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
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