  Orthogonal time frequency space (OTFS) modulation, a delay-Doppler (DD)
domain communication scheme exhibiting strong robustness against the Doppler
shifts, has the potentials to be employed in LEO satellite communications.
However, the performance comparison with the orthogonal frequency division
multiplexing (OFDM) modulation and the resource allocation scheme for multiuser
OTFS-based LEO satellite communication system have rarely been investigated. In
this paper, we conduct a performance comparison under various channel
conditions between the OTFS and OFDM modulations, encompassing evaluations of
sum-rate and bit error ratio (BER). Additionally, we investigate the joint
optimal allocation of power and delay-Doppler resource blocks aiming at
maximizing sum-rate for multiuser downlink OTFS-based LEO satellite
communication systems. Unlike the conventional modulations relaying on complex
input-output relations within the Time-Frequency (TF) domain, the OTFS
modulation exploits both time and frequency diversities, i.e., delay and
Doppler shifts remain constant during a OTFS frame, which facilitates a DD
domain input-output simple relation for our investigation. We transform the
resulting non-convex and combinatorial optimization problem into an equivalent
difference of convex problem by decoupling the conditional constraints, and
solve the transformed problem via penalty convex-concave procedure algorithm.
Simulation results demonstrate that the OTFS modulation is robust to carrier
frequency offsets (CFO) caused by high-mobility of LEO satellites, and has
superior performance to the OFDM modulation. Moreover, numerical results
indicate that our proposed resource allocation scheme has higher sum-rate than
existed schemes for the OTFS modulation, such as delay divided multiple access
and Doppler divided multiple access, especially in the high signal-to-noise
ratio (SNR) regime.
