Low-powder and High-rate Wireless Transmission Systems for Neural Implants

Book excerpt: Neural recording systems are fundamental to the advancement of brain-machine interfaces that can significantly improve the quality of lives of patients with neurological diseases, such as spinal cord injuries or quadriplegia. This thesis presents two newly developed wireless neural recording systems that are able to provide a high degree of usability and neural decoding accuracy. They are capable of simultaneously transmitting 32 to 96 channels of neural signals detected by an implanted neural sensor array. This work was carried out within the framework of the Hermes project and its technical design challenges will be addressed. The Hermes project is aimed at primarily developing hardware and software tools that extract neural information from the motor cortex. Those tools can enable practical prosthetic devices used to significantly ameliorate the life of patients with neurological impairments that directly affect motor functions. The first developed system, HermesD, is a 32-channel broadband transmission system using an FSK modulated carrier at 24 Mbit/s in the 3.7-4.1 GHz band. The link range extends beyond 20 m and the total power consumption is 142 mW. The HermesD system uses only COTS components and can be easily replicated. HermesD is fully operational and is currently used to transmit broadband neural data for neuroscience research in the Neural Prosthetic Systems Laboratory (NPSL) at Stanford University. HermesD is also planned as the base platform for future human trials to take place in the same laboratory. The second system that represents the next Hermes generation, HermesE, uses a novel UWB transmitter architecture implemented in a custom IC in the 65-nm CMOS technology. The transmitted signal bandwidth covers the 3.6 to 7.5 GHz frequency range. The time domain waveform is digitally programmable, allowing a very flexible control of the output spectrum to avoid interference and to allow multi-band operation. The UWB transmitter chip is part of a 96-channel broadband recording system delivering 40 Mbit/s. Its power consumption is 230 uW for a communication range of about 5 m. The antenna subsystems for these wireless recording devices presented a design challenge given the requirements for small size, large bandwidth and high efficiency. While HermesD has an operating FBW of 10%, HermesE is much more demanding in this respect, with 70% FBW, requiring unconventional antenna structures. The design techniques and performance of the antennas required to meet the specifications of both systems are also addressed in this work.