US20070171812A1  Ovsf code system and methods for cdma stations  Google Patents
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 US20070171812A1 US20070171812A1 US11566002 US56600206A US2007171812A1 US 20070171812 A1 US20070171812 A1 US 20070171812A1 US 11566002 US11566002 US 11566002 US 56600206 A US56600206 A US 56600206A US 2007171812 A1 US2007171812 A1 US 2007171812A1
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 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04J—MULTIPLEX COMMUNICATION
 H04J13/00—Code division multiplex systems
 H04J13/10—Code generation
 H04J13/12—Generation of orthogonal codes

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04J—MULTIPLEX COMMUNICATION
 H04J13/00—Code division multiplex systems
 H04J13/0007—Code type
 H04J13/004—Orthogonal
 H04J13/0044—OVSF [orthogonal variable spreading factor]

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04J—MULTIPLEX COMMUNICATION
 H04J13/00—Code division multiplex systems
 H04J13/16—Code allocation
 H04J13/18—Allocation of orthogonal codes
 H04J13/20—Allocation of orthogonal codes having an orthogonal variable spreading factor [OVSF]

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04J—MULTIPLEX COMMUNICATION
 H04J13/00—Code division multiplex systems
 H04J13/0007—Code type
 H04J13/004—Orthogonal
 H04J13/0048—Walsh
Abstract
A code indexing system for a CDMA communication station that uses orthogonal variable spreading factor (OVSF) codes having a single number mapped to each code. The new code number itself not only provides the code signature, but it is also used for the OVSF code generation. In addition, the system provides easy and fast generation of the available code list without the help of lookup table. This capability improves the dynamic code assignment.
Description
 This application is a continuation of U.S. patent application Ser. No. 10/831,254, filed on Apr. 23, 2004, which is a continuation of U.S. patent application Ser. No. 10/419,485, filed Apr. 21, 2003, which is a continuationinpart of U.S. patent application Ser. No. 10/040,513, filed Dec. 28, 2001, now U.S. Pat. No. 6,552,996 which in turn claims the benefit of U.S. Provisional Patent Application Ser. No. 60/323,120, filed Sep. 18, 2001 which all are incorporated by reference.
 The present invention relates CDMA communication systems and, in particular, to Orthogonal Variable Spreading Factor (OVSF) codes and methods for allocating, generating and determining orthogonality of OVSF codes of different data rates used in CDMA wireless communication stations.
 CDMA communication systems are well known in the art. Generally, such systems comprise communication stations which transmit and receive wireless communication signals between each other. Typically, base stations are provided which are capable of conducting wireless concurrent communications with a plurality of subscriber stations. In CDMA systems specified by the Third Generation Partnership Project (3GPP), base stations are called Node Bs, subscriber stations are called User Equipments (UEs) and the wireless interface between the Node Bs and UEs is known as the Uu interface.
FIG. 3 illustrates a typical 3GPP CDMA system.  Orthogonal variable spreading factor (OVSF) codes provide an orthogonal code set of variable spreading factors. In the prior art, methods exist for allocating a set of OVSF codes of different data rates employing Walsh codes of variable length. The code assignment is made on the basis of channel data rates in a manner that results in improved utilization of the available frequency spectrum.
 An alternative method to obtain OVSF codes based on the code tree structure is based on the modified Hadamard transformation, which requires two indices to indicate a specific code, (i.e., spreading factor and code number). In order to handle the code allocation process, an ASSIGNED list and a BUSY list are conventionally generated.
 These prior art methods have drawbacks in that they require a large amount of memory to store a large number of codes, or require fast processing speeds to generate the codes or effectively allocate the available codes.
 Communication stations for a CDMA system include a code indexing system and method for orthogonal variable spreading factor (OVSF) codes that introduces a single number mapped to each code. The new code number itself not only provides the code signature, but it is also used for the OVSF code generation. In addition, it provides easy and fast generation of the available code list without the help of a lookup table. This capability improves the dynamic code assignment.
 In general, each communication station is configured with a processor and an associated memory device so that OVSF codes are selected from a set of Walsh codes by using an index value p where p represents the (p+1)−2^{i}th Walsh code of the ith layer of Walsh codes where i is an integer such that 2^{i}≦p<2^{i+1}. Accordingly, the memory device includes an index P for the code values p where each successive index value represents one of the OVSF codes such that the index values representing any given layer of Walsh codes are less than the index values representing any Walsh code of any layer of Walsh codes successive to the given layer.
