AWR’s Visual System Simulator Co-Simulates with NI’s LabVIEW for Enhanced Signal Processing Capabilities
By Gent Paparisto, Ph.D., AWR Corporation

Achieving the highest possible performance from circuits used in third-and fourth-generation wireless systems is driving a tighter integration of previously disparate tools. Certainly, a level of software synergy is essential when designing circuits for use in today’s wireless systems that employ higher-order modulation techniques together with advanced technologies, such as Orthogonal Frequency Division Multiplexing (OFDM), multiple-input multiple-output (MIMO), and digital predistortion (DPD) circuits, to name a few. As this article illustrates, AWR’s Visual System Simulator (VSS) and National Instruments’ LabVieW graphical programming environment are now co-simulating to better enable designers to analyze, optimize, and verify complex RF circuits, subsystems and digital signal processing within a unified framework.
Before looking more closely into specific design scenarios, it is important to understand how this new and cohesive VSS/LabVIEW co-simulation environment works.

How it Works
The integration of VSS and LabVIEW is enabled via a new LabVIEW block within VSS. The simulation is driven by VSS and the LabVIEW block provides the interface to LabVIEW, allowing the exchange of data and parameters between the two platforms (Figure 1). This new block invokes a LabVIEW virtual instrument (VI) which in turn performs digital signal processing functions programmed through palettes provided in the graphical programming environment. This interface provides flexible configuration options with user-defined mapping of VSS nodes and parameters to the VI input and output ports. Multiple LabVIEW blocks may be placed on a VSS system diagram so that multiple VIs are executed at the same time.

This integration offers VSS and LabVIEW users increased capabilities, such as expanded math and baseband libraries. The link to the extensive library of LabVIEW VIs also enables access to a large collection of functions for RF instrument control and RF measurements as well as frequently used DSP functions and primitives, signal generation and analysis toolkits for standards such as WiMAX, WLAN, GSM/EDGE, WCDMA/HSPA+ and LTE. Additionally, VSS provides a platform that is known for its ease of use, and also includes a large number of standard and custom signal sources, signal processing capabilities, RF and digital hardware component modeling as well as an extensive set of measurements.

There are two options for interfacing with LabVIEW in VSS:

1. Use the LabVIEW Run-Time Engine for executing a VI provides for fast simulation times, but disables the creation, modification or debugging of VIs.

2. Connect to the LabVIEW Development System provides full flexibility for creation, modification and debugging of VIs as necessary.

This interface further allows the use of LabVIEW add-ons and toolkits, which provide standard signal sources and measurements; signal processing, analysis and connectivity; integration with development hardware; and many other capabilities. Taking a closer look as to how designers may utilize this new co-simulation environment, the following scenarios are presented:

Scenario 1: Signal Processing Blocks
VSS can generate signals required for the design and simulation of modern communication systems. Real, complex, and digital signal sources can be combined to provide real-world emulation of signals for a wide range of applications. Figure 2 shows a VSS system diagram with several of these signals feeding a LabVIEW block (red circle in Figure 2) and outputs being measured with test points (blue box inset into Figure 2). The LabVIEW block ties directly to a VI in LabVIEW. The link between VSS and LabVIEW is managed exclusively by this block. During VSS simulation, the time-domain data is fed into the LabVIEW data flow and the resulting VI ouput is resynchronized in time with the VSS simulation. Referring to Figure 2, the integer data in VSS goes to LabVIEW and its amplitude is calculated with a summing function and returned to VSS. The resulting output (Figure 3) -- relative to the input -- is scaled in amplitude according to the LabVIEW processing block. The real and complex signals have a similar operation.

LabVIEW VIs can also be used from within the VSS environment. Consider a simple VI that takes two input signals, adds them together, and scales the result by a variable gain that is set by a user-controlled slider (Figure 4). Here, this VI is converted to a “subVI” by assigning Input 1 and Input 2 numeric controls as input terminals and Output as an output terminal in the connector pane. This VI can then be used in a VSS system diagram with a LabVIEW block configured to the desired VI, and the input and output nodes that will interface with the VI are added. In essence, the controls and indicators in LabVIEW become inputs and outputs, respectively, in VSS. This is accomplished by clicking “Add” under the Input Ports section, selecting the nodes from available input ports defined in the VI, and if desired defining a name for each port. The output port is also added and the property propagation is defined.

This simulation also uses the LabVIEW Run-Time Engine and shows the VI front panel. The input to the VI consists of two tones: 1 and 1.1 GHz. The results are displayed on a graph in the LabVIEW front panel window, as defined in the VI (Figure 5). This window is automatically opened during the simulation. The results will display immediately on a VSS graph as well.

