NEW : Liang : Relationship has opportunities and challenges . The trip is Gates ' second to China since he became defense secretary . The relationship between the two sides suffered a setback last year . Comment:
NEW : Liang : Relationship has opportunities and challenges . The trip is Gates ' second to China since he became defense secretary . The relationship between the two sides suffered a setback last year . Gates has said he wants the two nations to hold more high-level visits .