  In this work we investigate the reassignment of User Equipments (UEs) between
adjacent small cells to concurrently enable spatial multiplexing gains through
Multi-User MIMO (MU-MIMO) and reductions in energy consumption though switching
emptied small cells to a sleep state. We consider a case where UEs can be
reassigned between adjacent small cells provided that the targeted neighbouring
cell contains a UE with which the reassigned UE can perform MU-MIMO without
experiencing excessive multi-user interference, and whilst achieving a minimum
expected gain in spectral efficiency over the previous original cell
transmissions as a result. We formulate the selection decision of which UEs to
reassign as a set covering problem with the objective of maximising the number
of small cell base stations to switch to a sleep state. Our results show that,
for both indoor and outdoor LTE small cell scenarios, the proposed
MU-MIMO-based reassignments achieve significant reductions in the required
number of active small cell base stations, whilst simultaneously achieving
increases in spectral efficiency.
