Abstract:
A communications system employing discrete multi-tone (“DMT”) modulated signals on a communications channel formed by inductively-coupled pipe. In one embodiment, the communications system includes a string of wired pipe joints that couple a transmitter to a receiver. Each pipe joint in the string possesses threaded ends configured for mechanically connecting with other pipe joints, a electrical or inductive coupler at each threaded end, and an insulated conductor that electrically connects the couplers. The transmitter and receiver communicate via DMT modulated signals transported by the wired pipe string. The system may further include multiple repeaters in the string. Each repeater may be configured to receive a DMT-modulated signal, to recover information from the DMT-modulated signal, and to re-transmit the information in the form of a DMT-modulated signal.

Description:
BACKGROUND 
   Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes earth formation characteristics as well as information concerning the borehole itself. The methods that can be used to collect information downhole include: wireline logging, logging-while-drilling (“LWD”), and seismic imaging. After a well has been completed, downhole data collection may be performed by permanently emplaced sensors, by wireline logging, and by seismic monitoring. 
   In conventional wireline logging, a probe (“sonde”) containing formation sensors is lowered into the borehole after some or all of the well has been drilled. The sonde&#39;s upper end is attached to a conductive wireline that suspends the sonde in the borehole. The conductive wireline transports power from the surface to the sonde&#39;s instrumentation, and transports information from the sonde&#39;s instrumentation to the surface. Wireline logging generally requires that the drill string or production tubing be removed from the borehole before logging can begin. Consequently, wireline logging cannot be performed during drilling or production operations. 
   In LWD, as the name suggests, data may be collected during the drilling process. Collecting and processing data during the drilling process eliminates the need to remove the drilling assembly to insert a wireline logging tool. LWD consequently provides the driller with better control, allowing performance optimization and minimizing downtime. Designs for measuring downhole conditions relating to the drilling assembly&#39;s movement and location have become known as “measurement-while-drilling” techniques, or “MWD”. LWD generally concentrates more on the measurement of formation parameters, but the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly. 
   In LWD, the logging instrumentation is typically located at the lower end of the drill string. The instrumentation may operate, continuously or intermittently, to monitor predetermined drilling parameters and formation data. Some form of telemetry is then used to transmit the information to a surface receiver. Various telemetry systems exist, including mud pulse systems and systems that transmit acoustic signals through the drill string. 
   In seismic imaging, seismic waves are transmitted through earth formations and reflected from various boundaries and discontinuities. Seismic imaging involves stringing hundreds of listening devices on the surface or in a wellbore near a location where an understanding of the underground formations is desired. Once the listening devices have been placed, a disturbance is created to generate seismic waves. As these waves travel through the formations and encounter strata boundaries, some wave energy reflects back to the surface. With the appropriate processing of the received signals, a three-dimensional representation of the subsurface formations can be constructed. 
   The foregoing data collection methods are primarily used to locate and tap hydrocarbon reservoirs. Once a reservoir has been tapped, the goal becomes removing as much of the hydrocarbons from the reservoir as possible. Sensors may be emplaced in the wellbore to monitor pressure, temperature, fluid composition and inflow volumes. Wireline tools may be used to search for previously missed completion opportunities. Finally, long-term seismic monitoring may be employed to identify fluid migration patterns in and around the reservoir. 
   Information is the key to being profitable in the oil and gas industry. The more information one has regarding location and migration patterns of hydrocarbons within a hydrocarbon reservoir, the more likely it is that that reservoir can be tapped at its optimal location and utilized to its full potential. To this end, new and more sophisticated sensor arrangements are routinely created and placed downhole, so much so that existing telemetry techniques are becoming inadequate. For these reasons it would be desirable to have a communication technique that can support high speed communications between downhole sensors and a surface installation. 