 Preferably, the OVSF code is selected on the basis of a spreading factor SF which is a power of 2 and a Walsh code is selected having an associated index p where SF≦p<2SF.
 The relative orthogonality of a selected Walsh code of layer i represented by index value p with another Walsh code of layer j represented by an index value q is determined by comparing the binary forms of p and q. The binary form of p is a sequence of i significant binary digits and the binary form of q is a sequence of j significant binary digits. The represented Walsh codes are determined to be not orthogonal if either the binary form of p is the same as the i most significant binary digits of the binary form of q or the binary form of q is the same as the j most significant binary digits of the binary form of p.
 A selected Walsh code represented by index value p is easily generated based upon the sequence of significant binary digits representing the binary form of p. Accordingly, the binary form of p is preferably the index P value stored in the memory device of the communication station. The selected Walsh code is generated as the Kronecker Product of i Walsh codes represented by index values 2 and 3 correspondingly to the sequence of i significant binary digits of the binary form of p where each binary digit 0 corresponds to the Walsh code of index value 2 and each binary digit 1 corresponds to the Walsh code of index value 3.
 Alternatively, the selected Walsh code is generated by the Kronecker product of two Walsh codes represented by index values q and r of respective layers of j and k where j+k=i. In such case, the binary form of p is the same as the binary form of q concatenated with the binary forms of (r−2^{k}). Thus the processor is preferably configured to use the binary form of q concatenated with the binary forms of (r−2^{k}) as the binary form of p to generate a selected Walsh code.
 In general, the processor of the communication station is configured to use and select OVSF codes based upon a spreading factor SF where SF is a positive power of 2, using an index value p from a set of codes where for each integer p>3 the corresponding code is defined by C(p)=C(m+2){circle around (×)}C(k), with p=2·k+m, where k and m are integers with m=0 or 1. The codes corresponding to p=1, 2 or 3 are C(1)=[1], C(2)=[1, 1], and C(3)=[1, −1]. Accordingly, p represents the (p+1)−2^{i}th code of an ith layer of codes for SF=2^{i }where i is the unique integer such that 2^{i}≦p<2^{i+1}.
 Other objects and advantages of the invention will be apparent to those skilled in the art from the following description.

FIG. 1 is a prior art OVSF code tree of Walsh codes. 
FIG. 2 is a table representing an indexing system according to the teaching of the present invention. 
FIG. 3 is a schematic diagram of a typical CDMA system in accordance with current 3GPP specifications. 
FIG. 4 is block diagram of a communications station made in accordance with the teachings or the present invention. 
FIG. 5 is diagram of a method in accordance with the teachings or the present invention.  OVSF codes are used in the encoding and decoding of communication signals between communication stations, such as between Node Bs and UEs of the CDMA system depicted in
FIG. 3 . A conventional OVSF code tree structure is shown inFIG. 1 which codes are referred to as Walsh codes herein. C_{SF}(n) denotes the OVSF code word with the spreading factor SF=2^{k}, where n is the code number and k is the layer number. The indices n and k are known as the Hadamard indices. The Walsh codes are conventionally generated recursively from the code tree as shown inFIG. 1 .  The mother codes are the lower layer codes on the path from the specific code to the root code C_{1}(0), and the descendent codes are those produced from the specific code. For example, the mother codes of C_{8}(2) is C_{4}(1), C_{2}(0) and C_{1}(0), and the descendent codes of C_{4}(1) are C_{8}(2), C_{8}(3) and their descendent codes.
 Two codes are orthogonal if, and only if, any one is not the mother code or the descendent code of another. When a specific code is assigned, its mother codes and descendent codes cannot be assigned in the same channel since they are not orthogonal to each other. In other words, two OVSF codes with different spreading factors are not orthogonal when they are on the same branch of the code tree.
 When a new call is requested with a specific data rate, the communication station needs to assign a code from an available set of codes with the corresponding spreading factor. Conventionally, in order to maintain the orthogonality between assigned codes, the set of available code list is updated whenever the new code is assigned. This code set is updated by removing the assigned code itself and all of its descendent and mother codes.
