Data communications receiver with automatic control of gain or threshold for soft decision decoding

• US 6,792,055 B1
• Filed: 07/21/2000
• Issued: 09/14/2004
• Est. Priority Date: 07/21/2000
• Status: Expired due to Term

• Current Assignee: Rockwell Collins, Inc. (Rtx Corporation)
• Original Assignee: Rockwell Collins, Inc. (Rtx Corporation)
• Inventors: Hart, Billy D.
• Primary Examiner(s): PHU, PHUONG M
• Application Number:
  US09/621,805
• Time in Patent Office: 1,516 Days
• Field of Search: 375/345, 375/340, 375/341, 375/262, 714/794, 714/795, 455/232.1, 455/234.1, 455/234.2, 455/235.1, 455/239.1, 455/240.1, 455/245.1, 455/242.2, 455/246.1, 455/247.1, 455/250.1
• US Class Current: 375/345
• CPC Class Codes: H03G 3/3036 in high-frequency amplifiers or in frequency-changers H03G3/3052, H03G3/32, H03G3/34 take precedence
• CPC Class Codes: H04L 25/067 providing soft decisions, i.e. decisions together with an estimate of reliability H04L25/068 and H04L25/069 take precedence; sequence estimation techniques H04L25/03178

Abstract

A data communications receiver incorporating soft decision error correction decoding and a means to set the amplitude or threshold for quantized soft decisions in a near-optimum manner to a soft decision decoder. In one embodiment the means for setting the amplitude or threshold measures the soft decisions from a quantizer and marks data bits as weak or strong. The gain or threshold is automatically adjusted to achieve a desired fraction if each marking. The desired fraction is chosen as the value that optimizes performance of the soft decision decoder.

Claims

1. A data communications receiver incorporating soft decision decoding, said data communications receiver comprising:

a demodulator for demodulating an input signal;

means connected to the demodulator for producing strong and weak data symbols from the demodulated input signal and for controlling a desired fraction of strong and weak data symbols; and

a soft decision decoder for receiving and decoding the strong and weak data symbols to produce a data output.

2. The data communications receiver of claim 1 wherein the means for producing the strong and weak data symbols from the demodulated signal comprises a soft decision quantizer that indicates a level of confidence that the demodulated input signal is a logic one or logic zero with a strong data symbol and a level in-between logic one and logic zero with a weak data symbol.

3. The data communications receiver of claim 2 wherein the means for controlling the desired fraction of strong and weak data symbols comprises:

a strong/weak indication function connected to the soft decision quantizer for providing strong/weak indications;

an averaging function connected to the strong/weak indication function for averaging the strong/weak indications;

a comparison function connected to the averaging function to compare an average of strong/weak indications to obtain a desired fraction of strong and weak signals and to generate a gain error signal;

a gain control function to generate a gain control signal from the gain error signal received from the comparison function; and

a variable gain element connected to a signal path of the receiver and to the gain control function to vary the gain with the gain control signal to maintain the desired fraction of strong and weak data symbols to the soft decision decoder.

4. The data communications receiver of claim 3 wherein the strong/weak indication function comprises an exclusive OR function having inputs connected to outputs of the soft decision quantizer and an output connected to the averaging function wherein said OR function provides an output signal for a strong indication and an opposite output signal for a weak indication.

5. The data communications receiver of claim 3 wherein the averaging function comprises a counter having an input connected to the strong/weak indication function and an output connected to the comparison function wherein said counter counts in one direction to increase gain with a weak indication and counts in an opposite direction to reduce gain with a strong indication.

6. The data communications receiver of claim 1 wherein the demodulator provides a multiple bit digital word output indicative of the demodulated input signal.

7. The data communications receiver of claim 6 wherein the means for producing strong and weak data symbols from the demodulated input signal and for controlling the desired fraction of strong and weak data symbols comprises:

a plurality of strong/weak indication functions each with an input connected to a hard decision bit output from the demodulator and another input connected to another output bit of the demodulator for providing strong/weak indications;

a plurality of averaging functions connected to outputs of the plurality of strong/weak indication functions for averaging the strong/weak indications;

a threshold value for setting a threshold to control the desired fraction of strong/weak indications;

a plurality of comparison functions connected to the averaging functions and to the threshold value to indicate which strong/weak indications exceed the threshold;

a logic function connected to the plurality of comparison functions to select an index number indicative of which strong/weak indications drop below a threshold and to generate a shift control signal; and

a bit shifter connected to the multiple bit digital word output of the demodulator and to the logic function to receive the shift control signal to select bits from the demodulator that provide the desired fraction of strong and weak data symbols to the soft decision decoder.

8. The data communications receiver of claim 7 wherein the strong/weak indication function comprises an exclusive OR function having inputs connected to outputs of the soft decision quantizer and an output connected to the averaging function wherein said OR function provides an output signal for a strong indication and an opposite output signal for a weak indication.

