With the lack of low-cost backhaul options one of the biggest challenges for mobile operators planning to deploy LTE small cells, several startup companies have emerged to fill the technology gap.

It's a tough space to fill, though: Any small-cell backhaul system has to be inexpensive to deploy and operate, have high enough capacity to support LTE and, of course, be reliable.

The good news for hopeful vendors, though, is that the operators are hungry for options: Light Reading Mobile understands that carriers are open to trying pretty much anything when it comes to small-cell backhaul technologies. Options include xDSL, FTTC, DOCSIS 3.0, microwave, millimeterwave or in-band backhaul (where part of an operator's radio access spectrum is used to provide backhaul capacity). (See Startup Tackles 4G Backhaul Bottleneck .)

So who are these startups? Here are three new companies targeting this niche market, each with a slightly different solution to the small-cell backhaul problem:

BLiNQ Networks Inc. -- Founded in June 2010 after acquiring intellectual property and assets from bankrupt Canadian vendor Nortel Networks Ltd. , this startup's backhaul pitch is a non-line-of-site wireless system that operates in sub-6GHz time division duplex (TDD) spectrum. The system, which comprises a backhaul hub and remote backhaul modules, is deployed below the roofline of buildings and can be configured in a point-to-multipoint or point-to-point setup. (See BLiNQ Backhauls Small Cells and BLiNQ Optimizes Backhaul.)

The company has developed a technology called Managed Adaptive Resource Allocation (MARA), which essentially introduces self-organizing network (SON) capabilities into the backhaul network. The technology coordinates the backhaul nodes and minimizes interference by evaluating the radio frequency environment where the backhaul system is deployed.

Blinq, which has a team of engineers from Nortel, has raised US$7.4 million in Series A funding from New Venture Partners LLC , Summerhill Venture Partners and the Business Development Bank of Canada . The company's CEO is Carleton Miller, who was president of the wireless network solutions group at Andrew Corp., which was acquired by Commscope in 2007. (See BLiNQ Secures $7.4M .)

Bluwan S.A. -- Spun off from Thales SA (Paris: TCFP.PA) in 2005, this company has developed an IP-based point-to-multipoint microwave system for wireless backhaul and broadband access, which it calls "fiber through the air" (FTTA). The backhaul system operates in the 42GHz frequency band and uses TDD spectrum. The company says its backhaul product can deliver multi-gigabit throughput to multiple base stations in a single sector.

Bluwan's first target market for the FTTA backhaul product is the U.K., where 42GHz spectrum has been licensed to EE , MLL Telecom Ltd. and UK Broadband Ltd.

Cambridge Communication Systems Ltd. -- This new outfit has developed a multipoint-to-multipoint microwave system with SON capabilities that operates in 26GHz, 28GHz, 32GHz and 42GHz frequency bands using TDD or frequency division duplex (FDD) spectrum. The system, which is deployed below roof line, has a network point of presence (POP) located at a macro base station site and smaller nodes that are collocated with the small cells out in the radio access network. It does not need line-of-sight between the small cells and the network POP, but each node needs line of sight to at least one other node. The system's range is up to 1,000 meters in high rain-fade areas.

Using SON, the small-cell backhaul network is self-healing if any nodes fail, automatically allocates capacity for dynamic traffic management and does not require frequency planning in TDD spectrum. Also, the system automatically accommodates any new nodes that are added.

The company's founders are John Porter and Steve Greaves, who also started Adaptive Broadband Corp. (Nasdaq: ADAP) in 1998 and Cambridge Broadband Networks Ltd. in 2000.

— Michelle Donegan, European Editor, Light Reading Mobile