Method and apparatus for multipath signal compensation in spread-spectrum communications systems
First Claim
1. An apparatus comprising:
- a compensation circuit adapted to receive input samples from a received multipath signal and compensate input samples using secondary signal estimates to form compensated samples;
an estimation circuit adapted to receive magnitude and phase information corresponding to a secondary path signal in the received multipath signal and output a current secondary signal estimate for a current input sample based on the magnitude and phase information and a selected one in a running sequence of past nominal phase values derived from prior input samples;
a hard-decision circuit adapted to determine a nominal phase value for each input sample, wherein said hard-decision circuit receives the compensated samples and outputs a selected one from a defined set of nominal phase values as the nominal phase value;
a buffer circuit adapted to maintain the running sequence of past nominal phase values in successive buffer positions corresponding to successively greater sample time offsets with respect to the current input sample; and
a buffer interface circuit adapted to receive path delay information corresponding to the secondary path signal and provide said estimation circuit with the selected past nominal phase value by retrieving the past nominal phase value currently held in one of said buffer positions corresponding to a sample time offset equal to the received path delay information;
wherein said apparatus is operative to remove secondary path signal interference from the input samples for output as compensated samples to an associated communications system.
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Abstract
An equalizer operates on chip or sub-chip resolution input samples of a received spread-spectrum multipath signal to remove interference from one or more secondary propagation path signals within the multipath signal. The equalizer may be configured for cancellation of secondary signals arriving before and after a main propagation path signal, referred to as pre- and post-cursor signals, respectively. An associated communications system provides the equalizer with a path coefficient and delay value for each secondary path signal for which cancellation is desired. With its unique architecture, the equalizer cancels secondary signals displaced in time by amounts as small as the input sample time resolution or by amounts exceeding multiple chips, or even multiple symbols. The length of sample delay buffers within the equalizer, in combination with the input sample rate, determines the maximum secondary signal delay accommodated by the equalizer. Post-cursor cancellation requires only a hard-decision value buffer, while pre-cursor cancellation requires an additional input or compensated sample buffer. The equalizer makes a hard-decision about the phase value of each input sample and buffers these hard-decision values for use in secondary signal cancellation. Hard-decision values delayed from current input samples by delay times corresponding to the path delays of the secondary signals for which cancellation is desired are used to rotate the phase of corresponding path coefficients. These adjusted values are fed back for subtraction from input samples for post-cursor cancellation, and fed forward for subtraction from delayed input samples for pre-cursor cancellation.
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Citations
49 Claims
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1. An apparatus comprising:
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a compensation circuit adapted to receive input samples from a received multipath signal and compensate input samples using secondary signal estimates to form compensated samples;
an estimation circuit adapted to receive magnitude and phase information corresponding to a secondary path signal in the received multipath signal and output a current secondary signal estimate for a current input sample based on the magnitude and phase information and a selected one in a running sequence of past nominal phase values derived from prior input samples;
a hard-decision circuit adapted to determine a nominal phase value for each input sample, wherein said hard-decision circuit receives the compensated samples and outputs a selected one from a defined set of nominal phase values as the nominal phase value;
a buffer circuit adapted to maintain the running sequence of past nominal phase values in successive buffer positions corresponding to successively greater sample time offsets with respect to the current input sample; and
a buffer interface circuit adapted to receive path delay information corresponding to the secondary path signal and provide said estimation circuit with the selected past nominal phase value by retrieving the past nominal phase value currently held in one of said buffer positions corresponding to a sample time offset equal to the received path delay information;
wherein said apparatus is operative to remove secondary path signal interference from the input samples for output as compensated samples to an associated communications system. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21)
a delay circuit adapted to receive compensated samples and to output delayed compensated samples; and
a second compensation circuit adapted to receive delayed compensated samples and second secondary signal estimates from said estimation circuit and to output finally compensated samples based the delayed compensated samples and the second secondary signal estimates, wherein the second secondary signal estimates correspond to one or more of a second type of secondary signals within the received multipath signal having a path delay less than the path delay of the main path signal.
