In this paper we analyze the performance degradation of a communications system combining the Orthogonal Frequency Division Multiplexing (OFDM) modulation with the simultaneous use of an array of antennas at both the transmitter and the receiver, configuring a Multiple-Input-Multiple-Output (MIMO) channel. The design is based on joint beamforming, where it is assumed that both sides of the communications link have estimates of the channel and the covariance matrices of the noise and the interferences at the receiver. When there is some error in these estimates, a degradation is expected from the optimum case in which the Channel State Information (CSI) is perfect. Here we focus our attention on the case in which the receiver has a perfect knowledge of the channel and covariance matrices, whereas the transmitter has the same information with some error. A theoretical upper bound of the relative Signal-to-Noise and Interference Ratio (SNIR) reduction is deduced. Some simulations results validating the tightness of this upper bound are provided, in addition to showing the performance degradation in terms of the Bit Error Rate(BER).