Near-Field Beamfocusing with Polarized Antennas
CoRR(2023)
摘要
One of the most relevant challenges in future 6G wireless networks is how to
support a massive spatial multiplexing of a large number of user terminals.
Recently, extremely large antenna arrays (ELAAs), also referred to as
extra-large MIMO (XL-MIMO), have emerged as an potential enabler of this type
of spatially multiplexed transmission. These massive configurations
substantially increase the number of available spatial degrees of freedom
(transmission modes) while also enabling to spatially focus the transmitted
energy into a very small region, thanks to the properties of near-field
propagation and the large number of transmitters. This work explores whether
multiplexing of multiple orthogonal polarizations can enhance the system
performance in the near-field. We concentrate on a simple scenario consisting
of a Uniform Linear Array (ULA) and a single antenna element user equipment
(UE). We demonstrate that the number of spatial degrees of freedom can be as
large as 3 in the near-field of a Line of Sight (LoS) channel when both
transmitter and receiver employ three orthogonal linear polarizations. In the
far-field, however, the maximum number of spatial degrees of freedom tends to
be only 2, due to the fact that the equivalent MIMO channel becomes rank
deficient. We provide an analytical approximation to the achievable rate, which
allows us to derive approximations to the optimal antenna spacing and array
size that maximize the achievable rate
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