Scenario 2: A Digital Predistortion (DPD) Example
Digital predistortion techniques, employed to overcome the nonlinear operation of base station amplifiers, have become a mandatory component to support today’s wider bandwidth signals, high power efficiency and output linearity requirements. VSS provides an environment for generating input signals, modeling the circuit-level amplifier, as well as performing the required measurements. It also offers the ability to design a DPD proof-of-concept and then convert it from floating-point to a realistic fixed-point implementation. LabVIEW includes many of these signal processing capabilities as well but also offers the option of implementing the resulting design in an FPGA. In this way, users are greatly aided by the joint VSS/LabVIEW environment in that it allows for faster design through the prototype and test of final DPD implementations.

In this scenario, the predistortion circuit uses digitally-controlled attenuators and phase shifters and its implementation is moved very early in the design process to LabVIEW given the new VSS/LabVIEW unified framework. Based on a circuit-level amplifier, a VI is created that takes a sample of the signal passing through the amplifier and creates a predistorted signal. This signal is passed back to VSS, where it enters the amplifier so the resulting signal is more linear. This process benefits designers in that it provides a seamless methodology that allows for successively adding more complexity to the circuit while concurrently evaluating its performance.

Figure 6 shows a 1900 MHz power amplifier from AWR’s library. The AM/AM and AM/PM curves are shown in Figure 7 along with the resulting spectrum with QPSK modulation at a power level with notable distortion (visible as spectral regrowth “side lobes”) simulated in VSS as a reference. There are two foundational aspects of digital predistortion that need to be addressed for the technique to work.

A sample of the amplifier’s input-output characteristics must be available in order to determine what must be corrected. This can be done by periodically running the digital predistortion circuit in a calibration mode to characterize the amplifier, or by sampling the amplifier’s output and building a characterization table dynamically over time.

In this case, the discussion concerns an early stage of the design process when only the amplifier is being simulated. A copy of the amplifier can be used to create the characterization dynamically before the signal reaches the actual amplifier simulation. This is shown in Figure 8, which is a VSS system diagram in which a copy of the 1900 MHz amplifier (element S1, circled) is placed early in the signal path where the signal is predistorted.

The second issue that must be addressed is the amount of correction required to predistort the signal. While this can be achieved algorithmically by understanding the AM/AM and AM/PM behavior, it is being done manually within the VI to illustrate the functionality.

Here, LabVIEW’s benefits become clear as the design flow moves toward actual hardware, as the VI can be made more realistic by adding corrective algorithms and extending this VI to measure the attenuators, phase shifters, and other components that would otherwise only be modeled in the VSS system diagram.

One of the benefits of the synergy between VSS and LabVIEW is in the ability of VSS to capture the simulated 1900 MHz amplifier and LabVIEW to capture the actual attenuators and phase shifters. The user can determine when and how to switch from manual to algorithmic behavior given the challenges and progress of the design flow. The LabVIEW VI is added to the VSS system diagram to create the predistorted signal (Figure 9). The VSS input signal is transferred through the LabVIEW block to the VI and separated into phase and magnitude. These signals are then used early in the development of the predistortion circuit to drive a look-up table or simple continuous algorithm corresponding to the phase and magnitude of the output signal. Operating on the complex signal (as phase and magnitude) allows direct implementation of the phase shifter and attenuator. As the digital phase shifter and attenuator are digitally controlled, the look-up table can be implemented in an FPGA for easy recalibration or updating as determined by system performance.

First-pass predistorter results are shown in Figure 10. The blue trace shows the signal as it would appear without predistortion, while the red trace shows an early manual correction using the LabVIEW VI to alter the phase and magnitude as a predistortion function. The improvement is seen in the spectral mask, where regrowth is reduced by about 25 dB. As the amplifier is driven harder, the performance should be even more pronounced, thereby maintaining the linearity of the amplifier at higher power levels.

Summary
Achieving the highest possible performance from circuits used in third and fourth-generation wireless systems requires the seamless integration of simulation and measurement at every stage of the design process. Designers who deviate from this are at risk, as the further in the design process that serious design issues are discovered, the more time (and money) it will take to remedy them. AWR’s VSS software and National Instruments’ LabVIEW significantly reduce the possibility that the latter scenario will occur, as the circuit and its components are passed back and forth seamlessly between the two tools. The result is better performance, shorter design time, and a minimum of frustration.

About the Author
Dr. Gent Paparisto is a Senior Systems Engineer at AWR. He received his Ph.D. in electrical engineering from the University of Southern California (USC) and has extensive experience in research, design, development, and implementation of communication systems and algorithms for wireless, satellite, and wireline applications.

Dr. Paparisto has authored a number of publications in international journals and conferences, served on the technical program committees of various IEEE conferences and contributed to the 3GPP GERAN standardization group.

AWR Corporation
www.awrcorp.com
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