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A digital modulation system provides enhanced multipath performance by using modified orthogonal codes with reduced autocorrelation sidelobes while maintaining the cross-correlation properties of the modified codes. For example, the modified orthogonal codes can reduce the autocorrelation level so a

A digital modulation system provides enhanced multipath performance by using modified orthogonal codes with reduced autocorrelation sidelobes while maintaining the cross-correlation properties of the modified codes. For example, the modified orthogonal codes can reduce the autocorrelation level so as not to exceed one-half the length of the modified orthogonal code. In certain embodiments, an M-ary orthogonal keying (MOK) system is used which modifies orthogonal Walsh codes using a complementary code to improve the auto-correlation properties of the Walsh codes, thereby enhancing the multipath performance of the MOK system while maintaining the orthogonality and low cross-correlation characteristics of the Walsh codes.

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The invention claimed is: 1. A method for modulating information bits over a radio frequency communication channel, comprising: grouping a number of information bits, based on the grouping, choosing one of M modified orthogonal codes that are produced by modifying an orthogonal code with a compleme

The invention claimed is: 1. A method for modulating information bits over a radio frequency communication channel, comprising: grouping a number of information bits, based on the grouping, choosing one of M modified orthogonal codes that are produced by modifying an orthogonal code with a complementary code, and modulating, by a transmitter, the phase of at least one carrier signal in accordance with the chosen modified orthogonal code; and wherein the complementary code is defined by the sequence ABAB′, such that A is a sequence of elements and B is a sequence of elements and wherein B′ is derived by inverting all elements in the sequence B. 2. The method of claim 1 further including: applying a phase shift to modulate at least one additional information bit on the at least one carrier signal. 3. The method of claim 1 wherein the orthogonal code is a Walsh code. 4. The method of claim 1, wherein the phase of the at least one carrier signal is QPSK modulated in accordance with the chosen one of M modified orthogonal codes. 5. The method of claim 1 further including: scrambling the information bits prior to grouping. 6. The method of claim 1, wherein modulating the phase of at least one carrier signal includes In-phase and Quadrature phase modulating the at least one carrier signal. 7. The method according to claim 1, wherein the complementary code has a length of 2X chips where X is a positive integer. 8. The method according to claim 1, wherein A={1 1} and B={1 0} such that the sequence ABAB′={1 1 1 0 1 1 0 1}. 9. The method according to claim 1, wherein the complementary code provides autocorrelation sidelobes suitable for multipath environments. 10. The method according to claim 1, wherein the modified orthogonal codes are stored in a look-up table. 11. The method according to claim 1, wherein the complementary code is characterized by a property that for shifts in the complementary code, the autocorrelations of the complementary codes sum to zero except for a main peak at zero shift. 12. The method according to claim 1, wherein the complementary code provides for a modified orthogonal code that has autocorrelation sidelobes which are equal to or less than one-half the length of the modified orthogonal code. 13. The method according to claim 1, wherein the complementary code is the sequence {11101101}. 14. The method according to claim 13, wherein the orthogonal code is a Walsh code and wherein the Walsh code is modified by multiplying the Walsh code by the complementary sequence {11101101}. 15. The method according to claim 1, further comprising the step of selecting a full data mode or a fallback mode, wherein the data rate in the full data mode is approximately twice the data rate in the fallback mode. 16. The method according to claim 15, wherein the full data mode is selected such that eight information bits are grouped in the grouping step and wherein each of the M modified orthogonal codes words contain eight chips. 17. The method according to claim 15, wherein the fallback data mode is selected such that four information bits are grouped in the grouping step and wherein each of the M modified orthogonal code words contain eight chips. 18. A method for demodulating a received signal that conveys information bits over a radio frequency communication channel, comprising: correlating, by a demodulator, the received signal against a code set of M symbols, M>1, the code set being produced by modifying an orthogonal code with a complementary code, selecting, by the demodulator, one of the M symbols in the code set based upon the correlating step; and wherein the complementary code provides for autocorrelation sidelobes in the code set which are equal to or less than one-half the length of the codes in the code set. 19. The method according to claim 18, wherein the complementary code has a length of 2X chips where X is a positive integer. 20. The method according to claim 18, wherein the complementary code is defined by the sequence ABAB′, such that A is a sequence of elements and B is a sequence of elements and wherein B′ is derived by inverting all elements in the sequence B. 21. The method according to claim 20, wherein A={1 1} and B={1 0} such that the sequence ABAB′={1 1 1 0 1 1 0 1}. 22. The method according to claim 18, wherein the complementary code provides autocorrelation sidelobes suitable for multipath environments. 23. The method according to claim 18, wherein the complementary code is characterized by a property that for shifts in the complementary code the autocorrelations of the complementary codes sum to zero except for a main peak at zero shift. 24. The method according to claim 18, wherein the decoding step decodes the information bits based upon the highest correlation magnitudes from the correlation step. 25. The method according to claim 18, wherein the decoding step decodes the information bits based upon the highest correlation complex magnitude from the correlation step. 26. The method according to claim 18, further comprising: detecting the phase of the code in the code set that generates the highest correlation magnitude, and decoding at least one bit per code based upon the detected phase. 27. The method according to claim 18, wherein the orthogonal code is a Walsh code and wherein the Walsh code is modified by multiplying the Walsh code by a complementary sequence {11101101}. 