graphic mechanisms, we encrypt the quadratic problem as-
sociated with MPC prior to the computations in the cloud,
and propose an efficient method to verify its solutions from
the cloud. We show that no false solutions can succeed in the
verification with a non-negligible probability.
b) Secondly, to guarantee stability and enhance resiliency, we
design a switching mechanism SMM using an event-triggered
MPC scheme and H
∞
-optimal control. Under this mecha-
nism, the NCSs switch to a Buffer Mode when control inputs
are unavailable from the cloud for a short duration, or switch
to a Safe Mode when control inputs are unavailable for a long
duration. We demonstrate that the SMM guarantees the sta-
bility of CE-NCSs.
c) Finally, both analytical and experimental results show that
our mechanism can boost efficiency for NCSs. The experi-
mental results also corroborate that the SMM can guarantee
stability for CE-NCSs when control inputs are unavailable
for either a short or long period of time.
1.4 Organization
The rest of the paper is organized as follows: Section 2 presents
the problem formulation, three cloud attack models, the design
goals, and the framework of the proposed mechanism. In Section 3,
we develop specific strategies and techniques to achieve data confi-
dentiality and integrity. In Section 4, we propose a switching mech-
anism to deal with availability issues. Both theoretical analysis and
experiment results are presented in Section 5. Finally, Section 6
concludes this paper.
2. PROBLEM STATEMENTS
Each CE-NCS has two layers: cyber layer and physical layer.
The cyber layer consists of wireless communications and a cloud,
while the physical layer incorporates a plant, actuators, and sensors,
and a controller. The integration of the controller with the cloud
constitutes a cloud-based controller. Fig. 2 illustrates a feedback
architecture of a CE-NCS. The controller of a CE-NCS is struc-
turally different from other general NCSs. Instead of solving an
off-line control problem, the controller of a CE-NCS formulates a
dynamic optimization problem based on the sensor data, and out-
sources the computations of the control decisions to a cloud. After
receiving the solutions from the cloud, the controller sends them to
the actuators of the physical system.
To describe in detail the architecture of a CE-NCS, we first intro-
duce the physical layer control problem. In this work, we present a
discrete-time linear system to capture the dynamics of the physical
plant and use Model Predictive Control (MPC) framework to de-
sign optimal control to stabilize the system. The computations of
the MPC control inputs are outsourced to the cloud, which can be
subject to adversarial attacks. To this end, the second part of this
section presents three attack models on the CE-NCS, and outline
the design goals and framework of the proposed mechanism.
2.1 System Dynamics and MPC Algorithm
MPC has been widely used in many domains of industries and
civil applications, such as process control of chemical plants and
oil refineries, energy systems in building, and autonomous vehicles
[1, 31]. MPC is a model-based control strategy that uses a sys-
tem model to predict the future behaviors of the system to establish
appropriate control inputs [28]. To achieve prediction, MPC strat-
egy provides a moving finite-horizon problem based on the system
model, and control inputs can be computed by solving this prob-
lem at each sampling instant. One advantage of MPC is that it
Figure 2: Architecture of a CE-NCS: The integrated system
consists of a plant, sensors, actuators, and a cloud-based con-
troller. The cloud-based controller is composed of a local con-
troller, a wireless network, and a cloud. The network is de-
ployed to transmit data of computations to the cloud and the
control solutions from the cloud. The controller aims to stabi-
lize the plant and achieve system objectives.
allows the system to operate constraints on control inputs or sys-
tem states [15], while it is difficult for the other control strategies,
e.g., Linear Quadratic Regulator (LQR) and H
∞
-optimal control,
to handle constraints. The second advantage is that MPC enables an
on-line design, which can deal with disturbance in real time, while
LQR and H
∞
-optimal control are off-line designs. In addition,
H
∞
-optimal control considers the worst-case disturbance, leading
to more conservative and expansive control inputs. A main chal-
lenge of MPC is that the controller needs to solve a optimization
problem at each sampling time, resulting in a higher computational
complexity. This motivates us to introduce a CE-NCS framework
to outsource the computations of the MPC problem to the cloud.
To apply MPC, we use a discrete-time linear system model to
describe the system dynamics of a control system, given by
x(k + 1) = Ax(k) + Bu(k), (1)
where x(k) ∈ R
n×1
is the state vector of the NCS, x(0) = x
0
∈
R
n×1
is the given initial state value, u(k) ∈ R
l×1
is the control
input, A ∈ R
n×n
and B ∈ R
n×l
are constant matrices.
At each sampling time k, a finite-horizon MPC problem P is a
minimization problem, which is given as follows:
P : min
U (k)
J(x(k), U (k))
=
N−1
X
τ =0
kˆx(k + τ |k)k
2
+ ηkˆu(k + τ |k)k
2
, (2)
subject to
ˆx(k + τ + 1|k) = Aˆx(k + τ |k) + B ˆu(k + τ |k),
ˆx(k|k) = x(k), ˆu(k + τ|k) ∈ U ,
U(k) = [ˆu
T
(k|k), ˆu
T
(k + 1|k), . . . , ˆu
T
(k + N − 1|k)]
T
,
where J : R
n×1
× R
lN×1
→ R is the objective function, η is a
tuning parameter for the desired control performance, and U(k) ∈
R
lN×1
is the solution sequence of the problem P. ˆx(k + τ|k)
denotes an estimate value of x(k + τ), based on the feedback state
x(k). Through out this paper, k · k represents the Euclidean norm.