Device and method for generating quasi-orthogonal code and spreading channel signals in mobile communication system

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a channel communication device and a method used in a CDMA communication system, and in particular, to a device and method for generating binary quasi-orthogonal codes and spreading channel signals using the generated binary quasi-orthogonal codes.

[0004] 2. Description of the Related Art

[0005] In general, a CDMA (Code Division Multiple Access) mobile communication system performs channel separation using orthogonal codes in order to increase channel capacity. Walsh orthogonal codes are typically used for the orthogonal codes. For example, a forward link specified by the IS-95/IS-95A standard separates the channels using the Walsh orthogonal codes.

[0006] FIG. 1 illustrates the IS-95/IS-95A forward link in which channels are separated by Walsh orthogonal codes. Referring to FIG. 1, channels are separated by unique Walsh orthogonal codes Wi (where i=0 to 63), respectively. The IS-95/IS-95A forward link uses a coding rate R=½ convolutional codes for channel coding, employs BPSK (Binary Phase Shift Keying) modulation for spreading the Walsh orthogonal codes, and has a bandwidth of 1.2288 MHz. Accordingly, the number of available channels is 1.2288 MHz/(9.6 KHz*2)=64. That is, the IS-95/IS-95A forward link can separate 64 channels using the Walsh orthogonal codes.

[0007] Therefore, the number of available Walsh orthogonal codes is dependent on the employed modulation method and the minimum data rate. However, future CDMA mobile communication systems will require a greater number of channels assigned to users in order to improve performance. To this end, future CDMA mobile communication systems will employ traffic channels, pilot channels and control channels, thereby increasing channel capacity.

[0008] However, there are a limited number of available orthogonal codes available for use. This limitation will restrict the increase in channel capacity. To overcome this disadvantage, it is desirable to generate quasi-orthogonal codes, which will limit interference with the orthogonal codes and a variable data rate.

[0009] It is, therefore, an object of the present invention to provide a device and method for generating quasi-orthogonal codes which have the least interference with Walsh orthogonal codes in a CDMA communication system using the Walsh orthogonal codes to perform channel spreading.

[0010] It is another object of the present invention to provide a device and method for generating binary quasi-orthogonal codes having excellent full correlation and partial correlation with Walsh orthogonal codes in a CDMA communication system.

[0011] It is further another object of the present invention to provide a device and method for spreading channel signals using quasi-orthogonal codes limiting interference with orthogonal codes in a CDMA communication system.

[0012] It is still another object of the present invention to provide a channel transmission device and method for spreading channel signals selectively using Walsh orthogonal codes and binary quasi-orthogonal codes in a CDMA communication system.

[0013] To achieve the above objects, there is provided a method for generating a binary quasi-orthogonal code in a CDMA communication system. The method comprises generating an m-sequence, and specific left and right sequences having a good full correlation property with the m-sequence; column permuting the specific left and right sequences in a column permutation method used when converting the m-sequence to a Walsh orthogonal code, and connecting the left sequences to the right sequences to generate mask candidates; operating the mask candidates and Walsh orthogonal codes having the same length as the mask candidates to generate quasi-orthogonal code candidate families; and selecting a quasi-orthogonal code out of the generated quasi-orthogonal code candidate families, the selected quasi-orthogonal code satisfying a partial correlation property with the Walsh orthogonal codes, and selecting a mask related to generation of the selected quasi-orthogonal code.

[0014] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[0015] FIG. 1 is a diagram illustrating channel separation using Walsh orthogonal codes in a conventional CDMA communication system;

[0016] FIG. 2 is a diagram illustrating a partial correlation property between a Walsh orthogonal code and a quasi-orthogonal code, satisfying Condition 3 according to an embodiment of the present invention;

[0017] FIG. 3 is a diagram illustrating a matrix Q for shifted sequences according to an embodiment of the present invention;

[0018] FIG. 4 is a diagram illustrating a matrix Q′ for quasi-orthogonal code candidates added to Walsh orthogonal codes after performing column permutation for the matrix Q of FIG. 3 according to an embodiment of the present invention;

[0019] FIG. 5 is a flow chart illustrating a procedure for generating binary quasi-orthogonal codes according to a first embodiment of the present invention;

[0020] FIG. 6 is a diagram illustrating channel separation using Walsh orthogonal codes and quasi-orthogonal codes generated according to an embodiment of the present invention, in a CDMA communication system;

[0021] FIG. 7 is a bock diagram illustrating a channel transmitter using quasi-orthogonal codes generated according to an embodiment of the present invention, in a CDMA communication system;

[0022] FIG. 8 is a block diagram illustrating a quasi-orthogonal code generator for the channel transmitter of FIG. 7 according to an embodiment of the present invention in a CDMA communication system;

[0023] FIG. 9 is a diagram illustrating a quasi-orthogonal code mask table in the quasi-orthogonal code generator of FIG. 8 according to an embodiment of the present invention, by way of example;

[0024] FIG. 10 is a diagram illustrating a Walsh orthogonal code table in the quasi-orthogonal code generator of FIG. 8 according to an embodiment of the present invention, by way of example;

[0025] FIG. 11 is a diagram illustrating a table for generating quasi-orthogonal codes in the quasi-orthogonal code generator of FIG. 8 according to another embodiment of the present invention;

[0026] FIG. 12 is a block diagram illustrating a channel transmitter for spreading channel signals using quasi-orthogonal codes and Walsh orthogonal codes in a CDMA communication system;

[0027] FIG. 13 is a detailed block diagram illustrating a scheme for spreading channel signals transmitted using quasi-orthogonal codes in FIG. 12; and

[0028] FIG. 14 is a detailed block diagram illustrating a scheme for spreading channel signals transmitted using quasi-orthogonal codes and Walsh orthogonal codes in FIG. 12.

[0029] A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the invention in unnecessary detail.

[0030] The present invention generates quasi-orthogonal codes, which will have limited interference with orthogonal codes, in order to increase the channel capacity or maximize a capacity of a single cell in a CDMA communication system.

