Backhaul is still a bugbear for small cells. Specifically for outdoor, public access small cells, backhaul remains one of the biggest challenges to deployment.

This is not surprising considering the tall order of cost, capacity and quality of service (QoS) requirements operators have for small cell backhaul. First and foremost, backhaul for public access small cells must be extremely low-cost. It must also offer high capacity (e.g., about 50 Mbit/s for LTE small cells). The physical equipment must be compact, lightweight, unobtrusive and low-power to meet city zoning restrictions. Products should also support some form of automated provisioning for easier installation and activation. While not exhaustive, that's a weighty wish list.

But the nature of the backhaul challenge has changed over the last 18 to 24 months. Where previously there was a lack of solutions, now there is a plethora of technology options. Operators and vendors are investing much time and resources into addressing the issues of small cell backhaul. The new challenge for operators is to choose which technologies best meet their needs and decide where certain requirements can be relaxed in order to make the business case for small cells.

As discussed in the new Heavy Reading 4G/LTE Insider, "Wireless Backhaul for Small Cells: Who's Doing What," operators need a variety of backhaul options -- both fixed-line and wireless -- depending on the locations and use cases of the small base stations. This is commonly referred to as the "toolkit" approach to small cell backhaul, which stems from operators' desire to use existing infrastructure wherever possible. Fixed-line technologies are typically the first choice. For example, an operator that has access to fiber in city centers will want to use that network to backhaul traffic from small cells. But in cases where fixed-line assets are not available, or too expensive to install, operators will need wireless backhaul solutions.

Wireless backhaul for outdoor, public access small cells has been a hotbed of innovation recently, as many startups have flocked to the opportunity. The report counts 16 new or relatively young companies that have developed wireless products specifically for small cell backhaul. With this bevy of startups plus the 11 traditional microwave vendors that are profiled in the report, there is an overwhelming amount of choice.

But it is early days for wireless backhaul for small cells; operators are currently trialing and evaluating various technologies. The next 18 to 24 months are expected to be dynamic, as operators clarify their backhaul requirements and consolidation among the many vendors begins.

Backhaul is certainly not the only challenge to outdoor small cells -- site acquisition and interference risks rank high on the list of issues, for example. But if the vision of small, outdoor base stations deployed in large volumes to add mobile data capacity is to be realized, viable backhaul is critical.

— Michelle Donegan, Contributing Analyst, Heavy Reading 4G/LTE Insider

This report,"Wireless Backhaul for Small Cells: Who's Doing What," is available as part of an annual subscription (6 issues per year) to Heavy Reading 4G/LTE Inside, priced at $1,595. Individual reports are available for $900. To subscribe, please visit:www.heavyreading.com/4glte.