Beyond Near-Field: Far-Field Location Division Multiple Access in Downlink MIMO Systems
arxiv(2024)
摘要
Exploring channel dimensions has been the driving force behind breakthroughs
in successive generations of mobile communication systems. In 5G, space
division multiple access (SDMA) leveraging massive MIMO has been crucial in
enhancing system capacity through spatial differentiation of users. However,
SDMA can only finely distinguish users at adjacent angles in ultra-dense
networks by extremely large-scale antenna arrays. For a long time, most
research has focused on the angle domain of the space, overlooking the
potential of the distance domain. Near-field location division multiple access
(LDMA) was proposed based on the beam-focusing effect yielded by near-field
spherical propagation model, partitioning channel resources by both angle and
distance. To achieve a similar idea in the far-field region, this paper
introduces a far-field LDMA scheme for wideband systems based on orthogonal
frequency division multiplexing (OFDM). Benefiting from frequency diverse
arrays (FDA), it becomes possible to manipulate beams in the distance domain.
Combined with OFDM, the inherent cyclic prefix ensures a complete OFDM symbol
can be received without losing distance information, while the matched filter
of OFDM helps eliminate the time-variance of FDA steering vectors. Theoretical
and simulation results show that LDMA can fully exploit the additional degrees
of freedom in the distance domain to significantly improve spectral efficiency,
especially in narrow sector multiple access (MA) scenarios. Moreover, LDMA can
maintain independence between array elements even in single-path channels,
making it stand out in MA schemes at millimeter-wave and higher frequency
bands.
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