[0031] Binary quasi-orthogonal codes should satisfy the following Equations: $\begin{array}{cc}\left|\stackrel{N}{\sum _{t=1}}{\left(-1\right)}^{S_i\left(t\right)+W_k\left(t\right)}\right|\le {\theta }_{min}\left(N\right)\dots \left(1\right)& \text{<Equation 1>}\\ \left|\stackrel{N}{\sum _{t=1}}{\left(-1\right)}^{S_i\left(t\right)+S_i^{\prime }\left(t\right)}\right|\le {\theta }_{min}\left(N\right)\dots \left(2\right)& \text{<Equation 2>}\\ \left|\stackrel{\frac{N}{M}\left(l+1\right)}{\sum _{t=1+\left(\frac{N}{M}l\right)}}{\left(-1\right)}^{S_i\left(t\right)+W_k\left(t\right)}\right|\le {\theta }_{min}\left(\frac{N}{M}\right)\dots \left(3\right)& \text{<Equation 3>}\end{array}$

[0032] where i=0,1,2, . . . ,log₂M−1.

[0033] In Equations (1) to (3), W_k(t) denotes the k-th sequence of a Walsh orthogonal code having length N (1≦k≦N) and S_i(t) denotes an i-th quasi-orthogonal code having length N (1≦i≦X), where X denotes the number of quasi-orthogonal codes satisfying Conditions 1 to 3. Condition 1 expressed by Equation (1) means that the full correlation between the k-th Walsh orthogonal code W_k(t) (1≦k≦N, 1≦t≦N) and the i-th quasi-orthogonal code S_i(t) (1≦i≦X, 1≦t≦N) should not exceed θ_min(N). Condition 2 expressed by Equation (2) means that the full correlation between an i-th line and an i′-th line of a quasi-orthogonal code should not exceed θ_min(N). Condition 3 expressed by Equation (3) means that a partial correlation should not exceed ${\theta }_{min}\left(\frac{N}{M}\right),$

[0034] when the partial correlation is taken for respective parts $\frac{N}{M}$

[0035] obtained by dividing, by M, the length N of a k-th line of an Walsh orthogonal code and an i-th line of a quasi-orthogonal code.

[0036] Here, Condition 1 of Equation (1) represents the full correlation property of a Walsh orthogonal code and a quasi-orthogonal code, and means the minimum correlation value that a quasi-orthogonal code can have theoretically as an absolute correlation value with a Walsh orthogonal code, wherein θ_min(N)={square root over (N)}. Condition 2 of Equation (2) represents a condition for a full correlation property between quasi-orthogonal codes. Condition 3 of Equation (3) represents a partial correlation property between a Walsh orthogonal code and a quasi-orthogonal code, as shown in FIG. 2. In FIG. 2, M=2^m(0≦m≦log₂N). The partial correlation of Equation 3, shown in FIG. 2, satisfies a correlation property for the case where the N/M parts of the orthogonal code are transmitted while the data rate increases during a data service. This partial correlation satisfies a correlation property at this moment. For example, when N=256, ${\theta }_{min}\left(\frac{N}{M}\right)$

[0037] values are shown in Table 1.

[0038] The results of Table 1 can be generally extended. For example, when N=1024 and M=2, for the partial correlation during length 512 out of an orthogonal code of length 1024, a full correlation bound θ_min(N) between an orthogonal code of length 512 and a sequence other than the orthogonal code should be considered. Table 2 shows the relationship between the length N and the minimum correlation value θ_min(N).

[0039] Active research has been carried out regarding the sequences satisfying Conditions 1 and 2. For these sequences, there are known Kasami sequences of length 2^2mand Gold sequences of length 2^2m+1. That is, both the Kasami sequence family and Gold sequence family have a good cross correlation property. A full correlation property for the above sequence families is well known.

[0040] No research has been conducted with regard to providing a sequence satisfying Condition 3. However, it is very important for the IS-95B standard or the future CDMA system supporting the variable data rate to satisfy Condition 3.

[0041] In an embodiment of the present invention, Kerdock code families are generated as Family A sequences so as to generate binary quasi-orthogonal code families having the increased number of code families satisfying the above conditions.

[0042] First, a reference will be made to a method for generating Kerdock codes. For formation, see A. R. Hammons, P. V. Kumar, A. R. Calderbank, N. J. A. Sloane, and P. Sole, “The Z4-linearity of Kerdock, Preparata, Goethals and related codes,”, IEEE Trans. Inform. Theory, vol.IT-40, pp. 301-319, Mar., 1994.

[0043] A binary Kerdock code of length 2^m−2 is expressed as the sum of a sequence of length 2^m−2 generated by repeating twice an m-sequence of length 2^m−1−1, and a specific sequence of length 2^m−2, wherein the specific sequence is generated by arranging left and right sequences each having length 2^m−1−1. First, a binary primitive polynomial f(x) of the (m-1)-th degree is selected to obtain a quasi-orthogonal code sequence of length 2^m. It is possible to construct a Galois Field GF(2^m−1) using the characteristic polynomial f(x). When a root of the characteristic polynomial f(x) is represented by β, Kerdock codeword can be expressed as Equation (4) below by using a trace function in the Galois Field GF(2^m−1).

[0044] <Equation 4>

[0045] c(t,x₁)=Q_γ(t,x₁)+L_η,c,c(t,x₁)  (4)

[0046] where, Q_γ(t,x₁)=σ₂(γα¹)+x₁σ₁(γα¹),

[0047] L_η,c,c(t,x₁)=σ₁(ηβ^t)+c₀x₁+c₁,

[0048] η,γ ∈{0,1,β,β²,β³, . . . ,β²⁻²},

[0049] c₀,c₁,x₁∈ GF(2), ${\sigma }_1\left(x\right)=T{\gamma }_{m-1}\left(x\right)=\underset{S=0}{\sum ^{m-2}}{x}^{2^S}\text{, and }{\sigma }_2\left(x\right)=\underset{S=0}{\sum ^{\left(m-2\right)\text{/}2}}T{\gamma }_{m-1}\left(x^{1+2^S}\right).$

[0050] In Equation (4), σ₁(ηβ^t) is an m-sequence of length 2^m−1−1, and a Walsh orthogonal code of length 2^m−1having symbols of “0” and “1” can be obtained by substituting “0” or β^t(0≦i≦2²⁻²) for η and then column permuting it.

