Electronic Mode Stirring for Improved Backscatter Communication Link Margin in a Reverberant Cavity Animal Cage Environment

Neuroscience research in nonhuman primates (NHPs) often requires multiday neural recordings from freely moving animals inside their home cages, making ultralow-power uplinks using wireless backscatter communication highly desirable. Previous work reveals that the channel transfer function (CTF) of a standard NHP home cage in the 915 MHz and 2.4 GHz industrial, scientific, and medical (ISM) bands resembles a resonant cavity exhibiting deep nulls throughout the cage volume, which are particularly acute for round-trip backscatter paths. In this work, we investigate a novel application of passive antenna mode stirring via switched parasitic antennas (SPAs) to reduce the magnitude and prevalence of deep nulls in the cage CTF. We present a system leveraging four cage-ceiling-mounted SPAs with two dynamically controlled impedance states each, yielding 16 total mode stirring configurations. We compare the frequency-domain power ratio measurements at 126 positions throughout the cage taken with and without passive antenna mode stirring. In the 915 MHz ISM band, the optimized SPA configuration improved the maximum two-way insertion loss in 68% of testing locations, reducing the worst case two-way insertion loss by 60.2 dB. In the 2.4 GHz ISM band, the maximum two-way insertion loss was improved in 53% of testing locations, reducing the worst case two-way insertion loss by 35.6 dB. This approach eliminates the deepest nulls in the cage volume and leads to significantly improved link margin for a backscatter-based wireless brain–computer interface (BCI).