Table 3. The number of the studies for each literature repository and keywords.

Literature repositories	Keywords						Total
	K1	K2	K3	K4	K5	K6
IEEE Xplore	59	29	2	32	6	29	157
ACM digital library	131	112	35	105	58	7	448
Scopus	64	62	3	48	9	2	188
Springer	105	71	25	70	28	36	335
Web of science	39	34	1	27	6	3	110
Science direct	35	30	11	31	15	15	137