The basic of multiple hops MIMO relay system is described from ideal channel models to the real channel model, from one-way transmissions to two-ways transmissions. Both amplify-and-forward and decode-and-forward schemes are respectively analyzed based on MAC layer, PHY layer and then MAC-PHY cross layer. The idea and the mathematical derivation are depicted clearly. Additionally, conventional schemes, e.g. CSMA/CA, network coding, cooperative relaying and so on, are introduced and compared to the proposed MAC-PHY cross layer.