Theses supervised by Doç. Dr. Ertuğrul Başar

System modellingSpatial modulationTransmitters

Advanced spatial modulation systems for future MIMO systems

Spatial modulation (SM) seems like a promising transmission technique for multiple-input multiple-output (MIMO) systems to meet the requirements of future wireless communication technologies. SM activates only one transmit antenna in each time slot and conveys additional information bits through the indices of active transmit antennas. Thus, the spectral efficiency increases without increasing the modulation order. Besides, SM requires only one radio frequency (RF) chain, which means low-level hardware complexity and inter-channel interference (ICI) cancellation due to the activation of only one transmit antenna. In recent years, the design of SM-based transmission techniques has become a general research topic because of the advantages of SM, and it is seen that clever SM-based designs can increase the spectral efficiency and bit error rate (BER) performance concerning SM. In this thesis, three novel SM-based transmission schemes are proposed for next-generation MIMO systems. Firstly, a novel transmission scheme named transmit antenna grouping quadrature SM (TAG-QSM) is proposed. The motivation of TAG-QSM is to increase the number of information bits in the spatial domain to achieve lower modulation order than the existing schemes in the literature. In TAG-QSM, transmit antenna groups with an equal number of transmit antennas are created. After that, first, the incoming bit group determines the indices of transmit antenna groups separately for the transmission of the real and imaginary parts of data symbol. The second incoming bit group chooses one transmit antenna from each of the determined transmit antenna groups. Data symbol is determined using the last bit group. To conclude, the increase in the number of additional information bits in the spatial domain results in lower modulation order and improved BER performance concerning the benchmark schemes. In this study, the theoretical upper bound is derived for BER, and Monte Carlo simulations are demonstrated for comparing the BER performance of TAG-QSM with the benchmark schemes and showing the consistency of the theoretical upper bound of BER derivation. Secondly, flexible spatial modulation with transmit antenna selection (FSM-TAS) is introduced for future multiple-input multiple-output (MIMO) systems. In this scheme, the number of active antennas varies in each time slot depending on the incoming bits. After determining the number of active antennas, channel coefficients corresponding to each possible active antenna combination are added up. Then, a certain number of antenna combinations with largest gains is selected to apply spatial modulation (SM). For the proposed system, complexity and outage probability analyses are performed. In addition, it has been shown by Monte Carlo simulations that FSM-TAS provides better bit error rate (BER) performance than the benchmark scheme, named enhanced spatial modulation with generalized antenna selection (ESM-GAS) \cite{qing2021enhanced}, under the same spectral efficiency, the same number of transmitter and receiver antennas. Thirdly, spatial modulation (SM) using signal space diversity (SM-SSD) is proposed for multiple-input multiple-output (MIMO) systems. In this scheme, consecutive time slots are processed jointly and signal and space diversity (SSD) technique is applied for the transmission of data symbols to obtain transmit diversity. In addition, a clever active antenna activation algorithm is introduced to prevent transmit diversity degradation. An upper bound BER derivation is performed and compared with Monte Carlo simulations. Besides, BER performance comparison is demonstrated with the reference schemes in the literature. Lastly, the suboptimal solution for the rotation angles is expressed to maximize minimum coding gain distance (MCGD).

Mehmet Akif Kurt

Koç University · Institute of Graduate Studies in Science

Frequency division multiplexingWireless systemsOrthogonal frequency division multiplexing

OFDM with index modulation-based alternative waveform designs for future wireless systems

Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which transmits information bits through ordinary signal constellation symbols and indices of active subcarriers, is a promising multicarrier transmission scheme and has attracted the attention of researchers due to its several benefits such as flexibility and simplicity. Nonetheless, OFDM-IM cannot satisfy many needs of future wireless communication services such as, ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB) since providing superior reliability, high data rates, and low complexity is challenging due to its null subcarriers and unsufficient error performance. Therefore, several novel waveforms have been proposed by utilizing the flexible nature of the OFDM-IM to meet the diverse needs of eMBB and URRLC services. In this thesis, two novel OFDM-IM based multicarrier transmission schemes have been proposed by utilizing the aforementioned advantages of OFDM-IM. Firstly, a novel transmission scheme named as coordinate interleaved OFDM with power distribution IM (CI-OFDM-PIM), which applies coordinate interleaving and power distribution in subcarriers to achieve a diversity gain that is equal to the number of subcarriers in a subblock, is proposed. The average bit error probability (ABEP) for the CI-OFDM-PIM was derived and the superior error performance of the proposed scheme over benchmarks has been shown via computer simulations. Next, a novel OFDM-IM based scheme called coordinate interleaved OFDM with repeated in phase/quadrature IM (CI-OFDM-RIQIM), which provides superior error performance and enhanced spectral efficiency due to its diversity order of two and clever subcarrier activation pattern (SAP) detection mechanism, is proposed. Furthermore, a log-likelihood ratio (LLR) based low-complexity detector is designed for the proposed scheme. Theoretical analyses are performed and an upper bound on the bit error probability is derived. Comprehensive computer simulations under perfect and imperfect channel state information (CSI), are conducted to compare the proposed and reference schemes. It is shown that CI-OFDM-RIQIM shows superior results and can be considered as a promising candidate for next generation wireless communication systems.

Ömer Furkan Tuğtekin

Koç University · Institute of Graduate Studies in Science

Energy efficiencyWireless networksChannel models+1

Active reconfigurable intelligent surface architectures for future wireless networks

Reconfigurable intelligent surface (RIS)-assisted communication has recently attracted the attention of the wireless communication community as a potential candidate for 6th generation (6G) of wireless networks. Various studies have been carried out on the RIS technology, which is capable of enabling the control of the signal propagation environment by network operators. However, when an RIS is used in its inherently passive structure, it appears to be only a supportive technology for communications, while suffering from a multiplicative path loss. Therefore, researchers have lately begun to focus on RIS hardware designs with minimal active elements to further boost the benefits of this technology. In this thesis, first, we present a simple RIS hardware architecture including a single and variable gain amplifier for reflection amplification to confront the multiplicative path loss. The end-to-end signal model for communication systems assisted with the proposed amplifying RIS design is presented, together with an analysis focusing on the capacity maximization and theoretical bit error probability performance, which is corroborated by computer simulations. In addition, the major advantages of the proposed amplifying RIS design compared to its passive counterpart are discussed. It is shown that the proposed RIS-based wireless system significantly eliminates the double fading problem appearing in conventional passive RIS-assisted systems and improves the communication energy efficiency. This thesis also introduces an RIS-assisted grant-free non-orthogonal multiple access (GF-NOMA) scheme. We propose a joint user equipment (UE) clustering and RIS assignment/alignment approach that ensures the power reception disparity required by the power domain NOMA (PD-NOMA). The proposed approach maximizes the network sum rate by judiciously pairing UE with distinct channel gains and assigning RISs to proper clusters. To alleviate the computational complexity of the joint approach, we decouple UE clustering and RIS assignment/alignment subproblems, which reduces run times 80 times while attaining almost the same performance. Once the proposed approaches acknowledge UEs with the cluster index, UEs are allowed to access corresponding resource blocks (RBs) at any time requiring neither further grant acquisitions from the base station (BS) nor power control as all UEs are requested to transmit at the same power. In addition to passive RISs containing only passive elements and giving an 18% better performance, a fully-connected active RIS structure that enhances the performance by 37% is also used to overcome the double path loss problem. The numerical results also investigate the impact of UE density, RIS deployment, RIS hardware specifications, and the fairness among the UEs in terms of bit-per-joule energy efficiency.

