# Speech Communication System Based on Piezoelectric Electret Mechanical Antenna
## 2 Materials and Methods
### Speech Communication System Design
#### 2.2.2 Communication Protocol  
The communication protocol refers to the OSI model [26] and adopts a layered design, dividing the mechanical antenna communication protocol into a user layer, a presentation layer, and a physical layer. The protocol stack is shown in Figure 3. The protocol stack is symmetrically organized on both the transmitter and receiver sides, with each layer offering services to its corresponding layer above and below.  
Figure 2: Hardware system of the speech communication system. (**a**) Architecture design. (**b**) A real-time photo of the measurement environment.  
The user layer processes the raw message of transmitting and receiving. Without extra data processing defined by the user, the original message is directly transmitted to the user. As the user chooses to perform data compression, encryption, and other processing at the user layer on the transmitter side, the user layer on the receiver side performs decompression and decryption. Meanwhile, the encrypting key, compression dictionary, and other information involved in these processing tasks are also transmitted by the user layer.  
The presentation layer protocol performs the binary encoding and decoding of data from the upper layer, as shown in Figure 4. The presentation layer is solely responsible for completing the encoding and decoding of the data, independent of the specific data content transmitted by the user layer.  
A valid data frame of the presentation layer protocol is built by the frame header, payload data, and frame tail. The frame header is formed of an excitation signal and a begin flag. The excitation signal consists of eight binary 1s (0 xFF), which are used as an excitation signal to indicate the beginning of signal transmission in a communication channel. The begin flag denotes the first byte of the frame header, which contains seven binary 0s and one binary 1 (0x1). Payload data is represented by a string of binary bits, with each eight-bit group forming one byte. The frame tail, represented by the End-of-File (EOF) marker (0x20), signifies the termination of the message transmission.  
The physical layer protocol is tasked with the actual transmission and reception. Specifically, the physical layer transmits the modulated signals on the transmitter side and demodulates the received signals on the receiver side. This speech communication system employs the Amplitude Shift Keying (ASK) modulation method at a frequency of 167.34 kHz for message transmission. For facilitating the observation of experimental results and compatibility with hardware devices, the frequency of modulating signals is set to 10 Hz, which means each bit of the modulated signal lasts 0.1 s. The binary 1 is modulated as a sine wave at 167.34 kHz, repeated 16,734 times, while the binary 0 is represented by a 0 V signal. At the receiver side, the physical layer receives the signals.  
Figure 4: Format of presentation layer data frame.  
Figure 3: Communication protocol stack.  
Following demodulation, the logic can be judged by comparing the received signal to the reference voltage.