  With the recent emergence of 5G era, heterogeneous cellular networks (HCNs)
have invoked a popular research interest. In this paper, we provide a
comprehensive analysis for multiantenna transmissions in a multi-tier downlink
HCN. We first propose a reliability-oriented threshold-based mobile association
policy, where each user connects to the strongest base station from which this
user can obtain the largest truncated long-term received power. Under our
mobile association policy, we derive analytical expressions for the exact
outage probability of an arbitrary randomly located user, along with
computationally convenient lower and upper bounds. Asymptotic analysis on the
outage probability shows that introducing a large access threshold into mobile
association significantly decreases the outage probability. We further
investigate the spectrum efficiency and the energy efficiency of the HCN. Our
theoretic analysis and numerical validations show that both the spectrum and
energy efficiencies can be improved by properly choosing the access threshold.