   SUMMARY 
   Accordingly, there is disclosed herein a communications system employing discrete multi-tone (“DMT”) modulated signals on a communications channel formed by inductively-coupled pipe. In one embodiment, the communications system includes a string of wired pipe joints that couple a transmitter to a receiver. Each pipe joint in the string possesses threaded ends configured for mechanically connecting with other pipe joints, a electrical or inductive coupler at each threaded end, and an insulated conductor that electrically connects the couplers. The transmitter and receiver communicate via DMT modulated signals transported by the wired pipe string. The system may further include multiple repeaters in the string. Each repeater may be configured to receive a DMT-modulated signal, to recover information from the DMT-modulated signal, and to re-transmit the information in the form of a DMT-modulated signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of various invention embodiments, reference will now be made to the accompanying drawings in which: 
       FIG. 1  shows an illustrative drilling environment; 
       FIG. 2  shows a cut-away view of a drill pipe connector; 
       FIG. 3  shows a cross-sectional view of a connection between drill pipes; 
       FIG. 4  shows a first illustrative drill pipe wiring configuration; 
       FIG. 5  shows a second illustrative drill pipe wiring configuration; 
       FIG. 6  shows a first illustrative repeater configuration; 
       FIG. 7  shows a second illustrative repeater configuration; 
       FIG. 8  shows illustrative receive signal spectra; 
       FIG. 9  shows an illustrative graph of signal strength versus position; 
       FIG. 10  shows a third illustrative repeater configuration; 
       FIG. 11  shows a receive signal spectrum for the repeater of  FIG. 10 ; 
       FIG. 12  shows a block diagram of an illustrative discrete multi-tone (“DMT”) transceiver; 
       FIG. 13  shows an illustrative method that may be used to establish a multi-link communications path; and 
       FIG. 14  shows an illustrative method that may be used to establish individual communication links. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
   NOTATION AND NOMENCLATURE 
   Certain terms are used throughout the following description and claims to refer to particular system-components. As one skilled in the art will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. The terms upstream and downstream refer generally, in the context of this disclosure, to the transmission of information from subsurface equipment to surface equipment, and from surface equipment to subsurface equipment, respectively. Additionally, the terms surface and subsurface are relative terms. The fact that a particular piece of hardware is described as being on the surface does not necessarily mean it must be physically above the surface of the Earth; but rather, describes only the relative location of the surface and subsurface pieces of equipment. 
   DETAILED DESCRIPTION 
     FIG. 1  shows an illustrative well during drilling operations. A drilling platform  2  is equipped with a derrick  4  that supports a hoist  6 . Drilling of oil and gas wells is typically carried out with drill pipes connected together by “tool” joints  7  so as to form a drill string  8 . The hoist  6  suspends a kelly  10  that is used to lower the drill string  8  through rotary table  12 . Connected to the lower end of the drill string  8  is a drill bit  14 . The bit  14  is rotated by rotating the drill string  8  or by operating a downhole motor near the drill bit. The rotation of the bit  14  extends the borehole. 
   Drilling fluid is pumped by recirculation equipment  16  through supply pipe  18 , through drilling kelly  10 , and down through the drill string  8  at high pressures and volumes to emerge through nozzles or jets in the drill bit  14 . The drilling fluid then travels back up the hole via the annulus between the exterior of the drill string  8  and the borehole wall  20 , through the blowout preventer (not specifically shown), and into a mud pit  24  on the surface. On the surface, the drilling fluid is cleaned and then recirculated by recirculation equipment  16 . The drilling fluid cools the drill bit  14 , carries drill cuttings to the surface, and balances the hydrostatic pressure in the rock formations. 
   Downhole instrument sub  26  may be coupled to a telemetry transmitter  28  that communicates with the surface to provide telemetry signals and receive command signals. A surface transceiver  30  may be coupled to the kelly  10  to receive transmitted telemetry signals and to transmit command signals downhole. Alternatively, the surface transceiver may be coupled to another portion of the rigging or to drillstring  8 . One or more repeater modules  32  may be provided along the drill string to receive and retransmit the telemetry and command signals. The surface transceiver  30  is coupled to a logging facility (not shown) that may gather, store, process, and analyze the telemetry information. 
