Theses supervised by Prof. Dr. Ezhan Karaşan
13 theses · İhsan Doğramacı Bilkent University
Hibrit röle-RIS destekli MU-MISO sistemleri için toplam veri oranı maksimizasyonu: Çoklu erişim teknikleri
Space Division Multiple Access (SDMA) is a pivotal multiple access technique in modern wireless communication systems. Additionally, Reconfigurable Intelligent Surfaces (RIS) is recognized as fundamental technology for next-generation wireless communications. This work investigates a multiuser downlink Multiple-Input Single-Output (MISO) system with an underloaded or critically loaded network. Here, a multiantenna base station (BS) communicates with multiple single-antenna users by leveraging a combination of a half-duplex decode-and-forward (DF) relay and a full-duplex RIS. In this study, the aim is to maximize the sum rate by joint design of active beamforming at the BS and the DF relay in addition to passive beamforming at the RIS under maximum power constraints, minimum SINR constraints at the relay, and unit-modulus constraint for the RIS elements. The complex design problem is addressed using Lagrangian Dual Transformation (LDT), Quadratic Transformation (QT), and Semidefinite Relaxation (SDR). Also, an alternating optimization algorithm is proposed. In the context of performance evaluation, benchmarks such as Non-Orthogonal Multiple Access (NOMA) and Orthogonal Multiple Access (OMA) are considered. Furthermore, Particle Swarm Optimization (PSO) is utilized for RIS phase profile optimization as a benchmark for the model-based proposed algorithm. This comparative analysis provides insights into the effectiveness of the proposed SDMA-enabled hybrid RIS-Relay communication system and the efficiency of the metaheuristic PSO algorithm for large sizes of the RIS.
Esnek optik ağlarda tek SRLG arızalarına karşı arızadan bağımsız yol koruma
In Elastic Optical Networks, flexi-grid spectrum allocation is used where the the spectrum is assigned to optical connections according to their bandwidth requirements so that the capacity is used more efficiently. Ensuring network survivability is one of the main problem in elastic optical networks. In this thesis, we study network survivability against failure of a single link or a single Shared Risk Link Group (SRLG), which is a group of links sharing a common risk of failure. We formulate the network survivability problem where the objective is to minimize the required capacity resources and maximize their efficient usage such that the elastic optical network can recover against all single-SRLG failures. We developed two formulations towards this end using flow and path formulation approaches, respectively. In both approaches, the aim is to use two paths called the active and backup paths for all connection demands. In the normal operations, the active path is used. It is switched to the backup path in case of a failure of the active path. The active and backup paths are chosen SRLG-disjoint so that the network can recover from the failure without knowing the location of the failure, which is called failure independent protection. For the spectrum allocation, an Adaptive Coding and Modulation (AMC) scheme, which assigns the appropriate AMC profile based on the path length, is used. The backup paths can be shared among active paths because concurrent failure of multiple SRLGs is neglected. In the Flow Formulation, an Integer Linear Programming (ILP) is used to calculate SRLG-disjoint active and backup paths according to a given network topology, the set of connection demands and the AMC profile. In the Path Formulation, an ILP is used to select active and backup paths from a pre-computed set of SRLG-disjoint path pairs. In both approaches, the aim is to minimize the resource usage. The formulations are tested for the 14-node NSFNET and the 24-node USANET topologies. Although the performance of the Flow Formulation is better than the Path Formulation, the Path Formulation has smaller execution times due to its simplicity. The Path Formulation finds a solution for all possible connection demands of the 14-node NSFNET and the 24-node USANET, but the Flow Formulation was not able to find a solution for the NSFNET topology when the number of demands is large and for the USANET topology even for low number of demands. Both formulations are tested for 10 randomly selected demand sets each with 50 connection requests for 14- node NSFNET and the performance of the Flow Formulation is 5% better than the Path Formulation on the average. In some cases, the Path Formulation gives a better solution than the Flow Formulation when the runtime is limited because of the quality of the pre-computed set of path pairs. The Path Formulation is tested by limiting the number of pre-computed path pairs for all possible demands in 24-node USANET. It is found that the optimal solution first decreases rapidly as the number of path pair increases, but then it saturates when the number of path pairs per connection exceeds 30..
