A 25 Mbps, 12.4 pJ/b DQPSK Backscatter Data Uplink for the NeuroDisc Brain-Computer Interface

Wireless brain-computer interfaces (BCIs) are used to study neural activity in freely moving non-human primates (NHPs). However, the high energy consumption of conventional active radios is proving to be an obstacle as research drives for wireless BCIs that can provide continuous high-rate data uplinks for longer durations (i.e. multiple days). We present a differential quadrature phase shift keying (DQPSK) backscatter uplink for the NeuroDisc BCI as an alternative to active radios. The uplink achieves a 25Mbps throughput while operating in the 915MHz industrial, scientific, and medical (ISM) band. The DQPSK backscatter modulator was measured to have an error-vector magnitude (EVM) of 9.7 and a measured power consumption of 309$\mu$W during continuous, full-rate transmissions, yielding an analog communication efficiency of 12.4pJ/bit. The NeuroDisc is capable of recording 16 channels of neural data with 16-bit resolution at up to 20kSps per channel with a measured input-referred noise of 2.35$\mu$V. In previous work, we demonstrated the DQPSK backscatter uplink, but bandwidth constraints in the signal chain limited the uplink rate to 6.25Mbps and the neural sampling rate to 5kSps. This work provides new innovations to increase the bandwidth of the system, including an ultra-high frequency (UHF) antenna design with a -10dB return loss bandwidth of 12.5MHz and a full-duplex receiver with an average self-jammer cancellation of 89dB. We present end-to-end characterization of the NeuroDisc and validate the backscatter uplink using pre-recorded neural data as well as in vivo recordings from a pigtail macaque.