 The inventors have recognized that the prior art codes can each be assigned via a single indice system instead of the dual indice system per the known Hadamard method. In the single indice system of the present invention, a sequential numerical code label p is assigned where p equals the sum of the code layer plus the code number of the conventional tree structure designation using Hadamard indices as reflected in step 30 of
FIG. 5 . As such, the code labels are sequential integers starting with the one code of layer 0 where SF=1 labeled as 1, the two codes of layer 1 where SF=2 labeled as 2 and 3, followed by the 2^{i }codes of layer i where SF=2^{i }labeled as the next 2^{i }integers for each successive layer i from 2 onward as represented inFIG. 2 for layers 0 through 3. Although only codes for spreading factors up to 8 are shown inFIG. 2 , the system is applicable for spreading codes of any power of 2. Accordingly, as illustrated inFIG. 4 , a processor 20 within a communication station 10 that selects the OVSF codes has an associated memory device 15 that includes an index P which contains the code label values p.  In general for each positive integer label p, there is a unique integer i, where 2^{i}≦p<2^{(i+1)}, and p represents the (p+1)−2^{i}th Walsh code of the ith layer of Walsh codes. For example, when p=87, i=6 since 64≦p<128, so that 87 represents the 24th Walsh code of the 6th layer of the Walsh codes. For p=1, i=0 since 2^{0}≦1<2, so 1 represents the first code of the zeroth layer. Generally, for a prior art code designated C_{N}(x), that code is the (x+1) th code of layer N, since the prior art code designations start for each layer with x=0.
 In lieu of using the prior art designations of
FIG. 1 , the prior art treestructured codes can be generated for each positive integer p by the recursive Kronecker procedure where for each integer p>3 the corresponding code is defined by:
C(p)=C(m+2){circle around (×)}C(k), Equation (1)
with
p=2·k+m, Equation (2)  where k and m are integers with m=0 or 1, and the codes corresponding to p=1, 2 or 3 are:
C(1)=[1], C(2)=[1, 1], and C(3)=[1, −1]. Equation (3)  As noted above, for any specified p there is a unique integer i such that 2^{i}≦p<2^{i+1}, so that each p represents a code of only one SF, namely SF=2^{i}. Also, the code represented by p is the (p+1)−2^{i}th code of an ith layer of codes starting with p=1 representing the first code of a zeroth layer.
 Codes generated in this manner meet the following three properties:
 Property 1: The OVSF code for code label p where SF≦p<2SF and SF=2^{L }can be factored into a Kronecker product with L terms of C(2) or C(3) as follows:
C(p)=C(α_{i−1}+2){circle around (×)} . . . {circle around (×)}C(α_{1}+2){circle around (×)}C(α_{0}+2) Equation (4)  where α_{0}=1 and each α_{i}, for i=1 to L−1, is 0 or 1 and
$\begin{array}{cc}p={a}_{0}\xb7{2}^{L1}+{a}_{1}\xb7{2}^{L2}+\dots +{a}_{L1}=\sum _{i=0}^{L1}\left({a}_{i}\xb7{2}^{\left(Li1\right)}\right)& \mathrm{Equation}\text{\hspace{1em}}\left(5\right)\end{array}$  Thus, α_{0}, α_{1 }. . . α_{L−1 }is the binary representation of p where α_{0}=1 and each α_{i}, for i>1, is the binary digit 1 or 0.
 Property 2: The mother codes of C(p) are all of the form:
[C(α_{L−m}+2){circle around (×)} . . . {circle around (×)}C(α_{1}+2){circle around (×)}C(α_{0}+2)] with m=2, 3, . . . L.  Property 3: The descendent codes of C(p) are all C(q){circle around (×)}C(p) with any positive integer q.
 For notational purposes, code designations of the present invention C(p) where p is in decimal form can also be represented as c(p_{binary}), i.e. c(α_{0 }. . . α_{N−1}) where α_{0 }. . . α_{N−1 }is the binary representation of p. For example, C(6)=c(110), since 6 in decimal notation equals 110 in binary notation.
