High-Speed Networks And 3G Wireless Hike Demands On Test Gear

For the design engineer, building communications equipment is a tough challenge. Speeds have increased to the multigigabit range and standards are changing every day. It's no wonder there's a pressing need for improved communications test equipment to validate their designs. Instrument manufacturers know this and are constantly innovating to meet these challenges.

We might consider how these innovations come about. Are research staffs of the big test-equipment companies busy predicting the future, so that they can turn out the perfect product at the right time? Probably not.

To keep pace with new communications developments, test-equipment manufacturers work closely with their customers. Sometimes this means developing test equipment side-by-side with the communications products. Other times it involves listening to customer feedback about the equipment.

We don't intend to cover the entire spectrum of communications test instruments here. Rather, we will cover some trends and examine a few new measurements required by the latest advances in communications technology.

Let's begin with general-purpose digital storage oscilloscopes (DSOs). These instruments, of course, test the complete gamut of circuitry, including communications designs. In order to facilitate communications testing, DSO manufacturers supply software for checking compliance the various communications standards.

One of the trends in this area is to increase the flexibility of the test software. An example is a recent innovation from LeCroy Corp. called a pass/fail mask-creation utility. Known as PolyMask, the utility runs on a PC. Users create pass/fail masks that define signal pass/fail boundaries. These masks are then loaded into a high-performance LeCroy oscilloscope.

With the PolyMask option, users can create complicated test masks, such as those used for testing 100Base-T network signals. Signals can be tested to lie within or outside the mask area.

While high-end general-purpose DSOs have bandwidths in the 1- to 2-GHz range, high-end sampling oscilloscopes have bandwidths in the 30- to 50-GHz range. Those oscilloscopes, referred to as communications analyzers, are targeted at designers who build transmitters for optical networks which operate at rates up to 10 Gbits/s.

As optical transmitter signal speeds increase, it becomes more difficult to distinguish the transmitter signal from the noise. One of the trends in the newest communications analyzers, therefore, is increased signal-acquisition fidelity. The Tektronix CSA8000, for instance, has short-term trigger jitter that's typically less than 1 ps and timebase stability of less than 0.1 ppm.

Communications analyzers, like the general-purpose DSOs, must be able to test to various standards. These instruments usually accept modules, which perform compliance tests against such standards as SONET/SDH, Gigabit Ethernet, and Fibre Channel.

An important feature is a module's ability to test for compliance against several standards. This ability eliminates the process of swapping out modules and recalibrating the test system for each standard tested.

Another trend with these high-end communications analyzers is to integrate all necessary test components into a module rather than connecting to external devices. For example, a sampling module might contain an optical receiver, power meter, and clock-recovery circuit. Abolished, then, is the need for additional cabling and add-on accessories to support these functions.

Besides the Tektronix model mentioned above, Agilent Technologies recently announced its 86100A Infinium Digital Communications Analyzer. Compared to its predecessor, the 86100A has faster throughput and improved accuracy and repeatability.

Let's move away from networks for a moment and turn our attention to wireless communications. A known fact is the global transition from second-generation (2G) wireless phones to third-generation (3G) sets (see "A Bump In The Path To 3G," p. 88). Designers typically use spectrum analyzers to make measurements on their wireless prototypes, both handsets and basestations.

Especially useful in making wireless measurements on CDMA and wideband CDMA signals is the real-time- spectrum analyzer (RTSA). Note the opening theme illustration for an example of measurements taken by this type of instrument. An RTSA differs from a conventional swept-spectrum analyzer. The latter sweeps through a series of frequency increments one at a time. It requires a steady signal, and takes time to accumulate enough information for a display. While time elapses, momentary events may occur unnoticed by the instrument. An RTSA, on the other hand, captures a broad band of frequencies instantaneously and continuously. That's why it can capture wireless signal "bursts" and brief transients.

Right now, Tektronix is the only manufacturer that makes an RTSA. Its top- of-the-line model, the 3086, has a 3-GHz bandwidth and a 30-MHz instantaneous capture capability (Fig. 1). Two new software options equip the 3086 to perform code-domain power and complementary cumulative-distribution-function (CCDF) measurements.

