An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Filter-Bank Multicarrier Wiretap Channel <s> Scenarios envisaged for mobile communication systems beyond 2020 imply that future applications and services will impose highly diverse requirements on the system design. To appropriately respond to these requirements while ensuring an efficient usage of available spectrum and system resources, the air interface of 5G system is expected to provide much more flexibility compared to today's systems. Filterbank based multi-carrier (FBMC) may be considered a potential enabler for that purpose, as it provides excellent waveform properties with additional degrees of freedom for the system design, thanks to the use of prototype filters that may be adjusted to meet desired system constraints. This paper elaborates on the challenges that still need to be solved to render FBMC an eligible candidate as fundamental technology for 5G mobile radio and presents some first approaches towards practical solutions. <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Filter-Bank Multicarrier Wiretap Channel <s> As the standardization activities are being formed to lay the foundation of 5G wireless networks, there is a common consensus on the need to replace the celebrated orthogonal frequency division multiplexing (OFDM) by a more effective air interface that better serves the challenging needs of 5G. To this end, in the recent past, a number of new waveforms have been introduced in the literature. Interestingly, and at the same time not surprising, these methods share a common fundamental principle with OFDM: each data packet is made up of a number of complex-valued sinusoidals (pure tones) that are modulated by the information symbols. In this tutorial article, we build a common framework based on the said OFDM principle and derive these new waveforms from this point of view. This derivation provides a new perspective that facilitates straightforward understanding of channel equalization and the application of these new waveforms to multiple-input multiple-output channels. It also facilitates derivation of new structures for more efficient synthesis/analysis of these waveforms than those that have been reported in the literature. <s> BIB002
OFDM systems face important challenges when they are adopted for more complex networks. For instance, it is very difficult to establish full synchronization for the signals corresponding to different users in the base station in an orthogonal frequency division multiple access (OFDMA) network. Furthermore, OFDM introduces significant out of band interference to other users when it is used to transmit over a set of non-contiguous frequency bands as in cognitive radio where transmission is limited to certain portions of the band and 5G wireless systems BIB002 . Filter bank multicarrier (FBMC) modulation is shown to be able to resolve the above problems by utilizing high quality filters for transmission, which makes it a potential candidate for future networks including 5G. With this motivation, recently, there has been some interest in the physical layer security aspects of FBMC based networks, as well. Schellmann et al. BIB001 present a filter hopping FBMC approach for security. They argue that the choices of filters for each symbol provide a major degree of freedom for the FBMC waveforms, which can be exploited for security purposes. Furthermore, a mismatch in the receive filter results in violation of the orthogonality of different subcarriers, and it causes the energy of a transmit symbol at a certain subcarrier to spread into the adjacent subcarriers. This effect is shown to be significant even for a slight mismatch of the filters between the transmitter and the receiver BIB001 , hence it can be exploited for providing physical layer security. For instance, in a practical system, Alice and Bob can exchange their filter designs by exchanging keys or using a pre-shared sequence of filters. Then, the transmitter can change the filters continuously, and make it extremely difficult for an eavesdropper to find the exact filter being used, hence securing the transmission.