   The telemetry and command signals may be carried by electrical conductors embedded in the drill pipes. Couplers in the tool joints couple the electrical conductors of one drill pipe to the electrical conductors of the next drill pipe. Various coupling techniques have been proposed and are outlined in Michael J. Jellison, et al., “Telemetry Drill Pipe: Enabling Technology for the Downhole Internet,” (SPE 79885) IADC/SPE Drilling Conference, Amsterdam, February 2003. The proposed coupling techniques include direct electrical connections and inductive coupling. Of particular interest to the present disclosure are inductively couplers. 
     FIG. 2  shows a cut-away view of a drill pipe connector (a “pin” end)  202 . The drill pipe connector  202  has a central bore  204  surrounded by a tapered and threaded surface  206 . At the end face of connector  202  is an inductive coupler  208  that surrounds the central bore  204 . The inductive coupler  208  is electrically connected to an inductive coupler at the opposite end of the drill pipe by electrical conductors  210 . 
     FIG. 3  shows a cross-sectional view of a tool joint formed by a pin end connector  202  and a mating “box end” connector  302 . Inductive coupler  208  closely abuts a matching inductive coupler  308  in box end connector  302 . Similar to inductive coupler  208 , inductive coupler  308  is electrically connected to an inductive coupler at the opposite end by electrical conductors  310 . The inductive couplers  208 ,  308 , may be free from any azimuthal orientation requirements. 
     FIG. 4  illustrates a first wiring configuration that may be employed with the inductive couplers. The wiring for two drill pipes  402 ,  404 , and adjoining connectors is shown. Drill pipe  402  has inductive couplers  408  and  410  at opposite ends. Coupler  408  matches with coupler  406  in an adjoining connector. Drill pipe  404  has inductive couplers  412  and  414  at opposite ends. Inductive coupler  412  matches with inductive coupler  410 , while coupler  414  matches with coupler  416  in an adjoining connector. The symmetric nature of the wiring configuration allows for bi-directional communication. 
   To understand the operation of the  FIG. 4  configuration, assume an alternating current “I” flows through inductive coupler  406 . Inductive coupler  406  is a multi-turn coil that produces a magnetic field in response to the current flow. Inductive coupler  408  is a multi-turn coil that resides in this magnetic field. The magnetic field induces a current flow in inductive coupler  408 , and the current is directed from coupler  408  to inductive coupler  410 , where the process repeats. In one implementation, the signal loss at each tool joint is a small fraction of a decibel, allowing a signal to be communicated across many drill pipes before any boost or active enhancement is required. 
     FIG. 5  illustrates another wiring configuration that may be employed with the inductive couplers. The wiring for two drill pipes  502 ,  504 , and adjoining connectors is shown. Drill pipe  502  has inductive couplers  508  and  510  at opposite ends. Coupler  508  matches with a coupler  506  in an adjoining connector. Drill pipe  504  has inductive couplers  512  and  514  at opposite ends. Inductive coupler  512  matches with coupler  510 , while coupler  514  matches with coupler  516  in an adjoining connector. Again, the symmetric nature of the wiring configuration allows for bi-directional communication. 
   To understand the operation of the  FIG. 5  configuration, assume an alternating current “I” flows through inductive coupler  506 . Inductive coupler  506  is a toroid that induces an electric field (or, if surrounded by conductive material, a current density distribution) in response to the current flow. Inductive coupler  508  is a toroid that resides in this electric field (or current density distribution). The electric field (or current density distribution) induces a current flow in inductive coupler  508 , and the current is directed from coupler  508  to coupler  510 , where the process repeats. An illustrative implementation is provided in U.S. Pat. No. 4,605,268 (“Transformer Cable Connector”). 
   A signal propagating through inductively-coupled wiring in a drilling string experiences losses from attenuation, impedance mismatches at the connectors, and imperfect coupling at the connectors. While these losses may be small for individual connectors and drill pipes, the cumulative losses may be significant enough to require the use of one or more repeaters, i.e., devices that receive and retransmit the information, thereby overcoming signal losses. 
