It has been shown that the tracking performance is connected with the antenna configuration in the colocated MIMO radar system. To achieve better tracking performance, the dynamic antenna configuration should be considered. The cognition technique perceives the environment change through the closed-loop feedback mechanism from the transmitter to the receiver, and then self-turns the working parameters. Its superiorities in MIMO radar resource management have been shown [
9,
10,
22,
23]. As such, in this section, a closed-loop antenna adjustment scheme is proposed. Since the PCRLB is predictive, the main idea is that the tracking information obtained from the previous time epoch is used to approximate the PCRLB in the next time epoch and serves as a guideline for the antenna configuration.
However, the number of antennas in the colocated MIMO radar is usually large, which makes the solution of the antenna selection problem expensive. Therefore, we firstly treat it as the antenna placement problem. Then, in the solution exploration, the whole solution region is discretized into many grids, which matches the antenna position in reality, to select the optimal antennas.
4.1. Objective Function Establishment
In practice, the antennas to be chosen in the colocated MIMO radar need to be well separated to satisfy the maintenance and safety requirements. On the other hand, the distance between antennas should be sufficiently small to ensure that the far-filed assumption is valid [
7]. The constraints are expressed as:
where
dminmn is the minimum distance constraint between the transmit antenna
m and receive antenna
n, and
dmaxmn can be explained accordingly. Δ
bmn is shown in Equation (A12).
Additionally, the center of both transmit antennas and receive antennas are located in the origin, and the transmit antenna and receive antenna should be within a region. Thus, the following constraints should be considered:
where
btmin and
btmax form the region wherein the transmit antenna should be located, and
brmin and
brmax form the region to which the receive antenna should be restricted.
Therefore, the optimization problem is established as:
Combined with
Appendix A, it is evident that the objective function in Equation (30) is non-convex [
7]. Standard convex approaches are powerless. Thus, a FDPSO is proposed as the solution to the optimization problem.
4.2. FDPSO Algorithm
Owing to its high efficiency and easy implementation, the PSO algorithm has been used to solve many engineering optimization problems. Refs. [
28,
29] proposes a novel PSO at an acceptable time cost for the endmember extraction in the hyperspectral image. Ref. [
30] presents an anarchic PSO for scheduling a distributed production network. Ref. [
31] uses the coevolutionary PSO (CPSO) method to identify the battery parameters. As such, in this section, a novel PSO variant is proposed for the solution to Equation (30).
The PSO is originally inspired by the social behaviors of bird flocking and fish schooling. During foraging, each individual in the swarm will change the position and velocity depending on its own experience and the interaction with others. As a result, all individuals will fly to the best position found by the swarm. Finally, through swarm cooperation and iterative optimization, the global optimum will finally be achieved. However, the traditional PSO is restricted by its large computational demand in the application of target tracking. Therefore, many optimization methods are put forward and integrated into the PSO, whose main aim is to meet real-time requirements while offering high-quality solutions.
Firstly, the whole solution region is discretized into many grid points to match the real antenna placement. Secondly, the penalty function is introduced into the PSO. When the particles do not satisfy the constraints, and additional punishment function will be introduced to the fitness value, leading to other particles staying far away from those individuals. In contrast, the particles will fly to those satisfying the constraints. Thereby, the exploration ability of the algorithm is enhanced. Last but not least, the update equation of PSO is modified to be concise enough, and a random and damping factor is put forward to diversify the population around the local optima. As such, the efficiency can be further improved. Meanwhile, the particles can be driven for exploring the global optimum, which guarantees the quality of solutions.
The flow chart of FDPSO is shown in
Figure 1.
Assuming that the population cardinality is
Npop, the number of variables in each particle is
Nvar, the smallest resolution unit is chosen as Δ
d (to discretize the solution region) and the vector of the lower bound and the upper bound of variables are
bL and
bU, respectively. Here,
bL is formed by
btmin and
brmin, and
bU is formed by
btmax and
brmax. Before running the FDPSO, the optimization problem should be reformulated as:
In population initialization, the Gaussian initializer is adopted:
where
xi,t is
ith particle’s position in
tth iteration. randi([
bL/Δ
d,
bU/Δ
d],1,
Nvar) means generating a 1 ×
Nvar dimensional vector with Gaussian distribution, and all the elements are restricted in the region of [
bL/Δ
d,
bU/Δ
d].
In the fitness value calculation, the punishment function in the convex optimizer is introduced:
where
φ1 and
φ2 are two relatively large numbers. It can be seen that when the particle does not meet the constraints shown in Equation (31), a relatively large value will be added to its fitness value, resulting in this particle possessing a bad fitness value. Therefore, according to the swarm update mechanism, other particles will stay away from this one. Conversely, the particles satisfying the constraints will have more chances to attract other individuals and update their positions.
The update equation of a traditional PSO is [
25,
26,
27]:
where
vi,t is the velocity vector of
ith particle in the
tth iteration,
pi,tbest is the best position achieved by
ith particle to date,
xi,t is the position vector of
ith particle, and
gtbest is the best position achieved by the swarm so far.
w is the inertia weight, which balances the particles’ exploration ability between the whole solution region and the local solution region.
c1 and
c2 determine the speed when the particle is flying to
pi,tbest and
gtbest, respectively. Usually,
c1 =
c2 = 2.
r1 and
r2 are random values which belong to (0,1).
However, such an updated equation may be a little redundant in the application for target tracking. Thus, the core idea in the PSO is reserved and the equation is modified as:
where
β is a predetermined parameter belonging to (0,1) which controls the convergence ratio of the algorithm. The small
β corresponds to slow convergence, whereas a large
β enables the fast convergence.
ρt is the random damping factor which is calculated by:
where
γ is the parameter belonging to (0,1). randn(1,
Nvar) generates a 1 ×
Nvar dimensional integer vector.
denotes the Hadamard product. Equation (36) shows that many futile parameters are removed on the basis of a traditional particle update equation, and a random item is introduced. Thereby, the algorithm is sufficiently useful for application. Meanwhile, particles will further explore better solutions around local optima, so that high-quality solutions can be determined. Additionally, the
ρt is diminishing along with the iteration, ensuring that the random disturbance term is also diminishing. Therefore, the convergence of the algorithm can be guaranteed.
4.4. Closed-Loop Feedback System for Target Tracking
To tackle the nonlinear transformation in the radar coordinate, a desirable filter must be chosen. The CKF [
41], which is based on the three-degree spherical cubature rule, transforms the nonlinear filtering problem into the integration calculation. Its merits of simple structure and high precision have been demonstrated in various estimate problems [
43,
44,
45,
46]. The SCKF introduces the matrix triangular factorization on the basis of CKF and avoids the recursive square root operation to the state error covariance matrix, so that the numerical stability and accuracy are further improved. Therefore, the SCKF is adopted.
Since the PCRLB is predictive, after obtaining the tracking result from the current time epoch, the PCRLB can be predicted in order to choose the optimal antenna configuration in the next time epoch. Then, antenna placement can be adjusted to improve the whole tracking performance.
As such, the procedure of the closed-loop tracking system can be summarized as follows:
Step 1—Obtain the state and the state estimate error covariance matrix of a target in current time epoch;
Step 2—Predict the PCRLB in the next time epoch, and call the proposed FDPSO algorithm to adjust antenna placement;
Step 3—The tracking results by the antenna adjustment are sent back to guide the antenna configuration in next time epoch, rendering it a closed-loop system.