  In an Ultra-dense network (UDN) where there are more base stations (BSs) than
active users, it is possible that many BSs are instantaneously left idle. Thus,
how to utilize these dormant BSs by means of cooperative transmission is an
interesting question. In this paper, we investigate the performance of a UDN
with two types of cooperation schemes: non-coherent joint transmission (JT)
without channel state information (CSI) and coherent JT with full CSI
knowledge. We consider a bounded dual-slope path loss model to describe UDN
environments where a user has several BSs in the near-field and the rest in the
far-field. Numerical results show that non-coherent JT cannot improve the user
spectral efficiency (SE) due to the simultaneous increment in signal and
interference powers. For coherent JT, the achievable SE gain depends on the
range of near-field, the relative densities of BSs and users, and the CSI
accuracy. Finally, we assess the energy efficiency (EE) of cooperation in UDN.
Despite costing extra energy consumption, cooperation can still improve EE
under certain conditions.