Recep Akif Taşçı

Koç University · Institute of Graduate Studies in Science

Koç University · Institute of Graduate Studies in Science

Reconfigurable intelligent surface-assisted wireless networks and channel modeling

Reconfigurable intelligent surface (RIS)-assisted communications is one of the promising candidates for next generation wireless networks by controlling the propagation environment dynamically. In this thesis, a channel modeling strategy for RISassisted wireless networks is introduced in sub-6 GHz bands by considering both far-field and near-field behaviours in transmission. We also proposed an open-source physical channel simulator for sub-6 GHz bands where operating frequency, propagation environment, terminal locations, RIS location and size can be adjusted. It is demonstrated via extensive computer simulations that an improved achievable rate performance is obtained in the presence of RISs for both near-field and far-field conditions. Furthermore, the spatial correlation between RIS elements are investigated in a millimeter wave (mmWave) transmission scenario. Conventional power domain non-orthogonal multiple-accessing (PD-NOMA) scheme require transmission power allocation on the user side, which creates computational complexity and signaling overhead that hinders NOMA implementation. This thesis also introduce an RIS-assisted grant-free non-orthogonal multiple access (GFNOMA) scheme. We propose a joint user equipment (UE) clustering and RIS assignment and alignment approach that ensures the power reception disparity required by the PD-NOMA. The proposed approach maximizes the network sum rate by judiciously pairing UE with distinct channel gains and assigning RISs to proper clusters. To alleviate the computational complexity of the joint approach, we decouple UE clustering and RIS assignment/alignment subproblems, which reduces run times 80 times while attaining almost the same performance. Once the proposed approaches acknowledge UEs with the cluster index, UEs are allowed to access corresponding resource blocks (RBs) at any time requiring neither further grant acquisitions from the base station (BS) nor power control as all UEs are requested to transmit at the same power. In addition the RIS assisted GF-NOMA scheme enhances the sumrate of the network by 18% compared to conventional PD-NOMA which requires transmit power allocation at the UE side. Our numerical results also investigate the impact of UE density, RIS deployment, RIS hardware specifications, and the fairness among the UEs in terms of bit-per-joule energy efficiency. Furthermore, this thesis introduces two hybrid transmission schemes combining a passive reconfigurable intelligent surface (RIS) with decode-and-forward (DF) relaying in a synergistic manner. The proposed schemes offer a flexible as well as cost- and power-efficient solution for coverage extension in future generation wireless networks. We present the end-to-end signal-to-noise ratio of both schemes and a sequential optimization algorithm for the power allocation and the RIS phase configurations. Our computer simulations demonstrate that the use of an RIS and relaying technologies enhance the achievable rate and error performance remarkably when working complementary to each other, rather than being considered as competing technologies.

Fatih Kılınç

Koç University · Institute of Graduate Studies in Science

DoctorateOpen AccessEN

Reconfigurable intelligent surface-based novel transceiver architectures and multiple access