     FIG. 6  shows one illustrative repeater configuration that could be used for inductively coupled drill pipe telemetry. A drill pipe  602  includes a repeater module  605  coupled to one or both of the electrical conductors by a-transformer  604 . The repeater module  605  includes a hybrid  606  and an echo canceller  608 . The hybrid  606  is an interface between a bi-directional channel (the electrical conductors of drill pipe  602 ) and two uni-directional channels. Hybrid  606  receives a transmit signal from echo canceller  608  and couples the transmit signal to transformer  604 . Hybrid  606  further detects a receive signal from transformer  604 , and provides the detected receive signal to echo canceller  608 . Echo canceller  608  includes a filter  610  that generates an estimate of the interference caused by the transmit signal, and a summer  612  subtracts the estimated interference from the receive signal. Filter  610  may be implemented as an adaptive filter in accordance with well-known techniques such as those described in Haykin&#39;s  Adaptive Filter Theory  textbook. 
   The illustrative configuration of  FIG. 6  uses transformer coupling between the repeater module  605  and the electrical conductors.  FIG. 7  shows an alternative repeater configuration in which the repeater module  605  is electrically connected between the electrical conductors of a drill pipe  702 . In both configurations, the electrical conductors are not terminated at the repeater module  605 . This circumstance allows signals to propagate along the drill string and past the repeater. Such propagation may allow for continued communications (albeit at a reduced rate) even when a repeater fails. 
   Repeater module  605  may include a transmitter and receiver configured to respectively send and receive discrete multi-tone (“DMT”) modulated signals. (The operation of an illustrative transmitter/receiver (“transceiver”) is discussed below in the description of  FIG. 12 .)  FIG. 8  shows illustrative receive signal spectra for a channel that has been divided into three frequency bands. The channel bandwidth is divided into a large number of frequency “bins”.) For illustrative purposes,  FIG. 8  shows sixty four frequency bins (numbered 0–63), but a typical embodiment may be expected to have at least 256 or 512 bins.) The lowest frequency bin or bins  801  may be unused to avoid interference from power and other low-frequency electronic signals. The remaining bins may be divided into three (or more) bands  802 ,  804 , and  806 . Between the bands, one or more bins  803 ,  805  may be reserved as guard bands to simplify signal separation. 
     FIG. 8  shows an illustrative receive signal spectrum in each of three bands  802 ,  804 ,  806 . Due to increased channel attenuation as a function of frequency, the higher-frequency bins may not support as high a bit rate as the lower bins. Accordingly, the higher-frequency bands may be allocated a greater number of bins than the lower-frequency bands. 
     FIG. 9A  shows a logarithmic graph of illustrative signal power as a function of position along the drill string. Seven positions along the drill string are identified: a surface transceiver, a downhole transceiver, and five intermediate repeaters. Two power levels are identified: a transmit power level, and a minimum receive power level. The surface transceiver transmits a signal  902  that attenuates as it propagates. The first repeater (RPTR 1 ) receives signal  902  from the surface transmitter, and signal  906  from the second repeater (RPTR 2 ). To avoid interference, the first repeater communicates with the surface transmitter in one frequency band, and communicates with the second repeater in a second frequency band. Information received from the surface transmitter in one frequency band is retransmitted in the second frequency band, and information received in the second frequency band is retransmitted in the first frequency band. 
   The first repeater transmits signal  904  which attenuates as it propagates in both directions. The second repeater receives signal  904  from the first repeater, and receives signal  908  from the third repeater (RPTR 3 ). To avoid interference with signal  902  at the first repeater, the third repeater does not transmit any information in the first frequency band. Communications between the first and second repeater are carried out in the second frequency band, while communications between the second and third repeater are carried out in a third frequency band. Information received in the second frequency band (from the first repeater) is retransmitted in the third frequency band, while information received in the third frequency band (from the third repeater) is retransmitted in the second frequency band. 
