Integration of Circuit and System Analysis for 802.11 Applications

Steve MaasApplied Wave Research, Inc.

1960 E. Grand Ave., Suite 430, El Segundo, CA 90245

Software architecture

Circuit simulation

How software architecture affects circuit simulation

Introduction

A component of the Microwave Office Suite

Acquired from IQCom, known as Accolade

Digital, time-domain, multirate simulator

Operates on both real and complex signal representations

Large model set; additional third-party libraries

Library for 802.11 applications

Visual System Simulator (VSS)

Tightly integrated design system

Simulators include linear, electromagnetic, harmonic balance, system.

Capabilities include full linear, nonlinear, system simulation and layout.

AWR Design Environment

Object oriented

Simulator objects and data objects can be shared throughout

COM based

Models and simulators connect through COM interfaces

Single-database architecture

Architecture

“Clean slate” design: no legacy software to accommodate.

Software architecture is fundamental to the system; “equal billing” with the computational capabilities.

The system is strongly based on three concepts:

1. Object-oriented design2. Single-database architecture

3. COM technology

Architectural Issues

Necessary for handling complexity.

Allows extraordinary versatility in large software systems.

Software consists of self-contained objects that initialize and destroy themselves, contain all necessary data, include functions to manipulate those data.

Object concept is broadly applicable

Simulators

Data structures

Optimizers,....

Essential for single-database architecture.

Object-Oriented Design

Simulators are objects, which are callable from other objects.

A simulator can call another simulator

Linear simulator can call a nonlinear simulator for transient data.

Nonlinear simulator can call the linear simulator to provide linear-subcircuit characterization

System simulator can call a linear or nonlinear circuit simulator to evaluate a block.

A data set can call a simulator to manipulate its data, or increase the size of the data set.

Example: S parameters can use a simulator for interpolation.

Circuit elements can call simulators to provide necessary parameters

Example: EM based microstrip models

Simulators as Objects

Use of a single database obviates the need for synchronization between modules

All input and output data are available to all simulators.

Functional components (e.g., layout, schematic are different views of the same data.

Single-Database Architecture

Occasionally simulation blocks need data that is not part of the basic simulation.

Example

System simulator operates on the complex envelope, which is independent of carrier frequency.

System block, such as an amplifier or S parameters, must know the center frequency and bandwidth in order to provide the correct data to VSS.

Parameter propagation

Sets of data are passed forward and backward along a cascade, and intelligently through more complex structures (e.g., feedback).

Data can be anything: center frequency, VSWR, data rates...

Parameter Propagation

Calculation of Mismatch Effects

Blocks in system simulators are typically somewhat idealized.

Property propagation can be used to model mismatch effects.

Input reflection coefficient is

Output reflection coefficient is

Start at the first stage, calculate Γout, add it to the property set, and continue down the cascade to the output.

Repeat the process, calculating Γin, starting at the output and continuing to the input.

Γin S11S21S12

1 S22ΓL–--------------------------+=

Γout S22S21S12

1 S11Γs–--------------------------+=

Mismatch Effects

At each block, we use a corrected gain, S21c:

S21c is modeled in the time domain by means of a finite impulse response (FIR) filter.

The filter’s coefficients are computed from the complex frequency-dependent mismatch loss.

We are also investigating the use of an infinite impulse response (IIR) filter or other digital signal processing techniques for the block gain correction.

S21cb2a1------

S211 S22ΓL–--------------------------= =

Microsoft calls this COM: Component Object Model.

Used extensively in Windows, Microsoft Office suite.

A standard for interfacing pieces of software on a binary level by defining their interfaces.

Language independent; can work over a network.

Allows individual developers to develop compatible products. Need know only the interfaces.

Simulator can consist of individual, user-selected COM objects from various sources, all tightly integrated.

Component Architecture

All element libraries connect to simulators through COM interfaces.

Separates the functions of modeling and simulation.

Allows model sets to be added, modified, or updated without changing the simulator.

Models reside in DLLs, which can be replaced independently of the simulator.

802.11 library was originally developed as a third-party library.

COM-Based Model Libraries

The nonlinear system block receives necessary data via parameter propagation

The NL block calls the HB simulator, which calls the linear simulator in turn, to obtain Pin/Pout and phase information.

Interpolation, generation of model characteristics is performed within the system block model.

How It Works...

NL Block

HB Sim

Circuit

Linear Sim

System Blk System Blk

Data

The nonlinear block is modeled in the system simulator by an AM-AM/AM-PM model.

One, two, or three-block models are also available

Fitting to characteristics calculated by HB is done behind the scenes; no user intervention.

Spline fitting is used by default.

Object dependency hierarchy determines whether the circuit needs to be reanalyzed; unnecessary reanalysis is avoided.

NL Block Model

AM-AMAM-PMH1(ω) H2(ω)

“Top-Down” Design

Avoid bad surprises late in the system-design process.

Methodology:

1. Create the system in a system simulator.

2. Generate specifications.

3. As components are designed, replace the system-simulator blocks with blocks representing the component’s predicted performance.

4. When the components are fabricated, replace the calculated performance, in the system simulator, with measured.

Effect of each component on the system can be assessed continuously

Requires good behavioral models, system simulator that includes all relevant effects

Wireless LAN standard; virtually identical to 802.11g, except operates at 5 GHz instead of 2.4 GHz.

Based on orthogonal frequency-division multiplexing (OFDM).

High tolerance for interference and multipath effects.

OFDM signal has high peak/average ratio; AM distortion is a great concern.

Nonlinear behavioral models that reproduce only gross characteristics (IP, P1dB, ...) are not adequate.

802.11a

802.11a System

Amplifier Implementation

Receiver Amplifier

Power Amplifier Model

LNA Model

Conclusions

System simulators can include

Tight integration with circuit simulation

Circuit-like capabilities, such as full S parameter characterization of linear blocks.

Software architecture design is critical to its scientific functionality.

The gap between system and circuit simulation is narrowing.