Image Processing Reference
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state every time is excited by the inputs. However, this is not always guaranteed so
in this case the system settles in a metastable state, affecting the functionality of the
design. This problem can be solved by adiabatic switching [44].
Experiments have shown that QCA cells created by using metallic dots operate at
cryogenic temperatures. By contrast, other realizations of QCA cells by using either
single molecules or nano-magnets can be more promising for room temperature
operation [18, 20, 26, 27, 30].
4.4
Methodology
The methodology that should be followed for the implementation of a new circuit,
consists of three basic steps: Analysis, Circuit Design and Simulation. These steps
are described in the following paragraphs.
4.4.1
Analysis
During the step of Analysis, any algorithm should be split into smaller procedures,
taking into account the advantages and the requirements of the QCA technology. It
is well known that an algorithm can be implemented in several ways. Usually, the
better implementation is the one that minimizes the computational time and/or the
area covered by the QCA circuit.
The research should focus on analyzing an algorithm to provide easier and better
QCA design in terms of space, stability and time. This can be achieved when the
advantages of QCA are taking into account during the analysis step. For example an
advantage is the fact that some operations such as the majority gate are easily imple-
mented by using QCA. So, the majority gate is a module that could be extensively
used for the analysis of several algorithms.
4.4.2
Circuit Design
The circuit design step is the major part of the research work. Designing with QCA
is a very interesting but challenging work for the following reasons. Theoretically,
all Boolean logic functions can be implemented on a QCA combinational circuit,
by properly placing QCA cells so that their coulombic interactions produce the re-
quested output [61]. But, in QCA circuit design it is very important to produce a
circuit characterized by advanced stability [2]. Many times small QCA designs suc-
ceed in simulation when they are not connected with other designs, but fail when
they are used as parts of a larger design. In other cases the designers tweak simula-
tion model parameters in order to manage their circuits to succeed in simulation. As
a result many QCA designs are not robust and consequently not operational.
There are some design concerns that should be kept into account to increase
the robustness of the QCA circuits [8, 22]. When generating QCA circuit designs,
a significant effort should be made to keep the length of a wire within a given
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