Methods and systems for forward error correction for measurement while drilling (MWD) communication systems

1. A method, in a communication system comprising a transmitter and a receiver, for communicating data from the transmitter to the receiver over a banded communication channel, the method comprising:

applying a forward error correction, FEC, code to data to be transmitted to obtain FEC-encoded data;

assigning the FEC-encoded data into a plurality of sub-channels, each sub-channel comprising a plurality of sub-carriers, each sub-carrier spaced apart in the frequency domain;

modulating the FEC-encoded data from each of the plurality of sub-channels into a corresponding one of a plurality of frequency sub-bands, the plurality of frequency sub-bands having spaced apart center frequencies, each frequency sub-band spread over a finite frequency range surrounding its center frequency and the modulated data for each sub-channel occupying the finite frequency range of its corresponding frequency sub-band; and

concurrently transmitting the modulated data from the plurality of frequency sub-bands onto a banded communication channel, the banded communication channel comprising one or more pass-bands and one or more stop-bands.

2. The method according to claim 1 wherein concurrently transmitting the modulated data from the plurality of sub-bands onto the banded communication channel comprises transmitting acoustic energy over the channel.

3. The method according to claim 2 wherein transmitting acoustic energy over the channel comprises transmitting acoustic energy over a drill string.

4. The method according to claim 1 wherein the FEC code can accommodate a percentage error rate and wherein a stop-band density over a frequency-domain range of each sub-band is less than the percentage error rate.

5. The method according to claim 1 comprising:

receiving, at the receiver and across the banded communication channel, the transmitted data; and

decoding the transmitted data received at the receiver in accordance with the FEC code to recover received data.

6. The method according to claim 1 comprising encoding the data to be transmitted according to a two-dimensional FEC encoding scheme, wherein encoding the data to be transmitted according to a two-dimensional FEC encoding scheme comprises, after assigning the FEC-encoded data into the plurality of sub-channels and prior to modulating the FEC-encoded data from each of the plurality of sub-channels, applying, to the FEC-encoded data assigned into each of the plurality of sub-channels, a corresponding in-stream FEC code.

7. The method according to claim 6 comprising:

receiving, at the receiver and across the banded communication channel, the transmitted data; and

for each sub-band, decoding the transmitted data received at the receiver in the sub-band in accordance with its corresponding in-stream FEC code to obtain sub-band decoded data; and

decoding the sub-band decoded data in accordance with the FEC code to recover received data.

8. The method according to claim 1 wherein modulating the FEC-encoded data from each of the plurality of sub-channels into its corresponding one of the plurality of sub-bands comprises using an orthogonal frequency division multiplexing (OFDM) scheme wherein, for each sub-channel, a plurality of bits from the sub-channel are modulated concurrently onto orthogonal acoustic sub-carriers having sub-carrier frequencies within the sub-band corresponding to the sub-channel.

9. The method according to claim 8 comprising:

receiving, at the receiver and across the banded communication channel, the transmitted data; and

decoding the transmitted data received at the receiver in accordance with the FEC code to recover received data.

10. The method according to claim 8 wherein using the OFDM scheme comprises applying, to the FEC-encoded data assigned to each of the plurality of sub-carriers, a corresponding sub-carrier FEC code.

11. The method according to claim 10 comprising:

receiving, at the receiver and across the banded communication channel, the transmitted data; and

for each sub-carrier, decoding the transmitted data received at the receiver in association with the sub-carrier in accordance with its corresponding sub-carrier FEC code to obtain sub-carrier decoded data; and

decoding the sub-carrier decoded data in accordance with the FEC code to recover received data.

12. The method according to claim 1 wherein modulating the FEC-encoded data from each of the plurality of sub-channels into its corresponding one of the plurality of sub-bands comprises assigning the center frequency of each sub-band to be within an estimated frequency-domain location of a corresponding pass-band in the channel.

13. The method according to claim 2 comprising receiving, from the receiver, the estimated frequency-domain locations of the pass-bands in the channel.

14. The method according to claim 1 wherein the sub-bands spread over distinct and non-overlapping ranges of frequencies.

