  Spatial division multiple access (SDMA) is essential to improve the spectrum
efficiency for multi-user multiple-input multiple-output (MIMO) communications.
The classical SDMA for massive MIMO with hybrid precoding heavily relies on the
angular orthogonality in the far field to distinguish multiple users at
different angles, which fails to fully exploit spatial resources in the
distance domain. With the dramatically increasing number of antennas, the
extremely large-scale antenna array (ELAA) introduces additional resolution in
the distance domain in the near field. In this paper, we propose the concept of
location division multiple access (LDMA) to provide a new possibility to
enhance spectrum efficiency compared with classical SDMA. The key idea is to
exploit extra spatial resources in the distance domain to serve different users
at different locations (determined by angles and distances) in the near field.
Specifically, the asymptotic orthogonality of near-field beam focusing vectors
in the distance domain is proved, which reveals that near-field beam focusing
is able to focus signals on specific locations with limited leakage energy at
other locations. This special property could be leveraged in hybrid precoding
to mitigate inter-user interferences for spectrum efficiency enhancement.
Moreover, we provide the spherical-domain codebook design method for LDMA
communications with the uniform planar array, which provides the sampling
method in the distance domain. Additionally, performance analysis of LDMA is
provided to reveal that the asymptotic optimal spectrum efficiency could be
achieved with the increasing number of antennas. Finally, simulation results
verify the superiority of the proposed LDMA over SDMA in different scenarios.
