Design Suite Bridges System/Circuit Gap

Over the course of the past 10 years, wireless-communications-system design has become increasingly difficult. The old way of doing it required engineers to deal with measurements based on things like S-parameters and third-order intercepts. Unfortunately, such measurements are no longer sufficient for digital radio design. As an alternative, engineers have now turned to options like error vector magnitude (EVM) and adjacent-channel power ratio (ACPR). Using them, engineers try to get accurate measurements to use in the design process. This is an ad-hoc solution, however, that plays host to another problem. Circuit-simulation tools were designed to deal with S-parameters and third-order intercepts—not new types of complex statistical measurements that require digital modulations. As a result, system-level and circuit-level engineers often are forced to resort to the costly process of overspecifying a design just to make sure that it works properly. And they must come up with a viable design solution in the absence of any common design environment.

Recognizing the difficulties of both camps, Applied Wave Research set out to devise a solution. Its first release, Virtual System Simulator 2002 (VSS2002), was introduced last year. It was a good step in the right direction. This year, though, the company has come to market with a revision of the earlier VSS tool. It directly addresses the complexity of wireless-communication design. In doing so, it gives the user capabilities that were never before possible.

Ad-hoc methods required a great deal of time. They also affected system performance. In contrast, VSS2003 shortens development time while cutting cost and mitigating risk. In addition, the tool brings test and measurement into the software arena. VSS2003 provides the circuit-level engineer with a product that literally replicates a laboratory environment. As a result, system engineers no longer have to hand the circuit-level engineer information that he or she cannot deal with using conventional tools. The system engineer also is relieved from having to worry that the circuit-level engineer cannot understand this information.

System- and circuit-level engineers can now work in the same design environment. The system engineer can specify his or her constraints, which in turn can be used to conduct system-level simulation. The results of this simulation can then be easily and quickly pulled into a transistor-level view of the design. Together, both engineers can work hand in hand while making tradeoffs to ensure that the design works appropriately and is fully optimized. As a result of this new process, the need to overspecify a design is all but eliminated. This is good news for project teams, as overspecification often leads to significantly increased costs and potential time delays.

At the heart of VSS2003 is the combination of two key technologies: AWR's discrete time system-level simulation engine and a comprehensive library of models, such as RF models that can drive Microwave Office for circuit-level accuracy. Together, these technologies culminate in an interactive product that helps system engineers perform top-down modeling, analysis, and optimization of analog and digital communications systems.

VSS2003 is characterized by an easy-to-use Windows interface and a fast system-simulation engine. This combination allows users to quickly build graphical block diagrams. They can then analyze the corresponding performance using built-in measurements and signal generators, which support virtually any modulation scheme. Improved parameter settings for existing VSS filters comprise another feature of VSS2003.

A number of optional, application-specific libraries also are new. They are intended for engineers who are conducting RF-link conformance testing (see figure). The libraries support 3G, GSM, EDGE, and 802.11a/g, as well as other emerging standards. VSS2003 itself contains an expanded library of core elements and mathematical primitives. They can be used to build an accurate representation of even the most complex communications systems. Included in this library are improved encoders/decoders (including Viterbi, Reed-Solomon, convolutional, and others); modulators/demodulators; and filters.

A slew of models is flaunted by VSS2003 as well. These models support features like sweeping any designated parameter, creating an RF signal directly from a complex envelope, creating bursts of data from any transmitted source, and resampling data to any given rate. Other features include enhancements to the bit-error-rate (BER) and EVM measurements. In addition, designers can now define spectrum- and network-analyzer measurements using familiar settings, such as resolution and video bandwidth. Enhanced RF behavioral models for amplifiers and mixers provide advantages for RF/analog engineers, who typically use multiple tones to analyze the non-linearities of a radio-frequency link.

The tool seamlessly integrates with AWR's TestWave 2003 software design tool. As a result, engineers can perform analysis at the system level. There, hardware measurements are incorporated through bi-directional links to popular test and measurement equipment.

Perhaps one of the most significant features of VSS2003 is its patent-pending technology for handling mismatches between components. In the idealized mathematical world of conventional simulators or spreadsheets, impedance mismatches are not typically taken into consideration. They must be accounted for, though, to avoid unnecessary degradation in system performance or the need to rework a design. With its new VSWR modeling capabilities, VSS2003 allows designers to account for impedance mismatches throughout the entire RF link. They also can monitor any resulting spurious signals. Inclusion of these "VSWR effects" is vital to the accurate analysis of RF/analog subsystems.

With VSS2003, the engineer can import both measured data and behavioral models. This is a crucial point, as the engineer often needs real-world signals to test devices like power amplifiers. Through such testing, he or she can get accurate measurements of things like ACPR. This capability also helps ensure that system measurements fully agree with measurements taken from the lab.

Another strength of this design suite is that it can generate virtually any signal that the engineer might want. Consider, for example, that specifications often change or get updated. Rather than having to purchase a specific signal-generator tool to accommodate these changes, VSS2003 can generate new signals resulting from the specification change. The engineer just has to create models or download a library from the AWR Web site. These signals can be downloaded into any standard signal generator.

Evaluation software of the VSS2003 communications-system design suite is available for download at www.appwave.com/freetrial/index.html. The fully functioning design suite also is available to customers. It supports the Windows 2000, NT, and XP platforms. The U.S. list price for a perpetual license ranges from $15,000 to $33,000, depending on the simulation capabilities required. The GSM/EDGE and 802.11a/g Analysis Library add-ons sell for $15,000.