Principles of Modern Radar - Volume 2 1891121537

17.5 2D-CCF Sidelobe Control 775superposition of the scattering contributions from the different parts of the body movingin different manners.The same analysis is then repeated by using only the signal received in the surveillancechannel, from which the direct signal is also reconstructed. The corresponding results arereported in Figures 17-16c–d using the recovered version of the transmitted signal obtainedby demodulating and remodulating the surveillance signal. The results obtained withthis approach are largely comparable to those obtained with the reference signal directlycollected at the output of the AP. The different tests performed showed that the detectioncapability of the considered system remains almost unchanged; the few differences arefrom the impossibility of reintroducing the exact behavior of the transmitter and thereceiver, thus including their selectivity and frequency accuracy.The proposed approach has a great interest in a wider range of PBR systems usingdigital waveforms (e.g., DVB-T, DAB, GSM signals). In fact, when the reference signalcan be directly recovered from the signal collected at the surveillance antenna, a dedicatedreceiving channel can be eliminated, thus significantly reducing the system complexity.Nevertheless, when the transmitted signal has to be collected by a directive antenna ora dedicated beam synthesized from an electronically steerable array of antennas to guaranteean acceptable SNR, the proposed approach is still able to significantly improveperformance since it might yield a cleaner version of the reference signal with reducedmultipath or other disturbance contributions. In this regard, this approach might be a successfulstrategy for recovering the transmitted signal in single-frequency networks likethose for DVB-T and DAB broadcast transmissions.17.5 2D-CCF SIDELOBE CONTROLPBR operation inherently implies that the transmitted waveform is not within the controlof the radar designer. This contrasts with the usual case of conventional radar systems,where the transmitted waveform is carefully designed to provide an AF with appropriateproperties (e.g., narrow peak in both range and Doppler and low sidelobes). The implicationsinclude bad behavior for the sidelobes of the 2D-CCF with specific features slightlydifferent in the cases of analog and digital transmissions.When analog transmissions are exploited (e.g., in an FM-based PBR) due to thevariable and unpredictable characteristics of the transmitted FM waveform, the sidelobesof the AF usually have a time-varying structure and exist at a level not greatly lower thanthat of the peak [3, 47]. Thus, target returns are likely to be masked by the high levelsidelobes resulting from the disturbance contributions (direct signal breakthrough andmultipath reflections), even in the presence of a significant range-Doppler separation andproper techniques should be applied to remove such undesired contributions, as previouslydescribed.When digital transmissions are used, the situation becomes even more severe. In fact,the presence of specific periodic or pseudo-periodic features in the modulation format ofthe signal of opportunity leads to a number of undesired peaks in the 2D-CCF. Possibleexamples of such specific features are the chipping sequence in a direct sequence spreadspectrum (DSSS) modulation [16, 18], the repetition of the symbols in the guard intervalsand the presence of pilots subcarriers in an OFDM modulation [6-9], and the multiplerepetitions of preassigned symbol sequences usually exploited to obtain synchronizationin many digital standards. The corresponding sidelobes structures present in the 2D-CCF