Downhole telemetry systems and methods with time-reversal pre-equalization
Specific embodiments of disclosed downhole telemetry systems and methods employ time-reversal pre-equalization. One downhole telemetry system embodiment includes an acoustic transducer and a digital signal processor. The acoustic transducer transmits an acoustic signal to a distant receiver via a string of drillpipes connected by tool joints. The digital signal processor drives the acoustic transducer with an electrical signal that represents modulated digital data convolved with a time-reversed channel response. Due to the use of time-reversal pre-equalization, the received signal exhibits substantially reduced intersymbol interference.
1. A downhole telemetry system that comprises:
an acoustic transducer that transmits an acoustic signal to a receiver via a string of drillpipes connected by tool joints such that the acoustic signal is transmitted along the string of drillpipes one drillpipe after another; and
a digital signal processor that drives the acoustic transducer with an electrical signal that represents modulated digital data convolved with a time-reversed channel response corresponding to the transmission along the string of drillpipes one drillpipe after another,
wherein the digital signal processor determines the time-reversed channel response based on a model, parameters of the model including an estimated number of drillpipes in the string.
2. The system of claim 1 , wherein the digital signal processor convolves a modulated signal with the time-reversed channel response to obtain said electrical signal.
3. The system of claim 2 , wherein the acoustic transducer is part of a transceiver, and wherein the digital signal processor processes received signals to determine the time-reversed channel response.
4. The system of claim 1 , wherein the acoustic transducer is part of a transceiver, and wherein the digital signal processor derives from a received signal a representation of each channel symbol, wherein the digital signal processor stores said representations, and wherein the digital signal processor generates said electrical signal by overlapping and adding said representations in a sequence.
5. The system of claim 1 , wherein the digital data is modulated with frequency-shift keying.
6. The system of claim 1 , wherein the digital data is modulated with amplitude-shift keying.
7. The system of claim 1 , wherein the digital data is modulated with phase-shift keying, quadrature amplitude modulation, or orthogonal frequency division multiplexing.
8. A downhole telemetry method that comprises:
generating an electrical signal that represents modulated digital data convolved with a time-reversed response of an acoustic channel that includes a string of drillpipes connected by tool joints, the time-reversed response corresponding to transmission along the string of drillpipes one drillpipe after another;
driving an acoustic transducer with the electrical signal to communicate the modulated digital data along the string of drillpipes one drillpipe after another to a receiver; and
determining the time-reversed response using a model, parameters of the model including a variable number of drillpipes in the acoustic channel.
9. The method of claim 8 , wherein said generating includes convolving a modulated signal with a channel response and time-reversing the result to obtain said electrical signal.
10. The method of claim 8 , wherein said generating includes convolving a modulated signal with the time-reversed response to obtain said electrical signal.
11. The method of claim 10 , further comprising:
processing received signals to extract a determined channel response; and
storing a time-reversed version of the determined channel response.
12. The method of claim 8 , further comprising:
extracting from a received signal a representation of each channel symbol;
storing a time-reversed version of each channel symbol representation; and
assembling a sequence of said stored channel symbol representations.
13. The method of claim 8 , wherein the modulated digital data is frequency-shift keyed.
14. The method of claim 8 , wherein the modulated digital data is amplitude-shift keyed.
15. The method of claim 8 , wherein the modulated digital data is phase-shift keyed, quadrature amplitude modulated, or modulated via orthogonal frequency division multiplexing.
16. A downhole telemetry method that comprises:
generating an electrical signal that represents modulated digital data convolved with a time-reversed response of an acoustic channel that includes a string of drillpipes connected by tool joints, the time-reversed response corresponding to transmission along the string of drillpipes one drillpipe after another;
driving an acoustic transducer with the electrical signal to communicate the modulated digital data along the string of drillpipes one drillpipe after another to a receiver;
determining the time-reversed response using a model, parameters of the model including a variable number of drillpipes in the acoustic channel;
obtaining a frequency-domain channel response and storing the frequency-domain channel response in memory;
generating each possible modulated channel symbol;
frequency transforming each modulated channel symbol;
multiplying each frequency transformed channel symbol with the frequency-domain channel response to obtain corresponding products;
inverse transforming the products to obtain time-domain convolutions;
time-reversing the time-domain convolutions to obtain channel symbol representations; and
assembling the channel symbol representations into a sequence to obtain said electrical signal.
17. The method of claim 16 , wherein the modulated channel symbols are frequency-shift keyed representations of a binary 0 and a binary 1.