Small cells are low-powered radio access nodes that may operate in both the licensed and unlicensed spectrum and that have a range of 10 meters to 200 meters, as compared to macrocells, which might have a range of a few kilometres. Small cells have long existed in the network with the purpose of filling in coverage gaps. The recent resurgence in interest in these small cells is being driven primarily by market demand for higher network capacity to host existing and new data services. Cell coverage continues to be an issue due to limited indoor penetration at higher transmission frequencies. For such cases, small cells are likely to play a vital role in providing indoor coverage and capacity. While small cells hold the promise of a faster deployment, deeper reach, and a much lower cost, there are significant challenges associated with their deployment. In order to overcome these deployment challenges, small cells must coexist with macro cells and other small cells located in the same vicinity, satisfy backhaul connectivity issues, and provide programmability to ease deployment and management to contain the operational complexity that they add to the wireless networks.
In parallel, macro cells have been evolving from a monolithic architecture to a distributed architecture, with a significant increase in investments to remote radio heads and active antenna technologies. This opens the door to using alternative architectures to increase network capacity and coverage. Therefore, the question of small cell adoption how fast and how many depends on how quickly the wireless industry overcomes the deployment hurdles for small cells as well as on the level of adoption of distributed macrocell architectures. This article discusses base station trends and their evolution, the rationale for using small cells, and the backhaul challenges that the growth of wireless networks will face in the coming years.
Traditional wireless infrastructure relies on a monolithic base station chassis sitting at the foot of the tower feeding signals back and forth to passive antennas mounted on the top of the tower. The connection between the base station chassis and passive antenna components is via a coaxial cable as shown conceptually in Figure 1.
Figure 1. Conceptual depiction of power savings
in distributed base station architecture.
A major shortcoming of this architecture is that the signal power transmitted by the base station cabinet to the passive antennas encounters a loss of approximately 3dB signal power. In other words, only half of the signal power transmitted by the base station chassis is received by the antennas. In order to solve this problem and conserve power, the industry has transitioned to a distributed base station architecture in which the radio cards (that host the power amplifiers) are removed from the base station chassis and mounted directly on the towers adjacent to the antennas. These radio cards are called remote radio heads (RRH). Use of RRH avoids the loss of signal power due to the power amplifiers residing in close proximity to the antennas. The RRH are connected to the channel cards in the base station chassis using optical fiber. The Common Public Radio Interface (CPRI) is one of the most commonly used protocols to transfer low power modulated baseband signals from the channel cards to the radio. Signal loss in a fiber link is negligible for low power signals when compared to transferring a high power signal over a coaxial cable.
Despite significant benefits in reducing operational costs, the transition to a distributed base station architecture has been gradual. Mounting RRH on top of the tower results in installation (higher weight and wind loading), maintenance, and reliability concerns. Truck roll becomes more expensive and the skill set of the repair crew has to change. Improvement in technology, higher integration, remote field-programmability an control, size, and weight reductions in the equipment are helping to overcome these hurdles. In addition, the potential of a distributed base station architecture has opened an avenue to solve another pressing problem faced by wireless network operators that is expected to accelerate the adoption in coming years. A distributed base station architecture offers a significant value in solving network capacity crunch by providing a highly flexible architecture to reach service hot spots effectively.
Wireless network operators have to continuously add capacity to the network to meet perpetual demand for higher data rates. Limited availability of new spectrum is a big hurdle in meeting growing demand for network capacity. Improved communication technology and use of multiple transmit and receive antennas (diversity, spatial multiplexing, beam forming) such as in LTE and LTE Advanced technology and increasing use of Wi-Fi offload provides ways to increase network capacity. Another technique is active antenna systems (AAS), an incarnation of RRH that have the same tower footprint as the existing antenna, support multiple active antenna elements for beam forming that help improve coverage and capacity. However, a boost in network capacity with improved technology may not be sufficient. Use of cells with smaller radius serving smaller number of users appears to be necessary to scale network capacity, particularly in dense urban areas where capacity crunch is most severe. New sites need to be located to roll out new base stations on an ongoing basis.
The acquisition of a new base station site (to locate the tower along with adjacent space to place the base station cabinet) is becoming an expensive and lengthy process due to the arduous complexities of attaining approvals from local/ municipal governments. A major part of the expense in installing a base station is in acquiring real estate and the costs associated with the civil works. In a typical installation, electronic equipment constitutes less than 20% of the total cost of installing a base station. Besides the scarcity problems associated with finding space for cell towers in congested urban/sub-urban areas, the visual impact of these towers is making matters worse and causing increasing hurdles in obtaining city approvals.
