Method and apparatus for differential phase encoding and decoding in spread-spectrum communication systems with continuous-phase modulation

• US 5,692,007 A
• Filed: 06/07/1995
• Issued: 11/25/1997
• Est. Priority Date: 09/09/1994
• Status: Expired due to Term

• Current Assignee: Intel Corporation
• Original Assignee: Omnipoint Corp. (Deutsche Telekom AG)
• Inventors: Burbach, Mark T., Durrant, Randolph L., Hoyt, Eugene P.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): GLUCK, JEFFREY WILLIAM
• Application Number:
  US08/477,480
• Time in Patent Office: 902 Days
• Field of Search: 375/200, 375/206, 375/273, 375/274, 375/283, 375/305
• US Class Current: 375/142
• 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 method and apparatus for phase encoding and decoding a CPM spread spectrum signal. A transmitter divides a data stream into a data symbol portion and an associated phase information portion. The data symbol portion is used to select one of a plurality of spread spectrum codes for transmission from a symbol table. The phase information portion is used to differentially phase encode the data symbol prior to transmission. The transmitter divides the phase encoded spread spectrum codes 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 derives a real correlation signal and an imaginary correlation signal. For each received symbol, the receiver determines which of a plurality of phase sectors the phase angle lies in, and compares the difference between the present phase sector and the immediately preceding phase sector. For biphase encoding, if the difference in closer to 0°, then the receiver outputs a first bit, and if the difference is closer to 180°, the receiver outputs a second bit. In a particular embodiment, a plurality of correlators operate in parallel to carry out phase decoding of the received signal.

Claims

1. An apparatus for differential phase encoding of signals, comprising:

a divider, an input of said divider comprising a plurality of bits, a first output of said divider comprising a first subset of said plurality of bits, and a second output of said divider comprising a second subset of said plurality of bits, wherein at least one bit of said second subset of bits is mutually exclusive from said first subset of bits;

a symbol table, said symbol table comprising a plurality of symbol codes, said symbol table having as an input said first subset of bits and said symbol table having as an output a symbol code;

a phase register having an output;

a phase determination circuit, said phase determination circuit having as a first input said second subset of bits, said phase determination circuit having as a second input said output of said phase register, and said phase determination circuit having as an output a phase selection signal;

said phase selection signal forming an input to said phase register; and

a phase selector, said phase selector having as a first input said phase selection signal, said phase selector having as a second input said symbol code, and said phase selector having as an output a phase encoded signal.

2. The apparatus of claim 1 wherein said phase encoded signal comprises a stream of chips, said apparatus further comprising:

a switch coupled to said phase encoded signal, outputting an even chip stream and an odd chip stream;

an odd waveform generator coupled to said odd chip stream and outputting a first waveform;

an even waveform generator coupled to said even chip stream and outputting a second waveform;

an odd modulator coupled to said first waveform and to a first carrier having a frequency ω

₀ ;

an even modulator coupled to said second waveform and a second carrier having a frequency ω

₀, said first carrier and said second carrier phase offset by 90 degrees; and

a summer coupled to an output of said odd modulator and to an output of said even modulator, said summer outputting a combined signal.

3. The apparatus of claim 2 wherein said combined signal is a continuous phase modulated signal.

4. The apparatus of claim 2, wherein said switch is comprised of a demultiplexer.

5. The apparatus of claim 1 further comprising a receiver, said receiver comprising:

a correlator having as outputs a real correlation signal and an imaginary correlation signal;

a phase discriminator coupled to said real correlation signal and to said imaginary correlation signal, said phase discriminator outputting a phase sector signal; and

a subtractor coupled to said phase sector signal and a previous phase sector signal, said subtractor outputting a sector difference signal.

6. The apparatus of claim 5, wherein said receiver further comprises means for recovering phase encoded information from a received signal based on said sector difference signal.

7. The apparatus of claim 6, wherein a phase angle of said received signals is within one of a plurality of predefined sectors and wherein said receiver further comprises means to generate a sector value for a respective phase angle of said received signal in response to said sector difference signal.