 The code indexing system implemented in CDMA communication stations in accordance with the present invention is illustrated in
FIG. 2 andFIG. 4 . The OVSF code words with their spreading factor up to 8 are shown with both the conventional index using the Hadamard indices and the new code index representations.  The OVSF code layer numbers are shown in the first column. The conventional OVSF code indices are shown in the second column, i.e., SF and code number. The third and fourth columns are the binary and decimal forms of the code labels of the present invention. Preferably, the binary code label values are stored in an index P of a memory device 15 associated with the communication station's processor 20 that selects and generates the Walsh codes.
 The code label index maps a code label to each codeword shown in the last column. The code words of
FIG. 2 correspond directly to the Walsh code sets of 1s and −1s inFIG. 1 with 0s in the codeword being substituted for each −1 of the corresponding Walsh code.  In view of properties 2 and 3, the mother codes and descendent codes of c(α0, α1, α2, α3) are {c(α0), c(α0, α1), c(α0, α1, α2)} and all the codes having binary indices starting with (α0, α1, α2, α3), i.e., c(α0, α1, α2, α3, X, X, X, . . . ) are easily identifiable.
 The code label indexing method in accordance with the present invention has several distinct advantages over prior art methods:
 1) Reduced Number of Bits for Identifying Codes and Increased Capacity
 The new indexing method reduces the memory size requirements for the communication stations since only L+1 bits are required to support the maximum spreading factor 2^{L}, while the conventional indexing requires L+┌log_{2}(L)−1┐ bits for the same case. For example, there is a 3 bit saving for the maximum spreading factor 512. For maximum spreading factor 512, the conventional method needs 4 bits to store ten spreading factors {1, 2, 4, 8, 16, 32, 64, 128, 256, 512} or ten layer numbers {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} correspondingly. In addition, the conventional method needs 9 bits to distinguish between the 512 codes in the 10th layer. Accordingly, a total of 13 bits are conventionally required to identify a particular code within a 10 layer system which supports spreading factors up to and including 512. In comparison, the new method needs only 10 bits to distinguish all the codes of 1023 for spreading factors up to and including 512. The reduction of 3 bits from 13 bits represents a nearly 25% increase in capacity.
 2) Easy to Generate Available Orthogonal Codes During Code Assignment
 As reflected in
FIG. 5 , with the new indexing, the available code with the specific spreading factor are generated directly with a processor in a straightforward way from the binary form of the indices of the assigned code without requiring the use of lookup tables.  For instance, if the code represented by 89 (or 1011001) is assigned, its mother codes and descendent codes cannot be assigned for the use simultaneously to maintain relative orthogonality of used codes. Those codes would normally be marked “BUSY” when code 89 is used. The BUSY codes are easily generated because mother codes of code#89 are code#70 (101100), code#22 (10110), code#11 (1011), code#5 (101), code#2 (10), code#1 (1), and its descendent codes, in a nine layer system, are code#178 (10110010), code#179 (10110011), code#356 (101100100), code#357 (101100101), code#358 (101100110), code#359 (101100111) according to properties 2 and 3.
 In general, each code index has a binary form which is represented by a sequence of significant binary digits of a length equal to the layer of the Walsh code it represents. To determine the relative orthogonality of one Walsh code of layer i, represented by index value p, with another Walsh code of layer j, represented by an index value q, the binary forms of p and q are compared. Since the binary form of p is a sequence of i significant binary digits and the binary form of q is a sequence of j significant binary digits, the represented Walsh codes are not orthogonal only if either the binary form of p is the same as the i most significant binary digits of the binary form of q or the binary form of q is the same as the j most significant binary digits of the binary form of p as reflected in step 32 of
FIG. 5 . For p=87, this condition is true only for q=1, 2, 5, 11, 22, 70, 178, 178, 356, 357, 358 or 359 in a nine layer system as referenced above.  3) Easy to Spread with the Long Code
 The spread sequence with a long code can be obtained by the multiple spreading with shorter spreading factors. The short spreading code numbers are directly extracted from the long code number.
 For example, the spreading code c(α_{0}, α_{1}, . . . , α_{M}) is the Kronecker product of c(α_{0}, α_{1}, . . . , α_{N}) and c(α_{0}, α_{N+1}, α_{N+2}, . . . , α_{M}) with N≦M. Hence, the long spreading can be obtained by two consecutive spreadings, first with c(α_{0}, α_{N+1}, α_{N+2}, . . . , α_{M}) and then with c(α_{0}, α_{1}, . . . , α_{N}).
