  Wireless Physical Layer Identification (WPLI) system aims at identifying or
classifying authorized devices based on the unique Radio Frequency Fingerprints
(RFFs) extracted from their radio frequency signals at the physical layer.
Current works of WPLI focus on demonstrating system feasibility based on
experimental error performance of WPLI with a fixed number of users. While an
important question remains to be answered: what's the user number that WPLI can
accommodate using different RFFs and receiving equipment. The user capacity of
the WPLI can be a major concern for practical system designers and can also be
a key metric to evaluate the classification performance of WPLI. In this work,
we establish a theoretical understanding on user capacity of WPLI in an
information-theoretic perspective. We apply information-theoretic modeling on
RFF features of WPLI. An information-theoretic approach is consequently
proposed based on mutual information between RFF and user identity to
characterize the user capacity of WPLI. Based on this theoretical tool, the
achievable user capacity of WPLI is characterized under practical constrains of
off-the-shelf receiving devices. Field experiments on classification error
performance are conducted for the validation of the information-theoretic user
capacity characterization.
