  We study multiple-input multiple-output (MIMO) based downlink heterogeneous
cellular network (HetNets) with joint transmit-receive diversity using
orthogonal space-time block codes at the base stations (BSs) and maximal-ratio
combining (MRC) at the users. MIMO diversity with MRC is especially appealing
in cellular networks due to the relatively low hardware complexity at both the
BS and user device. Using stochastic geometry, we develop a tractable
stochastic model for analyzing such HetNets taking into account the irregular
and multi-tier BS deployment. We derive the coverage probability for both
interference-blind (IB) and interference-aware (IA) MRC as a function of the
relevant tier-specific system parameters such as BS density and power, path
loss law, and number of transmit (Tx) antennas. Important insights arising from
our analysis for typical HetNets are for instance: (i) IA-MRC becomes less
favorable than IB-MRC with Tx diversity due to the smaller interference
variance and increased interference correlation across Rx antennas; (ii)
ignoring spatial interference correlation significantly overestimates the
performance of IA-MRC; (iii) for small number of Rx antennas, selection
combining may offer a better complexity-performance trade-off than MRC.
