Chaotic spreading codes and their generation
Generation of a set of spreading codes starts with determining first and second chaotic pseudo-random noise codes having delta-peak-like autocorrelation functions and a low cross-correlation function. Further codes are obtained by the steps: (a) generating a further pseudo-random noise code by computing D k =F(C 1 )+T k C 2 +F(C 2 ), where k represents a positive integral index, D k the generated pseudo-random noise code, C 1 the first code, C 2 the second code, F a binary function based on basic binary operations and T k the operator cyclically shifting a code by k chip positions; (b) adding code D k to the set of already determined pseudo-random noise codes if it has a delta-peak-like autocorrelation and low cross-correlation functions with the pseudo-random noise codes already determined; (c) discarding code D k if the conditions of step (b) are not satisfied; (d) modifying index k and repeating steps (a)-(d) until the cardinal number of the set of determined pseudo-random noise codes reaches the cardinal number of the set of spreading codes to be generated.
1. A method of generating a set of spreading codes, said set having a predefined cardinality, said method comprising:
determining a set containing a first and a second chaotic pseudo-random noise codes, said first and second codes having delta-peak-like autocorrelation functions and a low cross-correlation function;
determining further pseudo-random noise codes by carrying out the steps:
(a) generating a further pseudo-random noise code by computing
D k = F ( C 1 ) + T k C 2 + F ( C 2 ),
where k represents a positive integral index, D k represents the further pseudo-random noise being generated, C 1 represents the first code, C 2 represents the second code, F represents a binary function based on basic binary operations and T k represents the operator that cyclically shifts a code by k chip positions;
(b) adding the code D k to the set containing said first and second pseudo-random noise codes if said code has a delta-peak-like autocorrelation and low cross-correlation functions with the pseudo-random noise codes already determined;
(c) discarding the code D k if the conditions for being added to the set containing said first and second pseudo-random noise codes of step (b) are not satisfied;
(d) modifying index k and repeating steps (a)-(d) until a cardinal number of the set of determined pseudo-random noise codes reaches the predefined cardinality of the set of spreading codes to be generated.
2. Method according to claim 1 , wherein said first chaotic pseudo-random noise code is determined by generating a preliminary set of chaotic pseudo-random noise codes based upon an iterative chaotic map and choosing as said first chaotic pseudo-random noise code the code of said preliminary set that exhibits the best delta-peak-like auto-correlation function of the codes of said preliminary set.
3. The method according to claim 2 , wherein said second chaotic pseudo-random noise code is determined by selecting from said preliminary set a code having delta-peak-like autocorrelation and whose cross-correlation with said first chaotic pseudo-random noise code exhibits only one predominant peak for a certain delay, hereinafter denoted L, said delay preferably corresponding to about half the code length, flipping the first L chips of the selected code and maintaining the remaining chips of the selected code.
4. The method according to claim 2 , wherein said iterative chaotic map is at least one of a tent map, a split shift map, an n-way Bernoulli map.
5. The method according to claim 2 , wherein generating said preliminary set of chaotic pseudo-random noise codes comprises emulating said chaotic map by an extended linear feedback shift register.
6. The method according to claims 1 , wherein said binary function is based on at least one of shifting, flipping and reversing.
7. Method of using of a set of spreading codes generated according to the method of claim 1 in a CDMA system.
8. Method of using of a set of spreading codes generated according to the method of claim 1 in a satellite navigation system.
9. Computer program stored on a storage medium, said computer program contain computer-executable instructions for causing a computer to generate a set of spreading codes, said set having a predefined cardinality, by
determining a set containing a first and a second chaotic pseudo-random noise code, said first and second codes having delta-peak-like autocorrelation functions and a low cross-correlation function;
determining further pseudo-random noise codes by carrying out the steps:
(a) generating a further pseudo-random noise code by computing
+ D k = F ( C 1 )+ T k C 2 + F ( C 2 ),
where k represents a positive integral index, D k represents the further pseudo-random noise being generated, C 1 represents the first code, C 2 represents the second code, F represents a binary function based on basic binary operations and T k represents the operator that cyclically shifts a code by k chip positions;
b adding the code D k to the set containing said first and second pseudo-random noise codes if said code has a delta-peak-like autocorrelation and low cross-correlation functions with the pseudo-random noise codes already determined;
discarding the code D k if the conditions for being added to the set containing said first and second pseudo-random noise codes of step (b) are not satisfied;
(d) modifying index k and repeating steps (a)-(d) until a cardinal number of the set of determined pseudo-random noise codes reaches the predefined cardinality of the set of spreading codes to be generated.