  Large intelligent surface (LIS) has gained momentum as a potential
6G-enabling technology that expands the benefits of massive multiple-input
multiple-output (MIMO). On the other hand, orthogonal space-division
multiplexing (OSDM) may give a promising direction for efficient exploitation
of the spatial resources, analogous as what is achieved with orthogonal
frequency-division multiplexing (OFDM) in the frequency domain. To this end, we
study how to enforce channels orthogonality in a panel-based LIS scenario. Our
proposed method consists of having a subset of active LIS-panels coherently
serving a set of users, and another subset of LIS-panels operating in
semi-passive mode by implementing a receive and re-transmit (RRTx) process.
This results in an inter-symbol interference (ISI) channel, where we
characterize the semi-passive processing required to achieve simultaneous
orthogonality in time and space. We then employ the remaining degrees of
freedom (DoFs) from the orthogonality constraint to minimize the semi-passive
processing power, where we derive a closed-form global minimizer, allowing for
efficient implementation of the proposed scheme.
