[0001] The present invention relates to a downhole wireline communication system in general
and in particular to a high reliability downhole wireline communication system for
high speed communication with downhole equipment, sensors and devices.
[0002] Downhole operations normally require tools to be inserted in the downhole environment,
which tools are typically controlled from the surface.
[0003] In order to control downhole tools, a wired control interface is used, i.e. wireline
communication. This interface is used for sending and receiving control commands and
also to receive sensor data keeping track of the changing conditions downhole. The
distance from an uphole command center to a downhole tool can be measured in kilometres
which means that an extremely long wire needs to be used for the interface. These
extreme lengths of cabling introduce parasitic elements distorting the communication.
[0004] The ever changing environment downhole makes it essential to have full control of
the downhole tool. The communication between the uphole command center and the downhole
tool has to be reliable and bit-errors and lost data packets must be kept at a minimum.
[0005] Further to this, new sensors are emerging on the market and these sensors are much
more bandwidth demanding than older devices. The downhole tool may be equipped with
high resolution thermal imaging and high bitrates will be necessary in order to transfer
these images or streams of images to the uphole command center.
[0006] It is evident that there is a need for a high reliability, high speed communications
method for downhole tools.
[0007] In
US2014091943, a system providing data communication between a downhole tool and an uphole command
center is disclosed. The system introduces a coding algorithm that is used in conjunction
with automatic signal gain control at the receiver end which is specific to each cable
equalisation algorithm. This enables increased bitrates compared to legacy system
but there is still a need for increased reliability with high bitrates.
[0008] From the above, it is understood that there is room for improvements.
[0009] An object of the present invention is to provide a new type of method for downhole
data communication which is improved over prior art and which eliminates or at least
mitigates some of the drawbacks discussed above. More specifically, an object of the
invention is to provide a wireline data communication system that is capable of optimising
data transfer and to automatically adjust the bitrate. These objects are achieved
by the technique set forth in the appended independent claims with preferred embodiments
defined in the dependent claims related thereto.
[0010] In a first aspect, a method for downhole data communication in a downhole communication
system performed by a communication equipment configured to be arranged to transmit
and receive signals via an associated wireline at a bitrate is presented. The method
comprises the steps of determining, at one or more frequencies, one or more characteristics
of the wireline associated with each of the one or more frequencies, and adjusting
the bitrate based on the determined one or more characteristics. One advantage of
this method is that is allows for the bitrate to be adjusted to the characteristics
of the wireline and consequently adapt the performance of the communication system
to a desired level of speed and reliability.
[0011] In one embodiment, the method further comprises the step of estimating, from the
one or more characteristics, a wireline frequency response function associated with
each of the one or more frequencies. The step of adjusting the bitrate is further
based on the estimated wireline frequency response function. By estimating a wireline
frequency response function, it is possible to more accurately adjust the bitrate,
and the system design will require less design margin further increasing reliability
and speed in combination with the potential to reduce cost.
[0012] In a further embodiment of the method comprising the step of estimating, the step
of adjusting comprises comparing the estimated wireline frequency response function
with a first threshold and a second threshold. If the estimated wireline frequency
response function is above the first threshold, increase the bitrate, and if it is
below the second threshold, decrease the bitrate. One benefit of having these limits
is that the bitrate may be controlled in any number of steps.
[0013] In yet another embodiment of the method comprising the step of estimating, the step
of adjusting comprises comparing each of the values of the estimated wireline frequency
response function with a third threshold. For each value below the third threshold,
bar the frequencies being associated with such values from use. This has the advantage
that it is possible to avoid using bad frequencies that may reduce the system performance.
[0014] In one embodiment, the one or more characteristics of the wireline comprise a loss
of characteristic. This has at least the benefit of allowing the adjustment of the
bitrate as a function of the loss of the wireline.
[0015] One embodiment of the method comprises the step of determining transmitting and/or
receiving at least one single tone characterisation signal. In doing this, it is possible
to dynamically evaluate the characteristics of the wireline.
[0016] In a further embodiment with the single tone characterisation signal, more than one
single tone characterisation signal is sent, each single tone characterisation signal
having different frequency and/or amplitude. Using more than one single tone characterisation
signal enables the characterisation of the wireline across a number of different frequencies
and/or amplitudes.
[0017] The method is in one embodiment presented with the step of determining comprising
receiving one or more single tone characterisation signal(s). In this embodiment,
the step of estimating comprises comparing the one or more received single tone characterisation
signals to a reference characterisation signal. Using more than one single tone characterisation
signal enables the characterisation of the wireline across a number of different frequencies
and/or amplitudes and the comparison to a reference enables evaluation of wireline
effect on the single tone characterisation signal.
[0018] In an additional embodiment, the one or more single tone characterisation signals
are more than one single tone characterisation signal. The single tone characterisation
signal spaced in frequency between 1Hz and 10Mhz, preferably between 10Hz and 1MHz.
One benefit of characterising the wireline across a bandwidth is that higher bitrates
may be used since frequency response across the bandwidth is estimated.