[0051] Further, in Equation (4), x₁has a value of “0” and “1”, and it is possible to obtain (2^m−1−1) sequences of length 2^m−1by inserting “0” at the head of the sequence Q_γ(t,x₁) of length 2^m−1−1 for the respective x₁and then substituting β^t(0≦i≦2²−2) for γ. At this point, sequences of length 2^m−2 simultaneously satisfying Conditions 1 and 2 are generated by arranging left sequences of length 2^m−1−1 generated for x₁=0 and right sequences of length 2^m−1−1 generated for x₁=1, as shown in FIG. 9; there exist (2^m−1−1) sequences of length 2^m−2. Therefore, the sequences of length 2^m−2 are called mask candidates, and a mask candidate value of the sequence family for γ=β^twill be expressed as Si(t), as shown in Equation (7) below.

[0052] <Equation 5>

[0053] K=[S₀(t),S₁(t), . . . ,S₂₋₂(t)]  (5)

[0054] where t=0,1,2, . . . ,2^m−3.

[0055] It is possible to crease a matrix Q of FIG. 3 by inserting “0” at the head of each sequence in the set K of Equation (5). The matrix Q has (2^m−1−1)*2^mrows and 2^mcolumns. Here, through column permutation, there are generated orthogonal codes of length 2^mand (2^m−1−1) sequences satisfying Conditions 1 and 2; and it is possible to obtain (2^m−1−1)*2^mquasi-orthogonal code candidates satisfying Conditions 1 and 2 by combining the above sequences as masks with Walsh orthogonal codes. Next, sequences satisfying Condition 3 will be extracted from the (2^m−1−1)*2^msequences.

[0056] FIG. 5 illustrates a procedure for generating quasi-orthogonal candidate sequences of length 2^m.

[0057] Referring to FIG. 5, in step 511, sequences, Q_γ(t,x₁), t=0,1, . . . ,2^m−1−2, x₁=0,1, of length 2^m−1−1 for the respective x₁, i.e., the left sequences and the right sequences are generated.

[0058] In step 513, let m(t)=σ₁(ηβ^t)=Tγ_m−1(β^(t+τ)), when η=β^τ. By doing so, a column permutation function σ for a binary sequence can be defined as: $\begin{array}{cc}\sigma \text{:}\left\{0,1,2,\dots ,2^{m-1}-2\right\}->\left\{1,2,\dots 2^{m-1}-1\right\}\sigma \left(t\right)=\underset{i=0}{\sum ^{m-2}}m\left(t+i\right)2^{m-2-i}& \text{[Equation 6]}\end{array}$

[0059] Thereafter, in step 515, “0” is inserted at the head of each partial sequence of length 2^m−1−1 generated in step 511 to increase the length to 2^m−1and then, for each t, a sequence of length 2^mis created by arranging a partial sequence of length 2^m−1for x₁=0 and successively arranging a partial sequences of length 2^m−1for x₁=1. This can be defined as: $\begin{array}{cc}\left[d_i\left(t\right)|t=1,\dots ,2^m,i=1,2,\dots ,2^{m-1}-1\right]d_i\left(t\right)=\{\begin{array}{cc}0,& \mathrm{if}t=1\\ S\left(t+i-3\right),& \mathrm{if}t=2,3,\dots ,2^{m-1}\\ 0,& \mathrm{if}t=2^{m-1}+1\\ S\left(t+i-4\right),& \mathrm{if}t=2^{m-1}+2,2^{m-1}+3,\dots 2^m\end{array}& \text{[Equation 7]}\end{array}$

[0060] Then, in step 517, the leading 2^m−1part and the following 2^m−1part of the sequence generated in step 515 are independently column permuted by the column permutation function generated in step 513. That is, in step 517, new sequences are generated, which can be defined as: $\text{[Equation 8]}\left[e_i\left(t\right)|t=1,\dots ,2^m,i=1,2,\dots ,2^{m-1}-1\right]$ $e_i\left(t\right)=\{\begin{array}{cc}{d}_i\left(t\right),& \mathrm{if}t=1\\ d_i\left({\sigma }^{-1}\left(t-1\right)+2\right),& \mathrm{if}t=2,3,\dots ,2^{m-1}\\ d_i\left(t\right),& \mathrm{if}t=2^m\\ d_i\left({\sigma }^{-1}\left(t-1-2^{m-1}\right)+2+2^{m-1}\right),& \mathrm{if}t=2^{m-1}+2,2^{m-1}+3,\dots ,2^m\end{array}$

[0061] Thereafter, in step 519, quasi-orthogonal codes are arranged as shown in FIG. 4 using the sequences generated in the above process, in accordance with Equation (9) below.

[0062] [S_ij(t)|t=1,2, . . . ,2^m]S_ij(t)=e_i(t)+W_j(t)(mod 2), i=0,1,2, . . . ,2^m−1−2, j=0,1, . . . ,2^m−1  [Equation 9]

[0063] At this point, [W_j(t)|t=1,2, . . . ,2^m, j=0,1, . . . ,2^m−1] represents a Walsh sequence, being an orthogonal code, which is expressed with symbols of “0” and “1”.

[0064] A detailed description will be now made regarding the procedure for generating binary quasi-orthogonal code masks with reference to FIG. 5, by way of example.

[0065] Herein, it is assumed that 𝑓(x)=x³+x+1 is used for the binary primitive polynomial. Accordingly, in step 511, let the root of g(x) be β to determine specific sequences. That is, β³+β+1=0. For convenience, β, β², β³, β⁴, β⁵, β⁶and β⁷will first be determined, as follows.

