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Yayın Energy harvesting cooperative multiple access channel with data arrivals(IEEE, 2016) Gürakan, Berk; Kaya, Onur; Ulukuş, ŞennurWe consider an energy harvesting two user cooperative Gaussian multiple access channel (MAC), where both of the users harvest energy from nature. The data packets arrive intermittently over time. The users overhear each other's transmitted signals and can cooperate by forming common messages. We find the optimal offline transmit power and rate allocation policy that maximize the departure region. We first show that there exists an optimal policy, in which the single user rate constraints in each time slot are tight, yielding a one to one relation between the powers and rates. Then, we formulate the departure region maximization problem as a weighted sum rate maximization in terms of rates only. Next, we propose a sequential convex approximation method to approximate the problem at each step and show that it converges to the optimal solution. Finally, we solve the approximate problems using an inner outer decomposition method. Numerically, we observe that higher data rates can be supported with the same amount of energy.Yayın Enabling cooperation, resource allocation and receiver selection across cells: complementary fractional frequency reuse(IEEE, 2013) Bakşi, Saygın; Kaya, Onur; Bıyıkoğlu, TürkerFor a multi-cell multiple access channel, we develop a comprehensive cooperative communication framework: we propose a novel complementary fractional frequency reuse (FFR) strategy tailored specifically for pairwise user cooperation, also taking into account cell sectoring. This strategy allows the cell edge users not only to pool their resources and cooperate across cells, but also to choose the best receiver. We divide the users into cooperating inner and outer user pairs, and assign each pair orthogonal resources using OFDMA. We employ pairwise bidirectional cooperation based on block Markov superposition encoding among user pairs. We derive the achievable rates, while taking into account the geometry dependent interference at the users and the receiver. We find the jointly optimal power allocation, partner selection and receiver selection strategies that maximize the sum rate of the system. We then propose a heuristic matching algorithm, which operates based only on user and receiver locations. We compare the performance of our proposed strategies with several non-cooperative models, and demonstrate that the sum rate can nearly be doubled, while using the same resources.Yayın Jointly optimal chunk and power allocation in uplink SC-FDMA(IEEE, 2013) Mert, Teoman; Kaya, Onur; Çırpan, Hakan AliFor a single carrier frequency division multiple access (SC-FDMA) system, we obtain the jointly optimal power and chunk allocation policies which maximize the sum rate. Our solution is applicable to both localized and interleaved subcarrier mapping schemes. We solve the joint optimization problem by sequentially solving two sub-problems: power allocation and chunk allocation. Primarily, we use an optimal power allocation algorithm, which we derive from Karush-Kuhn-Tucker (KKT) conditions; and then we convert the optimum chunk assignment problem into a maximum weighted matching problem on a bipartite graph, and hence solve it in polynomial time. We also propose two greedy chunk allocation algorithms with lower complexity, and demonstrate that these algorithms produce near optimal results, especially for interleaved subcarrier mapping, when used in conjunction with optimal power control.Yayın Optimal primary-secondary user pairing and power allocation in cognitive cooperative multiple access channels(IEEE, 2014) Bakşi, Saygın; Kaya, OnurWe develop jointly optimal power control and primary-secondary user partnering strategies for a cognitive cooperative multiple access channel with K primary and K secondary users. For each primary user, a cooperating secondary user is assigned. We consider both underlay and overlay modes for cognition/cooperation. In overlay mode, each secondary user decodes and relays part of its assigned primary user's message, and simultaneously transmits its own independent message, while ensuring the primary user achieves at least its single user capacity with power control. The encoding is based on channel adaptive block Markov superposition coding, where the powers assigned to primary and secondary user codewords are optimized so as to maximize either the system's sum rate, or the sum of secondary users' rates. In underlay mode, each secondary user employs independent signalling and allocates its power to maximize its own rate, without decreasing its assigned primary user's rate. The partnering problem for either mode is reduced to a maximum weighted matching (MWM) problem on a bipartite graph, and solved jointly optimally with the power allocation problem.Yayın Optimum power control for transmitter cooperation in OFDMA based wireless networks(IEEE, 2011) Bakım, İsmail Sezi; Kaya, OnurFor a cooperative orthogonal frequency division multiple access (OFDMA) system with two transmitters (TXs), and full channel state information (CSI), we