MUI-free CDMA systems incorporating space-time coding and channel shortening

In this paper we consider a CDMA system equipped with multiple antenna transceivers to implement space-time block coding (STBC). The codes incorporate a cyclic prefix (CP), to facilitate channel equalization and simplify the rejection of multiuser interference (MUI) in broadband transmissions over frequency-selective channels. The only price paid for the introduction of CP is a rate reduction, depending on the relative length of the CP with respect to the code length. To limit, and possibly avoid, this loss, we propose a CDMA/STBC scheme using CP of length smaller than the channel. To prevent interblock interference which would require a sophisticated decoding procedure, we equip the receiver with a MIMO channel shortening filterbank. We derive the conditions under which we can achieve perfect shortening using an FIR filterbank. Finally, we show that the choice of a CP length represents a trade-off between the rate reduction factor and the SNR loss resulting from the insertion of the channel shortening filter.