Tam çift yönlü baz istasyonlu ve eski mobil istasyonlu kablosuz ağlarda kullanıcı gruplaması
Improving spectral efficiency is a key objective in next generation wireless networks. Recent advances in self-interference cancellation techniques made in-band full-duplex wireless communications possible. Unlike half-duplex systems which require orthogonal frequency or time resources to separate transmission and reception, in-band full-duplex radios utilize the channel bidirectionally and theoretically can double the ergodic capacity. However due to cost, power consumption and complexity constraints, mobile stations may not support this technology. In this work, operation of full-duplex base stations with legacy half-duplex mobile stations is considered. An inherent issue of this topology is the presence of significant inter-user interference between half-duplex mobile stations. In order to manage this at network level, an optimization problem is formulated for a cellular network topology. Solution methods and their corresponding sum throughput are compared with respect to the number of mobile stations. An analytic solution is presented to evaluate the throughput and full-duplex gains of random pairing method for the same scenario. Then the case of limited channel state information is evaluated and a learning strategy is introduced to extend the user pairing problem to a continuous case. Performance evaluation with 100 mobile stations show that the proposed learning strategy can reduce the overhead airtime more than 80\%. A weighted random sequential algorithm which is integrated to the learning process is proposed, and its performance evaluation under random walk and random waypoint mobility cases are performed.
Çoklu-hop iletişimli İHA ile havadan yer gözetimi
Cooperative utilization of unmanned aerial vehicles (UAVs) in public and military surveillance applications has attracted significant attention in recent years. Most UAVs are equipped with sensors that have limited coverage and wireless communication equipment with limited range. Such limitations pose challenging problems to monitor mobile targets. The thesis examines fulfilling surveillance objectives to achieve better coverage while building a reliable network between UAVs. Area coverage and cooperative multiple target tracking problems are investigated and linear integer programming models are presented for these problems. For the area coverage problem, the optimal placement of UAVs for a given area is investigated with varying coverage ranges and the results are discussed. Coverage of the map achieved by the UAVs and the maximum possible coverage area are compared under different coverage and communications constraints. For the cooperative multiple target tracking, the optimal placement of UAVs to monitor mobile targets where their mobility is modeled with the random waypoint mobility model is studied. The multiple target tracking problem is further extended by assuming a relay UAV within the fleet whose trajectory is planned in order to achieve a reliable connected network among all UAVs. Optimization problems are established for single-hop and multi-hop communications. Three algorithms are proposed for multi-hop communications and their performances are evaluated. The effect of the time horizon considered in the optimization problem is also studied. Performance evaluation results show that the trajectories planned for relay UAV by the proposed algorithms generates network topologies that remain connected for more than 90% of the maximum possible duration that the UAVs can be connected by an ideal relay.
Araçlar arası haberleşme için CSMA tabanlı yarı çift yönlü ve tam çift yönlü MAC protokollerinin performans analizi
Beaconing mechanism used in vehicular networks is an essential mechanism to improve the cooperative awareness between vehicles by periodically broadcasting vehicle kinematic information and other traffic entities. Cooperative awareness between vehicles affects the performance of safety applications in vehicular networks. Therefore, the Quality of Service (QoS) provided by the beaconing mechanism determines whether the strict timeliness and reliability requirements of safety applications are met. The most important state parameter of the beaconing mechanism is the beacon generation rate. Even if increasing it increases the cooperative awareness between vehicles, it also increases the channel congestion. This trade-off poses a challenge in the MAC protocol design of the beaconing mechanism. Moreover, the broadcast nature of beacon packets prevents the usage of acknowledgement mechanism in the beaconing mechanism, further complicating the problem. In this thesis, we examine the performance of two CSMA-based MAC protocols used for the beaconing mechanism. The first one makes use of Half-Duplex radios (HD-MAC), and it does not support collision detection, whereas the second one makes use of Full-Duplex radios (FD-MAC), and it supports collision detection and retransmission. These protocols are analyzed using Markov Chain based queueing models, taking queue state and beacon generation rate into account. FD-MAC model considers missed and false collision detection occuring due to non-ideal behavior of the self-interference cancellation. Using the Markov Chain models, QoS measures such as successful transmission probability, throughput, service delay, packet loss probability and age of information are derived. The accuracy of the Markov Chain based models are validated through simulations. It is shown that the FD-MAC protocol is superior to HD-MAC protocol in terms of throughput, packet loss probability, and age of information.
