Investigations of the Ambiguity Effect in the Estimation of Doppler Frequency and Directions in Channel Sounding Using Switched Tx and Rx Arrays

The improved-search-and-initialization spacealternating generalized expectation-maximization (ISI-SAGE) algorithm is proposed in [1] and [2] for joint estimation of the relative delay, the direction (i.e. azimuth and co-elevation) of departure, the direction of arrival, the Doppler frequency (DF) as well as the complex weight of the propagation paths from the transmitter (Tx) site to the receiver (Rx) site in mobile radio environments. A recent study [3] shows that the ISI-SAGE algorithm combined with a switched multiple-element Tx and Rx antenna sounding technique makes it possible to estimate the DF with an absolute value up to half the rate with which the pairs of Tx and Rx elements are switched (switching rate), rather than half the rate with which any fixed pair is switched (cycle rate) as commonly believed. This paper shows that so-called modulo-type switching modes (SMs) used with uniform linear and planar arrays lead to an ambiguity in the estimation of the DF and the directions, when the DF estimation range is extended from minus to plus half the switching rate. The SM of a switched array is the order with which the array elements are switched. Moreover, theoretical and experimental investigations show that the ambiguity problem can be avoided by using some specific SMs.