M-ary orthogonal keying system
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.
1. An M-ary orthogonal keying (MOK) modulator comprising:
a serial-to-parallel converter configured to generate data symbols based on received input data bits;
a first modulator configured to (i) receive a first n bits of each data symbol and (ii) select a corresponding length q orthogonal code modified by a complementary code based on the n bits;
a second modulator configured to (i) receive a subsequent m bits of each data symbol and (ii) select a corresponding length r orthogonal code modified by the complementary code based on the m bits;
a first logic device configured to, if a first subsequent bit of the data symbol is a first predefined value, change polarity of the modified length q orthogonal code generated by the first modulator;
a second logic device configured to, if a second subsequent bit of the data symbol is a second predefined value, change polarity of the modified length r orthogonal code generated by the second modulator;
a first mixer configured to modulate the modified length q orthogonal code with a carrier frequency;
a second mixer configured to modulate the modified length r orthogonal code with the carrier frequency shifted by 90 degrees;
wherein the modulated modified length q orthogonal code and the modulated modified length r orthogonal code are combined and transmitted.
2. The modulator of claim 1 , wherein the first modulator is configured to generate an In-Phase (I) component of the transmit signal and wherein the second modulator is configured to generate a Quadrature (Q) component of the transmit signal.
3. The modulator of claim 1 , wherein the first modulator and the second modulator are further configured to select the corresponding orthogonal code modified by a complementary code from one of M modified orthogonal codes.
4. The modulator of claim 3 , wherein the M modified orthogonal codes are stored in a lookup table.
5. The modulator of claim 3 , wherein the chosen one of the M modified orthogonal codes is defined according to the expression:
c={e j(Φ1+Φ2+Φ3+Φ4) , e j(Φ1+Φ3+Φ4) , e j(Φ1+Φ2+Φ4) , −e j(Φ1+Φ4) , e j(Φ1+Φ2+Φ3) , e j(Φ1+Φ3) , −e j(Φ1+Φ2) , e jΦ1 },
where Φi correspond to phase angles of code bits associated with the ith group of code bits.
6. The modulator of claim 3 wherein each of the M modified codes has N chips, and wherein M>N.
7. The modulator of claim 1 , comprising a scrambler in communication with the serial-to-parallel converter, the scrambler configured to receive the input data bits, generate scrambled input data bits and provide the scrambled input data bits to the serial-to-parallel converter.
8. The modulator of claim 1 , wherein the serial-to-parallel converter is a 1:8 multiplexer configured to generate an eight bit data symbol.
9. The modulator of claim 8 , wherein the serial-to-parallel converter operates at a clock rate of 1.375 MHz, and wherein the first modulator and the second modulator operate at a clock rate of 11 MHz.
10. The modulator of claim 1 , wherein n and m are equal to 3 bits and wherein q and r are equal to 8 bits.
11. The modulator of claim 1 wherein each orthogonal code is a Walsh code.
12. The modulator of claim 1 , wherein the phase of the carrier signal is QPSK modulated in accordance with the selected of the M modified orthogonal codes.
13. The modulator of claim 1 , wherein the complementary code has a length of 2 x chips where X is a positive integer.
14. The modulator of claim 1 , 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, and wherein A={11} and B={10} such that the sequence ABAB′={11101101}.