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    A flexible hybrid waveform
    (Institute of Electrical and Electronics Engineers Inc., 2017) Nemati, Mahyar; Takshi, Hengameh; Arslan, Hüseyin
    Cellular systems of fifth generation (5G) require a waveform with flexible guard interval length along with a low power consumption. Different cyclic prefix (CP) durations in long term evolution (LTE) result in loss of orthogonality between transmitted symbols and cause interference. In addition, CP-based waveforms consume more power for CP part. 'Zero tail DFT-spread OFDM' (ZT DFT-s-OFDM) and 'DFT-spread zero word OFDM' (DFT-s-ZW-OFDM) have been proposed recently with flexible guard intervals as a portion of the modulated signal. However, ZT DFT-s-OFDM suffers from intersymbol interference because of imperfect zero tails. On the other hand, although DFT-s-ZW-OFDM has a superior performance in multipath channels, it consumes more power than ZT DFT-s-OFDM. In this paper, we utilize the similarity between the transceivers of these two waveforms to propose an orthogonal hybrid waveform. The hybrid waveform has high flexibility in order to control the symbol power and bit error rate (BER) performance of the system along with having flexible guard interval. The complexity of the proposed hybrid waveform is the same as ZT DFT-s-OFDM and DFT-s-ZW-OFDM.
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    A novel one-base station hybrid positioning method
    (Institute of Electrical and Electronics Engineers Inc., 2017) Takshi, Hengameh; Nemati, Mahyar; Arvas, Ercüment; Baykaş, Tunçer
    In wireless networks, the need for accurate and low complexity localization methods are growing. Although many positioning methods based on signals' time difference of arrival (TDOA) and angle of arrival (AOA) have been proposed, these methods require multiple base stations (BS) and calculations with high complexity. Furthermore, the distance between a target and the BSs are usually larger than the distance between different BSs, which causes geometric dilution of precision (GDP) problem. To circumvent these issues, we propose a novel and linear method for positioning by only one BS. Our method uses both AOA and TDOA of incoming signals and called 'positioning using one BS (PuOB)'. In this method, we take measurements from one BS at different time instances instead of taking measurements from multiple BSs simultaneously. The ability to estimate the mobile transmitter position accurately by using one BS is the highlighted advantage of the PuOB over the conventional methods. The positioning accuracy of PuOB for different BS numbers are presented. According to simulation results, PuOB outperforms TDOA and AOA methods using three and two BSs, respectively.
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    Discrete fourier transform spread zero word OFDM
    (Institute of Electrical and Electronics Engineers Inc., 2017) Nemati, Mahyar; Takshi, Hengameh; Arslan, Hüseyin
    Fifth generation (5G) waveform must meet a superior performance in multipath channels along with low peak to average power ratio (PAPR), especially in up-link transmission. Among waveform candidates, zero tail (ZT) DFT-spread (s) OFDM has low PAPR. However, unlike its name, ZT-DFT-s-OFDM contains non-zero samples at its tails causing inter-symbol interference (ISI) in multipath channels. In this paper, we utilize redundant subcarriers concept in unique word (UW) OFDM to nullify the tail of ZT DFT-s-OFDM. The achieved waveform called 'DFT Spread (s) Zero Word (ZW) OFDM' benefits from high mitigation in the ISI compared to ZT DFT-s-OFDM. The remaining benefits in this approach include: (I) around 3 dB performance improvement in Bit Error Rate (BER) compared to cyclic prefix (CP)-OFDM in time dispersive channels, (II) around 0.7 dB lower PAPR compared to CP-OFDM and UW OFDM, (III) having a flexible guard interval length in IFFT output intrinsically. In order to support above claims, simulation results are provided which depict the accuracy and enhanced performance of the proposed waveform compared to the mentioned waveform candidates.
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    Low ICI symbol boundary alignment for 5g numerology design
    (Institute of Electrical and Electronics Engineers Inc., 2018) Nemati, Mahyar; Arslan, Hüseyin
    Symbol boundary alignment, with respect to waveform selection, has an important impact on the numerology design for fifth-generation mobile applications. The current symbol boundary alignment, along with orthogonal frequency division multiplexing (OFDM) waveform, strongly suffers from intercarrier interference (ICI) especially in unmanned aerial vehicles (UAV) communications. This happens when the mobility causes Doppler effect which results in loss of orthogonality in OFDM. The available solutions for overcoming the ICI problem suffer from high complexity, low spectral efficiency, and incompatibility with the current radio access technologies. This paper presents a novel symbol boundary alignment, called Low ICI Symbol (LICIS) boundary alignment numerology, to avoid the disadvantages of the available solutions. LICIS utilizes large subcarrier-spacing to reduce the ICI power (e.g., around 5-dB ICI power reduction with subcarrier spacing of 30 kHz in high-speed UAV communications). Moreover, LICIS is based on the same reference clock as local thermal equilibrium (LIE) which guarantees its compatibility with the current LIE numerology. In addition, this approach places only one guard-interval at the end of a sequence of OFDM symbols and creates a subslot. This leads to less overhead and preserves the spectral efficiency. Furthermore, a pre-fast Fourier transform (FFT) multipath channel equalizer is considered for removing the intersymbol interference between the OFDM symbols occurring within the subslot. Only one additional EFT and IEET operations are required for the equalizer which creates an acceptable complexity increment compared to the complexity of other available solutions. Numerical and analytical evaluations show the superior performance of the proposed technique in terms of reliability and spectral efficiency.
