# CoMP-Aware BBU Placements for 5G Radio Access Networks over Optical Aggregation Networks
## 1 Introduction  
A significant amount of research for determining specifications and architectures of a next-generation 5G mobile network has already been prompted by the exponential growth of wireless traffic. Fifth-generation mobile networks (5G) are expected to provide higher bandwidths, higher data rates with high level of security, and lower latency [1, 2]. Fifth-generation networks provide various types of communication methods, such as machine-to-machine (M2M), human-to-machine (H2M), device to-device (D2D), and Internet of Things (IoT) [3, 4]. In addition, recent research includes device-to-device (DoD) communication in 5G networks to maximize throughputs [5]. One proposed network architecture that promises to meet the 5G requirements is the Centralized Radio Access Network (C-RAN). Unlike Distributed RAN (D-RAN) architecture, where the processing units are placed together with the RF antennas in cell sites, in C-RAN, multiple baseband processing units (BBUs) are placed in a central location called BBU pool [6]. Several Remote Radio Heads (RRH) are linked to the BBU pool via an optical transmission network that supports the 'front-haul' deployed according to a given radio planning strategy [6, 7]. The BBU pool is connected to the core network through 'back-haul' links. C-RAN promises to allow for a substantial multiplexing gain in terms of hardware and processing power of the BBUs. Several studies have been conducted to examine thecost savings achieved by C-RANs from the point of view of Capital Expenditure (CapEx), Operating Expenditure (OpEx), and Total Cost of Ownership (TCO). Compared to D-RAN, C-RAN reduce the TCO up to 68% [8]. Despite the advantages offered by C-RAN, the front-haul network must have a very high bandwidth and very low latency, resulting in high transport-network costs. Optical aggregation networks based on wavelength division multiplexing (WDM) are considered to be the suitable candidate solution for meeting the requirements of the front-haul [9; 10]. C-RAN represents a convenient environment for applying advanced radio coordination techniques such as Coordinated Multi-Point (CoMP) Transmission/Reception, as D-RAN does not satisfy the tight latency constraint of the CoMP techniques (1 msec) [11]. CoMP is suggested as a promising mobile communication technology released in LTE-Advanced Rel-11 to enhance cell-edge user throughput by transforming inter-cell interference into useful signals. It falls into two main categories: Joint Processing (JP) and Coordinated Scheduling or Beamforming (CS/CB). Firstly, JP occurs when coordination exists between multiple base stations that transmit/receive simultaneously to/from the UEs using the same frequency and time resources at the cell edge. Second, CS/CB is a form of coordination where the adjacent cell sites cooperate by allocating different frequencies and beams at the cell edge [12]. From the point of view of the BBU pools, CoMP services can be classified into two types: (i) inter-BBU CoMP and (ii) intra-BBU CoMP. First, in inter-BBU CoMP, each cooperative RRH is linked to a different BBU, which means that the signaling data will be exchanged between the different BBUs over an X2 interface. This imposes a large back-haul bandwidth requirement. Second, in intra-BBU CoMP, Cooperative RRHs are connected to a single BBU. This eliminates signaling data exchanges, which result in saving bandwidth and reducing latency. In this paper, we discuss the BBU placement problem for C-RAN deployment over 5G optical aggregation networks, taking into consideration a reduction in the number of RRHs undergoing inter-CoMP. The main contribution of this paper is the decrease in the number of inter-CoMP users, which will save the X2 interface signalling overhead as well as provide improved coordination between RRHs engaged in CoMP Moreover, we aim to reduce the number of BBU pools and the number of wavelengths used.