WO2006014044A1

WO2006014044A1 - Apparatus and method for determining user code in quasi-synchronous cdma system, and apparatus and method for generating and spreading user code using the same

Info

Publication number: WO2006014044A1
Application number: PCT/KR2004/003409
Authority: WO
Inventors: Yun-Jeong Song; Ho-Kyom Kim; Deock-Gil Oh; Ho-Jin Lee; Kwon-Hue Choi
Original assignee: Electronics And Telecommunications Research Institute; Yeungnam Educational Foundation
Priority date: 2004-08-04
Filing date: 2004-12-22
Publication date: 2006-02-09
Also published as: KR100662506B1; EP1782548A1; KR20060012725A; EP1782548A4

Abstract

Provided are an apparatus and method for selecting a user code according to the number of access users in a quasi-synchronous CDMA system, and an apparatus and method for generating/spreading a user code using the same based on scrambling. The apparatus of the present researches minimize interference elements among user codes in a range of a synchronous error by classifying user codes into two code groups in a base station of a quasi-synchronous CDMA system, selecting a specific code group according to present access user numbers, transmitting the specific code group to a terminal, generating a user code according to the code group in the terminal, and using a scrambling code pattern minimizing a correlation characteristic among user codes. The user code selecting apparatus includes : an access user counting block for counting present access user number upon a user access/termination request signal from a terminal; a code group selecting/converting block for selecting a user to convert a code group of the present access user by checking a value of access user counting block upon a user access request signal from the terminal, selecting a corresponding user code group, and checking a value of the access user counting block and a code group of the termination request user upon a user access termination request signal from the terminal; and a code group selection/conversion signal transmitting block for transmitting the code group selection/conversion signal to the terminal upon a control of the code group selecting/converting block.

Description

APPARATUS AND METHOD FOR DETERMINING USER CODE IN QUASI- SYNCHRONOUS CDMA SYSTEM, AND APPARATUS AND METHOD FOR GENERATING AND SPREADING USER CODE USING THE SAME

Description

Technical Field

The present invention relates to an apparatus and the method for selecting a user code according to the number of access users in a quasi-synchronous Code Division Multiple Access (CDMA) system, and an apparatus and method for generating and spreading a user code using the same based on scrambling; and, more particularly, to a user code selecting apparatus and method according to the number of access users in a quasi-synchronous CDMA system to minimize an interference element among user codes in a range of a synchronous error by classifying user codes into two code groups in a base station of a quasi-synchronous CDMA system, selecting a specific code group according to the number of present access users, transmitting the specific code group to a terminal, generating a user code according to the selected code group in the terminal, and using a scrambling code pattern which minimizes a correlation characteristic among user codes, and a user code-generating and spreading apparatus and method using the same based on scrambling.

Background Art

Since a satellite communication channel has a large possibility that a line of sight exists in comparison with a terrestrial communication channel, and has a negligible multi-path interference element by a characteristic of a satellite channel structure, which has a little Inter-

Symbol-Interference (ISI) element in comparison with a terrestrial communication system. Recently, a method for reducing interference by maintaining user symbol synchronization in a reverse link, just as in a forward link, and using an orthogonal code in a satellite system using a Code Division Multiple Access (CDMA) method has been suggested based on the above method.

Since synchronization among users in a CDMA system is generally controlled by a feedback control loop of a base station, it is not possible to perfectly maintain a synchronous error among users by the dynamic delay time according to movement of a user and the finite resolution of synchronization control. Particularly, since a Walsh- Hadamard (WH) code, which was generally used as a conventional user orthogonal code, has an orthogonal characteristic in case that there is no synchronous error while a maximum value of a possible correlation value is very large in case that there are synchronization errors, the WH code is not suitable for a user code in case that a synchronous error exists among users.

That is, although an orthogonal code is used as a user code in a quasi-synchronous CDMA system, an interference element among users is generated due to imperfect synchronization among users. Therefore, the use of the conventional orthogonal code, which maintains the orthogonal characteristic only under assumption that synchronization is perfect, in the quasi-synchronous CDMA system seriously deteriorates performance.

Meanwhile, in an article by De Gaudenzi, R.;Elia C. ,-Viola,R, entitled "Bandlimited quasi-synchronous CDMA: a novel satellite access technique for mobile and personal communication systems, Institute of Electrical and Electronics Engineers (IEEE) journal on Selected Areas in Communications, Volume 10 Issue 2, Feb. 1992, pp. 328-343, performance of Preferably Phased Gold Code (PPGC) was analyzed when PPGC is used as a user code in a quasi- synchronous CDMA system. The PPGC is a gold code that a correlation value of all code couples satisfies -1 in all cases that synchronization is maintained, and has a maximum correlation value which is very small in comparison with WH code although synchronization among users is not maintained. Therefore, PPGC is considered as a very proper code method to a quasi-synchronous CDMA system.

