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Reader Forum: Rationalizing small cell backhaul

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There are a lot of opinions circulating that relate to small cells – how they will be deployed, how they will perform, what will they look like – but what’s become clear is there’s a consensus that small cells are something very much needed as a solution to meet spiking capacity demands on mobile networks around the globe. And, it’s only natural for people who see a business opportunity to insist that they have the best and most marketable solution, which is also understandably true as it relates to small cell backhaul. Yet, while a lot of ideas have been placed on the table – from copper based xDSL solutions, to fiber, to all manner of wireless solutions – it’s important to step back and take a rational approach based upon the realities of what works and at what cost, while assessing what’s needed comprehensively to ensure operators can reliably meet capacity demands today and in the future while remaining profitable.

For permanent indoor small cell deployments it’s widely accepted that utilizing a wired infrastructure that’s normally already in place makes the most sense. An exception might be a temporary network setup for a major event where an indoor wireless solution would eliminate the need to string a lot of cable. That said, when defining the criteria for broad small cell backhaul, the primary deployments to be considered are those that are needed at street level.

Outside deployments of small cells are almost never connected to a wired infrastructure and have their own unique set of challenges, in addition to the associated time and prohibitive cost that would be involved to establish wired connections. Clearly, the vast majority of these outdoor small cells will require a wireless backhaul solution, although one also needs to consider that not all wireless backhaul solutions are the same.

A first step in determining the criteria for outdoor small cell deployment is to examine the performance characteristics that are required. To model the capacity per site of a small cell, one can assume it is equivalent to a single sector of a similar macro site – i.e. 20 to 50 megabits per second for a 3G network and 50 to 150 Mbps for an LTE network. And, while a good degree of statistical multiplexing gain can be applied when dimensioning the backhaul links that are aggregating multiple sites, this sort of capacity must be provided by the final link to each site. Because delivering a significant increase in network capacity requires a fairly large ratio of small cells to macro cells, small cell site spacing needs to be on the order of 100 to 500 meters and spread more or less uniformly across a deployment area. What’s more, latency performance must be very low, especially for applications running over an LTE network.

When considering outdoor small cell deployments, the installation and commissioning effort required is a primary concern. Many people have suggested point-to-multipoint with non-line-of-sight capability as a simplified backhaul solution. Proponents of this track probably are thinking of Wi-Fi network set up, where no alignment of the wireless link is required and automated software controls allocated bandwidth between the hub site and the end nodes. And, while on the surface this option might seem viable, if you consider the capacity requirements outlined above, it would limit operators to only a few end sites per hub. In effect, the cost savings of having many cheap end sites per (expensive) hub site are nullified.

Another relevant issue with non-line-of-sight solutions is that very broad beam antennas are required. When coupled with long propagation distances it becomes very difficult or impossible to avoid interference from adjacent cell sites. Furthermore, these types of radios rely on a polling protocol that only allows the hub site to talk to each of the end sites in turn. Unfortunately, this adds unwanted latency that is often not acceptable for more demanding applications. This type of radio does perform well in small pilot networks, but it is difficult to envision it being used as the primary backhaul technology in a full-scale deployment.

As an alternative, almost all of the objectives for simplicity and performance can be met using point-to-point radio links with small (smaller than six inch) antennas. Not only are these antennas perfectly suited for the size requirements of municipal deployments, they also don’t require complex, dual-ended alignment because they employ wide enough beam widths – although still sufficiently narrow to ensure sound link-to-link isolation – to establish connections by simply pointing them at a receiving antenna. It is also likely that this alignment process can be automated, further simplifying matters. And, because they enable high modulation rates, the capacity and spectral efficiency characteristics of these radios are good as well. Of course, a combination of area licensed, non-licensed high frequency and low frequency non-line-of-site radios will be required, depending on the network topologies and frequency availability.

As we move forward as an industry with small cell deployments, it’s clear that a wireless backhaul solution needs to meet all the aforementioned criteria to be viable, and it has to work in large-scale deployments as well as small pilot runs. There is enough variability in the network requirements that there is undoubtedly room for almost every solution somewhere, but given the rationale involved, it seems highly likely the dominant solution will be point-to-point radios.

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