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Wireless communication is experiencing an exploding demand to enable the connectivity of everything and, consequently, the immediate response of wireless research is the development of 5G, 6G and beyond for cellular networks. The major challenge of deploying systems at mm-Wave or THz frequency bands is the high attenuation and channel fading caused by multi-path reflection and diffraction effects. Hence, multiple-element transceivers such as scalable beamforming phased array or massive multiple-input multiple-output (MIMO) systems are required for longer range communications and more precise angular resolution for imaging. Making a large and scalable array imposes a serious trade-off between the system performance and array size at mm-wave and THz.
In order to achieve scalable array size, several requirements need to be fulfilled such as: 1) the compact unit-element and unit-cell size given the spacing assumption between the antenna array, 2) coherency and local oscillator (LO) distribution, 3) intermediate frequency (IF) interfaces for taking all the signals from/to integrated circuits (ICs), 4) transition between antenna and IC (packaging), 5) transmitter efficiency and heating distribution, 6) receiver linearity, and 7) interference cancellation.
To address the LO synchronization and beamforming requirements for massive array, a power-efficient, parasitic insensitive, wideband 2x2 array, and highest frequency application of injection locked oscillators (ILO) for simultaneously frequency synchronization and beamforming for mm-wave scalable phased array (60 GHz and above) is proposed, removing the noisy and lossy phase shifter required at each scalable element.
Appeared in:
N. Ebrahimi, M. Bagheri, P. Wu and J. F. Buckwalter, “An E-band, Scalable 2×2 Phased-array Transceiver using High Isolation Injection Locked Oscillators in 90nm SiGe BiCMOS,” 2016 IEEE Radio Frequency Integrated Circuits Symposium (RFIC), San Francisco, CA, 2016, pp. 178‒181.
N. Ebrahimi, P. Wu, M. Bagheri and J. F. Buckwalter, “A 71–86-GHz Phased Array Transceiver Using Wideband Injection-Locked Oscillator Phase Shifters,” IEEE Transactions on Microwave Theory and Techniques (TMTT), vol. 65, no. 2, pp. 346‒361, Feb. 2017.
N. Ebrahimi and J. F. Buckwalter, “Robustness of Injection-locked Oscillators to CMOS Process Tolerances” in International Conference on Theory and Application in Nonlinear Dynamics, Springer Press Publication, 2016. (invited book chapter).
In order to reduce the required phase shifter bandwidth for mm-wave system, for the first time, an image selection sliding-IF Weaver architecture at mm-wave (60 GHz and above) is proposed, reducing the LO tuning range fractional bandwidth (FBW) from a conventional 20% for E-band system to 4%. In other words, the proposed image selection architecture (ISA) makes the two upper and lower band of mm-wave as images of each other relative to LO, doubling the bandwidth the system while requiring a narrowband LO phase shifter or IF phase shifter (only 4% FBW) and transferring the wide-bandwidth modulated signal from upper band to lower band and vice versa by employing a single bit phase inverter in LO-path.
In addition, employing ISA Weaver architecture for LO bandwidth reduction requires 2N numbers for I/Q mixers and 2N numbers for I/Q band pass filters (BPF) in an N-element array, making it a power-hungry and area-consuming solution. Therefore, the bidirectional N-path shared combiner/splitter that shares the second IF mixer as a down/up conversion between the array elements is proposed, reducing the required number of I/Q mixers to N+2 and only two I/Q BPF for an N-element array.
Appeared in:
N. Ebrahimi and J. F. Buckwalter, “A 71–86 GHz Bidirectional Image Selection Transceiver Architecture,” IEEE Radio Frequency Integrated Circuits Symposium (RFIC), Honolulu, HI, 2017, pp. 384‒387.
N. Ebrahimi and J. F. Buckwalter, “A High-Fractional-Bandwidth, Millimeter-Wave Bidirectional Image-Selection Architecture with Narrowband LO Tuning Requirements,” in IEEE Journal of Solid-State Circuits, (JSSC), vol. 53, no. 8, pp. 2164‒2176, Aug. 2018.
