1. A process for selecting a Precoding Matrix Index (PMI) in a Multiple In Multiple Out (MIMO) receiver used in a wireless communications system comprising a base station communicating with User Equipments through a downlink and uplink channel, said base station applying a precoding on a transmit symbol vector based on a matrix W i being selected from a set of predefined matrices and identified by a PMI index computed by said UE and forwarded to said base station via said uplink, said process comprising: estimating a MIMO channel matrix H of a given set of resources blocks comprising received symbol vectors; computing for each particular matrix W i comprised within said set of predefined matrices a value of a cost function F i (W i , H) representative of an orthogonality of said matrix MIMO channel matrix H; comparing computed values of said cost function F i ; and transmitting to said base station an index i of one of the predefined matrices maximizing or minimizing said F i cost function.