In this paper, we provide a comprehensive analysis of the multi-antenna transmission in a K-tier downlink heterogeneous cellular network (HCN). We first propose a reliability-oriented mobile access policy with an access threshold, in which each user connects to the strongest base station in terms of the truncated long-term received power. Under this policy, we derive for a random user explicit analytical expressions of the exact out-age probability along with its computational convenient lower and upper bounds. Asymptotic analysis on the outage probabilities shows that introducing the access threshold efficiently improves outage performance. We further investigate the area network throughput from the perspective of outage. Our theoretic analysis and numerical validations show that throughput performance can be enhanced by properly designing the access threshold.