   In like manner, the third repeater receives signals  906  and  910 , communicating with the second repeater in the third frequency band, and communicating with the fourth repeater (RPTR 4 ) in the first frequency band. The fourth repeater receives signals  908  and  912 , communicating with the third repeater in the first frequency band and communicating with the fifth repeater (RPTR 5 ) in the second frequency band. The fifth repeater receives signals  910  and  914 , communicating with the fourth repeater in the second frequency band and communicating with the downhole transceiver in the third frequency band. The downhole transceiver receives signal  912 , and communicates with the fifth repeater in the third frequency band. 
   The system may be designed so that the repeaters can detect signals from non-adjacent repeaters or transceivers. Thus, for example, the third repeater may be able to detect signals from not only the second and fourth repeaters, but also from the first and fifth repeaters. The signals from more distant sources may be typically overwhelmed by transmissions from nearby sources, but such a design may allow communications to be maintained even if a repeater-fails. 
   Thus, for example,  FIG. 9B  shows an illustrative graph of logarithmic signal power as a function of position when the second repeater has failed. For communications to be successful, the system should now avoid interference between signals  904 ,  910 , and  912 . In one embodiment, the system disables the fourth repeater, and uses three frequency bands for communication as before. (E.g., a first frequency band for communications between the surface transceiver and the first repeater, a second frequency band for communications between the first and third repeaters, a third frequency band for communications between the third and fifth repeaters, and the first frequency band for communications between the fifth repeater and the downhole transceiver.) 
   In another embodiment, the system reallocates the channel bandwidth to avoid interference and maximize the data rate past the failed receiver. The failed receiver creates a “bottleneck” in the system. The communications between the first and third repeaters are limited by channel attenuation, so it does little good to have large data rates between the other repeaters and transceivers. Accordingly, a somewhat larger bandwidth may be allocated for communications past the failed repeater, and those communications may take place in the frequency band with the smallest attenuation (typically the lowest frequency band). Hereafter, this bandwidth and frequency allocation is referred to as “channel  1 ”. Three other channels (channels  2 ,  3 , and  4 ), are also allocated for communications between other repeaters and transceivers. Compared with the three frequency bands used for the system of  FIG. 9A , it may be anticipated that channel  1  will have a significantly larger bandwidth, and channels  2 – 4  will have significantly smaller bandwidths. 
   Since channel  1  has been allocated for communication between the first and third repeaters, this is the starting point for assigning channels. Channel  4  may be used for communications between the surface transceiver and the first repeater. Channel  2  may be used for communication between the third and fourth repeaters, channel  3  may be used for communication between the fourth and fifth repeaters, and channel  4  may be used for communication between the fifth repeater and the downhole transceiver. 
   The repeater configurations of  FIGS. 6 and 7  offer robustness, i.e., the ability for the system to continue operating even if a repeater fails. The trade-off for this robustness is a need for time division multiplexing, frequency division multiplexing, or code division multiplexing to avoid interference between the transmit signals of different repeaters.  FIG. 10  shows a configuration that isolates the signals from different repeaters so that the full channel bandwidth may be used at each stage of the pipeline. 
     FIG. 10  shows a drill pipe  1002  having a repeater module  1005  connected to couplers at opposite ends of the drill pipe. Each coupler is connected to a respective hybrid  606 , which in turn is coupled to corresponding echo canceller  608  and a corresponding transceiver  1004 ,  1006 . A controller  1008  interfaces to each transceiver  1004 ,  1006 , and controls the information flow in both directions. Controller  1008  may be additionally coupled to various sensors (not shown) to detect drilling fluid and well bore conditions around the repeater. Controller  1008  may insert measurements from such sensors into the information flowing to the surface. Controller  1008  may further detect commands in the information stream flowing downhole and may adjust its operation in response to such commands. 
