International Core Journal of Engineering 2020-26 | Page 115

A. The application of spread spectrum technology in RFID
system
With the spread spectrum technology applied in RFID
system, traditional RFID access method is transformed to
multichannel orthogonal access, so that the access capability
of RFID air interface is greatly enhanced, thus, not the
bottleneck of system function anymore. Different orthogonal
sequences (spreading code) are applied to realize the
multiple accesses, so that information of various tags can be
communicated simultaneously. The information is multiplied
by spreading code sequence to realize spread spectrum, so
that each orthogonal spread spectrum sequence can provide
one channel. In reading signaling procedure, an orthogonal
sequence is selected randomly by tags for communicating
information after spread spectrum. When reader receives tag
information, the same orthogonal sequence pre-stored is
applied to dispread recovery data. The orthogonality of
spreading code allows multiple tags to work simultaneously
on the same carrier. The signaling procedure of its air
interference is shown in Figure 1. Compared to traditional
single-channel RFID, multi-channel access can reduce the
collision rate and pressure of collision decision in
communication protocol, and alleviate the tag response
collision to certain extent.
Fig. 2. The generator of 15-bit M-sequence
The circuit is to generate 31-bit orthogonal sequence.
According to its shift characteristic, the sequence after each
shift is orthogonal with original sequence, thus, 31-bit
orthogonal sequence can be obtained and 31 orthogonal
channels can be formed [8]. For example, one of sequence of
is
the
primitive
polynomial
G(x)=x 5 +x 2 +1
1001000010101110110001111100101, and other sequences
can be obtained after shift. The sequence family of same
primitive polynomial is conventionally used by the tag and
reader. After the instruction sent by reader, one M-sequence
is randomly selected by the tag from M-sequence generator
to establish orthogonal response channel.
D. The orthogonality of shift M-sequence
The correlation function of shift M-sequence is shown in
Equation (1).
1,
R(j) = ∑
φ(x )φ(x
) =
j=0
− , j≠0
(1)
Fig. 1. The signaling procedure of RFID with orthogonal spread spectrum
multi-channel access Wherein, p is the length of sequence, x i and x i+j are two
code elements at different phases. The values are taken as 0
and 1, φ(0) = 1, φ(1) = −1 . It can be seen that its
correlation depends on the difference value of the number of
same code element and different code elements [8]. The M-
sequence family generated from same primitive polynomial
is ergodic and balanced. According to additive and balanced
shift, bitewise XOR any two sequences and can still obtain
M-sequence, whereas, the difference in the number of “0”
and “1” in XOR result is 1.
B. Orthogonal sequence - M-sequence
M-sequence, the most basic PN sequence adopted in
CDMA system, is the abbreviation of Longest Linear
Feedback Shift Register Sequence. It is a typical pseudo-
random sequence, with better autocorrelation and cross
correlation characteristics. M-sequence is also characterized
by easy generation and multiple number of sequences, thus,
it is widely applied in communication [6], such as spreading
code in spread spectrum communication, simultaneousness,
scrambling and the measurement of bit error rate. The
orthogonality of M-sequence is the prerequisite of parallel
response of multiple channels, and the pseudo-random
characteristic also leads to the application of spread spectrum. During one period, one n-level M-sequence experiences
2 n -1 status, among which, there is one more number of “1” in
each sequence than that of “0”. The cross correlation of M-
sequence refers to the consistence of two different M-
sequences in same period. The closer to zero the cross-
correlation value is, the weaker the cross correlation is; on
the contrary, the stronger the cross correlation is. Therefore,
the cross correlation is − . With the increasing length of
sequence, its auto-correlation is closer to that of white noise.
From the analysis by Matlab, the correlation of M-sequence
is shown in Figure 3, where the 31-bit M-sequence is
1001000010101110110001111100101, and 15-bit M-
sequence is 110101111000100.
C. The generation principle of M-sequence
Shift M-sequence is generated on the basis of linear shift
register added with XOR feedback circuit. One n-level linear
feedback shift register consists of n-level register and a group
of linear feedback logic circuit. It is inappropriate to adopt
excessively long M-sequence in RFID system considering
the number of channels and communication rate. For
instance, 15-bit and 31-bit are both good choices. Taking 31-
bit M-sequence as an example, its generation method is
introduced. The primitive polynomial G(x) = x 5 +x 2 +1 is
selected, and the generation circuit is shown in Figure 2 [7]. In Figure 3, the auto-correlation of M-sequence is shown
in period point 31 and 15 [9], and it is good. Whereas, other
points show the cross correlation, and it is relatively weak.
By comparing 31-bit and 15-bit, it can be seen that the larger
the sequence length is, the more obvious the advantage is.
Considering the number of channels and communication
speed, 15-bit and 31-bit sequences are more ideal choices.
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