[0019] One embodiment presents the method as comprising, after the step of estimating, a
step of shaping the signal. The step of shaping comprises calculating and applying
one or more shaping parameters. One benefit of shaping the signal is that a received
shaped signal will have substantially the same behaviour as the signal sent before
it was shaped for the shaped parameters.
[0020] Further, in one embodiment, the method is initiated by the detection a characterisation
trigger. One benefit is that this enables the restarting and rerunning of the process
responsively to the characterisation trigger.
[0021] In another embodiment with the characterisation trigger, the characterisation trigger
comprises the detection of start up of the wireline transceiver. One benefit of this
embodiment is that it ensures a characterised wireline and desired bitrate at each
start up.
[0022] In one embodiment with the characterisation trigger, the characterisation trigger
comprises detecting a change in one or more environmental parameters. This is beneficial
since it allows automatic rerunning of the method on changes in environmental parameters.
[0023] In a further embodiment with the environmental parameters, the one or more environmental
parameters comprise(s) any or all of temperature, acidic concentration, air pressure,
humidity and cable changes. This enables adaptive and automatic adjustment of the
bitrate as the environmental conditions change.
[0024] In one aspect, a downhole data communication system is presented comprising at least
one communication equipment configured to perform the method according to any embodiment
of the method.
[0025] In yet another aspect, a communication equipment configured to be arranged to perform
the method according to any embodiments of the method is presented.
[0026] The invention and its many advantages will be described in more detail below with
reference to the accompanying schematic drawings, which for the purpose of illustration
show some non-limiting embodiments and in which:
Embodiments of the invention will be described in the following; references being
made to the appended diagrammatical drawings which illustrate non-limiting examples
of how the inventive concept can be reduced into practice.
Fig. 1a shows a partly cross-sectional view of a downhole system having a downhole
tool,
Fig. 1b is a schematic view of a downhole system including uphole/surface equipment,
Fig. 2 is a schematic view of a downhole communication system according to an embodiment,
Fig. 3 is a schematic view of a communication equipment of a downhole communication
system according to an embodiment,
Figs. 4a-c are diagrams showing signals before and after being subjected to a transfer
function according to different embodiments,
Fig. 5 is a diagram showing a data signal and a corresponding distorted signal, according
to an embodiment,
Figs. 6a-b are diagrams showing single tone characterisation signals according to
some embodiments,
Figs. 7a-d are diagrams showing how a gain curve can be used with a data signal, and
Fig. 8a-b are schematic views of a method according to some embodiments.
[0027] Hereinafter, certain embodiments will be described more fully with reference to the
accompanying drawings. The invention may, however, be embodied in many different forms
and should not be construed as limited to the embodiments set forth herein; rather,
these embodiments are provided by way of example so that this disclosure will be thorough
and complete, and will fully convey the scope of the invention, such as it is defined
in the appended claims, to those skilled in the art.
[0028] The word symbol is used to describe any communications symbol comprising one or more
bits. In e.g. a system using BPSK or 2-GFSK modulation, one symbol would equal one
bit. In for instance a system using QPSK, one symbol equals two bits and so on. This
means that symbol and bit may be used interchangeably with associated terms such as
symbol-rate and bitrate.
[0029] Fig. 1 shows a downhole system 100 comprising a downhole tool 110 being inserted
into a well tubular structure 120. The well tubular structure 120 is arranged for
producing hydrocarbon-containing fluid from a reservoir 130. The downhole system 100
comprises one or more sensors 140 that may be placed both outside the well tubular
structure 120, or be comprised in the downhole tool 110. The downhole tool 110 is
attached to a wireline 150 that comprises cables for communication, power cables,
fastening cables etc.
[0030] The downhole tool 110 is provided with a wireline communication equipment 210 to
form part of a downhole communications system 200, as will be explained in the following
description.
[0031] Fig. 1b reveals, schematically (and not to scale), a downhole operation system for
operating the downhole tool 110. The wireline 150 is attached to the downhole tool
110 and runs to a lowering means 170 located on a rig or vessel 160. The wireline
150 is arranged such that it enables communication and control between a surface data
acquisition system 180 and the downhole tool 110. Typically, the wireline 150 will
be provided from a spool and will unspool as the downhole tool 110 is lowered into
the well tubular structure 120 and re-spooled as it is raised.
[0032] In Fig. 2, a downhole communication system 200 is shown. The downhole communication
system 200 comprises at least one wireline communication equipment 210 in communication
over a wireline 150. For clarification, normal use would entail at least two wireline
communication equipment 210, one comprised in or at the downhole tool 110, and the
other comprised in e.g. the surface data acquisition system 180. However, in a start-up
and during characterisation phases, the system 200 may run with only one wireline
communication equipment 210, this will be further elaborated in the following sections.
The wireline 150 is not ideal and will consequently distort a data signal f(t) sent
over the wireline 150. Distortion may occur e.g. due to parasitic inductance and/or
capacitance of the individual cables comprised in the wireline 150. Distortion may
further occur due to capacitive or inductive loading between the individual cables
comprised in the wireline 150. This will be explained in more detail further on. As
explained with reference to Fig. 1b, the wireline 150 is typically provided on a spool
making it equivalent of a large coil. This means that the inductive effects, i.e.
high frequency loss, will be most significant when the cable is spooled. This is considered
the worst case, from an inductance standpoint, since the wireline 150 only gets better
as it is unspooled.
