@article {1066, title = {Sidelobe Suppression for Multicarrier Signals via Structured Spectral Precoding}, journal = {IEEE Transactions on Communications}, volume = {72}, year = {2024}, month = {06/2024}, pages = {3155-3168}, abstract = {
Reducing the large sidelobes of multicarrier signals
is crucial to prevent adjacent channel interference. Spectral
precoding is an effective approach toward this goal, at the
expense of throughput loss due to precoder redundancy; thus,
it is of interest to explore alternative precoder designs with
improved performance at lower redundancies. We present a
novel precoder which minimizes radiated power within a userselectable
frequency region. The structure of the precoding
matrix is chosen to allow efficient mitigation of in-band distortion
at the receiver by means of iterative and successive interference
cancellation, while completely avoiding distortion to protected
and pilot subcarriers. By exploiting the low-rank properties of
constituent blocks, computational complexity can be significantly
reduced with little impact on sidelobe reduction. Simulation
results show the benefits of the proposed design, which is
particularly effective in redundancy-limited settings targeting
high spectral efficiency.
}, keywords = {mayte, multicarrier communications, OFDM, spectrum shaping}, doi = {10.1109/TCOMM.2024.3357429}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce and Francesc Rey and Josep Sala and Javier Villares} } @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} } @article {1030, title = {Joint Precoder and Window Design for OFDM Sidelobe Suppression}, journal = {IEEE Communications Letters}, volume = {26}, year = {2022}, month = {12/2022}, pages = {3044-3048}, abstract = {

Spectral precoding and windowing are two effective approaches to reduce out-of-band radiation (OBR) in multicarrier systems. Their performance comes at the price of reduced throughput and additional computational complexity, so there is strong motivation for simultaneously using both techniques. We present a novel design that jointly optimizes the precoder and window coefficients to minimize radiated power within a user-selectable frequency region. Results show that the proposed design achieves a better OBR/throughput/complexity tradeoff than either of these individual techniques separately.

}, keywords = {rodin, spectrum shaping}, doi = {10.1109/LCOMM.2022.3203521}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce} } @conference {1025, title = {Orthogonal precoding with memory for sidelobe suppression in OFDM}, booktitle = {European Signal Processing Conference (EUSIPCO)}, year = {2022}, abstract = {
Orthogonal precoding is a very effective approach to reduce out-of-band radiation (OBR) in multicarrier systems in order to avoid adjacent channel interference. This is achieved at the cost of introducing precoder redundancy, which results in throughput loss. Introducing memory in the precoding operation has the potential to improve performance without sacrificing additional spectral efficiency, in exchange for extra computational complexity. We present a novel orthogonal memory precoder which minimizes OBR within a user-selectable frequency region and allows for controlling spectral overshoot. Decision-feedback decoding avoids symbol error rate degradation at the receiver.
}, keywords = {rodin, spectrum shaping}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce} } @article {995, title = {Mask-Compliant Orthogonal Precoding for Spectrally Efficient OFDM}, journal = {IEEE Transactions on Communications}, volume = {69}, year = {2021}, month = {03/2021}, pages = {1990-2001}, abstract = {
Orthogonal precoding constitutes a powerful technique to reduce spectrum sidelobes of multicarrier signals. This reduction is bought at the cost of introducing precoder redundancy, which results in some throughput loss and additional precoding/decoding complexity. When the goal is to meet some spectral emission mask constraints, it is desirable to avoid unnecessary sidelobe suppression in order to keep precoder redundancy at a minimum. In this context, we introduce a general framework under which we develop a novel Lagrange multiplier-based mask-compliant orthogonal precoder design targeting minimal redundancy. We also adapt to this framework two previously proposed
designs based on spectral notches and minimum out-of-band emission, respectively, to explicitly incorporate mask constraints. Simulation results are provided to show the effectiveness of the proposed designs under different practical masks for multicarrier wireless systems.
}, keywords = {rodin, spectrum shaping, winter}, doi = {10.1109/TCOMM.2020.3041351}, author = {Ravinder Kumar and Khawar Hussain and R. L{\'o}pez-Valcarce} } @conference {1003, title = {Memory Tricks: Improving Active Interference Cancellation for Out-of-Band power reduction in OFDM}, booktitle = {IEEE Int. Workshop Signal Process. Adv. Wireless Commun. (SPAWC)}, year = {2021}, address = {Lucca, Italy (Virtual event)}, abstract = {

Although active interference cancellation (AIC) is an appealing approach to reduce out-of-band radiation (OBR) in multicarrier systems without distorting data subcarriers, it usually requires a large number of reserved subcarriers to be effective. Introducing memory in AIC precoding may help alleviate this drawback in exchange for some increase in computational complexity. We propose a novel memory AIC precoder which, in contrast to previous schemes, minimizes OBR within a user-selectable frequency region and allows for controlling spectral overshoot.

