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TECHNICAL FIELD  
       [0001]     The application relates generally to a telemetry system for data communications between a downhole drilling assembly and a surface of a well. In particular, the application relates to a receiver for an acoustic telemetry system.  
       BACKGROUND  
       [0002]     During drilling operations for extraction of hydrocarbons, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The use of measurements while drilling (MWD) with real time data transmission provides substantial benefits during a drilling operation. For example, monitoring of downhole conditions allows for an immediate response to potential well control problems and improves mud programs.  
         [0003]     Measurement of parameters such as weight on bit, torque, wear and bearing condition in real time provides for more efficient drilling operations. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques.  
         [0004]     Currently, there are four major categories of telemetry systems that have been used in an attempt to provide real time data from the vicinity of the drill bit to the surface; namely, acoustic waves, mud pressure pulses, insulated conductors and electromagnetic waves.  
         [0005]     With regard to acoustic waves, typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission. Such reflective and refractive interference causes interbit interference among the bits of data being transmitted.  
         [0006]     In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar near the bit. This type of system typically transmits at one bit per second as the pressure pulse travels up the mud column at or near the velocity of sound in the mud. It is well known that mud pulse systems are intrinsically limited to a few bits per second due to attenuation and spreading of pulses.  
         [0007]     Insulated conductors or hard wire connection from the drill bit to the surface is an alternative method for establishing downhole communications. This type of system is capable of a high data rate and two-way communication is possible. It has been found, however, that this type of system requires a special drill pipe and special tool joint connectors that substantially increase the cost of a drilling operation. Also, these systems are prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string.  
         [0008]     The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. A current carrying downhole data signal is input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string. When a toroid is utilized, a primary winding, carrying the data for transmission, is wrapped around the toroid and a secondary is formed by the drill pipe. A receiver is connected to the ground at the surface where the electromagnetic data is picked up and recorded. It has been found, however, that in deep or noisy well applications, conventional electromagnetic systems are unable to generate a signal with sufficient intensity to be recovered at the surface.  
         [0009]     In general, the quality of an electromagnetic signal reaching the surface is measured in terms of signal to noise ratio. As the ratio drops, it becomes more difficult to recover or reconstruct the signal. While increasing the power of the transmitted signal is an obvious way of increasing the signal to noise ratio, this approach is limited by batteries suitable for the purpose and the desire to extend the time between battery replacements. These approaches have allowed development of commercial borehole electromagnetic telemetry systems that work at data rates of up to four bits per second and at depths of up to 4000 feet without repeaters in MWD applications. It would be desirable to transmit signals from deeper wells and with much higher data rates which will be required for logging while drilling, LWD, systems. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Embodiments of the invention may be best understood by referring to the following description and accompanying drawings which illustrate such embodiments. The numbering scheme for the Figures included herein are such that the leading number for a given reference number in a Figure is associated with the number of the Figure. For example, a system  100  can be located in  FIG. 1 . However, reference numbers are the same for those elements that are the same across different Figures. In the drawings:  
         [0011]      FIG. 1  illustrates a system for drilling operations, according to some embodiment of the invention.  
         [0012]      FIG. 2  illustrates a repeater along a drill string, according to some embodiments of the invention.  
         [0013]      FIG. 3  is a timing diagram of an acoustic signal received across a number of symbolic intervals, according to some embodiments of the invention.  
         [0014]      FIG. 4  illustrates a receiver for an acoustic telemetry system, according to some embodiments of the invention.  
         [0015]      FIG. 5  illustrates a flow diagram for operations of a receiver for an acoustic telemetry system, according to some embodiments of the invention.  
         [0016]      FIG. 6  illustrates an on-off key-based receiver for an acoustic telemetry system, according to some embodiments of the invention.  
         [0017]      FIG. 7  illustrates a flow diagram for operations of an OOK receiver, according to some embodiments of the invention.  
         [0018]      FIG. 8  illustrates a frequency shift key-based receiver for an acoustic telemetry system, according to some embodiments of the invention.  
         [0019]      FIGS. 9A-9B  illustrate a flow diagram for operations of an FSK receiver, according to some embodiments of the invention.  
         [0020]      FIG. 10  illustrates a phase shift key-based receiver for an acoustic telemetry system, according to some embodiments of the invention.  
         [0021]      FIGS. 11A-11B  illustrate a flow diagram for operations of a PSK receiver, according to some embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]     Methods, apparatus and systems for an acoustic telemetry receiver are described. In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description.  
         [0023]     While described with reference to transmitting downhole data to the surface during measurements while drilling (MWD), embodiments of the invention are not so limited. For example, some embodiments are applicable to transmission of data from the surface to equipment that is downhole. Additionally, some embodiments of the invention are applicable not only during drilling, but throughout the life of a wellbore including, but not limited to, during logging, drill stem testing, completing and production. Further, some embodiments of the invention can be in other noisy conditions, such as hydraulic fracturing and cementing.  
         [0024]     As further described below, embodiments of the invention attempt to minimize cross correlation between/among the different symbols to allow for the identification of the symbols. Embodiments of the invention allow for a more robust data recovery of acoustic telemetry through tubulars under various noisy conditions. Additionally, embodiments of the invention allowed for an increased data rate of acoustic telemetry through tubulars while maintaining reliable data recovery. Embodiments of the invention may remove intersymbol interference. This removal of intersymbol interference allows for correlation of a symbol with a database of acquired symbols to determine a value of a symbol.  
         [0025]      FIG. 1  illustrates a system for drilling operations, according to some embodiments of the invention. A system  100  includes a drilling rig  102  located at a surface  104  of a well. The drilling rig  102  provides support for a drill string  108 . The drill string  108  penetrates a rotary table  110  for drilling a borehole  112  through subsurface formations  114 . The drill string  108  includes a Kelly  116  (in the upper portion), a drill pipe  118  and a bottom hole assembly  120  (located at the lower portion of the drill pipe  118 ). The bottom hole assembly  120  may include a drill collar  122 , a downhole tool  124  and a drill bit  126 . The downhole tool  124  may be any of a number of different types of tools including Measurement While Drilling (MWD) tools, Logging While Drilling (LWD) tools, etc.  
