  A near-field integrated sensing, positioning, and communication (ISPAC)
framework is proposed, where a base station (BS) simultaneously serves multiple
communication users and carries out target sensing and positioning. A novel
double-array structure is proposed to enable the near-field ISPAC at the BS.
Specifically, a small-scale assisting transceiver (AT) is attached to the
large-scale main transceiver (MT) to empower the communication system with the
ability of sensing and positioning. Based on the proposed framework, the joint
angle and distance Cram\'er-Rao bound (CRB) is first derived. Then, the CRB is
minimized subject to the minimum communication rate requirement in both
downlink and uplink ISPAC scenarios: 1) For downlink ISPAC, a downlink target
positioning algorithm is proposed and a penalty dual decomposition (PDD)-based
double-loop algorithm is developed to tackle the non-convex optimization
problem. 2) For uplink ISPAC, an uplink target positioning algorithm is
proposed and an efficient alternating optimization algorithm is conceived to
solve the non-convex CRB minimization problem with coupled user communication
and target probing design. Both proposed optimization algorithms can converge
to a stationary point of the CRB minimization problem. Numerical results show
that: 1) The proposed ISPAC system can locate the target in both angle and
distance domains merely relying on single BS and limited bandwidths; and 2) the
positioning performance achieved by the hybrid-analog-and-digital ISPAC
approaches that achieved by fully digital ISPAC when the communication rate
requirement is not stringent.
