Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

160 Implementation Issues4.3.3.2 Robust Nonadaptive DesignA low-complex selection of the filter coefficients is to choose a fixed set of filter coefficientswhich is designed such that a great variety of power density spectra with differentshapes and maximum values is covered [37, 48]. No further adaptation to the time-variantchannel statistics is performed during the estimation process. A reasonable approach is toadapt the filters to uniform power density spectra. By choosing the filter parameter τ filterequal to the maximum expected delay of the channel τ max , the normalized delay powerdensity spectrum used for the filter design is given by⎧⎨ 1|τ| < τ filterρ filter (τ) = τ filter 2 .⎩0 otherwise(4.60)Furthermore, by choosing the filter parameter f D, filter equal to the maximum expectedDoppler frequency of the channel f D,max , the normalized Doppler power density spectrumused for the filter design is⎧⎨ 1|f D |