Maximum likelihood sequence estimation for a mobile digital cellular radio receiver

Claim 2

In a digital cellular receiver, a method of processing samples received from a channel, that are associated with a block of data that was transmitted within a time slot, the method comprising the steps of: processing the samples starting with a first received sample and proceeding beyond a final received sample of the time slot using a preselected maximum likelihood sequence estimation process to generate estimates of the transmitted data; simultaneously producing estimates of the characteristics of the impulse response of the channel which are used in the maximum likelihood sequence estimation process; and using the results of the preceding two processing steps to generate final decisions as to the transmitted data.

Claim 3

In a digital communications receiver, a method of compensating for fading in sequential samples transmitted within a time slot, the method comprising the steps of: storing samples received during a time slot in a time sequential manner; estimating the location within the time slot of maximum fade depth; processing the stored samples, starting with the first received sample and proceeding in a forward direction with respect to the time sequence in which the samples were stored, beyond the estimated location of the maximum fade depth, using a preselected maximum likelihood sequence estimation procedure to generate estimates of the transmitted data; processing the stored samples, starting with the final received sample and proceeding in a reverse direction with respect to the time sequence in which the samples were stored, beyond the estimated location of maximum fade depth, using the maximum likelihood sequence estimation procedure to generate estimates of the transmitted data; and processing the output of the preceding estimate generating steps to compensate for fading of the samples.

Claim 4

In a digital communications receiver, a method of compensating for fading in sequential samples transmitted within a time slot, the method comprising the steps of: storing samples received during a time slot in a time sequential manner; estimating the location within the time slot of maximum fade depth; processing the stored samples, starting with the first received sample and proceeding in a forward direction with respect to the time sequence in which the samples were stored, beyond the estimated location of the maximum fade depth, using a preselected maximum likelihood sequence estimation procedure to generate estimates of the transmitted data; processing the stored samples, starting with the final received sample and proceeding in a reverse direction with respect to the time sequence in which the samples were stored, beyond the estimated location of maximum fade depth, using the maximum likelihood sequence estimation procedure to generate estimates of the transmitted data; and processing the output of the preceding estimate generating steps to compensate for fading of the samples.

Claim 5

The method of Claim 2 comprising generating transmission channel impulse response estimates for use in the maximum likelihood sequence estimation procedures.

Claim 6

The method of Claim 3 wherein the steps of generating the channel impulse response estimates comprise 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, and 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.

Claim 7

The method of Claim 3 or 4 which further comprises the steps of: using a subset of the samples received during the time slot, and generating therefrom an error measurement comprising a first measure of the degree to which the estimated channel impulse response matches the actual channel impulse response; generating a plurality of similar measures utilizing simultaneously recorded samples having different time offsets wherein at least one sample is advanced and one sample is retarded in time relative to the first measure; and determining, by filtering a plurality of such measures, a bit time setting that minimizes the above error measurement and adjusting the sampling to reflect the newly determined bit time setting.

Claim 8

The method of Claim 2, 3, 4 or 5 which further comprises the steps of: recording at least two samples of at least one tap within the estimated channel impulse response at selected locations within the time slot; generating frequency offset estimates at each of a plurality of time slots from the phase difference in each time slot between the at least two samples; combining this plurality of frequency offset estimates by, for example, using a filtering process to generate a precise frequency offset estimate; and adjusting a controllable frequency source to compensate for the precise frequency offset estimate.

Claim 9

The method of Claim 2, 3, 4 or 5 wherein the samples further include a preamble and a postamble and which further comprises the steps of: during the reception of each time slot, selecting the half of the time slot that does not include the estimated location of maximum fade depth; recording two samples of each of the two estimated channel impulse response taps, a first just after the preamble or in the postamble, depending upon the location of the estimated location of maximum fade depth, and a second predetermined number of symbols later or earlier, depending upon the location of the estimated location of maximum fade depth; from information determined during the bit timing, 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 a plurality of bursts to generate an estimate of the carrier offset adapted for use as a feedback signal to adjust carrier tracking.

