Demos Serghiou

Postgraduate Research Student in Terahertz Channel


My research project


Ali Ali, Ali Araghi, Amirmasood Bagheri, Salman Behboudi Amlashi, Vikrant Singh, Demos Serghiou, Anton Tichenko, Seyed Ehsan Hosseininejad, Mohsen Khalily (2023)Electromagnetic Metasurfaces for 5G-and-beyond Networks

—This paper introduces the latest designed electromagnetic metasurfaces at the Institute for Communication Systems (ICS) for 5G-and-beyond networks. Various technologies and metasurfaces at different frequency ranges were developed to solve the drawbacks related to metasurfaces such as the limited bandwidth and Non-line-of-sight (NLoS) coverage issues. I. REFLECTIVE METASURFACES FOR 5G AND 6G COMMUNICATIONS One core objective of applying reflective metasurfaces in future communication systems is to provide electromagnetic (EM) coverage in the network's blind spots [1]. This happens by regulating the aperture response when it is illuminated by EM source(s), to purposefully reflect the incoming waves to the direction of interest. Controlling the aperture response can be done by the generalized Snell's law of reflection and holographic technique. In this section, we introduce two reflective metasurfaces based on these two techniques. A. Reconfigurable Intelligent Surface based on Generalized Snell's Law of Reflection A reconfigurable intelligent surface (RIS) is presented in [2] where the generalized Snell's law of reflection is applied to regulate the phase profile on the surface. This method requires knowledge about the location of the EM source and the direction of reflection, as well as the spacing between the unit cells on the surface. In a designed structure, the unit cell spacing (periodicity) is in general constant, but the location of the EM source and the direction of reflection can vary case by case. Hence, it is required to add a controllable component (varactor diode in [2]) to the physics of the unit cell to correspondingly customize the response of the surface and to make a reconfigurable structure. B. Reflective Metasurface based on Holography Technique A holographic-based reflective metasurface is presented in [3]. In the holography technique, the direction of the incoming waves must be known, and then, based on the direction of reflection, an interferogram will be obtained which is the so-called EM hologram. With this technique, it is possible to define more than one reflected beam, resulting in multi-spot coverage provisioning. Under this circumstance, the su-perposition of the desired reflecting beams will contribute to calculating the EM hologram. A dual-beam reflector is designed in [3] correspondingly. II. REFLECTIVE METASURFACE FOR OAM BEAMS GENERATION Orbital angular momentum (OAM) beams have been suggested as a strong solution to increase the channel capacity of a communication system by utilizing many orthogonal independent channels without using extra frequency resources [4]. Therefore, they can be used to solve the limited bandwidth drawback of metasurfaces. A. Reflective Metasurface with Steered OAM Beams Three environment-friendly reflective metasurfaces with single and dual-directed OAM beams to tackle the poor network coverage of THz waves in the absence of LoS communications are introduced in [5]. The integration between the OAM and THz RMTS technologies can improve spectral efficiency through a low-cost and low-profile solution. The presented metasurfaces of 90 × 90 mm were simulated, fabricated, and tested to verify the capability to control and steer the wavefront of the EM waves in the frequency range 90-110 GHz. B. THz reflectarray antenna with OAM multiplexing and beam-steering capabilities The unexplored potentials of reflectarray antennas to manipulate OAM beams are examined at 330 GHz in [6]. It investigated the maximum achievable angles by a planar meta-surface per single feed for a single OAM beam. That motivated the proposed work to investigate the possibility of generating multiple off-centered OAM beams of different modes with the maximal achievable angles for OAM multiplexing and beam-scanning applications through passive structures. The designed RAs can be envisaged for THz indoor communications. III. REPROGRAMMABLE GRAPHENE-BASED DIGITAL METASURFACE The metasurfaces using phase-only or amplitude-only engineering have limited the full functionality of the devices. In [7], a digital graphene-based metasurface simultaneously manipulating both amplitude and phase has been proposed to address this challenge in the terahertz (THz) band. As Fig. 1(c) presents conceptually, leveraging a 2/2-bit digital unit cell with independent control of 2-bit states of amplitude and phase, an efficient multi-focal meta-lens has been demonstrated. Moreover , the proposed metasurface has been applied to develop a

