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Vital signs detection by means of several topologies based on Doppler radar are proposed in this paper with the aim of overcoming the problem of null detection. They are based on phase and frequency diversity. The proposed topologies are an alternative to I/Q demodulation and double-sideband transmission. Four topologies are proposed, three of them based on phase diversity and one based on frequency diversity. The first topology is based on using two phase-shifted receivers. The second one is based on a physical antenna displacement between two receivers. A third system with a dynamic phase variation is also presented, which permits to work with only one receiver. The last topology is a radar with frequency diversity within the 2.4 GHz ISM band, which permits a dynamic change of the frequency dynamically in order to place the receiver always in a maximum. A system to empirically characterize nulls is also developed to validate the proposed topologies.