The digital challenge

As recently as a year ago, the requirements for testing digital networks were not a high priority for many wireless network operators. Many personal communication services hopefuls, after all, were-and in some cases still are-primarily concerned with securing operating licenses, striking deals for network infrastructure equipment and acquiring sites. On the cellular side, many carriers were still determining the best strategies for bolstering their networks in anticipation of the PCS onslaught.

All that has rapidly changed. Several digital wireless networks are now operational, including 800 MHz cellular systems that have been upgraded and new builds in the 1.9 GHz PCS range. As more digital networks continue to appear, the ability to test them has moved front and center in the collective consciousness of wireless network operators.

Testing becomes a crucial issue from the moment a network operator breaks ground on a new cell site. In the U.S., however, digital wireless network upgrade/buildout is such a new phenomenon that there is no reservoir of digital testing experience to tap into. Exacerbating that issue is the fact that the vendors manufacturing the infrastructure gear for these networks are highly competitive and reluctant to share technical information, even with test equipment developers.

Issues concerning proprietary technologies-and a lack of experience among carriers-may prove testing to be one of the biggest challenges wireless network operators face in going digital.

"The digital side is a whole new ball game," says George Brandle, applications support engineer for the Spokane division of Hewlett-Packard, one of several test equipment vendors tackling the digital wireless market. HP makes test gear for all three digital technologies: code division multiple access (CDMA), time division multiple access and PCS 1900, based on the GSM Pan-European digital cellular standard.

Part of HP's concern is that, in an effort to avoid sharing proprietary information, network infrastructure vendors are developing turnkey digital network solutions that attempt to encompass testing capabilities. That approach remains unproven and provides little flexibility, Brandle says. "Nobody really knows if it's going to work," he says, noting the virtually limitless approach carriers previously had in provisioning and operating their systems.

"They had a lot of choice the old way," Brandle says. "Now that it's at this stage, they basically have no choice. If you're an early adopter of digital technology, you have very few options on how to validate your business."

Taking aim During the installation of a base station, verification and optimization testing is required to validate the interface between that network element and the switch. After a network becomes operational, periodic testing is conducted to ensure optimal performance of the network.

Several companies, including HP, Tektronix, Sage, Tekelec, Anritsu Wiltron and Telecommunications Techniques Corp., provide equipment that performs these tasks. The Grayson Electronics division of Allen Telecom Group also recently introduced a test platform for digital wireless networks. Typically, these systems monitor and report variables such as signal strength, dropped calls and signal quality.

"In an analog network, once a call is connected, you only have to worry about whether you see any signal at all," says Dan Bantukal, senior applications engineer for Tekelec. "In the digital world, once you set up a voice path, it doesn't necessarily mean the voice path is set up correctly. You have to interpret what the signal means."

To test that signal and interpret its meaning, a carrier could simulate everything required to set up a call and send it into the base station, Bantukal says. The alternative is to constantly monitor what's going on between the base station and the network.

One of the issues carriers must grapple with in digital mode is power fluctuation, says Bob Buxton, product marketing manager for GSM products at Tektronix.

"With a technology like PCS 1900, the base station isn't continuously transmitting-it's ramping up and ramping down," Buxton says. Test equipment must therefore measure phase error in the base station transmission, which-if left unchecked-can lead to bit errors.

For the most part, the equipment used to test signals in analog wireless environments cannot be adapted for digital testing because of the different demodulation schemes required, says Jack Cowper, product marketing manager for CDMA products at Tektronix (Figure 1). With CDMA, for example, carriers must be equipped with a code domain analyzer to monitor all the users in the same frequency.

"It's such a wide bandwidth, and all callers are locked into the same frequency and assigned a different code," Cowper says.

Regardless of the air interface technology they select, wireless carriers also require spectrum analyzers to measure power level and bandwidth as well as out-of-band interference, Cowper says (Figure 2).

Anritsu Wiltron has developed spectrum analyzers while also focusing on the setup and optimization of antenna sites. In particular, the vendor has developed its Site Master equipment to allow carriers to perform in-service testing.

"It enables you to find out if your site is functioning properly and where you may have problems without powering down the antenna," says Al Fisher, product marketing manager for Anritsu Wiltron.

TTC focuses on the transmission side of testing-the interconnection of signals between cell sites and the backhaul of signals from cell sites to the mobile switching center. With analog networks, this process was not much different than what traditional landline carriers faced, says Mark Cortner, product line manager for TTC. Digital development has changed this process dramatically.

"Essentially there weren't a lot of specific requirements above and beyond what local exchange carriers had," Cortner says. "Now you have this digitally compressed signal that can travel over a T-1 or microwaves."

Vendors have had to develop products that can decode and test the digital signal. Because of differences in compression schemes, the testing functionality varies, depending on the technology used, Cortner says.

"The unique characteristics of a product are driven largely by the technology selected," he says. TTC's Interceptor 116, for example, is designed for the unique requirements of GSM/PCS 1900 networks, as is the hand-held T-Berd 107A. The company is planning similar equipment to support other digital technologies, Cortner says.

Practice makes perfect Development of digital test equipment is moving forward, but not without experiencing some of the same growing pains that the technologies themselves have faced. According to Tektronix, for example, there is a fuller range of equipment for testing GSM than for testing CDMA because GSM has experienced a longer development phase.

What will shape the market for digital testing will be the carriers themselves, Brandle of HP believes. As they learn more about the testing process required to install and optimize their equipment and validate digital service, they will force the vendors to open their code and allow third-party developers to adapt their equipment, he says.

In the meantime, carriers must equip themselves with new levels of understanding-sometimes provided by test vendors themselves-to become familiar with digital testing requirements, he says.

"Many times they don't understand the technology enough to really know what they're doing," Brandle says. "Without understanding it you don't know where to start, so we're trying to offer training where it's most needed."

That training ranges from in-house courses that teach carrier technicians how to use digital testing equipment to on-site engineering training, he says.

"A lot of people understand analog technology," Brandle says. "The number of people who understand digital is much smaller, so the amount of training required is much more substantial."