Segment yönlendirme ile trafik mühendisliği
Trafik mühendisliğinde segment yönlendirme, ağ oluşturma alanında nispeten yeni bir tekniktir. Segment yönlendirme, bir paketin izleyeceği segmentlerin paketin başlığına yazıldığı daha basit bir kaynak yönlendirme şeklidir. Segmentler, SID'ler olarak bilinen Segment Kimlikleri kullanılarak tanımlanır. Düğüm SID'leri ve Bitişiklik SID'leri, farklı segment türlerini tanımlar: Birincisi, en kısa yol segmentlerini tanımlar ve ikincisi, iki düğüm arasındaki en kısa olmayan doğrudan bağlantıyı tanımlar. Giriş yönlendiricileri, Eşit Maliyetli Çoklu Yolları (ECMP'ler) kullanarak paketleri hedeflerine yönlendirir. Son zamanlarda, segment yönlendirme kullanan trafik mühendisliği için çeşitli çözümler önerilmiştir. Bu formülasyonlardaki amaç, ağdaki Maksimum Bağlantı Kullanımını (MLU) en aza indirmektir. Bu Karışık Tamsayılı Doğrusal Programlama (MILP) tabanlı formülasyonlar tüm olası yolları dikkate almaz ve düğüm ve segment sayısı arttıkça hesaplama süreleri makul bir değerin ötesine geçer. Bu eksiklikleri göz önünde bulundurarak, problem için yeni formülasyonlar ve algoritmalar sunuyoruz. Tüm segment çiftlerini formülasyona dahil etmek için yola dayalı bir model $K$-MMILP tanıtıldı. Ayrıca, akış tabanlı bir model olan $K$-MsMILP de önerilmiştir. Bu formülasyonlar, tüm Bitişik SID'leri, Düğüm SID'lerini ve ECMP'leri içerir. Ayrıca, akışın takip ettiği maksimum yol uzunluğunu kısıtlamanın MLU ve hesaplama süresi üzerindeki etkisi analiz edilmiştir. Önerilen akış tabanlı formülasyon, uçtan uca yol başına maksimum 3 segment kullanarak 20 örneğin her biri için düşünülen tüm topolojiler için optimum sonuçlar üretir. Ayrıca tüm topolojiler için hesaplama süresini önemli ölçüde azaltır. Örneğin, $16$ düğümlü Alman Ağı için, hesaplama süresi ortalama olarak $14,9$ kat oranında azaltılır. Ayrıca, $27$-düğümlü Avrupa ağı için, $3$-sMILP 24 saat içinde optimum sonuçlar üretemezken, $3$-MsMILP ortalama $2268$ saniyede sonuç üretti.
RSMA kullanan enerji-verimli kablosuz ağlar için bilgi yaşına duyarlı güç tahsisi
With the commercial deployments of 5G, research in Beyond 5G (B5G) and 6G networks has started. Within the context of meeting all needs and demands of future generation networks, the predicted usage is envisaged in three cases: massive Machine-Type Communications (MTC), ultra-reliable low-latency communications, and enhanced mobile broadband. This thesis focuses on massive Machine-Type Communications (mMTC). Energy efficiency, under the banner of green communications and networking is one of the branches complementary to the research conducted on MTC. mMTC, industrial and medical Internet of Things (IoT) type technologies will demand not only networking capabilities for massive access, enhanced communications, but also sustainability and power efficiency. Rate Splitting Multiple Access (RSMA) presents a candidate massive access scheme with spectral efficiency, energy efficiency, reliability, Degree-of-Freedom (DoF) and Quality of Service (QoS) enhancements in most of user deployments and network loads over traditional access schemes used in 5G. Within the scope of the thesis, we propose an age-aware power allocation policy for minimizing the network's Weighted-Sum Average AoI (WSAoI). To our knowledge, this is the first work in the literature which combines the Age of Information (AoI) concept and RSMA framework. For downlink communication, we formulate the network's WSAoI minimization as a Markov Decision Process (MDP) and investigate an optimal as well as suboptimal policies for the Base Station (BS) to select a scheme among RSMA, Orthogonal Multiple Access (OMA), and Nonorthogonal Multiple Access (NOMA). We prove existence of an optimal policy. Complexity of computation is reduced by using an action elimination technique, and by using a sub-optimal policy with performance close to the optimal. We also investigate the tradeoff between energy and the WSAoI of the network. The adaptive RSMA only scheme outperforms adaptive RSMA/NOMA/OMA and OMA/NOMA on the basis of network's WSAoI. For example, when RSMA is selected, the performance metric, WSAoI, at 14, 15, and 16 dB SNR values, is on average, respectively 35.8%, 15.7%, and 12.7% less than the NOMA/OMA cases. Overall, it is seen that, the optimum policy becomes more likely to operate in the RSMA mode with an increase in Signal to Noise Ratio (SNR). By using RSMA scheme instead of NOMA/OMA scheme, power consumption can be saved in average 65.8%, 62.3%, and 59.6% for the selected WSAoI values of 4, 3, and 2, respectively.
