  In this paper, we propose a minimum mean square error (MMSE) based
transceiver design scheme for a downlink multiple-input multiple-output (MIMO)
two-tier heterogeneous network with general linear equality per-cell power
constraints. Three practical channel models with both perfect and imperfect
channel state information are used in simulations. In each channel model, we
consider two system configurations, two data transmission schemes and two
cellular cooperation scenarios. Our study shows that the proposed MMSE scheme
is more flexible than interference alignment (IA) based scheme. For the cases
where the IA-type scheme is applicable, the proposed scheme generally
outperforms IA-type scheme in terms of average sum rate and bit error rate
(BER), but is computationally more complex than the IA-type scheme.