 4) Easy to Generate the Long Code
 The long code c(α_{0}, α_{1}, . . . , α_{M}) can be obtained by spreading c(α_{0}, α_{1}, . . . , α_{N}) with c(α_{0}, α_{N+1}, α_{N+2}, . . . , α_{M}). There is no additional hardware complexity in generating the long code from the shorter code.
 For example, with references to
FIG. 2 :
c(1110)=c(11){circle around (×)}c(110) Equation (6)
since,
[1, −1, −1,1,1, −1, −1,1]=[1, −1]{circle around (×)}[1, −1,1, −1] Equation (7)
Also:
c(1110)=c(111){circle around (×)}c(10) Equation (8)
since,
[1, −1, −1,1,1, −1, −1,1]=[1, −1, −1,1]{circle around (×)}[1,1] Equation (9)  In general, any Walsh code of layer i, represented by an index value p, can be generated by the Kronecker product of two Walsh codes of layers j and k represented by respective index values of q and r where j+k=i. In such case the binary form of p is equal to the binary form of q concatenated with the binary form of (r−2^{k}).
 5) Reduced Memory Size for Code Table
 The whole spreading code set does not have to be tabulated in the memory. The above multistage spreading scheme needs a much smaller table supporting a lower spreading factor. In addition, there is no need to store the lookup table for the mother codes and descendent codes of all codes. They can be generated in a straightforward manner. For example, a 256 length OVSF code of layer 8 can be generated by two 16 length OVSF codes of layer 4. Hence a code table supporting codes up through layer 4 for a 16 SF is enough to support the easy generation of all codes through layer 8 for a 256 SF. Alternatively, all spreading codes can be generated using the layer two codes c(10) and c(11) per equation 4 above where c(10)=C(2)=[1,1] and c(11)=C(3)=[1, −1].
 6) Enable Easy and Fast Dynamic Channel Assignment (DCA)
 The indexing of the present invention benefits the dynamic code assignment for easy and fast generation of AVAILABLE and BUSY code lists. In conventional indexing methods, lookup tables are required to store and search through all the mother codes and descendent codes of all codes. Conventionally, the lookup tables take up large amounts of memory and the searching process is timeconsuming.
 With the new indexing method, there is no need for lookup tables. All the mother codes and descendent codes can be obtained directly in a straightforward manner from the assigned codes. This enables a easy and fast dynamic code assignment.
 Moreover, only a list of the index values of used codes need be maintained to determine whether an orthogonal code is available and to select such an orthogonal code. Where a code of spreading factor SF is needed and prior used codes indexes p_{1 }. . . p_{n }have been stored to a used code list, each value p from SF through 2SF−1 can be compared against the stored used code index values to determine the availability of an orthogonal code.
 For simplicity, p can be first set equal to SF and the binary form of p can be compared to the binary form of each of the stored used code index values to determine orthogonality as set forth above. If a comparison yields a determination of nonorthogonality, the comparison process can be stopped, p incremented by 1 and the comparison process repeated with the incremented p. The process continues until a p is found which represents a code orthogonal to all the used codes or until p is incremented to equal 2SF. In the first case, the code corresponding to p is selected for use as an orthogonal code and p is stored to the set of used codes. In the second case where p is incremented to equal 2SF, no orthogonal code is available.
 The new code index method is a method for assigning single number that indicates the layer number and the code number, and moreover it indicates the structure of the code and the information about the orthogonality to other codes.
Claims (17)
 1. A communication station configured for use in a communications system where orthogonal variable spreading factor (OVSF) codes are selected from a set of Walsh codes represented as a binary tree having multiple layers such that a zeroth layer has one Walsh code and each successive layer has twice the number of Walsh codes as the layer it succeeds, the communication station comprising:a memory device having an index of consecutive positive integer index values that have a binary form of significant bits where each successive index value represents one of the OVSF codes such that the index values representing any given layer of Walsh codes are less than the index values representing any Walsh code of any layer of Walsh codes successive to the given layer.