   Each of the transceivers  1004 ,  1006 , may be configured to transmit and receive DMT-modulated signals.  FIG. 11  shows an illustrative receive spectrum that may be supported by the repeater configuration of  FIG. 10 . The channel bandwidth is divided into many equally-space frequency bins, and the lowest frequency bin(s) may be left unused to avoid interference from nearby power signals. The remainder of the bins are grouped into a single channel band that can be used for bi-directional communication. 
   As mentioned previously, the illustrative telemetry system transports information using discrete multi-tone (DMT) modulation. DMT modulation is commonly employed in Asymmetrical Digital Subscriber Line (ADSL) systems, so a wealth of detail on DMT implementation is available in the ADSL literature. DMT modulation divides the frequency spectrum into many adjacent frequency bins (see, e.g.,  FIGS. 8 and 11 ). In the ideal case, frequency bin, would have the same data transmission rate as all the other frequency bins. However, the data rate for each bin depends on various factors. For example, interference having a particular frequency may affect bins centered near the frequency of the noise source. The affected bins will have lower signal to noise ratios and therefore their data carrying capacity will be lower than other bins. Another factor affecting data rates may be the frequency-dependent attenuation of the electrical conductors, which, due to capacitive effects, generally exhibit a smoothly increasing attenuation at higher frequencies. Other system components, such as transformers or imperfect impedance-matching connectors, may further aggravate attenuation at selected frequencies. ADSL systems generally include mechanisms for adjusting the data transmission rate for each bin to optimize the reliable data transmission rate. 
   In an inductively coupled pipe, the regularly spaced couplers create reflections and standing waves due to impedance mismatches. These reflections affect the transmission spectrum of the channel, creating many narrow pass bands and stop bands. DMT modulation techniques are particularly useful for exploiting the full channel capacity. DMT modulation allows the allocation of data to specific frequency bands (e.g., more data to bands having low attenuation, and little or no data to bands having high attenuation). In general, the width of the DMT frequency bands is inversely proportional to the DMT symbol length, so finer control of frequency usage is provided by longer DMT-symbols. 
   The length of the channel&#39;s impulse response may be used as a practical guide to determining a desirable DMT symbol length. It is desirable to make the DMT symbol substantially longer than the length of the impulse response (or at least the length of the non-negligible part of the impulse response). It may be desirable to employ DMT-symbols having lengths of 1024 or 2048 samples (not including the cyclic prefix). Such symbol lengths offer substantially reduced inter-symbol interference (ISI) relative to high bandwidth QAM signaling. 
   Also, the allocation of data to frequency bands is extremely flexible, making DMT modulation particularly adaptive to changing channel conditions. As the pipe string endures compressional and torsional loads, the couplers may experience coupling and/or impedance changes. DMT transceivers can quickly alter data allocations to maintain data rates in the face of such channel changes. The flexibility of DMT transceivers extends also to overall data rates, which may be adjusted as changes occur to the channel. If one or more repeaters fail, the remaining repeaters and transceivers can attempt to bypass the failed repeater(s) with no significant alteration of their communications algorithms. Such flexibility greatly enhances the reliability of the communications system. 
     FIG. 12  shows an illustrative transceiver  1006 , having a transmitter chain and a receiver chain. The transmitter chain includes a data framer  1202 , an error correction encoder  1204 , a tone mapper  1206 , an inverse Discrete Fourier Transform (IDFT) block  1208 , a cyclic prefix generator  1210 , and a line interface  1212 . The receiver chain includes a line interface  1214 , a cyclic prefix stripper  1216 , a Discrete Fourier Transform (DFT) block  1218 , a frequency domain equalizer  1220 , a demodulation and bit extraction block  1222 , an error correction decoder  1224 , and a CRC/deframing block  1226 . 
   In the transmitter chain, data framer  1202  groups bytes of uplink data together to form data frames. The data frames are then grouped together with a synchronization frame and a cyclic redundancy code (CRC) which is calculated from the contents of the data frames. The CRC provides one means for detecting errors in data received at the receiving end. An error correction encoder  1204  processes the data frames to add redundancy to the data stream. The redundancy may be used to detect and correct errors caused by channel interference. A Reed-Solomon (RS) code is suitable, but other error correction codes may be used. 
