M-ary FSK receiver

• US 6,002,725 A
• Filed: 08/20/1997
• Issued: 12/14/1999
• Est. Priority Date: 08/22/1996
• Status: Expired due to Fees

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Razzell, Charles J. H.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Maddox, Michael W.
• Application Number:
  US08/916,774
• Time in Patent Office: 846 Days
• Field of Search: 375/335, 375/334, 375/272, 375/275, 329/302, 329/300
• US Class Current: 375/335
• CPC Class Codes: H04L 27/1563 using transition or level detection

Abstract

An M-ary FSK receiver in which a frequency down-converted signal is treated as an M-ary DPSK signal with symbol repetition coding. The output of a frequency demodulator (20) is integrated (22) over an optimally chosen time interval which is a fraction of the symbol interval to obtain N phase change estimates per symbol. These N sub-symbols are manipulated (24, 26) to obtain corresponding soft decisions which are then summed (32, 34) over a symbol period (or a substantial fraction thereof) to provide an overall soft decision for each bit comprising an M-ary symbol. These outputs may be used directly as soft decisions for forward error correction or applied to a threshold circuit to obtain hard decisions.

Claims

1. An M-ary FSK receiver comprising means for frequency demodulating a modulated signal, means for estimating the rate of change in the demodulated signal, means for deriving N estimates of phase change per symbol, where N has a value of at least 2, by integrating the estimated rate of change over respective fractions of a symbol period, means for converting these N estimates of phase change to soft decisions for the transmitted bits, and means for additively combining these soft decisions over at least a substantial period to obtain soft decisions for each of log₂ M bits per symbol.

2. A receiver as claimed in claim 1, characterised in that at least one threshold circuit is coupled to the additive combining means to provide a hard bit decision for each of the log₂ M outputs.

3. A receiver as claimed in claim 1, characterised in that each of the log₂ M additively combined outputs are used directly as soft decisions for further forward error correction processing.

4. A receiver as claimed in claim 1, characterised by means for wrapping output samples of the integrating means, said wrapped samples being supplied to said converting means.

5. A receiver as claimed in claim 1, characterised in that the integrating means comprises a digital moving averaging means.

6. A method of recovering an M-ary FSK signal, comprising frequency demodulating a modulated signal, estimating the rate of phase change in the modulated signal, deriving N estimates of phase change per symbol, where N has a value of at least 2, by integrating the estimated rate of phase change over respective fractions of a symbol period, converting these N estimates of phase change to soft decisions for the transmitted bits, and additively combining these soft decisions over at least a substantial period to obtain soft decisions for each of log₂ M bits per symbol.

7. A method as claimed in claim 6, characterised by applying the additively combined signals to threshold means to obtain hard bit decisions for each of the log₂ M outputs.

8. A method as claimed in claim 6, characterised by using each of the log₂ M additively combined outputs directly as soft decisions for further forward error correction processing.

9. A method as claimed in claim 6, characterised by wrapping said N estimates to keep them in a range having a width of 2π

prior to converting them to soft decisions.

10. A method as claimed in claim 6, characterised by integrating the frequency demodulated signal to obtain a digital moving average.