Optical/IP Networks

Operators Eye LTE Metro Femtos

Major mobile operators are ready to take femtocells out of homes and offices and onto the streets as part of their Long-Term Evolution (LTE) network rollouts.

At the LTE World Summit in London, China Mobile Communications Corp. , T-Mobile International AG , and Telus Mobility backed the metro femto concept as a potential deployment scenario for realizing the full capacity gains that LTE, one of the so-called "4G" technologies, promises. (See China Mobile Preps LTE Network.)

The development shows that the vision first publicly revealed by Vodafone Group plc (NYSE: VOD) earlier this year is shared by other major carriers. (See Vodafone Dreams of Metro Femto.)

Femtocell silicon specialist Picochip takes Vodafone's vision even further with the proposition that all LTE deployments will comprise small femto base stations, which contrasts with the more common view that femtocells will be used only for initial LTE rollouts and for indoor coverage. PicoChip believes femtocells will be fundamental to LTE for both residential and public hotspot, or metro femto, deployments. (See picoChip Touts LTE Femto, Sniffer Femtos, and PicoChip Does LTE Femtos.)

"The macrocell is dead," says Doug Pulley, picoChip CTO. "It's a fallacy to think you can reuse existing cell sites to get LTE services. That whole premise is broken."

Both China Mobile and T-Mobile have said they plan to use existing 3G cell sites for their LTE networks. But Pulley contends a traditional macrocell deployment won't work because of the basic laws of physics [ed. note: which aren't so basic to this humble reporter].

Here's the deal: "User throughput rolls off the further you get from the base station," says Pulley. "[With] increased throughput, the signal becomes more sensitive to noise and interference. The further it has to travel, the weaker it gets."

So that means LTE cell sites need to be small and a have smaller radii than traditional macro sites to get the full data throughputs that LTE can offer, which will be up to three to four times higher than 3G HSDPA release 6, according to Adrian Scrase, CTO at the 3rd Generation Partnership Project (3GPP) .

"You've got to have new sites, and the capex implications of that are potentially horrible," says Pulley.

So future LTE femtos have to be cheap. PicoChip is developing a system-on-chip (SoC), code-named Feynman, that will enable a dualmode HSPA/LTE residential femtocell with a bill of materials of $70 in 2011. A lamp-post mounted, dualmode HSPA/LTE version of the femtocell will have a bill of materials of about $170.

"This is orders of magnitude cheaper than doing it any other way," says Pulley.

For equipment suppliers, though, the concept of small, cheap base stations, especially those that might supplant current base station models, is causing some tension, according to Pulley.

"Suppliers have vested interests. Why sell a cheap base station when you can sell a more expensive one?"

Carrier support
T-Mobile is keen on LTE femtocells. "Femtocells [will be] an important measure to supplement LTE for indoor coverage and capacity scenarios," says Frank Meywerk, senior vice president for radio networks at T-Mobile.

And China Mobile suggested that operators outside China could use the TD-LTE (Time Division LTE) version of LTE for capacity-enhancing femtocell deployments. That's an option for many European operators that, as part of their spectrum allocations for 3G UMTS services, have been awarded 5 MHz of time-division duplex (TDD) spectrum, along with their primary allocation of frequency-division duplex (FDD) spectrum.

TD-LTE would be a particularly useful choice for operators looking to maximize coverage in dense urban areas, as such deployments would "not interfere with FDD spectrum," according to Bill Huang, general manager at the China Mobile Research Institute. "I've heard that without having fixed spectrum allocated to femtos, it's not possible to deploy femtocells" because of interference issues.

Canada's Telus presented findings that femtocells do indeed have better performance in metro deployments.

"We're all here today to determine if there is a performance difference with going with very small cells," said Sam Luu, associate director of technology planning and strategy at Telus. "There is a significant performance difference at the edge and at the site."

But Luu cautioned that metro femtos are, for now, still only an interesting idea. "You can talk about technical better performance," he says. "But it still requires the R&D to get it off the ground. We're still in the early stages of evaluating the technology."

— Michelle Donegan, European Editor, Unstrung

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