Generally, most EEG-based wireless BCI
systems support multi-channel recording. To measure multi-channel signals simultaneously,
a multiplexer is needed to access all of the channels. Because the measured EEG signals
are analog signals, an ADC has to be included to process the recorded EEG data on the digital
circuits. This integrated circuit transforms the EEG analog signals into discrete digitized
data with a specific sampling frequency. The sampling frequency is determined by the
speed of the microprocessor, wireless transmission, and translated frequencies of EEG features.
Formally, researchers and system developers choose the sampling frequency between
about 100 Hz and 1000 Hz. The microprocessor makes data packets from the corrected EEG
data and hands them over to the wireless transmission unit. The microprocessor also manages
the components of the entire system. Some wireless BCI systems load the feature extraction
algorithm on the microprocessor to process the EEG signals internally [19][20][21]
[29][30]. Because the recorded multichannel EEG data is transmitted from the portable EEG
acquisition device to the host system, the wireless transmission unit is essential. Regarding
the protocol of wireless transmissions, various communication modules are employed for
transmission of the measured signals from the signal acquisition unit to the translation unit,
such as Bluetooth and IEEE 802.15.4 Zigbee. Bluetooth has many advantages such as sufficient
transmission rates and wide accessibility. Thus, many wireless BCI systems employ
this transmission module. Including analog front-end and digital system stage, the acquisition
unit of wireless BCI systems generally operates onboard power sources such as Li-ion,
Li-polymer, and NiMH batteries.