  A novel application of leaky-wave antennas (LWAs) as integrated antennas and
multiplexers/demultiplexers is proposed and experimentally demonstrated in the
millimeter-wave band at 60~GHz. The first application is demultiplexing of an
oblique-incident free-space wideband plane wave into $2N$ channels using $N$
LWAs with different beam-scanning laws. The second application is $N$--channel
multiplexing and demultiplexing using $N$ pairs of identical LWAs in each
other's far-field, where each LWA pair is designed such that the broadside
frequency is the center frequency of its respective channel. Friis transmission
equation is used to analytically demonstrate the two proposed applications
using Gaussian radiation beams. The LWAs are then implemented using
reflection-cancelling slot pairs using substrate integrated waveguide (SIW)
technology. Full-wave simulations are used to demonstrate the first application
by demultiplexing an incident plane wave into two and four channels using two
LWAs. Finally, the two proposed applications are experimentally demonstrated by
conducting horn-to-LWA and LWA-to-LWA transmission measurements and
successfully showing broadband frequency discrimination. The proposed LWAs
provide a simple, compact, high-efficiency, and low-profile
multiplexing/demultiplexing solution with high-integration capability with
other circuitry. Compared to conventional multiplexers, the proposed solution
does not require matching networks and can be directly scaled to higher
frequencies using the same architecture.