15. The method according to claim 1 wherein the center frequencies of the sub-bands are evenly spaced apart.

16. The method according to claim 1 wherein assigning the FEC-encoded data into a plurality of sub-channels comprises assigning the same FEC-encoded data into each of the plurality of sub-channels.

17. The method according to claim 1 wherein assigning the FEC-encoded data into a plurality of sub-channels comprises splitting the FEC-encoded data into a plurality of streams corresponding to the plurality of sub-channels and assigning each of the streams into the each of the corresponding plurality of sub-channels.

18. A communication system comprising a transmitter and a receiver for communicating data from the transmitter to the receiver over a banded communication channel, the communication system comprising:

a transmitter configured to:

apply a forward error correction, FEC, code to data to be transmitted to obtain FEC-encoded data;

assign the FEC-encoded data into a plurality of sub-channels, each sub-channel comprising a plurality of sub-carriers, each sub-carrier spaced apart in the frequency domain;

modulate the FEC-encoded data from each of the plurality of sub-channels into a corresponding one of a plurality of frequency sub-bands, the plurality of frequency sub-bands having spaced apart center frequencies, each frequency sub-band spread over a finite frequency range surrounding its center frequency and the modulated data for each sub-channel occupying the finite frequency range of its corresponding frequency sub-band;

concurrently transmit the modulated data from the plurality of frequency sub-bands onto a banded communication channel, the banded communication channel comprising one or more pass-bands and one or more stop-bands.

19. The system according to claim 18 comprising:

a receiver configured to:

receive, at the receiver and across the banded communication channel, the transmitted data; and

decode the transmitted data received at the receiver in accordance with the FEC code to recover received data.

20. A method for providing communication between a downhole location on a drill string and an uphole location on the drill string, the method comprising:

providing, at the downhole location, an acoustic transmitter connected for transmitting an acoustic signal into a communication channel comprising the drill string;

providing, at the uphole location, an acoustic receiver connected for receiving a transmitted acoustic signal from the communication channel;

applying a forward error correction, FEC, code to data to be transmitted to obtain FEC-encoded data;

assigning the FEC-encoded data into a plurality of sub-channels, each sub-channel comprising a plurality of sub-carriers, each sub-carrier spaced apart in the frequency domain;

modulating the FEC-encoded data from each of the plurality of sub-channels into a corresponding one of a plurality of frequency sub-bands, the plurality of frequency sub-bands having spaced apart center frequencies, each frequency sub-band spread over a finite frequency range surrounding its center frequency and the modulated data for each sub-channel occupying the finite frequency range of its corresponding frequency sub-band; and

concurrently transmitting the modulated data from the plurality of frequency sub-bands from the transmitter onto the communication channel, the communication channel comprising one or more pass-bands and one or more stop-bands.

21. A method according to claim 20 wherein modulating the data from each of the plurality of sub-channels into its corresponding one of the plurality of sub-bands comprises using an orthogonal frequency division multiplexing (OFDM) scheme wherein, for each sub-channel, a plurality of bits from the sub-channel are modulated concurrently onto orthogonal acoustic sub-carriers having sub-carrier frequencies within the sub-band corresponding to the sub-channel and wherein the method further comprises, for each sub-carrier, flagging the transmitted data received in association with the sub-carrier as erasures for handling during decoding based at least in part on measurements of receive signal power in the sub-carrier.

22. A method according to claim 20 wherein modulating the data from each of the plurality of sub-channels into its corresponding one of the plurality of sub-bands comprises selecting orthogonal acoustic sub-carriers having sub-carrier frequencies within an estimated frequency-domain location of a corresponding pass-band in the channel.

23. A method according to claim 20 wherein modulating the data from each of the plurality of sub-channels into its corresponding one of the plurality of sub-bands comprises using an orthogonal frequency division multiplexing (OFDM) scheme wherein, for each sub-channel, a plurality of bits from the sub-channel are modulated concurrently onto orthogonal acoustic sub-carriers having sub-carrier frequencies within the sub-band corresponding to the sub-channel; and wherein the method further comprises, for each sub-carrier, flagging the transmitted data received in association with the sub-carrier as erasures for handling during decoding of the sub-carrier decoded data based at least in part on measurements of receive signal power in the sub-carrier.