8. The apparatus of claim 7, wherein said receiver further comprises means for determining said phase encoded information is a first data value if said sector value represents a sector less than or equal to a predefined number of sectors from a previous sector value, and said phase encoded information is a second data value if said sector value represents a sector more than said predefined number of sectors from said previous sector value.

9. The apparatus of claim 5 wherein said correlator comprises a non-coherent CPM correlator.

10. The apparatus of claim 5 wherein said phase discriminator comprises:

means for determining a sign of said real correlation signal;

means for determining a sign of said imaginary correlation signal;

a comparator coupled to said real correlation signal and said imaginary correlation signal, said comparator outputting a magnitude comparison signal; and

means for determining a sector of a phase angle of a signal received by said receiver, based on a sign of said real correlation signal, a sign of said imaginary correlation signal, and said magnitude comparison signal.

11. The apparatus of claim 5 wherein said phase discriminator comprises:

means for forming an address from said real correlation signal and said imaginary correlation signal, and

a sector lookup table connected to said address.

12. The apparatus of claim 1, wherein said divider is comprised of a first register receiving and storing said first subset of bits, and a second register receiving and storing said second subset of bits.

13. A method for differential phase encoding of signals, comprising the steps of:

dividing a signal stream into a plurality of sets of bits, wherein a first subset of a set of bits comprises a data symbol, wherein a second subset of said set of bits comprises a phase select symbol, and wherein at least one bit of said phase select symbol is mutually exclusive from the bits comprising said data symbol;

selecting in response to said data symbol a symbol code from a plurality of symbol codes stored in a symbol table;

selecting a phase value in response to said phase select symbol and a previous phase value; and

phase encoding said symbol code with said phase value.

14. The method of claim 13, wherein the step of dividing the signal stream further comprises the step of storing said data symbol in a first register and the step of storing said phase select symbol in a second register.

15. The method of claim 13, wherein the step of phase encoding generates a phase encoded signal, said phase encoded signal comprising a stream of chips, and wherein said method further comprises the steps of:

dividing said stream of chips into an even chip steam and an odd chip stream;

generating a first waveform from said odd chip stream;

generating a second waveform from said even chip stream;

modulating said first waveform with a first carrier having a frequency ω

₀ ;

modulating said second waveform with a second carrier having a frequency ω

₀, said first carrier and said second carrier phase offset by 90 degrees; and

combining said modulated first and second waveforms to form a combined signal.

16. The method of claim 15 wherein said combined signal comprises a continuous phase modulated signal.

17. The method of claim 15 further comprising the steps of:

transmitting said combined signal;

receiving said combined signal at a receiver;

generating, in response to receiving said combined signal, a real correlation signal and an imaginary correlation signal;

generating a phase sector signal in response to said real correlation signal and said imaginary correlation signal; and

generating a phase sector difference signal in response to said phase sector signal and a previous phase sector signal.

18. The method of claim 17 wherein said real correlation signal and said imaginary correlation signal are generated by a non-coherent CPM correlator.

19. The method of claim 17 wherein said step of generating said phase sector signal further comprises the steps of:

determining a sign of said real correlation signal;

determining a sign of said imaginary correlation signal;

comparing a relative magnitude of said real correlation signal with a relative magnitude of said imaginary correlation signal, and generating a magnitude comparison signal in response thereto; and

determining a sector value of a phase angle of said received signal in response to a sign of said real correlation signal, a sign of said imaginary correlation signal, and said magnitude comparison signal.

20. The method of claim 17 wherein said step of generating said phase sector signal further comprises the steps of:

forming an address from said real correlation signal and said imaginary correlation signal, and

applying said address to a sector lookup table.

21. The method of claim 17 further comprising the step of recovering phase encoded information from said received signal in response to said sector difference signal.

22. The method of claim 21, wherein a phase angle of said received signal is within one of a plurality of predefined sectors, further comprising the step of generating a sector value for a respective phase angle of said received signal in response to said sector difference signal.

23. The method of claim 22, wherein said step of recovering phase encoded information further comprises the step of determining a phase encoded information value to be a first data value if said sector value represents a sector less than or equal to a predefined number of sectors from a previously generated sector value, and determining said phase encoded information value to be a second data value if said sector value represents a sector more than said predefined number of sectors from said previously generated sector value.

