## 2 Literature Review  
### Technological Fields and Spatial Features  
Patents as standardized data sources about technology have so far primarily applied across macro territories, often at the national level [17; 29; 30; 31; 32; 33; 34]. In addition, despite the wide use of patents as a proxy to measure technological innovation [35; 36], researchers recently argued that to simply equate the number of patents to innovative capability should be very cautious [37]. China is probably at the forefront of this controversy, as many of its patent applications are discarded during or after disposal or classified as a lower quality \"utility model\" or \"design\" instead of \"invention\" [6; 28]. Under the regulation of CNIPA, only those passed through substantive examination, during publication or obtaining grants, are valid patents (Figure 1). A utility model offers protection of \"minor inventions\" through a system similar to a patent but is overall cheaper to obtain or maintain, thus is often viewed as second-class patents [38; 39]. Earlier, in April 1985 when the Patent Law of the People's Republic of China was in force, utility model, design, and invention were all defined as patents, but this was not the case in the United States or United Kingdom. Even considering patent inventions alone, researchers have identified a skewed distribution in terms of their value and quality: a great portion are useless, a few have certain value, an extreme few are breakthroughs [40], and less than half have entered the market [41]. It has been noted that the patent type, e.g., the technological field in which the patent belongs, may play a role for innovation quality [40]. Normally, the more advanced the technology is, the more concentrated the patents produced [16], and the more likely the breakthroughs will happen. Knowledge spillover effects may well suggest that these concentrated, advanced technologies are highly inclined to form an innovation chain with other technologies, whereby the synergetic effects induce increasing colocation knowledge production [42]. To identify the concentration (usually measured by Gini coefficients) and colocation (can be measured by principal component analysis), the degree of patent production in line with their technological fields thus helps to unfold the innovative potential of a city-region. The patent classification system aids in this regard. The commonly used one by United States Patent and Trademark Office is the United States Patent Classification (USPC) system, which was first developed in 1900. Instead, CNIPA adopts the International Patent Classification (IPC) scheme established by the 1971 Strasbourg Agreement, which has been used by patent offices across over 100 countries. In 2010, an agreement between the USPTO and European Patent Office brought about the Cooperative Patent Classification system as harmonization. The system has a similar structure to IPC classification, thus resolved the inconsistency between major classification systems [11]. In this research, the four-digit IPC code proposed in the WIPO IPC-Technology Concordance Table will be applied to sort different technological fields. Figure 1: Procedures of patent applications under Chinese National Intellectual Property Administration. Source: author. Extant research using the IPC code to examine technological heterogeneity and their location features is primarily conducted at the national level. [PERSON] [16] and [PERSON], [PERSON] [17] find that in U.S., the most spatially concentrated technology fields are those advanced high-tech sectors, normally related to computer, biotechnology, and semiconductors, etc. Concentration implies internal flows of knowledge within the technology and different technology often concentrated in different cities.