Low cost DSSS communication system
A method and apparatus for direct sequence spread spectrum communication having low complexity. In the transmitter, the data bit is spreaded by a code sequence, a reference code is interleaved with the code sequence, and together they are transmitted by the transmitter. The corresponding receiver recovers the data signal from the received signal by transforming the received signal to a complex base band signal and differentially decoding the base band signal at the chip level. The spreaded signal is despreaded by the chip-level differential operation, thus eliminating the decorrelation operation, and so significantly reducing the cost of the overall system. Furthermore, this system is very robust to any carrier frequency drift since the chip-level differential operation is used, thus releasing the reference clock or crystal requirements, and so further reducing the cost of the system.
1. A method for generating a modulation sequence from a data value in a direct sequence spread spectrum transmitter, said method comprising:
generating a first and second code sequences by interleaving groups of M chips from a pseudo-noise sequence C=(C 1 , C 2 , C 3 , . . . , C N ) of length N with elements C i =±1, said first code sequence C a being
C a (2 jM+i )= C ( jM+i )
C a ((2 j +1) M+i )= C ( jM+i )
and said second code sequence C b being
C b (2 jM+i )= C ( jM+i )
C b ((2 j +1) M+i )=− C ( jM+i )
wherein j=0, . . . , N/M−1 and i=1, . . . , M, and M and N are integers; and
selecting said modulation sequence as said first or second sequence according to the data value.
2. A method as in claim 1 , wherein M=1, said first code sequence is given by C a =(C 1 , C 1 , C 2 , C 2 , C 3 , C 3 , . . . , C N , C N ) and said second code sequence is given by C b =(C 1 , −C 1 , C 2 , −C 2 , C 3 , −C 3 , . . . , C N , −C N ).
3. A method for generating a modulation sequence from a data value having a value ±1 in a direct sequence spread spectrum transmitter, said method comprising:
generating a pseudo-noise sequence C=(C 1 , C 2 , C 3 , . . . , C N ); wherein N is an integer;
up-sampling the pseudo-noise sequence by a factor of two to obtain a first sequence (C 1 , 0, C 2 , 0, C 3 , 0 , . . . , 0, C N , 0);
delaying the first sequence to give a second sequence (0, C 1 , 0, C 2 , 0, C 3 , 0, . . . , 0, C N );
multiplying said second sequence by the data value to obtain a third sequence; and
adding said first and third sequences to obtain said modulation signal.
4. A method for generating a modulation sequence from a data value in a direct sequence spread spectrum transmitter, said method comprising:
generating a pseudo-noise sequence C=(C 1 , C 2 , C 3 , . . . , C N ) of length N, where C i =±1;
generating an interleaved code sequence C b by interleaving groups of M chips from the pseudo-noise sequence C and the pseudo noise sequence −C as
C b (2 jM+i )= C ( jM+i )
C b ((2 j +1) M+i )=− C ( jM+i ),
wherein j=0, . . . , N/M−1 and i=1, . . . , M, and M and N are integers; and
selecting said modulation sequence as said pseudo-noise sequence or C said interleaved sequence according to the data value.
5. A method as in claim 4 , wherein M=1 and said interleaved code sequence is given by C b =(C 1 , −C 1 , C 2 , −C 2 , C 3 , −C 3 , . . . , C N , −C N ).
6. A direct sequence spread spectrum transmitter for generating a modulation sequence from a data value, the transmitter comprising:
means for generating a first code sequence C a having elements
C a (2 jM+i )= C ( jM+i )
C a ((2 j +1) M+i )= C ( jM+i )
where C=(C 1 , C 2 , C 3 , . . . , C N ) is a pseudo-noise sequence of length N with elements C i =±1, j=0, . . . , N/M−1, i=1, . . . , M wherein M and M<N and M and N are integers;
means for generating a second code sequence C b having elements
C b (2 jM+i )= C ( jM+i )
C b ((2 j +1) M+i )=− C ( jM+i );
means for selecting said modulation sequence as said first or second code sequence according to the data value.
7. A direct sequence spread spectrum transmitter as in claim 6 , wherein M=1, C a =(C 1 , C 1 , C 2 , C 2 , C 3 , C 3 , . . . , C N , C N ) and C b =(C 1 , −C 1 , C 2 , −C 2 , C 3 , −C 3 , . . . , C N , −C N ).
8. A direct sequence spread spectrum transmitter for generating a modulation sequence from a data value having a value ±1, the transmitter comprising:
a pseudo-noise generator for generating a sequence C=(C 1 , C 2 , C 3 , . . . , C N ), where N is an integer;
an up-sampler for up-sampling the pseudo-noise sequence by a factor of two to obtain a first sequence (C 1 , 0, C 2 , 0, C 3 , 0, . . . 0, C N , 0);
a delay element for delaying the first sequence to give a second sequence (0, C 1 , 0, C 2 , 0, C 3 , 0, . . . , 0, C N );
a multiplier for multiplying said second sequence by the data value to obtain a third sequence; and
an adder for adding said first and third sequences to obtain said modulation signal.
9. A direct sequence spread spectrum transmitter for generating a modulation sequence from a data value, the transmitter comprising:
pseudo-noise generator for generating a pseudo-noise sequence C=(C 1 , C 2 , C 3 , . . . , C N ) where C i =±1;
means for generating an interleaved code sequence C b having elements
C b (2 jM+i )= C ( jM+i )
C b ((2 j +1) M+i )=− C ( jM+i );
where C=(C 1 , C 2 , C 3 , . . . , C N ) is a pseudo-noise sequence of length N with elements C i =±1, j=0, . . . , N/M−1, i=1, . . . , M, wherein M<N, and M and N are integers; and
means for selecting said modulation sequence as said pseudo-noise sequence or said interleaved sequence according to the data value.
10. A direct sequence spread spectrum transmitter as in claim 9 , wherein M=1 and said interleaved code sequence is C b =(C 1 , −C 1 , C 2 , −C 2 , C 3 , −C 3 , . . . , C N , −C N ).