  Heterogeneous networks (HetNets) have strong interference due to spectrum
reuse. This affects the signal-to-interference ratio (SIR) of each user, and
hence is one of the limiting factors of network performance. However, in
previous works, interference management approaches in HetNets are mainly based
on interference level, and thus cannot effectively utilize the limited resource
to improve network performance. In this paper, we propose a user-centric
interference nulling (IN) scheme in downlink two-tier HetNets to improve
network performance by improving each user's SIR. This scheme has three design
parameters: the maximum degree of freedom for IN (IN DoF), and the IN
thresholds for the macro and pico users, respectively. Using tools from
stochastic geometry, we first obtain a tractable expression of the coverage
(equivalently outage) probability. Then, we characterize the asymptotic
behavior of the outage probability in the high reliability regime. The
asymptotic results show that the maximum IN DoF can affect the order gain of
the asymptotic outage probability, while the IN thresholds only affect the
coefficient of the asymptotic outage probability. Moreover, we show that the IN
scheme can linearly improve the outage performance, and characterize the
optimal maximum IN DoF which minimizes the asymptotic outage probability.