         [0026]     During drilling operations, the drill string  108  (including the Kelly  116 , the drill pipe  118  and the bottom hole assembly  120 ) may be rotated by the rotary table  110 . In addition or alternative to such rotation, the bottom hole assembly  120  may also be rotated by a motor (not shown) that is downhole. The drill collar  122  may be used to add weight to the drill bit  126 . The drill collar  122  also may stiffen the bottom hole assembly  120  to allow the bottom hole assembly  120  to transfer the weight to the drill bit  126 . Accordingly, this weight provided by the drill collar  122  also assists the drill bit  126  in the penetration of the surface  104  and the subsurface formations  114 .  
         [0027]     During drilling operations, a mud pump  132  may pump drilling fluid (known as “drilling mud”) from a mud pit  134  through a hose  136  into the drill pipe  118  down to the drill bit  126 . The drilling fluid can flow out from the drill bit  126  and return back to the surface through an annular area  140  between the drill pipe  118  and the sides of the borehole  112 . The drilling fluid may then be returned to the mud pit  134 , where such fluid is filtered. Accordingly, the drilling fluid can cool the drill bit  126  as well as provide for lubrication of the drill bit  126  during the drilling operation. Additionally, the drilling fluid removes the cuttings of the subsurface formations  114  created by the drill bit  126 .  
         [0028]     The drill string  108  may include one to a number of different sensors  151 , which monitor different downhole parameters. Such parameters may include the downhole temperature and pressure, the various characteristics of the subsurface formations (such as resistivity, density, porosity, etc.), the characteristics of the borehole (e.g., size, shape, etc.), etc. The drill string  108  may also include an acoustic telemetry transmitter  123  that transmits telemetry signals in the form of acoustic vibrations in the tubing wall of the drill sting  108 . An acoustic telemetry receiver  115  is coupled to the kelly  116  to receive transmitted telemetry signals. One or more repeaters  119  may be provided along the drill string  108  to receive and retransmit the telemetry signals. The repeaters  119  may include both an acoustic telemetry receiver and an acoustic telemetry transmitter configured similarly to the acoustic telemetry receiver  115  and the acoustic telemetry transmitter  123 .  
         [0029]      FIG. 2  illustrates a repeater along a drill string, according to some embodiments of the invention. In particular,  FIG. 2  illustrates one embodiment of the repeaters  119 . As shown, the repeaters  119  may include an acoustic telemetry transmitter  204  and an acoustic sensor  212  mounted on a piece of tubing  202 . One skilled in the art will understand that acoustic sensor  212  is configured to receive signals from a distant acoustic transmitter, and that the acoustic telemetry transmitter  204  is configured to transmit to a distant acoustic sensor. Consequently, although the acoustic telemetry transmitter  204  and the acoustic sensor  212  are shown in close proximity, they would only be so proximate in a repeater  119  or in a bi-directional communications system. Thus, for example, the acoustic telemetry transmitter  123  might only include the acoustic telemetry transmitter  204 , while the acoustic telemetry receiver  115  might only include sensor  212 , if so desired.  
         [0030]     The following discussion centers on acoustic signaling from acoustic telemetry transmitter  123  near the drill bit  126  to a sensor located some distance away along the drill string. Various acoustic transmitters are known in the art, as evidenced by U.S. Pat. Nos. 2,810,546, 3,588,804, 3,790,930, 3,813,656, 4,282,588, 4,283,779, 4,302,826, 4,314,365, and 6,137,747, which are hereby incorporated by reference. The transmitter  204  shown in  FIG. 2  has a stack of piezoelectric washers  206  sandwiched between two metal flanges  208 ,  210 . When the stack of piezoelectric washers  206  is driven electrically, the stack  206  expands and contracts to produce axial compression waves in tubing  202  that propagate axially along the drill string. Other transmitter configurations may be used to produce torsional waves, radial compression waves, or even transverse waves that propagate along the drill string.  
         [0031]     Various acoustic sensors are known in the art including pressure, velocity, and acceleration sensors. The sensor  212  preferably comprises a two-axis accelerometer that senses accelerations along the axial and circumferential directions. One skilled in the art will readily recognize that other sensor configurations are also possible. For example, the sensor  212  may comprise a three-axis accelerometer that also detects acceleration in the radial direction. A second sensor  214  may be provided 90 or 180 degrees away from the first sensor  212 . This second sensor  214  also preferably comprises a two or three axis accelerometer. Additional sensors may also be employed as needed.  
         [0032]     In some embodiments, the acoustic telemetry receiver receives an acoustic signal across a number of different symbolic intervals. In some embodiments, the acoustic telemetry receiver subtracts the tail of the acoustic signal of a previous symbolic interval from the acoustic signal of a current symbolic interval. To help illustrate,  FIG. 3  is a timing diagram of an acoustic signal received across a number of symbolic intervals, according to some embodiments of the invention.  FIG. 3  illustrates a timing diagram  300  for a first symbol  304 A that is represented by a solid line and a second symbol  304 B that is represented by a dashed line. The first symbol  304 A is received by the acoustic telemetry receiver in a symbolic interval  302 A. The second symbol  304 B is received by the acoustic telemetry receiver in a symbolic interval  302 B. As shown, a tail  306 A of the symbol  304 A carries over into the symbolic interval  302 B, thereby causing intersymbol interference with the symbol  304 B. A tail  306 B of the symbol  304 B carries over into a subsequent symbolic interval. Some embodiments of the invention may subtract the tail from the symbol for a previous symbolic interval from the symbol for the current symbolic interval to reduce the intersymbol interference.  
