Pulse or digital communications – Synchronizers – Synchronizing the sampling time of digital data
Reexamination Certificate
1998-12-08
2001-11-06
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Synchronizers
Synchronizing the sampling time of digital data
C375S324000
Reexamination Certificate
active
06314148
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a receiver demodulator using channel estimation and, more particularly, to maintenance of an optimum synchronization point for demodulation.
BACKGROUND OF THE INVENTION
In a typical RF communication system, a transmitted signal may travel from a transmitter to a receiver over multiple paths, for example, a direct path and also a reflected path. The signal on the reflected path typically arrives later than the signal on the direct path. Thus, the received signal exhibits distortion due to the time dispersive nature of a channel. Channel environments such as this are also known as multipath fading channels.
In digital advance mobile phone systems (DAMPS), an equalizer or demodulator typically operates based on the assumption that the transmitted signal encounters a symbol-spaced, two-tap multipath channel, regardless of the actual prevailing channel conditions. In order to demodulate the received signal, it is necessary to synchronize the receiver to a known synchronization sequence contained in the signal. This can be done initially by correlating the received waveform against a local version of the synch word. The synchronization point in a stream of oversampled received data is chosen which maximizes the sum of two points, one associated with each tap of the channel estimate, of a squared correlation taken at two different lags taken a symbol interval apart. By choosing the synchronization point that maximizes the sum, the correlated received power at symbol taps is maximized. This matches the symbol-spaced two-tap channel estimate in time to the actual channel in a maximum power sense.
When rapid fading is present, actual channel conditions at the beginning of a slot burst may substantially change over the transmission of that burst. The conditions for matching the channel estimate in time to the channel in a maximum power sense may change substantially as well. This can result in the optimum synchronization point shifting over the course of the burst. If the same symbol sampling phase is used, then the channel estimate becomes no longer matched in time to the actual channel in that maximum power sense.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
SUMMARY OF THE INVENTION
In accordance with the invention there is disclosed a synchronization tracking system and method that recalculates metrics in an equalizer using samples delayed from and in advance of a current sample.
Broadly, there is disclosed herein a demodulator for a receiver operating in a multipath channel carrying transmitted symbols. The demodulator is comprised of means to receive a signal representing a transmitted sequence of symbols and to produce output samples at a select sampling phase corresponding to a synchronization point. Means are provided for generating hypothetical samples from hypothetical sequences. A first means compares the output sample to the plural hypothetical samples to determine plural metrics and developing an original estimate of each transmitted sequence based on the hypothetical sample producing a best cumulative metric. A second means compares the hypothetical samples to the received sample at an early sampling phase in advance of the select sampling phase and to the received sample at a late sampling phase delayed from the select sampling phase and determining if a second estimate of the transmitted symbol produces a better metric than the original estimate, and in response to the second estimate producing a better metric adjusting the synchronization point.
It is a feature of the invention that the second means compares only the hypothetical sample producing the best metric to the received sample at the early sampling phase in advance of the select sampling phase and to the received sample at the late sampling phase delayed from the select sampling phase.
In accordance with another aspect of the invention there is disclosed a demodulator for a receiver operating in a multipath channel carrying a transmitted sequence of symbols. The demodulator includes means for sampling a received signal representing the transmitted sequence of symbols and developing an output signal at a select number of samples per symbol at a select sampling phase corresponding to a synchronization point. Means generate plural hypothetical sequences of symbols and associated hypothetical samples. A first means compares the output signal to the plural samples from hypothetical sequences of symbols to determine plural metrics and develop an original estimate of the transmitted sequence of symbols based on the hypothetical sequence producing a best metric. A second means compares the plural hypothetical samples to the received signal at an early sampling phase in advance of the select sampling phase and to the received signal at a late sampling phase delayed from the select sampling phase. Based on whether the metric calculated from this comparison is better than the metric from the original comparison, the synchronization point is adjusted.
It is a feature of the invention that the second means compares only the hypothetical sequence of symbols producing the best metric to the received signal at the early sampling phase in advance of the select sampling phase, and to the received signal at the late sampling phase delayed from the select sampling phase.
It is a further feature of the invention that the plural metrics determined by the first comparing means comprise cumulative metrics.
It is still another feature of the invention that the metrics produced by the second comparing means comprise cumulative metrics.
It is still another feature of the invention that the first comparing means comprises a maximum likelihood sequence estimator.
There is disclosed in accordance with another aspect of the invention a method of demodulating the received signal in a multipath channel carrying a transmitted sequence of symbols. The method comprises the steps of sampling the received signal representing the transmitted sequence of symbols and developing an output signal at a select number of samples per symbol at a select sampling phase corresponding to a synchronization point, generating plural hypothetical sequences of symbols and from these, generating plural hypothetical received samples based on the current estimation of the channel, comparing the output signal to the plural hypothetical samples to develop original metrics for the hypothetical received sequences, resampling the received signal at sampling phases earlier and later than the sampling phase corresponding to the synchronization point, comparing the hypothetical samples with the early and late samples to develop early and late metrics, and adjusting the synchronization point base on which of the original, the early, or the late metrics is a best metric.
More particularly, the demodulator maintains the optimum synchronization point even where placement of taps of the channel estimate are mismatched to the placement of taps in the actual channel, and the fading of the channel is rapid with respect to the length of the burst transmission. The demodulator makes use of the calculation of metrics which utilize received signal samples which are in advance of a current sampling point and samples which are delayed from a current sampling point. By comparing these metrics with the metrics calculated with the current, present sampling point, a timing offset is altered to steer the optimum synchronization point in a direction which shows the best calculated metric.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.
REFERENCES:
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patent: 5577068 (1996-11-01), Bottomley et al.
patent: 5596607 (1997-01-01), Larsson et al.
patent: 5619524 (1997-04-01), Ling et al.
patent: 0 757 458 A2 (1997-02-01), None
patent: WO 98/38742 (1998-03-01), None
Coats & Bennett
Ericsson Inc
Ghebretinsae Temesghen
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