Communications and Signal Processing Lab (ComSP)

Title	Spectral shaping techniques for OFDM signals
State	Finished
Author	Javier Giménez De La Cuesta
Director/s	José Antonio Cortés Arrabal ,  Luis Díez Del Río
University	Universidad de Málaga
Department	INGENIERIA DE COMUNICACIONES
Reading date	27/10/2025
File	PDF

Orthogonal frequency division multiplexing (OFDM) is a widespread modulation, but

suffers from low spectral confinement. A plethora of strategies have been proposed to mitigate

this effect. In particular, pulse shaping, adaptive symbol transition (AST), active interference

cancellation (AIC) and spectral precoding techniques are addressed in this thesis. Also, a

combination of several of these methods is used as foundation for the contributions of this

thesis, due to their computational simplicity and effectiveness in reducing the out-of-band

emissions (OOBE) of OFDM signals.

As first contribution, a strategy has been proposed to reduce the computational com-

plexity of the aforementioned spectral shaping strategies. It requires modifying the waveform

conventionally used in OFDM so that it has Hermitian symmetry. This adjustment is enough

for many AIC, AST and spectral precoding techniques to benefit from nearly halving their

optimization and implementation costs. Moreover, a design framework is set so that future

proposals can benefit from the same reduction.

In second place, a framework is devised to reduce the computational cost of recomputing

the solutions to the spectral shaping problem in systems where the power emission con-

straints can change dynamically with time. Most state-of-the-art spectral shaping techniques

are forced to solve an optimization problem each time the power spectral density (PSD)

constraints change. As an alternative, we propose a strategy in which a set of precomputed

solutions is seamlessly adapted to these changes by applying some simple transformations

that can be performed online within the system. This framework is based on AIC and AST

techniques, due to their ability to be applied transparently to the receiver.

Finally, a spectral shaping method that enhances the number of degrees of freedom avail-

able for the reduction of the OOBE in OFDM signals is proposed. It is also based on the AIC

and AST techniques, and consists in incorporating time-advanced and time-delayed versions

of these terms such that they overlap with preceding and succeeding OFDM symbols. The

additional degrees of freedom can be used to mitigate the data rate penalty associated to

AIC techniques. Alternatively, they can be used to further reduce the OOBE.