Over five generations of wireless communications networks, from 1G to 5G, the wireless channel was always a given system entity and not a design parameter. The wireless channel is dictated by the physical nature of the environment that contains the transmitter and receiver, the endpoints of a wireless network, which makes it random from their perspective. Hence, it cannot be manipulated, and the only way to deal with it is to compensate for its negative impact at the endpoints of the network. Recently, a new emerging technology called reconfigurable intelligent surfaces (RISs) has appeared to provide some degree of manipulation in the random wireless channel. With unprecedented capabilities, RISs somehow offer to make (even if partially) the random wireless channel as a design parameter that can be optimized jointly with the other conventional parameters to achieve a particular unified goal for the wireless network. An RIS is a metasurface in the form of a large array of passive and low-cost elements that can be controlled electronically to reshape the electromagnetic waves impinging the surface and absorb or reflect them to the opposite side. Owing to their unique properties of reconfigurability, low cost, and intelligence, recent literature envisions that RISs can find their way to integration with almost all of the existing wireless communication systems. Therefore, RISs has drawn growing attention in the wireless communication society as a promising emerging technology that can be a potential candidate for 6G and beyond wireless networks, which motivates this thesis. In this thesis, we investigate the potential of RISs to be used in the next generation of wireless networks as an enabling technology by examining how far it will impact the network's overall performance. Our investigation is carried out by proposing new solutions that redesign existing classical wireless communications systems to include RISs in their transceiver architectures and assess their performance compared to the classical systems. Specifically, this thesis proposes the design and performance analysis of seven RIS-assisted wireless communications systems, including single user and multiple users with multiple access, which can be categorized into three parts, as follows. In the first part of this thesis, we combine RISs with multiple-input multiple-output (MIMO) systems, where we consider vertical Bell Labs space-time (VBLAST) and Alamouti's schemes as the most common and practical MIMO schemes. For the VBLAST-based new system, an RIS is used to enhance the performance of the nulling and canceling-based sub-optimal detection procedure as well as to noticeably boost the spectral efficiency by performing index modulation (IM) at the RIS side. Furthermore, we propose an RIS-based transmitter for Alamouti's scheme that replaces the two radio frequency (RF) chains at the classical transmitter with a single RF signal generator. In the second part, we integrate RISs into non-orthogonal multiple access (NOMA) systems, as follows. First, we propose a novel NOMA solution with RIS partitioning with the aim of enhancing spectrum efficiency by improving the ergodic rate of all users and maximizing user fairness. In the proposed system, we distribute the physical resources among users such that the base station (BS) and RIS are dedicated to serving different clusters of users, thus, reducing the mutual interference between users' clusters. Second, in order to increase the coverage area, we consider the generalized version of RISs, namely, simultaneous transmitting and reflecting RISs (STAR-RISs). In particular, we investigate the BER performance of a NOMA network assisted by a STAR-RIS, where the STAR-RIS serves multiple non-orthogonal users located on either side of the surface by utilizing the mode-switching protocol. We derive the closed-form BER expressions in perfect and imperfect successive interference cancellation cases. Furthermore, asymptotic analyses are also conducted to provide further insights into the BER behavior in the high signal-to-noise ratio region. Third, we consider general system design and physical resources allocation problems for STAR-RIS-assisted NOMA networks. In particular, we propose a novel STAR-RIS-assisted NOMA system, where unlike most of the related works, we target scalable phase shift design that requires a reduced channel estimation overhead. The proposed system aims to maximize the overall sum rate while guaranteeing the quality-of-service requirements (QoS) for individual users. Finally, in the third part, we consider the RIS-assisted system design considering practical implementation problems associated with RISs. The first problem is the phase shift adjustment in RISs, which is associated with many issues in hardware implementation, limiting the RIS achievable gain. Therefore, we propose a low-cost, phase shift-free and novel passive beamforming (PB) scheme by only optimizing the on/off states of the RIS elements while fixing their phase shifts. The proposed PB scheme is shown to achieve the same scaling law (quadratic growth with the RIS size) for the signal-to-noise ratio as in the classical phase shift-based PB scheme, yet, with far less sensitivity to spatial correlation and phase errors. The second problem is the fact that RISs do not selectively reflect impinging electromagnetic waves; therefore, they can also reflect electromagnetic interference (EMI) from the surrounding environment to the receiver side. To solve this problem, we propose a novel EMI cancellation scheme to mitigate the impact of the EMI by exploiting its special time-domain structure and considering a clever passive beamforming method at the RIS. For all of the proposed system designs, we assess their performance using mathematical analyses by considering different relevant performance matrices, which are validated via simulation results. Also, using comprehensive computer simulations and under different system settings, we compare the performance of the proposed systems against their state-of-the-art counterparts using the same simulation parameters to guarantee fair comparison. The obtained mathematical and computer simulation results show that RISs have a huge potential to be a promising candidate for 6G and beyond wireless communications systems. RISs are shown to offer high degrees of freedom in terms of the system design, where they can be used as a supportive technology or main module within the existing wireless systems' architectures. Accordingly, they can offer system performance enhancement over different metrics, reduce the hardware design's complexity and cost, or even offer completely new features.

