Table 1.

Summary of typical approaches for FRs serving humans and WSNs.

Approach	FR Number	Dimension	Proactive or Reactive	Density, Location, or Distance-Based	Remark
[30]	Single	1D	Proactive	Location	Altitude optimization for coverage area
[52]	Single	3D	Proactive	Location	Minimizing transmitting power
[38]	Single	3D	Proactive	Location	Minimizing transmitting power
[39]	Single	3D	Proactive	Location	Maximizing covered user number
[43]	Single	2D	Reactive	Location	Tracking the center of users
[53]	Single	2D	Proactive	Location	Prolong WSN network lifetime
[25]	Multiple	2D	Proactive	Location	Minimizing robot number
[32]	Multiple	2D	Proactive	Density	Maximizing covered user number
[6]	Multiple	2D	Proactive	Density	Interference management
[37]	Multiple	2D	Proactive	Location	Recharge sensor nodes in cycle
[50]	Multiple	3D	Proactive	Location	Minimizing FR number
[51]	Multiple	3D	Proactive	Location	Minimizing FR number, maximizing data rate
[42]	Multiple	2D	Proactive	Density	Neural-based cost function
[31]	Multiple	2D	Proactive	Density	Decentralized robot-user distance minimization; connectivity
[40]	Multiple	2D	Proactive	Location	K-means clustering
[33]	Multiple	3D	Proactive	Location	Minimizing FR number; connectivity
[7]	Multiple	3D	Reactive	Location	Minimizing FR number; energy constrained
[44]	Multiple	2D	Reactive	Location	Exhaustive search moving direction
[48]	Multiple	2D	Reactive	Distance	Move towards weighted centers
[49]	Multiple	2D	Reactive	Location	Navigation based on virtual force
[55]	Single	2D	Proactive	Location	Selection charging node and sink node
[56]	Multiple	2D	Proactive	Location	Maximization of data collection utility
[57]	Single	2D	Proactive	Location	Varying energy consumption rates
[58]	Multiple	2D	Proactive	Location	Charging routes and sensor association
[26]	Multiple	3D	Proactive	Location	Minimizing FR number