System and method for spread spectrum based drill pipe communications

1. A method for use with a drill string positioned within a borehole, the method comprising:

receiving, by a receiver, a baseband signal transmitted via vibrations through the drill string using a plurality of elastic waves;

oversampling the received baseband signal at the receiver to duplicate a spectrum of the baseband signal a plurality of times,

determining whether a single passband of a plurality of available passbands has a predetermined signal to noise ratio and sufficient bandwidth to include an entire spectrum of the baseband signal of one of the plurality of spectrums of the baseband signal created by the oversampling;

selecting, by the receiver, if the single passband is available, the single passband from the plurality of passbands that are caused by the reflection of the baseband signal within the drill string based on one of the plurality of passbands having the predetermined signal to noise ratio and sufficient bandwidth to include the entire spectrum of the baseband signal of one of the plurality of spectrums of the baseband signal created by the oversampling, in order to recover the baseband signal from the duplicated spectrum;

selecting, by the receiver if the single passband is not available, multiple passbands from the plurality of passbands including different portions of the spectrum of the baseband signal;

combining portions of the spectrum of the baseband signal recovered from each of the selected multiple passbands to recover the entire spectrum of the baseband signal, the combination based upon a signal to noise ratio of each of the passbands, such that higher quality passbands contribute more to the combination than the passbands with lower quality signals; and

decoding, by the receiver, the baseband signal to recover a bit stream contained within the baseband signal.

2. The method of claim 1 further comprising:

measuring acoustic noise when no signal is being received;

estimating a noise power spectral density using the measured acoustic noise; and

using the noise power spectral density to find the signal to noise ratio.

3. The method of claim 1 wherein the multiple passbands contain overlapping portions of the baseband signal due to the duplicated spectrum.

4. The method of claim 1 wherein each of the multiple passbands is selected based on at least one of a signal to noise ratio of the passband or a width of the passband.

5. The method of claim 1 further comprising calculating a weight for each of the multiple passbands based on a signal to noise ratio of the passband for which the weight is being determined, wherein the weight calculated for each passband determines how that passband is used when combining portions of the baseband signal.

6. The method of claim 1 further comprising identifying the plurality of available passbands.

7. The method of claim 1 further comprising:

receiving a pilot training signal from a transmitter that is responsible for generating the baseband signal within the drill string;

determining a channel response within the at least one passband based on the pilot signal; and

equalizing a waveform obtained via the at least one passband using the channel response.

8. The method of claim 7 wherein multiple passbands are selected from the plurality of available passbands, and wherein the steps of determining a channel response and equalizing the passband are performed for each of the multiple passbands.

9. The method of claim 1 further comprising sampling the baseband signal at a rate that is a multiple of a rate at which the baseband signal was transmitted.

10. A method for use with a drill string positioned within a borehole, the method comprising:

receiving, by a receiver, a pilot signal transmitted through the drill string via vibrations using a first plurality of elastic waves;

modifying, by the receiver, an adaptive filter corresponding to at least one passband to compensate for variances between the received pilot signal and a known pilot signal;

receiving, by the receiver, a baseband signal transmitted through the drill string using a second plurality of elastic waves;

equalizing, by the receiver, the baseband signal using the adaptive filter to form a corrected baseband signal;

oversampling the received baseband signal at the receiver to duplicate a spectrum of the baseband signal a plurality of times,

determining whether a single one of a plurality of available passbands has a predetermined signal to noise ratio and sufficient bandwidth to include an entire spectrum of the baseband of the plurality of spectrums of the baseband signal created by the oversampling signal;

selecting, by the receiver, when the single passband is available, the single passband from the plurality of passbands that are caused by the reflection of the baseband signal within the drill string based on one of the plurality of passbands having the predetermined signal to noise ratio and sufficient bandwidth to include the entire spectrum of the baseband signal of the plurality of spectrums of the baseband signal created by the oversampling, in order to recover the baseband signal from the duplicated spectrum;

selecting, by the receiver if the single passband is not available, multiple passbands from the plurality of passbands;

combining portions of the spectrum of the baseband signal recovered from each of the multiple passbands to recover the entire spectrum of the baseband signal, the combination based upon a signal to noise ratio of each of the passbands, such that higher quality passbands contribute more to the combination than the passbands with lower quality signals; and

decoding, by the receiver, the baseband signal to recover a bit stream contained within the baseband signal.

11. The method of claim 10 wherein modifying the adaptive filter includes adjusting coefficients of the adaptive filter to minimize a mean square error between the received pilot signal and the known pilot signal.

12. A receiver for use in borehole communications comprising:

an input block configured to receive a plurality of duplicates of at least a portion of a spectrum of a baseband signal, the baseband signal includes a bit stream transmitted through a drill string positioned in a borehole via vibrations, the plurality of duplicates, caused by oversampling of the received baseband signal;

a channel selector configured to determine whether a single one of plurality of passbands has a predetermined signal to noise ratio and sufficient bandwidth to include the entire baseband signal of the plurality of spectrums of the baseband signal created by the oversampling, to select, if a single passband is available, the single passband from the plurality of passbands for use in recovering the baseband signal based on one of the plurality of passbands have the predetermined signal to noise ratio and the sufficient bandwidth to include the entire spectrum of the baseband signal of the plurality of spectrums of the baseband signal created by the oversampling, wherein the channel selector is configured to use the existence of the duplicates to select multiple passbands including different portions of the spectrum of the baseband signal from the plurality of available passbands if the single passband is not available to contain the entire spectrum of the baseband signal of the plurality of spectrums of the baseband signal created by the oversampling;

a fusion circuit for combining portions of the spectrum of the baseband signal recovered from each of the selected multiple passbands to recover the entire spectrum of the baseband signal, the combination based upon a signal to noise ratio of each of the passbands, such that higher quality passbands contribute more to the combination than the passbands with lower quality signals; and

a decoder configured to recover the bit stream from the baseband signal.

13. The receiver of claim 12 further comprising a channel equalizer configured to compensate for variances in the at least one passband based on differences between a known pilot signal and a received pilot signal.

14. The receiver of claim 12 further comprising a bit timing recovery block configured to provide frequency jitter correction information to the decoder.

15. The receiver of claim 12 further comprising an acoustic noise block configured to provide acoustic noise information to the channel selector for signal to noise ratio calculations.