Synchronization method and apparatus for modems based on jointly iterative turbo demodulation and decoding

• US 6,968,021 B1
• Filed: 09/24/2001
• Issued: 11/22/2005
• Est. Priority Date: 09/24/2001
• Status: Active Grant

• Current Assignee: Rockwell Collins, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: Rockwell Collins, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Mickelson, Rodney L., Tapp, Thomas L., White, Steven L., Whited, Joseph C.
• Primary Examiner(s): Patel, Jay K.
• Assistant Examiner(s): Munoz, Guillermo
• Application Number:
  US09/961,666
• Time in Patent Office: 1,520 Days
• Field of Search: 375/265, 375/324, 375/236, 375/343, 375/371, 375/340, 714/799
• US Class Current: 375/340
• CPC Class Codes: H04L 1/005 Iterative decoding, including iteration between signal detection and decoding operation
• CPC Class Codes: H04L 1/0054 Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
• CPC Class Codes: H04L 1/0055 MAP-decoding
• CPC Class Codes: H04L 1/0065 Serial concatenated codes
• CPC Class Codes: H04L 1/0071 Use of interleaving interleaving per se H03M13/27
• CPC Class Codes: H04L 2025/03407 Continuous phase
• CPC Class Codes: H04L 2027/0048 Frequency multiplication
• CPC Class Codes: H04L 25/03171 Arrangements involving maximum a posteriori probability [MAP] detection
• CPC Class Codes: H04L 25/03197 methods of calculation involving metrics
• CPC Class Codes: H04L 7/0062 detection of error based on data decision error, e.g. Mueller type detection

Abstract

A bandwidth efficient advanced modulation waveform modem using concatenated iterative turbo coding and continuous phase modulation is disclosed. A demodulator in the modem has a turbo decoder and a decision feedback carrier and time tracking algorithm to track a carrier and adjust timing. The decision feedback carrier and time tracking algorithm may use an APP decoder as a decision device to provide symbol decisions at a high error rate and low latency for a coded input data stream. A symbol phase estimator produces a symbol phase error estimate from the symbol decisions. An erasure decision function decides which symbol decisions are correct and which symbol decisions are erasures. A carrier tracking function receives the symbol phase error estimates when the symbol decisions are correct and receives erasure inputs when the symbol decisions are erasures to maintain carrier tracking.

Claims

1. A bandwidth efficient advanced modulation waveform modem comprising concatenated convolutional coding, interleaving, and continuous phase modulation and having a modulator connected to a transmitter to transmit signals having coded information bits over a communications channel and a demodulator connected to a receiver to receive the signals over the communications channel said demodulator comprising:

a multiplier for multiplying an input signal received from the receiver and having coded information bits and a carrier by a phase signal;

a memoryless demodulator connected to the multiplier for demodulating the multiplied input signal;

an iterative turbo decoder connected to the memoryless demodulator for decoding the multiplied input signal; and

a carrier and time tracking loop for providing the phase signal to track the input carrier and for adjusting time tracking, wherein said carrier and time tracking loop further comprises;

a correlator bank connected to the multiplier for providing correlator phase estimates;

a decision device connected to the memoryless demodulator for delivering alpha values representative of data symbols and hard decision phase information at a high error rate and low latency;

a multiplexer connected to the correlator bank and the decision device wherein an alpha value selects phase estimate from the correlator bank utilizing the multiplexer;

a symbol phase error estimator connected to the decision device and the multiplexer for producing a symbol phase error estimate from the hard decision phase information and the selected correlator phase estimate;

a carrier tracking loop filter connected to the symbol phase error estimator for receiving the symbol phase error estimate and providing an output frequency; and

a numerically controlled oscillator for receiving the output frequency from the carrier tracking loop filter and providing the phase signal to the multiplier to multiply the input signal to track the input carrier.

2. The modem of claim 1 wherein the carrier and time tracking loop further comprises:

an a posteriori probability (APP) decoder utilized as the decision device said APP decoder providing a decoder metrics output confidence level and operating with a reduced traceback depth to reduce latency;

an erasure decision function to compare the decoder metrics output confidence level from the APP decoder to a confidence threshold and the phase error estimate from the symbol phase error estimator with a phase threshold;

an erasure logic function that maintains knowledge of a last phase error and generates a next symbol phase error estimate; and

a multiplexer connected to the erasure decision function and erasure logic function that passes the symbol phase error estimate when the thresholds are met and the symbol decision is accepted as correct, passes the symbol phase error estimate when the thresholds are not met but symbol decision is accepted as correct, and inhibits the symbol phase error estimate when the threshold is not exceeded and an erasure is declared.