Aymen Khaleel

Koç University · Institute of Graduate Studies in Science

Practical implementation and real-world validation of reconfigurable intelligent surfaces

Reconfigurable intelligent surface (RIS)-empowered communications represent exciting prospects as one of the promising technologies capable of meeting the requirements of the sixth generation networks, such as low latency, reliability, and dense connectivity. The literature still lacks real-world experimental applications implementing scenarios that suggest applicability. However, validation of test cases and real-world experiments of RISs are imperative to their practical viability. To this end, this thesis presents the implementation and experimentation of the RIS in order to reveal the RIS's potential for future-generation networks in the possible use cases of RIS-assisted wireless communication networks, such as coverage enhancement, physical layer security, spectrum sensing, and wireless data link management. In Chapter 3, a physical demonstration of an RIS-assisted communication system is provided in an indoor environment in order to enhance coverage by increasing the received signal power. The performance of the RIS-assisted system is initially analyzed for a set of different locations of the receiver and observed around 10 dB improvement in the received signal power by careful RIS phase adjustments. Then, an efficient codebook design for RIS configurations is employed to adjust the RIS states on the move without feedback channels. The impact of an efficient grouping of RIS elements is investigated with the objective of reducing the training time needed to find the optimal RIS configuration. In the extensive experimental measurements, training time is reduced with the proposed grouping scheme from one-half to one-eighth by sacrificing only a few dBs in received signal power. In Chapter 4, the performance of the RIS is characterized according to the practical measurements to implement physical layer security (PLS), on which the transmitter (Alice), the intended user (Bob), and the eavesdropper (Eve) are located in an indoor environment. The measurement-based validation of the RIS deployment where the RIS is situated to maximize the secrecy capacity of the wireless communication system is still an insufficient stage although the theoretical framework of the PLS boosted by the RIS has been revealed in the literature. As of this point, it remains a challenge and an open question how the RIS can provide secure wireless communication in a practical measurement scenario by adjusting the RIS's phase shifts in such a way that the difference between the received signal power of Bob and Eve is increased to maximize the level of secrecy. The measurement campaign of the proposed system model of the RIS-aided PLS scheme demonstrates that the secrecy capacity can be significantly improved by introducing the RIS into the system with the assumption of the availability of Eve's power measurements. In Chapter 5, an experimental demonstration of an RIS-empowered spectrum sensing system is presented, in which the object detection approach by deep-learning (DL) methods is utilized to detect the signal's type and location in the frequency and time domain. The state-of-art DL-based detectors of Detectron2 and YOLOv7 are trained with the spectrograms of the synthesized signal dataset of 4G LTE and 5G NR, in which the signals are treated as objects on an image. Introducing the RIS into the proposed measurement setup significantly improves the performance of the detectors due to the direct correlation between the received signal power and the performance of the spectrum sensing process. In Chapter 6, the performance of a millimeter wave (mmWave) RIS prototype is validated with a vector network analyzer in an anechoic chamber as well as the evaluation of the considered RIS is performed with the fifth-generation (5G) waveform operating at the mmWave frequency band in a data center. In fact, link management in a data center is one of the possible playgrounds of RIS employment since rack cabinets cause severe blockage for the wireless communication system in the room. The measurements in the data center demonstrate that deploying the mmWave RIS into the data center provides improvements in received signal power.

Sefa Kayraklık

Koç University · Institute of Graduate Studies in Science

Communications for the planet mars: Past, present, and future

Space exploration has been on the rise since the 1960s. Along with the other planets such as Mercury, Venus, Saturn, and Jupiter, Mars certainly plays a significant role in the history of space exploration and has the potential to be the first extraterrestrial planet to host human life. In this context, tremendous effort has been put into developing new technologies to photograph, measure, and analyze the red planet. As the amount of data collected from science instruments around and on Mars increased, the need for fast and reliable communication between Earth and space probes has emerged. However, communicating over deep space has always been a big challenge due to the propagation characteristics of radio waves. Nowadays, the collaboration of private companies like SpaceX with space agencies to make Mars colonization a reality, introduces even more challenges, such as providing high data rate, low latency, energy-efficient, reliable, and mobility-resistant communication infrastructures in the Martian environment. Propagation medium and wireless channel characteristics of Mars should be extensively studied to achieve these goals. This thesis presents a comprehensive overview of the Mars missions and channel modeling studies of the near-Earth, interstellar, and near-planet links. Studies featuring three-dimensional (3D) channel modeling simulations on the Martian surface are also reviewed. We have also presented our own computer simulations considering various scenarios based on realistic 3D Martian terrains using the Wireless Insite software. Path loss exponent, power delay profile, and root-mean-square delay spread for these scenarios are calculated and tabularized in this study. Furthermore, future insights on emerging communication technologies for Mars are given. Additionally, the thesis explores the potential of reconfigurable intelligent surfaces (RIS) to enhance communication coverage in the Martian environment, considering diverse scenarios like canyons, craters, mountains, and plateaus. It also investigates the application of RIS-assisted localization proposing a framework for user position estimation in challenging Martian terrains. The findings contribute to the advancement of communication technologies for Mars missions and lay the groundwork for future improvements in interplanetary communication networks.

