Operators in Korea and Japan have developed a unique formula to scale wireless networks. In those markets, urban real estate and power is expensive (hence reliance on nuclear energy). The deployment of new sites is long and cumbersome. On the other hand, government policy and regulatory environment allowed operators to deploy extensive fiber networks and, where operators don’t own fiber assets, to lease dark fiber at very competitive rates. Pragmatic operators seized the opportunity to centralise the radio access network in fibre or data centres leaving only remote radios and antennas on site, all the while realizing a 30% – 50% reduction in operational expenditure.
The combination of centralised and virtualised baseband results in Cloud RAN which promises to have a disruptive impact on the wireless industry. To illustrate, imagine switching between base station vendors at the click of a button!
Centralisation paves the path to capital cost savings brought about by virtualisation of the baseband. Virtualised baseband relies on general purpose processors instead of dedicated silicon to run the air interface. Using commercial servers leverages the IT industry cost curve. The processing is pooled which leads to further cost savings because we do not need to provision for maximum site capacity as is the case in distributed network architecture (as much as 75% less processing is required according to some studies). Wireless operators have embraced virtualisation as witnessed by programs like Domain 2.0 at AT&T and UNICA at Telefonica. The combination of centralised and virtualised baseband results in Cloud RAN which promises to have a disruptive impact on the wireless industry. To illustrate, imagine switching between base station vendors at the click of a button! While this may be a utopian view of Cloud RAN, it helps to imagine the possibilities and power Cloud RAN provides.
Cloud RAN provides another benefit: performance improvement, especially for heterogeneous networks (HetNets). This is a critical point because when LTE was first conceived in the middle of the last decade HetNets were not at the forefront of the agenda which was dominated by developing a flat IP architecture optimized for broadband connectivity. This focus reversed the centralisation of high-layer functions, such as scheduling in 3G networks, in favour of a fully distributed architecture. Such decisions were taken just prior to the explosion in demand mobile data services initiated by the iPhone. As a result, the performance of early small cells has not been consistent, and varied from stellar success to downright damaging. Improving the performance of small cells through interference coordination progressed through latter releases of LTE, but the gain in performance remains closely tied to centralisation of baseband processing.
Given the great advantages that Cloud RAN brings one wonders what’s the hold up?
Given the great advantages that Cloud RAN brings one wonders what’s the hold up? In fact, there are two issues to resolve. The first issue is fronthaul which is an impediment towards centralisation. Fronthaul is the link between the baseband units and the remote radios. Fronthaul capacity, delay and synchronisation requirements are stringent. A single 20 MHz 2×2 MIMO LTE channel requires about 2.5 Gbps. This capacity adds up quickly when tens even hundreds of baseband modules are collocated in a data center. A fiber network is required to connect the sites to the data centres which not all operators possess. The cost of fibre breaks the business case for operators without their own fibre assets. But operators recognize the need for fibre and there has been a strong drive by many operators to acquire and build up their fibre assets.
The second challenge centres on virtualisation of the physical layer (PHY) which involves real-time processes and high computational load functions. General purpose processors are less efficient in running these functions for commercially scalable networks than dedicated processors resulting in high power consumption. Dedicated processors can have as much at 10x the performance per Watt of general purpose processors for PHY functions. A possible solution is to offload real-time and computationally expensive functions to hardware accelerators. But there are other solutions.
Since it is possible to draw the line between distributed and centralised functions along different points in the protocol stack, we are set to witness a proliferation of Cloud RAN implementations
The fronthaul and virtualisation challenges are coupled in a manner that a solution to both can be arrived at by judiciously choosing the functional split between centralised and distributed functions. For example, it is possible to place the physical layer at the remote radio while placing higher layers in the data centre. This significantly reduces fronthaul capacity and timing requirements, and consequently the cost of fronthaul, but also reduces the performance gain over traditional distributed architecture. Since it is possible to draw the line between distributed and centralised functions along different points in the protocol stack, we are set to witness a proliferation of Cloud RAN implementations (expect to hear more about ‘What is Cloud RAN’!).
While the industry works at resolving the above challenges in Cloud RAN, small Cloud RAN systems are set to emerge. Such systems would be targeted at venues, for example, a stadium or a convention centre, and deployed in a similar way to distributed antenna systems (DAS). More than eight different solutions are in development with various vendors, so the hype cycle is beginning. Virtualisation could spread at the ‘micro’ level, while the technology and economic challenges are resolved for the ‘macro’ level (wide-area Cloud RAN networks). Small Cloud RAN systems could eventually disrupt the wireless industry with virtualisation moving up into ever larger deployments – a familiar path taken by disruptive technologies in other fields ranging from steel to excavation equipment.
An indicator of the promise of Cloud RAN is manifested in the current thinking around 5G networks which emphasis scalability and energy efficiency (as much as 90% of power consumption of mobile operators is due to the radio access network). HetNets are a central feature of 5G networks which imposes requirements for simplification of all aspects of network operation and management. These facts combined point to the importance of virtualising the radio access network and formulating an architecture and deployment scenarios that leverage the benefits of Cloud RAN.
The trends in wireless network evolution indicate that we are currently at the cusp of bifurcation in network architecture and technologies, and even commercial practices. Operational processes remain rigidly entrenched. Operators will have to look for new means to meet the demands placed upon them by customers and investors more efficiently and effectively. Cloud RAN has the elements to provide the flexibility necessary in future wireless networks, and in the indoor market. Emerging solutions may break new ground towards a low-cost, high-density mobile infrastructure.
About the Authors:
Frank Rayal is Partner, XONA Partners, Joe Madden is Founder, Mobile Experts.
Access their co-written report Cloud RAN: Enabling NFV in Mobile Networks .