Principles of Modern Radar - Volume 2 1891121537

770 CHAPTER 17 Advanced Processing Methods for Passive Bistatic Radar Systems0+ ++w 0,0 × w 0,1 × w 0,2 × w 0,M−1 ×TTTs ref,0 [k] s ref,0 [k − 1] s ref,0 [k − 2] s ref,0 [k − M + 1]1+ ++w 1,0 × w 1,1 × w 1,2 ×w 1,M-1 ×s ref,1 [k]Ts ref,1 [k − 1]Ts ref,1 [k − 2]Ts ref,1 [k − M + 1]+y[k]N − 1+ ++w N−1,0 × w N−1,1 × w N−1,2 × w N−1,M−1 ×TTTs ref,N−1 [k] s ref,N−1 [k − 1] s ref,N−1 [k − 2] s ref,N−1 [k − M + 1]FIGURE 17-14Sketch of the space-time CMA approach.filter taps, a fully ST-CMA is obtained that is expected to operate as a combination of theprevious two versions (T-CMA and S-CMA) by contemporaneously using the temporaland spatial DOF.An extensive performance comparison among the different monodimensional CMAversions shows that both approaches allow effective equalization of the reference signal ina number of different multipath scenarios [41]. However, both approaches also have beenshown to yield significant loss in particular situations. Specifically, the T-CMA yieldsgood performance for a limited multipath replica’s delay, while the S-CMA yields aneffective equalization only when the multipath echo DOA is far enough from the directsignal’s DOA. Besides these considerations, the performance of the different versions ofthe CMA for multiple multipath replicas affecting the reference signal strongly dependson the particular phase shift combination between the replicas. The performance of themonodimensional CMA versions might be slightly improved increasing the number ofDOF in the temporal and spatial dimension, respectively; however, this is paid in termsof computational load and system complexity. Nevertheless, a residual loss still mightbe experienced in certain scenarios even if significantly increasing the number of DOF.This is especially evident when a certain degree of a priori knowledge on the multipathscenario is not available or is time varying. It is then apparent that the joint exploitation ofthe temporal and spatial DOF can be of great potential interest for multipath removal onthe reference signal exploited by a passive radar. The analysis demonstrated that in mostcases the ST-CMA correctly equalizes the reference signal, thus yielding a pure referencesignal to be used for the following cancellation stage; moreover, thanks to the advantageof a space-time processing, the ST-CMA has been shown to be robust even when operatingwith a small number of antennas.As an example, Table 17-4 reports the cancellation loss obtained after the applicationof all the considered CMA versions in different multipath scenarios (T-CMA with L = 100,S-CMA with M = 4, and ST-CMA with L = 100 and M = 4). Also the CA loss obtainedwithout any multipath cancellation algorithm is shown for comparison. Specifically, two