Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/0007—Code type
  • H04J13/004—Orthogonal
• • 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
• • 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

Definitions

• This invention relates to the field of communications systems and, in particular, to the transmission of spread coded message signals within communications systems.
• each vector w x is a column vector using a 0/1 alphabet or, equivalently, a -1/+1 alphabet.
• a set of code vectors using the 0/1 alphabet is expressed as W b n and a set using the -1/+1 alphabet is expressed as W n .
• the total transmission signal S transmitted by a base station to a mobile station is: k
• the mobile station can multiply signal S by the transpose of its own Walsh code vector.
• the separation of the transmitted message signals at the receiver depends on a zero correlation between the wanted signal and all of the other message signals.
• n message signals and separate them are available, as previously described.
• quasi orthogonal functions are vectors that are in addition to the n orthogonal vectors. Quasi orthogonal vectors have been selected from the remaining code vectors in the total binary 2" vector space in order to provide as little interference as possible. Specifically, quasi orthogonal vectors are selected to provide a level of interference that is within acceptable limits, even though the level of interference is not zero.
• a computer search can be performed within the total 2" vector space for binary (+1/-1 alphabet) masks.
• the masks can be applied to the orthogonal vectors to form a new set of vectors that are quasi orthogonal vectors.
• Applying a total of M masks to a set of Walsh code vectors w n the number of quasi orthogonal functions produced is: (M + 1) n.
• the interference resulting from the use of the new code vectors can be tested and the code vectors that provide the lowest correlation can be selected to provide a set of quasi orthogonal vectors.
• a plurality of such masking functions can be found in order to provide a plurality of sets of quasi orthogonal vectors from a single set of orthogonal vectors.
• the quasi orthogonal vectors should be mutually orthogonal with respect to each other. There is a non-zero correlation between at least one code vector in the orthogonal set and one vector in the quasi orthogonal set.
• the goal in picking quasi orthogonal vectors v is to pick the vectors such that max l ⁇ * ⁇ n ⁇
• the normalized correlation between two code vectors x and y_ can be defined as:
• the correlation between two orthogonal vectors is zero.
• a lower absolute value of correlation results in better separation between message signals mixed with the orthogonal vectors and the ones mixed with quasi orthogonal vectors.
• Better signal separation results in lower interference between the signals at the time of decoding.
• the mean square correlation between orthogonal vectors and their corresponding quasi orthogonal vectors where n is a power of two is 1/n.
• the lower bound on the absolute value of correlation can be shown to have the value, 1/ T • This quantity is referred to as the Holtzman lower bound.
• Masks have been found that meet the lower bound for cases wherein n is an even power of two. However, in cases where n is an odd power of two this bound has not been met with an equality.
• the lowest correlation found in the latter case is - I i . Therefore, the interference of the best quasi orthogonal vectors found in the odd power of two case using the computer search technique is J times the theoretical limit.
• a transmission method in a communications system has an orthogonal vector and a quasi orthogonal masking function for obtaining a quasi orthogonal vector from the orthogonal vector.
• Message signals are transmitted according to the quasi orthogonal vector.
• the method includes receiving the quasi orthogonal masking function and permuting the quasi orthogonal masking function to provide a further quasi orthogonal masking function.
• the further quasi orthogonal masking function is applied to the orthogonal vector to provide a further quasi orthogonal vector.
• the further quasi orthogonal vector is applied to the message signal to provide an encoded message signal for transmitting the encoded message signal within the communications system.
• Fig. 1 shows a block diagram representation of a permutation matrix algorithm suitable for use in the method of the present invention
• Fig. 2 shows a block diagram representation of the quasi orthogonal mask generation algorithm of the present invention
• Fig. 3 shows a block diagram representation of a method for mapping vectors that is suitable for use in the method of the present invention
• Fig. 4 is a block diagram representation of the quasi orthogonal mask generation algorithm of the present invention in a form suitable for use in the binary case
• Fig. 5 is a more detailed representation of a matrix generation step of one embodiment of the mask generation algorithm of Fig. 4;
• Fig. 6 is a more detailed representation of the matrix generation step in another embodiment of the mask generation algorithm of Fig. 4.;
• Fig. 7 is a flow chart representation of a permutation that can be performed upon the quasi orthogonal vectors of the present invention.
• masks m are constructed and applied to orthogonal code vectors in order to provide quasi orthogonal code vectors, wherein the masks are four phase or quaternary phase shift keying (QSPK) masks.
• the signal transmission method of the present invention can require two masks m when transmitting a message signal. One of the two masks can be used for the in phase (I) channel and one can be used for the out of phase (Q) channel.
• the new masks m can be generated using linear feedback shift registers (LFSR).
• LFSR linear feedback shift registers
• a 2 fc -ary LFSR sequence s[t] is a sequence having symbols ⁇ 0, 1, ... , 2 fc -l ⁇ where k is limited to the value 1 in the binary case and two in the quaternary case.
