# Open RAN--Radio Access Network Evolution, Benefits and Market Trends
## 1 Introduction  
The development of mobile cellular systems is a continuous process that began in the 1970s [1, 2]. Over the years, many standards of those systems have been published and released. Evolution from analogue to the digital era happened in the 1980s. Then, in the late nineties, the third generation of mobile networks facilitated packet switch communication. Until 5G, all legacy generations had some commonalities: at the least, (a) they were mainly intended for the consumer market; (b) the main goal was to deliver higher throughput to end users; (c) they were able to utilize wider bandwidth and new frequency bands than previously; (d) the main application scenarios were related to voice communication and Internet connectivity. Comparing these legacy systems, new requirement areas were set for 5G [3, 4]. By design, 5G RAN has to be at least compliant with an energy-efficient paradigm, needs to be extremely reliable, and has to support 1,000,000 users per km\({}^{2}\). Such an approach should allow to deliver mobile/wireless services not only for the consumer market but also for industries and private and corporate users. As many scenarios are expected to be supported, during specification, the decision was made that RAN architecture needs to be more modular than before [5]. In the past, attempts were made to deploy base stations in a decomposed way and by more than a single vendor per site, based on standardized reference architectures. Wideband Code-Division Multiple Access (WCDMA) in the 3G era brings the split of RAN to Radio Network Controller (RNC) and base station (called NodeB), enabling one-to-many connections among those entities. Fourth Generation (4G) Long Term Evolution (LTE), deployments, multi-vendor connections between RAN base stations (eNB), and core networks (CN) based on a standardized interface all have commonalities. However, interfaces applied in RAN among proper nodes are still proprietary. Despite the advances and complexity of the mobile access network, the mature solutions implemented in base stations are delivered by a single vendor, which used to be defined as vendor lock-in. Although the Third Generation Partnership Project (3 GPP) has defined some interfaces and released public specifications, vendors are deploying proprietary solutions and interfaces. This has finally led to the implementation of RAN networks in given bands and in a given geographical area by a single supplier. Currently, we are observing the single-vendor monolithic approach to RAN (eNBs) deployment around the world, i.e., networks are not open across the defined protocol stack. The amount of subscribers of mobile networks is higher than the world total population. It is expected that the number of cells and sites will continue to grow in the future due to network densification and implementation of new frequency bands. According to [6], it is forecasted that 8.8 billion mobile subscriptions will be active in 2026, including 3.5 billion subscriptions of 5G. It shows that we need flexible and more centralized deployed networks by design. Flexibility may be guaranteed by network element decomposition within the architecture of radio access protocol stack. However, such an approach does not still mean openness from the proprietary perspective. In the last decade, the need for flexible and _open_ approach in RAN development was identified by mobile network operators and network equipment suppliers, which has finally been called Open RAN. Open RAN is a broad term that encompasses the willingness of industry players (operators, vendors, neutral hosts, integrators, private business, etc.) to open the RAN architecture with the aim to facilitate deployment and integration of multi-vendor mobile radio access networks, eliminating potential blockers in further developments.