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Can wi-fi really be used to backhaul cellular voice?

Louis Au | Jan. 31, 2012
Under the strain of a mobile data onslaught, the move to small cells is opening the door to new and unexpected uses of smarter wi-fi.

As operators are confronted with insane demand for mobile data, there are at least four distinct areas where congestion may appear:

  • the access radio network,
  • the signalling and control portions of the network,
  • the network packet core and
  • the backhaul network.

Each of these choke points pose a unique challenge to the operator and generally can be addressed in one of three ways:

1.      increasing capacity of the affected network resource,

2.      offloading the network resource to relieve congestion, or

3.      doing both.

The move to smaller cells to augment existing macro networks is widely viewed as a potential panacea to the access radio network congestion these problems but also creates a new one: backhaul. This has become one of the telecom industry's hottest debates.

Mobile operators are planning their LTE networks as a combination of macro cells and an 'underlay' network of smaller micro and picocells. To achieve the capacity density required by rapidly rising mobile Internet bandwidth demand, these small cells will need to be much larger in number in a given area than is the case in current cellular networks. This presents a new and very significant backhaul challenge - because the mounting locations of these small cell nodes (such as utility poles or other street-level assets) will very rarely be a natural fit for fibre or microwave solutions.

The concept of LTE self-backhaul or meshing is one possible solution, but as with early mesh wi-fi networks that attempted to provide access and meshing all within the same spectrum band, this approach rapidly consumes scarce (and expensive) LTE access-capable spectrum with backhaul traffic. The option of using a 5 GHz 802.11n point-to-point solution is a very attractive alternative- easily delivering the more than 100 Mbps of backhaul capacity an LTE cell will need.

Getting smaller

Small cells are low-powered, multi-radio access points (cellular/wi-fi/backhaul) that improve indoor and outdoor coverage to increase capacity and offload traffic - as much as 80 percent during peak times. While small cells benefit 3G service deployments today, their importance will only grow as the industry moves towards higher capacity 4G / LTE, especially in urban environments. According to In-Stat's latest report, Femtocells and Small Cells: Making the Most of Megahertz, small cell shipments will reach $14 billion in 2015.

The problem is, as network operators continue to increase coverage and capacity and look to offload data to relieve traffic pressures, they also increase the stress on their cell site backhaul connectivity. In this small cell world, conventional point-to-point microwave, bonded copper, and fibre-based backhaul solutions can quickly become impractical or uneconomical.

While microwave point-to-point equipment costs have come down in recent years, it generally requires a line-of-sight (LOS) link with the connecting backhaul hub, a condition many small-cell locations will be unable to meet. Sub-6 GHz NLOS solutions using a point-to-multipoint architecture are better suited for dense underlays, but when using licensed spectrum, narrow bandwidth channels put strict limits on backhaul capacity, and most sub-6 GHz spectrum bands are expensive and frequently not available for licensing.


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