Pulse or digital communications – Repeaters – Testing
Patent
1989-01-13
1991-07-02
Safourek, Benedict V.
Pulse or digital communications
Repeaters
Testing
371 43, 375 39, 375101, H04L 2738
Patent
active
050291866
DESCRIPTION:
BRIEF SUMMARY
This invention relates to digital communication systems in which multipath propogation occurs, and in which Viterbi or other algorithms for calculating reference values are employed, such as algorithms of the Fano or Stack types. These reference values are used for calculating metrics in such algorithms, e.g. the Viterbi algorithm or the like.
Examples of fields of use of the invention are underwater communication, troposcatter communication, shortwave radio, mobile communication and communication via transmission lines. The invention can also be used in communication systems in which multipath propogation is caused by filters in the transmitter or the receiver, in addition to that being caused by the channel between transmitter and receiver. The method can be used in combination with digital phase modulation (DPM), digital frequency modulation (DFM), "tamed frequency modulation" (TFM), Gaussian "minimum shift keying" (GMSK) and other modulation methods, in which the transmitter employs pulses having lengths larger than one symbol interval (Partial response signalling), in particular those having a compact power spectrum.
Receivers for digital transmission through channels with multipath propagation are known (see list of references at the end of this specification). These are based on the use of DFE-equalizers {1} and Viterbi decoders {2, 9} and methods only scanning a limited number of possible data sequences {3}. It is also known to calculate reference values by only utilizing a portion of the multipath response {5} as well as methods for calculating the performance of this approximation {4}. Moreover there is known {6} a method for reducing the number of filters (Averaged Matched Filter) being required in a receiver for single path propagation as an alternative to Viterbi decoding. As a general background, reference can be made to chapter 6 in the book "Error-Correction Coding for Digital Communications" {8}.
In digital communications it is expected in many situations that such large differences will occur between the individual multipath delays, that a practical receiver will not be able to handle these situations. Therefore it is important to make the best possible use of the limited number (.nu.) consecutive data symbols which can be treated in the receiver. In other words a practical receiver, which cannot have an unreasonably high complexity, can only take into account a limited number of data symbols. In this connection these are denoted "known" data symbols.
The number of possible sequences increases exponentially (M.sup..nu.) when the number of known data symbols having M possible values, increases. With multipath transmission the difference in delay between the multiple paths to be handled by a receiver will be proportional to the number of data symbols considered to be known, i.e. to .nu.. The number of possible sequences therefore will increase exponentially with the difference in delay between the multiple paths, and it is therefore very important to be able to handle as large multipath delay differences as possible without increasing the number of data symbols considered to be known.
A receiver employing previously known methods will not be able to handle larger differences between the individual multipath delays in the number of data symbols, than the difference between the number of data symbols considered to be known in the receiver, and the number which influences the (known) transmitted symbol when the part of the multipaths profile which is longer than this difference, includes a significant portion of the total power of the profile. This is a situation often occurring in practice.
The primary object of the invention is to provide an improved demodulation in digital communication systems without making the receiver so complex that it cannot be implemented in practice. This is obtained by means of a method as defined more closely in the claims.
In short the solution presented here is based on the fact that knowledge of the signal in which more than .nu. data symbols are included, is n
REFERENCES:
patent: 4503545 (1985-03-01), Bremer et al.
patent: 4536878 (1985-08-01), Rattlingourd et al.
patent: 4555790 (1985-11-01), Betts et al.
patent: 4631735 (1986-12-01), Qureshi
Maseng Torleiv
Trandem Odd
Safourek Benedict V.
Stiftelsen For Industriell Og Teknisk Forskning Ved Norges Tekni
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