# A Wireless EEG System for Neurofeedback Training
## 2 Related Work  
### Real-Time EEG Acquisition Platforms  
There are several commercial electronic components that are used for noninvasive monitoring of brain waves (EEG devices) [20]. The creation of a new high-precision analog front-end design (AFE) for discrediting the analog EEG signal is still under experimentation [18]. Prototype systems for long-term continuous EEG monitoring of epileptic patients are based on wearable modules with battery-powered AFEs (ADS1299) with a sampling rate Fs of 250 ksps (up to 1 kbps), a 1 GHz microprocessor (DM3730) (ARM-cortex-A8), 32 active dry electrodes, and wireless data transmission (802.11 b/g) to a host computer [1; 16] (Table 1). The problems of this prototype were the maximum battery life and long cables due to the non-optimal system size [1]. Another portable EEG system [22] used for speller application on locked-in syndrome patients also integrated ADS1299 AFEs to support 16 independent channels (Fs, 250 Hz). This EEGu2 system had a more powerful processing element: BeagleBone Black with a 1 GHz processor (AM358) (ARM Cortex-A8). The sufficient processing capability of data with high accuracy (25 dB SNR) and long life operation of the battery (up to 12 h) enable rapid embedded deployment of the speller application. This prototype needs its real-time response, total required power, size, and cost improved (Table 1).  
\begin{table}
\begin{tabular}{l l l l l l l l l l} \hline \hline  
In order to protect the system from various AC and other noise sources, the EEG is transmitted to the host computer via a wireless connection. Wireless data transfer replaces long cables to electrodes and limits cable wobble, which introduces signal noise and artifacts [1]. Other systems based on a single ADS1299 have different microcontrollers such as SAM G55 [17] for continuous patient monitoring with schizophrenia, and STM32F4 [9] for different brain-computer applications, sending data over Bluetooth. The Bluetooth transfer of the EEG data due to the low power device consumption has bottlenecks, such as a shorter connection range and low data rates at higher EEG channel numbers or higher sample rates. In the EEG system [15], the bottleneck is changed by higher-bandwidth Wi-Fi 802.11 communication technology [15], but the limiting factor is the speed of the microcontroller (MCU). Other decisions include the use of two communication components: (1) a Bluetooth mini module HM-11 BLE 4.0 and (2) a Wi-Fi module ESP8266-12E through universal asynchronous receiver-transmitter and serial peripheral interfaces (SPIs) [19; 21]. A wireless communication protocol based on SPI was also implemented in 16-channel EEG systems with ADS1298 AFEs and a 16 MHz Atmega2560 MCU [19], as well as ADS1299 and an 84 MHz ATSAM3X8E MCU (based on the ARM Cortex-M3 processor) [23]. In these EEG systems, the rejection ratios of the common mode (CMRR) for signals up to 70 Hz were \(-\)97 dB for ADS1298 [19] and \(-\)110 dB for ADS1299 [23]. Since the high channel number usually implies a low sampling rate under the available transmission bandwidth, these systems can rarely work simultaneously with high channel numbers and high sampling frequencies (Table 1).