  As a promising 6G technology, integrated sensing and communication (ISAC)
gains growing interest. ISAC provides integration gain via sharing spectrum,
hardware, and software. However, concerns exist regarding its sensing
performance when compared to dedicated radar systems. To address this issue,
the advantages of widely deployed networks should be utilized, and this paper
proposes networked collaborative sensing (NCS) using multi-domain measurements
(MM), including range, Doppler, and two-dimension angle of arrival. In the
NCS-MM architecture, this paper proposes a novel multi-domain decoupling model
and a novel guard band-based protocol. The proposed model simplifies
multi-domain derivations and algorithm designs, and the proposed protocol
conserves resources and mitigates NCS interference. To determine the
performance limits, this paper derives the Cram\'er-Rao lower bound (CRLB) of
three-dimension position and velocity in NCS-MM. An accumulated
single-dimension channel model is used to obtain the CRLB of MM, which is
proven to be equivalent to that of the multi-dimension model. The algorithms of
both MM estimation and fusion are proposed. An arbitrary-dimension Newtonized
orthogonal matched pursuit (AD-NOMP) is proposed to accurately estimate
grid-less MM. The degree-of-freedom (DoF) of MM is analyzed, and a novel
DoF-based two-stage weighted least squares (TSWLS) is proposed to reduce
equations without DoF loss. The numerical results show that the performances of
the proposed algorithms are close to their performance limits.
