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

166 Implementation Issues10 0MSE10 −1Rural area (RA)Typical urban (TU)Hilly terrain (HT)Bad urban (BU)no mismatch10 −210 −30 5 10 15 20 25 30E s /N 0 in dBFigure 4-26MSE for 2 × 1D channel estimation with model mismatch; v = 3km/hIn the second step, the influence of different Doppler power density spectra is considered.Furthermore, the effect when the condition of two-times oversampling is notfulfilled in mobile radio channels with Doppler frequencies larger than 333.3 Hz is investigated.Figure 4-27 shows the mean square error of the presented channel estimation with2 × 1−D filtering and 2 × 5 taps for different velocities v of the terminal station in theCOST 207 channel model HT versus the SNR. As a reference, the mean square errorcurve without model mismatch is given. It can be observed that there is approximately nochange in the mean square error with different velocities v of the mobile station as long astwo-times oversampling of the fading process is guaranteed. For the defined multi-carriersystem, two-times oversampling is given for a velocity of 200 km/h. As soon as the ruleof two-times oversampling is not fulfilled, shown for velocity of v = 250 km/h, the meansquare error increases considerably.Figure 4-28 shows the mean square error obtained with the proposed channel estimationusing 2 × 1−D filtering and 2 × 5 taps when the autocorrelation matrix is optimized foran SNR of 10 dB. During the channel estimation no information about the actual varianceσ 2 is used to adapt the filter coefficients optimally. It can be seen that accurate results areobtained in a span of about 10 dB with mean at 10 dB. For higher SNRs, the mean squareerror obtained with the simplified channel estimation flattens out. Thus, application of thesimplified channel estimation that requires no estimation of σ 2 depends on whether theerror floor is acceptable or not. Otherwise, it depends on the SNR dynamic range at theinput of the receiver whether the simplified channel estimation is applicable or not.