Browsing by Author "Altay, Can."

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    Distributed spectrum sensing via energy detection in cognitive radios
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2013., 2013.) Altay, Can.; Deliç, Hakan.
    Energy Detection is widely studied spectrum sensing system to detect signal energy due to its simplicity and lack of requirement signal characteristics. This thesis focuses on two main problems on distributed detection of energy detectors. The first problem is threshold finding in the nodes due to achieve constant false alarm probability (PF) for binary decisions. The second problem is to derive optimum quantization of node observations due to minimum distortion and to give a expression on number of bits should nodes quantize their observations. For adaptive threshold nding, two algorithms are proposed. The first algorithm depends on minimizing the distance between instantenous PF and desired PF by Netwon's method. This algorithm is implemented for both mathematically calculated and estimated PF values. The second algorithm depends on generating a discrete random variable and comparing it with the previous decision in every iteration. By this algorithm, system adjusts its threshold to any changes in the noise variance and works totally independent from PF ( ). In the second focus of the thesis, minimum distortion quantization of two hypothesis together is derived for Lloyd-Max algorithm and its performance is observed. Number of quantized bits of observations is also in the scope of this thesis. A lower bound on rate-distortion function is derived for the signal which consists of two hypotheses. Another approach on number of bits is handled by Cherno bound which allows to compare performance of different bit quantizatons.
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    Interference management in energy-efficient heterogeneous networks
    (Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Altay, Can.; Koca, Mutlu.
    By the densification of the cellular networks, the performance of the users, who are close to coverage boundaries, significantly degrades due to severe interference. The statistical characteristics of the interfering signal gets complicated by heterogeneity of the base station deployment and employing non- orthogonal multiple access (NOMA). In this thesis, stochastic geometry analysis is presented to reveal the statistical characteristics of the user performance. The interference experienced by the user includes the signals coming from the neighbor base stations with different transmit power capabilities and also from user’s own serving base station for NOMA. Coverage probabilities of strict fractional frequency reuse (FFR) and soft frequency reuse (SFR) techniques are derived for NOMA-based heterogeneous network to indicate their performance under such interference. Parameter optimization problems for strict FFR and almost blank sub-frame (ABS) are modelled for maximization of energy efficiency by the capacity derivations of stochastic geometry analysis. A hybrid multiple access method, where NOMA and orthogonal multiple access (OMA) jointly deployed for improving edge user performance, is proposed and compared with conventional NOMA. For uplink NOMA, a decode order optimization problem is presented with its solution for avoidance of intra-cell interference under the user transmit power constraints.