The 16-element compact antenna-IC packaging has been recently designed and demonstrated using U-M Radiation Laboratory. The proposed 2x2 antenna array configuration employs wideband four-layer aperture coupled technique to eliminate the need for multi-layer vias with large loss. In addition, in order to have a symmetric and compact LO distribution network between multiple dies and low-mismatch current injection between the ILO arrays, a novel compact feed network is proposed employing a differential wideband aperture coupled technique for electric field distribution with minimum amplitude and phase mismatch. The proposed 16-element system demonstrates 30 dBm EIRP and 1.5 GHz modulation bandwidth for 64 QAM (9 Gb/s) with minimum error vector magnitude (EVM) and EIRP variation over the entire E-band,
Related Submission:
N. Ebrahimi, K. Sarabandi, J. F. Buckwalter, “A 71-76 and 81-86GHz, Scaled 16-Element Transceiver Phased Array with Shared Image Selection Weaver Architecture, 25% EIRP to PDC, and Low EVM Variation” accepted to IEEE Radio Frequency Circuit and System Symposium (RFIC 2020).
Moreover, the next generation of wireless world is expected to have over one trillion Internet of Things (IoT) devices connected, which requires scalable security protocols. In addition, by advancing quantum computers/supercomputers, the conventional encryption algorithms with fixed secret keys can be broken in a fraction of a second. One of my main research focus has been on advancing the security of the IoT system to address their vulnerability to being attacked or compromised by advancement of future supercomputers and to regularly update the secret key.
In the proposed research idea, by focusing on IoT applications, each IoT node generates a phase-modulated random key/data and transmits it to a master node in the presence of an eavesdropper, Eve. The master node, simultaneously, broadcasts a high power signal using an omni-directional antenna, which is received as interference by Eve, and resulting in higher bit-error rate (BER) at Eve’s. The two legitimate intended nodes communicate in a full-duplex manner and, consequently, subtract their transmitted signals, as a known reference, from the received signal (self-interference cancellation). The proposed protocol does not require any knowledge of the node locations. Also, it is proven that in our novel technique, the possible eavesdropping region, defined by the region with BER < 10-1, is always smaller than the reliable communication region with BER < 10-3.
Related Publications:
N. Ebrahimi, B. Yektakhah, K. Sarabandi, H. Kim, D. Wentzloff, D. Blaauw, “A Novel Physical Layer Security Technique Using Master-Slave Full Duplex Communication,” in proceeding of 2019 IEEE MTT-S International Microwave Symposium (IMS), Boston, MA, USA, 2019, pp. 1096-1099.
N. Ebrahimi, H. Kim, D. Blaauw, “Simultaneous Interference-Data Transmission for Secret Key Generation in Distributed IoT Sensor Networks” submitted to IEEE Transaction on Microwave Theory and Techniques (TMTT).
The next generation of ultra-dense connectivity and cognitive wireless sensor networks requires low power sensor nodes that can cooperatively communicate with each other, leading to solutions for physically scalable network. However, time synchronization and collision avoidance could be very challenging for such dense distributed networks. Pulse based synchronization is a promising scalable protocol, where each node transmits its periodic pulses while adjusting its local reference based on received pulses from other nodes. In ongoing research, with the initial theoretical proof of concept published at ISIT 2019 and GLOBECOM 2018, such scalable pulse-coupled synchronization protocols is aimed to be employed to generate secret keys between clusters of cooperative sensor nodes. Based on the pulsed coupled oscillator theory presented at ISIT 2019, the number of cycles before synchronization occurs can be used as a common secret key between the IoT nodes.
Furthermore, an intruder to the network will change the dynamic of the system and result in a change of the synchronization time constant. Therefore, any unwanted intruder can be detected by the proposed cooperative dynamical-based synchronization protocol. Based on this idea it is aimed to initiate research on dynamical-based sensor nodes that are cooperatively synchronized and localized to update their dynamics (time and amplitude) in order to detect any malicious activity collaboratively.
Related Publications:
N. Ebrahimi, H. Mahdavifar “A Novel Approach to Secure Communication in Physical Layer via Coupled Dynamical Systems,” in Proceeding IEEE GLOBECOM 2018, Abu Dhabi, UAE, 2018.
H. Mahdavifar and N. Ebrahimi, “Secret Key Generation via Pulse-Coupled Synchronization,” in 2019 IEEE International Symposium on Information Theory (ISIT), Paris, France, 2019, pp. 3037-3041.
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