YUE et al.: MULTIOBJECTIVE PARTICLE SWARM OPTIMIZER USING RING TOPOLOGY FOR SOLVING MULTIMODAL MULTIOBJECTIVE PROBLEMS 807
Fig. 2. Illustration of multimodal multiobjective problem.
approximate the PS and the PF. However, this algorithm can
only deal with multiobjective problems of class II [22]. Other
drawback of the method is the technique performs poorly when
the PS is a linear manifold.
Liang et al. [4] defined the multimodal multiobjective
optimization problem, which could be succinctly illustrated
through the diagram in Fig. 2. The figure shows a simple
diagram illustrating a case, where there are two PSs cor-
responding to the same PF. Liang et al. [4] proposed an
algorithm called DN-NSGAII conceived for the purpose of
locating more PSs. The technique first ranks solutions accord-
ing to nondominated sorting scheme and the solutions in
the same front are then sorted based on a decision-space
CD. Therefore, in DN-NSGAII the nondominated and less-
crowded solutions in the decision space are preferred. It has
been demonstrated that this algorithm can obtain more solu-
tions than NSGAII [23] and various other commonly used
multiobjective optimization approaches. However, the distri-
bution of solutions in the decision space is not very good (see
Fig. 9 (a) and (c) in [4]).
B. Framework of Particle Swarm Optimization
PSO is a population-based algorithm, which makes it par-
ticularly well suited for solving multimodal optimization
problems. PSO was inspired by the social behavior of birds
within a flock [14]. In this method, the historical personal best
position of a point (also called particle) is denoted pbest, and
the historical best position of its neighborhood is called nbest.
Each particle in the swarm is led by pbest and nbest to fly from
a starting position to a better area. Let x
i
(t) and v
i
(t) denote
the position and velocity of particle p
i
of the tth generation.
They are updated according to the following equations:
x
i
(t) =x
i
(t − 1) +v
i
(t) (4)
and
v
i
(t) = Wv
i
(t − 1) + C
1
r
1
x
pbest
i
−x
i
(t)
+ C
2
r
2
x
nbest
i
−x
i
(t)
(5)
where W is the inertia weight (often set to 0.7298, according
to [24]), C
1
and C
2
are constants (which satisfy the equal-
ity C
1
+ C
2
= 4.1[24]) used to balance exploration and
exploitation processes, and r
1
and r
2
are random values uni-
formly generated in the range [0, 1]. The key step in the
PSO methodology is the selection of leaders for the current
particles [25].
When extending PSO algorithms from the single-objective
to the multiobjective case, three issues become of particu-
lar significance: 1) the selection of a leader particle; 2) the
management of the distribution properties; and 3) the man-
agement of the convergence speed. Regarding the first issue,
the leader-particle selection is a relatively straightforward pro-
cedure in single-objective optimization, where one can simply
declare that particle with the largest fitness value is the best
candidates to serve as leaders. In multiobjective optimization,
however, the identification of a best leader among all the solu-
tions may become a challenging task due to inherent conflicts
introduced by the multiple objectives. In that case it is natu-
ral to give preference to nondominated solutions to serve as
candidates for the leader designation. Concerning the second
issue, it is necessary to develop of a methodology that can
ensure a good distribution of solutions in the decision space
and in the corresponding objective space. Finally, regarding
the third issue, the PSO convergence speed has been addressed
through a number of communication topologies. In particular,
the star, ring, and von Neumann topologies are known to be
effective in avoiding premature convergence. The topology-
based PSO algorithms are reviewed and compared in [26].
However, these topological approaches cannot induce stable
niches, and as a consequence in multimodal problems the
population converges to a single solution instead of multiple
solutions. In contrast, r3pso described in [12], which incor-
porates an index-based ring topology, has been demonstrated
through experimental results that it can form stable niches.
In addition, the algorithm does not require the introduction
of niching parameters, which is an attractive feature. The
three issues described here are addressed in the new algorithm
described in the next section.
III. D
ESCRIPTION OF MO_RING_PSO_SCD
As a population-based algorithm, PSO has the capability,
and hence the natural advantage of searching for multiple
optima in a single run. In multimodal multiobjective opti-
mization problems, where a large number of Pareto-optimal
solutions should be searched for and maintained, PSO is there-
fore a natural choice for use as an optimizer. Inspired by the
single-objective particle swarm optimizer using ring topology
proposed by Li [12], this paper proposes a multiobjective PSO
algorithm with ring topology, and includes a SCD, which the
authors denote by the acronym MO_Ring_PSO_SCD. In this
section, the
MO_Ring_PSO_SCD procedure is presented fol-
lowed by a discussion of its underlying mechanism for the
purpose of analyzing how and why the proposed algorithm
can successfully address multimodal multiobjective problems.
Finally, the novelty of the proposed algorithm is described
and the convergence behavior of MO_Ring_PSO_SCD is
compared with that of alternative algorithms.
A. Procedure of MO_Ring_PSO_SCD
In MO_Ring_PSO_SCD, the personal best archive (PBA)
and the neighborhood best archive (NBA) are first estab-
lished, and then the pbest and gbest are selected from the two
respective archives. Ring topology is used to induce multiple