Ultra wideband antenna array processing under spatial aliasing

Given a certain transmission frequency, Shannon spatial sampling limit de¯nes\nan upper bound for the antenna element spacing. Beyond this bound, the exceeded\nambiguity avoids correct estimation of the signal parameters (i.e., array manifold\ncrossing). This spacing limit is inversely proportional to the frequency of transmis-\nsion. Therefore, to meet a wider spectral support, the element spacing should be\ndecreased. However, practical implementations of closely spaced elements result in a\ndetrimental increase in electromagnetic mutual couplings among the sensors. Further-\nmore, decreasing the spacing reduces the array angle resolution. In this dissertation,\nthe problem of Direction of Arrival (DOA) estimation of broadband sources is ad-\ndressed when the element spacing of a Uniform Array Antenna (ULA) is inordinate.\nIt is illustrated that one can resolve the aliasing ambiguity by utilizing the frequency\ndiversity of the broadband sources. An algorithm, based on Maximum Likelihood\nEstimator (MLE), is proposed to estimate the transmitted data signal and the DOA\nof each source. In the sequel, a subspace-based algorithm is developed and the prob-\nlem of order estimation is discussed. The adopted signaling framework assumes a\nsubband hopping transmission in order to resolve the problem of source associations\nand system identi¯cation. The proposed algorithms relax the stringent maximum\nelement-spacing constraint of the arrays pertinent to the upper-bound of frequency\ntransmission and suggest that, under some mild constraints, the element spacing can be conveniently increased. An approximate expression for the estimation error has\nalso been developed to gauge the behavior of the proposed algorithms. Through con-\n¯rmatory simulation, it is shown that the performance gain of the proposed setup\nis potentially signi¯cant, speci¯cally when the transmitters are closely spaced and\nunder low Signal to Noise Ratio (SNR), which makes it applicable to license-free\ncommunication.