[0066] β=β

[0067] β²=β²

[0068] β³=β+1

[0069] β⁴=β²+β

[0070] β⁵=β³+β²=(β+1)+β²=β²+β+1

[0071] β⁶=β³+β²+β=(β+1)+β²+β=β²+1

[0072] β⁷=β³+β=(β+1)+β=1

[0073] When γ=β⁰=1, σ₁(1·β^t) and σ₂(1·β^t) will be determined prior to determining Q₁(t,x₁)=σ₂(1·β^t)+x₁σ₁(1·β^t). ${\sigma }_1\left({\beta }^t\right)=\underset{S=0}{\sum ^2}{\left({\beta }^t\right)}^{2^S}$

[0074] will be determined as follows: $\mathrm{for}t=0,T\left(1\right)=\underset{S=0}{\sum ^2}{1}^{2^S}=1+1+1=1$ $\mathrm{for}t=1,T\left(\beta \right)=\underset{S=0}{\sum ^2}{\beta }^{2^S}=\beta +{\beta }^2+{\beta }^4=\beta +{\beta }^2+\left(\beta +{\beta }^2\right)=0$ $\mathrm{for}t=2,T\left({\beta }^2\right)=\underset{S=0}{\sum ^2}{\left({\beta }^2\right)}^{2^S}={\beta }^2+{\beta }^4+{\beta }^8={\beta }^2+{\beta }^4+\beta =0$ $\mathrm{for}t=3,T\left({\beta }^3\right)=\underset{S=0}{\sum ^2}{\left({\beta }^3\right)}^{2^S}={\beta }^3+{\beta }^6+{\beta }^{12}={\beta }^3+{\beta }^6+{\beta }^5=1$ $\mathrm{for}t=4,T\left({\beta }^4\right)=\underset{S=0}{\sum ^2}{\left({\beta }^4\right)}^{2^S}={\beta }^4+{\beta }^8+{\beta }^{16}={\beta }^4+\beta +{\beta }^2=0$ $\mathrm{for}t=5,T\left({\beta }^5\right)=\underset{S=0}{\sum ^2}{\left({\beta }^5\right)}^{2^S}={\beta }^5+{\beta }^{10}+{\beta }^{20}={\beta }^5+{\beta }^3+{\beta }^6=1$ $\mathrm{for}t=6,T\left({\beta }^6\right)=\underset{S=0}{\sum ^2}{\left({\beta }^6\right)}^{2^S}={\beta }^6+{\beta }^{12}+{\beta }^{24}={\beta }^6+{\beta }^5+{\beta }^3=1$ ${\sigma }_2\left({\beta }^t\right)=\underset{S=0}{\sum ^{\left(m-2\right)\text{/}2}}T\gamma \left({\left({\beta }^t\right)}^{1+2^S}\right)$ $\mathrm{for}t=0,{\sigma }_2\left(1\right)=\underset{S=0}{\sum ^1}T\gamma \left(1^{1+2^S}\right)=T\gamma \left(1\right)+T\gamma \left(1\right)=1+1=0$ $\mathrm{for}t=1,{\sigma }_2\left(\beta \right)=\underset{S=0}{\sum ^1}T\gamma \left({\beta }^{1+2^S}\right)=T\gamma \left(\beta \right)+T\gamma \left({\beta }^3\right)=0+1=1\mathrm{for}t=2,{\sigma }_2\left({\beta }^2\right)=\underset{S=0}{\sum ^1}T\gamma \left({\left({\beta }^2\right)}^{1+2^S}\right)=T\gamma \left({\beta }^2\right)+T\gamma \left({\beta }^6\right)=0+1=1$ $\mathrm{for}t=3,{\sigma }_2\left({\beta }^3\right)=\underset{S=0}{\sum ^1}T\gamma \left({\left({\beta }^3\right)}^{1+2^S}\right)=T\gamma \left({\beta }^3\right)+T\gamma \left({\beta }^9\right)=T\gamma \left({\beta }^3\right)+T\gamma \left({\beta }^2\right)=1+0=1$ $\mathrm{for}t=4,{\sigma }_2\left({\beta }^4\right)=\underset{S=0}{\sum ^1}T\gamma \left({\left({\beta }^4\right)}^{1+2^S}\right)=T\gamma \left({\beta }^4\right)+T\gamma \left({\beta }^{12}\right)=T\gamma \left({\beta }^4\right)+T\gamma \left({\beta }^5\right)=0+1=1$ $\mathrm{for}t=5,{\sigma }_2\left({\beta }^5\right)=\underset{S=0}{\sum ^1}T\gamma \left({\left({\beta }^5\right)}^{1+2^S}\right)=T\gamma \left({\beta }^5\right)+T\gamma \left({\beta }^{15}\right)=T\gamma \left({\beta }^5\right)+T\gamma \left(\beta \right)=1+0=1$ $\mathrm{for}t=6,{\sigma }_2\left({\beta }^6\right)=\underset{S=0}{\sum ^1}T\gamma \left({\left({\beta }^6\right)}^{1+2^S}\right)=T\gamma \left({\beta }^6\right)+T\gamma \left({\beta }^{18}\right)=T\gamma \left({\beta }^6\right)+T\gamma \left({\beta }^4\right)=1+0=1$

[0075] Therefore, when γ=β⁰=1, the left sequence (for x₁=0) becomes Q₁(t,0)=σ₂(β^t)=1001011 and the right sequence (for x₁=1) becomes Q₁(t,1)=σ₂(β^t)+σ₁(β^t)=1110100.

[0076] In addition, when γ=β¹=β, Q_β(t,x₁)=σ₂(β·β^t)+x₁σ₁(β·β^t)=σ₂(β^t+1)+x₁σ₁(β^t+1)=Q₁(t+1,x₁). Therefore, the sequence Q_β(t,x₁) for γ=β¹=β is equal to the result obtained by shifting once the sequence Q₁(t,x₁) for γ=β⁰=1.

[0077] In this manner, it is possible to determine the left sequence 1001011, the right sequence 1110100 and their shifted sequences. Herein, a sequence shifted i times will be referred to as S_i.

