JP2000224073A - Spread spectrum communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate many kinds of diffusion codes, to increase the number of subscribers without generating the deterioration of the synchronous characteristic of a diffusion code and performance related to a multiplex number and to improve secret story nature in a system by using a vent function as a diffusion code. SOLUTION: Relating to the SS transmitter 10 of a transmitting side, an information modulating part 11 first performs information modulation of information to be transmitted. Although any kind of modulation system is acceptable as for the information modulation in theory, when information is a digital signal e.g. QPSK modulation, etc., is used. Next, a diffusion modulating part 12 performs diffusion modulation using a diffusion code of modulated information. For the diffusion modulation, for instance, BPSK modulation, etc., is used. For a diffusion code, a proper code has to be allocated to each user performing communication and a diffusion code using a vent function generated by a vent function generating part 13 is used. An SS receiver 15 of a receiving side performs diffusion demodulation by the diffusion code using the vent function of a received signal, while synchronizing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spread spectrum communication system used in mobile communication, wireless LAN, and the like.
It is an object of the present invention to provide a spread spectrum communication system capable of increasing the number of users and enhancing confidentiality between users.

[0002]

2. Description of the Related Art Spread spectrum is one of the modulation techniques for communication data, and is generally defined by the following two items. (1) A signal is spread and transmitted in a band wider than the bandwidth required for information transmission. (2) Spreading is performed using a signal (spreading code) unrelated to the information signal.

The general features of the communication system using the spread spectrum include the following.
That is, (1) it is hard to receive interference or interference. (2) It is difficult to cause interference or interference. (3) Signal confidentiality is high. (4) The confidentiality of the signal is high. (5) Distance measurement and time synchronization are possible. (6) Multiplexing (CDMA) is possible. (7) It is strong in a multipath environment.

The above features (4) to (7) are effects caused by using a special spreading code in communication using spread spectrum. That is, it is difficult to demodulate information without knowing the spreading code on the transmitting side, and multiplexing (CDMA) at the same frequency becomes possible by using different spreading codes. The type, period,
The combination of chip rates (one bit of the spreading code is called a chip) determines the performance of the spread spectrum communication system. Where CDMA (Code Division Multiple Access)
Is FDMA (Frequency Division Multiple Access), TD
This is one of the multiple access methods that are in line with MA (Time Division Multiple Access). The multiple access capability of the CDMA system
Obtained by coding techniques. A unique code sequence (spreading code) is assigned to each user, and each user encodes and decodes information signals using this. In this case, since the bandwidth of the code sequence is wider than the bandwidth of the information signal, the bandwidth is expanded (spread) during the encoding process. This encoding process is spread spectrum, and it can be said that "CDMA is a multiple access scheme using spread spectrum." recent years,
Spread spectrum is used in mobile communications, wireless LANs, and the like, and the demands on systems such as an increase in the number of multiplexes are increasing.

FIG. 4 shows a configuration example of a conventional spread spectrum communication system. FIG. 4A is a block diagram on the transmitter side, and FIG. 4B is a block diagram on the receiver side. In FIG. 7A, the transmitting side (SS transmitter 40) first modulates information to be transmitted by the information modulating unit 41. The information modulation used here may be any modulation method in principle, but when the information is a digital signal, for example, QPSK modulation or the like is used. Next, spread modulation is performed in the spread modulator 42 using the spread code generated by the spread code generator 43. For the spread modulation, for example, BPSK modulation or the like is used. In a communication system using spread spectrum, it is necessary to assign a unique spreading code to each user who performs communication. Conventionally, an M sequence or a GOLD sequence has been used as the spreading code.

The receiving side (SS receiver 45) shown in FIG.
Then, spread demodulation is performed on the received signal using the same spreading code as that on the transmitting side. That is, the spread code generator 47 generates the same spread code as that on the transmission side, and the spread demodulator 46 performs spread demodulation using this. In spread demodulation, since the bandwidth returns to the value before spreading, this is sometimes called despreading. In despreading, it is necessary to match not only the type of spreading code but also its timing, that is, to synchronize the spreading code. After the spread demodulation, the information demodulation section 48 demodulates the information to obtain the original information signal.

[0007]

SUMMARY OF THE INVENTION The performance of a spread spectrum communication system depends on the spreading code used. That is, the properties required for the spreading code include the following. (1) Good autocorrelation characteristics (sharp peaks, other values are small). (2) Good cross-correlation characteristics (small cross-correlation with spreading codes of other users). (3) There are many types of codes.