Code-domain power quantifies a basestation's response to instructions from the network. When configured with this option, the 3086 measures code-domain power to the published specifications for W-CDMA experimental version 1.1. This measurement will also be available for analysis of 3GPP signals.

While other instruments can measure code-domain power, Tektronix believes the wide-bandwidth, real-time acquisition and deep-capture memory of the 3086 are perfectly suited for critical 3G measurements. The instrument can record 10 full frames—that's 160 time slots of 100 ms—of information from one trigger event. This gives the designer a wide window of viewable frames. Conventional spectrum analyzers, the company says, require a hit-and-miss, frame-by-frame examination of the data.

"For power control measurements, major mobile equipment manufacturers have been looking for a code-domain measurement tool that captures many W-CDMA frames in one acquisition," says Steve Stanton, product manager for Tektronix. "The 3086 spectrum analyzer with Option 16 is the first solution to really meet that need."

The CCDF measurement ensures that the multiple carrier signals used for design verification are accurate and repeatable. This measurement is particularly useful to designers evaluating the distortion performance of their products. According to the company, the 3086 is the only instrument able to perform CCDF measurements for W-CDMA.

Standard spectrum analyzers are helpful in taking such measurements as adjacent-channel power ratio (ACPR). Again, 3G wireless systems have prompted manufacturers to enhance their equipment. Rhode & Schwarz, for instance, boosted the dynamic range of its FSE family of spectrum analyzers to 75 dB. This enhancement allows the instruments to meet ACPR measurement requirements for W-CDMA.

Additionally, the company improved its FSIQ models. These instruments now contain better phase-noise and linearity specifications. Phase noise is currently rated at ≤ −156 dBc at a 10-MHz offset. Linearity now stands at <0.2 dB across a 0- to −70-dB range, which the company states is an industry-leading figure. These attributes qualify the FSIQ for ACPR tests on W-CDMA and cdma2000 equipment, and for exacting spurious-noise measurements on GSM devices.

The FSE and FSIQ families have a new option too—an attenuator with 1-dB attenuation steps. Tektronix has said that the fixed, coarse 10-dB steps on most conventional RF attenuators can degrade the net dynamic range. An application of 10 dB of attenuation when only 4 dB might be needed is one such instance. This attenuator preserves the dynamic range needed for accurate RF measurements on 3G basestation transmitters. With it, designers can apply the 75-dB ACPR measurement over a range of input powers.

Working with the hard-to-measure burst, hopped, and modulated signals presents an ongoing challenge to designers of high-performance communication systems. When you're creating a new design, it takes time to ferret out the real reason why a circuit doesn't work. A more exotic type of instrument that's designed to solve these problems is a vector-signal analyzer (VSA). This instrument combines time-, frequency-, and modulation-domain analysis, and its measurements and displays help designers spot problems faster.

A new VSA platform from Agilent, the 89600 series, is an integrated instrument that combines VXI hardware and measurement software (Fig. 2). The software resides on a PC using Windows NT. Connection to the 89600 processing software in the PC is over an IEEE-1394 interface. The company says the 89600 is the first PC-hosted VSA for communications design. Engineers can use these measurement tools in the same operating environment in which most of their design work takes place.

This series has 36-MHz bandwidth capacity for measuring RF signals up to 2.7 GHz. These signals encompass cellular and satellite communications, digital video, and local multipoint distribution services (LMDSs). The 89600 series also is available with a VXI-based front end using one or two baseband inputs and covering bandwidths to 40 MHz. The company says this capability is critical for RF wireless and emerging communication systems design and verification.

The most interesting aspect of this VSA, though, is an optional link to Agilent's Advanced Design System EDA software from the company's EEsof product group. This feature ensures a tightly linked, software/hardware design environment for the engineer. It means that the VSA can accompany the designer through all phases of design and evaluation. Designers can evaluate computational results from simulation software as well as measured results from hardware with the same processing algorithms. During the simulation phase, the designer can drop in an icon of the VSA and generate test results.

