# A Study on Design Requirement Development and Satisfaction for Future Virtual World Systems
## 1 Introduction  
Virtual worlds, including game worlds and social worlds, are synthetic, persistent, and immersive environments, facilitated by computer networks connecting multiple users, in the form of avatars, who interact in (near) real-time [1, 2]. Virtual worlds have been evolving from text-based games (e.g., early multiuser dungeon games which only support text-based user interaction) to highly immersive three-dimensional graphic environments [3]. In the evolution of their forms and contents, virtual worlds have developed some important features, including interactivity, immersion, presence, persistency, embodiment, shared temporality, and spatiality, etc. [2]. Supported by these features, the growth of virtual worlds has brought the participation of millions of people and innovations in many fields, including education [4], medicine [5], tourism [6], commerce [7], and entertainment [8]. Like the realworld, virtual worlds often consist of economic systems and social systems [9]. These systems can increasingly influence the real-world economy and society, for instance, through the approaches of real-money trade of virtual assets [10] and personal identity online representation [11]. The evolution of virtual worlds is closely related to their underlying technologies, which have a wide range over the whole information and communications technology (ICT) sector. For example, hardware and network platforms provide the infrastructure for running virtual world systems [2]. Application software provides the content for fulfilling user's business purpose [12]. Virtual reality techniques promote a sense of presence in a virtual environment [13]. Peripherals and social connection enrich user and in-world interactions [3]. Artificial intelligence (AI) can make a synthetic world and its components more autonomous [14]. The evolution of the underlying technologies spirally drives the change of virtual worlds and brings them new features. The emerging technologies, such as mobile computing, user content generation, wearable devices, virtual reality, and machine learning, can advance virtual world system development, improving virtual world accessibility, diversity, interactivity, and autonomy. Thus, to accommodate technology innovations, it is important to study the changing requirements and quest for a more reasonable architectural design for future virtual worlds. This paper aims at developing future virtual world system design requirements and addressing the requirement satisfaction issue. It first discusses how virtual worlds are developing and evolving by studying the trends of related technologies with industrial and market data for eliciting new system architectural design requirements, which complement existing requirements of consistency [15], scalability [16], and security [17]. Following the trends and their implications (i.e., the increase of client mobility, computation complexity, content creation, and data generation), new features of virtual worlds are suggested and summarized as emerging design requirements: sufficiency, reliability, persistency, and credibility, by synthesizing the implications to system architectural design. Then, the existing virtual world architectures are scrutinized based on the emerging requirements by qualitatively studying their solutions to the design issues. By classifying and comparing them in our proposed requirement satisfaction, we find that none of the existing system architectures can fully satisfy all requirements due to their intrinsic limitations, including a single point of failure for (partial) centralized architectures and instability for (partial) decentralized architectures. To better satisfy the new requirements, we introduce an ongoing research, a new distributed architecture, which leverages the advantages of existing architectures and compares it with other architectures at the end for eliciting new discussions on virtual world system design and development. This paper has made three contributions.