• the coefficients c belong to the set ⁇ 0, 1, ... , 2 fc -l ⁇ and c r ⁇ 0.
• the sequence s[fj is periodic with a period that is less than or equal to 2 r -l. If the period of the sequence s[t] reaches the maximum value 2 r -l, the characteristic polynomial of s[t] is defined as a primitive polynomial and the sequence s[t] is an m-sequence. Sequences of this type are taught in S. W. Golomb, "Shift Register Sequences," Holden Day, San Francisco, CA, 1967.
• a code matrix C includes one period of an m- sequence and one period of each of its cyclic shifts.
• the size of the code matrix C is 2 r -l.
• the code matrix C can be extended by appending a zero bit to each code word in matrix C. The zero is appended at the same bit location of each code word. The inclusion of an all zero vector in this manner forms the code matrix C from the code matrix C.
• the code matrix C has a length T and a size 2 r .
• the code matrix C can be columnwise and rowwise permuted to create the Walsh code w br of size 2 r .
• it is sufficient to obtain permutation matrix P such that the set of row vectors of the matrix product CP are the same as the set of row vectors of w b.r .
• permutation matrix algorithm 10 which is suitable for use in the present invention.
• a submatrix W of matrix w *,r is formed as shown in block 12.
• the submatrix W includes r rows having indices 1, 2, 4, ... , 2 r .
• the indexing of W b is zero based and ranges from 0 to 2'-l.
• Matrix W therefore has r rows and 2' columns. Every column of matrix W is distinct from all of the other columns.
• a submatrix M of code matrix C is then formed as shown in block 14 of permutation matrix algorithm 10.
• Submatrix M has r rows and 2 r columns.
• submatrix M In order to form submatrix M an intermediate submatrix M' having r rows and 2 r -1 columns is formed. Submatrix M' is formed by adding a column containing all zeros to submatrix M.
• the first row of submatrix M' can be any cyclic shift of the m - sequence used in constructing code C.
• the r- ⁇ rows of submatrix M' following the first row are successive shifts by one time unit in each case beginning with the first row. Every column of submatrix M is distinct.
• permutation matrix P can be determined using a matrix computation technique. It will be understood by those skilled in the art that the rows of the matrix CP are the same as the rows of ⁇ br .
• a sequence known as Family A can be determined.
• the Family A sequence is taught, for example, in S. Boztas, P. N. Kumar, R, Hammons, "4-Phase Sequences with Near-Optimum Correlation Properties," IEEE Transactions on Information Theory, IT-38 No. 3 (May 1992), pp 1101-1113.
• c(y) be a binary primitive polynomial of degree r.
• Such a lift of the binary polynomial c(x) to the quaternary polynomial g(x) is a special case of the Hensel lift of polynomials.
• the LFSR sequence with the characteristic polynomial g(x) is defined to be a Family A sequence.
• the sequence has a period 2 r -l.
• quasi orthogonal mask generation algorithm 50 there is shown quasi orthogonal mask generation algorithm 50.
• Quasi orthogonal mask generation algorithm 50 can be used to construct four-phase masks for forming quasi orthogonal vectors of length 2 r .
• a binary primitive polynomial c(x) of degree r is provided as shown in block 52.
• a period of an m - sequence is constructed as shown in block 56.
• Matrix M' having dimensions (2 r -l) X (2 r -l) is constructed as shown in block 58.
• the rows of matrix M' each contain a period of the m-sequence of block 56 along with all of its cyclic shifts.
• Matrix M' is then extended to form matrix M as shown in block 62.
• the extension of matrix M' is performed by adding an all zero column and an all zero row to matrix M'.
• the dimensions of matrix M are therefore 2 r X 21.
• the first column of matrix M can be the all zero column.
• a permutation P is found which column permutes the matrix M to contain the same row vectors as those contained in W.
• ⁇ r ⁇ e permutation matrix method taught hereinabove, or any other method known to those skilled in the art, can be used to perform the operations of block 66.
• a Hensel lift is then performed on the primitive polynomial c(x) obtained in block 62 of mask generation algorithm 50 to provide the polynomial g(x) as described hereinabove.
• the Hensel lift operation is shown in block 72.
• One period of the Family A sequences with the polynomial g(x) as its characteristic polynomial is generated as shown in block 78.
• a sequence of the Family A sequences is selected. The selected sequence can be any one of the Family A sequences having at least one symbol equal to one or three.
• a vector N' of length (2 -l) is constructed.
• the vector N' consists of a period of the Family A sequence selected according to block 78.
• a vector N of length 21 is formed by appending a zero bit at the first bit location to vector N'.
• the vector N is then column permuted using the permute P found in block 66.
