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From RF Link and Component Design to Implementation with Cadence VSS and Rohde & Schwarz VSESIM-VSS Software

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by Dr. Gent Paparisto and David Vye, Cadence and Markus Loerner, Rohde & Schwarz

Simulation with electronic design automation (EDA) software is critical to the design of modern RF front-end electronics and wireless systems found in 5G, radar, and wideband satellite link applications. To ensure product development success and minimize the number of design iterations, engineers rely on accurate device modeling and simulations to predict the resulting component/system-level performance using key performance indicators (KPIs) such as error vector magnitude (EVM) bit error rate, etc.

As the latest technologies adopt more complex waveforms and support wider bandwidths for high data throughput, accurately determining these KPIs requires simulations with standard-compliant waveforms (5G, Wi-Fi, etc.) or realistic user-defined wideband digital modulation schemes. Furthermore, in addition to applying standard-compliant signals to the simulation models, designers need to align their simulation setup with the measurement system used to validate KPIs of the manufactured device.

To address these needs, Cadence and Rohde & Schwarz have collaborated to develop and support the integration of signal generation (R&S®WinIQSim2™) and analysis (R&S VSE) software from Rohde & Schwarz with Cadence® Visual System Simulator™ (VSS) system design software that operates within the Cadence AWR Design Environment® platform. The new product, R&S VSESIM-VSS, will be sold and distributed by Rohde & Schwarz, and will allow users to bring signals as used in commercial test instruments into VSS software or drive the VSS output into the test and measurement (T&M) instrument and demodulate the signals successfully. The signals offered in Rohde & Schwarz software products are the same ones used in their instruments, ensuring that simulation signals are fully compliant with standard specifications and constantly up to date. The R&S VSESIM-VSS allows licensing via either a hardware dongle or soft­ware and for signal generation (R&S WinIQSim2) and analysis (R&S VSE), along with add-on installs of RS_SRC as data source and RS_SNK as data sink component (model) blocks with symbols for placement in a VSS schemat­ic testbench (Figure 1).

Figure 1: RS_SRC (data source) and RS_SNK (data sink) shown as the far left and far right component symbols placed in a VSS testbench

Sharing Standard Waveforms Across Simulation and Measurement

VSS software is used to plan and design RF radar and communication systems. While investigating and developing target system characteristics, it enables users to examine the required performance of function blocks or sub-modules, such as power amplifier (PA) stages, filters, or antenna arrays, and determine the impact on the overall RF link performance. Connecting VSS software with the signal creation know-how in the R&S WinIQSIM2 simulation software and the analysis capabilities of R&S VSE software enables a direct link to all different digital systems from 5G to the latest Wi-Fi variants or ultra-wideband (UWB). In addition, using the exact same algorithms in simulation and real hardware measurements mitigates uncertainty when comparing results from both the simulation and test worlds.

R&S VSESIM-VSS brings full access to all standards supported by R&S WinIQSIM2 and R&S VSE for use in VSS software. R&S WinIQSim2 software generates encrypted standards-compliant waveforms that can be replayed in Rohde & Schwarz test instruments. The R&S VSE signal analyzer software for standard waveforms can decrypt R&S WinIQSim2 waveforms and generate standard measurements.

Figure 2: Performance KPIs such as constellation plots, complementary cumulative distribution function (CCDF), and adjacent channel power leakage (ACPL) within the R&S VSE signal analyzer using the results of the VSS simulation testbench

The new R&S VSESIM-VSS product includes a plugin for VSS software. Both R&S WinIQSIM2 and R&S VSE are represented by function blocks in the simulation environment to serve as a data source and data sink in the EDA environment. The function blocks act as interfaces between VSS software and Rohde & Schwarz software tools, thereby connecting the simulation environment with the device under test (DUT) to the actual measurement software and T&M equipment (Figure 2). Once plugged into the AWR Design Environment platform via VSS software, the R&S tools can also be used together with other Cadence products such as Microwave Office® RF/microwave circuit design software for monolithic microwave IC (MMIC) gallium arsenide (GaAs)/gallium nitride (GaN) and PCB-based amplifier design through the circuit/system co-simulation capability in Cadence AWR® software.

VSS software offers capabilities that can be used at various stages of the RF system design flow (Figure 3). To run a system-level performance test, designers can use the RS_ SNK block as a data sink in VSS software to grab the signal anywhere in the signal chain. While pushing the extracted waveform to a vector signal generator such as the R&S SMW200A, users have the connection to a real RF signal going into their already available hardware realization.

Figure 3: The individual RF components of the double up-conversion RF link placed in the VSS testbench driven by the RS_SRC data source

Applying DPD to PA Simulations

In addition to supplying standard-compliant or user-defined signal generation and analysis, R&S VSESIM-VSS brings digital predistortion (DPD) techniques to VSS so designers can predict the performance of nonlinear devices such as power amplifiers with active linearization. Most PAs operate in their nonlinear range in order to deliver the output power required by the transmitter and to maximize the power-added efficiency performance. Unfortunately, PAs driven into deep compression distort the RF signal, as is evident through the spectral regrowth shown in Figure 4. Different PA design strategies can be employed to improve linearity and efficiency performance, including the use of different architectures. VSS software, along with Microwave Office circuit design software, provides designers with the ability to simulate PAs employing different linearization technologies.

Figure 4: VSS simulation of spectral regrowth associated with the nonlinearity of a PA in an RF link excited by signal generated with R&S WinIQSim2 source

In order to maintain signal quality in this operating region, many transmitters employ DPD, a common method used to linearize the output signal of a PA. Implementing real-time DPD in a transmitter is a challenging task that includes RF PA design and modeling that is specific to the signal transmitted. During the design phase of a PA, it is important to quickly analyze the performance of the PA design under DPD conditions to understand possible system-level performance. VSS software offers virtual testbenches for investigating the relationship between the PA performance and the input signals of numerous wireless communication standards provided by R&S VSESIM-VSS.

The R&S FSW-K18D extension to the amplifier test application adds direct DPD functionality to the R&S FSW signal analyzer and is also now available to VSS/Microwave Office PA designs in the EDA environment within R&S VSESIM-VSS. The direct DPD application compares the simulated output of a VSS PA model to the ideal reference signal on a sample-by-sample basis and modifies each sample individually in amplitude and phase to derive the predistorted signal that can be applied to the VSS PA model through the RS_SRC in a VSS testbench, resulting in a highly linearized output signal. VSS software and direct DPD as part of R&S VSESIM-VSS work together to predict what can be achieved in terms of linear performance for an RF amplifier.

Summary

Today’s communication and radar systems achieve their high data rates and range resolution through wide bandwidths and complex modulation waveforms that must be accounted for during simulation to ensure the accurate prediction of component/system RF performance and reduce the number of design iterations. By sharing standard-compliant waveforms between the test instruments used in product validation and the excitation signals used during simulation, designers can eliminate waveform inconsistencies that lead to discrepancies between measured and simulated KPIs. Through integration of the signal generation and analysis software in the Rohde & Schwarz test platform and Cadence’s AWR Design Environment software, developers of RF front-end and system electronics can accelerate their design to implementation turnaround time.

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