28. A digital modulation system for modulating data bits, comprising: a processor that groups the data bits, a modulator that chooses a code having N chips in response to the group of data bits, the code being a member of a code set that is produced by modifying an orthogonal code with a complementary code; and wherein the complementary code is defined by the sequence ABAB′, such that A is a sequence of elements and B is a sequence of elements and wherein B′ is derived by inverting all elements in the sequence B. 29. The digital modulation system according to claim 28, further comprising a mixer that modulates a carrier signal in accordance with the chosen code. 30. The digital modulation system according to claim 29, wherein the mixer modulates the phase of at least one carrier signal in accordance with the chosen code. 31. The digital modulation system according to claim 30, wherein the phase of the at least one carrier signal is QPSK modulated in accordance with the chosen code. 32. The digital modulation system according to claim 28, further comprising a scrambler for scrambling the group of data bits. 33. The digital modulation system according to claim 28, wherein A={1 1} and B={1 0} such that the sequence ABAB′={1 1 1 0 1 1 0 1}. 34. The digital modulation system according to claim 28, wherein the complementary code provides autocorrelation sidelobes suitable for multipath environments. 35. The digital modulation system according to claim 28, further comprising a look-up table for storing the code set. 36. The digital modulation system according to claim 28, wherein the complementary code is characterized by a property that for shifts in the complementary code, the autocorrelations of the complementary codes sum to zero except for a main peak at zero shift. 37. The digital modulation system according to claim 28, wherein the complementary code provides for autocorrelation sidelobes in the code set which are equal to or less than one-half the length of the N chip code. 38. The digital modulation system according to claim 28, wherein the orthogonal code is a Walsh code. 39. The digital modulation system according to claim 38, wherein the Walsh code is modified by multiplying the Walsh code by a complementary sequence {11101101}. 40. The digital modulation system according to claim 28, wherein the modulator operates to select a full data mode or a fallback mode, and further wherein the data rate in the full data mode is approximately twice the data rate in the fallback mode. 41. The digital modulation system according to claim 28, wherein a full data mode is selected such that the group of data bits comprises eight bits and wherein each of the modified orthogonal code words contain eight chips. 42. The digital modulation system according to claim 28, wherein a fallback data mode is selected such that the group of data bits comprises four bits and wherein each of the M modified orthogonal code words contain eight chips. 43. A digital modulation system for modulating a group of data bits, comprising: a scrambler for scrambling the group of data bits, a modulator that chooses a code having N chips in response to the group of data bits, the code being a member of a code set that that is produced by modifying an orthogonal code with a complementary code; and wherein the complementary code provides for autocorrelation sidelobes in the code set which are equal to or less than one-half the length of the N chip code. 44. The digital modulation system according to claim 43, wherein the complementary code is defined by the sequence ABAB′, such that A is a sequence of elements and B is a sequence of elements and wherein B′ is derived by inverting all elements in the sequence B. 45. The digital modulation system according to claim 44, wherein A={11} and B={10} such that the sequence ABAB′={1 1 1 0 1 1 0 1}. 46. The digital modulation system according to claim 43, wherein the complementary code provides autocorrelation sidelobes suitable for multipath environments. 47. The digital modulation system according to claim 43, further comprising a look-up table for storing the code set. 48. The digital modulation system according to claim 43, wherein the complementary code is characterized by a property that for shifts in the complementary code, the autocorrelations of the complementary codes sum to zero except for a main peak at zero shift. 49. The digital modulation system according to claim 43, wherein the orthogonal code is a Walsh code. 50. The digital modulation system according to claim 49, wherein the Walsh code is modified by multiplying the Walsh code by a complementary sequence {11101101}. 51. A digital demodulator for demodulating a received signal that conveys information bits over a radio frequency communication channel, comprising: a correlator block for correlating the received signal against a code set of M symbols, M>1, the code set being produced by modifying an orthogonal code with a complementary code, a find code block for selecting one of the M symbols in the code set based upon the correlations of the received signal and the code set; and wherein the complementary code is defined by the sequence ABAB′, such that A is a sequence of elements and B is a sequence of elements and wherein B′ is derived by inverting all elements in the sequence B. 52. The digital demodulator according to claim 51, wherein A ={1 1} and B={1 0} such that the sequence ABAB′={1 1 1 0 1 1 0 1}. 53. The digital demodulator according to claim 51, wherein the complementary code provides autocorrelation sidelobes suitable for multipath environments. 54. The digital demodulator according to claim 51, wherein the complementary code is characterized by a property that for shifts in the complementary code the autocorrelations of the complementary codes sum to zero except for a main peak at zero shift. 55. The digital demodulator according to claim 51, wherein the complementary code provides for autocorrelation sidelobes in the code set which are equal to or less than one-half the length of the N chip code. 56. The digital demodulator according to claim 51, further comprising a phase detector that detects the phase of the code in the code set that generates the highest correlation magnitude and that decodes an extra 2 bits per code based upon the detected phase. 57. The digital demodulator according to claim 51, wherein the orthogonal code is a Walsh code and wherein the Walsh code is modified by multiplying the Walsh code by a complementary sequence {11101101}. 58. The method according to claim 18, further including the step of receiving an RF signal from the radio frequency communication channel with an antenna. 59. The method of claim 58, further including the steps of: multiplying the received RF signal by a signal at a mixer prior to the correlating step, and low pass filtering the output of the mixer. 60. The method of claim 58, further including the step of transmitting the RF signal over the radio frequency communication channel. 61. The method of claim 60, further including the step of forming the transmitted RF signal by choosing a code that is produced by modifying an orthogonal code with a complementary code. 62. The digital demodulator according to claim 51, further including an antenna for receiving an RF signal from the radio frequency communication channel. 63. The digital demodulator according to claim 51, further including: a mixer for multiplying the received RF signal by a signal, and a low pass filter for filtering the output of the mixer. 64. The digital demodulator according to claim 62, further including a transmitter for transmitting the RF signal over the radio frequency communication channel, the transmitter comprising: a modulator that chooses a code having N chips, the code being produced by modifying an orthogonal code with a complementary code.