9. The data communications receiver of claim 1 wherein the means for producing strong and weak data symbols from the demodulated input signal and the means for controlling the desired fraction of strong and weak data symbols further comprises:

a first greater than test function having a first input connected to the demodulator to receive the demodulated input signal and a second input connected to a reference to provide a hard decision indication;

an absolute value circuit connected to the demodulator to receive the demodulated input signal to provide an absolute value output of the demodulated input signal;

a plurality of greater than test functions with a first input of each of the greater than test functions connected to the absolute value output;

a second greater than test function with a first input connected to the absolute value output and for providing a strong/weak indication;

a threshold control function connected to the strong/weak indication for providing a variable threshold output level in accordance with the strong/weak indication; and

a plurality of scaling functions connected to the variable threshold output of the threshold control function and to a second input of the plurality of greater than test functions and a second input of the second greater than test function for providing a desired fraction of strong and weak data symbols from the plurality of greater than test functions to the soft decision decoder.

10. A data communications receiver incorporating soft decision decoding and a gain or amplitude control for maintaining a desired fraction of strong and weak symbols comprising:

a demodulator for demodulating an input signal;

a soft decision quantizer for quantizing the demodulated input signal received from the demodulator into strong and weak data symbols indicative of a confidence level that the demodulated input signal is a logic one or zero or a level in-between logic one and logic zero;

a soft decision decoder connected to the soft decision quantizer for decoding the strong and weak data symbols;

a strong/weak indication function connected to the soft decision quantizer for providing a strong/weak indication from the strong and weak data symbols;

an averaging function connected to the strong/weak indication function for averaging the strong/weak indications;

a comparison function connected to the averaging function for comparing a desired fraction of weak signals and to generate a gain error signal;

a gain control function to generate a gain control signal from the gain error signal received from the comparison function; and

a variable gain element connected to the input of the receiver and to the gain control function to vary the gain to maintain the desired fraction of strong and weak data symbols into the soft decision decoder.

11. The data communications receiver of claim 10 wherein the strong/weak indication function comprises an exclusive OR function having inputs connected to outputs of the soft decision quantizer and an output connected to the averaging function wherein said OR function provides an output signal for a strong indication and an opposite output signal for a weak indication.

12. The data communications receiver of claim 10 wherein the soft decision quantizer is a three-bit quantizer.

13. The data communications receiver of claim 10 wherein the averaging function is a counter.

14. A method of controlling an input to a soft decision decoder in a data communications receiver to produce a data output comprising the steps of:

demodulating an input signal;

producing strong and weak data symbols from the demodulated input signal with a soft decision quantizer;

indicating with the soft decision quantizer a level of confidence that the demodulated input signal is a logic one or logic zero with a strong data symbol and a level in-between logic one and logic zero with a weak data symbol;

controlling a desired fraction of strong and weak data symbols input to the soft decision decoder; and

decoding the strong and weak data symbols input to the soft decision decoder to produce the data output.

15. The method of claim 14 wherein the step of controlling the desired fraction of strong and weak data symbols input to the soft decision decoder further comprises:

providing strong/weak indications from a strong/weak indication function connected to the soft decision quantizer;

averaging the strong/weak indications in an averaging function connected to the strong/weak indication function;

comparing a desired fraction of weak signals from the average of strong/weak indications to generate a gain error signal;

generating a gain control signal from the gain error signal in a gain control function; and

varying gain with the gain control signal in a variable gain element connected to the input of the receiver to maintain the desired fraction of strong and weak symbols input to the soft decision decoder.

16. The method of claim 14 wherein the step of demodulating the input signal further comprises providing a multiple bit digital word output indicative of the demodulated input signal.

17. The method of claim 16 wherein the steps of producing strong and weak data symbols of the demodulated input signal and controlling the desired fraction of strong and weak symbols provided to the soft decision decoder further comprise:

providing strong/weak indications from a plurality of strong/weak indication functions;

averaging the strong/weak indications with a plurality of averaging functions;

setting a threshold with a threshold value to control the desired fraction of strong/weak indications;

indicating which strong/weak indications exceed the threshold with a plurality of comparison functions connected to the averaging functions and to the threshold value;

selecting an index number indicative of which strong/weak indications drop below a threshold in a logic function connected to the plurality of comparison functions to generate a shift control signal; and

selecting bits from the demodulator that provide the desired fraction of weak and strong symbols to the soft decision decoder with a bit shifter connected to the multiple bit digital word output of the demodulator and to the logic function in accordance with the shift control signal.

18. The method of claim 14 wherein the steps of producing strong and weak data symbols of the demodulated input signal and of controlling the desired fraction of strong and weak data symbols further comprises:

providing a hard decision indication in a first greater than test function with a first input connected to the demodulator to receive the demodulated input signal and a second input connected to a reference;

providing an absolute value output of the demodulated input signal with an absolute value circuit connected to the demodulator to receive the demodulated input signal;

providing a strong/weak indication from a second greater than test function in a plurality of greater than test functions with a first input of each of the plurality of greater than test functions connected to the absolute value output;

providing a variable threshold output level in accordance with the strong/weak indication with a threshold control function connected to the strong/weak indication; and

providing a desired fraction of strong and weak data symbols from the plurality of greater than test functions to the soft decision decoder with a plurality of scaling functions connected to the variable threshold output of the threshold control function and to a second input of the plurality of greater than test functions.