Low cost DSSS communication system

• US 20030152137A1
• Filed: 03/03/2003
• Published: 08/14/2003
• Est. Priority Date: 12/21/2001
• Status: Active Grant

• Current Assignee: Motorola Solutions, Inc.
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: OʼDea, Robert J., Shi, Qicai
• Granted Patent: US 6,985,509 B2
• Time in Patent Office: Days
• US Class Current: 375/146
• CPC Class Codes: H04B 1/70718 with asynchronous demodulation, i.e. not requiring code synchronisation

Abstract

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.

Claims

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=(C₁, C₂, C₃, . . . , C_N) of length N with elements C_i=±

1, said first code sequence C_a being

C_a(2jM+i)=C(jM+i)C_a((2j+1)M+i)=C(jM+i)

and said second code sequence C_b being

C_b(2jM+i)=C(jM+i)C_b((2j+1)M+i)=−

C(jM+i)

where j=0, . . . , N/M−

1 and i=1, . . . , M; 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=(C₁, C₁, C₂, C₂, C₃, C₃, . . . , C_N, C_N) and said second code sequence is given by C_b=(C₁, −

C₁, C₂, −

C₂, C₃, −

C₃, . . . , 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=(C₁, C₂, C₃, . . . , C_N);

up-sampling the pseudo-noise sequence by a factor of two to obtain a first sequence (C₁, 0, C₂, 0, C₃, 0 , . . . , 0, C_N,0);

delaying the first sequence to give a second sequence (0, C₁, 0, C₂, 0, C₃, 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=(C₁, C₂, C₃, . . . , 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(2jM+i)=C(jM+i)C_b((2j+1)M+i)=−

C(jM+i),

where j=0, . . . , N/M−

1 and i=1, . . . , M; and

selecting said modulation sequence as said pseudo-noise sequence or 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=(C₁, −

C₁, C₂, −

C₂, C₃, −

C₃, . . . , C_N, −

C_N).

6. A method for recovering a data value from a received signal in a direct sequence spread spectrum receiver, said received signal having a chip period T_c, a bit period T_b and being modulated by first code sequence C_a or second code sequence C_b generated by interleaving groups of M chips from a pseudo-noise sequence C=(C₁, C₂, C₃, . . . , C_N) of length N with elements C_i=±

1, said first code sequence C_a having elements

C_a(2jM+i)=C(jM+i)C_a((2j+1)M+i)=C(jM+i)

and said second code sequence C_b having elements

C_b(2jM+i)=C(jM+i)C_b((2j+1)M+i)=−

C(jM+i)

where j=0, . . . , N/M−

1 and i=1, . . . , M;

said method comprising;

transforming said received signal to complex baseband signal having in-phase and quadrature components;

delaying said complex baseband signal by a time MT_c equal to M chip periods to obtain a complex delayed signal;

multiplying the complex baseband signal by the conjugate of the complex delayed signal to obtain a complex product signal;

integrating the real part of said complex product signal over the bit period T_b to obtain an integrated value; and

determining said data value from said integrated value.

7. A method as in claim 6, wherein M=1, C_a=(C₁, C₁, C₂, C₂, C₃, C₃, . . . , C_N, C_N) and C_b=(C₁, −

C₁, C₂, −

C₂, C₃, −

C₃, . . . , C_N, −

C_N).

8. A method as in claim 6, wherein said determining comprises comparing said integrated value to a predetermined threshold value.

9. A method as in claim 6, further comprising sampling said baseband complex signal using an analog-to-digital converter.

10. A method for recovering a data value from a received signal in a direct sequence spread spectrum receiver, said received signal having a chip period T_c, a bit period T_b and being modulated by a code C=(C₁, C₂, C₃, . . . , C_N) or a code sequence C_b=(C₁, −

C₁, C₂, −

C₂, C₃, −

C₃, . . . , C_N, −

C_N), where C_i=±

1 and C is a pseudo-noise sequence, said method comprising;

transforming said received signal to complex baseband signal having in-phase and quadrature components;

delaying said complex baseband signal by a time equal to the chip period T_c;

to obtain a complex delayed signal;

multiplying the complex baseband signal by the conjugate of the complex delayed signal to obtain a complex product signal;

integrating the real part of said complex product signal over the bit period T_b to obtain an integrated value; and

determining said data value from said integrated value.