The PPGC considers only a case that additional scrambling is not preformed to a spread code. The correlation characteristic among codes is not changed in case that the synchronization among users is perfectly maintained although a scrambling is performed additionally, but the correlation characteristic among user codes can be changed by scrambling in case that the synchronization among users is not maintained.

Therefore, a correlation characteristic change by scrambling should be considered in a code design of the quasi-synchronous CDMA system, and a method for improving a correlation characteristic of a code among users by optimizing scrambling is essentially required.

Disclosure Technical Problem

It is, therefore, an object of the present invention to provide a user code selecting apparatus and method according to the number of access users in a quasi- synchronous Code Division Multiple Access (CDMA) system that can minimize an interference element among user codes in a range of a synchronous error by classifying user codes into two code groups in a base station of a quasi- synchronous CDMA system, selecting a specific code group according to the number of present access users, transmitting the specific code group to a terminal, generating a user code according to the selected code group in the terminal, and using a scrambling code pattern which minimizes a correlation characteristic among user codes, and a user code-generating and spreading apparatus and method using the same based on scrambling.

Technical Solution

In accordance with one aspect of the present invention, there is provided an apparatus for selecting a user code according to the number of access users in a quasi- synchronous Code Division Multiple Access (CDMA) system, including: an access user counting block for counting the number of present access users when a user access/termination request signal is transmitted from a terminal; a code group selecting/converting block for selecting a user to convert a code group among the present access users by checking a value of access user counting block when a user access request signal is transmitted from the terminal, selecting a corresponding user code group, and checking the number of the access users counting block and a code group of the termination request user when a user access termination request signal is received from the terminal; and a code group selection/conversion signal transmitting block for transmitting the code group selection/conversion signal to the terminal according to a control of the code group selecting/converting block.

In accordance with another aspect of the present invention, there is provided an apparatus for generating and spreading a user code based on scrambling in a quasi- synchronous CDMA system, including: a code generating block for selectively outputting a user code included in a first code group or a second code group according to a code group selection/conversion signal from a base station; a first multiplying block for spreading a user code outputted from the code generating block by multiplying the user code to the user's data symbols; and a second multiplying block for multiplying a signal from the first multiplying block by the optimized scrambling pattern.

In accordance with another aspect of the present invention, there is provided a method for selecting a user code according to the number of access users in a quasi- synchronous CDMA system, including the steps of: a) checking whether the number of present access users is less than a half of a total user code number after increasing the number of the present access user number by 1 whenever an access request is received; b) selecting a first code group for the newly accessing user when the number of the present access users is less than a half of a total user code number; c) selecting a second code group for the newly accessing user when the number of the present access users is larger than a half of total user code number; and d) selecting a code group conversion user by checking the number of the present access users, the code group of termination user and the code groups of other access users when a user access termination request is received.

In accordance with another aspect of the present invention, there is provided a method for generating and spreading a user code based on scrambling in a quasi- synchronous CDMA system, including the step of: a) generating one user code among codes of the corresponding code group according to a code group selection/conversion signal from a base station; and b) spreading the generated user code by multiplying the user code to the user's data symbols thereby produce a spread code, and multiplying the spread code by the optimized scrambling code pattern.

Advantageous Effects

The present invention can minimize an interference element among user codes in a range of synchronous error by classifying user codes into two code groups in a base station of a quasi-synchronous Code Division Multiple Access (CDMA) system, selecting the specific code group according to the number of present access users whenever a new access request is received, acknowledging the selected code group to the accessing user terminal, generating a user code according to the selected code group in the terminal, and scrambling the spread signal by using a scrambling code pattern which minimizes a correlation characteristic among user codes.