         [0033]     Different embodiments of an acoustic telemetry receiver are now described. Such embodiments may be different embodiments of the acoustic telemetry receiver  115 . In particular,  FIGS. 4 and 5  illustrate an embodiment of the acoustic telemetry receiver  115  and an embodiment of the operations thereof, respectively.  FIGS. 6 and 7  illustrate an on-off key-based embodiment of the acoustic telemetry receiver  115  and an embodiment of the operations thereof, respectively.  FIGS. 8 and 9  illustrate frequency shift key-based embodiment of the acoustic telemetry receiver  115  and an embodiment of the operations thereof, respectively.  FIGS. 10 and 11  illustrate a phase shift key--based embodiment of the acoustic telemetry receiver  115  and an embodiment of the operations thereof, respectively.  
         [0034]      FIG. 4  illustrates a receiver for an acoustic telemetry system, according to some embodiments of the invention. In particular,  FIG. 4  illustrates a receiver  400  that includes a correlation logic  402  and a detection logic  404 . The correlation logic  402  is coupled to receive a telemetry signal. For example, the telemetry signal may be an acoustic signal that is propagated along a drill string. The correlation logic  402  may perform one to a number of correlations to stored telemetry signals to determine degrees of correlation. The output of the correlation logic  402  is coupled to the input of the detection logic  404 . The detection logic  404  may determine the symbol within the telemetry signal based on the degrees of correlation. The output of the detection logic  404  may be the symbolic values. Such symbolic values may represent communications (such as communications from downhole).  
         [0035]     One embodiment of the operations of the receiver  400  is now described in more detail in conjunction with a flow diagram  500  of  FIG. 5 . In particular,  FIG. 5  illustrates a flow diagram for operations of a receiver for an acoustic telemetry system, according to some embodiments of the invention.  
         [0036]     In block  502 , a telemetry signal that is transmitted along a transmission channel (having a transmission channel characteristic) is received. With reference to the embodiment of  FIG. 4 , the correlation logic  402  receive the telemetry signal. In some embodiments, the correlation logic  402  may receive this signal during drilling operations. The telemetry signal may be an acoustic signal (that is transmitted from an acoustic telemetry transmitter downhole) along the drill string  108 . The transmission channel chacteristic may include the different physical characteristics of the drill sting (including, length, thickness, shape, number of sections of drill pipe that is part of the drill string, etc.). Control continues at block  504 .  
         [0037]     In block  504 , the telemetry signal is correlated to a first stored telemetry signal that includes the transmission channel characteristic to output a first degree of correlation. With reference to the embodiment of  FIG. 4 , the correlation logic  402  performs this correlation. The correlation logic  402  may compare the signals and output a degree of correlation that may be a value indicative of such comparison. In some embodiments, logic (not shown in  FIG. 4 ) may also remove intersymbol interference from the received telemetry signal prior to this correlation. Such operations are described in more detail below. The first stored telemetry signal may be one of a number of stored telemetry signal (such as from a library of signals) that is stored. This library of signals may be generated during an approximately noise free environment (such as when drilling operations are not being performed).  
         [0038]     For example, the acoustic telemetry transmitter may generate a sequence of different symbols that are received by the receiver  400  during a period when no drilling operations are performed. The received symbols include the different characteristics of the drill string. In particular, the received symbols include the distortions made thereto as a result of the characteristics of the drill string. Control continues at block  506 .  
         [0039]     In block  506 , the telemetry signal is correlated to a second stored telemetry signal that includes the transmission channel characteristic to output a second degree of correlation. With reference to the embodiment of  FIG. 4 , the correlation logic  402  performs this correlation. Control continues at block  508 .  
         [0040]     In block  508 , the telemetry signal is marked as a particular symbolic value based on the first degree of correlation and the second degree of correlation. With reference to the embodiment of  FIG. 4 , the detection logic  404  marks the telemetry signal. The detection logic  404  may mark this telemetry signal based on either or both of the degrees of correlation. For example, if the telemetry signal received may be one of two symbols, the detection logic  404  may mark the telemetry signal as a first symbol if the first degree of correlation is above a maximum threshold and if the second degree of correlation is below a minimum threshold. In other words, the telemetry signal may be marked as a given symbol base on the correlation with one stored telemetry signal and the lack of correlation with a second stored telemetry signal. A more detailed description of such correlation comparisons is provided below.  
         [0041]     While the flow diagram  500  illustrates the correlation with two stored telemetry signals, embodiments of the invention may correlate with a lesser or greater number of such signals. For example, the received telemetry signal may be correlated with any of a number of the signals stored in a library of signals.  
         [0042]      FIG. 6  illustrates an on-off key-based receiver for an acoustic telemetry system, according to some embodiments of the invention. In particular,  FIG. 6  illustrates an on-off key (OOK) receiver  600  that includes a bandpass filter  608 , a switch  610 , a tail subtract logic  612 , a timing recovery logic  614 , a training logic  615 , a correlation logic  618 , a memory  619  and a detection logic  620 .  
         [0043]     The bandpass filter  608  receives an on-off key (OOK) signal  602 . The switch  610  receives a tail signal  604 . The tail signal  604  is a tail from a previous timing interval for a tone pulse. The training logic  615  receives a training OOK signal  601 . The training logic  615  is coupled to the memory  619 . The memory  619  is coupled to a first input of the correlation logic  618  and a first input of the timing recovery logic  614 . An output from the bandpass filter  608  is coupled to a first input of the tail subtract logic  612  and a second input of the timing recovery logic  614 .  
         [0044]     The timing recovery logic  614  may determine the time of the symbolic interval. In some embodiments, the output of the timing recovery logic  614  peaks after the received input most closely matches the shape of the training pulse  617 . While the timing recovery logic  614  may be any of a number of different timing circuits, in some embodiments, the timing recovery logic  614  is an early-late-gate correlation timing circuit.  
         [0045]     An output of the switch is coupled to a second input of the tail subtract logic  612 . An output of the tail recovery logic is coupled to a third input of the tail subtract logic  612 , a second input of the correlation logic  618  and a detection logic  620 . An output of the tail subtract logic  612  is coupled to a third input of the correlation logic  618 .  
