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
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7073—Synchronisation aspects
  • H04B1/7075—Synchronisation aspects with code phase acquisition
• • 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
  • H04B—TRANSMISSION
  • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
  • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
  • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
  • H04B2201/7097—Direct sequence modulation interference
  • H04B2201/709709—Methods of preventing interference

Definitions

• the present invention relates to improvements in or relating to spreading codes for quasisynchronous code division multiple access systems, and is more particularly concerned with the optimisation of code assignment for such systems.
• This "synchronisation” can be achieved, for example, by providing each user of the system with a global positioning system (GPS) receiver and triggering the user bit epochs by a GPS clock. The relative delays of received signals can then be held to within a few chips.
• GPS global positioning system
• the candidate spreading codes are m -length sequences, the small Kasami set, the Gold codes and the large Kasami set. These terms are well known in the relevant technical field and no further discussion will be given here. These codes correspond to periodic binary sequences.
• a method of generating codes for a multiplicity of cliques in which the properties within the cliques are orthogonal or pseudo orthogonal and the cross-correlation of pairs selected from different cliques exhibit pseudo orthogonal properties within specific time windows.
• 'clique' is meant a fully connected subset of constraints as will be readily understood by a person of ordinary skill in the art.
• the pairs may be selected from different cliques are such that both elements of each pair are in the small Kasami set.
• the pairs selected from different cliques are such that one element of each pair is in the small Kasami set and the other element of each pair is a Gold code.
• the pairs selected from different cliques are such that each element of each pair is generated by a simplex code.
• the small Kasami set has good cross-correlation properties and it is possible to use several different phases of the same sequence for different users.
• the small Kasami set contains an m-length sequence, and is embedded within the large Kasami set which also contains a set of Gold codes. Whilst it may be thought that the small Kasami set is too small to be useful in a system with many users, this assumption ignores the following:-
• the small Kasami set contains an m -length sequence and some of the most important pairs of transmitters can be assigned different phases of this sequence. Should the small Kasami set prove inadequate, some transmitters can be assigned Gold codes, and the correlation of a small Kasami code-Gold code pair is no worse than that of a Gold code pair. Such an assignment is made to a pair where the interference potential is relatively limited.
• ⁇ x y ⁇ is generally required to be small for all ⁇ . In quasisynchronous systems, it may be simply required that, for any user codes x and , then ⁇ x y ⁇ ) is small for 0 ⁇
• the correlation functions can also be defined in the same way when the periods of the sequences are divisors of p .
• the correlation functions can also be defined over windows of length w ⁇ p . In this case the definitions are:
• K_ ( «) ⁇ -., « ⁇ w, u ⁇ Tw,u® T 2 w...,u® T " 2 ⁇ 2 > ⁇ ,
• the small Kasami set is contained within the large Kasami set, which also includes Gold codes.
• These Gold codes can be expressed as:
• the assignment of spreading codes in CDMA systems appears to have received almost no attention. There appear to be two basic reasons for this. The first reason is concerned with the need for code re-use. In many circumstances, particularly with long spreading codes, the number of codes available may be sufficiently large that code assignment is not a critical problem. An example of this is the long (scrambling) codes proposed for cells in UMTS mobile telephone systems. Code re-use is clearly more critical when the small Kasami set is used. The second reason is concerned with the assignment of codes so that constraints on the codes assigned to pairs of transmitters are satisfied. This does not generally arise when codes are used for which the maximum correlation between all pairs of codes is the same. This does not hold for the codes described here.
• Electromagnetic Compatibility Vol. 43, No. 2, pages210 to 218, 2001 can be modified for spreading code assignment. If f(t,) denote the code assigned to a transmitter t, . Pairs of transmitters t,,t 2 have one of the following:
• the frequency domains (or sets of available frequencies) in the frequency assignment algorithms are replaced by codewords which are equally spaced phases (separated by at least 2-- max +1 cyclic shifts) of the various sequences.
• the cost function is simply a (weighted) sum of the number of constraint violations. Although it is not possible to give a theoretical estimate of the reduction of the ' multiple access interference in the system without making many assumptions, the assignment of the best code pairs to the most critical interfering pairs should ensure that the overall reduction is substantial.
• X. D. Lin and K. H. Chang have proposed a construction for spreading sequences for QS-CDMA ("Optimal PN sequence design for quasisynchronous CDMA communication systems", IEEE Trans.
• ⁇ xA ⁇ > where ⁇ m (d) is the cross-correlation function of certain seed sequences u, v .
• ⁇ x y -1 for the chosen pair of vectors X,Y , but it is easy to see that if m > 3 , this cannot be the case for every pair.
• ⁇ be a primitive element of the Galois field, GF(2"), and
• Tr n '](x) _" l "' ⁇ x ⁇ 2" " be the trace function from GF(2") to GE(2 m ), then the properties of the trace function are as described in MacWilliams and Sloane referenced above.
• the sequence X e of length 2"-l for a balanced seed vector can be constructed from a (2 m -l)xT array with columns labelled 0, 1, ..., T- ⁇ as follows:- If s, - ⁇ , then the tth column is a column of zeros. If -., ⁇ ⁇ , then the / th column is the transpose of (e Sj , e ⁇ +1 , ..., c , +2 *»_ 2mod(2 *. ) ) a d thus ' s a cyclic shift of e .
• ⁇ e ( e s 0 s e S ⁇ ⁇ > — ⁇ > e ⁇ ⁇ . t • > e ⁇ ⁇ > • •• ⁇ > e !2(7 ._ l) > e *--. ⁇ > •• • ⁇ > e ⁇ i ,
• ⁇ ef (0) -1 for any pair of seed vectors. It is also required that all the vectors must be cyclically distinct.
• a simplex code is the dual of a Hamming code as discussed by F. J.
• MacWilliams and N. J. A. Sloane in "The theory of error-correcting codes", Amsterdam, Elsevier 1996, 9 th Edition. It is a linear code with a generator matrix with columns formed from the set of all distinct, non-zero (0, 1) vectors of length m in some order. The code has parameters (2 m -1, m, 2 (m_1) ) . Thus a simplex code is an equidistant code and all 2"' -l non-zero codewords have weight 2"' "1 . It follows that if these 2"' -l non-zero codewords are used as seed vectors, then ⁇ ef (0) -l for any pair.

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• 2003
  • 2003-12-18 WO PCT/GB2003/005512 patent/WO2004057784A1/en not_active Ceased
  • 2003-12-18 EP EP03780406A patent/EP1576752A1/de not_active Withdrawn
  • 2003-12-18 AU AU2003288567A patent/AU2003288567A1/en not_active Abandoned

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