Spread spectrum code pulse position modulated receiver having delay spread compensation

• US 5,623,511 A
• Filed: 11/28/1994
• Issued: 04/22/1997
• Est. Priority Date: 08/30/1994
• Status: Expired due to Term

• Current Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Krishnamoorthy, Rajeev, Bar-David, Israel
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Ghebretinsae, T.
• Application Number:
  US08/345,110
• Time in Patent Office: 876 Days
• Field of Search: 375/207, 375/200, 375/239, 375/343, 380/34, 370/10, 332/112
• US Class Current: 375/143
• CPC Class Codes: H04B 1/69 Spread spectrum techniques
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7093 Matched filter type
• CPC Class Codes: H04B 14/026 using pulse time characteristics modulation, e.g. width, position, interval

Abstract

A spread spectrum code pulse position modulated communication system is disclosed. The communication system includes a spread spectrum code pulse position modulated receiver that compensates for the delay spread of a transmission medium, such as a wireless radio channel of a local area network. Delay spread may cause signal components of one transmitted spread spectrum codeword to spill over into chip positions of adjacent symbols, causing intersymbol interference, or into chip positions on quadrature channels of a given symbol interval, such as in a QPSK implementation, causing interchip interference. The disclosed receiver decodes a transmitted spread spectrum code pulse position modulated signal in the presence of such delay spread, utilizing a tentative symbol estimator to sample the matched filter data. The tentative symbol estimator will assign a weight to each potential symbol value, taking into account the cross-rail or bias influence caused by the residual coupling between the signals on quadrature channels for a given symbol duration. In addition, the disclosed receiver includes a mode sifter which evaluates the weighted values for each symbol estimate and assigns final estimates to each symbol value, taking into account the influence of codewords transmitted during one or more successive symbol intervals on one another.

Claims

1. An apparatus for receiving a received signal comprising information symbols transmitted across a communication channel, each of said symbols conveying a plurality of bits transmitted during a given symbol duration, each of said symbols being encoded using a first and a second spread spectrum code pulse position modulated signal transmitted during said associated symbol duration to a generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing residual coupling between said first and second spread spectrum code pulse position modulated signals of a given symbol duration and causing energy associated with a first symbol transmitted during a first symbol duration to disperse into one or more adjacent symbol durations, said receiver apparatus comprising:

a filter matched to said spread spectrum codewords;

a channel matched filter matched to said delay spread characteristics of said communication channel;

a tentative symbol estimator for sampling output signals of said channel matched filter to estimate the value of each symbol, said estimation assigning a weight to each possible value of said symbol, each of said weights taking into account said residual coupling influence between said first and second spread spectrum code pulse position modulated signals; and

a mode sifter for evaluating said weights assigned by said tentative symbol estimator to select a final estimate for said symbol value, said final estimate selection taking into account the influence of said one or more adjacent transmitted symbols on one another.

2. The receiver apparatus of claim 1, wherein each of said spread spectrum codewords comprises an orthogonal pulse position modulated signal.

3. The receiver apparatus of claim 1, wherein said communication channel is a wireless radio channel.

4. The receiver apparatus of claim 1, wherein said first and second spread spectrum code pulse position modulated signals are each modulated by a carrier signal having a distinct phase, amplitude or frequency.

5. The receiver apparatus of claim 1, wherein said symbols are defined by the polarity and position of the main lobe of said first and second spread spectrum code pulse position modulated signals upon said matched filtering.

6. The receiver apparatus of claim 1, wherein said mode sifter evaluates a reduced number of said weights assigned by said tentative symbol estimator to assign said final estimates.

7. The receiver apparatus of claim 1, wherein said mode sifter waits for a predefined window period before making a determination about said final estimate of a symbol.

8. The receiver apparatus of claim 1, wherein said filter matched to said delay spread characteristics of said channel compensates for the phase shifts and loss of energy of the received signal caused by said communication channel.

9. The receiver apparatus of claim 1, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating an estimate of the impulse response of said channel for utilization by said filter matched to said channel characteristics.

10. The receiver apparatus of claim 1, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating a series of bias correction terms utilized by said tentative symbol estimator to compensate for said residual coupling.

