# Wireless Control Combining Myoelectric Signal and Human Body Communication for Wearable Robots
## 2 Tx and Rx Module Structure  
The basic structure of the HBC module for wearable robot control has already been reported in [11; 21]. The current version aims for further miniaturization and longer transmission distance. In order to make effective use of the wide bandwidth, we adopted a wide band impulse radio (IR) scheme, which directly transmits pulse signals without using a carrier. Information bits are represented by the temporal position of multiple pulses, called multi-pulse position modulation (MPPM). This modulation scheme contributes to a simple circuit structure and low power consumption. To improve the communication performance for achieving longer transmission distances, IR-MPPM spreads one information bit over multiple pulses. Figure 2 shows the time waveform with a spreading rate of 16, where 16 pulses are used to represent one information bit. This technique can effectively improve the energy per bit to noise power spectral density ratio and bit error rate (BER) performance. By changing the spreading rate, the data rate can be changed according to the communication environment and transmission speed requirements. To demodulate the IP-MPPM signal, we adopted an energy detection method by comparing the total energy of the pulses in the orange period and the blue period, as shown in Figure 3. This method does not require a threshold to determine \"0\" and \"1\".  
Figure 4 shows the block diagram of the Tx module, and Table 2 shows the specifications of the Tx module. First, an analog to digital converter (ADC) digitizes the myoelectric  
Figure 3: How to demodulate information bit by energy detection.  
Figure 2: Conceptual time waveform of IR-MPPM with a spreading rate of 16. (**a**) Information bit “0”. (**b**) Information bit “1”.  
signal from the detector. The modulator then modulates the digitized myoelectric signal by the IR-MPPM. The packet generator generates the packets, and each packet consists of a 4-bit long start bit and a 36-bit long payload. The packet rate is set to 2 kHz, which is the same as the sampling rate of the ADC. The parallel-serial converter outputs pulses at a pulse rate of 20 Mp/s. Since the spreading rate is 16, the data rate will be 1.25 Mb/s. The output pulses pass through the spectrally shaped BPF and are then transmitted from the transmitting electrode.  
Figure 5 shows the block diagram of the Rx module with a physical size of 48 \(\times\) 35 mm\({}^{2}\). The Rx module consists of a BPF, an automatic gain controller (AGC), an ADC, a signal processing module, a data storage, and a digital to analog converter (DAC). First, the received signal is filtered by the BPF and amplified by the AGC to an appropriate level. The ADC then digitizes the amplified signal at a sampling frequency of 200 MHz. The signal processing module extracts the envelope and calculate the peak and energy values. The peak and energy values are used to detect the start bits and demodulate the information bits, respectively. Finally, the demodulated information bits are stored in the data storage, and output from the DAC is taken for use in the wireless control of robot hand.  
Figure 4: Block diagram of the Tx module implemented in a field programmable gate array (FPGA).  
Figure 5: Block diagram of the Rx module.  
\begin{table}
\begin{tabular}{l c} \hline Frequency band & 10–60 MHz \\ Modulation method & Impulse radio-Multi-pulse position modulation \\ Pulse rate & 20 Mp/s \\ Date rate & 1.25 Mb/s \\ Spreading rate & 16 \\ Output power & \(-\)15 dBm/MHz \\ Power supply & 3.3 V/2.7 V \\ Consumption current & 17 mA \\ Physical size & 44 \(\times\) 33 mm\({}^{2}\) (including myoelectric signal detection unit) \\ \hline \end{tabular}
\end{table}
Table 2: Specifications of the Tx module.