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The lowdown on small cells

Posted: 21 Nov 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Small cells? radio access nodes? macrocells?

Small cells are low-powered radio access nodes that may operate in both the licensed and unlicensed spectrum. They also have a range of 10m to 200m, 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 C how fast and how many C 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 fibre. The Common Public Radio Interface (CPRI) is one of the most commonly used protocols to transfer low power modulated base band signals from the channel cards to the radio. Signal loss in a fibre 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.

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