[0008] The above (1) is a property necessary for synchronizing spread codes, and (2) is a property necessary for increasing the multiplexing number (the number of terminals capable of simultaneous communication). Also, (3)
Increases the number of subscribers (the number of terminals that can connect to the system),
This property is necessary for improving the confidentiality between system users.

However, in the conventional spread spectrum communication system, since the M sequence and the GOLD sequence are used as the spreading code, the above (1) and (2) are satisfied.
There is a problem that the type of code (3) is not sufficient.
For example, in the case of the M-sequence, if the peak of the autocorrelation characteristic is 1, the others are 1 / n (n is the code length), which is almost ideal. However, in a practical system in which the number of shift registers for code generation is about 10 stages, the types of codes to be generated are as small as several tens. Therefore, there is a problem that the number of subscribers cannot be increased, codes can be easily estimated, and confidentiality between system users is reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the conventional spread spectrum communication system as described above. It is an object of the present invention to provide a spread spectrum communication system capable of increasing the number of subscribers and improving secrecy in the system.

[0011]

In order to achieve the above object, the present invention uses a spreading code using a Bent function as a spreading code. By using a vent function as a spreading code, it is possible to generate many types of spreading codes, thereby increasing the number of subscribers of the spread spectrum communication system and improving the confidentiality in the system.

The present invention also provides a linear feedback shift register that generates an M-sequence that operates in synchronization with a clock input, a code number setting unit that sets a number for distinguishing a spreading code, An arithmetic circuit that performs an exclusive OR operation on each register value and the value set in the code number setting unit for each bit, and a result obtained by applying an operation by a vent function to the operation result of the arithmetic circuit as a spread code And a spreading code generating circuit for outputting.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on one embodiment shown in the drawings. FIGS. 1A and 1B show an embodiment of a spread spectrum communication system according to the present invention. FIG. 1A is a block diagram on the transmitter side, and FIG. 1B is a block diagram on the receiver side. In these figures, SS transmitter 1
0 is an information modulator 11 that modulates information to be transmitted by an arbitrary modulation scheme, a spread modulator 12 that spreads and modulates the information modulated by using a given spreading code by an arbitrary modulation scheme, and a vent function The above-mentioned spread modulation unit 12
And a vent function generating unit 13 for generating a spread code to be given to the computer.

The SS receiver 15 uses the same spreading code as that used on the transmitting side to spread-demodulate the received information in a method corresponding to the transmitting side.
6. A vent function generator 17 for generating a spreading code to be provided to the spreading demodulator 16 using a bent function, and an information demodulator 18 for demodulating the spread demodulated information by a method corresponding to the transmitting side.

Next, the present invention will be specifically described according to a communication procedure using the spread spectrum communication system. In FIG. 1A, on the transmitting side (SS transmitter 10), an information modulator 11 first modulates information to be transmitted with information. The information modulation may be in principle any modulation method, but when the information is a digital signal, for example, QPSK
Modulation or the like is used. Next, in the spread modulation unit 12, spread modulation is performed on the modulated information using a spread code. For the spread modulation, for example, BPSK modulation or the like is used.

As the spreading code, it is necessary to assign a unique code to each user who performs communication. In the present invention, a spreading code using a vent function generated by the vent function generator 13 is used. The circuit and properties of the spreading code using the bent function will be described later.

On the receiving side (SS receiver 15), spread demodulation (despreading) using the same spreading code as the transmitting side, that is, a spreading code using a bent function, is synchronized in the spreading demodulation section 16 with the received signal. Perform while taking. The generation of the spread code is performed by the vent function generator 17. Next, after the spread demodulation, the information demodulation unit 18 performs information demodulation to obtain an original information signal.

FIG. 2 shows vent function generators 13 and 18,
That is, an example of the configuration of a circuit for generating a spreading code using a bent function is shown. In the figure, a vent function generator 200 is shown.
Is an n-stage linear feedback shift register 201,
It comprises a code number setting unit 202, an n-bit input 1-bit output vent function circuit 203, and an exclusive OR operation circuit 204. An n-stage linear feedback shift register 201 generates an M-sequence code sequence, and changes its state in response to a clock Cp. The frequency of the clock Cp is equal to the chip speed. Each register value of the linear feedback shift register 201 is calculated for each bit with the n-bit value set in the code number setting unit 202 in the exclusive OR operation circuit 204, and is input to the n-bit input vent function circuit 203. You. The vent function circuit 203 outputs a 1-bit value in response to an n-bit input. Thus, each time the clock Cp is input, a 1-bit value is output from the vent function circuit 203 as one chip of the spread code.

