Finite Impulse Response Cyclic Shift Transmit Diversity for Broadband Mobile OFDM

This paper introduces an open-loop transmit diversity method called finite impulse response cyclic shift diversity (FIR-CSD) for broadband mobile OFDM systems. Ordinary CSD operates in OFDM by cyclically shifting the same time- domain signal by different cyclic shift values on each transmit antenna branch. Unfortunately, in some channels (such as a line of sight channel or a heavily spatially correlated channel), ordinary CSD introduces deep spectral nulls in the composite frequency response seen by the receiver, which results in a performance loss relative to the single transmit antenna case. FIR-CSD is an extension of the CSD concept wherein multiple weighted and circular shifted copies of the time-domain signal are combined and transmitted on each antenna. Each transmit antenna essentially uses a different FIR filter that combines multiple weighted and circularly-shifted copies of the time-domain signal to be transmitted. Careful selection of the circular shift values and weighting factors on each transmit antenna can significantly reduce or eliminate the deep spectral nulls that ordinary CSD would produce in the above mentioned channels. This paper shows that FIR-CSD provides a significant performance improvement over the original cyclic shift diversity method, especially in correlated channels and with high-rate coding.