ASK ANY CELLULAR infrastructure vendor, they'll tell you, it's an FDD world. Frequency division duplexing has taken center stage in the world of mobile communications for more than half a decade as CDMA rose to prominence and AT&T Wireless and Cingular phased out their old TDMA networks. The last vestiges of TDD still lie in the GSM technology that dominates the world's mobile networks, but since 1999, the future has been charted on an FDD path.

But a small company called IPWireless recently has emerged to challenge the notion of FDD dominance. Its time division-CDMA (TD-CDMA) technology made little impact at first — going into the networks of small ISPs in the far corners of the globe. But in the last nine months, industry heavyweights like Orange and Sprint have launched trials of its technology, and T-Mobile has even launched TD-CDMA networks commercially in the Czech Republic. Although TD-CDMA is hardly a mainstream technology, the industry has certainly taken notice.

“We've gone through an evolution of technologies,” said Bill Jones, IPWireless co-founder and chief operating officer. “The capabilities our technology can provide are now what the industry is demanding.”

FDD technologies like wideband-CDMA (W-CDMA) separate the uplink and downlink transmissions over different frequencies, requiring two dedicated spectral bands to operate. TDD, however, uses the same spectrum for the uplink and downlink dividing the frequency into separate up and down time slots. There are advantages to that kind of setup. If there is more traffic coming downstream than upstream, TD-CDMA can dynamically allocate more timeslots to the downlink. Meanwhile, by separating the downlink and uplink over separate frequency bands, W-CDMA lets capacity lie fallow if there is a disproportionate amount of traffic going to and from the base station. Another advantage TD-CDMA has is its ability to allow multiple sessions to share a channel.

SO IF TD-CDMA had so many advantages, why all the focus on W-CDMA? The answer is that TD-CDMA's advantages are only really advantages if you're running a data network. “Wideband-CDMA was primarily about giving carriers new voice capacity,” said Jon Hambidge, vice president of marketing for IPWireless. “That's why it retained so many properties of circuit-switched voice networks.”

The ironic thing about 3G was it was always billed as data technology. But the primary goal of most providers launching CDMA 1X and UMTS networks was to gain greater voice capacity and spectral efficiency. With those goals in mind, W-CDMA fits the bill perfectly. Voice, unlike data, always has perfect symmetry on the uplink and downlink, and although voice over IP is being bantered about as the eventual successor for circuit-switched voice, there's no question that dedicated robust channels are necessary to deliver a good-quality conversation over the air.

In fact, when Jones and Roger Quayle, fellow co-founder and current chief technology officer, founded IPWireless in 1999, they focused on broadband wireless access, creating a non-line-of-sight high-capacity access system they could bill as a DSL replacement, Jones said. They focused on the 2.5 GHz licenses held by U.S. broadband wireless carriers.

IPWireless is still targeting those customers (Sprint is now the largest holder of 2.5 GHz in the U.S. and an IPWireless investor), and it has expanded into the global broadband access marketplace, landing deals with ISPs as distant as New Zealand and South Africa. But over the last two years, the vendor has been squaring its technology with carriers' plans for 3G mobility. That transition isn't because of any new development in the industry. In fact, the demand for TD-CDMA has been lingering below the radar for years. Until now, no one has taken advantage of it.

TD-CDMA ISN'T EXACTLY a proprietary interloper. In fact, TD-CDMA is built into UMTS standards. When 3GPP finalized UMTS standards in 1999, it developed two distinct tracks, one for FDD/UMTS, which was based on W-CDMA, and another for TDD/UMTS, based on TD-CDMA. W-CDMA obviously became the powerhouse. It was allotted swathes of new spectrum, launched by any carrier that won the spectrum and supported by every infrastructure vendor in wireless. But governments in Europe and Asia also allotted small amounts of TDD/UMTS spectrum. Carriers, for the most part, just sat on these bands. Some even turned down the TDD licenses entirely.

But now carriers holding those licenses have started to give them a second look. Last summer, T-Mobile chose to deploy a broadband access network in Prague, Czech Republic, over its allotted TDD/UMTS spectrum in 1.9 GHz frequencies using IPWireless technology. For a short time, it was the fastest 3G in the world, coming out ahead of the first high-speed downlink packet access networks (HSDPA) launches, with a theoretical peak of 4.5 Mb/s and average throughput of 512 kb/s. But T-Mobile went one step further, it also used the technology — this time in a paired-band configuration — to launch broadband access services over 872 MHz in other areas of the Czech Republic, showing that TD-CDMA could be used as a replacement for FDD technologies.

Soon after, Orange had trials of the TD-CDMA trials running alongside its HSDPA trials in France and Slovakia using the TDD spectrum. And in June, IPWireless won a contract to supply its core TD-CDMA infrastructure to an unnamed vendor building IPMobile's mobile broadband network in Japan. And IPWireless claims to be in trials or discussions with almost every other carrier sitting on those dormant licenses.

WHILE OTHER COMPANIES, like Interdigital, have developed TD-CDMA technologies, IPWireless and its OEM partner UTStarcom are the only ones so far to produce a TD-CDMA portfolio. And the Tier 1 vendors Telephony talked to had no current plans to develop their own TD-CDMA lines.