• the resulting permuted code word can be used as a masking function for generating quasi orthogonal vectors according to the method of the present invention.
• the quasi orthogonal vectors generated in this manner can be used with symbol mapping to (+1, - 1, +j, -j).
• a total of 127 masks can be generated in this manner for a length 128 Walsh code.
• Two of the masks generated according to quasi orthogonal mask algorithm 50 are set forth in Table I.
• vector mapping function 100 As shown in vector mapping function 100 a quasi orthogonal vector mask can be represented equivalently with symbols from the ⁇ 0, 1, 2, 3 ⁇ alphabet or with symbols from ⁇ +1, -1, -f-j, -j ⁇ alphabet of Table I using the mapping:
• mapping block 108 The output of mapping block 108 can be applied to coded QPSK symbols by mixer 110 to provide a coded message signal output for transmission.
• the method of quasi orthogonal mask generation algorithm 50 can be generalized to all powers of two to yield the optimal quasi orthogonal vectors for each power of two.
• Table II sets forth the correlations and the number of masks provided according to the method of the present invention for several powers of two.
• the present invention provides for the construction and transmission of binary quasi orthogonal code vectors using masking functions obtained using the present invention.
• the length of a masking function in the binary case is an even power of two
• the method of the present invention provides quasi orthogonal functions having the optimal correlation with the Walsh code.
• the length of the masking function is an odd power of two, the correlation between any pair of sets is as least as good as the known results using binary alphabets.
• s[t] is a sequence with symbols ⁇ 0, 1, ... ,2*-l ⁇ that satisfies the relationship set forth hereinabove.
• Such a sequence s[t] has a characteristic polynomial c(x) that is defined as also set forth hereinabove.
• the method for forming binary quasi orthogonal vectors is restricted to the case corresponding to k - 1.
• k - 1 the sequence s[t] is periodic with a period less than or equal to 2 r -l.
• the characteristic polynomial of s[t] can be defined as a binary primitive polynomial.
• the sequence s[t] is defined as an m- sequence.
• a code matrix C can be defined to consist of one period of an m- sequence ml with characteristic polynomial c(x) and one period of all of the cyclic shifts of the m- sequence ml. Thus the size of the code matrix C is 21- 1.
• the code matrix C can be extended by appending a zero bit to each code word within matrix C at the same bit location of each code word. In the preferred embodiment, the appended zero bits can be placed at the first bit location of each code word within matrix C.
• Code matrix C has a size of 2 r x 2 r .
• Code matrix C can be columnwise and rowwise permuted to form the Walsh code ⁇ b . 2 r arid a record of the permutation operations required to form ⁇ b can be made.
• the permutation matrix P can be applied to form the product CP and obtain the same vectors as the set of row vectors of w w ⁇
• Binary quasi orthogonal mask generation algorithm 120 can be used to construct two phase masks for forming quasi orthogonal vectors of length 2 r .
• a binary primitive polynomial c(x) of degree r is provided as shown in block 122.
• primitive polynomial c(x) as its characteristic polynomial, a period of an m-sequence is constructed as shown in block 126.
• Matrix M' having dimensions (2 r -l) X (2 r -l) is constructed as shown in block 128.
• the rows of matrix M' each contain a period of the m - sequence of block 126 along with all of the cyclic shifts of the m- sequence.
• Matrix M' is then extended to form matrix M as shown in block 132.
• the extension of matrix M' is performed by adding an all zero column and an all zero row to the matrix M'.
• the dimensions of matrix M are therefore 2 r X 2 r .
• the first column of the matrix M can be the all zero column.
• a permutation P is found that column permutes the matrix M to contain the same row vectors as those contained in W b and a record of the required permutation operations can be made.
• a code matrix C G or a code matrix C ⁇ is then formed as shown in block 142 of binary quasi orthogonal mask generation algorithm 120.
• the code matrix C G is formed in cases where the degree of the primitive polynomial r is odd and the code matrix C ⁇ is formed when r is even.
• a sequence m2 that forms a preferred pair with the m- sequence ml is obtained as shown in block 160.
• a preferred pair of m-sequences is a pair of m-sequences having a period 2 m -1.
• the preferred pair has the preferred three-valued cross correlation function ⁇ -1 + 2 2 ,-1 ⁇ when m is odd.
• the construction of preferred pairs of m-sequences is taught, for example, by D. Sarwate and M. Pursley, "Crosscorrelation Properties of Pseudorandom and Related Sequences," Proceedings of the IEEE, pp. 593-620, May 1980.
• the sequence m2 has a period 2 - 1.
• the code matrix C' G is then formed from the sequence ra2 as shown in block 168. It is a matrix having one period of each of the m-sequence 2 and all of its distinct cyclic shifts.
• the number of rows in matrix C' G is 2 r - 1 and the number of columns is 2 r - 1.
• the code matrix C G is formed from the code matrix C G by extending the matrix C' G as shown in block 172.
