GMSK spread modulation receiver

1. A method for demodulating a received signal comprising:

receiving an approximate direct sequence spread spectrum GMSK signal generated at a transmitter that has modulated a sequence of data symbols with a pseudo-noise (PN) coded signal, wherein said spread spectrum GMSK signal is approximated by a pulse amplitude modulation (PAM) signal having a main pulse exhibiting antipodal signaling;

filtering the received spread spectrum GMSK signal using at least one matched filter to produce at least one filtered signal;

correlating the at least one filtered signal with the PN coded signal corresponding to the spread spectrum GMSK signal to produce at least one correlation output; and

evaluating the at least one correlation output to demodulate the sequence of data symbols.

2. The method of claim 1 wherein the at least one matched filter is matched to a main Laurent pulse corresponding to an approximation of the spread spectrum GMSK signal.

3. The method of claim 1 further comprising a step of sampling the at least one filtered signal before the correlating step.

4. The method of claim 3 wherein even ones and odd ones of the at least one filtered and sampled signals are separately correlated with even and odd portions of the PN coded signal corresponding to the spread spectrum GMSK signal to produce even and odd correlated signals.

5. The method of claim 4 wherein the even and odd correlated signals are combined into a complex signal before the evaluating step.

6. The method of claim 5 wherein the complex signal is phase-adjusted to compensate for a phase differential before the evaluating step.

7. The method of claim 1 , wherein the main pulse further exhibits at least one signal transformation based on a current symbol in the main pulse, a first chip associated with the current symbol in the main pulse, a previous symbol in the main pulse, and a last chip associated with the previous symbol in the main pulse.

8. The method of claim 1 , wherein the main pulse further exhibits at least one signal transformation based on the following mapping:

α m =α k+(n−1)K =(−1) m ( {tilde over (b)} −1 {tilde over (d)} n−1 )·( {tilde over (b)} 0 {tilde over (d)} n ) k= 0, and

α m =α k+(n−1)K =(−1) m {tilde over (b)} k−1 {tilde over (b)} k k> 0

where α m is a pre-modulation chip sequence of length m, {tilde over (b)} k =2b k −1 is a translation of logic values, {tilde over (d)} n =2d n −1 is a translation of the logic values, {tilde over (b)} −1 is a last PN chip associated with a previous data symbol d n−1 , d n is a data symbol at index n, b k is a spread spectrum chip at index k, m is a chip counter, n is an integer and k is a chip count.

9. An apparatus for demodulating a received signal comprising:

at least one matched filter configured to:

receive an approximate direct sequence spread spectrum GMSK signal generated at a transmitter that has modulated a sequence of data symbols with a pseudo-noise (PN) coded signal, wherein said spread spectrum GMSK signal is approximated by a pulse amplitude modulation (PAM) signal having a main pulse exhibiting antipodal signaling, and

to produce at least one filtered signal;

at least one correlator for correlating the at least one filtered signal with the PN coded signal corresponding to the spread spectrum GMSK signal to produce at least one correlation output; and

a decision unit for evaluating the at least one correlation output to demodulate the sequence of data symbols.

10. The apparatus of claim 9 wherein the at least one matched filter is matched to a main Laurent pulse corresponding to an approximation of the spread spectrum GMSK signal.

11. The apparatus of claim 9 further comprising a sampler for sampling the at least one filtered signal before the correlation is performed by the at least one correlator.

12. The apparatus of claim 11 wherein even ones and odd ones of the at least one filtered and sampled signals are separately correlated with even and odd portions of the PN coded signal corresponding to the spread spectrum GMSK signal to produce even and odd correlated signals.

13. The apparatus of claim 12 wherein the even and odd correlated signals are combined into a complex signal before the evaluation is performed by the decision unit.

14. The apparatus of claim 13 wherein the complex signal is phase-adjusted to compensate for a phase differential before the evaluation is performed by the decision unit.

15. The apparatus of claim 9 , wherein the main pulse further exhibits at least one signal transformation based on a current symbol in the main pulse, a first chip associated with the current symbol in the main pulse, a previous symbol in the main pulse, and a last chip associated with the previous symbol in the main pulse.

16. The apparatus of claim 9 , wherein the main pulse further exhibits at least one signal transformation based on the following mapping:

α m =α k+(n−1)K =(−1) m ( {tilde over (b)} −1 {tilde over (d)} n−1 )·( {tilde over (b)} 0 {tilde over (d)} n ) k= 0, and

α m =α k+(n−1)K =(−1) m {tilde over (b)} k−1 {tilde over (b)} k k> 0,

where α m is a pre-modulation chip sequence of length m, {tilde over (b)} k =2b k −1 is a translation of the logic values, {tilde over (d)} n =2d n −1 is a translation of logic values, {tilde over (b)} −1 is a last PN chip associated with a previous data symbol d n−1 , d n is a data symbol at index n, b k is a spread spectrum chip at index k, m is a chip counter, n is an integer and k is a chip count.

17. A system for demodulating a received signal comprising:

means for receiving an approximate direct sequence spread spectrum GMSK signal generated at a transmitter that has modulated a sequence of data symbols with a pseudo-noise (PN) coded signal, wherein said spread spectrum GMSK signal is approximated by a pulse amplitude modulation (PAM) signal having a main pulse exhibiting antipodal signaling;

means for processing the received spread spectrum GMSK signal using at least one matched filter to produce at least one filtered signal;

means for correlating the at least one filtered signal with the PN coded signal corresponding to the spread spectrum GMSK signal to produce at least one correlation output; and

means for evaluating the at least one correlation output to demodulate the sequence of data symbols.