Dr Demos Serghiou

Postgraduate Research Student in Terahertz Channel


My research project


Demos Serghiou, Tim W C Brown Dataset and code for reproducing the results of "Autoregressive Modelling for Sub-THz Spatio-Temporal Wideband Scattered Reflection Coefficient Measurement of Complex Structures", In: Autoregressive Modelling for Sub-THz Spatio-Temporal Wideband Scattered Reflection Coefficient Measurement of Complex Structures University of Surrey

The dataset includes separate excel files for the received complex valued distributions of the first 8 delay taps of the power delay profile for each scenario presented in the paper titled "Autoregressive Modelling for Sub-THz Spatio-Temporal Wideband Scattered Reflection Coefficient Measurement of Complex Structures". This is accompanied by example matlab code that allows reproduction of the results.

Demos Serghiou, Mohsen Khalily, Ali Ali, Tim Brown, Rahim Tafazolli (2024)Autoregressive Modelling for Sub-THz Spatio-Temporal Wideband Scattered Reflection Coefficient Measurement of Complex Structures, In: Dataset and code for reproducing the results of "Autoregressive Modelling for Sub-THz Spatio-Temporal Wideband Scattered Reflection Coefficient Measurement of Complex Structures" Wiley

This paper presents spatio-temporally resolved wideband measurements of Sub-Terahertz (Sub-THz) reflection coefficients in the frequency range of 92-110 GHz. A stochastic model for single reflection fixed links that is capable of modelling random scattering from small-scale discontinuities such as those encountered in complex structures in walls and partitions of buildings is presented. The model auto-regressively produces filter coefficients that are fed into an Infinite-Impulse-Response (IIR) filter which convolves them with the spatio-temporal series in order to generate the next output sample based on previous observations. The IIR filter allows for flexible stochastic generation of samples, and its parameters can be adjusted as needed to suit different channel conditions. A total of 20 suitable start-up filter coefficients are generated from 21.7 % of the sample size for each complex delay tap distribution, which corresponds to 50 complex instances of the channel. These coefficients are then utilised to validate the remaining measured sample set. The model is in quantitative agreement with measurement statistics and can be used to construct relatively simple modified reflection coefficients that can be used in micro-cellular ray-optical network planning tools.

Demos Serghiou, Mohsen Khalily, Tim W. C. Brown, Rahim Tafazolli (2022)Terahertz Channel Propagation Phenomena, Measurement Techniques and Modeling for 6G Wireless Communication Applications: A Survey, Open Challenges and Future Research Directions, In: IEEE Communications surveys and tutorials24(4)pp. 1957-1996 IEEE

The Terahertz (THz) band (0.3-3.0 THz), spans a great portion of the Radio Frequency (RF) spectrum that is mostly unoccupied and unregulated. It is a potential candidate for application in Sixth-Generation (6G) wireless networks, as it has the capabilities of satisfying the high data rate and capacity requirements of future wireless communication systems. Profound knowledge of the propagation channel is crucial in communication systems design, which nonetheless is still at its infancy, as channel modeling at THz frequencies has been mostly limited to characterizing fixed Point-to-Point (PtP) scenarios up to 300 GHz. Provided the technology matures enough and models adapt to the distinctive characteristics of the THz wave, future wireless communication systems will enable a plethora of new use cases and applications to be realized, in addition to delivering higher spectral efficiencies that would ultimately enhance the Quality-of-Service (QoS) to the end user. In this paper, we provide an insight into THz channel propagation characteristics, measurement capabilities, and modeling techniques for 6G communication applications, along with guidelines and recommendations that will aid future characterization efforts in the THz band. We survey the most recent and important measurement campaigns and modeling efforts found in literature, based on the use cases and system requirements identified. Finally, we discuss the challenges and limitations of measurement and modeling at such high frequencies and contemplate the future research outlook toward realizing the 6G vision.

D. Serghiou, M. Khalily, S. Johny, M. Stanley, Fatadin, T. W. C. Brown, N. Ridler, R. Tafazolli (2021)Comparison of Diffuse Roughness Scattering from Material Reflections at 500-750 GHz, In: 2021 15TH EUROPEAN CONFERENCE ON ANTENNAS AND PROPAGATION (EUCAP) IEEE

In this paper, an ultra-wideband Terahertz (THz) channel measurement campaign in the 500-750 GHz frequency band is presented. Power levels received from signal transmission by reflections off 14 different materials were measured in an indoor environment at Non-Line-of-Sight (NLoS) between the Transmitter (Tx) and Receiver (Rx), and compared to power levels received at Line-of-Sight (LoS) transmission. Frequency up-converters were used to transmit the signal using 26 dBi horn antennas at the Tx and Rx side and the signal was measured using a Vector Network Analyzer (VNA). From the data collected, the signal losses due to absorption and diffuse scattering from the rough surface of each Material Under Test (MUT) are calculated. The power delay profile (PDP) is presented, where multipath clustering due to diffuse scattering is observed for materials which have a high frequency selectivity, while less scattering and mostly specular reflection is shown for materials with low frequency selectivity.

Demos Serghiou, Mohsen Khalily, Ali Ali, Tim Brown, Rahim Tafazolli (2022)Characterisation of Frequency Selective Reflections off Indoor Surfaces for 92-110 GHz, In: 2022 16TH EUROPEAN CONFERENCE ON ANTENNAS AND PROPAGATION (EUCAP) IEEE

In this paper, we investigate the reflection properties of different interior surfaces in the 92-110 GHz sub-Terahertz (THz) band. The measurements were conducted in an indoor environment by placing the surface in a specular configuration between the Transmitter (Tx) and Receiver (Rx) and collecting a large set of data by offsetting the Tx and Rx in parallel to the surface. The measurements were performed using an Agilent N5230A Vector-Network-Analyzer (VNA). In particular, we present a statistical analysis in the frequency domain to show how frequency selective each surface reflection is and how constant this behaviour is across the whole data set. We introduce the Power-Delay-Profile (PDP) to characterize the multipath behaviour of the channel and calculate the Root-Mean-Square (RMS) delay spread. The measurement results provide a good insight for future propagation work to be done for the development of indoor communications systems at sub-THz frequencies.

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.