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In OFDM system, with fixed cyclic prefix (CP) length, inter block interference (IBI) and inter carrier interference will be present if the channel impulse response (CIR) is larger than the CP length. This problem can be mitigated by design a time domain filter at the receiver, which can shorten or shape the effective CIR to become less than the CP length. This paper proposes a novel method to design a channel shortening prefilter (CSP) for MIMO-OFDM systems, by defining a measurement model which has an underdetermined set of equations. This model is always applicable when the number of transmit antennas (actually transmit streams) is less than the number of receive antennas. The proposed Null-space Exploiting CSP (NE-CSP) removes the IBI and ICI nearly completely, but in the simplest form, has a loss in diversity. It is possible to reclaim this diversity by using the multiple layers (or filters) within the NE-CSP structure. Simulation results indicate that, compared to existing channel truncation methods, the NE-CSP is not only computationally attractive, but can also gives a gain of 3dB to 5dB in bit error rate performance (BER) performance at high SNRs.