• the extension of the matrix C' G can be performed by appending a zero bit at the same bit location of each code word in the matrix C' G .
• the bit location used for appending the zero can be the first bit location in the preferred embodiment.
• the number of rows in code matrix C G is 2 r -l and the number of columns is 2 r .
• FIG. 6 there is shown a more detailed representation of block 142 of binary quasi orthogonal mask generation algorithm 120 in the case wherein the degree of the primitive polynomial r is even.
• block 142 of binary quasi orthogonal mask generation algorithm 120 in the case wherein the degree of the primitive polynomial r is even.
• a code matrix C is obtained as previously described.
• the code matrix C is then decimated by a factor of 1 + 2 r/2 .
• the sequence m3 is the m- sequence obtained from the m-sequence ml by decimating the sequence ml by a factor 1 + 2 r/2 .
• the sequence m3 has a period of 2 r 2 - 1.
• This decimation operation is an operation C ⁇ , 2 r/ , that is known in the art.
• a code matrix C ⁇ ' is formed as shown in block 188. The formation of the matrix C ⁇ ' is begun by inserting one period of length 2 r/2 - 1 of the m 3 sequence and all its distinct cyclic shifts. This forms the first 2' /2 - 1 columns of code matrix C ⁇ '. The first 2 r/2 - 1 columns are then repeated 2' + 1 times as shown in block 192.
• a code matrix C ⁇ can then be obtained by appending a zero bit at the first location of each code word in the preferred embodiment within the matrix C ⁇ ' as shown in block 200.
• the size of the code matrix C ⁇ can be 2 r 2 - 1 wherein the size is understood to indicate the number of row vectors.
• the number of masking functions obtained using binary quasi orthogonal mask generation algorithm 120 for some exemplary values of length n are set forth in Table III along with the maximum absolute correlation with the Walsh code and the correlation spectrum.
• a row vector f of either code matrix C G or code matrix C ⁇ is selected.
• the permutation P determined herein is then applied to the row vector f as shown in block 140.
• a mask can be provided according to the permutation.
• the mask can be applied to orthogonal vectors to provide quasi orthogonal vectors.
• a lower bound on the minimum correlation between a quasi orthogonal function set and the set of Walsh codes is known as previously described . Furthermore, the minimum correlation has been attained for all code lengths that are a power of two as also described above. Additionally, the constructions of the four phase symbols, the constellation alphabet, is known in the set ⁇ +1, -I, +/, -j ⁇ . However, the masking functions obtained are not necessarily optimized for some specific cases, such as some higher data rate cases. For example, in the so called fat-pipe environment where data rates are increased by giving the user two Walsh codes of length n/2 rather than the original Walsh code of length n some of the masking functions are not optimum.
• n the length of any code vector in a Walsh code wherein n is an integer power of two.
• a vector v (v v ..., v modifier) is a unit vector if:
• W n is the Walsh code of length n and the vector v has symbols that are complex roots of unity.
• v l k [v., v 1+1 , ..., vj.
• n ⁇ ⁇ n be an integer power of two and let j be a positive integer such that j-n j ⁇ n.
• the objective is to obtain the mask v of length n such that, for every such integer n lr j, the following is satisfied:
• Fig 7 there is shown mask generation algorithm 220, the masking functions obtained above can be further permuted in to WO 00/14916 plausible _, PCT/US99/20340
• any permutation within the automorphism group of the first order Reed-Muller codes of the same length can be applied to the masking function that leaves the correlation with the Walsh code of same length unchanged. These permutations can also be applied systematically to obtain permuted masking functions optimal for fat pipe transmission in the sense of the latter equation.
• block b having a length L chips as shown in block 224.
• the length of L is an integer power of two.
• the block b its first chip at chip location k.
• the block bR is a reflection of the block b about an integer point, where L ⁇ x ⁇ n, if the block bR has length L chips, and the first chip of bR is at the chip location x+k. While any kind of permutation known to those skilled in the art can be used one very useful permutation is one that leaves the correlation with the Walsh code of same length unchanged.
• This permutation is obtained by swapping subblocks of length L chips, where L is an integer power of 2, by their reflections w.r.t. around the center point as shown in block 232. It will be understood that reflections around other points can also be used.
• a complex quasi orthogonal function mask of a required length is constructed using the methods taught herein.
• subblocks of length two can be optimized. Procedures for performing this optimization include swapping symbols as previously described. Subblocks of length two obtained previously are swapped to provide subblocks of length four. The subblocks with length four are obtained to provide optimal correlation with the corresponding Walsh code of length four.
• step k+1 sub-blocks of length 2k with optimal correlation with the Walsh code of length 2k are obtained as shown in block 236.
• Up to log 2 n steps can be required in order to obtain the fatpipe optimal quasi orthogonal functions.

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