Çoklu erişim kanalı kullanarak zamanında verim maksimizasyonu
Latency and reliability capabilities of currently available fourth-generation (4G) wireless networks paved the path towards massively connected devices requiring much lower latency and much higher reliability. In the fifth-generation (5G) wireless networks, the concept of ultra-reliable and low-latency communications (URLLC) is introduced to fulfill these demands. URLLC aims to deliver short packets with 1 ms latency with a reliability rate of 99.999%. The cellular Internet of Things (IoT) is a framework for conceptualizing such massive connectivity while addressing fundamental challenges such as the ever-increasing number of interconnected devices, latency constraints, and high-throughput demands. One of the challenging tasks for cellular IoT applications is the delivery of deadline-constrained information to densely deployed IoT devices. Increasing demand for delivering timing-critical information in cellular IoT networks poses a URLLC-oriented challenge for both academia and industry. With this motivation, this thesis aims to develop techniques for reliably transferring short packets to densely deployed devices within a given deadline. In this thesis, we address the problem of latency-constrained communications with strict deadlines under average power constraint using Multiple Access (MA) schemes. The first MA scheme considered in the thesis is Hybrid MA, which consists of both Orthogonal MA (OMA) and power domain Non-Orthogonal MA (NOMA) as transmission scheme options. The second MA scheme studied in the thesis is Rate-Splitting Multiple Access (RSMA), which generalizes OMA, NOMA and Space-Division MA (SDMA) schemes. We maximize the timely throughput, which represents the average number of successfully transmitted packets before deadline expiration, where expired packets are dropped from the buffer. We use Lyapunov stochastic optimization methods to develop a dynamic power assignment algorithm for minimizing the packet drop rate while satisfying time average power constraints. Moreover, we propose a flexible packet dropping mechanism called Early Packet Dropping (EPD) to detect likely to become expired packets and drop them proactively. Finally, we propose a simple heuristic to reduce the computational load of the proposed algorithm. Numerical results show that Hybrid MA improves the timely throughput compared to conventional OMA by up to 46% and on average by more than 21%. With EPD, these timely throughput gains improve to 53% and 24.5%, respectively. Utilization of RSMA with EPD further improves timely throughput by up to 5.95% and on the average by about 3.12% compared to Hybrid MA with EPD. Simulation results indicate that the proposed heuristic significantly reduces the computational load at the cost of a small loss in the timely throughput performance.
5g heterojen ağlarda küçük hücre baz istasyonlarının enerji verimliliği arttırımı için aktivite yönetimi algoritması
Heterogeneous networks (HetNets) are proposed in order to meet the increasing demand for next generation cellular wireless networks, but they also increase the energy consumption of the base stations. In this thesis, an activity management algorithm for improving the energy efficiency of HetNets is proposed. A smart sleep strategy is employed for the operator deployed pico base stations to enter sleep and active modes. According to that strategy, when the number of users exceeds the turn on threshold, the pico node becomes active and when the number of users drop below the turn off threshold, it goes into sleep mode. Mobile users dynamically enter and leave the cells, triggering the activation and deactivation of pico base stations. The performance of the system is examined for three different cellular network architectures: cell on edge (COE), uniformly distributed cells (UDC) and macro cell only network (MoNet). Two different user distributions are considered: uniform and hotspot. The effects of number of hotspot users and sleep energies of pico nodes on the energy efficiency are also investigated. The proposed activity management algorithm increases the energy efficiency, measured in bits/J, by 20%. The average bit rates achieved by HetNet users increase by 29% compared with the MoNet architecture. Thus, the proposed activity control algorithm increases the spectral efficiency of the network while consuming the energy more efficiently.
