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US 5,471,501 AGranted
Enhanced digital communications receiver using channel impulse estimates
AssigneeHughes Aircraft Company
InventorsMichael I. Parr, John L. Watson, T. G. Vishwanath
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Abstract
The present invention provides for a receiver that employs integrated symbol timing adjustment and carrier tracking algorithms based on channel impulse response estimates. It uses variable coefficient least mean square tracking during estimation of the transmission channel's impulse response. Maximum likelihood sequence estimation equalization can also be integrated into the system using a non-real time operation that permits and exploits "time-reversed" equalization to significantly enhance bit-error rate performance and permits implementation of all the functions in a reasonably simple receiver.
Claims29
In a digital communications receiver in which signals received sequentially within time slots of a transmission channel, having a carrier and an impulse response are sampled, a method for adjusting timing of the signal sampling and for tracking offset of the transmission channel carrier comprising the steps of:
- storing samples of the sequentially received signals in a time sequential manner;
- generating transmission channel impulse response estimates associated with the stored samples, the estimates having associated taps;
- generating from the samples an error measurement comprising a first measure of the degree to which a channel impulse response estimate matches the actual channel impulse response;
- determining a sample time setting that minimizes the above error measurement and adjusting the timing of the signal sampling in accordance with the newly determined sample time setting;
- recording at least two samples of at least one tap of a channel impulse response estimate at at least one selected location within a time slot;
- estimating the location within the time slot at which any fade in power received over the channel is a maximum;
- the time slot having a preamble and a postamble;
- during the reception of each time slot, selecting the half of the time slot that does not include the estimated location of maximum power fade;
- recording two samples of each of two taps of the channel impulse response estimates, the first tap being just after the preamble or in the postamble, depending upon the estimated location of maximum power fade, and the second tap being a predetermined number of symbols later or earlier, depending upon the estimated location of maximum power fade;
- selecting a dominant tap;
- using recorded settings for the selected dominant tap, calculating a phase change, yielding an estimate of the frequency offset;
- filtering the estimate of the frequency offset over many time slots to generating an estimate of the offset of the transmission channel carrier adapted for use as a feedback signal to adjust carrier tracking;
- and
- adjusting a controllable frequency source to track the offset of the transmission channel carrier using the feedback signal.
11 more elements
A digital communications receiver in which signals received sequentially within time slots of a transmission channel having a carrier and an impulse response are sampled comprising:
- a memory for sorting samples of the sequentially received signals in a time sequential manner, each sample having an associated phase;
- a channel impulse response estimate generator coupled to the memory;
- means coupled to the memory for generating from the stored samples an error measurement comprising a first measure of the degree to which a channel impulse response estimate matches the actual channel impulse response;
- a second memory for recording at least two samples of at least one tap of a channel impulse response estimate at at least one selected location within a time slot;
- a fade locator for estimating the location within the time slot at which any fade in power received over the channel is a maximum the time slot having a preamble and a postamble;
- means for selecting the half of the time slot that does not include the estimated location of maximum power fade wherein the second memory records two samples of each of two taps of the channel impulse response estimates, the first tap being just after the preamble or in the postamble, depending upon the estimated location of maximum power fade, and the second tap being a predetermined number of symbols later or earlier, depending upon the estimated location of maximum power fade;
- a dominant tap selector;
- means for using recorded settings from the second memory for the selected dominant tap, calculating a phase change, yielding an estimate of the frequency offset;
- and
- a frequency offset filter for filtering the estimate of the frequency offset over many time slots to generate an estimate of the offset of the transmission channel carrier adapted for use as a feedback signal to adjust carrier tracking;
- and
- a controllable frequency source adjustable to track the offset of the transmission channel carrier using the feedback signal.
