Doubling Up On Voice

In an industry where 3G is quickly becoming the “legacy” network, 2G innovations don't get much attention. Vendors may be applying most of their R&D efforts on long-term evolution and evolved packet cores, but they still have a few tricks left for GSM. Although 2G may not do much for data these days, you can certainly shove a lot of voice onto those networks. A new technology called orthogonal sub channel, or OSC, intends to double that voice capacity.

The GSM community did this doubling act before when it introduced half-rate technologies, which allow for the transmission of two simultaneous calls with the same GSM time slot. A 200 kHz GSM channel has eight time slots, each used for the transmission or reception of a single phone call. Half-rate managed to insert an extra call into each time slot. Now, OSC doubles up on half-rate, cramming four simultaneous calls into the same time slot.

“We're packing voice more tightly into the GSM channel,” said Kai Sahala, head of radio access strategic solution sales for Nokia Siemens Networks. “Another way to look at it is OSC offers the same capacity with less equipment, which is particularly important in markets where W-CDMA has come into the picture. Carriers want to find more spectrum for W-CDMA.”

The technique currently is being batted about in the standards bodies, scheduled for Release 9 of the 3GPP's UMTS specification, which is still quite a ways off. But NSN grounded the technology a little more thoroughly in the present when it announced in January it had successfully completed an OSC mobile drive test, where it made four simultaneous phone calls over the same GSM time slot. NSN is hoping to apply OSC as a software upgrade in 2010 to its current generation of Flexi base stations.

The infrastructure is where the tricky economic questions come into play. OSC can support any newer handset with adaptive multirate (AMR) codecs with single antenna interference cancellation (SAIC), of which NSN estimates there about a billion. But most GSM network infrastructure is relatively old and can't support the software upgrades necessary to provide OSC.

Sahala said there will be definite cases for even established GSM operators to use the technology. Many 2G-only operators are replacing older GSM gear with EDGE base stations, making them prime OSC customers. And greater voice usage and increased population requires operators to expand existing networks with 2G as well as with 3G. For those operators, OSC essentially can give them two base stations for the price of one, Sahala said. But there are operational considerations. As power bills climb and operators face pressure to be green, the value of greater efficiency rises, he said.

“We aren't increasing the power by increasing the call capacity,” he said. “The same capacity requires half the energy. That's a much greener network.”