  We consider the downlink of cell-free massive multiple-input multiple-output
(MIMO) systems with orthogonal time frequency space (OTFS) modulation. Two
pilot-based channel estimation schemes, namely superimposed pilot-based
(SP-CHE) and embedded pilot-based channel estimation (EP-CHE), are applied to
estimate the channels at the access points (APs). The SP-CHE scheme
superimposes low power pilots onto the data symbols in the delay-Doppler domain
to avoid the spectral efficiency (SE) loss due to null guard intervals used in
the EP-CHE scheme. In the case of SP-CHE scheme, we consider a max-min fairness
optimization problem to jointly optimize the peruser pilot/data power
allocation coefficients and per-AP power control coefficients. The complicated
non-convex problem is then iteratively solved through two decoupled
sub-problems. Moreover, a max-min fairness problem is cast for the EP-CHE
scheme, where the optimization variables are the per-AP power control
coefficients. Numerical results show that the proposed resource allocation
approaches provide at most 42 and 5-fold increase in the 95%-likely per-user SE
for the SP-CHE and EP-CHE scheme, respectively, compared with the uniform power
control and in correlated shadowing fading channels.
