KR950003473B1 - Code rate generating method for cdma - Google Patents

Abstract

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Description

Code String Generation Method for Code Division Multiple Access

1 is a basic conceptual diagram of a code division multiple communication scheme.

2 is a block diagram of a conventional longest code heat generator.

3 is an explanatory diagram showing an example of a conventional longest code string.

4 is an explanatory diagram of autocorrelation values of a conventional longest code string.

5 is an explanatory diagram showing an example of the longest code string of the present invention.

6 is an explanatory diagram of autocorrelation value of the longest code string of the present invention.

7 is a structural example of a code generator of the present invention.

* Explanation of symbols for main parts of the drawings

11: longest code heat generator 12: counter

13: selector

The present invention relates to a code division multiple access scheme (CDMA) that extends a transmission band by using a code string. In particular, the present invention has a completely orthogonal characteristic such that a cross-correlation value of a code string is "0", thereby transmitting a transmission signal of each subscriber. The present invention relates to a code string generation method for a code division multiple access method that enables simultaneous transmission without mutual interference.

The code division multiple access scheme, unlike the conventional time division multiple access scheme (TDMA) or frequency division multiple access scheme, is a spreading scheme in which a specific code string is assigned to each user in the same frequency band and the same time zone without requiring accurate time synchronization or frequency adjustment. As a band multiple access method, in this connection method, there are many advantages such as reducing the impact on noise and jamming, increasing information incompatibility, and making full use of a given transmission line capacity by extending the transmission band. Research is being actively conducted to apply to the construction of Communications Network, Local Area Network, and Wireless Communication Network.

1 is a basic conceptual diagram of a code division multiple communication scheme, and as shown therein, data (d ₁ , d ₂ , ..., d _k ) to be transmitted by all users in the multipliers 1a, 1b,. Each output signal transmitted to a transmission line, multiplied by the orthogonal code strings q ₁ , q ₂ , ..., q _k , where data and code strings appear as bipolar signals with different signs. Is delayed by a predetermined time (T ₁ , T ₂ , ..., T _k ), attenuated by a certain size (α ₁ , α ₂ , ..., α _k ) and transmitted to the receiver with the noise signal n added. In this way, the signal transmitted to the receiving side is multiplied by a specific code string in the multiplier 2 and then integrated in the integrator 3 so that unwanted signals that are orthogonal to each other are not output, and only the desired signal is output.

Here, d _k (t) is a data signal of the k-th user, which is a non-return-to-zero (NRZ) signal having a duration T _b and unit amplitude, and q _k (t) is a k-th signal. It is a code of a code string assigned to a user, and the code signal is an NRZ signal having a duration T _c and a unit amplitude, and is expressed as in Equation (1) below.

Here, qj (k) is the value of the jth bit of the kth user code, and has one of +1 and -1, and P (t) has the following value.

P (t) = 1, 0 <t <T _c

0, otherwise

Assuming that the data duration (T _b ) is divided by the code signal duration (T _c ) to be the code period (L), one period of code strings per data symbol a ₀ (k), a ₁ (k),... , a _L-1 (k) corresponds. At this time, the transmission signal S _k (t) of the k-th user is represented by the following equation (2).

Here, P is the power of the transmission signal of each user, W _ct is the carrier phase of the k-th user, θ _k is a carrier frequency shared by all users.

In addition, the received signal r (t) reached to the receiver after being delayed and attenuated by a predetermined time is expressed by the following equation (3).

Where k is the number of concurrent users, P is the power of the signal, d _k (t) is the code signal, and τ _k represents the delay time. Therefore, ø _k = θ _k -W _c τ _k, and τ _k becomes 0 when synchronous, so ø _k becomes equal to θ _k . And n (t) represents additive white Gaussian noise with a power spectral density of No / 2.

On the other hand, since the relative phase shift (modulo 2π) and the time delay (modulo T _b ) affect the performance of the system, it is assumed that θ ₁ = τ ₁ = 0 when k ≠ i, and 0≤τ _k ≤T _b , 0≤ Considering only θ _k ≤ 2π, since the generality is not lost in the analysis, the i-th correlation receiving output Z (T _b ) = Z ₁ becomes as shown in Equation (4) below.

In the above equation (4), the first term on the right is the term due to white Gaussian noise, the second term is the desired signal to be detected, and the third term is k-1 other users, {S _k ( t): Interference signal component generated by 1 ≦ k ≦ k, k ≠}.

As a result, as shown in Equation (4), when the synchronization is matched, if the orthogonality of the user code is perfect, the interference term is "0", but if the orthogonality is not perfect, the interference term exists. It is a factor that causes performance degradation of a CDMA system.

However, conventionally developed code strings include GOLD and KASAMI code strings, but they do not guarantee perfect orthogonality and have a limited number of code strings that can be used.

In addition, in Korean Patent Publication No. 86-1747 and US Pat. No. 4,701,905, the longest code string is used to obtain sufficient orthogonal characteristics in the "near-field communication method using code division multiplexing".

2 is a block diagram of a conventional longest code string generator, and as shown therein, the longest code string can be simply generated by the binary arithmetic element 4 and the shift register 5.

3 is an explanatory view showing an example of a longest code string having a number of code strings of seven, and the longest code string C ₁ as shown in FIG. 3 (a) is shown in FIG. 3 (b). By delaying bit by bit, the longest code string C ₁ -C ₇ by the longest code string period N = 7 can be generated.