Esnek optik ağlarda adaptif modülasyon ve kodlama ile yenileyici yerleştirilmesi
Due to the rapid and diverse increase in the traffic load on the optical networks, efficient utilization of the network resources becomes an important issue. Using different modulation formats and coding rates in optical signal transmission, it is possible to assign different spectral efficiency and optical reach for each traffic requests. To satisfy the quality of transmission (QoT) for the distances beyond optical reach, optical - electronic - optical (O/E/O) 3R regeneration of the optical signal is required. During the regeneration process, the spectral efficiency and thus optical reach of the resultant signal can also be set. In these circumstances, by selecting specific regenerator node locations and assigning different line rates for each traffic request, the network utilization can be optimized. Joint selection of regenerator placement (RP), routing and adaptive modulation and coding (AMC) profile in elastic optical networks (EON) is studied to propose an offline RP algorithm for a given network topology with link length and link capacity constraints. For a given RP, an Integer Linear Programming (ILP) model is formulated to perform routing and AMC profile assignment for each traffic demand. We use two different approaches for determining candidate paths for routing: In the first set, k shortest paths (KSP) are utilized for all cases. In the second set, namely regenerator location dependent path selection (RLDPS), the candidate paths are determined according to the given RP. To find the minimum cost RP among all possibilities, Tabu Search based regenerator placement algorithm (TSRPA) is proposed. Results show that adaptively selecting the candidate paths based on the regenerator locations reduces network utilization either by decreasing the number of regenerator nodes by up to 66.6% or decreasing link capacity utilization by up to 5.09% as compared to selecting candidate paths as fixed k shortest paths. The regenerator node location distribution obtained with RLDPS concentrated on smaller number of nodes compared to the results obtained with KSP. By placing regenerators at a significantly less number of nodes, capital expenditures (CAPEX) are reduced by RLDPS.
5g Çokturel ağlarda enerj, verimliliği ve adaleti iyileştirmek için dinamik kaynak atama ve faaliyet yönetimi
The higher energy consumption of Heterogeneous Networks (HetNet) compared to Macro Only Networks (MONET) raises a great concern about the energy e ciency of HetNets. In this thesis a dynamic activation strategy is proposed which changes the state of small cells between Active and Idle according to the dynamically changing user tra c in order to increase the energy e ciency of HetNets. Moreover, both inter-tier and inter-cell resource allocations are adjusted dynamically. The proposed strategy, Dynamic Bandwidth Allocation Dynamic Activation (DBADA), is applied in a small cell deployment where HotSpot regions are located at the cell edge and a small cell is located at the center of each HotSpot. The objective is to maximize the sum utility of the network with minimum energy consumption. To ensure proportional fairness in the network, the logarithmic utility function is employed. To evaluate the performance of the DBADA strategy over the proposed network topology, the median and 10- percent rates and the energy consumed per unit of these metrics are studied. Our simulation results reveal that the DBADA strategy can achieve the highest median and 10-percent rates among other scenarios. In addition, compared to always active scenario for small cells, DBADA decreases the energy consumption per unit of median and 10-percent rates by at least 26% and 21%, respectively.
