In simple terms, Open RAN is an industrial concept that includes; virtualization, automation, and RAN internal open interfaces. Disaggregation and automation of RAN can be achieved in a number of different ways, including by solutions offered by traditional vendors. In this sense, Open RAN can be realized by 3GPP and O-RAN alliance specifications or proprietary non-standard-based solutions.
O-RAN Alliance, in addition to what 3GPP enables, also specifies open RAN internal interfaces between the main building blocks of the radio access networks – Radio Unit (RU), Distributed Unit (DU), and Centralized Unit (CU). This would enable a mix and match between RAN components from different vendors. It is also essential to consider that since RAN is only one part of the mobile network, 3GPP is the only available specification that standardizes end-to-end a fully functional mobile network (2G/3G/4G/5G) that is interoperable and multi-vendor interoperable. O-RAN alliance specifications are only focused on RAN and currently only cover 5G.
The three main building blocks of the RAN are:
- Radio Unit (RU) – it transmits, receives, amplifies, and digitizes the radio frequencies. It is located near or integrated into the antenna.
- Distributed Unit (DU) – it is responsible for real-time L1 and L2 scheduling functions. In a centralized 5G cloud RAN deployment, the DU’s physical and software layers are hosted in a central office or an edge cloud data center.
- Centralized Unit (CU) – it is responsible for non-real-time L2 and L3 functions. In a centralized 5G cloud RAN deployment, the CU physical and software layers can be co-located with the DU or hosted in the regional cloud data center.
O-RAN alliance specifications that build on top of 3GPP RAN specification add additional functions and open RAN internal interfaces. If based on a specification like the O-RAN alliance, these functions and interfaces can potentially ensure multi-vendor interoperability and hence allow network operators to mix and match RAN internal components.
To achieve multi-vendor interoperability, individual vendors that offer products compliant with O-RAN alliance specifications must perform interoperability tests between different vendors’ solutions (and future software updates) that may or may not be built on proprietary vendor implementations. Mobile network operators can have integrated RAN products in one portion of the network and O-RAN compliant RAN products in another geographical part of the network.
Additionally, the disaggregated hardware and software RAN components enable the network operator to upgrade software functionality without ripping out & replacing the underlying physical infrastructure, i.e., future-proof their 5G hardware infrastructure investments.
However, since RAN disaggregation is possible in several ways, this possibility can be realized based on 3GPP, O-RAN Alliance, or proprietary solutions. In other words, if this the objective OpenRAN as defined by O-RAN alliance specifications is not the only avenue to achieve this outcome and presence of RAN internal interfaces is not a necessary condition to achieve benefits associated with virtualization and automation of RAN.