  In a heterogeneous network consisting of macro base stations (MBSs) and small
base stations (SBSs), the traditional cell association policy, i.e., coupled
access (CA), is far from optimal, due to the significant difference between the
coverage and transmit powers of MBSs and SBSs. Hence, users may choose to
associate with different types of BSs in downlink (DL) and uplink (UL), i.e.,
decoupled access (DA), to enhance spectral efficiency. In this paper, DA in
multiuser MIMO communications is investigated in terms of UL spectral
efficiency. Firstly, we obtain the UL association probabilities. In contrast to
the CA scenario, association probabilities for DA scenario only depend on the
densities of BSs. Hence, DA allows UL and DL to be totally independent.
Secondly, we derive lower bounds on the spectral efficiency. The lower bounds
show that, different from CA, the UL spectral efficiency for DA scenario is
irrelative with the transmit powers of BSs, which implies DA allows users to
associate with any BSs that can achieve the highest UL spectral efficiency.
Finally, the spectral efficiencies for DA and CA scenarios are compared via
simulation results, where it can be concluded that the spectral efficiency in
multiuser MIMO systems is improved by DA.