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21. The apparatus of claim 20 wherein said estimation circuit is further adapted to receive magnitude and phase information for one or more of the second type of secondary signal and said buffer interface circuit is further adapted to receive path delay information for the one or more of the second type of secondary signal, and said estimation circuit is further adapted to output the second secondary signal estimates based on receiving additional selected ones from said buffer circuit via said buffer interface circuit based on the path delay information for the one or more of the second type of secondary path signal.
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22. An apparatus comprising:
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a first compensation circuit adapted to receive input samples from a received multipath signal and compensate input samples using first secondary signal estimates to form first compensated samples;
a delay circuit adapted to receive the first compensated samples and output delayed first compensated samples;
a second compensation circuit adapted to receive said delayed first compensated samples and compensate delayed first compensated samples using second secondary signal estimates to form second compensated samples;
an estimation circuit adapted to receive magnitude and phase information corresponding to one or more of first and second types of secondary path signals in the received multipath signal and output current first and second secondary signal estimates for a current input sample and a current delayed first compensated sample, respectively, based on the magnitude and phase information and selected ones in a running sequence of past nominal phase values derived from prior input samples;
a hard-decision circuit adapted to determine a nominal phase value for each input sample, wherein said hard-decision circuit outputs a selected one from a defined set of nominal phase values as the nominal phase value;
a buffer circuit adapted to maintain the running sequence of past nominal phase values in successive buffer positions corresponding to successively greater sample time offsets with respect to the current input sample based on receiving the nominal phase values output from said hard-decision circuit; and
a buffer interface circuit adapted to receive path delay information corresponding to one or more of the first and second types of secondary path signals, and further adapted to provide said estimation circuit with the selected past nominal phase values by retrieving the past nominal phase values currently held in said buffer circuit from said buffer positions corresponding to sample time offsets equal to the received path delay information;
wherein said apparatus is operative to remove secondary path signal interference for one or more of the first and second types of secondary path signals, further wherein the first type of secondary signal has a path delay greater than a path delay of a main path signal in the received multipath signal and the second type of secondary path signal has a path delay less than the path delay of the main path signal. - View Dependent Claims (23, 24, 25, 26, 27, 28, 29, 30, 31)
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32. An apparatus comprising:
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a means for receiving input samples taken from a received multipath signal;
a means for subtracting a first secondary signal estimate from each input sample to form a first compensated sample;
a means for determining a nominal phase value for each first compensated sample;
a means buffering a running sequence of past nominal phase values determined for a like number of most-recent input samples in a number D of successive buffer positions, and an associated means of updating the running sequence with each successive nominal phase value derived from successive input samples;
a means for delaying each first compensated sample by an amount d;
a means for subtracting a second secondary signal estimate from each delayed first compensated sample to form a second compensated sample;
a means for receiving magnitude and phase information for a first and second secondary signal in the received main path signal relative to a main path signal in the received multipath signal, and an associated means for generating the first and second secondary signal estimates for each input sample and delayed first compensated sample, respectively, using the magnitude and phase information and selected ones from the running sequence of nominal phase values; and
a means for receiving path delay information for the first and second secondary signals and an associated means for selecting particular ones of said successive buffer positions based on the path delay information and providing said means for generating the first and second secondary signal estimates with nominal phase values retrieved from said particular ones of said successive buffer positions;
wherein said apparatus is operative to remove secondary signal interference resulting from the first and second secondary signals from the received multipath signal. - View Dependent Claims (33)
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34. An apparatus comprising:
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a delay circuit adapted to receive input samples from a received multipath signal and output delayed samples;
a hard-decision circuit adapted to receive input samples and output a nominal phase value for each input sample;
a compensation circuit adapted to receive delayed samples and compensate the delayed samples using secondary signal estimates to form compensated samples;
an estimation circuit adapted to receive magnitude and phase information corresponding a secondary path signal in the received multipath signal and output a secondary signal estimate for each delayed sample based on the magnitude and phase information and a past nominal phase value selected from a running sequence of past nominal phase values derived from prior input samples;
a buffer circuit adapted to maintain the running sequence of past nominal phase values in successive buffer positions corresponding to successively greater sample time offsets with respect to the current input sample based on receiving past nominal phase values output from the hard-decision circuit; and
a buffer interface circuit adapted to receive path delay information corresponding to the secondary path signal and provide said estimation circuit with the selected past nominal phase value by retrieving the past nominal phase value currently held in said buffer circuit from the buffer position corresponding to a sample time offset equal to the received path delay information;
wherein said apparatus is operative to remove secondary path signal interference from the received multipath signal due to the secondary path signal.
