Note: Descriptions are shown in the official language in which they were submitted.
84185956
A DIFFERENTIAL CHAOS SHIFT KEYING (DCSK) BASED ON HYBRID
CHAOTIC SYSTEM
TECHNICAL FIELD
The invention belongs to the field of spread spectrum communication
technology, and
relates to a differential chaos shill keying (DCSK) communication method based
on a hybrid
system.
BACKGROUND ART
With the problems such as resource shortage, environment pollution monitoring,
national
security, defense, and so on, being serious the development and exploration of
marine
resources becomes the focus of scientific and technological research. As
electromagnetic
waves cannot propagate over long distances in the water, underwater acoustic
communication
is only feasible technique for the long distance communication in the water.
However, the reliability of acoustic communications is susceptible to acoustic
channel
constraints, such as multipath propagation, amplitude damping, time-varying
parameters, and
ambient noise. It is significant to find reliable methods for the underwater
acoustic
communication.
SUMMARY
The purpose of the present invention is to provide a DCSK communication method
based on a hybrid system to improve the reliability of the underwater acoustic
communication
as compared to the existing methods.
The proposed invention provides a DCSK communication method based on a hybrid
system, it includes the following steps:
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Step 1: Set up parameters of a communication system
Assume that bit transmission rate is Rb bits/s, and corresponding bit duration
is Tb, at the
same time, a base frequency of the hybrid system is f, symbol rate in chaotic
signal is R,
bits/s, which corresponds symbol duration, Te=1/Rd=1/f, and spreading gain in
this case is
defined as L=R,IRb=(11T,)1 (11Tb)=TbiTc;
Step 2: Prepare binary information to be transmitted
Set binary infoiniationBn={4,b2õbn} , where b, represent +1 or -1, i=1, n
represent the ith binary information to be transmitted and n is the number of
the binary
information;
Step 3: Generate of the chaotic signal ui(t)
The hybrid system given by:
(t)-2flit1 (t)+ (co' +132)(u1 (t)¨s)= 0
is used to generate the chaotic signal ui(t), and discrete state s is defined
by guard condition,
and the discrete state s is set as sgn(ui(t)), if and only if ü (t) =0;
otherwise, s keeps
unaltered, where sgn(-) is defined as:
+1, ul 0
sgn =
¨1, ui < 0
co=27-tf, /3=j1n2 are parameters, f is the base frequency of the chaotic
signal and a switching
period of the discrete state s is denoted as T, =---271- I co =11 f ;
Step 4: Modulate transmitted signal
For the ith binary information to be transmitted, the transmitted signal,
u3(t), composing
of ui(t) in first half slot and u2(t) in second half slot, in the first time
slot
(i-1)Tb<t<(i-1)Tb+Tb/2, reference signal piece, ui(t), generated by the hybrid
system is sent;
the same reference piece is multiplied by an information bit to be
transmitted, i.e., +1 or -1, to
form the signal piece, which is sent in the second time slot (i-
1)Tb+T1,12<t<iTb;
Step 5: Match filter for received signal
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The received signal, v(t), is firstly fed into the match filter to filter
ambient noise and
decrease effect of interference, and the match filter is given by:
{(t)+ 2/3(t) + (co2 + 132 )g (t) ¨ 77(0 = 0
,
where 4t) is a filter output and '1(0 is an intermediate state;
Step 6: Recover the received information bits
The filter output signal, 4t), for the ith binary information, (1-1)Tb<t<Tb,
is divided into
two equal time slots , given by
1,(t)=(t+(i-1)Tb)
'21(t)---- 4t+(i-1)Tb+Tb12), Ot< TbI2,
and a correlation output is given by:
r Tb /2
Zi (t)=¨ j i (r) i (t ¨ r)dr ,
0
and the binary information bit could be detected by i), = sgn(Z, (Tb /2)).
The advantages of the present disclosure are as follows:
1) The proposed disclosure does not need the chaotic synchronization, channel
estimation, and complicated equalization technology, which are necessary for
the conventional
wireless communication methods. Different from other enhanced versions of DCSK
that
demands extra hardware or more complicated algorithm, the proposed method
recovers the
received information bits at the low cost by using the simple algorithm, thus
facilitating the
real-time applications.
2) A chaotic match filter corresponding to the hybrid system is used to
relieve the effect
of interference and to improve signal to noise (SNR) ratio of the received
signal. By this way,
the proposed communication scheme has a good performance in underwater
acoustic channel,
in the sense of low Bit Error Rate (BER).
3) The base frequency of the hybrid system can be conveniently adjusted by
changing
the system parameter in order to adapt to the available communication channel
bandwidth.
