Lucrative roaming solution

Mid-Tex Cellular launches CDMA and GMS on the same infrastructure using Vanu gear

Right smack in the middle of Texas, away from the urban centers of Austin and Dallas, one of the most sophisticated cellular networks is operating. Mid-Tex Cellular has launched the first commercial software-defined radio wireless network in the country, allowing the company to support both CDMA and GSM networks over the same radio access equipment.

The Mid-Tex launch is the pride of its gear vendor, Vanu, which has championed software radios for most of the decade. Now Vanu is hoping to use Mid-Tex's dual network as a springboard to the rest of the country, targeting the hundreds of smaller carriers that depend on roaming for their livelihoods.

Software-defined radio (SDR) hasn't had a direct impact on operators until recently. Its primary uses have been in the military, where the U.S. Defense Department needed to solve the problem of bridging its numerous and disparate spectrum with a single radio technology. As its name implies, SDR doesn't use a specific embedded radio technology. Rather, the radio tuning and modulation is performed by software — a different software stack is used for each radio protocol.

For the military the benefits were obvious. The Defense Department wanted to produce a single radio that could receive and transmit on Army, Navy and Air Force frequencies as well as communicate via satellite — which spanned bands from 400 MHz to 2 GHz and a host of different modulation schemes. Instead of embedding multiple radio chips in every device, the military's SDR project, called SpeakEasy, created a single radio with a software stack capable of handling all frequencies and all protocols. What was a godsend for the military, however, wasn't necessarily one for commercial wireless.

Operators typically adopt a single radio technology, and while they may be operating on multiple frequencies, the cost of embedding dual-band radios into handset and base station infrastructure is much less than using SDR. Some vendors have begun using SDR in their own development, notably Alcatel-Lucent, but mainly as a way to cut development costs rather than as a product option for their customers. Before its merger with Lucent Technologies, Alcatel decided to shift its base station design to a single configurable chassis, allowing it to build a single base station that it then could program with the appropriate software for GSM, UMTS or WiMAX at the appropriate frequencies for each individual product line.

Mid-Tex's deployment, however, highlights a business case in which a dual-radio base station makes sense for an operator. Unlike the Tier 1 operators, which run their networks entirely to support their own retail services, rural and regional operators such as Mid-Tex make a good deal of their money through roaming agreements with the Tier 1 providers. The more large operators with which it can sign agreements, the more money it stands to make. In this instance, having a single network that supports CDMA and GSM makes sense, said Toney Prather, CEO of Mid-Tex.

“We were doing a lot of roaming business with all four [nationwide operators] back in the TDMA and analog days,” Prather said. “But when there was a fork in the road, we had to choose which way to fork. … If you build a network and partner up, you have to pick a partner. Who do you pick: AT&T or Verizon? I don't want to have to make that decision.”

Mid-Tex picked AT&T, following that company's previous incarnations AT&T Wireless and Cingular down the migration path from TDMA to GSM. Mid-Tex continued to offer analog roaming to CDMA operators, but as CDMA became more pervasive and new phones stopped including analog radios, those revenues steeply declined, Prather said.

But Mid-Tex had deployed its GSM network using Vanu's SDR base station, which gave it an eventual migration path for CDMA. Mid-Tex had to wait, however. In 2003, Vanu had yet to develop the CDMA software stack to support the technology. In January 2006 though, Vanu signed a CDMA licensing agreement with Qualcomm. Three months later at CTIA, Vanu showed off its first triple-radio-protocol SDR base station. Vanu uploaded the software into Mid-Tex's network in February 2007, using the same base stations and transmitters it previously had installed as well as the same ADC wireless backhaul. Only in the core did the two networks separate, splitting the communications stream off into separate CDMA and GSM switches.

“Our CDMA carrier business has jumped five-fold,” Prather said.

Marty Gilbert, vice president of marketing for Vanu, said that Vanu has been targeting rural operators for years, trying to convince them that SDR isn't just an application for the military. While large operators expressed initial interest in SDR, their large vendors never delivered on the technology, as it would have required transforming their infrastructure businesses into software businesses. Big operators never pushed their vendors to deliver SDR solutions, primarily because those vendors were able to combine multiple-radio gear cheaply, or in the case of CDMA, install radio upgrades on the same equipment.

“Quite honestly, I think some of the smaller carriers are more open to innovation than the large carriers,” Gilbert said. Unlike the Tier 1 operators, which have little interest in supporting a competing radio protocol on their networks, small operators have every interest in doing so. “More than the Tier 1s, they are dependent on roaming,” he said.

That actually may change though as the Tier 1 operators flesh out their 4G plans. New technologies such as long-term evolution and WiMAX will require new networks, which might make SDR more attractive. But in those scenarios, the base station architecture will just be a small part of their worries. 4G networks will have open-IP architectures, operate on new frequencies, use much wider channels and in some cases use unpaired spectrum instead of the standard split-channel configuration of current cellular networks. Being data-centric, they will require different footprints than their voice-centric cellular counterparts, and new smart antenna technologies will require entirely new configurations on the cell tower.

SDR can address some of those problems. It already negotiates the different channel sizes of CDMA and GSM networks over the same spectrum, but it can't physically re-architect the network from core to transmitter.

But Vanu is still a long way off from thinking about 4G. It now has three software stacks under its belt — CDMA, GSM and integrated digital enhanced network — and its road map is closely following that of the rural and regional operators it is targeting. The rural operators are far from worrying about 4G. For them, the pressing issue — and the focus of Vanu's next software releases — is 3G.