Demos Serghiou, Mohsen Khalily, Vikrant Singh, Ali Araghi, Rahim Tafazolli (2020)Sub-6 GHz Dual-Band 8×8 MIMO Antenna for 5G Smartphones, In: IEEE Antennas and Wireless Propagation Letters Institute of Electrical and Electronics Engineers (IEEE)

In this letter, a dual-band 8x8 MIMO antenna that operates in the sub-6 GHz spectrum for future 5G multiple-input multiple-output (MIMO) smartphone applications is presented. The design consists of a fully grounded plane with closely spaced orthogonal pairs of antennas placed symmetrically along the long edges and on the corners of the smartphone. The orthogonal pairs are connected by a 7.8 mm short neutral line for mutual coupling reduction at both bands. Each antenna element consists of a folded monopole with dimensions 17.85 x 5mm2 and can operate in 3100-3850 MHz for the low band and 4800-6000 MHz for the high band ([S11] ˂ -10dB). The fabricated antenna prototype is tested and offers good performance in terms of Envelope Correlation Coefficient (ECC), Mean Effective Gain (MEG), total efficiency and channel capacity. Finally, the user effects on the antenna and the Specific Absorption Rate (SAR) are also presented.

Ali Ali, Mohsen Khalily, Demos Serghiou, Rahim Tafazolli (2023)Reflective Metasurface with Steered OAM Beams for THz Communications, In: IEEE Access Institute of Electrical and Electronics Engineers (IEEE)

The abundant spectrum resources and low beam divergence of the terahertz (THz) band can be combined with the orthogonal propagation property of orbital angular momentum (OAM) beams to multi-fold the capacity of wireless communication systems. Here, a reflective metasurface (RMTS) is utilized to enhance the coverage of the high gain THz OAM beams by enabling the non-line-of-sight (NLoS) component by reshaping the planar wavefront of the incident wave into the helical wavefront, so that it is redirected towards the direction of interest. This can contribute to alleviating the concern of the small aperture size, since improving the channel capacity can be achieved at the low spectrum blocks of the THz band (larger aperture size). For validation, three 90 × 90 mm RMTSs are simulated, fabricated, and tested in the frequency range 90-110 GHz, to re-direct single and dual OAM beams towards the desired location.

Demos Serghiou, Mohsen Khalily, S. Johny, M. Stanley, I. Fatadin, T.W.C Brown, N. Ridler, Rahim Tafazolli (2020)Ultra-Wideband Terahertz Channel Propagation Measurements from 500 to 750 GHz, In: 2020 International Conference on UK-China Emerging Technologies (UCET)

This paper presents empirically based ultrawideband and directional channel measurements, performed in the Terahertz (THz) frequency range over 250 GHz bandwidth from 500 GHz to 750 GHz. Measurement setup calibration technique is presented for free-space measurements taken at Line-of-Sight (LoS) between the transmitter (Tx) and receiver(Rx) in an indoor environment. The atmospheric effects on signal propagation in terms of molecular absorption by oxygen and water molecules are calculated and normalized. Channel impulse responses (CIRs) are acquired for the LoS scenario for different antenna separation distances. From the CIRs the Power Delay Profile (PDP) is presented where multiple delay taps can be observed caused due to group delay products and reflections from the measurement bench.

Matthew Bridges, Mohsen Khalily, Mohammad Abediankasgari, Demos Serghiou, Pei Xiao, Rahim Tafazolli (2020)High Isolation 8?8 MIMO Antenna Design for 5G Sub-6 GHz Smartphone Applications, In: 2020 International Conference on UK-China Emerging Technologies (UCET)

In this paper, an 8×8 Multiple Input Multiple Output (MIMO) antenna design for Fifth Generation (5G) sub- 6GHz smartphone applications is presented. The antenna elements are based off a folded quarter wavelength monopole that operate at 3.4-3.8GHz. Isolation between antenna elements is provided through physical distancing. The fabricated antenna prototype outer casing is made from Rogers R04003C with dimensions based on future 5G enabled phones. Measured results show an operating bandwidth of 3.32 to 3.925GHz (S11 < 6dB) with a transmission coefficient < -14.7dB. A high total efficiency for an antenna array is also obtained at 70-85.6%. The design is suitable for MIMO communications exhibited by an Envelope Correlation Coefficient (ECC) < 0.014. To conclude a Specific Absorption Rate (SAR) model has been constructed and presented showing the user’s effects on the antenna’s Sparameter results. Measurements of the amount of power absorbed by the head and hand during operation have also been simulated.