[0078] In step 513, an m-sequence is generated in accordance with the polynomial 𝑓(x)=x³+x+1. Here, the m-sequence according to the polynomial 𝑓(x)=x³+x+1 will become 1001011. A column permutation function for converting the m-sequence to a Walsh orthogonal code in accordance with Equation (8) is calculated using the m-sequence 1001011. Equation (8) implies that the m-sequence is converted into decimal numbers by grouping the m-sequence by three consecutive terms. That is, the first three terms 100 are converted to decimal number 4; the second three terms 001 are converted to decimal number 1; the third three terms 010 are converted to decimal number 2, the fourth three terms 101 are converted to decimal number 5; the fifth three terms 011 are converted to decimal number 3; the sixth three terms 111 are converted to decimal number 7; and the seventh three terms 110 are converted to decimal number 6. The foregoing results can be expressed as: $\mathrm{for}t=0,\sigma \left(0\right)=\underset{S=0}{\sum ^2}m\left(0+S\right)2^{2-S}=4\times m\left(0\right)+2\times m\left(1\right)+m\left(2\right)={\left(100\right)}_2=4$ $\mathrm{for}t=1,\sigma \left(1\right)=\underset{S=0}{\sum ^2}m\left(1+S\right)2^{2-S}=4\times m\left(1\right)+2\times m\left(2\right)+m\left(3\right)={\left(001\right)}_2=1$ $\mathrm{for}t=2,\sigma \left(2\right)=\underset{S=0}{\sum ^2}m\left(2+S\right)2^{2-S}=4\times m\left(2\right)+2\times m\left(3\right)+m\left(4\right)={\left(010\right)}_2=2$ $\mathrm{for}t=3,\sigma \left(3\right)=\underset{S=0}{\sum ^2}m\left(3+S\right)2^{2-S}=4\times m\left(3\right)+2\times m\left(4\right)+m\left(5\right)={\left(101\right)}_2=5$ $\mathrm{for}t=4,\sigma \left(4\right)=\underset{S=0}{\sum ^2}m\left(4+S\right)2^{2-S}=4\times m\left(4\right)+2\times m\left(5\right)+m\left(6\right)={\left(011\right)}_2=3$ $\mathrm{for}t=5,\sigma \left(5\right)=\underset{S=0}{\sum ^2}m\left(5+S\right)2^{2-s}=4\times m\left(5\right)+2\times m\left(6\right)+m\left(7\right)={\left(111\right)}_2=7$ $\mathrm{for}t=6,\sigma \left(6\right)=\underset{S=0}{\sum ^2}m\left(6+S\right)2^{2-S}=4\times m\left(6\right)+2\times m\left(7\right)+m\left(8\right)={\left(110\right)}_2=6$

[0079] The calculated column permutation functions are shown in Table 3A.

[0080] In step 515, “0” is added at the head of every left sequence determined in step 511 and then arranged; succeedingly, “0” is added at the head of every right sequence and then arranged. With regard to the expression of d_i(t) in accordance with S_i(t), when i=0, d₀(t) is the binary sequence S₀(t), at the head of which “0” is added, determined in step 511 for γ=β⁰=1. That is, if S₀(1)=1, S₀(2)=0, S₀(3)=0, S₀(4)=1, S₀(5)=0, S₀(6)=1, S₀(7)=1, S₀(8)=1, S₀(9)=1, S₀(10)=1, S₀(11)=0, S₀(12)=1, S₀(13)=0 and S₀(14)=0 as determined in step 511, then d₀(t) can be determined as follows:

[0081] d₀(1)=0

[0082] d₀(2)=S₀(2−1)=S₀(1)=1

[0083] d₀(3)=S₀(3−1)=S₀(2)=0

[0084] d₀(4)=S₀(4−1)=S₀(3)=0

[0085] d₀(5)=S₀(5−1)=S₀(4)=1

[0086] d₀(6)=S₀(6−1)=S₀(5)=0

[0087] d₀(7)=S₀(7−1)=S₀(6)=1

[0088] d₀(8)=S₀(8−1)=S₀(7)=1

[0089] d₀(9)=0

[0090] d₀(10)=S₀(10−2)=S₀(8)=1

[0091] d₀(11)=S₀(11−2)=S₀(9)=1

[0092] d₀(12)=S₀(12−2)=S₀(12)=1

[0093] d₀(13)=S₀(13−2)=S₀(11)=1

[0094] d₀(14)=S₀(14−2)=S₀(12)=1

[0095] d₀(15)=S₀(15−2)=S₀(13)=1

[0096] d₀(16)=S₀(16−2)=S₀(14)=1

[0097] In addition, when i=1, d₀(t) is the binary sequence S₁(t), at the head of which “0” is added, determined in step 511 for γ=β¹=β. That is, when S₁(1)=0, S₁(2)=0, S₁(3)=1, S₁(4)=0, S₁(5)=1, S₁(6)=1, S₁(7)=1, S₁(8)=1, S₁(9)=1, S₁(10)=0, S₁(11)=1, S₁(12)=0, S₁(13)=0 and S₁(14)=1 as determined in step 511, d₁(t) can be determined as follows.

[0098] d₁(1)=0

[0099] d₁(2)=S₁(2−1)=S₁(1)=0

[0100] d₁(3)=S₁(3−1)=S₁(2)=0

[0101] d₁(4)=S₁(4−1)=S₁(3)=1

[0102] d₁(5)=S₁(5−1)=S₁(4)=0

[0103] d₁(6)=S₁(6−1)=S₁(5)=1

[0104] d₁(7)=S₁(7−1)=S₁(6)=1

[0105] d₁(8)=S₁(8−1)=S₁(7)=1

[0106] d₁(9)=0

[0107] d₁(10)=S₁(10−2)=S₁(8)=1

[0108] d₁(11)=S₁(11−2)=S₁(9)=1

[0109] d₁(12)=S₁(12−2)=S₁(10)=0

[0110] d₁(13)=S₁(13−2)=S₁(11)=1

[0111] d₁(14)=S₁(14−2)=S₁(12)=0

[0112] d₁(15)=S₁(15-2)=S₁(13)=0

[0113] d₁(16)=S₁(16-2)=S₁(14)=1

[0114] Table 3B below shows the forgoing results, wherein c_i(i=0 to 15) denotes an i-th column.

