  Orthogonal time frequency space (OTFS) modulation is a recent modulation
scheme designed in the delay-Doppler domain. It has been shown to achieve
superior performance compared to conventional multicarrier modulation schemes
designed in the time-frequency domain. In this paper, we consider OTFS based
multiple access (OTFS-MA), where delay-Doppler bins serve as the resource
blocks for multiple access. Different delay-Doppler resource blocks (DDRBs) in
the delay-Doppler grid are allocated to different users for multiple access. We
consider three different DDRB allocation schemes. While Scheme 1 multiplexes
the users along the delay axis, Scheme 2 multiplexes them along the Doppler
axis. In both these schemes, each user's signal spans the entire time-frequency
plane. Scheme 3 allocates the DDRBs in such a way that each user's signal is
limited to span only over a subset of the time-frequency plane. We study the
performance of OTFS-MA in high mobility environments on the uplink and compare
it with those of OFDMA and SC-FDMA. Our results show that OTFS-MA (with
maximum-likelihood detection in small dimension systems and with a message
passing based detection in large dimension systems) achieves better performance
compared to OFDMA and SC-FDMA. We also present the performance of a multiuser
channel estimation scheme using pilot symbols placed in the delay-Doppler grid.
