%&tex
\chapter{Background}
\label{chap:background}
Engineers have many different types of design methods available in their fields.
Examples of these are Agile, Spiral, V model, and Waterfall.
Each of these design methods start with a need and develop a product to satisfy that need.
From an extremely basic point of view, these methods start with a preliminary design where the need is translated into an initial design, requirements, and specifications.
This initial design is implemented into a product and the product is tested.
The preliminary design is often similar between the different design methods, but the methods differentiate on their implementation and testing phase. 
\textcite{broenink_rapid_2019} do not provide a complete design method and focus on their implementation and testing method.
To create a complete design plan that can be used in the case study, I used the waterfall model in the \ac{se} approach as a basis for the design plan.
The techniques of the \ridm replace the implementation and testing phase of the waterfall model.
This chapter will introduce the basics of \ac{se} and the waterfall model, and analyse what the \ridm provides.

\section{Systems Engineering}
\label{sec:SE}
\begin{marginfigure}
    \centering
    \includegraphics[width=2.9cm]{graphics/waterfall.pdf}
    \caption{Minimalistic implementation of waterfall model.}
    \label{fig:waterfall}
\end{marginfigure}
\textcite{blanchard_systems_2014} describe \ac{se} in their book as: "an interdisciplinary approach and means to enable the realization of successful systems."
Their book extensively covers multiple design methods and design steps in detail.
The simplest of these design method is the waterfall model.
This waterfall model consists of number of steps that are all successively executed as shown in \autoref{fig:waterfall}.
The successive steps make it possible to insert or replace the steps in the design method.

\section{Rapid Iterative Design Method}
\label{sec:RIDM}
The \ridm by \textcite{broenink_rapid_2019} describes a methodology using two core components for the implementation: the rapid development cycle and the variable detail approach.
The design method also describes the preparation steps that are required prior to this implementation.
In short, the preparation prepares a list of features.
These features are implemented one by one with in the rapid development cycle using the variable detail approach.
This section discusses each of these three parts and how they fit in the waterfall model.

\subsection{Rapid Development Cycle} 
\label{sec:background_rdc}
The goal of the rapid development cycle is sequential implementation of the features in a system.
Each iteration of the rapid development incorporates the following steps:
\begin{enumerate}
    \item Create an initial design and corresponding tests for the next feature.
    \item Implement and test that feature.
\end{enumerate}
The first step is to create an initial design and tests that are used to verify the requirements of the current feature.
During the second step, the initial design is developed into a detailed design of the feature.
This detailed design of the feature is develop with the variable detail approach, in which the level of detail is stepwise incremented.
When the second step is completed, the implemented feature contains all the required details and passes all the tests as defined in the first step.
From this point the rapid development cycle is repeated for the next feature, or, when no features are left, finish the development.

\subsection{Variable Detail Approach}
The variable detail approach starts with a low-detailed model and increases the detail discretely over multiple iterations.
The low-detailed model is for example a single transfer function of the system.
In the following iteration, the detail of the model is increased by adding, for example, non-linearity, non-continuity or parasitic elements.
The tests, as specified in the first step of the rapid development cycle, are performed after each addition of detail.
If the tests show that the added detail is not conform the specifications, the added detail is reviewed or redesigned.
When the added detail passes the tests, the process is repeated to add more detail.
The variable detail approach is finished when all the tests are passed and all the detail is added.

\subsection{Preparation}
Although the \ridm does not specify the complete steps for the preparation, it does state some requirements.
The rapid development cycle requires a list of features that can be implemented one by one.
These features are gained by partitioning the functionality of the system, such that features can be implemented and tested individually.
For each feature it is required to specify the feature requirements and the corresponding test protocol.
The feature requirements are based on the system requirements and the tests are used to validate that the feature meets its requirements.
About the order of implementation, the \ridm states that critical features should be implemented first, as these features have an increased chance of invalidating the complete design.
Would such a feature fail, the investment loss is limited, because the development is still in an early stage.

\section{Combination}
\begin{marginfigure}
    \centering
    \includegraphics[width=6cm]{graphics/design_flow.pdf}
    \caption{Combined design plan, where the first three steps are based on the waterfall model and the other four steps are taken from the \ac{ridm} \autocite{broenink_rapid_2019}}
    \label{fig:design_flow}
\end{marginfigure}
As the \ridm integrates the implementation and testing steps together, it replaces these steps in the waterfall model.
The first three steps as seen in \autoref{fig:waterfall} are unchanged and the last two steps are replaced by the \ac{ridm}.
A combined design flow of the both design methods is shown in \autoref{fig:design_flow}.
