"We investigate the spectral efficiency (SE) of an uplink multi-user multiple-input multiple-output (MIMO) system with orthogonal time frequency space (OTFS) modulation. Two multiple access schemes are considered hereafter, namely, delay division multiple access (DDMA) and Doppler division multiple access (DoDMA). 
To avoid multi-user interference (MUI), we separate the delay Doppler (DD) domain resource blocks assigned to different users by guard bands. We design a minimum-mean-square-error (MMSE) receiver to combine the received signals for the detection of different users. We analyze the SE for the considered MIMO-OTFS system and quantify the performance gains achieved over a MIMO system with orthogonal frequency-division multiple access (OFDMA). Our simulation results demonstrate a noticeable improvement in the performance of MIMO-OTFS over MIMO-OFDMA."
 

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