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Overloaded Time-Frequency Modulation Schema for 5-6 G Mobile Systems

Shamsi, Mahdi | 2022

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 55800 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Marvasti, Farokh
  7. Abstract:
  8. Increasing dynamics of mobile users intensifies the Doppler effect and degrades the performance of the communication system. Considering Fourier kernel-based modulation, in this research, we focus on dealing with the Doppler effect and compensating non-ideal communication channel response suffered from the Doppler-delay spread. In the first step, we study Zak transform and present a new transform for 2-D representation of time signals. This mapping is a well-known transformation that can be considered as a generalized discrete-time Fourier transform in the field of signal processing and can be used to represent a signal in the delay-Doppler domain. After providing a new 2-D transform, we prove its properties such as signal information conservation, relation to the Fourier transform, and the effect of delay and the Doppler shift on the 2-D representation of an arbitrary continuoustime complex-valued signal. By introducing the new 2-D representation, we provide a method to introduce a class of 2-D kernel-based modulation techniques in the delay-Doppler domain, with the ability to compensate for the Doppler effect and delay spread. With the help of these properties and by presenting a class of overloaded modulation schemes, it is shown that the proposed schemes can be equivalent to Orthogonal Frequency Division Multiplexing and even achieve better performance in terms of spectral efficiency. However, the increase in the dimensionality inevitably results in large frames and a multiplied amount of delay, corresponding to the added dimensionality of the kernel. By imposing high delay and requiring large bandwidth, these large frames necessitate a trade-off between frame duration and modulation characteristics such as quality of service or spectral efficiency. Hence, by considering the concept of the Mazo Limit and faster than Nyquist signaling, this thesis presents a class of overloaded modulations in the delay-Doppler domain. To improve the performance of the orthogonal frequency division modulation in the presence of the Doppler shift, we incorporate a cyclic prefix in the frequency domain which enables us to compensate the effect of the Doppler channel. Additionally, using an iterative soft decision detector enables the communication system to overcome the delay spread. We also study the effect of adding frequency-domain cyclic prefix to spectrally efficient frequency division modulation in the delay-Doppler channel. For the purpose of detection, we focus on iterative solutions to the inverse system problem and improve the performance by providing a 2-D version of the sphere decoding. The two-dimensional representation of the frames results in a significant reduction in the required calculations, and is shown to be superior to the common methods based on conversion of two-dimensional to one-dimensional frames. The implementation aspects of the proposed 2-D modulation schemes, specifically the required dynamic range, are also
    investigated
  9. Keywords:
  10. Orthogonal Frequency Division Multiplexing (OFDM) ; Inverse System ; Iterative Methods ; Sphere Decoding ; Delay-Doppler Modulation ; Mazo Limit

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