         [0046]     An output of the correlation logic  618  is coupled to a second input of the detection logic  620 . The output of the detection logic  620  is an output signal  622  of the OOK receiver  600 . The output signal  622  is coupled an input of the switch  610 .  
         [0047]     One embodiment of the operations of the OOK receiver  600  is now described in more detail in conjunction with a flow diagram  700  of  FIG. 7 . In particular,  FIG. 7  illustrates a flow diagram for operations of an OOK receiver, according to some embodiments of the invention.  
         [0048]     In block  702 , a training tone pulse for an OOK signal during a training period is determined. With reference to the embodiment of  FIG. 6 , the training logic  615  may make this determination. For binary signaling, the OOK signal  602  may be a tone pulse over a symbolic interval for data “one” and a gap over a symbolic interval for data “zero”. Accordingly, the training OOK signal  601  may be a sequence of approximately identical widely spaced tone pulses sent by the acoustic telemetry transmitter  123 . In particular, the sequence of tone pulses is widely spaced such that there is no interference between the pulses. The training logic  615  may receive the training OOK signal  601  during an approximately noise free operating environment. For example, the drill string  108  is not in motion to turn/move the drill bit (as is typical during normal drilling operations). The training logic  615  may store these trained tone pulses into the memory  619 . As further described below, the correlation logic  618  may correlate these trained tone pulses with the acoustic signals received during normal drilling operations. Additionally, the timing recovery logic  614  may determine the time of the symbolic interval during this training period. Control continues at block  704 .  
         [0049]     In block  704 , an OOK signal is received during a current symbolic interval during normal operations. With reference to the embodiment of  FIG. 6 , the bandpass filter  608  may receive the OOK signal  602 . Normal operations may include drilling operations or operations related thereto (e.g., trip operations, etc.). The location of the current symbolic interval may be based on the timing of such interval (received from the timing recovery logic  614 ). Control continues at block  706 .  
         [0050]     In block  706 , a bandpass filter operation is performed on the OOK signal in the current symbolic interval. With reference to the embodiment of  FIG. 6 , the bandpass filter  608  may perform this bandpass filter operation. The OOK signal  602  is bandpass filtered to remove any out-of-band noise. Such out-of-band noise may be introduced into the OOK signal  602  by the multiple joints along the drill string  108 , drilling operations (such as the noise from the drill bit), etc. Control continues at block  708 .  
         [0051]     In block  708 , a determination is made of whether the previous symbol is a tone pulse. With reference to the embodiment of  FIG. 6 , the switch  610  makes this determination. As shown, the output from the detection logic  620  is inputted into the switch  610 . This output is an indication of whether the symbol is a tone pulse (representing a first value, such as a binary one) or a non-tone pulse (representing a second value, such as a binary zero). Accordingly, the switch  610  may make this determination based on the output from the previous symbolic interval. Upon determining that the previous symbol is a non-tone pulse, there is no need to subtract a tail of this symbol from the current symbol because there is no intersymbol interference. Therefore, control continues at block  712 , which is described in more detail below. In one such embodiment, the switch  610  does not input the tail signal  604  (which is representative of a tail of a tone pulse) into the tail subtract logic  612 . Upon determining that the previous symbol is a tone pulse, the switch  610  may input the tail signal  604  into the tail subtract logic  604 . Additionally, upon determining that the previous symbol is a tone pulse, control continues at block  710 .  
         [0052]     In block  710 , the tail of symbol in a previous symbolic interval is subtracted from the symbol in the current symbolic interval to generate a corrected symbol for the current symbolic interval. With reference to the embodiment of  FIG. 6 , the tail subtract logic  612  may perform this operation. The tail subtract logic  612  may subtract the tail signal  604  from the symbol in the current symbolic interval. Returning to  FIG. 3 , for the symbolic interval  302 B, the tail  306 A of the first symbol  304 A (which has carried over into the symbolic interval  302 B) is subtracted therefrom. Accordingly, the symbol  304 B remains in the symbolic interval  302 B. Control continues at block  712 .  
         [0053]     In block  712 , the corrected symbol is correlated with the training tone pulse. With reference to the embodiment of  FIG. 6 , the correlation logic  618  correlates the corrected signal with the training tone pulse. The correlation logic  618  may perform this correlation by multiplying the corrected signal by the training tone pulse to generate a multiplied output. Control continues at block  714 .  
         [0054]     In block  714 , a determination is made of whether the correlation is above a threshold. With reference to the embodiment of  FIG. 6 , the detection logic  620  may make this determination. The detection logic  620  may make this determination by determining if the multiplied output is greater than the threshold. In some embodiments this threshold is a configurable value that may be set based on the environment of operation. For example, a drilling operation may have a lower threshold value in comparison a drill stem test operation.  
         [0055]     In block  716 , upon determining that the correlation is above a threshold, the corrected symbol is marked as a tone pulse. With reference to the embodiment of  FIG. 6 , the detection logic  620  marks the corrected symbol as a tone pulse. Therefore, if the tone pulse is defined as a binary one, the corrected symbol is marked as a binary one. Control continues at block  720 , which is described in more detail below.  
         [0056]     In block  718 , upon determining that the correlation is not above a threshold, the corrected symbol is marked as a non-tone pulse. With reference to the embodiment of  FIG. 6 , the detection logic  620  marks the corrected symbol as a non-tone pulse. Therefore, if the non-tone pulse is defined as a binary zero, the corrected symbol is marked as a binary zero. Accordingly, data communications from downhole may be interpreted in light of a sequence of symbols received. Control continues at block  720 .  
         [0057]     In block  720 , the value of the corrected symbol is stored. With reference to the embodiment of  FIG. 6 , the detection logic  620  may store this value into a memory (not shown) internal or external to the OOK receiver  600 . Such value may then be further processed to interpret the communications based on such symbols. Additionally, the detection logic  620  may store this value into a memory within the switch  610 . Accordingly, for the subsequent symbolic interval, the switch  610  may or may not input the tail signal  604  into the tail subtract logic  612  depending on whether this symbol was a tone pulse or a non-tone pulse, respectively (as described in block  708 ). Control continues at block  704 , where another OOK signal is received for the subsequent symbolic interval.  
