Aerospace and Electronic Systems Magazine February 2018 - 8

Cyber-Secure Multiuser Superposition Covert Avionics System

Figure 3.

Transmitter design.

higher transmission power is assigned to a signal whose receiver
has a lower normalized channel gain, whereas a lower transmission
power is assigned to a signal whose receiver has a relatively higher
normalized channel gain. The superposed signal S will be further
noise modulated and be transmitted over the whole frequency band
through a polarized antenna.
The use of NOMA can support multiple users on the same frequency band simultaneously, which significantly improves the system spectral efficiency and throughput. In this article, we denote
the channel gain between the transmitter and receiver k as hk, for k
= 1, 2, · · ·, N. A backhaul feedback link is usually established to
feed back the channel state information from the receivers to the
transmitter [25]. It is usually categorized into wired (leased lines or
copper and fiber) or wireless (point-to-point or point-to-multipoint
over high-capacity radio links). In this article, we assume the receivers will estimate the channel condition and feed back to the
transmitter via a backhaul link intact so that the channel condition
is perfectly available. Without loss of generality, the channel gains
are normalized by noise and interference, and have been ordered
h1

<<

. Herein, N0,k, for k = 1, 2, · · ·, N, is
N 0,1 N 0,2
N 0, N
defined as the noise and interference at the receiver k. For the user
with a higher channel gain normalized by noise and interference
(called simply channel gain in the remaining), it will be allocated a
relatively lower transmission power and vice versa. Therefore, the
power allocation strategy can be formed by the order of p1 > p2 > ·
N
· · > pN, where  k =1 pk = P, and P is the total transmission power
at the transmitter. Accordingly, power allocation may be optimized
in (1).

{ p1 , p2 ,, pN } = arg max ( R1,NOMA + R2, NOMA +  RN ,NOMA )
p1 , p2 ,, pN

subject to
N

p
k =1

= P;

p1 > p2 >  > pN .

(1)

Here, Rk,NOMA for k = 1, · · ·, N, represents the achievable
data rate of user k with NOMA operation. By dynamically selecting the transmission power, the superposed signal is formed as
S = p1 s1 + p2 s2 +  + pN s N .. Afterward, in the noise modulation stage, S is modulated with a carrier frequency of fc. Meanwhile, a zero-mean band-limited Gaussian noise with center
frequency fn is intelligently generated by Key 1 as the reference
waveform. This Gaussian noise is then passed through a bandpass filter to ensure that it is confined within certain operating
frequency range with a center frequency of fn. Furthermore, the
band-limited Gaussian noise passes through a power divider and
splits in two components. One is mixed with modulated signal S,
and the lower sideband signal is selected (centered at fc − fn). It is
further transmitted through a vertically polarized antenna, which
is denoted as V(t).
Concurrently, the other component of the band-limited Gaussian noise is connected with a delay line with a predetermined and
controllable delay τ, which is generated by Key 2. The delayed
noise signal is transmitted through a horizontally polarized antenna as the reference noise signal, which is denoted as H(t − τ). By
judiciously choosing fc = 2fn, the vertically polarized signal V(t)
is centered at fn as well, in the same range as the reference noise
signal H(t − τ).
At the receiver side illustrated in Figure 4, the vertically polarized signal V (t ) is first passed through a delay line with the exact
delay time τ as the transmitted horizontally polarized signal. This
brings the two orthogonally polarized signals H (t − τ ) and V (t − τ )
into time synchronization. If the delay does not exactly match the
corresponding transmit delay, then no message can be extracted
from the noise-modulated signal. Hence, only a friendly receiver
knows the exact value of the delay time. Any adversary without
knowledge of the delay value will not be able to perform the proper
correlation to decode the hidden message. By mixing the two polarized signals together and passing them through a bandpass filter
centered at fc, only the sum frequency component of the mixed signal can be caught. Then, assuming perfect carrier synchronization,
the baseband signal can be obtained at the output of a low-pass
filer. To recover the superposed baseband signal, SIC is applied

IEEE A&E SYSTEMS MAGAZINE

FEBRUARY 2018

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