obtain the optimal power allocation (PA) policies which maximize the rate region achievable by a recently introduced version of block Markov superposition encoding (BMSE): inter-subchannel cooperative encoding (ISCE) [1]. We provide the optimality conditions that need to be satisfied by the powers associated with the transmitted codewords. We propose two algorithms that yield the optimal power distribution: a subgradient algorithm which achieves an arbitrary rate point on the achievable rate region boundary, and an iterative waterfilling-like algorithm which maximizes the sum rate, and converges much faster. We observe that, utilization of power control to take advantage of the diversity offered by the cooperative OFDMA system, not only leads to a remarkable improvement in achievable rates, but also may help determine how the subchannels have to be instantaneously allocated to various tasks in cooperation.Yayın Energy and data cooperation in energy harvesting multiple access channel(IEEE, 2016) Gürakan, Berk; Şişman, Berrak; Kaya, Onur; Ulukuş, SennurWe consider the energy harvesting two user Gaussian multiple access channel (MAC), where both users harvest energy from nature. The users cooperate at the physical layer (data cooperation) by establishing common messages through overheard signals and then cooperatively sending them. In addition, the users cooperate at the battery level (energy cooperation) by wirelessly transferring energy to each other. We find the jointly optimal offline transmit power and rate allocation policy together with the energy transfer policy that maximizes the departure region. We provide necessary conditions for energy transfer, and prove some properties of the optimal transmit policy, thereby shedding some light on the interplay between energy and data cooperation.Yayın Achievable rates for two user cooperative OFDMA(IEEE, 2010) Bakım, İsmail Sezi; Kaya, OnurWe propose two encoding strategies for a two user cooperative Orthogonal Frequency Division Multiple Access (OFDMA) system, based on block Markov superposition encoding. We obtain the expressions for the resulting achievable rate regions for both encoding strategies, and evaluate them through simulations. We show that, by allowing for re-partitioning and re-encoding of the cooperative messages across subchannels, it is possible to better exploit the diversity created by OFDMA, and higher rates can be achieved.Yayın Cooperative multiple access under energy harvesting constraints(Institute of Electrical and Electronics Engineers Inc, 2015) Su, Nuğman; Kaya, Onur; Ulukuş, Şennur; Koca, MutluWe consider a cooperative multiple access channel (MAC) with two energy harvesting transmitters. The transmitters perform delay constrained transmission, i.e., every information block is encoded, transmitted and decoded between two consecutive energy harvests. We aim to maximize the achievable departure region over a finite transmission duration. We formulate the departure region maximization as a convex optimization problem. We propose an iterative algorithm which uses a directional waterfilling strategy to calculate the optimal power components. The departure region obtained by cooperation is shown to be significantly larger than that of a MAC without cooperation under the same energy arrival patterns. As a special case, we also analyze an energy harvesting relay channel with full duplex cooperation.Yayın Optimal and near-optimal partner selection algorithms in cooperative OFDMA(IEEE, 2012) Bakşi, Saygın; Kaya, Onur; Bıyıkoğlu, TürkerWe obtain the jointly optimal power allocation and partner selection policies, that maximize the sum rate of a cooperative OFDMA system with mutually cooperating pairs of users. We show that the power allocation and partner selection steps can be performed sequentially, and the latter step can be formulated as a maximum weighted matching problem on an undirected graph, which can be solved in polynomial time. We further propose practical algorithms, and compare their performances to the optimal matching algorithm, and demonstrate that very simple and low complexity algorithms based on user-user and user-receiver distances may provide near-optimum rate performance. Moreover, we observe that algorithms that achieve superior sum-rate performance, surprisingly pair the cell edge users, with the strong users near the base station.Yayın Age of Information in Practice(Cambridge University Press, 2023) Uysal, Elif; Kaya, Onur; Baghaee, Sajjad; Beytur, Hasan BurhanWhile age of Information (AoI) has gained importance as a metric characterizing the freshness of information in information-update systems and time-critical applications, most previous studies on AoI have been theoretical. In this chapter, we compile a set of recent works reporting AoI measurements in real-life networks and experimental testbeds, and investigating practical issues such as synchronization, the role of various transport layer protocols, congestion control mechanisms, application of machine learning for adaptation to network conditions, and device related bottlenecks such as limited processing power.
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