# Open RAN--Radio Access Network Evolution, Benefits and Market Trends
## 2 Mobile Network Architecture Evolution  
In the early days, mobile network deployment was based on a monolithic approach in which baseband processing units and radio modules where placed near an antenna. These two main elements constitute which we commonly call RAN--which is a principal part of mobile networks next to core network, terminals and transport. In 4G/LTE, vendors started to deliver eNB nodes disaggregated into Radio Remote Heads (RRHs) and Baseband Units (BBUs), which are placed within close site proximity, which is commonly referred to as distributed RAN (D-RAN)--see Figure 1. Note that RRH is named Remote Unit (RU) in 3 GPP 5G New Radio (5G NR) related specifications. BBU performs computing-hungry operations related to the RAN stack such as signal processing for many cells, managing resources, correcting transmission errors, etc. [7]. In such D-RAN deployments, proprietary interfaces are used to provide communication between RRHs and BBUs, e.g., based on Common Public Radio Interface (CPRI) or Open Base Station Architecture Initiative (OBSAI) protocols [8; 9]. Taking into account statistical behaviours of users connected to cells, i.e.,call time and instantaneous data transfer, to mitigate resource overestimation in a baseband, resources may be pooled together. Thus, network densification motivates to move those resources to centralized offices and data centres. The concept of centralized RAN (C-RAN) is about moving part of the baseband processing to a central office (hub). C-RAN was proposed by China Mobile in 2009 to support high-bandwidth applications [10]. BBU centralization is motivated by maximization of data transmission efficiency and coordination of resources, among others. Another step towards decomposition of RAN was achieved by Small Cell Forum, which identified and announced the benefits from virtualization and centralization in small cells networks together with implementation of MAC/PHY split in 2015 [11]. From that time, many evolutions of potential future network architecture functional splitting were conducted. Functional splitting determines how much the network is centralized. According to [12], RAN functional decomposition is needed to diverse transport demands in terms of different performance requirements and to adjust to various traffic types of different performance goals, e.g., very high throughput vs. extremely low latency. The authors of [13] reviewed different studies concerning functional splits and evaluated advantages and drawbacks of those splits. According to [14], centralized baseband processing enables better inter-cell coordination and, by these means, allows for load balancing and cost reduction due to increased hardware/software pooling and decreased site rental costs. 3 GPP in an early phase of 5G NR specification works (i.e., in Release 14) evaluated an extended approach to functional splits, which is reflected in splitting the architecture into Centralized Unit (CU), Distributed Unit (DU), and RU entities. In this approach, the CU and DU take a role of a baseband unit and are mutual linked via a midhaul F1 interface [15], where CU performs non-real time L2/L3 operations and DU realizes near-real time L1/L2 functions, e.g., scheduling. RU is responsible for converting signals to and from radio frequency (RF), and is either connected to an antenna or integrated with an antenna, e.g., in the case of multi-input multi-output (mMIMO) systems. According to 3 GPP TS 38.470 specifications [16], the F1 interface is open and facilitates connection of a CU and a DU supplied by different manufacturers. CU and DU may be collocated and linked internally. 3 GPP TR 38.913 [17] states that different functional split options should be allowed in the RAN. Particular split options are presented in Figure 2.  
Figure 1: Difference between D-RAN, D-RAN partially cloudified and C-RAN. C-RAN does not define whether the 3 GPP radio stack is running in Cloud (Cloud-RAN).