Why mobile broadband needs femto nets

A SOPHISTICATED ECOSYSTEM of femtocell home base stations will be required to achieve the next big leap in mobile broadband performance, according to wireless chipmaker Qualcomm.

The current focus is on the parallel evolution of two next-generation cellular radio technologies, Long Term Evolution (LTE) and High Speed Packet Access Plus - HSPA+, which includes the HSDPA+ downlink and HSUPA+ uplink protocols.

Both will hit similar limits in throughput and how efficiently they use available bandwidth, said Rasmus Hellberg, technical marketing director of Qualcomm, which makes chipsets for both technologies and has developed one supporting both.

HSPA+ was generally the most efficient way of upgrading use of bandwidth already in use and was likely to dominate in the short term at least, with an estimated 1.4 billion subscribers worldwide by 2013, around ten times the estimated take-up of LTE.

HSPA+ release 7, which became available last year, uses MIMO technology like that in 11n Wifi to help take the peak downlink throughput to 28Mbps, with 11Mbps on the uplink. Release 8, for which chipsets will become available this year, aggregates two carrier signals to bring peak data rates to 42Mbps on the downlink.

Release 9 will put two MIMO streams on each of two 5MHz carriers, aggregated to produce a 10MHz data pipe delivering 84Mbps on the downlink; the uplink uses simple aggregation to 23Mbps. A projected Release 10 would bring the peak downlink speed to 168Mbps, though this would require 20MHz carriers only available in the 2.5GHz and 2.6GHz bands.

Hellberg said that with the bursty traffic typical of mobile browsing, doubling up carriers can increase data rates for all users, even those using single channels, because the bandwidth is used more intensively. Peak data rates would be nothing like users would get in practice but with HSPA+ Release 8, a "median user" could expect to get a little under 8Mbps while someone using only a single carrier could still expect half that speed.

However operators had the option of holding the data rates down in order to support more simultaneous users.

Qualcomm tests showed there was very little difference between LTE and HSPA+ in spectral efficiency, peak data rates, and round-trip latency, Hellberg said in a briefing evidently anticipating next month's Mobile World Congress in Barcelona, where the two technologies are expected to loom large (see slide above).

LTE was best deployed in areas where new, wider spectrum was available, which was why the likes of China Mobile were interested in it. LTE would also be used to increase data capacity in dense urban areas, but that still meant you would need services like HSPA+ because LTE does not support voice. This meant it was very important to have a chipset that supported both technologies.

Hellberg said performance improvements in the radio technology would get smaller as the theoretical limits were reached, which means the next big gain would have to come from improving the system topology. "This brings bringing the network closer to the user to get a better user experience and increase capacity. It's all about bringing many more low power [network] nodes closer to the users... and this means in practice bringing femtocells to the user."

The use of a residential femtocell, effectively a private base station, is currently restricted to people within a home, partly because it uses the home broadband connection for the "backhaul" - the land link to the operator. Another problem, Hellberg said, is that a femtocell can interfere with the wider network resulting in a poorer service for other users. But with proper management to reduce interference femtocells can reduce the load on the main network and improve the service for all users.

HSPA+ Release 8 includes basic support for femtocells, which is extended in release 9, allowing them to be deployed on a big scale with interference management.

"There is an evolution towards what I call femto networks, with dense deployment and open access," said Hellberg. The femtocells would include hand-over technology that would allow passing users to move from one to the other without dropping a connection. "So you are getting something close to the traditional pico network [of small footprint cells] but coming from a different direction."

The difference is that deployment could be unplanned and the femtocells would be self-configuring and self optimising to avoid conflicts with neighbours and the wider cellular network. Femtocell users would benefit by getting optimal data rates, while operators would increase the capacity of their networks without having to upgrade their backhauls. µ