Taşıyıcı dinleyen çoklu erişim protokolünün gelecek nesil kablosuz ağlar için performans analizi
Variants of the carrier-sense multiple access (CSMA) protocol has been employed in many communications protocols such as the IEEE 802.11 and Ethernet standards. CSMA based medium access control (MAC) mechanisms have been recently proposed for other communications scenarios such as sensor networks and acoustical underwater networks. Despite its widespread use, the performance of the CSMA protocol is not well-studied from the perspective of these newly encountered networking scenarios. We here investigate the performance of the CSMA protocol from the point of three different aspects: throughput in networks with large propagation delay, short-term fairness for delay sensitive applications in large networks and energy efficiency-throughput trade-off in networks with battery operated devices. Firstly, we investigate the performance of the CSMA protocol for channels with large propagation delay. Such channels are recently encountered in underwater acoustic networks and in terrestrial wireless networks covering larger areas. However, a mathematical model of CSMA performance in such networks is not known. We propose a semi-Markov model for a 2-node CSMA channel and then extend this model for arbitrary number of users. Using this model, we obtain the optimum symmetric probing rate that achieves the maximum network throughput as a function of the average propagation delay, d, and the number of nodes sharing the channel, N. The proposed model predicts that the total capacity decreases with 1/d as N goes to infinity when all nodes probe the channel at the optimum rate. The optimum probing rate for each node decreases with 1/N and the total optimum probing rate decreases faster than 1/d as N goes to infinity. Secondly, we investigate whether the short-term fairness of a large CSMA network degrades with the network size and density. Our results suggest that (a) the throughput region that can be achieved within the acceptable limits of short-term fairness reduces as the number of contending neighboring nodes increases for random regular conflict graphs, (b) short-term fair capacity weakly depends on the network size for a random regular conflict graph but a stronger dependence is observed for a grid topology. We also present related results from the statistical physics literature on long-range correlations in large systems and point out the relation between these results and short-term fairness of CSMA systems. Thirdly, we investigate the energy efficiency of a CSMA network proposing a model for the energy consumption of a node as a function of its throughput. We show that operating the CSMA network at a very high or at a very low throughput is energy inefficient because of increasing carrier-sensing and sleeping costs, respectively. Achieving a balance between these two opposite operating regimes, we derive the energy-optimum carrier-sensing rate and the energy-optimum throughput which maximize the number of transmitted bits for a given energy budget. For the single-hop case, we show that the energy-optimum total throughput increases as the number of nodes sharing the channel increases. For the multi-hop case, we show that the energy-optimum throughput decreases as the degree of the conflict graph of the network increases. For both cases, the energy-optimum throughput reduces as the power required for carrier-sensing increases. The energy-optimum throughput is also shown to be substantially lower than the maximum throughput and the gap increases as the degree of the conflict graph increases for multi-hop networks. Key words: Wireless Networking, Wireless Multiple Access, Carrier-sense Multiple Access, Energy E ciency, Underwater Networks, Short-term Fairness, Propagation Delay.
Esnek optik ağlarda yönlendirme, spektrum tahsisi ve yenileyici yerleştirilmesi
Tremendous increase in the number of wireless devices has been resulting in huge growth in the Internet traffic. This growth necessitates efficient usage of resources in the optical networks, which form the backbone of the Internet. Recently proposed flexible optical networks can adjust the optical layer transmission parameters to take advantage of existing channel conditions thereby increasing the resource utilization efficiency. Therefore, flexible optical network is a promising solution to fulfill growing future demand of IP traffic. Apart from efficient usage of the optical spectrum, the degradation of the optical signal as it propagates over the fiber is another problem. In such cases, the optical signal must be regenerated when a lightpath travels longer than the maximum optical reach. However, regenerators are expensive devices with high operational costs. Therefore, they should be placed carefully to reduce the capital and operational network costs. In this dissertation, we deal with the joint routing, spectrum allocation and regenerator placement (RSA-RP) problem for flexible optical networks. Our aim is to find the route and allocate spectrum for each traffic demand by assigning minimum number of nodes as regenerator sites. Firstly, we introduce a novel mixed integer linear programming (MILP) formulation for the joint RSA-RP problem. Since this formulation is not practical for large networks, we propose a decoupled formulation where the RSA-RP problem is decomposed into two phases. In the first step, we find routes and locations of regenerators assuming a full wavelength converting network. Then, we allocate the spectrum to each demand in the second phase. The decoupled model can be used to solve the RSA-RP problem for reasonably sized optical networks. We show that the decoupled model can find optimum solutions for 92% of the all cases tested for the NSFNET topology and 99% of the all cases tested for the Deutsche Telecom topology. We also show that the locations of regenerator sites significantly depend on network parameters such as the node degree and lengths of the links adjacent to the node.