Jennifer Lynch, program manager at Agilent Technologies, explains, "Designers can use the same measurement tool to evaluate the model on their PC as they use to evaluate their actual prototype. This will give huge benefits to designers by reducing test time and speeding the movement from design on a PC through the prototype stages. Any demodulation a designer would want to look at—eye diagrams, EVM, all of that—appears on the computer screen."

Lynch continues, "If you wanted to check different spots in your design, if you have, say, several stages and you're not sure where the error is being introduced, you can just drop that icon of the analyzer anywhere along the way and figure out exactly which bit is wrong. If the model on your PC is the same as the model that you build in silicon, then test results should be exactly the same."

With the 89600 series, measurement, design simulation, modeling, and documentation are all available to the engineer in one integrated interface. Data may easily be moved back and forth between the analyzer and most Windows applications via a simple cut-and-paste operation. Furthermore, the 89600 series VSA can accept data from various software and hardware sources. Models developed using Advanced Design System, MATLAB, or other design software can be evaluated using the 89600 series VSA software.

Let's move on now to investigate the role of emulators in communications testing. 3G wireless receivers utilize time-sensitive algorithms such as rake-finger management and wide-band channel estimation. These critical functions must be evaluated in a dynamic, mobile propagation environment. Test specifications for 3G wireless equipment include a new generation of channel models designed to evaluate key receiver performance metrics.

A typical piece of test equipment that carries out these tests is an RF-channel emulator, such as the 4500 FLEX5 from Telecom Analysis Systems. To deal with new 3G demands, the company has enhanced the capabilities of this emulator with a dynamic-channel modeling feature. FLEX5's 3GPDP (Third-Generation Power-Delay Profile) emulation mode can be programmed to provide a wide range of time-varying RF channel profiles. Equipped with this new feature, the FLEX5 meets and exceeds the requirements outlined in cdma2000 and W-CDMA test specifications.

The 3GPDP feature implements models known as moving-propagation and birth-death channels as referenced in the 3G specifications. These two new classes of channel models emulate the temporal variations in the propagation channel by changing delay-spread characteristics versus time. This feature also provides the ability to go beyond the two-path dynamic models defined in these minimum-performance standards. The 3GPDP accomplishes this by letting all of its paths be independently varied over time.

"3G test specifications are employing more-sophisticated channel models to evaluate the technology's complex receiver requirements," says Rob Van Brunt, TAS product manager of wireless test instruments. "3GPDP simplifies testing to these new standards and will help to speed the development and deployment of next-generation wireless communications technology."

Now, let's return to the discussion about wired networks. A communications test for someone like a network-equipment designer often means employing an instrument that simulates Internet traffic. Typically, designers need to know the per-port metrics of their prototypes. Latency, throughput, packet loss, and the ability to handle bursts of traffic have been the important metrics for several years.

Recently, however, more complex measurements have been required. This results from testing conducted at higher levels of the International Standards Organization's Open Systems Interconnect reference model. At layer 3, for example, it's all about latency, latency variation, and sequence tracking of individual packets that form one of the millions of flows.

The test equipment generates and receives the traffic. Essentially, packets sent out to the network are received from many different destinations. The test equipment must unravel where they came from and when they were initiated. The instrument helps designers understand the relationship between an individual set of packets making up a flow of data and the millions of flows in which that packet sits. That's real complexity.

Besides this, designers have to execute data over TCP, voice over IP (VoIP), and router testing. This requires measuring what's going on in the control plane and how it interacts with the data plane. In a world that's placing increasing emphasis on VoIP, designers still want to know gauge the voice quality by looking at jitter and latency. But now they also want to know if the user can make the call. In other words, they want to know how many calls per second can be set up and torn down. Or, in multicast IP, designers want to know how many leaves and joins are on the network, instead of just the amount of data travelling across it. The key issue is the interplay in these advanced networks between the control and data planes. And the key metrics are leaves, joins, setups, tear downs, and so on.

