Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Hüseyin Arslan, Ph.D.

Co-Major Professor

Selçuk Köse, Ph.D.

Committee Member

Kwang-Cheng Chen, Ph.D.

Committee Member

Dmitry B. Goldgof, Ph.D.

Committee Member

Gangaram S. Ladde, Ph.D.

Keywords

Interference Elimination, Interference Suppression, Multiple Access Interference, Orthogonal Frequency Division Multiplexing, Pulse Shaping Methods

Abstract

Newer cellular communication generations are planned to allow asynchronous transmission of multiple numerologies (waveforms with different parameters) in adjacent bands, creating unavoidable adjacent channel interference (ACI). Contemporary windowed-orthogonal frequency division multiplexing (W-OFDM) algorithms have limited ACI rejection capability under high delay spread and small fast Fourier transformation (FFT) sizes. CP is designed to be longer than the maximum excess delay (MED) of the channel to accommodate such algorithms in current standards. Most prior work on windowing assume additional extensions reserved for windowing, which does

not comply with standards. The robustness of these algorithms can only be improved against these conditions by adopting additional extensions in a new backward incompatible standard. Whether windowing should be applied at the transmitter or the receiver was not questioned. Such extensions would deteriorate the performance of high mobility vehicular communication systems in particular.

In this dissertation, algorithms that enable minimum, even insufficient guards are discussed to achieve the spectral efficiency and latency requirements of cellular communication systems beyond 5G. This leads to interference in both time and frequency domains.

First, a partial-non-orthogonal multiple accessing (NOMA) scenario in which the desired user is experiencing both intersymbol interference (ISI) due to insufficient CP and ACI caused by asynchronous transmitters using non-orthogonal numerologies in adjacent bands is investigated. ISI and ACI depend on the power offset between desired and interfering users, the instantaneous channel

impulse responses (CIRs) of interfering users and transmitter and receiver window functions. Therefore, joint and adaptive utilization of CP requires real-time calculation of ISI and ACI. Analytical expressions for expected ISI and ACI at each subcarrier of the desired user are derived to minimize their combination. Accordingly, an adaptive algorithm consisting of windowing each subcarrier at the receiver with window length that minimizes the combined interference at that subcarrier by optimally exchanging ISI and ACI is proposed. Interference reduction performances of current, outdated and average optimal window length raised cosine receiver windows are assessed and compared to fixed and no receiver windowing. Windowing reduces interference even when CP is shorter than the channel if window length is determined using the proposed design guidelines.

Second, two independent algorithms are proposed that are implemented at the transmitter and receiver, respectively. These algorithms estimate the transmitter and receiver windowing duration of each RE with an aim to improve fair proportional network throughput. While doing so, solely the available extension that was defined in the standard is utilized. Presented standard-compliant algorithms also do not require any modifications on the counterparts or control signaling. Furthermore, computationally efficient techniques to apply per-RE transmitter and receiver windowing to signals synthesized and analyzed using conventional CP-OFDM are derived and their

computational complexities are analyzed. The spectrotemporal relations between optimum window durations at either side, as well as functions of the excess SNRs, the subcarrier spacings and the throughput gains provided over previous similar techniques are numerically verified.

Third, a low-complexity Hann receiver windowed-orthogonal frequency division multiplexing (RW-OFDM) scheme that provides resistance against ACI without requiring any ISI-free redundancies is presented. While this scheme is backward compatible with current and legacy standards and requires no changes to the conventionally transmitted signals, it also paves the way towards future spectrotemporally localized and efficient schemes suitable for higher mobility vehicular communications. A Hann window effectively rejects unstructured ACI at the expense of structured and limited inter-carrier interference (ICI) across data carriers. A simple maximum

ratio combining (MRC)-successive interference cancellation (SIC) receiver is therefore proposed to resolve this induced ICI and receive symbols transmitted by standard transmitters currently in use. The computational complexity of the proposed scheme is comparable to that of contemporary RW-OFDM algorithms, while ACI rejection and BER performance is superior in both long and short delay spreads. Channel estimation using Hann RW-OFDM symbols is also discussed.

Finally, the extension of this flexible signaling approach to other radio access technologies (RATs), such as characteristics that could be exploited in the cellular structure and application of these approaches to NOMAs schemes are discussed, and such an extension is exemplified using practical multiple-input-multiple-output (MIMO) systems. Practical MIMO systems depend on a predefined set of precoders to provide spatial multiplexing gain. This limitation on the flexibility of the precoders affects the overall performance. Here, we propose a transmission scheme that can reduce the effect of mismatch between users’ channels and precoders. The scheme uses the channel knowledge to generate an artificial signal, which realigns the predefined precoder to the actual channel. Moreover, the scheme can provide an additional level of secrecy for the communication link. The performance of the proposed scheme is evaluated using BER, EVM, and secrecy capacity. The results show a significant improvement for the legitimate user, along with a degradation for the eavesdropper.

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