Another choice, fibre, is clearly the preferred backhaul option for mobile operators (if you can get it). But pulling fibre to every small cell location is, well, just not going to happen. It's simply too expensive, disruptive and time consuming. Consequently, traditional cellular backhaul solutions must now be rethought in the context of moving to smaller cells.
Wanted: New backhaul options
New backhaul options, well suited for dense urban environments and for close-to-the ground equipment (both line of sight and non-line of sight), are required to make small cells viable.
Counter-intuitive to most, unlicensed smart wi-fi has become a viable and affordable option to solve this problem and looks to play a crucial role in backhauling licensed small cell traffic. Yes, cellular traffic. Here's why:
Assume a mobile network operator (MNO) deploys an infill underlay radio network of small cells to add access capacity to areas where there is a high density of mobile data users, perhaps in an urban city centre such as in London, New York, or Hong Kong.
Today this small cell network would likely be comprised of lower-powered 3G and/or wi-fi nodes, or possibly in the future LTE radio nodes. No matter what the access radio technology is used, how does the operator get the data from the access radio node back to the network?
One obvious high performance solution is fibre, assuming that it's available. The operator may have to lease this fibre from a fixed line carrier which drives up operational costs, but perhaps more significantly there is the very real possibility that the fibre POPs will not exist in specific locations where the MNO needs to place the small cell.
The reality is that small cells only increase network capacity if you place them in close proximity to subscribers trying to access the network. Therefore site acquisition becomes a major determinant in the relative effectiveness of the small cell deployment.
But this then poses a very real problem - given the constraints of where operators must place small cells. It is highly unlikely that a fibre POP will exist in all of those locations. And given the cost and time delays of provisioning new fibre runs to each small cell location, an alternative solution is clearly needed.
Microwave radio links are of course a well-understood alternative technology that can be used to at least partially address the problem. But while microwave point-to-point (PtP) links are high performance, reliable workhorses for backhauling data and voice traffic, they have issues.
First and foremost, PtP microwave solutions generally rely on licensed radio bands for transmission. This improves reliability, however acquiring new licensed spectrum takes deep pockets filled with lots of cash. Also radio capacity is directly related to how much spectrum is used for the radio transmissions. This means deploying more capacity on the access radio side exacerbates both the cost and the shortage of spectrum for the backhaul radio network. Add to this the problem that PtP radio links require highly skilled installation to aim or align the radio nodes. In a crowded urban area or near street level, this quickly becomes an onerous task.
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