Description of Drawings

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a block diagram showing a user code selecting apparatus according to the number of access users in a quasi-synchronous Code Division Multiple Access (CDMA) system, and a user code generating/spreading apparatus using the same based on scrambling in accordance with an embodiment of the present invention;

Fig. 2 is a flowchart showing a method for selecting a user code according to the number of access users in a quasi-synchronous CDMA system, and a method for generating and spreading a user code using the same based on scrambling in accordance with an embodiment of the present invention; Fig. 3 is a flowchart illustrating a code group selecting process when a user access request signal is received in a method for selecting a user code based on scrambling in a quasi-synchronous CDMA system in accordance with an embodiment of the present invention; Fig. 4 is a flowchart illustrating a code group converting process when a user access termination request signal is received in a method for selecting a user code according to the number of access users in a quasi- synchronous CDMA system in accordance with an embodiment of the present invention;

Fig. 5 is a 3-dimensional (3D) graph showing correlation values of all possible code couples when using a scrambling code which can maintain a code correlation to be 0 for the number of users less than N/2 in case that the user code length N is 32 and the maximum synchronization error among user codes is 1 chip duration in accordance with an embodiment of the present invention; and

Fig. 6 is a graph showing maximum correlation values among codes according to the number of users for Preferably Phased Gold Code (PPGC), Walsh-Hadamard (WH) code, PPGC with optimized scrambling and WH code with optimized scrambling when the user code length N is 32 in accordance with an embodiment of the present invention.

Best Mode for the Invention

Other objects and advantages of the present invention will become apparent from the following description of the embodiments with reference to the accompanying drawings. Therefore, those skilled in the art that the present invention is included can embody the technological concept and scope of the invention easily. In addition, if it is considered that detailed description on prior art may blur the points of the present invention, the detailed description will not be provided herein. The preferred embodiments of the present invention will be described in detail hereinafter with reference to the attached drawings.

Fig. 1 is a block diagram showing a user code selecting apparatus according to the number of access users in a quasi-synchronous Code Division Multiple Access (CDMA) system, and a user code generating/spreading apparatus using the same based on scrambling in accordance with an embodiment of the present invention.

As shown in Fig. 1, an apparatus for selecting a user code according to the number of access users and an apparatus for generating/spreading a user code using the same based on scrambling are separately provided to a base station and a terminal.

A user code selecting apparatus 100 according to the number of access users, which is provided to a base station, receives user access request signal and the number of present access users, selects a code group of a present access request user, receives a user termination request signal and the number of present access users, selects one user of the present access users according to the number of present access users and a code group of termination users, and converts the code group of the selected user.

That is, the user code selecting apparatus 100 includes an access user counter 110, a code group selecting/converting unit 120 and a code group selecting/converting signal transmitter 130. The access user counter 110 counts the number of present access users when a user access/termination request signal is received from the quasi-synchronous terminal. The code group selecting/converting unit 120 checks a value of the user access counter 100 when a user access request signal is transmitted from the terminal, selects a user code group of a present access request user, checks a value of the access user counter 110 a code group of the termination request user when a user access termination request signal is received from the terminal, and selects a user to convert a code group of the present access users. The code group selecting/converting signal transmitter 130 transmits a code group selecting/converting signal to the corresponding terminal according to a control of the code group selecting/converting unit 120. Meanwhile, the user code generating/spreading apparatus 200 based on scrambling, which is provided to a terminal, includes a code generator 210, a first multiplier 220 and a second multiplier 230. The code generator 210 selectively outputs a Walsh-Hadamard (WH) code included in a code group A or a code group B when a code group selection signal is transmitted from the user code selecting apparatus 100. The first multiplier 220 multiplies and spreads the WH code outputted from the code generator 210 to the data symbols from the user terminal. The second multiplier 230 outputs a signal which is spread by multiplying an optimized scrambling pattern to the signal from the first multiplier.

The WH code used as a user code in the present invention is divided into two groups, e.g., an group A and a group B. Herein, when the degree of the WH code, i.e., a length of the user code, is N, it is considered that the code group A includes codes corresponding to 1^st to N/2^th rows, and the code group B includes codes corresponding to l+N/2^th to N^th rows in an N-order WH code matrix. Each code of the code group A or the code group B is assigned to all users one by one, and a code group is selectively used according to a command of a base station.

That is, when the number of present access users is less than a half of the entire code number, the base station commands all users to use a code of the same group. For example, when the number of users is less than a half of the entire code number, only a code of the code group A, i.e., the codes corresponding to 1^st to N/2^th rows in the N- th order WH code matrix, are assigned to the users, or only a code of the code group B includes a code corresponding to l+N/2^th to N^th rows in the N-order WH code matrix, is assigned.

Each user signal spread by the assigned user code is multiplied by the same scrambling pattern and transmitted. Herein, a scrambling code used in the present invention has a characteristic as shown in Fig. 5, and satisfies characteristics as follows.

