The wireless infrastructure network is going through a critical phase of technology evolution. A slew of equipment form factors are being rolled out to meet perpetual growth in capacity demand. All these solutions are gravitating toward maximizing the potential of precious and limited spectrum resource.
The 3rd Generation Partnership Project (3GPP) is a collaboration among groups of telecommunications associations. Its standardization efforts are devising ways to pack more and more bits into available spectrum within the capacity constraints governed by Shannon's law, but the wireless radio network is on the move to create topologies that allow fewer users per node to extract more bandwidth from the same spectrum.
There are two major macro trends pushing the network expansion in completely opposite directions. The first macro trend is deployment of an underlay of tens of small cells per macro base station, initially to improve coverage and then to deliver capacity by serving a small set of users. This trend enables the radio access network to support a higher density of callers for a given area, but it also creates complexity and scale challenges in the backhaul network. The second macro trend is splitting the traditional centralized base station into a network. The radios are located remotely, and the base station chassis is made solely of baseband functions. This split into distributed base stations allows ease of scalability in terms of increasing the density of baseband processing as well as the number of connected remote radio heads to address coverage and capacity needs effectively.
Cloud-RAN (also referred to as C-RAN and Centralized-RAN) is a new cellular network architecture for the future mobile network infrastructure. It is a network of high-density base stations connected to a large number of distributed remote radio heads. The concept is mobilizing technologies from wired networks to pool baseband resources using virtualization technologies. This is resulting in significant changes in baseband card architecture and design. Also, every radio node within Cloud-RAN is designed to connect to any channel card. The connectivity and algorithmic functions on the radio are also changing to leverage resource sharing effectively for load balancing and network failover. The Cloud-RAN trend simplifies backhaul but adds complexity to the connectivity between the base station chassis and the multitude of remote radio heads, also referred to as the fronthaul network.
There is no clear winner between these two trends. Both distributed base stations and micro/pico cells have been in use for some time, and they are more than likely to continue to coexist. The resulting heterogeneity in the network and varying complexities in backhaul and radio access networks pose significant network management challenges. Cloud-RAN network technologies can be used to manage the underlying heterogeneity and leverage associated intelligence to run networks more effectively and create viable service platforms. Operators and system vendors will need to work collectively in standardizing some critical elements of the wireless infrastructure to bring about a cohesive framework that eases adoption and guides a series of innovations to realize the full potential benefits.
Traditional base station architecture.
Distributed base station and remote radio heads.
Unlike wired networks, wireless radio access networks comprising base stations and associated access connectivity abound in proprietary and pseudo-standards. The transition to standards-based connectivity and synchronization is an important step for performance improvement, interoperability, and economies of scale. This is a critical milestone for the realization of Cloud-RAN. Gradual adoption of Ethernet as the standard connectivity within a radio access network, and timing over packets, is helping fuel innovation in the right direction. Care has to be taken to create solutions that aptly serve the need for coexistence with legacy equipment, cost of deployment, and scalability concerns.
Timing and synchronization are key elements to keep all the nodes in the Cloud-RAN synchronized and coordinated. Small cells have similar needs. Base stations synchronize with the core network using a combination of multiple timing and synchronization inputs. GPS and legacy TDM networks, such as T1/E1 lines, continue to be used along with the packet timing protocol (PTP 1588v2) and synchronous Ethernet. Today PTP and synchronous Ethernet are mainstream technologies that manage synchronization within the wireless infrastructure.
The clock and control module within a base station leverages these mechanisms to achieve frequency, phase, and time-of-day accuracy within the network. In turn, the base station clock control module provides the synchronized clocking information to the radio elements. In traditional centralized base stations, it is easy to achieve synchronization, because the radio elements reside within the same chassis. Successful distribution of synchronized clocks becomes challenging in high density distributed base stations (Cloud-RAN), where radio elements are located remotely at varying distances connected via fiber or microwave/millimeter wave point-to-point link.
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