Transmitter Precoding for Spread-spectrum Signals in Frequency-selective Fading Channels

Spread-spectrum systems are used to transmit over frequency selective fading channels. The RAKE receiver is a well-established technique to exploit the multipath diversity. However, if the channel impulse response (CIR) is known a priori in the transmitter, the signal can be precoded to increase the SNR at the receiver while the transmit power remains constant. Because the channel can be assumed to be identical in the uplink and downlink in TDD systems the CIR is available in the transmitter. In this paper, different FIR filter precoding schemes for one antenna (Pre-RAKE, Preand Post-RAKE and Eigenprecoder) and multiple transmit antennas are compared, their decision variables are given in vector notation and the SNRs are presented analytically. The performance of the precoding schemes is evaluated numerically for codes with good autocorrelation behavior and for 3GPP TDD codes.