## 2 Related Research
### Technological Distance Measure  
The radical performance of RI is usually the result of introduction of other domain knowledge or technology to solve difficult problems; however, it does not mean that cross-domain knowledge must come from far away [48; 49]. Because even though the learning and recombination of external knowledge will bring engineers and designers more creative inspiration and opportunities, firms may not have the necessary absorptive capacity for learning and mastering to take place [50]. The relationship between technological distance and innovative performance is an inverted U-shaped [51; 52] and has been corroborated by many empirical studies [48; 53]. Intuitively, if the technological distance between two firms is too close together, the high similar knowledge bases will not lead to the generation of novelty value. On the contrary, if the technological distance is too far away from each other, it is difficult for firms to absorb and utilize the cross-domain knowledge to achieve radical ideals [54; 55]. Therefore, a moderate level of technological distance is desirable for firms to use cross-domain knowledge for RCG effectively.  
The measure of technological distance was pioneered by [PERSON] to describe the effect of knowledge spillover [56; 57]. The Euclidean Distance and Cosine Angle are two common measures along with the introduction of patent vector [56; 58], and both take all the patent classes into the calculation according to their own calculation rules [49]. However, [PERSON] and [PERSON] [59] argued that collaboration, technology transfer, knowledge spillover, or R&D competition between a target firm with some empty patent classes and other firms should depend on patent classes in which the target firm has some patent holdings, rather than the other firm allocation of patents in classes that are empty for the target firm. This is particularly important because small innovative firms are often concentrated in a few classes and not likely to have very broad patent layout, whereas technology giants may own dozens of patent classes. Obviously, technological distance is an objective existence and should not be affected by the size of the firm. In order to eliminate the influence of this asymmetry in technological distance measure, [PERSON] and [PERSON] [59] developed a new measure, named minimum complement distance, as an alternative measure of technological distance. When using this method, the share of invention overlap will be subtracted from the whole universe of firm's inventions. The remaining part has a share of non-overlapping inventions, which are interpreted as the technological distance. In the research of innovation collaborations, [PERSON] et al. [49] showed that the use of the minimum complement distance seems to be reasonable due to the intuitive property of the independence of irrelated patent classes in cross-industry collaboration settings.  
Cross-domain knowledge for RCG comes from firms of different sizes firms, either a small enterprise or a technology giant. Technological distance measure can quantitatively identify cross-industry partners and contribute to valuable partnering decisions. Considering that effective quantitative methods are necessary to identify cross-domain knowledge for RCG, we try to integrate the minimum complement distance into the process of the selection systematically to improve the effectiveness of RCG. Simultaneously, we also hope the potential of the measure is fully explored.