Imagine a wireless service provider can provide service anywhere there’s Internet connectivity at the click of a button. All they need is a small remote radio connected to the Internet. The baseband modems no longer reside at cell sites, but are hosted in data centers where powerful commercial servers execute all the functions of todays base station baseband units. The remote radios can even be self-deployed by the mobile subscribers. The service provider would have full control of the service features which can be configurable on the fly. Sounds like science fiction to some, but this is what virtualization of the radio access network aims to achieve.
No wonder then that the term Cloud RAN which meant centralization and virtualization of baseband became an overused term to denote all types of architectures. We live in the world of Anything as a Service (XaaS). Which investors would put money now in a hardware company?
But we can also ask why there are so many architectures to begin with? Unfortunately, the distributed architecture of LTE limits the performance of HetNets which is essential to meet future capacity demand. Centralization, of at least part of the protocol stack, becomes necessary to improve performance and run application services the edge. Therefore, if we are centralizing some functions, why not virtualizing them to gain advantage of the economies of scale of the IT/data center world?
In all, a confusing array of architectures and nomenclatures has emerged. In my new report on RAN virtualization*, I introduce the following definitions:
Virtual RAN: An architecture where general purpose processors and servers are used to run air interface protocol stack in a central location (Layer 1 and above). Various architectures and implementations of Virtual RAN exist including the use of accelerators, bare metal, and fully virtualized hypervisor-based implementations.
Hybrid RAN: A split baseband architecture where some modem functions run on GPPs in the center (virtual mode) while other baseband functions, such as Layer 1 or parts of Layer 2, run on programmable or hardware devices at the remote radio (FPGAs, DSPs, NPUs ASICs and SoCs). The split can occur at different locations and is a vendor specific design. Hybrid RAN is an architecture that optimizes cost and performance but does not have the same disruptive potential as Virtual RAN.
Clustered RAN: An architecture where baseband modules are located in a central location as is done in today’s base station hotels. The air interface protocol stack runs on programmable or hardware devices. This is the most basic form of centralization targeted for OPEX reduction in certain Asian markets and used for practical considerations in other parts of the world where it is not possible to collocate the baseband with the remote radio due to lack of space, access, security, etc.. Clustered RAN is the name given by SK Telecom to Phase 1 of their roadmap to implements Virtual RAN.
Centralized RAN: An architecture where the baseband modules are located in a central location, similar to Clustered RAN, but with two variations:
- All the baseband functions of the air interface protocol stack are centralized (full centralization). In this case, the difference from Clustered RAN lies in the integration of baseband processing to save cost among different modems and to improve performance through coordination of resources.
- Part of the upper layers of the protocol stack are centralized while the lower layers are distributed at the remote radio (partial centralization) – essentially a hybrid architecture without virtualized baseband.
In either case, the baseband processing is based on programmable and hardware devices to run all air interface modem functions. The architecture supports a 1:1 relationship between a radio its baseband modem.
Many other terms are used in the industry to denote a level of coordination among base stations for interference management such as Cooperative, Collaborative and Elastic RAN (Ericsson). These can be classified according to one of the above categories.
** For additional information, see RAN Virtualization: Unleashing Opportunities for Market Disruption