  In this paper, we propound a solution named Cognitive Sub-Small Cell for
Sojourners (CSCS) in allusion to a broadly representative small cell scenario,
where users can be categorized into two groups: sojourners and inhabitants.
CSCS contributes to save energy, enhance the number of concurrently supportable
users and enshield inhabitants. We consider two design issues in CSCS: i)
determining the number of transmit antennas on sub-small cell APs; ii)
controlling downlink inter-sub-small cell interference. For issue i), we
excogitate an algorithm helped by the probability distribution of the number of
concurrent sojourners. For issue ii), we propose an interference control scheme
named BDBF: Block Diagonalization (BD) Precoding based on uncertain channel
state information in conjunction with auxiliary optimal Beamformer (BF). In the
simulation, we delve into the issue: how the factors impact the number of
transmit antennas on sub-small cell APs. Moreover, we verify a significant
conclusion: Using BDBF gains more capacity than using optimal BF alone within a
bearably large radius of uncertainty region.