FIG. 3 shows an example of a specific configuration of the bent function circuit. As shown, the vent function circuit 300 includes n
/ 2 AND operation unit 301 and exclusive OR operation unit 3
02. Exclusive OR operation circuit 20 in FIG.
The n-bit outputs (inputs 1 to n) from 4 are input to the AND operation unit 301, where adjacent bits are subjected to AND operation. The output from each AND operation unit 301 is input to an exclusive OR operation unit 302, where it is subjected to an exclusive OR operation to obtain a 1-bit result as a spreading code.

Next, the nature of the vent function will be described. The bent function is a Boolean function having n (even number) bits input and 1 bit output, and has a property of satisfying PC (n). PC (n) is the Propagation Criterion of order
n, the Boolean function f becomes a∈ {0,1} ⁿ , a ≠ (0,
・ ・, 0)

[0021]

(Equation 1) Point to meet. Where the symbol

[Outside 1] Indicates an exclusive OR operation between bits. The properties of the PC (n) and the vent function are described in Seongtack Chee, Sangjin Le
e, kwangjo Kim, "Semi-Bent functions," Ad
vances in Cryptology-ASIACRYPT'94, LNCS 914, Sprin
ger-Verlag, pp. 107-118, 1995.

From the above equation (1), equation (2) can be derived using a1, a2 such that a1, a2∈ {0,1} ⁿ and a1 ≠ a2.

[0023]

(Equation 2)

The above equation (2) can be interpreted as an equation indicating the cross-correlation between codes. That is, all states (0,..., 0) to (1,.
Is changed so that appears at most once, and the output of the vent function f during that time is used as a sign, there is no cross-correlation between the two signs.

Therefore, each code is calculated using the vent function f.

[Outside 2] If x is the generated M-sequence described above, the cross-correlation between codes (in a synchronous system) will be very small for the reasons described above. Also, each code is a∈ {0,
1} It is obtained by changing ^n, and exists in a large number (about the code length). Further, the bent function is known as a cryptographic function, and has a feature that it is difficult to estimate a code.

As described above, the property of the vent function is PC (n)
Therefore, it satisfies the property required for the above-described spread code. In particular, in the case of a practical system in the type of code, for example, in a shift register of 10 stages, the number of types of code is 2 ¹⁰ = 1024, which is very large.

As described above, according to the present invention, by using a spreading code using a bent function as a spreading code, the properties of the spreading code are improved. As a result, the number of subscribers of the spread spectrum communication system increases and the number of subscribers in the system increases. The confidentiality could be improved.

The embodiment of the present invention has been described with reference to the drawings. However, it is apparent that the present invention is not limited to the matters described in the above embodiments, and that changes, improvements, and the like can be made based on the description in the claims.

[0029]

As described above, according to the present invention, it is possible to increase the number of subscribers and improve the secrecy in the system as compared with the conventional system, and to improve the performance of the spread spectrum communication system. Has a great effect.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of a spread spectrum communication system according to the present invention.

FIG. 2 is a block diagram showing a circuit for generating a spreading code using a bent function in FIG. 1;

FIG. 3 is a block diagram illustrating an example of a bent function circuit in FIG. 2;

FIG. 4 is a block diagram showing a conventional spread spectrum communication system.

[Explanation of symbols]

 Reference Signs List 10 SS transmitter 11 Information modulator 12 Spread modulator 13 Bent function generator 15 SS receiver 16 Spread demodulator 17 Bent function generator 18 Information demodulator 200 Bent function generator 201 Linear feedback shift register 202 Code number setting unit 203 Bent function circuit 204 Exclusive OR operation circuit 300 Bent function circuit 301 AND operation unit 302 Exclusive OR operation unit

Claims (2)

[Claims] 1. A spread spectrum communication system, wherein a spread code using a bent function is used as a spread code. 2. A linear feedback shift register for generating an M-sequence that operates in synchronization with a clock input, a code number setting unit for setting a number for distinguishing a spreading code, and a register value of the linear feedback shift register And an arithmetic circuit for performing an exclusive OR operation on each of the bits and the value set in the code number setting unit, and a spreading unit that outputs a result obtained by applying an operation by a vent function to the operation result of the arithmetic circuit as a spreading code. A spread spectrum communication system, comprising: a code generation circuit.