  In this paper, we consider a passive radar system that estimates the
positions and velocities of multiple moving targets by using OFDM signals
transmitted by a totally un-coordinated and un-synchronizated illuminator and
multiple receivers. It is assumed that data demodulation is performed
separately based on the direct-path signal, and the error-prone estimated data
symbols are made available to the passive radar receivers, which estimate the
positions and velocities of the targets in two stages. First, we formulate a
problem of joint estimation of the delay-Doppler of reflectors and the
demodulation errors, by exploiting two types of sparsities of the system,
namely, the numbers of reflectors (i.e., targets and clutters) and demodulation
errors are both small. This problem is non-convex and a conjugate gradient
descent method is proposed to solve it. Then in the second stage we determine
the positions and velocities of targets based on the estimated delay-Doppler in
the first stage. And two methods are proposed: the first is based on
numerically solving a set of nonlinear equations, while the second is based on
the back propagation neural network, which is more efficient. The performance
of the proposed passive OFDM radar receiver algorithm is evaluated through
extensive simulations.
