International Core Journal of Engineering 2020-26 | Page 116

A. The synchronizing signal acquisition The tags response information of reader is 75­s after the reader send out instruction, and the synchronizing signaling acquisition is performed before and after 75­s after the instruction, so 70̚80­s is appropriately taken as the region of search. Perform related operations within this region and find out the correlation peak at the end of region. If the largest value is large than threshold decision, the location succeeds. The location of correlation peak occurs is the starting point of tag response data. The correlation peak possibly occurs at any point, so the determination of correlation peak is required to be set at the end of region. For instance, at the end of region, correlation peak occurs at 72­ s after comparison, when response data shall begin despreading from 72­s. However, the data at 72̚80­s has been lost, thus, the response information within this region shall be prestored. (a) The correlation of 31-bit M-sequence B. The despreading realized by parallel adder The spread spectrum data received by reader is stored in shift register and updated when each clock signal arrives. When the code element of local M-sequence is “0”, the taken value of “1” is multiplied by corresponding code element of tag data; when the code element of local M-sequence is “1”, the taken value of “-1” is multiplied by corresponding code element of tag data. The algorithm is realized by parallel adder. The value setting of each code element is the coefficient of adder, as shown in Figure 5. The summation results will be sent for threshold decision, and if it is more than the set threshold value, the data is the spreading channel data of M-sequence. On the contrary, this data is not this channel data. (b) The correlation of 15-bit M-sequence Fig. 3. The correlation of M-sequence III. T HE M ULTI - CHANNEL I NFORMATION BY R EADER In communication of reader and tag, when various tags receive the instruction of reader, tags randomly select one M- sequence as their own response orthogonal sequence, and send back to reader after spread spectrum operation by tag code data. When multiple tags response simultaneously, there is aliasing in data. When reader receives data, it performs sampling first, and perform related operations with M-sequences in all channels for despreading (Fig. 4). Related operations shall be conducted on tag data for n times, and n is the set number of all channels [10]. According to Figure 3, when tag responds in certain channel, the value of related operation results will be relatively larger [11], and decorrelation will succeed. On the contrary, when there is no response by tag in this channel, the correlation between the sequence in this channel and tag data is low. The correlation value fails to reach the set threshold value, and decorrelation fails. Fig. 5. The parallel despreading by reader IV. C ONCLUSION In this paper, M-sequence is taken as orthogonal sequence and introduced to RFID system, which thus becomes the CDMA system. Multiple tags are accessed to down link, so as to respond simultaneously, reduce the collision and improve reading efficiency. The characteristics of better orthogonality and efficient number of orthogonal codes make shift M-sequence become the optimal choice of this scheme. If this technology is widely applied, the tag collision problem in traditional RFID system can be alleviated to great extent, bringing new research direction for the development of traditional RFID. In practical industry application, more studies and demonstration are required to promote this concept to further drive the development of this Fig. 4. Despreading of multi-channel tag data by reader 94