   Tone mapper  1206  takes bits from the data stream and assigns them to frequency bins. For each frequency bin, the bits are used to determine a Discrete Fourier Transform (DFT) coefficient that specifies a frequency amplitude. The number of bits assigned to each frequency bin is variable (i.e., the number may be different for each bin) and dynamic (i.e., the number may change over time), and the number depends on the estimated error rate for each frequency. Microcontrollers or software (not specifically shown) at each end cooperate to determine the error rate detected by the receiver in each frequency band, and to adjust the tone mapper accordingly. 
   The coefficients provided by the tone mapper  1206  are processed by IDFT block  1208  to generate a time-domain signal carrying the desired information at each frequency. Cyclic prefix block  712  duplicates the end portion of the time-domain signal and prepends it to the beginning of the time domain signal. As discussed further below, this permits frequency domain equalization of the signal at the receiving end. The signal-with-prefix is then converted into analog form, filtered, and amplified for transmission across the communications channel by line interface  1212 . 
   In the receiver chain, line interface  1214  filters the received signal, converts it to digital form, and performs any desired time domain equalization. The time domain equalization at least partially compensates for distortion introduced by the channel, but it is likely that at least some intersymbol interference will remain. Stripper block  1216  removes the cyclic prefixes that were added by the source&#39;s prefix block (the counterpart of block  1210 ), but trailing intersymbol interference from the cyclic prefix remains in the signal. DFT block  1218  performs a DFT on the signal to obtain the frequency coefficients. If desired, frequency domain equalization may be performed by block  1220  to compensate for the remaining intersymbol interference. It is noted that frequency domain equalization on DFT coefficients is a cyclic convolution operation which would lead to incorrect equalization results had the cyclic prefix not been transmitted across the channel. 
   Block  1222  extracts the data bits from the frequency coefficients using an inverse mapping of the source&#39;s tone mapper (counterpart to mapper  1206 ). Decoder  1224  decodes the data stream correcting such errors as are within its correcting ability. De-framer  1226  then identifies and removes synchronization information, and determines if the CRC indicates the presence of any errors. If error free, the data is forwarded to the output. Otherwise, the controller is notified of errors in the data. 
     FIG. 13  shows an illustrative method which may be implemented by the surface transceiver&#39;s controller to initialize the various transceivers (hereafter, the term “transceiver” includes any repeaters) and establish a complete communications path between the surface and downhole transceivers. Each link between transceivers is assigned a unique index and a correspondingly unique trigger signal. The trigger signal may be a tone or combination of tones that are used to alert the appropriate transceivers to establish contact over the link. In one implementation, the index is a binary word with a bit for each transceiver. The link is then identified by setting two of the bit values to “1”, namely, the two bit values associated with the transceivers at the ends of the link. The trigger signal may then be a pair of tones, each tone indicating one of the transceivers. 
   Beginning in block  1302 , the controller sets the link index to some initial value. Thereafter, as the link index is incremented in block  1304 , the link index will cycle through the links between transceivers from the surface to the bottom hole assembly. In block  1306 , the surface transceiver sends the corresponding trigger signal. After sending the trigger signal, the controller will either receive a report of successful training, or will time-out after waiting a predetermined time. In block  1308 , the controller tests to determine whether a success report was received. If so, the controller tests in block  1310  to determine if more links need to be established. If more links need to be established, the controller returns to block  1304 . Otherwise, the controller transmits a start command in block  1312  to complete the communications path. 
   If in block  1308 , the controller determines that no success report was received, the controller attempts to establish a bypass link beginning in block  1314 . In block  1314 , the controller sets the bypass index to some initial value. Thereafter, as the bypass index is incremented in block  1316 , the bypass index will cycle through bypass links from the last successful link to the bottom hole assembly. As with the link index, the bypass index may be implemented as a binary word with a bit for each transceiver. The bypass link is identified by setting two non-adjacent bit values to “1”, namely, the two bit values associated with the transceivers at the end of the bypass link. The trigger signal for bypass links may be implemented in the same fashion as before. 