The distributed base station architecture allows for locating the base station cabinet miles away from the RRH, thereby providing a much higher flexibility in mounting smaller RRH units on the side of buildings, electric poles, etc. The RRH units reduce visual impact and improve reach (proximity to end users) significantly. Eventually, a centralized site hosting a bank of base station cabinets far removed from RRH units (also called CloudRAN) may become viable, opening an opportunity to use high-performance hardware shared by multiple operators via virtualization. As a futuristic vision, this would allow base stations to be located inside a data center leveraging close proximity to application servers and application/user data to gain network efficiencies and create new possibilities/services. On the other hand, remote radio heads would continue to move closer and closer to the end users.
Figure 2. Distributed base station architecture showing
RRH mounted on the side of the buildings.
Distributed base station architecture deployment does have a shortcoming in scaling the front-haul network and connectivity between the base stations and remote radio heads. Currently, dedicated fibers are being laid out to connect RRH to base station chassis. Technology innovations need to happen to allow sharing of a common media by incorporating class of service prioritization to maintain appropriate levels of deterministic latency and end to end delay management.
While distributed base station architectures have been making a gradual progress in providing a viable solution for network capacity, small cells have arisen as an alternative recently generating a new wave of industry hype and series of innovations. No industry standard definition exists for small cells. In general terms, a small cell is a complete base station in small and compact form factor enclosure that can be easily deployed on light poles and on the sides of buildings. The transmit power of a small cell is in the range of 250 milliWatts to 5 Watts, with cell range varying from 50m to 5 km. A small cell radius of 100s of meters supporting 32/64+ users is likely to be most common in dense urban settings. A small cell is extremely low cost and is expected to be in the range of 1/10th of the cost of a typical macrocell. A cluster of 5 to 10 (or higher numbers) of small cells are expected to exist within a macrocell as an underlay to enhance network capacity, primarily expected to cater to data services. A small cell gateway is expected to control small cells to work cooperatively with neighbors and with the mother macrocell. Load balancing and inter cell interference management are expected to be crucial parameters controlled by the small cell gateway.
Figure 3. Small cell deployment as a cluster underlay to a macrocell.
While small cells have a lot of benefits and are generating a tremendous excitement fueling innovation within the industry, there are considerable challenges for wider deployments. Small cell backhaul is a major hurdle. Based on deployment scenarios, the ability to choose a backhauling solution from among a toolkit of backhaul solutions is critical to ease deployment. This creates a considerable challenge in integrating backhaul into small cells. A solution using two boxes, one for the small cell and other for the backhaul, may not be viable in the long run. Small cell radio access network (RAN) creates an additional layer in the mobile backhaul hierarchy, further stressing an already constrained mobile backhaul access network. Another significant barrier for small cells is the need for an industry standard and a truly interoperable framework to ascertain coexistence of a small cell with neighboring cells. Small cells are expected to work as well-behaved, self-optimizing network entities. Inter-cell interference coordination and dynamic load balancing are issues; if not properly managed and controlled, they can lead to poor overall network performance. Despite all the challenges, small cell technology holds tremendous promise in providing a good solution to perpetual growth in demand for network capacity.
In Summary, small cell deployment numbers and the speed of deployment will depend on how quickly the industry comes up with solutions to the challenges associated with small cells and the extent of distributed base station architecture adoption and penetration. Remote radio heads and active antenna systems, being an extension of the existing macrocells, are equally attractive in many situations. There is no doubt that both forms of innovation distributed base station architectures and small cells will be deployed and coexist in increasing network capacity, be they based on FPGAs, ASSPs or ASICs. Ease of deployment, cost of installing and maintaining a small cell backhaul network, and overall performance of small cell clusters in conjunction with macrocell network are some of the main factors that are expected to influence small cell adoption and volume in the coming years.About the author
Harpinder S. Matharu, manages Mobile Backhaul business and the Connectivity IP product portfolio for the Wireless Infrastructure Group at Xilinx. He has over 25 years of experience working in different capacities in the high technology industry focusing on the Embedded and Communications markets. Prior to joining Xilinx, he was Principal Systems Architect at Integrated Device Technology, Inc. where he managed the Integrated Communication Processors and Switch products.
Mr. Matharu has published many technical papers, spoken at industry events, and has chaired several Standards technical/marketing bodies. He holds a Bachelor of Engineering in Electronics & Communication degree from Delhi Institute of Technology, and business degrees and certifications from both the University of Phoenix and Stanford University.
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