11. The receiver apparatus of claim 1, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating a series of trellis correction terms utilized by said mode sifter to compensate for said dispersion of said energy of a first symbol into one or more adjacent symbol durations.

12. An apparatus for receiving an information symbol transmitted across a communication channel, said symbol conveying a plurality of bits transmitted during a symbol duration, said symbol being encoded using a first and a second spread spectrum code pulse position modulated signal transmitted during said associated symbol duration to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing residual coupling between said first and second spread spectrum code pulse position modulated signals, said receiver apparatus comprising:

a filter matched to said spread spectrum codeword;

a channel matched filter matched to said delay spread characteristics of said communication channel;

a tentative symbol estimator for sampling output signals of said channel matched filter to estimate the value of said symbol, said estimation assigning a weight to each possible value of said symbol, each of said weights taking into account said residual coupling influence between said first and second spread spectrum code pulse position modulated signals on one another; and

;

a mode sifter for evaluating said weights assigned by said tentative symbol estimator to select a final estimate for said symbol value, said final estimate being said possible value of said symbol having the greatest weight.

13. The receiver apparatus of claim 12, wherein said symbols are defined by the polarity and position of the main lobe of said first and second spread spectrum code pulse position modulated signals upon said matched filtering.

14. The receiver apparatus of claim 12, wherein said mode sifter evaluates a reduced number of said weights assigned by said tentative symbol estimator to assign said final estimates.

15. The receiver apparatus of claim 12, wherein said mode sifter waits for a predefined window period before making a determination about said final estimate of a symbol.

16. The receiver apparatus of claim 12, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating an estimate of the impulse response of said channel for utilization by said filter matched to said channel characteristics.

17. The receiver apparatus of claim 12, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating a series of bias correction terms utilized by said tentative symbol estimator to compensate for said residual coupling.

18. An apparatus for receiving information symbols transmitted across a communication channel, each of said symbols conveying a plurality of bits transmitted during a given symbol duration, each of said symbols being encoded using a spread spectrum code pulse position modulated signal to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing energy associated with a first symbol transmitted during a first symbol duration to disperse into one or more adjacent symbol durations, said receiver apparatus comprising:

a filter matched to said spread spectrum codewords;

a channel matched filter matched to said delay spread characteristics of said communication channel;

a tentative symbol estimator for sampling output signals of said channel matched filter for a given symbol duration to estimate the value of each of said transmitted symbols, said estimation assigning a weight to each possible value of said symbol based on said sampled values; and

a mode sifter for evaluating said weights assigned by said tentative symbol estimator to select a final estimate for said symbol value, said final estimate selection taking into account the influence of said one or more adjacent transmitted symbols on one another.

19. The receiver apparatus of claim 18, wherein said mode sifter evaluates a reduced number of said weights assigned by said tentative symbol estimator to assign said final estimates.

20. The receiver apparatus of claim 18, wherein said mode sifter waits for a predefined window period before making a determination about said final estimate of a symbol.

21. The receiver apparatus of claim 18, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating an estimate of the impulse response of said channel for utilization by said filter matched to said channel characteristics.

22. The receiver apparatus of claim 18, further comprising a channel parameter estimator for analyzing one or more training symbols transmitted through said communication channel, said channel parameter estimator generating a series of trellis correction terms utilized by said mode sifter to compensate for said dispersion of said energy of a first symbol into one or more adjacent symbol durations.

23. An apparatus for receiving information symbols transmitted across a communication channel, each of said symbols conveying a plurality of bits transmitted during a given symbol duration, each of said symbols being encoded using a first and a second spread spectrum code pulse position modulated signal transmitted during said associated symbol duration to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing residual coupling between said first and second spread spectrum code pulse potion modulated signals of a given symbol duration and causing energy associated with a first symbol transmitted during a first symbol duration to disperse into one or more adjacent symbol durations, said receiver apparatus comprising:

a filter matched to said spread spectrum codewords;

a channel matched filter matched to said delay spread characteristics of said communication channel;

a tentative symbol estimator for sampling output signals of said channel matched filter to estimate the value of said symbol, said estimation assigning a weight to each possible value of said symbol, each of said weights taking into account said residual coupling influence between said first and second spread spectrum code pulse position modulated signals of a given symbol duration on one another and said influence of said one or more adjacent transmitted symbols on one another; and

a mode sifter for evaluating said weights assigned by said tentative symbol estimator to select a final estimate for each of said symbol values, said final estimate being said possible value of said symbol having the greatest weight.