1. A cycle of a scrambling code is the same as a length of a WH code. 2. A scrambling code is multiplied to the user spread signal with the synchronized code phase with the user spread code.

3. When a scramble code is S (i) ,i=l,2,3,...N, the scrambling code satisfies patterns of the following equations 1 or 2 when the number of access users is less than a half of a total user code number, i.e., less than N/2.

S((N/2)+i) = (-I)¹S(D, i=l,2,3,...N/2 Eq. 1 S((N/2)+i) = -(-I)¹S(I), i=l,2,3,...N/2 Eq. 2

wherein N is a natural number.

For example, when N is 32, a pattern of [S (17) ,S (18) , ...,S (32)]=[S(I),-S (2),S (3),-S (4),...,S (15),-S (16)] is satisfied, or a pattern of [S (17) ,S (18) , ...,S (32) ]=[-S (1) , S(2),-S(3), S (4) ,...,-S (15) , S(lβ)] is satisfied.

4. When the number of users is more than a half of a total user code number, i.e., more than N/2, an optimal pattern which minimizes a correlation value with the maximum synchronization error among users equal to one chip duration for all possible code couples is found and used. The optimal pattern can be found through a computation by a computer.

As above, the scrambling code pattern used in the present invention is optimized to minimize a correlation characteristic among user. From the characteristics, the present invention can generate a user code that correlation performance is improved in comparison with the conventional PPGC. An entire operation process of the present invention will be described hereinafter referring to Fig. 2.

Fig. 2 is a flowchart showing a method for selecting a user code according to the number of access users in a quasi-synchronous CDMA system, and a method for generating and spreading a user code using the same based on scrambling in accordance with an embodiment of the present invention.

If a base station, i.e., the user code selecting apparatus 100, receives a user access request signal at step S201, the base station selects a code group according to the number of present access users and transmits the code group to a terminal at step S202.

At step S203, the terminal, which is a user code generating/spreading apparatus, generates a user code of the determined code group, which is a WH code.

The generated user code is spread by being multiplied by the data symbol at step S204, and an optimized scrambling code pattern is multiplied at step S205.

Meanwhile, when the base station, which is the user code selecting apparatus 100, receives a user access termination request signal (not shown in Fig. 2) , the base station terminates the user access, and a code group conversion process of a user using a present code group B is preformed when the terminated user code is a code group A. That is, it is checked whether there is a user who uses a code group B, and simultaneously has a code group A, which is the same as the code group A that a terminated user used, and the code of the user is converted into a code group A. Detailed process will be described hereinafter referring to Fig. 4.

Fig. 3 is a flowchart illustrating a code group selecting process when a user access request signal is received in a method for selecting a user code based on scrambling in a quasi-synchronous CDMA system in accordance with an embodiment of the present invention. At step S301, the number of present access users is increased by 1, and at step S302, it is checked whether the number of present access users is less than a half of a total user code number. In the check result of the step S302, when the number of present access users is less than a half of the total user code number, a code group A is selected at step S303.

Meanwhile, in the check result of the step S302, when the number of present access users is larger than a half of the total user code number, a code group B is selected at step S304.

Fig. 4 is a flowchart illustrating a code group converting process when a user access termination request is received in a method for selecting a user code according to the number of access users in a quasi-synchronous CDMA system in accordance with an embodiment of the present invention.

When the user access termination request signal is received at step S401, the access is terminated at step S402 after the number of present access users by 1 is decreased. At step S403, a code group is converted after the number of present access users, a code of a terminated user and a code of other access users are checked. That is, it is checked whether there is a user who uses a code group B, and simultaneously has a code group A, which is the same as a code group A that a terminated user used, and the code of the user is converted into the code group A.

Meanwhile, as shown in Fig. 6, the present invention can minimize an interference element among user codes in a range of a synchronous error, and the WH code is more proper than the PPGC to the present invention.

While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims,

Claims

What is claimed is:

1. An apparatus for selecting a user code according to the number of access users in a quasi-synchronous Code Division Multiple Access (CDMA) system, comprising: an access user counting means for counting the number of present access users when a user access/termination request signal is transmitted from a terminal; a code group selecting/converting means for selecting a user to convert a code group among the present access users by checking a value of access user counting means when a user access request signal is transmitted from the terminal, selecting a corresponding user code group, and checking the number of the access users counting means and a code group of the termination request user when a user access termination request signal is received from the terminal; and a code group selection/conversion signal transmitting means for transmitting the code group selection/conversion signal to the terminal according to a control of the code group selecting/converting means.

