All-Pass Pre-Filtering in Pre-Post-Rake Schemes for Spread-Spectrum Communications

Pre-Post-Rake is a known technique in spread-spectrum communications that uses one rake filter at the transmitter side (to partially precompensate for the wireless channel dispersion) and one rake filter at the receiver side (matched to the entire transmitter plus channel chain). A novel scheme based on transmitter all-pass filtering has been employed by the authors to enhance the performance in the downlink of direct sequence code division multiple access (DS/CDMA) system. In this correspondence, we explain by theoretical analysis why the structure performs well. In particular, we show that the interference can be reduced by all-pass prefiltering. In fact, it basically acts as a (phase-only) preequalizer of the multipath channel that combats intersymbol interference. On the other hand, it better preserves the orthogonality between near orthogonal pseudorandom codes in multiuser synchronous DS/CDMA environments. The performance is analyzed for wireless multipath channels, simulated by realistic models, to evidence its gain (in terms of dispersion index) compared to conventional pre-post-rake.