By the way, the maximum of the bit stream is the third number of the total number of the code chip 2 ⁿ -1 gaejung "-1" 2 ^n-1 -1 numbered, as can be seen from the road, "1", the number of 2 ^n-1 of the open-circuit " The number of -1 "is always one less than the number of" 1 ", and as shown in Figure 4 of the autocorrelation value explanatory drawing, the autocorrelation value becomes" 1 "only when the synchronization is completely matched. In case of deviation of more than one chip, the autocorrelation value is always -1 / N.

In addition, since the autocorrelation property of the longest code string delayed is the same as the autocorrelation property of the longest code string, the cross-correlation value between any two longest code strings is -1 / N.

However, for perfect code division multiplexing, the orthogonality of the cross-correlation value of the used code strings should be guaranteed to be "0". However, in the longest code string as described above, the cross-correlation value of the code string is -1 / N, which is perfect. Since it does not guarantee the interference, inter-channel interference occurs. Accordingly, when the number of users increases, the cross-correlation value accumulates as the number of users, which adversely affects the quality of the communication line such as thermal noise.

Accordingly, it is an object of the present invention to provide a code string generating method for generating a code string for a code division multiple access scheme in which cross-correlation value between code strings is " 0 "

This object of the present invention is achieved by adding one bit of "-1" to the last bit of the longest code string.

The longest code string having a length of 2 ^N −1 is generated by the N-stage shift register of the longest code string generator, and at the same time, the number of the longest code string generating clocks is counted, and the count value is 2; ^At the instant of ^N -1, the longest code string signal of the longest code string generator is not output, and a signal of "-1" is forcibly added to the longest code string signal and then output. This is achieved by generating the longest code string again from the value, which will be described in detail with reference to the accompanying drawings.

The fifth turning the bit stream number of an explanatory view showing an example 7 of the present invention up to a bit stream, in as much as the longest code string (C ₁₎ a longest code string cycle by a time delay one bit (N = 7) as shown In when generating the maximum code string (C ₁ -C ₇₎ adds a bit of "-1" to the last bits of the longest code string (C ₁ -C ₇₎ as a single force.

Thus configured longest code stream is the total number and the number of "1" of the total number of the code chip 2 ⁿ -1 gaejung "-1" and the same open-circuit 2 ^n-1 As can be seen in FIG. 5, the cross-correlation value yiettara Becomes "0".

6 is an explanatory diagram of the autocorrelation value of the longest code sequence of the present invention. As shown in FIG. 6, the autocorrelation value becomes "1" only when the synchronization is completely coincident. It becomes "0", and thus the cross-correlation value between any code strings becomes "0" and thus has a perfect orthogonal characteristic that always becomes "0" regardless of the increase in the number of users.

The seventh turning the invention Code Division as a configuration example of a multiple access code string generator for the Figure, and thus receives the longest code string generating a clock signal (CK) as shown, the length by the mobile storage device of the N-stage 2 ^N - The longest code string generator 11 generating the longest code string equal to 1 and the longest code string generation clock signal CK are counted and a bit signal of "-1" is output when the count value becomes 2 ^N -1. In addition, the counter 12 outputs a control signal to reset the initial value to the longest code string generator 11 after a 1-bit delay, and selects and outputs the longest code string generated by the longest code string generator 11. And a selector 13 which selects and outputs the bit signal of "-1" when the bit signal of "-1" is output from the counter 12. Referring to FIG. The code string generating method will be described.

The longest code string generating clock CK is inputted from the longest code string generator 11 to generate and output the longest code string having a length of 2 ^N -1 from the initial value by the N-stage mobile storage element. The longest code string generation clock signal CK is received by the counter 12 and counted, and the maximum code string generator 11 is generated and outputted before the count value of the counter 12 becomes 2 ⁿ -1. The code string is selected by the selector 13 and output as the longest code string signal Ci, and the bit signal of " -1 " at the counter 12 at the moment when the count value of the counter 12 becomes ^2N- 1. The bit signal of " -1 " is selected by the selector 13 and output as the longest code string signal Ci. Therefore, at this time, the longest code string signal Ci is in a state where a bit of " -1 " is forcibly added to the last bit of the original final code string signal as shown in FIG. On the other hand, after outputting the bit of "-1" in the counter 12, an initial value reset control signal is applied to the longest code string generator 11 to reset the initial value, and at the same time, the counter 12 The above process is repeated by starting to count the longest code string clock signal CK again.

As described in detail above, the cross-correlation value of the present invention has a perfect orthogonal characteristic of always being "0" regardless of an increase in the number of users, so that it is easy to achieve synchronization using a correlator when used in a code division multiple access system. Therefore, since there is no influence of mutual interference between users, the degradation of transmission quality due to mutual interference between channels can be prevented, so that the system performance of the code division multiple access method can be significantly increased.

Claims (1)

By using the N-stage mobile memory device of the longest code string generator, the longest code string having a code string length of 2 ^N -1 is generated, and at the same time, the number of the longest code string generating clocks is counted, and the count value is 2 ^N -1. Code division multiple access method comprising the step of preventing the longest code string signal of the longest code string generator from being outputted and adding a chip of "-1" to the last chip of the longest code string signal. Code string generation method.