         [0058]      FIG. 8  illustrates a frequency shift key-based receiver for an acoustic telemetry system, according to some embodiments of the invention. In particular,  FIG. 8  illustrates a frequency shift key (FSK) receiver  800  that includes a bandpass filter  802 , a f 1  timing recovery logic  810 , a f 2  timing recovery logic  812 , a switch  814 , a training logic  815 , a tail subtract logic  816 , a f 1  correlation logic  818 , a memory  819 , a f 2  correlation logic  820  and a detection logic  824 .  
         [0059]     The training logic  815  receives a training OOK signal  801 . The training logic  815  is coupled to the memory  819 . The memory  819  is coupled to a first input of the f 1  timing recovery logic  810 , a first input of the f 2  timing recovery logic  812 , a first input of the f 1  correlation logic  818  and a first input of the f 2  correlation logic  820 .  
         [0060]     The bandpass filter  808  receives a FSK signal  802 . The switch  814  receives a T(f 1 ) signal  804  and a T(f 2 ) signal  806 . The T(f 1 ) signal  804  and the T(f 2 ) signal  806  are tails from a previous timing interval for a first data representation and a second data representation, respectively. An output of the bandpass filter  808  is coupled to a first input of the tail subtract logic  816 , a second input of the f 1  timing recovery logic  810  and a second input of the f 2  timing recovery logic  812 . An output of the switch i 814  is coupled to a second input of the tail subtract logic  816 . An output of the f 1  timing recovery logic  810  is coupled to a second input of the f 1  correlation logic  818 . An output of the f 2  timing recovery logic  812  is coupled to a second input of the f 2  correlation logic  820 . The output of the tail subtract logic  816  is coupled to a second input of the f 1  correlation logic  818  and to a second input of the f 2  correlation logic  820 . An output of the f 1  correlation logic  818  and an output of the f 2  correlation logic  820  are coupled as inputs into the detection logic  824 . The output of the detection logic  824  is an output signal  826  of the FSK receiver  800 . The output signal  826  is coupled to a third input of the switch  814 .  
         [0061]     One embodiment of the operations of the FSK receiver  800  is now described in more detail in conjunction with a flow diagram  900  of  FIGS. 9A-9B . In particular,  FIGS. 9A-9B  illustrate a flow diagram for operations of an FSK receiver, according to some embodiments of the invention.  
         [0062]     In block  902 , a training tone pulse at a first frequency and a training tone pulse at a second frequency for a FSK signal during a training period are determined. With reference to the embodiment of  FIG. 8 , the training logic  815  may make this determination. For binary signaling, the FSK signal  802  may be a tone pulse over a symbolic interval at a first frequency for data “one” and a tone pulse over a symbolic interval at a second (different) frequency for data “zero”. Accordingly, the training FSK signal  801  may be a sequence of approximately identical widely spaced tone pulses at a first frequency and a sequence of approximately identical widely spaced tone pulses at a second frequency sent by the acoustic telemetry transmitter  123 . In particular, the sequence of tone pulses at the first and second frequencies is widely spaced such that there is no interference between the pulses. The training logic  815  may receive the training the FSK signal  801  during an approximately noise free operating environment. For example, the drill string  108  is not in motion to turn/move the drill bit (as is typical during normal drilling operations). The training logic  815  may store these trained tone pulses into the memory  819 . As further described below, the f 1  correlation logic  818 , and the f 2  correlation logic  820  may correlate these trained tone pulses with the acoustic signals received during normal drilling operations. Additionally, the f 1  timing recovery logic  810  and the f 2  timing recovery logic  812  may determine the current symbolic interval for the first frequency and the second frequency during this training period. Control continues at block  904 .  
         [0063]     In block  904 , a FSK signal is received during a current symbolic interval during normal operations. With reference to the embodiment of  FIG. 8 , the bandpass filter  808  may receive the FSK signal  802 . Normal operations may include drilling operations or operations related thereto (e.g., trip operations, etc.). The location of the current symbolic interval may be based on the timing of such interval (received from the f 1  timing recovery logic  810  and the f 2  timing recovery logic  812 ). Control continues at block  906 .  
         [0064]     In block  906 , bandpass filter operations are performed on the FSK signal in the current symbolic interval with regard to the first frequency and the second frequency. With reference to the embodiment of  FIG. 8 , the bandpass filter  808  may perform this bandpass filter operation. The FSK signal  802  at the first frequency may have a different bandpass region in comparison to the FSK  802  signal at the second frequency. Accordingly, the bandpass filter  808  may perform the bandpass operation at the first frequency separate from the bandpass operation at the second frequency for the FSK signal  802 . Control continues at block  908 .  
         [0065]     In block  908 , a determination is made of whether the previous symbol is at the first frequency. With reference to the embodiment of  FIG. 8 , the switch  814  may make this determination. As shown, the output signal  826  from the detection logic  824  is inputted into the switch  814 . The output signal  826  is an indication of whether the symbol is a tone pulse at the first frequency or a tone pulse at the second frequency (representing a first value, such as a binary one, or a second value, such as a binary zero, respectively). Accordingly, the switch  814  may make this determination based on the output from the previous symbolic interval.  
         [0066]     In block  910 , upon determining that the previous symbol is at the first frequency, the tail of a symbol at the first frequency is subtracted from the symbol in the current symbolic interval to generate a corrected symbol for the current symbolic interval. With reference to the embodiment of  FIG. 8 , the tail subtract logic  816  may perform this operation. The switch  814  may input the T(f 1 ) signal  804  (which is a tail at the first frequency) into the tail subtract logic  816  if the previous symbol is at the first frequency. The tail subtract logic  816  may subtract the T(f 1 ) signal  804  from the symbol in the current symbolic interval. Control continues at block  914 , which is described in more detail below.  
