## 2 Literature Review
### Quality Function Deployment  
Quality Function Deployment (QFD), first introduced in 1972 at the Mitsubishi Kobe Shipyard in Japan, is a systematic process of translating customer requirements into product characteristics, design elements, and production details, with the aim of developing products that satisfy customers' needs [39]. The core concept of QFD lies in analyzing and quantifying the relationship between user requirements and design elements; it uses the \"quality house\" to calculate and classify them, which guides product design and development by prioritizing critical requirements [40]. This process effectively improves design quality, shortens engineering cycles, optimizes the design and manufacturing process, and reduces the number of product parts. The key tool of QFD, the \"Quality House\", is used to map the relationships between customer requirements and design elements [41]. This includes user requirements, design elements, the correlation matrix, competitive analysis, and requirement importance ratings [42]. Experts assess the relevance of requirements to design elements through multi-criteria decision analysis and provide a basis for subsequent design decisions [43].  
However, QFD has some limitations, especially in terms of real-time prototype feedback and rapid iteration. Although QFD is capable of comprehensively translating user requirements in the early stages of design, it relies on static requirement inputs and lacks the ability to respond to real-time changes in the design process. In addition, QFD usually relies on expert judgment and lacks automated quantitative support for some complex emotional requirements, which may lead to the neglect or misunderstanding of emotional requirements.  
In the Dual-Path Optimization Model, the combination of QFD with AHP, TRIZ and KE compensates for these shortcomings. First, AHP provides requirement prioritization for QFD, thus ensuring that the design solution addresses user requirements according to priority. By prioritizing emotional and functional requirements through the AHP, QFD translates these requirements into specific design elements and clarifies the relationships between the elements through the Quality House. This integration ensures that the design meets both functional and emotional needs. Second, QFD helps designers identify potential conflicts by creating a mapping diagram between requirements and design elements, providing a basis for TRIZ to resolve technical conflicts and ensuring that the design is always in line with the user's needs while resolving technical conflicts. Finally, QFD ensures that emotional needs are reflected in product design by transforming emotional needs into specific design elements, while KE provides accurate measurements of emotional needs to ensure that customer needs in QFD not only include functional needs, but also reflect the user's emotional demands. In this way, the design not only meets the technical requirements, but also resonates with the user's emotions.  
As one of the core methods of the Dual-Path Optimization Model, QFD can not only systematically analyze and process user requirements but also synergize with AHP, TRIZ, and KE to ensure that the design solution comprehensively and accurately meets user requirements, realizes the organic unity of functionality, emotion, and technology, and enhances the science and accuracy of design decisions, which further promotes the development of the design process. The process is made scientific and accurate, further promoting design innovation and the optimization of the user experience.