Optimization of 5G Fronthaul Based on Functional Splitting at PHY Layer

5G is coming with a promise to provide ubiquitous coverage with high data rate availability. To do so, densification of access points to enhance the system capacity is anticipated. For managing such densely populated network, 5G will be employing Centralized Radio Access Network (CRAN), where most of the Radio Access Network (RAN) functionalities are centralized in a central processing unit. This centralization reduces operational costs and eases implementation of advanced technologies, such as, Cooperative multipoint (CoMP) and enhanced inter-cell interference coordination (eICIC), in a cost efficient way. However, CRAN imposes stringent requirements on the fronthaul, i.e. the link connecting access points to the central unit, in terms of capacity and latency. Furthermore, future fronthaul networks are expected to rely on wireless technologies, since wired options are costly, not scalable and not always suitable for all scenarios. Therefore, meeting the expected requirements of fronthaul network utilizing capacity-limited wireless technologies may become an inescapable bottleneck. In this paper, we study different functional splits at the PHY layer in terms of data rate requirements and operational cost, and discuss the combination of different splits aimed at minimizing the overall cost and maximizing the centralization gains, while keeping the capacity requirements below the limit of the fronthaul.

I. Introduction

Popularity of mobile devices and data-rich applications are putting high capacity demand on the future wireless networks. Hence, instead of futuristic vision, Fifth Generation (5G) is becoming today's reality [1], and is preparing to meet thousand times system capacity/km², hundred times data rate and number of connected devices compared to Long Term Evolution (LTE) [2]. To meet the expected performance, Centralized Radio Access Network (CRAN) is identified as a key enabler, where most of the Radio Access Network (RAN) functionalities are centralized in a Baseband Unit (BBU) and the Access Points (AP), known as Remote Radio Heads (RRH), perform basic Radio Frequency (RF) functionalities. Additionally to the demand of ultra dense network composed of densely deployed APs to serve huge number of users expecting very high data rate in the access network, CRAN will be asking for a very high capacity and low latency fronthaul (FH) network connecting BBU to the RRHs. To tackle the stated challenge, the industry and the academia studied different functional splits, where the distribution of PHY functions between the RRH and the BBU varies. The more functions are given to the BBU, the more flexible is the architecture, but the requirements imposed to the FH are harder to meet. Figure 1 illustrates the basic architecture of future CRAN, where the links connecting the BBUs to the subsequent part of the network are referred to as Backhaul (BH). Fig. 1.

Basic CRAN architecture.