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35. A method of substantially canceling at least one secondary path signal in a multipath signal received through a main radio signal propagation path and at least one secondary radio signal propagation path, said method comprising:
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providing a path coefficient associated with a magnitude and phase of the at least one secondary path signal relative to a main path signal and a time offset associated with a path delay of the secondary radio signal propagation path relative to the main radio signal propagation path;
receiving multipath signal samples;
generating a compensation value for a current multipath signal sample based on the path coefficient and a nominal phase value determined for a prior multipath signal sample received a number of sample times substantially equal to the time offset earlier than the current multipath signal sample;
compensating the current multipath signal sample with a compensation value to form a compensated sample;
determining a nominal phase value for the compensated sample;
storing the nominal phase value for the compensated sample for use in generating a compensation value for a subsequent multipath signal sample received a number of sample times substantially equal to the time offset later than the current multipath signal sample; and
outputting the compensated sample for use in an associated communications system. - View Dependent Claims (38, 39, 40, 41, 42, 43, 44, 45, 46, 47)
providing a path coefficient and corresponding time offset for at least one of a second type of secondary signal;
delaying the compensated samples by an amount d corresponding to a maximum delay associated with the second type of secondary signal;
generating a second compensation value for each delayed compensated sample based on the path coefficient and a nominal phase value determined for a prior multipath signal sample received a number of sample times equal to the time offset associated with the at least one of the second type of secondary signal; and
compensating each delayed compensated sample with a second compensation value to form a finally compensated sample.
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- 36. The method of 35 further comprising providing a defined set of nominal phase values defined for a modulation scheme associated with the received multipath signal.
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48. A method of compensating a received multipath signal for interference arising from selected secondary path signals with the received multipath signal, said method comprising:
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providing a complex coefficient representing a magnitude and phase relative to a main path signal within the received multipath signal and an associated delay value representing a path delay relative to the main path signal, for each of the selected multipath signals;
receiving input samples at a defined sample rate taken from the received multipath signal;
selecting a past nominal phase value determined for a prior input sample for each of the selected secondary signals based on the delay value associated with each secondary signal;
adjusting a phase value of each complex coefficient with a corresponding one of the selected past nominal phase values;
combining a first number of complex coefficients corresponding to a first group of secondary path signal signals within the selected secondary signals to form a first compensation value, and combining a remaining number of complex coefficients corresponding to a second group of secondary path signal signals within the selected secondary signals to form a second compensation value;
subtracting the first compensation value from a current input sample to form a first compensated sample;
subtracting the second compensation value from a delayed first compensated sample representing a first compensated sample determined a time d earlier for a prior input sample to form a final compensated sample;
delaying the first compensated sample for use in forming a subsequent final compensated sample;
determining a nominal phase for the current first compensated sample; and
updating a running sequence of past nominal phase values with the nominal phase value determined for the current first compensated sample;
wherein the first group of secondary signals are those selected secondary signals within the received multipath signal having path delays longer than a path delay of the main path signal, and the second group of secondary signals are those selected secondary signals within the received multipath signal having path delays shorter than the path delay of the main path signal. - View Dependent Claims (49)
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Specification