Spectrum shaping using code-hopping CDMA

• US 20070030883A1
• Filed: 08/02/2005
• Published: 02/08/2007
• Est. Priority Date: 08/02/2005
• Status: Active Grant

• Current Assignee: Mitre Corporation
• Original Assignee: Mitre Corporation
• Inventors: Saidi, Ali
• Granted Patent: US 7,711,028 B2
• Time in Patent Office: Days
• US Class Current: 375/133
• CPC Class Codes: H04J 13/00 Code division multiplex systems for frequency hopping H04B1/713
• CPC Class Codes: H04J 13/16 Code allocation

Abstract

A method for spectrally shaping a communication signal is provided. The method comprises generating a plurality of spreading codes, assigning a probability to each of the plurality of spreading codes, and modulating the communication signal by hopping between the plurality of spreading codes such that a time-average proportion of time spent using any one of the plurality of spreading codes is equal to the probability assigned to that spreading code.

Claims

1. A method for spectrally shaping a communication signal, comprising:

(a) generating a plurality of spreading codes;

(b) assigning a probability to each of the plurality of spreading codes; and

(c) modulating the communication signal by hopping between the plurality of spreading codes such that a time-average proportion of time spent using any one of the plurality of spreading codes to modulate the communication signal is equal to the probability assigned to that spreading code.

2. The method of claim 1, wherein the plurality of spreading codes are short-code CDMA sequences.

3. The method of claim 1, wherein the probability assigned to each of the plurality of spreading codes represents the probability that the spreading code will be selected to modulate an information bit of the communication signal.

4. The method of claim 3, wherein a power spectral density function of the communication signal is a weighted sum of power spectral density functions of the spreading codes, wherein code weights in said weighted sum correspond respectively to the probabilities assigned to the spreading codes.

5. The method of claim 4, wherein the spectrum of the communication signal is shaped by varying the probabilities assigned to the spreading codes.

6. The method of claim 5, wherein the spectrum of the communication signal is shaped without varying the shape of a pulse function used to pulse modulate the communication signal prior to code modulation in step (c).

7. The method of claim 5, wherein the spectrum of the communication signal is shaped to approximate a desired spectrum by biasing the probabilities toward spreading codes having desired spectrum components and away from spreading codes having undesired spectrum components.

8. The method of claim 7, wherein the desired spectrum components are selected to provide sufficient spectral separation between the communication signal and signals in adjacent communication bands, thereby minimizing adjacent-band interference.

9. The method of claim 5, wherein the spectrum of the communication signal is flattened by assigning an equal probability to each of the plurality of spreading codes.

10. A method for spreading a communication signal having a plurality of information bits, comprising:

(a) selecting a spreading code from a plurality of spreading codes;

(b) multiplying a first information bit of said plurality of information bits with the selected spreading code; and

(c) repeating steps (a) and (b) for each subsequent information bit of said plurality of information bits,

wherein the spreading code is selected according to a pre-determined probability distribution function for selecting spreading codes.

11. The method of claim 10, wherein the pre-determined probability distribution function assigns for each spreading code of said plurality of spreading codes a probability that said spreading code will be selected.

12. The method of claim 11, wherein the probability of each spreading code for selection corresponds to a time-average proportion of time that said spreading code is used for spreading the communication signal.

13. The method of claim 10, wherein the spreading code is selected for a duration equal to an information bit period.

14. The method of claim 10, wherein the spreading codes are pairwise orthogonal.

15. The method of claim 10, wherein the spreading codes are pairwise quasi-orthogonal.

16. The method of claim 10, wherein the spreading codes are short-code CDMA sequences.

17. A multiple access communication method for use in a spectrally-constrained multiple access communication system, the method comprising:

(a) generating a plurality of spreading codes;

(b) selecting a hopping pattern, said hopping pattern defining, for each spreading code, a time-average proportion of time that said spreading code will be used to modulate information at each of a plurality of transmitters;

(c) modulating information, according to said hopping pattern, at the plurality of transmitters to generate a plurality of modulated signals; and

(d) transmitting the modulated signals to a receiver such that any two simultaneously transmitted signals are modulated using different spreading codes.

18. The method of claim 17, further comprising:

(e) receiving the modulated signals at the receiver; and

(f) demodulating the received signals, based on a priori knowledge of the spreading codes, to restore the information encoded therein.

19. The method of claim 17, wherein the spectrum of the transmitted signals is dependent on the selected hopping pattern.

20. The method of claim 19, wherein the hopping pattern is selected to conform the spectrum of the transmitted signals to an allocated frequency band, thereby minimizing interference to adjacent frequency bands.

21. The method of claim 18, wherein the spreading codes are short-code CDMA sequences.

22. The method of claim 18, wherein the spreading codes are pairwise orthogonal or pairwise quasi-orthogonal.