An asymmetrical pulse shaping technique to combat delay spread

Residual error floors caused by delay spread are reduced with a new pulse-shaping (or filtering) technique described in the present paper. Delay spread, which causes frequency-selective fading, is a major cause of error floors in high-speed digital mobile radio systems. The authors propose a new class of nonlinear-phase intersymbol interference (ISI) free filters that lead to asymmetrical pulse shapes and are more robust against frequency-selective multipath fading. The nonlinear-phase filters do not require redundant signalling bandwidth in comparison to raised-cosine filters which are more robust against delay spread. A piece-wise linear-phase filter with cosine roll-off is designed to demonstrate the robustness of nonlinear-phase filters against delay spread. Coherent quarternary phase-shift keying (QPSK) modems and the two-ray Rayleigh fading model are assumed in the analyses. For the filter roll-off factor α=1, the delay tolerance increases from τ=0.5 T (where τ is the relative delay of two paths and T is the symbol duration) for the raised-cosine filter to 0.65 T for the nonlinear-phase filter if bit error rate floors (BERF)=3% is used as a benchmark and the equal-power two-ray Rayleigh fading model is assumed