         [0067]     In block  912 , upon determining that the previous symbol is not at the first frequency (rather the second frequency), the tail of a symbol at the second frequency is subtracted from the symbol in the current symbolic interval to generate a corrected symbol for the current symbolic interval. With reference to the embodiment of  FIG. 8 , the tail subtract logic  816  may perform this operation. The switch  814  may input the T(f 2 ) signal  806  (which is a tail at the second frequency) into the tail subtract logic  816  if the previous symbol is at the second frequency. The tail subtract logic  816  may subtract the T(f 2 ) signal  806  from the symbol in the current symbolic interval. Control continues at block  914 .  
         [0068]     In block  914 , the corrected symbol is correlated with the training tone pulse at the first frequency to generate a first correlated output. With reference to the embodiment of  FIG. 8 , the f 1  correlation logic  818  may correlate the corrected signal with the training tone pulse at the first frequency. The f 1  correlation logic  818  compares the corrected signal with the training tone pulse at the first frequency to determine the correlation there between. Control continues at block  916 .  
         [0069]     In block  916 , the corrected symbol is correlated with the training tone pulse at the second frequency to generate a second correlated output. With reference to the embodiment of  FIG. 6 , the f 2  correlation logic  620  may correlate the corrected signal with the training tone pulse at the second frequency. The f 2  correlation logic  620  compares the corrected signal with the training tone pulse at the second frequency to determine the correlation there between. Control continues at block  918 .  
         [0070]     In block  918 , the second correlated output is subtracted from the first correlated output to generate a subtracted output. With reference to the embodiment of  FIG. 6 , the detection logic  624  may perform this subtraction. Control continues at block  920 .  
         [0071]     In block  920 , a determination is made of whether the polarity of the subtracted output is positive. With reference to the embodiment of  FIG. 6 , the detection logic  624  may make this determination.  
         [0072]     In block  922 , upon determining that the polarity of the subtracted output is positive, the corrected symbol is marked as a “data one.” With reference to the embodiment of  FIG. 6 , the detection logic  624  may mark the corrected symbol. Control continues at block  926 , which is described in more detail below.  
         [0073]     In block  924 , upon determining that the polarity of the subtracted output is not positive, the corrected symbol is marked as a “data zero.” With reference to the embodiment of  FIG. 6 , the detection logic  624  may mark the corrected symbol. Control continues at block  926 .  
         [0074]     In block  926 , the value of the corrected symbol is stored. With reference to the embodiment of  FIG. 6 , the detection logic  624  may store this value into a memory (not shown) internal or external to the FSK receiver  600 . Such value may then be further processed to interpret the communications based on such symbols. Additionally, the detection logic  624  may store this value into a memory within the switch  614 . Accordingly, for the subsequent symbolic interval, the switch  614  may input the T(f 1 ) signal  604  or the T(f 2 ) signal  606  depending on whether this symbol was at a first frequency or a second frequency, respectively (as described in blocks  910  and  912 ). Control continues at block  904 , where another FSK signal is received for the subsequent symbolic interval.  
         [0075]      FIG. 10  illustrates a phase shift key-based receiver for an acoustic telemetry system, according to some embodiments of the invention. In particular,  FIG. 10  illustrates a phase shift key (PSK) receiver  1000  that includes a bandpass filter  10010 , a switch  1010 , a tail subtract logic  1012 , a timing recovery logic  1014 , a training logic  1015 , a memory  1019 , a ( phi-1 ) correlation logic  1028 , a ( phi-2 ) correlation logic  1030  and a detection logic  1034 .  
         [0076]     The training logic  1015  receives a training PSK signal  1001 . The training logic  1015  is coupled to the memory  1019 . The memory  1019  is coupled to a first input of the timing recovery logic  1014 , a first input of the ( phi-1 ) correlation logic  1028  and a first input of the ( phi-2 ) correlation logic  1030 .  
         [0077]     The bandpass filter  1008  receives a PSK signal  1002 . The switch  1010  receives a T( phi-1 ) signal  1004  and a T( phi-2 ) signal  1006 . The T( phi-1 ) signal  1004  and the T( phi-2 ) signal  1006  are tails from a first data representation and a second data representation, respectively.  
         [0078]     An output of the bandpass filter  1008  is coupled to a first input of the tail subtract logic  1012  and an input of the timing recovery logic  1014 . An output of the switch  1010  is coupled as a second input of the tail subtract logic  1012 .  
         [0079]     A first output of the timing recovery logic  1014  is a timing signal for the first phase, which is a second input of the (phi-1) correlation logic  1028 . A second output of the timing recovery logic  1014  is a timing signal for the second phase, which is a second input of the (phi-2) correlation logic  1030 .  
         [0080]     An output of the tail subtract logic  1012  is coupled to a third input of the (phi-1) correlation logic  1028  and to a third input of the (phi-2) correlation logic  1030 . An output of the (phi-1) correlation logic  1028  is coupled to a first input of the detection logic  1034 . An output of the (phi-2) correlation logic  1030  is coupled to a second input of the detection logic  1034 . The output of the detection logic  1034  is an output signal  1036  of the PSK receiver  1000 . The output signal  1036  is coupled to an input of the switch  1010 .  
         [0081]     One embodiment of the operations of the PSK receiver  1000  is now described in more detail in conjunction with a flow diagram  1100  of  FIGS. 11A-11B . In particular,  FIGS. 11A-11B  illustrate a flow diagram for operations of a PSK receiver, according to some embodiments of the invention.  
         [0082]     In block  1102 , a training tone pulse at a first phase and a training tone pulse at a second phase for a PSK signal during a training period are determined. With reference to the embodiment of  FIG. 10 , the training logic  1015  may make this determination. For binary signaling, the PSK signal  1002  may be a tone pulse over a symbolic interval at a first phase for data “one” and a tone pulse over a symbolic interval at a second (different) frequency for data “zero”. In some embodiments, the first phase is shifted approximately 180 degrees relative to the second phase.  
