  We propose a novel method for blind bistatic radar parameter estimation
(RPE), which enables integrated sensing and communications (ISAC) by allowing
passive (receive) base stations (BSs) to extract radar parameters (ranges and
velocities of targets), without requiring knowledge of the information sent by
an active (transmit) BS to its users. The contributed method is formulated with
basis on the covariance of received signals, and under a generalized
doubly-dispersive channel model compatible with most of the waveforms typically
considered for ISAC, such as orthogonal frequency division multiplexing (OFDM),
orthogonal time frequency space (OTFS) and affine frequency division
multiplexing (AFDM). The original non-convex problem, which includes an
$\ell_0$-norm regularization term in order to mitigate clutter, is solved not
by relaxation to an $\ell_1$-norm, but by introducing an arbitrarily-tight
approximation then relaxed via fractional programming (FP). Simulation results
show that the performance of the proposed method approaches that of an ideal
system with perfect knowledge of the transmit signal covariance with an
increasing number of transmit frames.