[0115] In step 517, the shifted arranged specific sequences of Table 3B are column permutated with the column permutation functions determined in step 513. Shown in Table 3C below are the binary sequences of Table 3B which are column permuted with the column permutation function determined in step 513.

[0116] By connecting the above left and right sequences of length 8, it is possible to create binary quasi-orthogonal code mask candidates of length 16 shown in Table 3D.

[0117] In summary, the left and right binary sequences are independently column shifted and arranged. Then, “0” is added to every left and right sequence to crease left and right sequences of length 8. Thereafter, the left and right sequences are column permuted according to the column permutation function, respectively. The column permuted left and right sequences are connected to crease quasi-orthogonal code mask candidate functions of length 16.

[0118] Alternatively, after column shifting the left and right binary sequences, arranging the column shifted sequences and then column permuting the arranged left and right sequences according to the column permutation function, “0” can be added at the head of the column shifted left and right sequences to generate the left and right sequences of length 8.

[0119] The created binary quasi-orthogonal code mask candidates are added to Walsh orthogonal codes to verify whether Condition 3 is satisfied.

[0120] Thereafter, in step 519, sequences satisfying Condition 3 are selected from the (2^m−1−1)*2^mquasi-orthogonal code candidates, and the used quasi-orthogonal code mask candidate is selected as a quasi-orthogonal code mask. That is, after the process of step 517, in step 519, sequences satisfying Condition 3, are selected from the determined mask candidate families. With regard to the selecting method, every Walsh orthogonal code and a correlation for every length are determined to examine whether Condition 3 is satisfied, and accordingly selected as a mask when condition 3 is satisfied.

[0121] For example, for an orthogonal code of length 128, a partial correlation with every Walsh orthogonal code of length 64 is determined to examine whether the partial correlation exceeds 8. When the partial correlation exceeds 8, it is not selected as a mask; otherwise, when satisfied, a partial correlation is calculated again for partial length 32 with respect to the mask candidate. Then, it is examined whether the partial correlation value exceeds 8. When the partial correlation exceeds 8, it is not selected as a mask; otherwise, when satisfied, the same operation is performed for the next length. After performing the above process for the partial length of up to 4, a mask candidate having passed all of the above conditions is selected as a quasi-orthogonal code mask candidate satisfying Conditions 1 to 3.

[0122] Quasi-orthogonal code sequences generated using the process of FIG. 5 are determined by the mask function e_i(t). That is, when the mask function e_i(t) satisfy Conditions 1 to 3, it is, possible to obtain 2^mquasi-orthogonal codes. Therefore, if k masks satisfying Conditions 1 to 3 exist, it is possible to obtain k×2^mquasi-orthogonal codes. Table 4 below shows the number of quasi-orthogonal codes according to the m-sequence. Further, Tables 5, 6 and 7 below show the quasi-orthogonal code sequence mask functions e_i(t) for m=6, m=7 and m=8, respectively, determined in the above process.

[0123] Therefore, when an increased number of orthogonal codes are required in a system using Walsh orthogonal codes, it is possible to increase channel capacity, by using the quasi-orthogonal codes generated according to the present invention. In such a case, the quasi-orthogonal codes have limited interference with the Walsh orthogonal codes and provide a fixed correlation value. For example, when N=64, a correlation value between the quasi-orthogonal code and the Walsh orthogonal code is 8 or −8. Further, even when N=256, a partial correlation value (for length N=64) is also 8 or −8. This means that it is possible to accurately predict the interference, providing excellent characteristics.

[0124] The above quasi-orthogonal codes can be used for every link in the CDMA system using the Walsh orthogonal codes. When the quasi-orthogonal codes are used together with the orthogonal codes, the following three options can be considered.

[0125] Option 1

[0126] In a system using the Walsh orthogonal codes and providing a service at a variable data rate, it is possible to freely use the binary quasi-orthogonal codes without restriction as to the length. Further, it is possible to use every binary quasi-orthogonal code sequence at full length.

[0127] Option 2

[0128] One Walsh orthogonal code group and a binary quasi-orthogonal code group is selected to make two orthogonal sets, and the two groups both can provide a service at the variable data rate.

[0129] Option 3

[0130] It is possible to use the Walsh orthogonal code group and every binary quasi-orthogonal code group as one group to allow every code group to support the variable data rate. In this case, a random code characteristic between the binary quasi-orthogonal code groups may occur.

[0131] It is preferable to use the quasi-orthogonal codes according to the application, taking into consideration the above three options. In general, when only the Walsh orthogonal codes are used, the modulating side exchanges a predetermined orthogonal code number with the demodulating side. Therefore, when the orthogonal codes and the quasi-orthogonal codes are used, it is necessary to exchange a predetermined orthogonal code number and a group number (i.e., an index i of the Q′ matrix e_i(t) shown in FIG. 4). In this case, the orthogonal code group is defined as a Group 0, and subsequently, the group numbers are redefined up to 2^m−1.

[0132] A description will now be made regarding a method for applying the quasi-orthogonal code group to a system supporting the variable data rate, like the orthogonal code group. Each element of the quasi-orthogonal code group is comprised of an orthogonal code number and a group number. The group number indicates which e_i(t) is selected in FIG. 4. To service the variable data rate using the quasi-orthogonal code group, a previously allocated orthogonal code number is used as a Walsh orthogonal code number and then, the allocated e_i(t) is added to it for every length N.

