  In bi-static Ambient Backscatter Communications (AmBC) systems, the direct
path from the ambient source to the receiver can be several orders of magnitude
stronger than the scattered path modulated by the AmBC device. Because of the
large power difference between these two signals, the receiver needs to operate
at a large dynamic range. In this paper, we propose a novel analog-digital
hybrid null-steering beamformer which allows the backscatter receiver to detect
and decode the weak AmBC-modulated signal buried in the strong direct path
signals and the noise without requiring the instantaneous channel state
information. The analog cancellation of the strong signal components allow the
receiver automatic gain control to adjust to the level of the weak AmBC
signals. This hence allows common analog-to-digital converters to be used for
sampling the signal. After cancelling the strong components, the ambient source
signal appears as zero mean fast fading from the AmBC system point of view. We
use the direct path signal component to track the phase of the unknown ambient
signal. In order to avoid channel estimation, we propose AmBC to use orthogonal
channelization codes.