         [0083]     The training PSK signal  1001  may be a sequence of approximately identical widely spaced tone pulses at a first phase and a sequence of approximately identical widely spaced tone pulses at a second phase sent by the acoustic telemetry transmitter  123 . In particular, the sequence of tone pulses at the first and second phases is widely spaced such that there is no interference between the pulses. The training logic  1015  may receive the training the PSK signal  1001  during an approximately noise free operating environment. The training logic  1015  may store these trained tone pulses into the memory  1019 . As further described below, the ( phi-1 ) correlation logic  1028  and the ( phi-2 ) correlation logic  1030  may correlate these trained tone pulses with the acoustic signals received during normal drilling operations. Additionally, the timing recovery logic  1014  may determine the current symbolic interval for the first phase and the second phase during this training period (as described above). Control continues at block  1104 .  
         [0084]     In block  1104 , a PSK signal is received during a current symbolic interval during normal operations. With reference to the embodiment of  FIG. 10 , the bandpass filter  1008  may receive the PSK signal  1002 . The location of the current symbolic interval may be based on the timing of such interval (received from the timing recovery logic  1014 ). Control continues at block  1106 .  
         [0085]     In block  1106 , bandpass filter operations are performed on the PSK signal in the current symbolic interval with regard to the first phase and the second phase. With reference to the embodiment of  FIG. 10 , the bandpass filter  1008  may perform these bandpass filter operations. Control continues at block  1108 .  
         [0086]     In block  1108 , a determination is made of whether the previous symbol is at the first phase. With reference to the embodiment of  FIG. 10 , the switch  1010  may make this determination. As shown, the output signal from the detection logic  1034  is inputted into the switch  1010 . This output signal is an indication of whether the symbol is a tone pulse at the first phase or a tone pulse at the second phase (representing a first value, such as a binary one, or a second value, such as a binary zero, respectively). Accordingly, the switch  1010  may make this determination based on the output from the previous symbolic interval.  
         [0087]     In block  1110 , upon determining that the previous symbol is at the first phase, the tail of a symbol at the first phase is subtracted from the symbol in the current symbolic interval to generate a corrected symbol for the current symbolic interval. With reference to the embodiment of  FIG. 10 , the tail subtract logic  1012  may perform this operation. The switch  1010  may input the T( phi-1 ) signal  1004  (which is a tail at the first phase) into the tail subtract logic  1012  if the previous symbol is at the first phase. The tail subtract logic  1012  may subtract the T( phi-1 ) signal  1004  from the symbol in the current symbolic interval. Control continues at block  1114 , which is described in more detail below.  
         [0088]     In block  1112 , upon determining that the previous symbol is not at the first phase (rather the second phase), the tail of a symbol at the second phase is subtracted from the symbol in the current symbolic interval to generate a corrected symbol for the current symbolic interval. With reference to the embodiment of  FIG. 10 , the tail subtract logic  1012  may perform this operation. The switch  1010  may input the T( phi-2 ) signal  1006  (which is a tail at the second phase) into the tail subtract logic  1010  if the previous symbol is at the second phase. The tail subtract logic  1012  may subtract the T( phi-2 ) signal  1006  from the symbol in the current symbolic interval. Control continues at block  1114 .  
         [0089]     In block  1114 , the corrected symbol is correlated with the training tone pulse at the first phase to generate a first correlated output. With reference to the embodiment of  FIG. 10 , the (phi-1) correlation logic  1028  correlates the corrected signal with the training tone pulse at the first phase. The (phi-1) correlation logic  1028  compares the corrected signal with the training tone pulse at the first phase to determine the correlation there between. Control continues at block  1116 .  
         [0090]     In block  1116 , the corrected symbol is correlated with the training tone pulse at the second phase to generate a second correlated output. With reference to the embodiment of  FIG. 10 , the (phi-2) correlation logic  1030  correlates the corrected signal with the training tone pulse at the second phase. The (phi-2) correlation logic  1030  compares the corrected signal with the training tone pulse at the second phase to determine the correlation there between. Control continues at block  1118 .  
         [0091]     In block  1117 , a determination is made of whether the correlation for the first phase (the first correlated output) is above a maximum first phase threshold. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may make this determination. Upon determining that the correlation for the first phase is not above the maximum first phase threshold, control continues at block  1121 , which is described in more detail below.  
         [0092]     In block  1118 , upon determining that the correlation for the first phase is above the maximum first phase threshold, a determination is made of whether the correlation for the second phase (the second correlated output) is below a minimum second phase threshold. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may make this determination. Accordingly, in some embodiments, both correlation outputs (for the two different phases) may be analyzed in the determinations related to whether the corrected symbol is at the first phase (shown in blocks  1117 / 1118 ). However, embodiments of the invention are not so limited as either one of the correlations alone may be used in this determination. Upon determining that the correlation for the second phase is not below the minimum second phase threshold, control continues at block  1121 , which is described in more detail below.  
         [0093]     In block  1120 , upon determining that the correlation for the second phase is not below the minimum second phase threshold, the corrected symbol is marked as a symbol representing the first phase. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may mark the corrected symbol. Therefore, if the symbol for the first phase is defined as a binary one, the corrected symbol is marked as a binary one. Control continues at block  1128 , which is described in more detail below.  
         [0094]     In block  1121 , upon determining that the correlation for the first phase is not above the maximum first phase threshold or that the correlation for the second phase is not below a minimum second phase threshold, a determination is made of whether the correlation for the second phase (the second correlated output) is above a maximum second phase threshold. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may make this determination. Upon determining that the correlation for the first phase is not above the maximum first phase threshold, control continues at block  1126 , which is described in more detail below.  
         [0095]     In block  1122 , upon determining that the correlation for the second phase is above a maximum second phase threshold, a determination is made of whether the correlation for the first phase (the first correlated output) is below a minimum first phase threshold. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may make this determination. Accordingly, in some embodiments, both correlation outputs (for the two different phases) may be analyzed in the determinations related to whether the corrected symbol is at the second phase (shown in blocks  1121 / 1122 ). However, embodiments of the invention are not so limited as either one of the correlations alone may be used in this determination. Upon determining that the correlation for the first phase is not below the minimum first phase threshold, control continues at block  1126 , which is described in more detail below.  