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    On the design of a flexible waveform and low ici symbol boundary alignment
    (İstanbul Medipol Üniversitesi Fen Bilimleri Enstitüsü, 2017) Nemati, Mahyar; Arslan, Hüseyin
    Cellular systems of fifth generation (5G) radio access technology is expected to support a wide variety of service requirements in different applications. High reliability, flexibility, spectral efficiency, and low power consumption are some of the service requirements. In order to support these requirements, symbol boundary alignment design and the waveform selection play important roles. The current symbol boundary alignment, along with orthogonal frequency division multiplexing (OFDM) waveform, has some disadvantages, such as non-flexible guard interval (e.g., hard coded cyclic pre x (CP)), and severe intercarrier interference (ICI) in high speed communications like in unmanned aerial vehicles (UAV). In the literature, multiple di erent waveforms are proposed to be used instead of the OFDM in 5G. Although they try to prevent from the drawbacks of OFDM, they create other problems such as high complexity. Among the available waveform candidates, zero tail (ZT) DFT-spread (s) OFDM has flexible GI and low power consumption along with a low complex transceiver. However, unlike its name, ZT DFT-s OFDM contains non-zero samples at its tail causing intersymbol interference (ISI) in multipath channels. Additionally, ZT DFT-s OFDM does not solve the ICI problem of high speed communications. Regarding to these issues,this thesis presents two separate solutions as follows. First, an improved version of ZT DFT-s OFDM, called DFT-s zero word (ZW) OFDM, is proposed to reduce the ISI power. In DFT-s ZW OFDM, we utilize redundant subcarriers concept, like in unique word (UW) OFDM, to nullify the tail of ZT DFT-s OFDM. The achieved waveform bene ts from high mitigation in the ISI power compared to ZT DFT-s OFDM. Although DFT-s ZW OFDM has a superior performance in multipath channels, it consumes slightly more power than ZT DFT-s OFDM. Therefore, a hybrid waveform, constructed by ZT DFT-s OFDM and DFT-s ZW OFDM, is designed which provides a high flexibility in order to control the symbol power and bit error rate (BER) performance of the system. The hybrid waveform utilizes the similarity between the transceivers of these two waveforms to deploy them in one resource block (RB) for a user. Thus, it can control the symbol power, reliability, and even peak to average power ration (PAPR) of the system by tuning the dedicated subcarriers to each waveform with respect to the channel characteristics. The second part of this thesis focuses on ICI reduction in current LTE numerologies and presents a novel symbol boundary alignment called \Low ICI Symbol boundary alignment numerology (LICIS)'. LICIS utilizes large subcarrierspacing to reduce the ICI power (e.g. around 5 dB ICI power reduction with subcarrier-spacing of 30 kHz in high speed UAV communications). Moreover, LICIS is based on the same reference clock as LTE which guarantees its compatibility with the current LTE numerology. Additionally, this approach places only one guard-interval (GI) at the end of a sequence of OFDM symbols and creates a sub-slot. It leads to less overhead and preserves the spectral e ciency. Furthermore, a pre-FFT multipath channel equalizer is considered for preventing the intersymbol interference (ISI) between the OFDM symbols occurring within the sub-slot. However, only one additional FFT and IFFT operations are required for the equalizer which creates an acceptable complexity increment compared to the complexity of other available solutions. Numerical and analytical evaluations show the superior performance of the proposed technique in terms of reliability and spectral effciency.
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    Performance of TDOA and AOA localization techniques for different base-stations topologies
    (Institute of Electrical and Electronics Engineers Inc., 2019) Nemati, Mahyar; Baykaş, Tunçer; Choi, Jinho
    Time difference of arrival (TDOA) and angle ofarrival (AOA) localization techniques provide satisfactory positioning performance utilizing multiple base-stations (BSs). However, their performance depends on the BSs topology, whichmay restrict localization zones. The significance of the BSstopology increases when the localization needs line-of-sight (LoS)communication and the transmitter utilizes directional antennasfor signal transmission – e.g., unmanned aerial vehicles (UAVs)with directional antennas. In this work, in order to study theimpact of the BSs topology on the performance of the aeriallocalization with LoS communication, we model two well-knownTDOA and one AOA localization techniques with the minimumnumber of required BSs. After analyzing their system model,we derive a closed form expression for the covered area onthe ground by the directional antenna of an aerial transmitter.The covered area determines a region where the BSs should belocated in to be exposed to the LoS communication. From theMATLAB simulation results, we can determine the impact of theBSs topology on the localization accuracy and explain the role ofthe BSs topology in conjunction with the geometric dilution ofprecision (GDOP) and standard deviation (SD) of TDOA/AOAmeasurements.