Channel shortening in OFDM system using differential space-frequency block encoding

Alamouti code with differential encoding has been recently proposed for OFDM systems. It allows full-rate and full-diversity communication and can be decoded with a linear complexity when using two transmit and one receive antennas. However, these advantages assume no interbloc interference (IBI) and no intersymbol interference (ISI) which occur only if the Guard Interval (GI) size is larger than the channel length. In order to relax this condition and to increase the effective flow, we adopt firstly linear channel shortening techniques to reduce the length of the channel at the length of the GI. We show here that this technique is not efficient for a 2 × 1 system and we propose a new non linear method based on the knowledge of the first emitted symbol.