A major player in this area is Netcom Systems, which is known for its SmartBits line of testers. "If you want to know what we really test," states Mark Fishburn, vice president of marketing at Netcom Systems, "you might say we really test IP and the complexity in the protocols that go above it, running across different kinds of physical topologies underneath it. But in addition, we increasingly test millions of flows, the quality of service (QoS), and how things like multicast IP, routing, and VoIP traffic works over those networks. And that applies whether it's a high-speed core network or one of the access technologies such as DSL or cable modems."

Usually, Netcom Systems develops new boards to go into existing chassis. The only time company ever changed its chassis was for scalability reasons. Netcom Systems has four platforms that address large-scale systems at one end and flexibility with lots of different technologies at the other. One of the usual operations of its products is to induce errors that don't comply to a particular standard to find how other devices will react.

"Today's hottest issue for us is data over TCP testing," Fishburn adds. "We test how well, for instance, bandwidth managers and load-balancing devices deliver the optimization and performance improvements they claim. Certainly over the next few months, higher-layer testing will be important, such as router testing, VoIP, and high-speed packet over SONET. After that, probably the middle of the year, will be OC-192 packet over SONET," adds Fishburn.

Recently, Netcom Systems announced what it says is the industry's first network test application designed to measure the quality, performance, and reliability of VoIP over policy-based networks. The new product, SmartVoIPQoS, generates and analyzes thousands of VoIP flows and millions of IP data flows simultaneously, in real time. This application is designed to objectively analyze the QoS delivered by the new breed of policy-based network devices critical to high-quality converged services. The QoS is reported for both the voice and data flows, and the corresponding voice-quality scores are reported for voice flows.

As we mentioned earlier, we don't intend to cover all the different types of communications test equipment here. But we do want to touch on a few more topics. First, there is battery power. As you might expect, cell-phone designers pay close attention to battery power management. Yet, what is the best way to test this part of the design? Battery and charger simulators come in handy (see "Testing A Cell-Phone's Roller Coaster Power Needs," p. 94). Keithley Instruments is a major player in this area with its battery/charger simulator products.

Another area is general PC-based test equipment. Since the communications market moves very fast, designers often can't wait for new test equipment to hit the market. Sometimes they put together new systems or even reuse the old ones containing the core board components. Then they create new software or reuse what they had before, simply modifying the waveforms, the modulation schemes, the test patterns, the protocols, and so forth.

For example, National Instruments (NI) manufactures the 5911 Flex Digitizer and 5411 Arbitrary Waveform board. These two together can actually execute many of the tests required for ADSL products. One such product is the ADSL modem. There are over 150 parametric measurements that need to be completed. The company has actually coded about 10% of the most frequently used tests into LabVIEW to assist designers in producing their own ADSL test sets. "For ADSL, we do have some measurement VIs \[virtual instruments\] that can be used for doing ADSL tests, and we're in the process of packaging those into a starter kit," says Hall Martin, marketing development director at National Instruments.

Martin emphasizes that NI presently performs parametric testing but not protocol testing. Still, the cost of the testing for ADSL parametrics is very compelling, with the ability to modify and customize the software to address specification changes from one revision to the next. "We're able to move more quickly than other people in those cases," Martin states. "Design engineers like it because they don't have to wait for a test box to come out. They can put something together very quickly. Time-to-market is really the driving issue here. They've got to get products out very fast," he remarks.

With cell phone testing, the signal that results after an RF signal is down-converted is what many designers want to evaluate. They test the specific modulation schemes and custom protocols of this signal (Fig. 3). "We have tools that can take a down- converted signal and then process it. We can just plug off the back of that RF box and start doing further analysis on that signal," Martin says. "And, since people are playing with modulation schemes, that's something they really need access to."

This communications test tour can't conclude without mentioning how the Internet is rapidly becoming an extension of communications test. Since most test equipment is computer-based, it's easy to establish an Internet connection. The avenue is usually an Ethernet card, but sometimes it's a standard or wireless modem instead.

Design teams are no longer centralized in one building. They span the globe in some cases. Engineers can collaborate via the Internet. Hence, they may post reports and perform such tasks as remote monitoring, control, and data collection. This is a natural fit for communications test.