3. The modem of claim 2 wherein the erasure logic function drives the carrier tracking loop filter with one of a zero phase error signal, a repeat of the last phase error signal, and a decayed version of the last phase error signal when the erasure is declared.

4. The modem of claim 1 wherein the correlator bank further comprises a plurality of complex correlators each of said complex correlators further comprising:

a shift register for loading complex symbol samples from the multiplier;

a plurality of correlator multipliers connected to the shift register outputs for comparing the complex samples to correlator coefficients; and

a summing junction connected to the plurality of correlator multipliers for summing the comparison results to give a complex correlation output.

5. The modem of claim 1 wherein the symbol phase estimator comprises a sine-cosine mixer for producing a symbol phase error estimate from the phase estimate and the hard decision phase information.

6. The modem of claim 1 wherein the symbol phase estimator comprises an arctangent discriminator for producing a symbol phase error estimate from the phase estimate and the hard decision phase information.

7. The modem of claim 1 wherein the carrier and time tracking loop further comprises:

an inverter connected to the decision device alpha decision output to invert the alpha value;

a multiplier connected to the inverter to negate the inverted alpha value by multiplying by a minus one; and

a second multiplier connected to the multiplier to multiply the inverted and negated alpha value by the symbol phase error estimate to produce a timing adjustment value.

8. The modem of claim 1 wherein the memoryless demodulator comprises a demodulator branch metric computer that further comprises a plurality of branch metrics computers for computing branch metric values for a plurality of phase states and a plurality of alpha values wherein each branch metric is computed as a square of a Euclidean distance between an actual symbol value and an ideal symbol corresponding to a phase state and alpha value.

9. The modem of claim 1 wherein the turbo decoder further comprises:

an inner code decoder connected to the memoryless demodulator for decoding the continuous phase shift modulation to generate probabilities for decoded information bits and new probabilities for the coded information bits;

a deinterleaver connected to the inner code decoder for reordering the decoded information bit probabilities;

an outer code decoder connected to the deinterleaver to generate probabilities for the information bits and new probability estimates for the coded bits; and

an interleaver connected to the outer code decoder and the inner code decoder for reordering the new probability estimates for the coded bits and delivering said probability estimates to the inner code decoder.

10. A modem using iterative turbo coding, a carrier and time tracking algorithm, and continuous phase modulation, said modem comprising:

a multiplier for multiplying an input signal having coded information bits and a carrier by a phase signal;

a demodulator branch metric computer connected the multiplier for demodulating the multiplied input signal;

a correlator bank connected to the multiplexer for providing correlator phase estimates;

an inner code a posterior probability (APP) decoder connected to the demodulator branch metrics computer for delivering alpha values representative of data symbols hard decision phase start information, and a decoder metrics output at a high error rate and low latency wherein the alpha values select a correlator phase estimate from the correlator bank;

a symbol phase error estimator for producing a symbol phase error estimate from the correlator phase estimate and the hard decision phase information;

a carrier tracking loop filter for receiving the symbol phase error estimate and providing an output frequency; and

a numerically controlled oscillator for receiving the output frequency from the carrier tracking loop filter and providing the phase signal to the multiplier to multiply the input signal to track the input carrier.

11. The modem of claim 10 further comprising:

an erasure decision function to compare the decoder metrics from the APP decoder and the phase error estimate from the phase error estimator with thresholds and to provide an erasure decision;

an erasure logic function that maintains knowledge of a last phase error and generates a next symbol phase error estimate; and

a multiplexer connected to the erasure decision function and erasure logic function that passes the symbol phase error estimate when the thresholds are met and the symbol decision is accepted as correct, passes the symbol phase error estimate when the thresholds are not met but symbol decision is accepted as correct, and inhibits the symbol phase error estimate when the threshold is not exceeded and an erasure is declared.

12. The modem of claim 11 wherein the erasure logic function drives the carrier tracking loop filter with one of a zero phase error signal, a repeat of the last phase error signal, or a decayed version of the last phase error signal when the erasure is declared.

13. The modem of claim 10 wherein the symbol phase estimator comprises a sine-cosine mixer for producing a symbol phase error estimate from the phase estimate with the phase start information.

14. The modem of claim 10 wherein the symbol phase estimator comprises an arctangent discriminator for producing a symbol phase error estimate from the phase estimate with the phase start information.

15. The modem of claim 10 wherein the demodulator branch metric computer further comprises a plurality of branch metrics computers for computing branch metric values for a plurality of phase states and a plurality of alpha values wherein each branch metric is computed as a square of a Euclidean distance between an actual symbol value and an ideal symbol corresponding to a phase state and alpha value.