24. A method for receiving a received signal comprising information symbols transmitted across a communication channel, each of said symbols conveying a plurality of bits transmitted during a given symbol duration, each of said symbols being encoded using a first and a second spread spectrum code pulse position modulated signal transmitted during said associated symbol duration to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing residual coupling between said first and second spread spectrum code pulse position modulated signals of a given symbol duration and causing energy associated with a first symbol transmitted during a first symbol duration to disperse into one or more adjacent symbol durations, said receiving method comprising the steps of:

filtering said received signal with a filter matched to said spread spectrum codewords;

filtering said received signal with a channel matched filter matched to said delay spread characteristics of said communication channel;

sampling output signals of said channel matched filter to estimate the value of each symbol, and assigning a weight to each possible value of said symbol, each of said weights taking into account said residual coupling influence between said first and second spread spectrum code pulse position modulated signals; and

evaluating said weights to select a final estimate for said symbol value, said final estimate selection taking into account the influence of said one or more adjacent transmitted symbols on one another.

25. A method for receiving a received signal comprising an information symbol transmitted across a communication channel, said symbol conveying a plurality of bits transmitted during a symbol duration, said symbol being encoded using a first and a second spread spectrum code pulse position modulated signal transmitted during said associated symbol duration to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing residual coupling between said first and second spread spectrum code pulse position modulated signals, said receiving method comprising the steps of:

filtering said received signal with a filter matched to said spread spectrum codeword;

filtering said signal with a channel matched filter matched to said delay spread characteristics of said communication channel;

sampling output signals of said channel matched filter to estimate the value of said symbol, and assigning a weight to each possible value of said symbol, each of said weights taking into account said residual coupling influence between said first and second spread spectrum code pulse position modulated signals on one another; and

evaluating said weights to select a final estimate for said symbol value, said final estimate being said possible value of said symbol having the greatest weight.

26. A method for receiving a received signal comprising information symbols transmitted across a communication channel, each of said symbols conveying a plurality of bits transmitted during a given symbol duration, each of said symbols being encoded using a spread spectrum code pulse position modulated signal to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing energy associated with a first symbol transmitted during a first symbol duration to disperse into one or more adjacent symbol durations, said receiving method comprising the steps of:

filtering said received signal with a filter matched to said spread spectrum codewords;

filtering said received signal with a channel matched filter matched to said delay spread characteristics of said communication channel;

sampling output signals of said channel matched filter for a given symbol duration to estimate the value of each of said transmitted symbols, and assigning a weight to each possible value of said symbol based on said sampled values; and

evaluating said weights to select a final estimate for said symbol value, said final estimate selection taking into account the influence of said one or more adjacent transmitted symbols on one another.

27. A method for receiving a received signal comprising information symbols transmitted across a communication channel, each of said symbols conveying a plurality of bits transmitted during a given symbol duration, each of said symbols being encoded using a first and a second spread spectrum code pulse position modulated signal transmitted during said associated symbol duration to generate a spread spectrum codeword, said communication channel exhibiting delay spread characteristics, said delay spread causing residual coupling between said first and second spread spectrum code pulse position modulated signals of a given symbol duration and causing energy associated with a first symbol transmitted during a first symbol duration to disperse into one or more adjacent symbol durations, said receiving method comprising the steps of:

filtering said received signal with a filter matched to said spread spectrum codewords;

filtering said received signal with a channel matched filter matched to said delay spread characteristics of said communication channel;

sampling output signals of said channel matched filter to estimate the value of said symbol, and assigning a weight to each possible value of said symbol, each of said weights taking into account said residual coupling influence between said first and second spread spectrum code pulse position modulated signals of a given symbol duration on one another and said influence of said one or more adjacent transmitted symbols on one another; and

evaluating said weights to select a final estimate for each of said symbol values, said final estimate being said possible value of said symbol having the greatest weight.