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All-digital method and apparatus for demodulating an analog signal
| Details |
Inventors: Ezran, Philippe; Le Clec'h, Michel; Jeanclaude, Serge;
Assignee: France Telecom (Paris, FR)
Primary Examiner: Chin; Stephen
Assistant Examiner: Phan; Hai H.
Attorney, Agent or Firm: Larson and Taylor
For digitally demodulating a PSK analog signal transmitted as a linearly modulated carrier and representing successive symbols at a rate 1/T, the signal is sampled at a rate 1/.tau. at least twice the symbol rate 1/T, successive samples sampled with different filters selected from a plurality of filters corresponding to instants of no IES that are mutually offset by a fraction of the sampling .tau. that is optimum as regards coincidence between the respective sampling instant and an instant at which there is no ISI. A mean symbol demodulation rate equal to the symbol transmission rate is however retained. The signal as delivered by the selected filters in succession is decoded. |
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DETAILED DESCRIPTION It is an object of the invention to provide a demodulator that is totally digital in structure, and thus in which ISI-eliminating filtering must take place after the signal has been sampled and digitized.
This approach a priori appears to be out of the question due to several difficulties: the instant at which there is no ISI in the period allocated to each symbol is initially unknown.
Even if it coincides with a sample for a given symbol, coincidence will disappear for the following symbol unless the sampling frequency is an integer multiple of the symbol transmission rate.
For example, in a demodulator incorporated in a modem for transmission at 2400 bauds with transmission over a PCM link that is sampled at 8000 Hz, asssuming that sample k coincides with an instant at which there is no ISI, then the following instant of no ISI will be situated between the instants at which samples k+3 and k+4 are obtained.
It is a more specific object of the invention to provide a fully digital method and apparatus overcoming such difficulties.
To this end, there is provided, in particular, a method including the step of performing, after the signal has been sampled, with a filter selected from a plurality of filters corresponding to instants of no ISI that are mutually offset by a fraction of the sampling period that is optimum as regards coincidence between the sampling instant and the instant at which there is no ISI, while maintaining a mean symbol demodulation rate equal to the symbol transmission rate.
The method can be implemented in numerous different ways.
A first solution consists in using a plurality of distinct filters, all filters having coefficients such that the instants of no ISI appear at the same rate, equal to the symbol transmission rate, but giving instants of no ISI that are time offset from one filter to the next.
The filter used for a given symbol and the sample representative of said symbol are selected, for each symbol, by using a criterion that is based on the filter which was used and the sample which was selected as being representative of the preceding symbol
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Multiplexer-related 
