Method and apparatus for coherent serial correlation of a spread spectrum signal

• US 5,832,028 A
• Filed: 06/07/1995
• Issued: 11/03/1998
• Est. Priority Date: 09/09/1994
• Status: Expired due to Fees

• Current Assignee: Intel Corporation
• Original Assignee: Omnipoint Corp. (Deutsche Telekom AG)
• Inventors: Durrant, Randolph L., Burbach, Mark
• Primary Examiner(s): Tse, Young T.
• Assistant Examiner(s): VO, DON NGUYEN
• Application Number:
  US08/480,914
• Time in Patent Office: 1,245 Days
• Field of Search: 375/200, 375/206, 375/208, 375/209, 375/367, 375/286, 375/289, 375/343, 370/107
• US Class Current: 375/150
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/708 Parallel implementation
• CPC Class Codes: H04B 1/7093 Matched filter type
• CPC Class Codes: H04B 2201/70701 featuring pilot assisted reception
• CPC Class Codes: H04J 13/10 Code generation
• CPC Class Codes: H04J 14/005 Optical Code Multiplex
• CPC Class Codes: H04L 27/2014 in which the phase changes in a piecewise linear manner during each symbol period, e.g. minimum shift keying, fast frequency shift keying H04L27/201 takes precedence
• CPC Class Codes: H04L 27/2278 using correlation techniques, e.g. for spread spectrum signals
• CPC Class Codes: H04L 27/2332 using a non-coherent carrier

Abstract

A technique for modulating and demodulating CPM spread spectrum signals and variations of CPM spread spectrum signals. A transmitter divides a signal data stream into a plurality of data streams (such as I and Q data streams), independently modulates the I and Q data streams using CPM or a related technique, and superposes the plurality of resultants for transmission. A receiver receives the superposed spread spectrum signal and simultaneously attempts to correlate for a plurality of chip sequences (such as I and Q chip sequences), and interleaves the correlated I and Q data streams into a unified signal data stream. In one embodiment, the receiver separates the received spread spectrum signal into real and imaginary parts, attempts to correlate both real and imaginary parts for a plurality of chip sequences, and combines the real I, real Q, imaginary I, and imaginary Q signals into a unified signal data stream. The demodulating reference signal in the receiver may or may not be frequency or phase matched with the transmitter. In other embodiments, single bit or multi-bit digitization of the received spread spectrum signal for both I and Q signals (or real I, real Q, imaginary I, and imaginary Q signals) is carried out prior to correlation. In another embodiment, the transmitter differentially phase encodes the information to be transmitted, and the receiver decodes the phase encoded information by determining a quantized phase angle of the received signal.

Claims

1. A coherent serial CPM correlator for despreading a received continuous phase modulated spread spectrum signal comprising:

a power divider for dividing a continuous phase modulated spread spectrum signal into a first signal and a second signal, said first signal and second signal being duplicates of one another,

means for multiplying said first signal with a first coherent local reference signal and an odd chip signal representing the odd chips of a chip sequence and for generating an I product signal having a value of zero for each even chip in said chip sequence,

means for multiplying said second signal with a second coherent local reference signal and an even chip signal representing the even chips of said chip sequence and for generating a Q product signal having a value of zero for each odd chip in said chip sequence, said second coherent local reference signal having the same frequency as said first coherent local reference signal but phase offset therefrom by 90 degrees, and said odd chip signal having non-zero chip values staggered in time with respect to non-zero chip values in said even chip signal,

an I integrator for integrating said I product signal and generating an I correlation signal,

a Q integrator for integrating said Q product signal and generating a Q correlation signal, and

a summer for combining said I correlation signal and said Q correlation signal.

2. The apparatus of claim 1 wherein said odd chip signal and said even chip signal each comprises a tri-valued return-to-zero waveform.

3. A method for despreading a received continuous phase modulated spread spectrum signal comprising the steps of:

dividing a continuous phase modulated spread spectrum signal into a first signal and a second signal, said first signal and second signal being duplicates of one another,

multiplying said first signal with a first coherent local reference signal and an odd chip signal representing the odd chips of a chip sequence to generate an I product signal having a value of zero for each even chip in said chip sequence,

multiplying said second signal with a second coherent local reference signal and an even chip signal representing the even chips of said chip sequence to generate a Q product signal having a value of zero for each odd chip in said chip sequence, said second coherent local reference signal having the same frequency as said first coherent local reference signal but phase offset therefrom by 90 degrees, and said odd chip signal having non-zero chip values staggered in time with respect to non-zero chip values in said even chip signal,

integrating said I product signal to generate an I correlation signal,

integrating said Q product signal to generate a Q correlation signal, and

summing said I correlation signal and said Q correlation signal to generate a combined correlation signal.

4. The method of claim 3 wherein said odd chip signal and said even chip signal each comprises a tri-valued return-to-zero waveform.