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4) The present invention has a good performance to resist narrowband
interference,
especially in the case of the interference frequency is larger than the base
frequency of the
hybrid system.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is the block diagram of the proposed DCSK invention;
Fig. 2 is the binary information to be transmitted, `-1-1' during [0, 11s, and
`-1' during
[1,2[s;
Fig. 3 is the continuous state variable and the discrete variable in the
hybrid system;
Fig. 4 is the transmitted signal for information bits [1,-1];
Fig. 5 is the received signal after being transmitted through the
communication channel
and the transmitted one;
Fig. 6 is the filter out signal after the match filter;
Fig. 7 is the correlation output of the first information bit;
Fig. 8 is the correlation output of the second information bit;
Fig. 9 is the bit error rate (BER) performance over an additive white Gaussian
noise
(AWGN) channel and comparison with the conventional DCSK;
Fig. 10 is the BER performance over an underwater acoustic channel and
comparison
with the conventional DCSK;
Fig. 11 is the BER performance under the different interference amplitudes;
Fig. 12 is the BER performance under the different interference frequencies.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
Certain exemplary embodiments of the present invention will be described in
greater
detail with reference to the accompanying drawings.
Refer to Fig. 1, the bit duration is divided into two equal time slots, in the
first time slot,
the reference signal piece, ul, generated by the hybrid system is sent; the
same reference
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signal piece is multiplied by the information bit bi, i.e., one bit in the
binary information
sequence Bn={b1,b2,...,b,}, to be transmitted to form the signal piece, u2,
which is sent in the
second time slot; The transmitted signal, u3=[221, 222], composing of u1 in
first half time slot and
122 in second half time slot is sent over the communication channel; The
received signal, v, is
firstly fed into the match filter to filter the noise and decrease the effect
of interference. The
filter output signal for the ith bit information, 4t)(i-1)Tb<t<iTb, is divided
into two equal time
slots as done at the transmitter, i.e., and 61,
then two time slots are used for correlation
operation, the correlation output is sampled at the specified instant, the
sign function of the
sampled value is used to recover the received information bits.
Based on the above principle, the invention includes the following steps:
Step 1: Set up parameters of a communication system
Assume that bit transmission rate is Rb bits/s, and corresponding bit duration
is Tb (the
duration for transmitting one bit), at the same time, a base frequency of the
hybrid system is f,
symbol rate in chaotic signal is Rc. bits/s, which corresponds symbol
duration, 71=1/Rc=1/f,
and spreading gain in this case is defined as L=RcIRb=(1171)1 (11Tb)=TbITc;
Set the base frequency f=8Hz in exemplary embodiment, bit duration Tb=ls, this
means
that the spreading gain L=8;
Step 2: Prepare binary information to be transmitted
Set binary information Bn = , b2, .. bõ1, where b, represent +1 or -1, i=1,
n
represent the ith binary information to be transmitted and n is the number of
the binary
information;
Assume the bits to be transmitted is B2=1+1,-11 as shown in Fig. 2, where the
bit
duration Tb=ls in the exemplary embodiment;
Step 3: Generate of the chaotic signal ut(i)
The hybrid system given by:
(t)-2Piti(t)+(co2 +p2 )(,11 (t)¨ s)= 0 (1)
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is used to generate the chaotic signal ui(t), and discrete state s is defined
by guard condition,
and the discrete state s is set as sgn(ui(t)), if and only if U1(t) = 0;
otherwise, s keeps
unaltered, where sgn() is defined as:
+1 u 0
sgn (u, ) = (2)
,-13 U1 < 0
co=27rf, fl=fin2 are parameters, f is the base frequency of the chaotic signal
and a switching
period of the discrete state s is denoted as T, = 2irl co =1 I f ;
As shown in Fig. 3, the chaotic signal generated by system (1) with f=8Hz and
the
discrete symbol, s, are shown in the solid line and dotted line, respectively,
we can see that
eight discrete symbols are generated in one second.
Step 4: Modulate transmitted signal
For the ith binary information to be transmitted, the transmitted signal,
u3(t), composing
of ui(t) in first half slot and u2(t) in second half slot, in the first time
slot
(i-1)Tb<K(i-1)Tb+Tb/2, reference signal piece, ui(t), generated by the hybrid
system is sent;
the same reference piece is multiplied by an information bit to be
transmitted, i.e., +1 or -1, to
form the signal piece, which is sent in the second time slot (i-
1)Th+Th/2<t<iTb;
In the exemplary embodiment, the first time slot, [0,0.5]s, for the first bit,
the reference
signal piece ui(t), corresponding to ui(t) during [0,0.5]s, is sent, and the
same piece signal
multiplied by first information bit, i.e., "1", is sent in the second time
slot [0.5,1]s. For the
second information bit, "A", in the first time slot, i.e., [1, 1.5]s, the
signal piece of ui(t) during
[0.5, 1 ]s is sent as the reference signal, and the same piece of reference
signal multiplied by
the second information bit, i.e., "4", is sent duration [1.5,2]s, as shown in
Fig. 4.