[0133] FIG. 6 illustrates a channel separation method using the Walsh orthogonal codes and the quasi-orthogonal codes in an IS-95/IS-95A forward link to extend the channel capacity according to an embodiment of the present invention. FIG. 6 shows an exemplary embodiment where the Walsh orthogonal codes represented by Wi (i=0 to 63) are assigned to various channels and the quasi-orthogonal codes of length 256 represented by Sj (j=0 to 255) are assigned to traffic channels. Therefore, an IS-95/IS-95A forward link can separate 64 channels using the Walsh orthogonal codes, and 256 channels, which is 4 times the number of the Walsh orthogonal codes, using the quasi-orthogonal codes. Therefore, it is possible to increase the channel capacity by using the Walsh orthogonal codes and the quasi-orthogonal codes.

[0134] FIG. 7 illustrates a channel transmitter which separates channels using the quasi-orthogonal codes generated according to the present invention. The mobile communication system shown in FIG. 7 constructs the channel bit streams in a method different from the IS-95 standard.

[0135] Referring to FIG. 7, a signal converter (or demultiplexing & signal mapping part) 711 receiving a data bit stream X, separates the received data bit stream into odd-numbered bits and even-numbered bits to demultiplex the separated signals into I and Q signals, and then converts the separated I and Q signals by converting a signal “0” to “+1” and a signal “1” to “−1”. A quasi-orthogonal code (QOC) generator 713, receiving quasi-orthogonal code mask index and a Walsh orthogonal code (WOC) mask index, generates a quasi-orthogonal code QOFk. An orthogonal spreader 715 channel spreads the signals output from the signal converter 711 with the quasi-orthogonal code QOFk generated from the quasi-orthogonal code generator 713. A PN code generator 717 generates complex PN codes PNi and PNq. A PN spreader 719 PN masks the orthogonally spread signals di and dq with PNi and PNq, respectively. A baseband filter 721 filters the PN spread signals Xi and Xq into baseband signals. A frequency shifter 723 up-converts the baseband filtered signals Xi and Xq to RF signals.

[0136] Although FIG. 7 illustrates one channel transmitter using the quasi-orthogonal code in a CDMA communication system, when the transmitter includes N channels, the corresponding number of orthogonal spreaders should be provided which use the quasi-orthogonal codes.

[0137] Referring to FIG. 7, binary input signals expressed as “0” and “1” are demultiplexed into I and Q signals by the signal converter 711, and then converted in a such manner that a signal “0” is converted to “+1” and a signal “1” to “−1”. The quasi-orthogonal code generator 713 then generates a quasi-orthogonal code QOFk according to the received quasi-orthogonal code mask index and Walsh orthogonal code mask index. The orthogonal spreader 715 multiplies the signals output from the signal converter 711 by the quasi-orthogonal code QOFk to spread a transmission signal on the corresponding channel.

[0138] The quasi-orthogonal code generator 713 can be implemented using two different methods.

[0139] Using a first method, the quasi-orthogonal code generator 713 includes a memory shown in FIG. 11 to generate the quasi-orthogonal codes. That is, every quasi-orthogonal sequence is stored in the memory as shown in FIG. 11 to output the quasi-orthogonal sequence according to the quasi-orthogonal code mask index and the Walsh orthogonal code index. Further, the quasi-orthogonal code generator 713 stores all the available quasi-orthogonal sequences in the memory, and upon receipt of a quasi-orthogonal code index q and an orthogonal code index p, outputs (q*2^m+p)-th quasi-orthogonal sequence QOF_q×2+p. In FIG. 11, upper 2^msequences are Walsh orthogonal code sequences, and the other sequences are quasi-orthogonal code sequences.

[0140] In a second method, the quasi-orthogonal code generator 713 is implemented as shown in FIG. 8.

[0141] Referring to FIG. 8, a quasi-orthogonal code mask generator 811 storing quasi-orthogonal code masks e₀-e_N, outputs a quasi-orthogonal code mask e_iaccording to a received quasi-orthogonal code mask index. That is, the quasi-orthogonal code mask generator 811 includes a table for quasi-orthogonal code masks e_i, which can be implemented as shown in FIG. 9. Referring to FIG. 9, since e₀=0, when e₀is selected, a Walsh orthogonal code is output which is generated from a Walsh orthogonal code generator 813. The Walsh orthogonal code generator 813 storing Walsh orthogonal codes, outputs a Walsh orthogonal code W_jaccording to a received Walsh orthogonal code index. The Walsh orthogonal code generator 813 includes a Walsh orthogonal code table, which can be implemented as shown in FIG. 10. An adder 815 adds the quasi-orthogonal code mask e_ioutput from the quasi-orthogonal code mask generator 811 and the Walsh orthogonal code W_joutput from the Walsh orthogonal code generator 813 to generate a quasi-orthogonal code QOFk.

[0142] A signal spread with the quasi-orthogonal code is applied to the PN spreader 719. The PN spreader 719 then spreads the signals di and dq output from the orthogonal spreader 715 with corresponding PN codes PNi and PNq, thereby performing PN masking. The PN masked signals Xi and Xq are baseband filtered by the baseband filter 721, and the baseband filtered signals are converted to RF signals by the frequency shifter 723.

[0143] FIG. 12 illustrates a channel transmitter which uses a Walsh orthogonal code and a quasi-orthogonal code according to an embodiment of the present invention, in a CDMA communication system. FIG. 12 shows an example in which when transmitting at least two transmission signals, the transmission signals of the respective channel transmitters are spread with unique quasi-orthogonal codes or unique Walsh orthogonal codes. In FIG. 12, a first channel signal and a second channel signal can be signals on dedicated channels, or signals on a dedicated channel and a common channel. The dedicated channel includes a fundamental channel, a supplemental channel and a dedicated control channel, and the common channel includes a common control channel, a pilot channel, a sync channel and a paging channel. In the embodiment, the common channels use Walsh orthogonal codes, and the dedicated channels use either the Walsh orthogonal codes or the quasi-orthogonal codes.

[0144] Referring to FIG. 12, a first signal converter 1211. demultiplexes a received first channel beat stream to I and Q arms, and then converts the demultiplexed signals. That is, the first signal converter 1211 converts a signal “0” of the received first channel bit stream to “+1” and a signal “1” to “−1”, and provides the converted signals to an orthogonal code spreading and PN masking part 1219. Similarly, a second signal converter 1213 demultiplexes a received second channel bit stream to I and Q arms, and then converts the demultiplexed signals. The second signal converter 1213 converts a signal “0” of the received second channel bit stream to “+1” and a signal “1” to “−1”, and provides the converted signals to the orthogonal code spreading and PN masking part 1219.

