@mastersthesis {1076, title = {Spectrum Shaping Techniques for Advanced Multicarrier Communication Systems}, volume = {PhD}, year = {2023}, month = {06/2023}, school = {Universidade de Vigo}, address = {Vigo}, abstract = {
Mobile communication has evolved from 1G voice-focused systems to 4G
data and mobile broadband services, with the exponential increase in mobile
data volume and connected devices leading to the development of the Fifth
Generation (5G) mobile network. The 5G network is expected to improve
key performance indicators such as data rate, massive connectivity, latency,
network reliability, and energy efficiency. To meet these requirements, the
5G air interface must be highly flexible, spectrally efficient, reliable, scalable,
and energy-efficient.
Among various waveforms proposed for the 5G radio interface, CPOFDM
was chosen due to its inherent advantages such as robustness to
multipath, compatibility with MIMO, low complexity, and high spectral efficiency.
However, it suffers from large spectrum sidelobes, peak-to-averagepower-
ratio, and sensitivity to synchronization errors. The focus of this
thesis is on the large spectrum sidelobes of OFDM that cause high out-ofband
radiation resulting in adjacent channel interference. Therefore, reducing
spectral leakage to adjacent channels is crucial for efficient spectrum
utilization.
Numerous techniques for reducing out-of-band radiation (OBR) have
been proposed in the literature, each with its advantages and drawbacks.
The performance of an OBR reduction technique is not only measured by
its ability to suppress out-of-band emission but also by key performance indicators
(KPIs) such as computational complexity, spectral efficiency, in-band
distortion, and spectrum overshoot. Balancing key performance indicators
(KPIs) is crucial when reducing OBR since it typically involves a tradeoff
between one or more performance parameters. However, the tradeoffs are
not uniform across different OBR reduction techniques. Thus, it is essential
to strike a balance between the KPIs to meet the varying requirements of
different use-case scenarios.
The thesis presents multiple innovative designs for OBR suppression in
multicarrier systems to prevent adjacent channel interference. These designs
offer the flexibility to select a frequency range through a spectral weighting
function that prioritizes certain frequencies. Additionally, the proposed designs
are adaptable enough to enable tradeoffs among the aforementioned
KPIs to fulfill the diverse requirements of various applications and scenarios.
An effective time-domain windowing technique is proposed that that is
both effective and simple, and transparent at the receiver. The proposed optimal
window design has the flexibility to shape the power spectral density
to focus on OBR at the frequency ranges of interest, unlike traditional windowing
approaches. Additionally, the design optimizes OBR performance
across different frequency regions based on the system{\textquoteright}s requirements. The
thesis also presents novel frequency-domain spectral precoding approaches
that can tradeoff between OBR performance and complexity, which is especially
of interest for battery-operated or low-power devices. The proposed
design allows for control over the amount of distortion on data subcarriers,
which is determined by a user-selectable parameter, and influences receiver
complexity.
A joint spectral precoding and windowing design is also presented that
leverages the advantages of both approaches. By optimizing the precoder
and window coefficients to reduce radiated power within a specified frequency
range, the design achieves a superior trade-off between out-of-band
radiation (OBR), throughput, and complexity compared to using these techniques
individually. The results demonstrate the effectiveness of this approach.
Finally, the thesis presents memory-based precoding approach to
mitigate the issue of large sidelobes. By incorporating memory into the precoder,
out-of-band radiation (OBR) performance can be improved without
compromising spectral efficiency, albeit at the expense of increased computational
complexity. The novel memory-based precoder is designed to
minimize the total OBR within a user-defined frequency range. Additionally,
this technique enables control over spectral peaks, thereby allowing for
flexible OBR levels.

}, author = {Khawar Hussain} }