 2. The communication station according to
claim 1 further comprising:a processor associated with said memory device configured to select codes from the set of OVSF codes; andsaid processor configured to select a second code that is orthogonal to a previously selected first code such that the binary form of the index value of the first code is not the same as the most significant bits of the binary form of the index value of the second code and the binary form of the index value of the second code is not the same as the most significant bits of the binary form of the index value of the first code.  3. The communication station according to
claim 2 wherein the processor is configured to select a second code by:first indicating as busy the index value of the previously selected first code and the index values of all other codes where the index value of the first code is the same as the most significant bits of the binary form of the index value of the other code or the binary form of the index value of the other code is the same as the most significant bits of the binary form of the index value of the first code, andthen selecting a code having an index value that is not indicated as busy as the second code.  4. The communication station according to
claim 2 wherein said processor is configured to determine the relative orthogonality of a selected OVSF code represented by a first index value with another OVSF code represented by a second index value by comparing the binary forms of the first and second index values.  5. The communication station of
claim 4 wherein said processor is configured to determine OVSF codes to be not orthogonal if either the binary form of the index value of one code is the same as the most significant binary digits of the binary form of the index value of another code or the binary form of the index value of the another code is the same as the most significant binary digits of the binary form of the index value of the one code.  6. The communication station of
claim 2 wherein said processor is configured to generate OVSF codes as the Kronecker Product of codes represented by index values 2 and 3 corresponding to a sequence of significant binary digits of the binary form of its index value where each binary digit 0 corresponds to the code of index value 2 and each binary digit 1 corresponds to the code of index value 3.  7. The communication station according to
claim 1 wherein said communication station is a User Equipment.  8. The communication station according to
claim 1 wherein said communication station is a Node B.  9. The communication station according to
claim 2 wherein said communication station is a User Equipment.  10. The communication station according to
claim 2 wherein said communication station is a Node B.  11. A method for a communication station configured for use in a communications system where orthogonal variable spreading factor (OVSF) codes are selected from a set of Walsh codes represented as a binary tree having multiple layers such that a zeroth layer has one Walsh code and each successive layer has twice the number of Walsh codes as the layer it succeeds, the method comprising:a providing an index of consecutive positive integer index values that have a binary form of significant bits where each successive index value represents one of the OVSF codes such that the index values representing any given layer of Walsh codes are less than the index values representing any Walsh code of any layer of Walsh codes successive to the given layer.
 12. The method according to
claim 11 further comprising:selecting codes from the set of OVSF codes such that a second code that is orthogonal to a previously selected first code is selected such that the binary form of the index value of the first code is not the same as the most significant bits of the binary form of the index value of the second code and the binary form of the index value of the second code is not the same as the most significant bits of the binary form of the index value of the first code.  13. The method according to
claim 12 wherein the selecting a second code is by:first indicating as busy the index value of the previously selected first code and the index values of all other codes where the index value of the first code is the same as the most significant bits of the binary form of the index value of the other code or the binary form of the index value of the other code is the same as the most significant bits of the binary form of the index value of the first code, andthen selecting a code having an index value that is not indicated as busy as the second code.  14. The method according to
claim 12 wherein the relative orthogonality of a selected OVSF code represented by a first index value with another OVSF code represented by a second index value is determined by comparing the binary forms of the first and second index values.  15. The method of
claim 14 wherein OVSF codes are determined to be not orthogonal if either the binary form of the index value of one code is the same as the most significant binary digits of the binary form of the index value of another code or the binary form of the index value of the another code is the same as the most significant binary digits of the binary form of the index value of the one code.  16. The method of
claim 12 wherein OVSF codes are generated as the Kronecker Product of codes represented by index values 2 and 3 corresponding to a sequence of significant binary digits of the binary form of its index value where each binary digit 0 corresponds to the code of index value 2 and each binary digit 1 corresponds to the code of index value 3.  17. The method of
claim 11 wherein OVSF codes are generated as the Kronecker Product of codes represented by index values 2 and 3 corresponding to a sequence of significant binary digits of the binary form of its index value where each binary digit 0 corresponds to the code of index value 2 and each binary digit 1 corresponds to the code of index value 3.
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US20070064590A1 (en) *  20050823  20070322  Tomasz Prokop  Bufferbased generation of OVSF code sequences 
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