Receiver algorithm for the length 4 CFC
View Patent ↗Decoding CFC codewords in the form of sums of QPSK-modulated spread symbols in a spread spectrum communications systems using comma-free codes with elements made of sums of QPSK-modulated secondary synchronization codes.
1. A method for decoding received sums of QPSK-modulated spreading codes corresponding to elements of CFC codewords, comprising:
(a) despreading received sums of QPSK-modulated spreading codes with each of said spreading codes;
(b) forming linear combinations with coefficients ±1 and ±j of the results of step (a), said combinations corresponding to possible sums as elements of CFC codewords;
(c) finding the maximum of said combinations of step (b);
(d) determining a codeword and cyclic shift from the results of step (c).
2. The method of claim 1 , wherein:
(a) said sums of step (a) of claim 1 are each sums of three QPSK-modulated spreading codes.
3. The method of claim 2 , wherein:
(a) said sums of step (a) of claim 2 are selected from the group consisting of the sums indicated in slot columns of FIG. 1 b wherein C 0 , C 1 , . . . , C 11 represent said spreading codes.
4. The method of claim 2 , wherein:
(a) said sums of step (a) of claim 2 are selected from the group consisting of the sums indicated in the slot columns of FIG. 6 wherein C 0 , C 1 , . . . , C 5 represent said spreading codes.
5. The method of claim 2 , wherein:
(a) said sums of step (a) of claim 2 are selected from the group consisting of the sums indicated in the slot columns of FIG. 7 a wherein C 0 , C 1 , . . . , C 15 represent said spreading codes.
6. The method of claim 1 , wherein:
(a) said sums of step (a) of claim 1 are in time slots of a time-division-duplex transmission.
7. A method for frame synchronization in a time-division duplex code division multiple access system, comprising:
(a) correlating, with each of a set of synchronization codes, received linear combinations of said synchronization codes in time slots, said linear combinations elements of an alphabet for codewords of a common-free code (CFC);
(b) determining a codeword from the results of step (a);
(c) determining frame synchronization from the results of step (b).
8. The method of claim 7 , wherein:
(a) said linear combinations of step (a) of claim 7 are selected from the group of linear combinations of the form b 1 c 1 +b 2 c 2 +b 3 c 3 where each of said b 1 , b 2 , and b 3 is equal to one of ±1 or ±j, and where each of said c 1 , c 2 , and c 3 is selected from said set of synchronization codes.
9. The method of claim 8 , wherein:
(a) said codewords of step (a) of claim 7 have length 4; and
(b) said linear combinations of step (a) of claim 8 are indicated in FIG. 1 b where said C 0 , C 1 , . . . , C 11 represent said set of synchronization codes.
10. The method of claim 8 , wherein:
(a) said codewords of step (a) of claim 7 have length 2; and
(b) said linear combinations of step (a) of claim 8 are indicated in FIG. 6 where said C 0 , C 1 , . . . , C 5 represent said set of synchronization codes.
11. The method of claim 8 , wherein:
(a) said codewords of step (a) of claim 7 have length 4; and
(b) said linear combinations of step (a) of claim 8 are indicated in FIG. 7 a where said C 0 , C 1 , . . . , C 15 represent said set of synchronization codes.