Challenge:relay selection 上游充通大警 SHANGHAI JIAO TONG UNIVERSITY Minimum-length tree is formulated as the Steiner Tree Problem,NP-hard in graph theory relay addition tree length decrease A 1 B 1 Fig.1 Without relay,tree length is 2 Fig.2 With relay,tree length is V3 10Challenge: relay selection ❑ Minimum-length tree is formulated as the Steiner Tree Problem, NP-hard in graph theory relay addition tree length decrease 10 Fig. 1 Without relay, tree length is 2 Fig. 2 With relay, tree length is 3