Claim 10

In a digital communications receiver in which samples are received sequentially within a time slot of a transmission channel, a method for adjusting sample timing comprising the steps of: storing samples received during a time slot in a time sequential manner; generating estimates of the transmission channel impulse response; using a subset of the samples received during the time slot, and generating therefrom an error measurement comprising a first measure of the degree to which the estimated channel impulse response matches the actual channel impulse response; generating a plurality of similar measures utilizing simultaneously recorded samples having different time offsets wherein at least one sample is advanced and one sample is retarded in time relative to the first measure; and determining, by filtering a plurality of such measures, a bit time setting that minimizes the above error measurement and adjusting the sampling to reflect the newly determined bit time setting.

Claim 11

In a digital communications receiver in which samples are received sequentially within a time slot of a transmission channel, a method for adjusting sample timing comprising the steps of: storing samples received during a time slot in a time sequential manner; generating estimates of the transmission channel impulse response; using a subset of the samples received during the time slot, and generating therefrom an error measurement comprising a first measure of the degree to which the estimated channel impulse response matches the actual channel impulse response; generating a plurality of similar measures utilizing simultaneously recorded samples having different time offsets wherein at least one sample is advanced and one sample is retarded in time relative to the first measure; and determining, by filtering a plurality of such measures, a bit time setting that minimizes the above error measurement and adjusting the sampling to reflect the newly determined bit time setting.

Claim 12

In a digital communications receiver in which samples are received sequentially within a time slot of a transmission channel, a carrier offset tracking method comprising the steps of: storing samples received during a time slot in a time sequential manner; generating estimates of the transmission channel, impulse response; recording at least two samples of at least one tap within the estimated channel impulse response at selected symbol locations within the time slot; generating frequency offset estimates at each of a plurality of time slots from the phase difference in each time slot between the at least two samples; combining this plurality of frequency offset estimates by, for example, using a filtering process to generate a precise frequency offset estimate; and adjusting a controllable frequency source to compensate for the precise frequency offset estimate.

Claim 13

In a digital communications receiver in which samples are received sequentially within a time slot of a transmission channel, a carrier offset tracking method comprising the steps of: storing samples received during a time slot in a time sequential manner; generating estimates of the transmission channel, impulse response; recording at least two samples of at least one tap within the estimated channel impulse response at selected symbol locations within the time slot; generating frequency offset estimates at each of a plurality of time slots from the phase difference in each time slot between the at least two samples; combining this plurality of frequency offset estimates by, for example, using a filtering process to generate a precise frequency offset estimate; and adjusting a controllable frequency source to compensate for the precise frequency offset estimate.

Claim 14

In a digital communications receiver in which samples, which include preambles and postambles are received sequentially within a time slot of a transmission channel, a carrier offset tracking method comprising the steps of: storing samples received during a time slot in a time sequential manner; estimating the location within the time slot of maximum fade depth; generating estimates of the transmission channel, impulse response; during the reception of each time slot, selecting the half of the time slot that does not include the estimated location of maximum fade depth; recording two samples of each of the two estimated channel impulse response taps, a first just after the preamble or in the postamble, depending upon the location of the estimated location of maximum fade depth, and a second predetermined number of symbols later or earlier, depending upon the location of the estimated location of maximum fade depth; from information determined during the bit timing, 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 bursts to generate an estimate of the carrier offset adapted for use as a feedback signal to adjust carrier tracking.

Claim 15

In a digital communications receiver in which samples, which include preambles and postambles are received sequentially within a time slot of a transmission channel, a carrier offset tracking method comprising the steps of: storing samples received during a time slot in a time sequential manner; estimating the location within the time slot of maximum fade depth; generating estimates of the transmission channel, impulse response; during the reception of each time slot, selecting the half of the time slot that does not include the estimated location of maximum fade depth; recording two samples of each of the two estimated channel impulse response taps, a first just after the preamble or in the postamble, depending upon the location of the estimated location of maximum fade depth, and a second predetermined number of symbols later or earlier, depending upon the location of the estimated location of maximum fade depth; from information determined during the bit timing, 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 bursts to generate an estimate of the carrier offset adapted for use as a feedback signal to adjust carrier tracking.