5. A correlator comprising

a power divider having as an input a received continuous phase modulated (CPM) signal, and further having a first power divider output and a second power divider output,

a first multiplier connected to said first power divider output,

a second multiplier connected to said second power divider output,

an odd chip signal connected to an input of said first multiplier, said odd chip signal comprising the odd chips of a chip sequence and having a value of zero for the even chips of said chip sequence,

an even chip signal connected to an input of said second multiplier, said even chip signal comprising the even chips of said chip sequence and having a value of zero for the odd chips of said chip sequence, said odd chip signal and said even chip signal having chip values staggered in time with respect to one another,

an I integrator connected to an output of said first multiplier,

a Q integrator connected to an output of said second multiplier, and

a summer connected to said I integrator and said Q integrator.

6. The correlator of claim 5 wherein said odd chip signal and said even chip signal each comprises a tri-valued return-to-zero waveform.

7. A receiver comprising

means for receiving a continuous phase modulated (CPM) signal, said CPM signal comprising an odd chip sequence modulated with a first carrier signal and an even chip sequence modulated with a second carrier signal, said first carrier signal orthogonal with respect to said second carrier signal, wherein said odd chip sequence comprises the odd chips of a spreading sequence but having twice the duration thereof, and said even chip sequence comprises the even chips of said spreading sequence but having twice the duration thereof, said odd chip sequence offset in time from said even chip sequence by a single chip interval of said chip sequence,

a first demodulator connected to said CPM signal and to an odd chip signal, said odd chip signal alternating between said odd chips and zero chip values,

a second demodulator connected to said CPM signal and to an even chip signal, said even chip signal alternating between said even chips and zero chip values,

a first integrator connected to said first demodulator,

a second integrator connected to said second demodulator, and

a summer connected to said first integrator and said second integrator.

8. The receiver of claim 7 wherein said odd chip signal comprises a first local carrier signal, and said even chip signal comprises a second local carrier signal orthogonal to said first local carrier signal.

9. A correlator comprising

means for receiving a continuous phase modulated (CPM) signal,

a first multiplier connected to said CPM signal and to a first signal, said first signal comprising the product of a first local carrier signal and a first tri-valued return-to-zero (RTZ) waveform,

a second multiplier connected to said second power divider output, said second signal comprising the product of a second local carrier signal and a second tri-valued RTZ waveform, said second local carrier signal orthogonal to said first local carrier signal,

an I integrator connected to an output of said first multiplier,

a Q integrator connected to an output of said second multiplier, and

a summer connected to said I integrator and said Q integrator.

10. The correlator of claim 9 wherein said first tri-valued RTZ waveform comprises a first sequence alternating between odd chips and zero chip values, and wherein said second tri-valued RTZ waveform comprises a second sequence alternating between even chips and zero chip values, said first sequence and second sequence offset in time such that the even chips of said second sequence occur during the zero chip values of said first sequence, and the odd chips of said first sequence occur during the zero chip values of said second sequence.

11. A method of correlating a received continuous phase modulated (CPM) spread spectrum signal comprising the steps of:

receiving a CPM spread spectrum signal,

multiplying an odd chip sequence with a first coherent local reference signal to generate a first demodulating signal, said odd chip sequence comprising a first alternating sequence of the odd chips of a chip sequence and zero chip values,

multiplying an even chip sequence with a second coherent local reference signal to generate a second demodulating signal, said even chip sequence comprising a second alternating sequence of the even chips of said chip sequence and zero chip values, said even chips in said even chip sequence occurring during the zero chips of said odd chip sequence, and said odd chips in said odd chip sequence occurring during the zero chips of said even chip sequence, and said second coherent local reference signal being orthogonal to said first coherent local reference signal,

demodulating said CPM spread spectrum signal using said first demodulating signal to generate an I product signal,

demodulating said CPM spread spectrum signal with said second demodulating signal to generate a Q product signal,

integrating said I product signal to generate an I correlation signal,

integrating said Q product signal to generate a Q correlation signal, and

summing said I correlation signal and said Q correlation signal to generate a combined correlation signal.

12. The method of claim 11 wherein said odd chip sequence and said even chip sequence each comprises a tri-valued return-to-zero waveform.

13. A method for despreading a received continuous phase modulated (CPM) spread spectrum signal comprising the steps of:

receiving a CPM spread spectrum signal,

dividing said CPM spread spectrum signal into a first signal and a second signal, said first signal and second signal being duplicates of one another,

demodulating said first signal with a first coherent local reference signal to obtain a first demodulated signal,

demodulating said second signal with a second coherent local reference signal to obtain a second demodulated signal, said second coherent local reference signal having the same frequency as said first coherent local reference signal but phase offset therefrom by 90 degrees,

alternately integrating said first demodulated signal and said second demodulated signal by integrating said first demodulated signal according to the value of odd chips of a chip sequence and integrating said second demodulated signal according to the value of even chips of said chip sequence, said steps of integrating said first demodulated signal and integrating said second demodulated signal alternating for each chip in said chip sequence,

generating an I correlation signal from integrating said first demodulated signal,

generating a Q correlation signal from integrating said second demodulated signal, and

summing said I correlation signal and said Q correlation signal to generate a combined correlation signal.

14. The method of claim 13 further comprising the steps of generating a first tri-valued return-to-zero waveform comprising said odd chips and generating a second tri-valued return-to-zero waveform comprising said even chips.