2. The apparatus for selecting a user code as recited in claim 1, wherein the code group selecting/converting means selects a user code group by checking whether the number of present access users is less than a half of the number of total user codes and selecting a first code group when the number of present access users is less than a half of the number of total user codes, or selecting a second code group when the number of present access users is larger than a half of the number of total user codes.

3. The apparatus for selecting a user code as recited in claim 2, wherein when a code that a user uses is of the first code group, the code group selecting/converting means determines a user who uses a code of the second code group and has a same code as the code of the first code group used by the termination request user as the first code group is selected as a code group conversion user upon a user access termination request received from the terminal.

4. The apparatus for selecting a user code as recited in claim 1, wherein the first code group includes a user code corresponding to 1^st to N/2^th rows in an N-order Walsh- Hadamard code matrix, N being a natural number; and the second code group includes a user code corresponding to l+N/2 to N rows in an N-order WH code matrix.

5. An apparatus for generating and spreading a user code based on scrambling in a quasi-synchronous Code

Division Multiple Access (CDMA) system, comprising: a code generating means for selectively outputting a user code included in a first code group or a second code group according to a code group selection/conversion signal from a base station; a first multiplying means for spreading a user code outputted from the code generating means by multiplying the user code by the user's data symbols; and a second multiplying means for multiplying a signal from the first multiplying means by the optimized scrambling pattern.

6. The apparatus for generating and spreading a user code as recited in claim 5, wherein the first code group includes a user code corresponding to 1^st to N/2^th rows in an N-order Walsh-Hadamard code matrix, N being a natural number; and the second code group includes a user code corresponding to l+N/2 to N rows in an N-order WH code matrix.

7. The apparatus for generating and spreading a user code as recited in claim 6, wherein a cycle of the optimized scrambling pattern is same as a length of a user code.

8. The apparatus for generating and spreading a user code as recited in claim 7, wherein the optimized scrambling pattern satisfies a condition of: S ( (N/2)+i)= (-I)¹S(I)₇I=I, 2, 3,...,N/2 or, S( (N/2)+i)=- (-I)¹S(I) ,i=l,2,3,...,N/2 and S is a scrambling code.

9. The apparatus for generating and spreading a user code as recited in claim 5, wherein the optimized scrambling pattern is an optimal scrambling pattern minimizing a correlation value of a case that 1 chip duration exists for all possible code couples of a user code when the number of the present access users exceeds a half of a total code number N/2.

10. The apparatus for generating and spreading a user code as recited in claim 5, wherein the user code is a WH code.

11. A method for selecting a user code according to the number of access users in a quasi-synchronous Code Division Multiple Access (CDMA) system, comprising the steps of: a) checking whether the number of present access users is less than a half of a total user code number after the number of the present access user is increased by 1; b) selecting a first code group when the number of the present access users is less than a half of a total user code number; c) selecting a second code group when the number of the present access users is larger than a half of a total user code number; and d) selecting a code group conversion user by checking the number of the present access users, a code group of termination users and a code group of other access users when a user access termination request signal is received.

12. The method for selecting a user code as recited in claim 11, wherein the number of the present access users is decreased by 1 when the user access termination request is received, and when a code that a user used is of a first code group, a user who uses a code of the second code group and has a same code as the code of the first code group, which is used by the termination request users, as the first code group is selected as a code group conversion user when a user access termination request signal is received in the step d) .

13. A method for generating and spreading a user code based on a scrambling in a quasi-synchronous Code Division Multiple Access (CDMA) system, comprising the step of: a) generating one user code among user codes among codes of a corresponding code group according to a code group selection/conversion signal from a base station; and b) spreading the generated user code by multiplying the user code by a data symbol to thereby produce a spread code, and multiplying the spread code by an optimized scrambling code pattern.

14. The method for generating and spreading a user code as recited in claim 13, wherein the first code group includes a user code corresponding to 1^st to N/2^th rows in an N-order Walsh-Hadamard code matrix, N being a natural number; and the second code group includes a user code corresponding to l+N/2 to N rows in an N-order WH code matrix.

15. The method for generating and spreading a user code as recited in claim 13, wherein a cycle of the optimized scrambling pattern is same as a length of a user code.

16. The method for generating and spreading a user code as recited in claim 14, wherein the optimized scrambling pattern satisfies a condition of:

S((N/2)+i)= (-l)ⁱS(i),i=l,2,3,...,N/2 or, S( (N/2)+i)=-(-l)ⁱS(i) ,i=l,2, 3,...,N/2 and S is a scrambling code.