         [0096]     In block  1124 , upon determining that the correlation for the second phase is above the maximum second phase threshold and that the correlation for the first phase is below a minimum first phase threshold, the corrected symbol is marked as a symbol representing the second phase. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may mark the corrected symbol. Therefore, if the symbol for the second phase is defined as a binary zero, the corrected symbol is marked as a binary zero. Control continues at block  1128 , which is described in more detail below.  
         [0097]     In block  1126 , upon determining that the correlation for the second phase is not above the maximum second phase threshold or that the correlation for the first phase is not below a minimum first phase threshold, the corrected symbol is marked as undefined. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may mark the corrected symbol. Therefore, if based on the correlation outputs and the thresholds the detection logic  1034  cannot determine whether the corrected symbol is a symbol representing either of the phases, the corrected symbol is set as undefined. For example, the correct symbol may be undefined because of an excessive amount of noise in the system. In some embodiments, if N number of corrected symbols are set as undefined in a predefined period, the PSK receiver  1000  may set an alarm and/or reboot and re-determine the training tone pulses for the first phase and the second phase. In some embodiments, if N number of corrected symbols are consecutively set as undefined, the PSK receiver  1000  may set an alarm and/or reboot and re-determine the training tone pulses for the first phase and the second phase. Control continues at block  1128 .  
         [0098]     In block  1128 , the value of the corrected symbol is stored. With reference to the embodiment of  FIG. 10 , the detection logic  1034  may store this value into a memory (not shown) internal or external to the PSK receiver  1000 . Such value may then be further processed to interpret the communications based on such symbols. Additionally, the detection logic  1034  may store this value into a memory within the switch  1010 . Accordingly, for the subsequent symbolic interval, the switch  614  may input the T( phi-1 ) signal  1004  and a T( phi-2 ) signal  1006  depending on whether this symbol was at a first phase or a second phase, respectively (as described in blocks  1110  and  1112 ). Control continues at block  1104 , where another PSK signal is received for the subsequent symbolic interval. In some embodiments, these different thresholds (e.g., the maximum first threshold, the maximum second threshold, the minimum first threshold and the minimum second threshold) are configurable values that may be set based on the environment of operation.  
         [0099]     While the flow diagrams  700 ,  900  and  1100  illustrate the generation of the training pulses during an initial training period, such training may be subsequently re-executed. For example, the tails generated during training may be affected by different physical characteristics of the drill string (e.g., the length). In particular, after a given time of drilling operations, the drill string may be physically altered because of the stresses applied thereto during such operations. Additionally, the physical characteristics may be altered by the removal or addition of a section of drill pipe on the drill string. Accordingly, if a section of the drill string is removed or added, the training may be re-executed. The training may also be re-executed after a given time of drilling operations (e.g., 100 hours of operation).  
         [0100]     Moreover, while described with reference to an OOK signal, a FSK signal and a PSK signal, embodiments of the invention are not so limited. Any of a number of different types of signaling can be used that allows for different symbols. For example, symbols may be different shaped envelopes, different levels and/or different chirp pulses that represent different values.  
         [0101]     In the description, numerous specific details such as logic implementations, opcodes, means to specify operands, resource partitioning/sharing/duplication implementations, types and interrelationships of system components, and logic partitioning/integration choices are set forth in order to provide a more thorough understanding of the present invention. It will be appreciated, however, by one skilled in the art that embodiments of the invention may be practiced without such specific details. In other instances, control structures, gate level circuits and full software instruction sequences have not been shown in detail in order not to obscure the embodiments of the invention. Those of ordinary skill in the art, with the included descriptions will be able to implement appropriate functionality without undue experimentation.  
         [0102]     References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.  
         [0103]     Embodiments of the invention include features, methods or processes that may be embodied within machine-executable instructions provided by a machine-readable medium. A machine-readable medium includes any mechanism which provides (i.e., stores and/or transmits) information in a form accessible by a machine (e.g., a computer, a network device, a personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). In an exemplary embodiment, a machine-readable medium includes volatile and/or non-volatile media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.), as well as electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.).  
         [0104]     Such instructions are utilized to cause a general or special purpose processor, programmed with the instructions, to perform methods or processes of the embodiments of the invention. Alternatively, the features or operations of embodiments of the invention are performed by specific hardware components which contain hard-wired logic for performing the operations, or by any combination of programmed data processing components and specific hardware components. Embodiments of the invention include software, data processing hardware, data processing system-implemented methods, and various processing operations, further described herein.  
         [0105]     A number of figures show block diagrams of systems and apparatus for an acoustic telemetry receiver, in accordance with some embodiments of the invention. A number of figures show flow diagrams illustrating operations for an acoustic telemetry receiver, in accordance with some embodiments of the invention. The operations of the flow diagrams are described with references to the systems/apparatus shown in the block diagrams. However, it should be understood that the operations of the flow diagrams could be performed by embodiments of systems and apparatus other than those discussed with reference to the block diagrams, and embodiments discussed with reference to the systems/apparatus could perform operations different than those discussed with reference to the flow diagrams.  
         [0106]     In view of the wide variety of permutations to the embodiments described herein, this detailed description is intended to be illustrative only, and should not be taken as limiting the scope of the invention. For example, embodiments of the invention are described in reference to correlations between two different values based on different attributes (phase, frequency, etc.). However, embodiments of the invention are not so limited. Embodiments of the invention may correlate among N number of different values based on a number of different attributes. For example, the pulses may be on multiple frequencies, multiple phases and/or multiple channels. Accordingly, these different pulses may have each have a training pulse for correlations during the acoustic telemetry operations. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto. Therefore, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Summary:
One embodiment includes a method comprising receiving an acoustic signal that is propagated along a drill string. The method also includes correlating the acoustic signal to a first stored acoustic signal representing a first symbol, wherein the first stored acoustic signal is acquired from a propagation along the drill string in an approximately noise free environment.