Principles of Modern Radar - Volume 2 1891121537

786 CHAPTER 17 Advanced Processing Methods for Passive Bistatic Radar Systemsthe original WiMAX signal with its prefiltered version after the application of the proposedpeaks removal approach for the same case of Figure 17-21. A conventional weighting networkalso has been applied in the frequency domain to improve the PSLR resulting fromthe rectangular shaped power spectral density (PSD) of the OFDM signal. The consideredapproach is able to effectively remove the undesired deterministic peaks due to thepresence of guard interval and pilot subcarriers in the OFDM symbols, with only a verylimited SNR loss.The side peaks in the Doppler dimension resulting from the temporal separation betweenthe integrated pulses (i.e., a low equivalent PRF) cannot be counteracted and yieldDoppler ambiguities. Such ambiguities become less severe when decreasing the frameduration as foreseen for WiMAX. However, after applying the proposed filter, only thezero-delay peaks at each ambiguous Doppler frequency survive, whereas the side peakset previously present at positive delays is correctly removed. The reported results referto particular selections of the WiMAX standard parameters according to the availabletechnical literature; since the WiMAX standard definition has not been completed yet,additional transmission modes could be designed in the future requiring a dedicated analysis.However, the considered analysis approach and the proposed filtering strategy willbe still valid for any transmission mode exploiting an OFDM modulation. As an example,the application of this approach to WiFi signals transmitted using an OFDM scheme ispresented in [21]. In the next subsection its application to DVB-T signals is considered.Finally, when considering practical applications of a WiMAX-based PBR, additionalissues should be taken in account. Since a dynamic bandwidth allocation is adopted, accordingto the traffic in the network, a corresponding performance degradation should beexpected at least in terms of range resolution. Moreover, a base station might be transmittinga great variety of signals in FUSC, PUSC, or optional modalities; consequently,average AF characteristic should be expected among the different transmission modes consideredin this subsection, and proper processing strategies should be designed to correctlymanage mixed transmissions.17.5.3 2D-CCF Shape Control for DVB-T SignalsWith reference to long-range passive radar surveillance, the linear filter for sidelobe reductionpresented in the previous subsection is now applied to DVB OFDM signals.Alternative nonlinear approaches are also described to cope with the masking effect dueto undesired peaks appearing in the AF of such signals.The DVB-T standard is proliferating and rapidly replacing its analog counterpart[53]. Many countries have already switched or planned to switch to the DVB-T standard.A number of studies have looked at the possibility of using the DVB-T transmissionsas waveform of opportunity for PBR systems [6–10]. These signals show both goodcoverage and wide frequency bandwidth and result in a reasonably long-range surveillancecapability with remarkable range resolution.The DVB-T signal exploits an OFDM modulation whose main parameters are listed inTable17-7 for typical 8 MHz channels, distinguishing between the 2k mode and 8k modespecifications. The data stream is modulated over a number of evenly spaced orthogonalsubcarrier frequencies, contemporaneously transmitted for a time interval equal to T U .A guard interval of duration T G is inserted by cyclically extending each OFDM symbolso the total symbol duration is equal to T S = T U + T G . The subcarriers are separated byf = 1/T U and the number of sub-carriers depends upon the considered mode. However,