of machine devices may transmit simultaneously in response to an event occurring in the system. Supporting massive device transmission while maintaining low congestion and low access delay is a challenging problem. We propose a new transmission control scheme based on slotted ALOHA, with a practical consideration of partial information available at the data aggregator about the system.
Machine learning for IoT applications
We consider many important enabling technologies and applications in the future Internet of Things (IoT) paradigm, such as wireless indoor localization, indoor people counting, energy management for smart homes, etc. We propose machine learning based methods using wireless signatures such as channel state information (CSI) as features. We develop visual analytics techniques to understand the working mechanism of deep neural networks (DNNs), convolutional neural networks (CNNs), generative adversarial networks (GANs), etc. for wireless indoor localization.
Figure: Experiments for device-free wireless indoor localization.
Figure: The 2D visualization of (a) the raw CSI samples, (b) the last DNN hidden layer activations before training (with random initializations), and (c) the last DNN hidden layer activations after training. For each location, the training samples are shown with a darker shade to distinguish from the testing samples with a lighter shade of the same color. The silhouette scores (calculated for the training samples only) for (a)-(c) are 0.22, 0.09, and 0.66, respectively.
Figure: Deep convolutional generative adversarial network (DCGAN) model for semi-supervised device-free fingerprinting indoor localization.
Figure: (a)–(d) Fake CSI samples generated by G of DCGAN in four different epochs of training, i.e., epoch 0 (initialization), epoch 1, epoch 10, and epoch 100 (end of training), respectively, and (e) real CSI samples, for location p2. (f)–(j) plot the same for location p8.
Figure: A mobile micro-robot explores an unknown area to collect data to construct an energy harvesting map of the environment (left) and the traces of robotic exploration (right).
and assistive technologies
Wireless technology, such as short-range wireless systems, has found
many healthcare applications such as blood pressure or body temperature
monitoring, audio/video transmission, etc. This research focuses on
enhancing the features of wireless technology, such as low energy
consumption and small transceiver size, from an application-driven
Wireless technology, such as short-range wireless systems, has found many healthcare applications such as blood pressure or body temperature monitoring, audio/video transmission, etc. This research focuses on enhancing the features of wireless technology, such as low energy consumption and small transceiver size, from an application-driven perspective.
People with hearing loss often experience more difficulty communicating over the cell phone than over the landline and face-to-face communication. This is because of the interference between the cell phone and the hearing-assisting device, the heightened noise due to mobility and environment, and the limited phone bandwidth. This research focuses on improving hearing-impaired people's experience with the cell phone.
Figure: Modeling of speech perception in cochlear implants.
The frequency modulation (FM) system allows a person with hearing loss to receive the voice signals of the speaker directly transmitted to the receiver worn by the person with hearing loss. This is similar to the FM radio. The FM system can enhance the signal-to-noise ratio (SNR) by about 12-20 dB, greatly reducing the influence of background noise on the perception of voice messages. The FM system is widely used in educational settings such as classrooms to help children with hearing loss to listen to the teacher with less difficulty.
Figure: The FM system.
We propose a new implementation of FM systems based on smartphones using wireless technologies ("SmartHear"), which offers better accessibility, affordability, and customization and extension potentials, and is stigma-free. The proposed system (video1, video2) is included as an assistive technology by Resource Portal of Assistive Technology (衛福部輔具中心，link) and covered by the local media, magazine, and book (link1, link2, link3, link4). The SmartHear 2.0 (or 智慧聽 2.0, if the smartphone's language setting is Chinese) mobile app is available for download for free on Google Play store.
Figure: The proposed hearing assistive system and the mobile application's intuitive user interface.
Figure: SmartHear's logo.
External Research Grants
【Distributed Beamforming and Power Allocation in Wireless-Powered Dense IoT Networks / 使用無線供電技術之密集物聯網的分散式波束成型與功率分配】Subsidy for Short-Term Research Abroad for Technologists, Ministry of Science and Technology, Taiwan 科技部補助科學與技術人員國外短期研究 (2021-2022) (PI)
【GIDA: Green, Intelligent, Decentralized, and Autonomous D2D/M2M Communications for Smart IoT / GIDA: 實現智慧物聯網的節能、智能、分散、自主裝置間/機器間通訊】Ministry of Science and Technology, Taiwan (2020-2023) (PI)
【Green Smart Living Enabler: Wireless Energy Harvesting Internet of Things / 綠能與智能生活的推手：使用無線能量採集之物聯網技術】Outstanding Young Scholar Award, Ministry of Science and Technology, Taiwan 科技部優秀年輕學者研究計畫 (2017-2020) (PI)
【Green Radio, Great Potential: Simultaneous Information and Power Transfer in Multiuser Wireless Networks / 綠能無線，潛能無限：多用戶無線網路之同步訊息與電力傳輸】Outstanding Young Scholar Award, Ministry of Science and Technology, Taiwan 科技部優秀年輕學者研究計畫 (2015-2017) (PI)
【Implementing Remote Microphone Hearing Assistance Technology on Smart Phone Platform (2/2) / 遠端麥克風助聽技術於智慧型手機平台之實現 (2/2)】Research Projects on Assistive Technologies for Individuals with Disabilities, Ministry of Science and Technology, Taiwan 科技部身心障礙輔助科技技術發展研究專案計畫 (2015-2016) (Co-PI)
【Implementing Remote Microphone Hearing Assistance Technology on Smart Phone Platform (1/2) / 遠端麥克風助聽技術於智慧型手機平台之實現 (1/2)】Research Projects on Assistive Technologies for Individuals with Disabilities, Ministry of Science and Technology, Taiwan 科技部身心障礙輔助科技技術發展研究專案計畫 (2014-2015) (Co-PI)
【Communication Protocol Design and Analysis for Physical-Layer Network Coded Wireless Relay Networks / 使用物理層網路編碼之中繼站無線網路的通訊協定設計與分析】Ministry of Science and Technology, Taiwan (2014-2015) (PI)
【Study of Space Shift Keying Modulation for MIMO Systems and Its Unequal Error Protection Applications / 多重輸出入系統之空間偏移調變及其不對等錯誤保護之研究】National Science Council, Taiwan (2013-2014) (PI)