16. The modem of claim 10 further comprising:

an inverter connected to the decision device alpha decision output to invert the alpha value;

a multiplier connected to the inverter to negate the inverted alpha value by multiplying by a minus one; and

a second multiplier connected to the multiplier to multiply the inverted and negated alpha value by the symbol phase error estimate to produce a timing adjustment value.

17. A bandwidth efficient advanced modulation waveform modem using iterative turbo coding and continuous phase modulation said modem having a demodulator comprising a turbo decoder and a decision feedback carrier and time tracking algorithm, said modem comprising:

an a posteriori probability (APP) decoder utilized as a decision device for providing symbol decisions at a high error rate and low latency for a coded input data stream having a carrier, said APP decoder providing a decoder metrics output confidence level and operating with a reduced traceback depth to reduce latency;

a symbol phase estimator for producing a symbol phase error estimate from the symbol decisions;

an erasure decision function for deciding which symbol decisions are correct and which symbol decisions are erasures, said erasure decision function comparing the decoder metrics output confidence level from the APP decoder to a confidence threshold and the phase error estimate from the symbol phase error estimator with a phase threshold;

an erasure logic function that maintains knowledge of a last phase error and generates a next symbol phase error estimate;

a multiplexer connected to the erasure decision function and erasure logic function that passes the symbol phase error estimate when the thresholds are met and the symbol decision is accented as correct, passes the symbol phase error estimate when the thresholds are not met but symbol decision is accepted as correct, and inhibits the symbol phase error estimate when the threshold is not exceeded and an erasure is declared; and

a carrier tracking function for receiving the symbol phase error estimates when the symbol decisions are correct and receiving an erasure input when the symbol decisions are erasures to maintain carrier tracking.

18. The bandwidth efficient advanced modulation waveform modem of claim 17 further comprising:

a carrier tracking loop filter for receiving the symbol phase error estimate from the symbol phase estimator and providing an output frequency proportional to the symbol phase error estimate;

a numerically controlled oscillator for receiving the output frequency from the carrier tracking loop filter and providing a phase signal output; and

a multiplier for multiplying the coded input data stream by the phase signal to provide an output to the decision device and to maintain carrier tracking.

19. The bandwidth efficient advanced modulation waveform modem of claim 18 further comprising:

a correlator bank connected to the multiplier for providing phase estimates;

a symbol delay connected to the correlator bank to compensate for a traceback depth of the decision device; and

a multiplexer connected to the symbol delay for providing a phase estimate output from the correlator bank as selected by the alpha value output of the decision device.

20. The bandwidth efficient advanced modulation waveform modem of claim 18 further comprising a branch metrics computer connected to the multiplier for demodulating the multiplied coded input data stream.

21. The bandwidth efficient advanced modulation waveform modem of claim 17 wherein the erasure logic function drives the carrier tracking loop filter with one of a zero phase error signal, a repeat of the last phase error signal, and a decayed version of the last phase error signal when the erasure is declared.

22. A bandwidth efficient advanced modulation waveform modem using iterative turbo coding and continuous phase modulation said modem having a demodulator comprising a turbo decoder and a carrier and time tracking algorithm said carrier and time tracking algorithm performing the steps comprising:

multiplying an input signal having coded information bits and a carrier in a multiplier;

demodulating the multiplied input signal in a branch metric computer connected to the multiplier;

providing correlator phase estimates from a correlator bank connected to the multiplier;

delivering alpha values representative of date symbols, hard decision phase information, and a decoder metrics output confidence levels at a high error rate and low latency from an inner code a posteriori probability (APP) decoder connected to the branch metric computer;

selecting correlator phase estimates from the correlator bank in a multiplexer with the alpha values from the inner code APP decoder;

producing a symbol phase error estimate from the correlator phase estimate and the hard decision phase information in a symbol phase estimator;

providing an output frequency proportional to the symbol phase error estimate from a carrier tracking loop filter; and

providing a phase signal from a numerically controlled oscillator to the multiplier to multiply the input signal to track the input carrier.

23. The modem of claim 22 wherein the carrier and time tracking algorithm further performs the steps comprising:

comparing the decoder metrics output confidence level from the APP decoder to a confidence threshold and the phase error estimate from the symbol phase error estimator with a phase threshold in an erasure decision function;

maintaining knowledge of a last phase error in an erasure logic function;

generating a next symbol phase error estimate in the erasure logic function;

passing the symbol phase error estimate when the confidence and phase thresholds are met and the symbol decision is accepted as correct through a multiplexer connected to the erasure decision function and erasure logic function;

passing the symbol phase error estimate when the confidence and phase thresholds are not met but symbol decision is accepted as correct through the multiplexer; and

inhibiting the symbol phase error estimate when the confidence and phase threshold are not met and an erasure is declared.