9 more elements
In a digital communications receiver in which signals received sequentially within time slots of a transmission channel, having a carrier and an impulse response are sampled, a method for adjusting timing of the signal sampling and for tracking offset of the transmission channel carrier comprising the steps of:
- storing samples of the sequentially received signals in a time sequential manner;
- generating transmission channel impulse response estimates associated with the stored samples, the estimates having associated taps;
- using variable tap coefficients that are determined by estimating tap settings for the estimated channel impulse response by minimizing the square of the difference between actual received samples and those synthesized by passing known transmitted signals through the estimated channel, the processing being done in an iterative manner by combining previous estimates of channel impulse response and new estimates thereof based on recent estimates, and by varying the ratio of the contributions from the previous and new estimates as a function of location within the time slot, the ratio being performed by varying the values of coefficients K.sub.1 and K.sub.2 as a function of symbol location (k) by solving the following equations for (k+1) and C.sub.1 (k+1):
- ##EQU5## where C.sub.0 (k) and C.sub.1 (k) are complex values of estimated channel impulse response taps, C.sub.s0 (k) and C.sub.n (k) are complex intermediate values related to the estimated channel impulse response taps, permitting second order operation, K.sub.1 and K.sub.2 are the real gain values controlling the tracking rate of the channel impulse response estimation process, z(k) are complex symbol spaced sampled outputs of a receiver matched filter, and a(k) are complex estimated or known values of transmitted symbols;
- generating from the samples an error measurement comprising a first measure of the degree to which a channel impulse response estimate matches the actual channel impulse response;
- determining a sample time setting that minimizes the above error measurement and adjusting the timing of the signal sampling in accordance with the newly determined sample time setting;
- recording at least two samples of at least one tap of a channel impulse response estimate at least one selected location within a time slot;
- generating a frequency offset estimate from the phase difference in the time slot between the at least two samples using the recorded tap samples;
- and
- adjusting a controllable frequency source to track the offset of the transmission channel carrier in accordance with the frequency offset estimate.
7 more elements
A digital communications receiver in which signals received sequentially within time slots of a transmission channel having a carrier and an impulse response are sampled comprising:
- a memory for storing samples of the sequentially received signals in a time sequential manner, each sample having an associated phase;
- a channel impulse response estimate generator coupled to the memory, comprising means for using variable tap coefficients that are determined by estimating tap settings for the estimated channel impulse response estimates by minimizing the square of the difference between actual received samples and those synthesized by passing known transmitted signals through the estimated channel, wherein the processing is done in an iterative manner by combining previous estimates of channel impulse response and new estimates thereof based on recent estimates, and by varying the ratio of the contributions from the previous and new estimates as a function of location within the time slot by varying the values of coefficients K.sub.1 and K.sub.2 as a function of symbol location (k) by solving the following equations for (k+1) and C.sub.1 (k+1):
- ##EQU6## where C.sub.0 (k) and C.sub.1 (k) are complex values of estimated channel impulse response taps, C.sub.s0 (k) and C.sub.n (k) are complex intermediate values related to the estimated channel impulse response taps, permitting second order operation, K.sub.1 and K.sub.2 are the real gain values controlling the tracking rate of the channel impulse response estimation process, z(k) are complex symbol spaced sampled outputs of a receiver matched filter, and a(k) are complex estimated or known values of transmitted symbols;
- means coupled to the memory for generating from the stored samples an error measurement comprising a first measure of the degree to which a channel impulse response estimate matches the actual channel impulse response;
- means for determining a sample time setting that minimizes the above error measurement and for adjusting the timing of the signal sampling in accordance with the newly determined sample time setting;
- a second memory for recording at least two samples of at least one tap of a channel impulse response estimate at least one selected location within a time slot;
- means for generating a frequency offset estimate from the phase difference in the time slot between the at least two samples using the recorded tap samples;
- and
- a controllable frequency source adjustable to track the offset of the transmission channel carrier in accordance with the frequency offset estimate.
6 more elements
In a digital communications receiver in which signals received sequentially within time slots of a transmission channel, having a carrier and an impulse response are sampled, a method for adjusting timing of the signal sampling and for tracking offset of the transmission channel carrier comprising the steps of:
- storing samples of the sequentially received signals in a time sequential manner;
- generating transmission channel impulse response estimates associated with the stored samples, the estimates having associated taps;
- generating from the samples an error measurement comprising a first measure of the degree to which a channel impulse response estimate matches the actual channel impulse response;
- determining a sample time setting that minimizes the above error measurement and adjusting the timing of the signal sampling in accordance with the newly determined sample time setting;
- recording at least two samples of at least one tap of a channel impulse response estimate at at least one selected location within a time slot;
- generating frequency offset estimates for each of a plurality of time slots from the phase difference in each time slot between at least two samples of a single tap of a channel impulse response estimate;
- combining this plurality of frequency offset estimates to generate a precise frequency offset estimate by applying f.sub.-- offset.sub.-- estimate.sub.k+1 =(1-K.sub.fo) f.sub.-- estimate.sub.k +K.sub.fo freq.sub.-- observed, where freq.sub.-- observed is derived from the observed phase change, the constant K.sub.fo controls the convergence rate of the estimation process, f.sub.-- offset.sub.-- estimate.sub.k is the estimated frequency offset at time slot "k", and K.sub.of is a constant controlling the convergence rate of the frequency tracking;
- and
- adjusting a controllable frequency source to track the offset of transmission channel carrier in accordance with the precise frequency offset estimate.
6 more elements
Dependencies are inferred from the claim text.