  The impact of large bandwidth on near-filed sensing (NISE) is analyzed in
multi-carrier systems. The fundamental Cramer-Rao bounds (CRBs) for wideband
NISE are characterized. In particular, the closed-form CRBs are derived for
both uniform linear arrays (ULAs) and uniform circular arrays (UCAs). Then, the
asymptotic CRBs are analyzed. It is rigorously proved that: 1) as the number of
antennas N increases, the maximum decay rates of asymptotic CRBs are 1/N for
ULAs and 1/N^2 for UCAs; 2) as the number of subcarriers M increases, the
asymptotic CRBs decay as 1/M^3 for both ULAs and UCAs; and 3) CRBs are
inversely proportional to the beamforming gain. Based on the analytical
results, two practical beamforming approaches are proposed for near-field
wideband integrated sensing and communication (ISAC), namely independent and
joint approaches. For the independent approach, the beamformer on each
subcarrier is designed exclusively for either sensing or communication. For the
joint approach, the beamformer on each subcarrier is jointly optimized for both
functions through a low-complexity iterative algorithm. Finally, numerical
results show that 1) large bandwidth sets an estimation error ceiling for NISE;
2) NISE performance converges to far-field sensing performance when the
bandwidth is extremely large; 3) there is a tradeoff between array size and
system bandwidth for achieving a given sensing performance; and 4) the simple
independent beamforming approach achieves an ISAC performance close to the
complex joint beamforming approach.