Enes Köktaş

Koç University · Institute of Graduate Studies in Science

Novel OTFS system designs for 6G communication networks

Conventional modulation methods, though effective in many scenarios, encounter difficulties when dealing with high-speed communication in dynamic environments. The rapid movement of devices, along with the impacts of multipath propagation and Doppler spread, can introduce complexities that degrade communication quality. To address these challenges, the orthogonal time frequency space (OTFS) waveform emerges as an innovative solution, bridging the gap between the demands of next-generation communication technologies and the intricacies of high-speed data exchange. OTFS represents a groundbreaking modulation technique that reimagines the approach to communication within the time-frequency domain. Unlike traditional methods, OTFS adopts a more comprehensive approach by simultaneously considering both time and frequency domains. However, there is still a need for the design of more sophisticated OTFS-based modulation schemes to meet the rigid requirements of the 6G standard. In this thesis, three performance improvement methods for OTFS are presented. In Chapter 3, a deep learning (DL)-based technique named autoencoder (AE)-based enhanced OTFS (AEE-OTFS) to improve the error performance of OTFS is proposed. An AE structure is exploited to learn a set of high-dimensional symbols and maximize the squared minimum Euclidean distance (SMED) between them. Unlike complex doubly-dispersive channels, this AE is trained under the simpler conditions of an additive white Gaussian noise (AWGN) channel. The encoder and decoder of AE are then repurposed as the mapper and demapper blocks in a real-time OTFS system. The approach also establishes a theoretical frame error rate (FER) upper bound. Simulation results demonstrate that AEE-OTFS outperforms conventional OTFS in terms of FER performance. The addition of a learned detector (LD) significantly reduces decoding complexity. These findings suggest that AEE-OTFS holds promise for certain 6G applications, particularly in high-mobility and flexible scenarios. In Chapter 4, a block-wise index modulation (IM) technique named joint delay-Doppler IM-based OTFS (JDDIM-OTFS) is designed to enhance the error performance of OTFS. In this chapter, a theoretical upper bound on the bit error rate (BER) is also established. Computer simulations reveal that JDDIM-OTFS surpasses conventional OTFS, OTFS-IM, and delay-IM OTFS (DeIM-OTFS) systems in terms of BER when combined with matched-filtered Gauss-Seidel (MFGS) detector with a maximum likelihood detector (MFGS-ML) and MFGS with a greedy detector (MFGS-GRD). The adoption of a Greedy detector within MFGS substantially reduces decoding complexity. JDDIM-OTFS exhibits remarkable error performance in scenarios characterized by high mobility and flexibility, making it an exciting choice for specific 6G use cases. Finally, in Chapter 5, for reducing the peak-to-average power ratio (PAPR) of OTFS, a method is suggested that utilizing unitary transformations such as Walsh-Hadamard transform (WHT), Zadoff-Chu transform (ZCT), and discrete cosine transform (DCT). The performance of this method is evaluated across various frame sizes and compared to clipping and discrete Fourier transform-spread (DFT-s OTFS) techniques. Additionally, the impact of this approach on BER performance is assessed against other methods. Computer simulations demonstrate that this proposed method significantly reduces PAPR while introducing a compromise of approximately 1.5 dB in error performance. The substantial PAPR reduction, coupled with the absence of the need for receiver-side information, highlights the significance of this approach.

