  The heterogeneous cellular network (HCN) is a promising approach to the
deployment of 5G cellular networks. This paper comprehensively studies physical
layer security in a multi-tier HCN where base stations (BSs), authorized users
and eavesdroppers are all randomly located. We first propose an access
threshold based secrecy mobile association policy that associates each user
with the BS providing the maximum \emph{truncated average received signal
power} beyond a threshold. Under the proposed policy, we investigate the
connection probability and secrecy probability of a randomly located user, and
provide tractable expressions for the two metrics. Asymptotic analysis reveals
that setting a larger access threshold increases the connection probability
while decreases the secrecy probability. We further evaluate the network-wide
secrecy throughput and the minimum secrecy throughput per user with both
connection and secrecy probability constraints. We show that introducing a
properly chosen access threshold significantly enhances the secrecy throughput
performance of a HCN.
