ity. FBMC eliminates the CP and introduces filter
banks to the OFDM system. More specifically, in-
stead of using a CP, FBMC uses an array of filters to
reduce the OOB power leakage and increase the spec-
tral efficiency at low costs (He and Schmeink, 2015).
However, the security aspects of FBMC-based sys-
tems have not been addressed in the literature so far.
Contributions. In this paper, we leverage the
random nature of the physical layer to enhance
the security of FBMC systems without impacting
its performance. More specifically, we propose a
new channel-based link-to-link encryption technique
for FBMC systems based on a pseudo-random per-
mutation. The proposed cipher scheme depends
on the unique pseudo-random channel characteris-
tics between two communicating users, which in-
creases the security level and robustness against ad-
versaries. Specifically, after offset quadrature ampli-
tude modulation (OQAM), frequency-domain sym-
bols are transformed into time-domain symbols using
the inverse fast Fourier transform (IFFT). The result-
ing time-domain symbols are randomly shuffled, and
then, filtered using a poly-phase network (PPN) fil-
ter bank, which also acts as a diffusion layer. Conse-
quently, this leads to a more secure FBMC system, as
demonstrated through experimental simulations and
cryptanalysis. To the best of our knowledge, this is the
first work that addresses and analyzes possible physi-
cal layer security (PLS) solutions for FBMC systems.
The rest of this paper is organized as follows.
Section 2 presents some basic concepts of the filter
bank and its system model. Section 3 presents the
proposed confidentiality scheme for the filter bank
system based on PLS. Section 4 analyzes the se-
curity properties of the proposed scheme, and as-
sesses it performance against different security at-
tacks. Section 5 studies the performance of the pro-
posed scheme in terms of execution time and error
propagation. Finally, section 6 concludes this work
and discusses its future prospects.
2 BACKGROUND
Figure 1 shows a block diagram of an FBMC sys-
tem. At the transmitter side, the symbols are first
modulated using offset QAM, and then, filtered us-
ing a Synthesis Filter Bank (SFB), which includes the
IFFT block and the poly-phase network. Similarly,
at the receiver, a reversed operation is performed, in
which time-domain symbols are recovered using an
Analysis Filter Bank (AFB) (which includes an FFT
block and PPN) and then demodulated (OQAM post-
processing). The SFB and AFB consist of an array of
filters equal in number to available sub-carriers.There
are two types of FBMC implementations—frequency
spreading (FS-FBMC) and poly-phase network (PPN-
FBMC). The latter is most common in the literature
as well as in this work, since it reduces the high
complexity that results from extra filtering (He and
Schmeink, 2015; Franzin and Lopes, 2017).
2.1 Offset Quadrature Amplitude
Modulation (OQAM)
In FBMC systems, orthogonality is achieved through
OQAM modulation where real and imaginary compo-
nents of symbols are transmitted in a staggered way.
The OQAM pre-processing block is based on a two-
step operation. The first step is converting complex
data into real data by separating the real and imag-
inary components of a complex-valued symbol into
two symbols. This increases the sample rate by a fac-
tor of 2 (Viholainen et al., 2009). Afterwards, the two
symbols are multiplied by the sequence θ
n,m
, given
by:
θ
n,m
= e
j
π
2
(n+m)
= j
n+m
(1)
where n is the sub-carrier index, and m is the time in-
dex at OQAM sub-symbol rate. Moreover, the time
index m depends on whether n is even or odd. If n is
even, then the time indices of the real and imaginary
parts of a complex symbol would be m and m + 1, re-
spectively. However, when n is odd, the time indices
of the real and imaginary components would be m+1
and m, respectively.
Accordingly, this technique avoids the interfer-
ence between consecutive sub-channels since in each
time interval, either the real or the imaginary part of
the original symbol is transmitted on a sub-carrier.
At the receiver, a reversed operation is performed
where the received symbols are multiplied by θ
∗
n,m
,
and the real part of the symbols are extracted. After-
wards, two consecutive real-value symbols are com-
bined to form the original complex-value symbol
(real-to-complex conversion).
2.2 Poly-Phase Network (PPN)
FBMC systems mainly depend on the poly-phase im-
plementation, which can be realized using different
filters such as finite impulse response filters (FIR)
or the more recent PHYDYAS prototype filters (Bel-
langer et al., 2010). In this paper, the PHYDYAS pro-
totype filter is considered since it reduces the high
complexity introduced by the extra filtering opera-
tions at the transmitter and receiver.
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