  Phase wrapping is a major problem in direction-of-arrival (DOA) estimation
using phase-difference observations. For a sensor pair with an inter-sensor
spacing greater than half of the wavelength ($\lambda/2$) of the signal, phase
wrapping occurs at certain DOA angles leading to phase-difference ambiguities.
Existing phase unwrapping methods exploit either frequency or spatial
diversity. These techniques work by imposing restrictions on the utilized
frequencies or the receiver array geometry. In addition to sensitivity to noise
and calibration errors, these methods may also have high computational
complexity. We propose a grid-less \emph{phase-difference projection} (PDP) DOA
algorithm to overcome these issues. The concept of \emph{wrapped
phased-difference pattern} (WPDP) is introduced, which allows the proposed
algorithm to compute most of the parameters required for DOA estimation in an
offline manner, hence resulting in a superior computational speed in realtime.
Simulation results demonstrate the excellent performance of the proposed
algorithm, both in terms of accuracy and speed.