Step 5: Match filter for received signal
The received signal, v(t), is firstly fed into the match filter to filter
ambient noise and
decrease effect of interference, and the match filter is given by:
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{ (t)+ 2 fie(t)+ (co' + p2.)((t)- To), 0
7j(t)= v(t + Tc) ¨ v(t) , (3)
where ".(t) is a filter output and )7(0 is an intermediate state;
After the transmitted signal, u3, passes through the wireless communication
channel
(affected from multipath propagation, amplitude damping, time-varying
characteristics, and
ambient noise) with SNR=-10dB, the received signal, v(t), is shown in Fig. 5,
where the
transmitted signal and the received signal (amplified 40 times for easy
comparison) are shown
in dotted line and solid line, respectively. The received signal, v(i), is fed
into the match filter,
and the output of the match filter, (t), is shown in Fig. 6. It can be seen
that the filter relieves
the effect of interference and ambient noises significantly.
Step 6: Recover the received information bits
The filter output signal, (t), for the ith binary information, (i-1)Tb<t<iTb,
is divided into
two equal time slots , given by
6 i(t)=(t+(i-1) Tb)
&(0= 4t+(i-1)Tb+Tb12), 05J< Th/2,
and a correlation output is given by:
,Tb/2
4 (i), j , (r)2,(t¨r)cir , (4)
0
and the binary information bit could be detected by -b-, = sgn (Z, (Tb / 2)) ,
i.e., if Zr(Tb/2)>0,
E., = +1; otherwise, if Z1(Tb/2)<O, -6, = ¨1.
In the exemplary embodiment, the filter out signal 40 of the first bit
t=[0,1]s is divided
into two equal time slots, i.e., 4i (0= (t), t=[0,0.5]s and 21 (0= (t+Tb/2)=
(t+0.5),
t=[0,0.5]s. The filter out signal 40 of the second bit t=[1,2]s is divided
into .12 (0= (t+Tb),
1=[0,0.5]s and 2 (0= (1 1.5Tb), t=[0,0.5]s.
The two binary information bit could be decoded by Eq. (4), and the result are
1-51 = sgn (Z1 (0.5)) and ic = sgn (Z2 (0.5)) . Z1(0.5) and Z2(0.5) are shown
in Figs.7 and 8
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with a star mark.
Embodiments
1) Ability to resist to noise
The proposed invention has a better ability to resist to noise than the
conventional
DCSK due to the usage of the chaotic signal generated by the hybrid system and
the
corresponding match filter. The BER versus SNR for the proposed method and the
conventional DCSK using Logistic map over an additive Gaussian white noise
(AWGN)
channel are shown in Fig. 9. The base frequency of the hybrid systemf=50kHz
and spreading
gain L=50. From Fig. 9, we know that the proposed invention is more robust to
the noise
compared with the conventional DCSK methods, due to the usage of the match
filter
corresponding to the chaotic signal at the receiver end.
2) BER perfounance in underwater acoustic channel
The characteristics of the underwater acoustic channel require that the
communication
system resists more serious multipath propagation, ambient noise, and
interference as
compared to the air-based counterpart. The BER versus SNR for the proposed
method and the
conventional DCSK in the underwater acoustic channel are given in Fig. 10 for
comparison,
where the base frequency f=50kHz and spreading gain L=50. In Fig. 10, we learn
that the
conventional DCSK has higher BER even though the SNR is high, while the
proposed
invention gets lower BER.
3) The ability to resist to narrowband interference
Since the broad band characteristic of the chaotic signal, it is able to
resist narrowband
interference. We consider the fixed amplitude and frequency of a sinusoidal
interference. Here
the chaotic signal base frequency is f=50kHz. Figure 11 shows the BER
performance versus
SNR under the different interference amplitude A51õ. We find that the BER
increases with the
increasing interference amplitude.
Figure 12 shows the BER versus SNR using different interface frequency, where
interference signal amplitude 24,,,=1. we obtain that i) with the decreasing
of the interference
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frequency F, BER increases; ii) the BER is not changed significantly when the
interference
frequency is larger than the base frequency of the chaotic signal.
In conclusion, the proposed invention used the continuous state of the hybrid
chaotic
system as the modulated signal, at the same time, the spread gain is
determined by the discrete
symbols; The base frequency of the hybrid system can be conveniently adjusted
by changing
the system parameter in order to adapt to available channel bandwidth. At the
receiver end, a
special match filter corresponding to the hybrid chaotic system is used to
relieve the effect of
interference and to improve SNR of the received signal.
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