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

Cooperative Transmissions in Ultra-Dense Networks under a Bounded Dual-Slope Path Loss Model