Yusuf İslam Tek

Koç University · Institute of Graduate Studies in Science

DoctorateOpen AccessEN

Reconfigurable intelligent surface-centric communication networks: A comprehensive exploration of channel modeling and coverage extension

Reconfigurable intelligent surface (RIS)-empowered communication has received growing interest from the wireless research community due to its undeniable potential in extending the coverage, enhancing the link capacity, mitigating interference, deep fading, and Doppler effects, and increasing the physical layer security. RISs enable the control of wireless propagation through their unique electromagnetic functionalities and provide a new degree of freedom in the system design. Delving into the potential of RISs, this thesis unfolds across four detailed chapters, each contributing distinct insights into the deployment and application of RIS in enhancing wireless communication systems. Each chapter is crafted to provide a different perspective on RIS deployment and explain how this technology can be innovatively applied to support various aspects of wireless communication networks. This thesis takes a journey from the theoretical foundations of RIS to its envisioned practical applications and provides insights into how RIS can reshape wireless communication infrastructures to meet the demanding requirements of 6G networks. In the scope of this thesis, the RIS is introduced as a groundbreaking technology, reshaping the future of wireless communications by offering enhanced network capabilities, crucial for meeting the advanced requirements of next-generation applications. First, a comprehensive discussion of the intricacies of RISs in channel modeling is embarked on, which is a cornerstone for the effective deployment of sixth-generation (6G) wireless networks. In this regard, the physical channel modeling methodologies are provided for emerging RIS-empowered networks and aimed to fill an important gap in the open literature by providing an open-source and widely applicable physical channel model for mmWaves. Moreover, the open-source SimRIS Channel Simulator MATLAB package is introduced, which can be used in channel modeling and computer simulations of RIS-based communication systems with tunable operating frequency, terminal and RIS locations, number of RIS elements, and environments. The upcoming parts go beyond merely introducing the technology, delving into how RIS can be strategically employed to address some of the most challenging aspects of modern wireless communication, such as coverage extension, signal quality improvement, and network throughput enhancement. The following chapters build upon this foundation, each focusing on specific applications and implications of RIS in the realm of wireless communications. From intricate channel modeling to practical deployment strategies for coverage extension and advanced beamforming designs, the study offers a comprehensive and nuanced understanding of the multifaceted role of RIS in future wireless networks. This work stands as a testament to the transformative impact of RIS technology within the 6G landscape, offering an extensive exploration of its applications in channel modeling, coverage extension, and more. By weaving RIS into the fabric of various network design aspects, from sophisticated beamforming to coverage optimization, the study sets a robust framework for the advancement of future wireless communications, paving the way for the realization of the 6G vision.

İbrahim Yıldırım

2024

Koç University · Institute of Graduate Studies in Science

Information harvesting: Leveraging wireless power transfer for next-generation communication frameworks

As wireless information transmission (WIT) progresses into its sixth generation (6G), the challenge of sustaining terminal operations with limited batteries for Internet-of-things (IoT) platforms arises. Wireless power transfer (WPT) emerges as a solution, empowering battery-less infrastructures and enabling nodes to harvest energy for sustainable operations. The integration of WPT with WIT mechanisms becomes crucial to mitigate the need for battery replacements while ensuring secure and reliable communication. A novel protocol, Information Harvesting (IH), amalgamates WIT and WPT through index modulation (IM) techniques atop the existing far-field WPT mechanism to address challenges in wireless information and power transfer (WIPT). Moreover, innovative modes of information and power transfer are explored to meet the rising demand for energy and spectrum resources in next-generation IoT systems. One potential solution involves harnessing the active transmission capability of devices to facilitate data transmission and wireless energy harvesting (WEH) for backscatter communication (BC), forming a symbiotic radio (SR) environment. Furthermore, incorporating reconfigurable intelligent surfaces (RISs) into the SR environment enhances the reliability of backscatter communication, reinforcing symbiotic relationships between active and passive devices. This thesis explores novel and synergistic methods for coordinating WPT and WIT in next-generation communication systems through state-of-the-art technologies. In Chapter 3, a unified framework for index modulation (IM)-based IH mechanisms is presented, evaluating their energy harvesting capability, bit error rate (BER), and ergodic secrecy rate (ESR) performance for diverse IM schemes. The findings indicate significant potential in facilitating reliable data communication within existing far-field WPT systems, underscoring promising refinements in green and secure communication paradigms for next-generation IoT wireless networks. Chapter 4 introduces a new IH mechanism atop the orthogonal frequency-division multiplexing (OFDM)-based far-field WPT mechanism, investigating its benefits in terms of harvested energy, achievable rates, and reliability. Finally, Chapter 5 presents a novel SR system, where a standalone RIS sustains its functions through WEH based on a low-power RIS structure. Mutualistic symbiosis is established by utilizing a signal conveyed by the primary transmitter (PTx) to assist ongoing transmissions and convey information to the primary receiver (PRx). The PTx employs time index modulation (TIM) to transmit information to the PRx and power to the RIS and energy harvester (EH). A log-likelihood ratio (LLR)-based detector is presented to address challenges in the TIM scheme. The performance of the proposed scheme is investigated in terms of harvested direct current (DC) power at the RIS and EH, as well as the BER at the PRx.