   In block  1318 , the controller transmits a trigger signal for the bypass link. After sending the trigger signal, the controller will either receive a report of successful training, or will time-out after waiting a predetermined time. In block  1320 , the controller tests to determine whether a success report was received. If so, the controller returns to block  1310 . Otherwise, the controller in block  1322  tests to determine whether anymore bypass links may be attempted. If so, the controller returns to block  1316 . Otherwise, the controller reports the failure to establish a complete communications path in block  1324 . 
     FIG. 14  shows an illustrative method which may be implemented by the controllers of each of the transceivers (including the surface transceiver). In block  1402 , the controller waits for a trigger signal. In block  1404 , the controller determines whether the trigger signal identifies one of the links or bypass links associated with the transceiver. If the transceiver is not associated with the identified link, the controller returns to block  1402 . Otherwise, the controller enters an activation phase in block  1406 . 
   Assuming that both of the transceivers associated with a link have received the trigger signal, both transceivers will be in the activation phase. In the activation phase, each of the two transceivers transmit single tones to establish contact and determine which one will control the timing on the link. In block  1408 , the transceivers individually determine whether the other transceiver has been successfully contacted, and if not, the transceivers revert to the listening mode in block  1402 . 
   If contact has been established, the transceivers exchange wideband signals in block  1410 . The wideband signals allow each unit to calculate the received power spectral density, to adjust automatic gain controls, and to perform initial training of the equalizers in each receiver. A quiet period may also be provided to allow each transceiver to determine line noise or to train echo cancellation equalizers. In block  1412 , the transceivers determine whether training has been successfully completed, and if not, they revert to the listening state in block  1402 . 
   If training is successful, the transceivers provide each other with basic information in block  1414 . The basic information includes capabilities of each receiver and proposed data rates. Additional training signals are also sent to permit further equalization training. In block  1416 , the transceivers may exchange information regarding channel measurements, desired data rates, and other configuration parameters to be used for subsequent communications. 
   Once the communication parameters have been exchanged, transceivers in block  1418  each transmit a signal indicating that communications have been successfully established. The transceivers then enter a forwarding mode in block  1420 . In the forwarding mode, each transceiver listens for trigger signals and success reports. Upon receiving either, a transceiver in forwarding mode re-transmits the trigger signal or success report. To avoid unduly replicating the trigger signals or success reports, each transceiver will ignore any trigger signals or success reports received within a predetermined interval after re-transmitting such a signal. 
   Each transceiver that enters the forwarding mode will exit from the forwarding mode for any of the following: the expiration of a predetermined delay, reception of a start command, or receipt of a trigger signal identifying a link associated with that transceiver. 
   In block  1422 , the transceiver determines whether the predetermined time has expired, and if so, the transceiver reverts to the listening state in block  1402 . Otherwise, in block  1424 , the transceiver determines whether a start command has been received. If so, then the transceiver enters a communications mode in block  426 . Otherwise, in block  1425 , the transceiver determines whether an associated trigger signal has been received. If so, the transceiver transitions to block  1406 . If not, the transceiver re-enters the forwarding mode  1420 . 
   In block  1426 , the transceivers begin communicating over the links, translating messages between frequency bands and re-transmitting as described above with reference to  FIGS. 9A–9B . Upon losing power or receiving a trigger signal, the transceiver exits the communication mode. In block  1428 , the transceiver determines whether a trigger signal has been received, and if so, reverts to the listening state in block  1402 . 
   The systems and method disclosed herein are expected to provide reliable, high-data-rate communications with downhole sensors. Such communications are expected to greatly enhance overall safety by quickly alerting drillers to downhole conditions, including sudden fluid influxes and critical pore pressure information, both of which relate to blowout prevention. Such information may make near-balanced drilling feasible and safe, allowing for much more economical drilling operations. 
   The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous other variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.