When it comes to small cells, multimode capabilities is the number one request a lot of vendors like Broadcom are hearing from wireless operators. It's also one reason small cell deployments have been slow to make their way out of the labs.

Multimode small cells combine 3G, 4G/LTE and Wi-Fi into one tiny base station with the goal of increasing capacity in highly trafficked areas, especially indoors where the challenges are more acute.

The reasons why multimode is attractive are simple: Wi-Fi is a relatively cheap addition to small cells, which are – in turn – cheaper to deploy than macrocells. Wi-Fi has also become the cornerstone of a lot of operators' strategy for data offload.

AT&T, for example, is working on Multi-Standard Metrocells (MSMs) in AT&T Labs and plans to deploy the first versions in the field in 2014. The small cells will combine 3G, 4G and Wi-Fi, support carrier aggregation between the 700 MHz and AWS spectrum bands and will eventually make up the entirety of the small cells it deploys. (See 3G, 4G, Wi-Fi: AT&T Plans a Small Cell Threesome and CTIA: AT&T Works on Wi-Fi Integration.)

But, there's a reason MSMs will take so long to make their way out of AT&T's labs, too: The implementation is anything but simple. There are technology challenges to multimode such as the coexistence of separate RF radios, the ability to intelligently select the best network to use in real time, avoiding interference and the fact that Wi-Fi and cellular don't map out one-to-one.

"The operators would like multimode small cells at the moment, but the market can't yet deliver them as products," Heavy Reading analyst Gabriel Brown explains.

When licensed spectrum is used with unlicensed, the network is tasked with intelligently selecting which connection makes the most sense for the users online at that point in time. That could mean connecting over the macro or micro basestation and a 3G or 4G cellular connection versus a Wi-Fi link.

All of these network elements have to work together seamlessly and without interference. Intelligent network selection is a big part of what operators are working on now with their vendor partners. Ericsson is leading the charge here with its recently announced network-based access control where connectivity is controlled from within the network. (See Ericsson Integrates Wi-Fi With Cellular.)

Another challenge carriers will run into, according to Broadcom's VP and GM of broadband carrier access Greg Fischer, is that Wi-Fi hotspots are deployed at a different density than cellular hotspots. Forcing together Wi-Fi and cellular presents some potential issues.

"Cellular tends to have more sensitivity and robustness and better capability on distance from the same output power level than Wi-Fi," Fischer says. "If you collocate Wi-Fi, you may still have areas where there is not enough Wi-Fi coverage."

That means most operators will require about a two-to-one ratio of Wi-Fi access points to multimode small cells even when every small cell has Wi-Fi included, Brown predicts.

Despite the challenges, vendors are beginning to get close to multimode with boxes that support both 3G and 4G connections. Broadcom introduced its first combo 3G/4G small cells at February's Mobile World Congress. Qualcomm, too, last month introduced its first combo Wi-Fi, 3G and LTE chipset for small cells, set to launch commercially later this year. Cavium, Freescale, Mindspeed and Texas Instruments are among the other vendors that also offer small cell systems on a chip.

And, unlike with 3G, small cells will be a part of LTE deployments from the start. Operators won't tack them on as an afterthought; they see them as integral to their network plans. That's part of the reason they are leaning hard on their vendor partners to nail the multimode equation early on.

"With LTE accelerating and seeing data traffic and challenges with frequency bands and getting infrastructure in place, small cells aren't an afterthought; they are part of the network," Fischer says. "It's a tool everyone acknowledges is available, and it is part of that deployment."

— Sarah Reedy, Senior Editor, Light Reading