“There are areas that TDD shows some advantages — in its spectral efficiencies and the way it dynamically allocates [capacity],” said Carel Bekker, vice president of technical sales for Siemens Mobile Networks. But when it comes down to overall market opportunities, TD-CDMA is a niche technology, he said. “We're an FDD company,” he said. “We believe in an FDD path for 3G technologies.”

In fact, the gap between spectrum designated for TDD and spectrum designated for FDD is immense. Many European operators received 30 MHz of FDD spectrum for every 5 MHz of TDD spectrum. Although many operators have launched W-CDMA over 10 MHz of that spectrum, they have a lot of room to grow. You can't make that case of TD-CDMA. Although many carriers want to use that dormant spectrum, it will be difficult for them to justify the cost of a new network architecture without the promise of future expansion, said Michael Thelander, principal analyst and founder of Signals Research Group.

“I don't want to call this a niche, but I don't think every operator is going to rush out and deploy a TD-CDMA network just yet,” he said. “What IPWireless needs is a clearly defined market opportunity. They have a lot of potential market opportunities, but not a single one you can wrap your arms around.”

IPWireless acknowledges carriers may have some reservations about deploying a new technology when their choices are limited going forward, but the vendor also has developed specific products to assuage those concerns. In January, IPWireless unveiled a mobile TV architecture based on its TD-CDMA technology. Called TDtv, the platform allows carriers to multicast 50 100 kb/s video channels over that 5 MHz chunk of frequencies. Because the network only transmits and doesn't receive, the scaled-down base stations cost 70% less than the full TD-CDMA base station unit. And the TV services would operate separately from data, so carriers wouldn't be faced with the conundrum of running two separate 3G data networks.

Orange and Sprint agreed to trial the TV system, giving IPWireless cause to believe the platform could be a viable competitor to some of the more visible mobile TV technologies like digital broadcast video-hand-held (DVB-H) and Qualcomm's Forward Link Only (FLO). But TDtv is still one technology among a dozen other, more established, platforms. Mobile TV isn't where IPWireless sees the biggest prospects for TD-CDMA. In fact, the vendor sees the most promise for its technology not in the current standards but the future ones.

IPWIRELESS BELIEVES that TD-CDMA could become a critical element in the next stage of 3G evolution, in a standard the 3GPP recently dubbed long-term evolution (LTE). This standard is intended to pick up where high-speed uplink packet access (HSUPA) leaves off at the end of the decade, creating a data-centric platform built off of new orthogonal frequency division multiplexing (OFDM) and multiple input/multiple output (MIMO) smart antenna technologies. One of the requirements that carriers insisted be included in the preliminary standard, however, is simultaneous support for both TDD and FDD duplexing schemes.

What that means, said IP Wireless' Quayle, is that W-CDMA's 3G path ends after HSUPA. An FDD system simply cannot operate over a single frequency band, but the same isn't true the other way around. TD-CDMA can easily incorporate an FDD scheme. What's more, IPWireless's technology seems tailor-made for LTE's other requirements. OFDM's larger time frame structure matches perfectly with TD-CDMA time frames. That means a carrier could deploy IPWireless's current TD-CDMA architecture and gradually overlay OFDM and MIMO, creating a network that supported TD-CDMA and LTE subscribers simultaneously, Quayle said.

IPWireless hopes to be the first vendor with a live LTE trial on the books. Sprint has agreed to test the vendor's next-generation platform, which meets many of performance requirements laid out by the LTE standards. Later this year — if the trial progresses — IPWireless hopes to provide the OFDM/MIMO upgrades that would bring the network into full standards compliance.

IPWireless has a lot riding on that trial, and Sprint has a lot riding on IPWireless, specifically $14 million in direct investment. But Sprint is testing several other technologies, including Qualcomm's Flarion-developed Flash-OFDM, Samsung's WiBro and Motorola's Mobile WiMAX as well as multi-carrier EV-DO, and Sprint officials claim they intend to play no favorites.

“The lab trials we've done with IPWireless have gone very well,” said Ali Tabassi, vice president of innovative technologies for Sprint. “But we want to make sure we're looking at all of the next-generation technologies out there. It behooves us to look at the best technology available.”

IPWireless Commercial Trials and Deployments March 2006

COMMERCIAL DEPLOYMENTS

1: T-Mobile, Czech Republic

2: UK Broadband (PCCW), U.K.

3: Woosh Wireless, New Zealand

4: Sentech, South Africa

5: AIRDATA, Germany

6: Netcom, Nigeria

7: IPMobile, Japan (not yet in service)

8: Nelte, Lithuania

PUBLIC COMMERCIAL TRIALS

A: Orange, France (UMTS TD-CDMA)

B: Orange, Slovakia (UMTS TD-CDMA)

C: Orange, U.K. (TDtv)

D: Sprint Nextel, USA

E: Maxis, Malaysia

F: M1, Singapore

G: Aksoran, Kazakhstan

H: IQ Connect, Australia

I: EmilNet, Mozambique

J: Douala1, Cameroon

K: Cats-Net, Tanzania NetZAP, Indonesia