Low-complexity Linear Equalization for \(2\times 2\) MIMO-OTFS Signals

Orthogonal time frequency space (OTFS) modulation is a two-dimensional modulation
scheme which has superior performance compared to conventional multicarrier
modulation schemes. In this paper, we propose low-complexity linear equalizers
for \(2\times 2\) multiple-input-multiple-output (MIMO) OTFS system. The proposed
equalizers are designed by exploiting the structure of the effective delay-Doppler
MIMO channel matrix in a MIMO-OTFS system. The channel matrix in a MIMO-OTFS system
is a block matrix composed of blocks which have a block circulant with circulant
block structure. The proposed approach makes use of the properties of block matrices
and block circulant matrices to reduce the computational complexity of linear
equalizers. For a \(2\times 2\) MIMO-OTFS system that uses \(N\times M\) OTFS
modulation, where \(N\) and \(M\) denote the number of Doppler and delay bins,
respectively, the proposed linear equalizers provide exact solution with a
computational complexity of \(O(MN\log MN)\), whereas conventional
linear equalizers require a complexity of \(O(M^3N^3)\).