[0145] An orthogonal code generator 1215 receiving a quasi-orthogonal code (QOC) mask index and a Walsh orthogonal code (WOC) index, generates a Walsh orthogonal code W_jor a quasi-orthogonal code S_iusing the received indexes and provides the generated code to the orthogonal code spreading and PN masking part 1219. Here, the number of orthogonal code generators 1215 should equal the number of the channel transmitters. For example, in FIG. 12, to separately provide orthogonal codes for the first channel and the second channel, there are required two orthogonal code generators. In addition, the orthogonal code generator 1215 should be able to generate quasi-orthogonal codes and Walsh orthogonal codes according to the quasi-orthogonal code mask index and the Walsh orthogonal code index designated by an upper processor, as shown in FIG. 8.

[0146] A PN code generator 1217 generates real and imaginary PN codes PNi and PNq and provides them to the orthogonal code spreading and PN masking part 1219. The orthogonal code spreading and PN masking part 1219 multiplies outputs of the first and second signal converters 1211 and 1213 by associated unique Walsh orthogonal codes Wj or quasi-orthogonal codes Si to spread the input signals, and again multiplies the spread signals by the PN codes PNi and PNq to generate PN masked signals Xi and Xq. A baseband filter 1221 filters the signals Xi and Xq output from the orthogonal code spreading and PN masking part 1219 to baseband signals. A frequency shifter 1223 frequency-shifts the signals output from the baseband filter 1221 to RF signals.

[0147] To ensure a high correlation property for multipath delay, the orthogonal code spreading and PN masking part 1219 can be implemented in various configurations. FIG. 13 illustrates an orthogonal code spreading and PN masking part implemented by a complex multiplier 1319 for complex PN masking, which uses a quasi-orthogonal code Si for the first channel and a Walsh orthogonal code W_ifor the second channel. In addition, the complex multiplier 1319 can also be equally applied to the case where the quasi-orthogonal codes S_iare used for both the first and second channels to perform complex PN masking. FIG. 14 illustrates an orthogonal code spreading and PN masking part, which employs non-complex PN masking and uses a Walsh orthogonal code W_ifor the first channel and a quasi-orthogonal code S_ifor the second channel. In addition, the same scheme can also be equally applied to the case where the quasi-orthogonal codes S_iare used for both the first and second channels and complex PN masking is not employed. In FIGS. 13 and 14, the number of channel spreaders and orthogonal code generators provided as equal the number of channels. Further in FIG. 13, there is required only one complex multiplier.

[0148] Referring to FIG. 13, a first spreader 1311 receiving a first channel signal, multiplies the received first channel signal by a quasi-orthogonal code S_ito output spread signals di1 and dq1. A second spreader 1313 receiving a second channel signal, multiplies the received second channel signal by a Walsh orthogonal code W_ito output spread signals di2 and dq2. A repeater 1317 repeats the PN codes PNi and PNq output from the PN code generator 1217 at predetermined times. The complex multiplier 1319 adds the signal di1 to the signal di2 and the signal dq1 to dq2, output from the first and second spreaders 1311 and 1313; and complex multiplies the added signals di1+di2 and dq1+dq2 by the PN codes PNi and PNq output from the repeater 1317, respectively, to generate PN masked signals Xi and Xq. The complex multiplier 1319 performs complex PN masking by performing the complex operation shown in FIG. 13.

[0149] In FIG. 13, with regard to the quasi-orthogonal code S_iand the Walsh orthogonal code W_iassigned to the first and second channels, sub-codes constituting the orthogonal codes should also be different from each other. Therefore, in the case where the orthogonal code spreading and PN masking part 1219 is implemented in this manner, it is possible to acquire accurate arrival time synchronization between the first channel and the second channel, thereby removing mutual interference between them.

[0150] FIG. 14 illustrates the orthogonal code spreading and PN masking part 1219, wherein the Walsh orthogonal code W_iis used for the first channel and the quasi-orthogonal code S_iis used for the second channel, and non-complex PN masking is employed.

[0151] Referring to FIG. 14, a first spreader 1411 receiving a first channel signal, multiplies the received first channel signal by a Walsh orthogonal code W_ito output spread signals di1 and dq1. Similarly, a second spreader 1413 receiving a second channel signal, multiplies the received second channel signal by a quasi-orthogonal signal S_ito output spread signals di2 and dq2. An adder 1415 adds the spread signal di1 output from the first spreader 1411 to the spread signal di2 output from the second spreader 1413 to generate a signal di1+di2. An adder 1417 adds the spread signal dq1 output from the first spreader 1411 to the spread signal dq2 output from the second spreader 1413 to generate a signal dq1+dq2. A repeater 1421 repeats the PN codes PNi and PNq output from the PN code generator 1217 at predetermined times. A multiplier 1423 multiplies the spread signal di1+di2 output from the adder 1415 by the PN code PNi output from the repeater 1421 to generate a masked signal Xi. A multiplier 1425 multiplies the spread signal dq1+dq2 output from the adder 1417 by the PN code PNq to generate a masked signal Xq.

[0152] In FIG. 14, as stated above, the Walsh orthogonal code W_iassigned to the first channel should be different from the quasi-orthogonal code S_iassigned to the second channel. In the case where the orthogonal code spreading and PN masking part 1219 is implemented in this manner, it is possible to acquire accurate arrival time synchronization between the first channel and the second channel, thereby removing mutual interference between them.

[0153] As described above, the embodiment can generate quasi-orthogonal codes having the least interference with Walsh orthogonal codes. Further, in a mobile communication system which separates channels using the Walsh orthogonal codes, it is possible to increase channel capacity by using the quasi-orthogonal codes regardless of the limitation on the Walsh orthogonal codes.

[0154] While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.