Femtocells cause coverage holes: Telstra

Femtocell devices designed to boost indoor 3G network coverage could actually create coverage blackspots, according to Telstra's chief technology officer Dr Hugh Bradlow.

Femtocell devices are deployed into homes and buildings where coverage is poor, acting as mini cell towers. Only 3G devices that are approved to access the femtocell can leverage a household's fixed-line broadband coverage to provide improved voice and data coverage. Optus launched a trial of an Alcatel-Lucent femtocell called 3G Home Zone with consumers earlier this year, while Vodafone has begun a limited trial of Huawei-branded devices with business customers.

According to Bradlow, Telstra is reluctant to join its rivals in offering femtocell devices because he said they interfere with normal network coverage for other users.

"[Femtocells] do actually cause holes in the macro coverage, so that because a femtocell is basically a private base station, you have to register handsets or dongles onto that base station for use by it. If you happen to be in someone else's house and their femtocell is actually sucking away capacity from the macrocell, then you actually can't get coverage at all," Bradlow told Twisted Wire.

Telstra has a philosophy of providing "always 3G" coverage for both indoor and outdoor, according to Bradlow. He also said Telstra addresses indoor coverage issues by using the more penetrative 850MHz spectrum in some places, as well as carrier-controlled small indoor cells in other areas. Furthermore, there was a buffer built into the network to allow better indoor coverage.

"We designed the network with an indoor margin built into it so that when the signal hits the side of a building there is at least 15dB that it can use to get inside the building and get coverage in the building," he said.

According to international group Femto Forum, however, the femtocell devices in use today do not interfere with macrocell coverage.

"They are extremely intelligent devices capable of sensing the use of frequencies around them and adapting their operation to minimise any disturbance while staying within parameters which are set by the operator. Ultimately, they benefit from the certainty of operation on licensed frequencies which can be carefully managed by the operator, unlike the licence-exempt frequencies used for Wi-Fi. Calculations and trials have demonstrated that femtocells actually reduce the overall interference levels in a network by reducing the loading of the outdoor network."

Bradlow also rejected industry speculation that customer use of femtocell devices would reduce network capacity issues for telcos.

"A lot of the femtocell manufacturers will claim that it does give you extra capacity, but we've done studies on that and the penetration of femtocells would have to be so significantly high to get any impact on network capacity that it is a meaningless concept," he said, adding that one third of households would need to have femtocell devices for it to have any noticeable effect.

Optus told ZDNet Australia that feedback from customers for its femtocell trial have been positive and that the company had addressed interference.

"We addressed the issue of interference as part of the planning and design of our femtocell trial," Optus said.

Despite Telstra's hesitation to use femtocell devices, the telco doesn't appear to have completely ruled them out, with a clause regarding the devices making its way into the $11 billion agreement between the telco and the National Broadband Network Company (NBN Co).

While Telstra can overbuild NBN Co fibre for use on its own mobile network, those femtocells (if the telco uses it) have to be hooked up to NBN Co's fibre, not Telstra's:

Telstra has agreed that certain types of wireless "femtocells" supplied for use primarily in residential premises or buildings will not be considered Telstra network elements. This means that, subject to any other applicable exceptions, the network preference for the NBN will apply to fixed-line connections to those "femtocells" in the NBN fibre footprint.

Last week it was revealed that early models of Vodafone UK's femtocell device could potentially be exploited and would allow people to listen in on other people's calls, or even pretend to be other Vodafone customers' phone numbers. According to Vodafone, however, this exploit has since been patched.