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

TL;DR

This paper examines cooperative transmission strategies in ultra-dense networks using a bounded dual-slope path loss model, analyzing their impact on spectral and energy efficiency with different cooperation schemes.

Contribution

It introduces a dual-slope path loss model for UDNs and evaluates the performance of non-coherent and coherent joint transmission schemes.

Findings

Non-coherent JT does not improve spectral efficiency.

Coherent JT can enhance spectral efficiency depending on conditions.

Cooperation can improve energy efficiency under certain scenarios.

Abstract

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.