Principles of Modern Radar - Volume 2 1891121537

17.6 Multichannel Processing for Detection Performance Improvement 791estimation of the AF in multiple previous time intervals, also frequency offsets and othersynchronization errors are intrinsically removed.The effects of synchronization errors on the different techniques have been analyzedin [54] with reference to synchronization error, carrier frequency offset, and samplingfrequency offset. Specifically, time synchronization errors are shown to yield a strongperformance degradation in terms of PSLR achievable with the nonlinear approaches;the performance degradation starts for time synchronization error lower than the guardinterval duration T G . Similarly, the PSLR obtained with the nonlinear approaches rapidlydecreases also for low frequency offset values; when the frequency offset is equal or higherthan 1/T U (subcarrier spacing) (i.e., integer carrier frequency offset) PSLR decreases toits lower bound, represented by the PSLR of the standard DVB-T AF. In contrast, thePSLR obtained with the linear filter approach is independent of time synchronization andfrequency offset errors. Nevertheless, it shows a significant performance degradation whena sampling frequency offset higher than 3 ppm is present, whereas the nonlinear methodsappear to be more robust. However, this degradation is inessential in most practical casesof interest, because typical sampling error values are lower or equal to ±1 ppm.17.6 MULTICHANNEL PROCESSING FORDETECTION PERFORMANCE IMPROVEMENTIn this section we show that multichannel processing has significant potential to improvePBR performance. Multichannel processing can be obtained by exploiting parallel channelsthat differ by frequency, polarization, time, and space. Although there is not spacehere to comprehensively treat all possibilities, we show how the detection performance ofPBR can be improved by jointly exploiting multiple receiving channels with reference toa selection of three multichannel types for an FM-based PBR:• Multiple frequencies. With reference to multiple radio channels, we explore differentways to combine them and to control the adaptive detection threshold. Performancesare evaluated against live data.• Multiple polarizations. For this case we consider how to exploit polarizations to increasedetection rate. The experimental results are shown again for FM radio-based PBR.• A receiving antenna array. For this case we consider the signals received at the multiplereceiving elements of the antenna array and show the potential approaches to exploitthem to (a) reduce the direct signal contribution and (b) steer the receiving beam on thedesired target region. Some hints are given for the array configuration optimization.17.6.1 Linear–Nonlinear Integration of Multiple Frequency PassiveRadar ChannelsDespite the effectiveness of the processing techniques introduced in the previous sections,the PBR detection and localization performance is time varying and largely depends onthe following:• The instantaneous characteristics of the transmitted waveform• The instantaneous characteristics of the electromagnetic environment (i.e., the radiopropagation channel)