11. A method as in claim 10, wherein said determining comprises comparing said integrated value to a predetermined threshold value.

12. A method as in claim 10, further comprising sampling said baseband complex signal using an analog-to-digital converter.

13. A direct sequence spread spectrum transmitter for generating a modulation sequence from a data value, said transmitter comprising:

means for generating a first code sequence C_a having elements

C_a(2jM+i)=C(jM+i)C_a((2j+1)M+i)=C(jM+i)

where C=(C₁, C₂, C₃, . . . , C_N) is a pseudo-noise sequence of length N with elements C_i=±

1, j=0, . . . , N/M−

1, i=1, . . . , M and M<

N is an integer;

means for generating a second code sequence C_b having elements

C_b(2jM+i)=C(jM+i)C_b((2j+1)M+i)=−

C(jM+i);

means for selecting said modulation sequence as said first or second sequence according to the data value.

14. A direct sequence spread spectrum transmitter as in claim 13, wherein M=1, C_a=(C₁, C₁, C₂, C₂, C₃, C₃, . . . , C_N, C_N) and C_b=(C₁, −

C₁, C₂, −

C₂, C₃, −

C₃, . . . , C_N, −

C_N).

15. A direct sequence spread spectrum transmitter for generating a modulation sequence from a data value having a value ±

1, said transmitter comprising;

a pseudo-noise generator for generating a sequence C=(C₁, C₂, C₃, . . . , C_N);

an up-sampler for up-sampling the pseudo-noise sequence by a factor of two to obtain a first sequence (C₁, 0, C₂, 0, C₃, 0, . . . 0, C_N,0);

a delay element for delaying the first sequence to give a second sequence (0, C₁, 0, C₂, 0, C₃, 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.

16. A direct sequence spread spectrum transmitter for generating a modulation sequence from a data value, said transmitter comprising:

pseudo-noise generator for generating a pseudo-noise sequence C=(C₁, C₂, C₃, . . . , C_N) where C_i=±

1;

means for generating an interleaved code sequence C_b having elements

C_b(2jM+i)=C(jM+i)C_b((2j+1)M+i)=−

C(jM+i);

where C=(C₁, C₂, C₃, . . . , C_N) is a pseudo-noise sequence of length N with elements C_i=±

1, j=0, . . . , N/M−

1, . . . , M and M<

N is an integer; and

means for selecting said modulation sequence as said pseudo-noise sequence or said interleaved sequence according to the data value.

17. A direct sequence spread spectrum transmitter as in claim 16, wherein M=1 and said interleaved code sequence is C_b=(C₁, −

C₁, C₂, −

C₂, C₃, −

C₃, . . . , C_N, −

C_N).

18. A direct sequence spread spectrum receiver for recovering a data value from a received signal in a direct sequence spread spectrum receiver, said received signal having a chip period T_c, a bit period T_b and being modulated by first code sequence C_a or second code sequence C_b generated by interleaving groups of M chips from a pseudo-noise sequence C=(C₁, C₂, C₃, . . . , C_N) of length N with elements C_i=±

1, said second code sequence C_b having elements

C_b(2jM+i)=C(jM+i)C_b((2j+1)M+i)=−

C(jM+i)

where j=0, . . . , N/M−

1 and i=1, . . . , M;

said receiver comprising;

a converter operable to transform said received signal to a complex baseband signal having in-phase and quadrature components;

a delay element operable to delay said complex baseband signal by a time MT_c, equal to M chip periods, to obtain a complex delayed signal;

a multiplier operable to multiply the complex baseband signal by the conjugate of the complex delayed signal to obtain a complex product signal;

an integrator operable to integrate the real part of said complex product signal over the bit period T_b to obtain an integrated value; and

a decision logic circuit operable to determine said data value from said integrated value.

19. A direct sequence spread spectrum receiver as in claim 18, wherein said first code sequence C_a has length 2N and elements

C_a(2jM+i)=C(jM+i)C_a((2j+1)M+i)=C(jM+i)

20. A direct sequence spread spectrum receiver as in claim 18, wherein said first code sequence C_a is C and has length N.