  In recent years, continuous-aperture multiple-input multiple-output
(CAP-MIMO) is reinvestigated to achieve improved communication performance with
limited antenna apertures. Unlike the classical MIMO composed of discrete
antennas, CAP-MIMO has a continuous antenna surface, which is expected to
generate any current distribution (i.e., pattern) and induce controllable
spatial electromagnetic waves. In this way, the information can be modulated on
the electromagnetic waves, which makes it promising to approach the ultimate
capacity of finite apertures. The pattern design for CAP-MIMO is the key factor
to determine the communication performance, but it has not been well studied in
the literature. In this paper, we propose the pattern-division multiplexing to
design the patterns for CAP-MIMO. Specifically, we first derive the system
model of a typical multi-user CAP-MIMO system, which allows us to formulate the
sum-rate maximization problem. Then, we propose a general pattern-division
multiplexing technique to transform the design of continuous pattern functions
to the design of their projection lengths on finite orthogonal bases. Based on
this technique, we further propose a pattern design scheme to solve the
formulated sum-rate maximization problem. Simulation results show that, the
sum-rate achieved by the proposed scheme is about 260% higher than that
achieved by the benchmark scheme.