Mehmet Ertuğ Pıhtılı

2024

Koç University · Institute of Graduate Studies in Science

Information harvesting: Leveraging wireless power transfer for next-generation communication frameworks

Mehmet Ertuğ Pıhtılı

2024

Koç University · Institute of Graduate Studies in Science

5G ve ötesi kablosuz iletişim ağları için OFDM tabanlı özgün dalga formu tasarımı

Orthogonal frequency division multiplexing (OFDM) with index modulation (OFDMIM) appears as a promising multi-carrier waveform candidate for 5G and beyond wireless networks due to its attractive advantages such as superior error performance, operational flexibility, and ease of implementation. OFDM-IM itself may not be a proper choice for 5G services such as enhanced mobile broadband (eMBB) and ultra-reliable and low-latency communications (URLLC) since achieving high data rates is challenging because of its null subcarriers and also its error performance is not at a sufficient level for mission critical applications. However, the flexible structure of OFDM-IM has a great potential and more sophisticated OFDM-IM-based waveforms can be designed to meet the requirements of future eMBB and URLLC applications. For eMBB, one solution to enhance the spectral efficiency of OFDM-IM is the employment of multiple distinguishable constellations (modes) by also exploiting its null subcarriers for data transmission. Furthermore, an OFDM-IM block with a sparse structure can be utilized to generate a suitable waveform for URLLC applications. In this thesis, application-based novel waveforms are designed by enjoying the appealing features of OFDM-IM. In Chapter 1, a novel IM technique called super-mode OFDM-IM (SuM-OFDM-IM) is proposed, where mode activation patterns (MAPs) and subcarrier activation patterns (SAPs) are jointly selected and conventional data symbols are repetition coded over multiple subcarriers to achieve a diversity gain. For the proposed scheme, a low-complexity detector is designed, theoretical analyses are performed and a bit error rate (BER) upper bound is derived. The performance of the proposed system is also investigated through real-time experiments using a software-de fined radio (SDR) based prototype. It is shown that SuM-OFDM-IM exhibits promising results in terms of spectral efficiency and error performance; thus, appears as a potential candidate for 5G and beyond communication systems. In Chapter 2, OFDM with power distribution index modulation (OFDM-PIM) is proposed where a unique power distribution technique is applied to index modulated subcarriers. Furthermore, OFDM with in-phase/quadrature PIM (OFDM-I/Q-PIM), which independently performs the same power distribution process on the real and imaginary parts of the data symbols, is proposed. Average bit error probabilities (ABEP) of OFDM-PIM and OFDM-I/Q-PIM are obtained and the superiority of the proposed schemes over reference schemes is demonstrated via extensive computer simulations. Finally, in Chapter 3, a low-complexity encoding/decoding algorithm is proposed for the sparse vector coding (SVC) scheme, which allows the use of all possible activation patterns (APs) without the need for a look-up table. Computer simulation results reveal that the proposed low-complexity algorithm provides not only a superior BER performance but also improved spectral efficiency compared to the ordinary SVC approach.

Ali Tuğberk Doğukan

2020