}, keywords = {rodin, spectrum shaping}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce} } @conference {981, title = {Optimal window design for W-OFDM}, booktitle = {IEEE Int. Conf. Acoustics, Speech and Signal Processing (ICASSP)}, year = {2020}, month = {05/2020}, abstract = {
Windowing is an effective approach to reduce out-of-band radiation (OBR) in multicarrier systems in order to avoid adjacent channel interference. However, commonly used window functions are chosen in an ad hoc manner and fixed. We present an optimal window design for windowed-OFDM minimizing OBR within a given frequency region. The proposed technique is flexible in that it allows to specify the range of frequencies involved in the minimization and to apply spectral weighting. The optimal window can be computed offline.
}, keywords = {spectrum shaping, winter}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce} } @conference {969, title = {Flexible spectral precoding for OFDM systems}, booktitle = {2nd XoveTIC Conference}, year = {2019}, month = {09/2019}, address = {A Coru{\~n}a (Spain)}, abstract = {
Spectral precoding is a popular approach to reduce out-of-band radiation (OBR) in
multicarrier systems in order to avoid adjacent channel interference. However, it introduces
distortion in the data, appropriate decoding is required at the receiver side. Thus, trading off
between implementation complexity, OBR reduction and error rate is important. We present a novel
linear precoder design with flexibility to trade off OBR reduction, precoding/decoding complexity,
and error rate at the receiver. Moreover, the constraint can be imposed on each subcarrier individually
to provide more flexibility. The precoding matrices have low rank, which translates into significant
computational savings. In this way, the requirements of different systems can be satisfied with
varying complexity levels
}, keywords = {spectrum shaping, winter}, doi = {10.3390/proceedings2019021024}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce} } @conference {958, title = {Flexible spectral precoding for sidelobe suppression in OFDM systems}, booktitle = {IEEE Int. Conf. Acoustics, Speech and Signal Processing (ICASSP)}, year = {2019}, month = {04/2019}, pages = {4789 - 4793}, publisher = {IEEE}, organization = {IEEE}, address = {Brighton, UK}, abstract = {

Spectral precoding is a popular approach to reduce out-of-band radiation (OBR) in multicarrier systems in order to avoid adjacent channel interference. Since precoding will introduce signal distortion, appropriate decoding is required at the receiver side. We present a novel linear precoder design with flexibility to trade off OBR reduction, precoding/decoding complexity, and error rate at the receiver. The precoding matrices have low rank, which translates into significant computational savings. In this way, the requirements of different systems can be satisfied with varying complexity levels.

}, keywords = {spectrum shaping, winter}, doi = {10.1109/ICASSP.2019.8683162}, author = {Khawar Hussain and Ana Lojo and R. L{\'o}pez-Valcarce} } @conference {959, title = {OFDM spectral precoding with per-subcarrier distortion constraints}, booktitle = {European Signal Processing Conference (EUSIPCO)}, year = {2019}, month = {09/2019}, address = {A Coru{\~n}a, Spain}, abstract = {
Spectral precoding is effective in reducing out-of-band radiation
(OBR) in multicarrier systems, but it introduces distortion
in the data, requiring a suitable decoder at the receiver.
Thus, trading off OBR reduction, implementation complexity,
and error rate is of paramount importance. We present a precoder
design which minimizes OBR under constraints on the
distortion at each individual data subcarrier. By judiciously
choosing the distortion profile, the decoding task can be significantly
alleviated, with a sizable improvement in terms of
implementation complexity with respect to previous designs.
}, keywords = {spectrum shaping, winter}, author = {Khawar Hussain and R. L{\'o}pez-Valcarce} }