24. A method for communication, comprising the steps of:

generating, in response to a signal stream, a sequence of data segments, each respective data segment comprised of a set of bits of said signal stream;

for each respective data segment, selecting a symbol code in response to a first subset of bits of said data segment;

for each respective data segment, selecting a phase of said symbol code in response to a phase of a previous symbol code and in response to a second subset of bits of said data segment, wherein at least one bit of said second subset of bits is mutually exclusive from said first subset of bits;

generating a phase encoded symbol code for each respective data segment in response to said symbol code of said data segment and said phase of said symbol code;

continuous phase modulating each of said phase encoded symbol codes generated for each of said respective data segments to generate a signal;

transmitting said signal;

receiving said signal;

correlating said signal with a continuous phase modulation correlator and generating a real correlation signal and an imaginary correlation signal thereby; and

determining, in response to a sign and a relative magnitude of said real correlation signal, and in response to a sign and a relative magnitude of said imaginary correlation signal, a phase of said received signal.

25. The method of claim 24 further comprising the step of recovering a second subset of bits of a respective data segment by comparing a phase of said received signal with a previous phase of said received signal.

26. The method of claim 24, wherein a phase of said received signal is quantized within one of a predefined plurality of phase ranges.

27. The method of claim 24 wherein said transmission signal is transmitted during a time slot of a time division duplex communication link.

28. An apparatus for communication, comprising:

a data register coupled to a signal stream;

a clock signal coupled to said data register, whereby a first predetermined number of bits of said signal stream are entered and stored in said data register;

a symbol table coupled to said data register, said symbol table comprising a plurality of symbol codes, whereby said bits stored in said data register are used to select a symbol code;

a phase determination circuit coupled to said signal stream, whereby a second predetermined number of bits of said signal stream are used to generate a phase value, wherein at least one of said second predetermined number of bits is mutually exclusive from said first predetermined number of bits;

a phase register coupled to said phase determination circuit, whereby an output of said phase register is used to generate said phase value and whereby said phase value is thereafter stored in said phase register;

a phase selector coupled to said symbol table and to said phase register, whereby said selected symbol code is phase encoded with said phase value.

29. The apparatus of claim 28 wherein said phase determination circuit comprises an exclusive-OR gate.

30. The apparatus of claim 28 wherein said phase selector comprises an exclusive-OR gate.

31. The apparatus of claim 28 wherein an output of said phase selector is coupled to a continuous phase modulation waveform transmitter, whereby a continuous phase modulation waveform is generated.

32. The apparatus of claim 31 wherein said phase encoded symbol code comprises a plurality of chips, and wherein said continuous phase modulation waveform transmitter comprises:

means for separating said chips into an odd chip stream and an even chip stream;

an odd waveform generator coupled to said odd chip stream;

an even waveform generator coupled to said even chip stream; and

a summer coupled to an output of said odd waveform generator and an output of said even waveform generator.

33. A method for communicating, comprising the steps of:

dividing a signal stream into multi-bit segments;

for each of said segments, loading a first subset of bits of said segment into a data symbol register;

for each of said segments, loading a second subset of bits of said segment into a phase selection register, wherein at least one of said second subset of bits is mutually exclusive from said first subset of bits;

selecting a symbol code from a plurality of symbol codes in response to the contents of said data symbol register;

determining a phase of said symbol code in response to the contents of said phase selection register and in response to a phase of a previous symbol code; and

phase encoding said symbol code with said phase of said symbol code.

34. The method of claim 33 further comprising the step of continuous phase modulating said phase encoded symbol code.

35. The method of claim 34 wherein said phase encoded symbol code comprises a plurality of chips, and wherein said step of continuous phase modulating said phase encoded symbol code further comprises the steps of:

separating said chips into an odd chip stream and an even chip stream;

generating a first sinusoidal waveform in response to said odd chip stream;

generating a second sinusoidal waveform in response to said even chip stream; and

combining said first sinusoidal waveform and said second sinusoidal waveform.

36. The method of claim 33 wherein said plurality of symbol codes are stored in a symbol code table, and the contents of said data symbol register are applied as an address to said symbol code table.