[0033] Looking at Fig. 3, a schematic view of one example of a wireline communication equipment
210 for downhole wireline communication is shown. The wireline communication equipment
210 comprises a wireline transceiver 320 and a controller 310. The wireline communication
equipment 210 is connected to the at least part of the wireline 150. The controller
310 is adapted to be arranged to control the transceiver 320 such that the wireline
communication equipment 210 may send and receive data packets across the wireline
150. The skilled person is well aware that the schematic view presented in Fig. 3
does not fully convey a working wireline communication equipment 210. Details such
as power supply, memory, various interfaces etc. are left uncommented as they are
well known in the art.
[0034] As mentioned earlier, the wireline 150 is not ideal but will affect the data signals
f(t) transmitted through the wireline 150. This affect can be described with a wireline
transfer function h(t) that describes how the wireline affect the data signal f(t).
[0035] The left hand side of Fig. 4a shows an example of a data signal f(t) transmitted
by the wireline communication equipment 210. The signal f(t) is subjected to the transfer
function h(t), i.e. transmitted through the wireline 150, and a distorted signal h(f(t))
is received by another wireline communication equipment 210. In the example shown
in Fig. 4a, the distortion, i.e. the transfer function h(t) of the wireline 150, is
manifested as low pass filtering and attenuation. A similar example is given in Fig.
4b wherein the distorted signal h(f(t)) is manifested with an oscillating amplitude
indicative of a LC load in the wireline 150. In Fig. 4c, another example of a distorted
signal h(f(t)) is shown, visualising a rise time T
R, a fall time T
F and a time per symbol T
S. From Fig 4c, it can be seen that if the rise time T
R or the fall time T
F becomes a significant part of the time per symbol T
S, the amplitude of the distorted signal h(f(t)) will drop rapidly to a point where
it will not be possible to decode the distorted signal h(f(t)). The possibility to
decode the distorted signal is also dependent on noise in the system, typically Additive
White Gaussian Noise, AWGN. This, among other factors, causes the ability to decode
a distorted signal to be a random function and the term Bit Error Rate, BER, is used
to signal the possibility that a bit is incorrectly decoded. The BER is a function
of noise and Energy per Symbol E
S. As can seen in Fig. 4c, as the amplitude of the distorted signal h(f(t)) drops due
to increased distortion, the energy of the received bit will decrease thus increasing
the BER. In Fig 4c, the energy of the distorted signal h(f(t)) is integrated function
h(f(t)) over the time per symbol T
S, i.e. the area of the distorted signal h(f(t)).
[0036] The example given with reference to Fig. 4c is just one example used to simplify
the understanding of how distortion affects the BER. The example was given with a
distortion, i.e. transfer function of the wireline 150, with a low pass characteristic,
but other types of distortion will have similar effect on BER. After inventive and
insightful thinking by the inventors, it was concluded that increasing the time per
symbol T
S as a function of the distortion would greatly improve the reliability of the downhole
communications system 200. This is equivalent with decreasing the bitrate since the
symbol-rate is the inverse of the time per symbol T
S.
[0037] In Fig. 5, the data signal f(t) is shown in the same diagram as the distorted signal
h(f(t)). From the data given in Fig. 5, it would be possible to generate a compensation
as the difference between the data signal f(t) and the known received distorted signal
h(f(t)). However, doing these compensations in the time domain is very costly in terms
of processing power and a better approach is to do compensation in the frequency domain.
Also, using a chirp function or even a step function, as illustrated by the data signal
f(t) in Fig. 5, would require rather heavy computational resources and a more cost
effective and robust, although slightly more time consuming, method is to send single
tone characterisation signals 610.
[0038] The single tone characterisation signal 610 is a signal of only one frequency. Fig
6a depicts a single tone characterisation signal 610 transmitted at a first reference
frequency f
0 with a transmit power P
Tx. The wireline transfer function h(t) can be Fouirer transformed into a wireline frequency
response function H(f). When the characterisation signal 610 is subjected to the frequency
response function H(f) of the wireline 150, it will change the amplitude, which means
that the attenuation of the wireline 150 at the first frequency f
0 may be described according to Eqn. 1.
[0039] The same scenario applies if several characterisation signals are used as depicted
in Fig. 6b. In Fig 6b, single tone characterisation signals 610 are transmitted at
the frequencies f
0 to f
n-1. The frequency response may be different at each of the frequencies as shown on the
right side of Fig. 6b, where the single tone characterisation signals 610 are shown
after being subjected to the wireline 150 frequency response function H(f). The attenuation
A
j at each of the transmitted frequencies f
0 to f
n-1 may be calculated according to Eqn. 2.
[0040] By characterising the wireline 150 by single tone characterisation signals 610 and
calculating the corresponding attenuation A
j, it is possible to compensate for the attenuation of the wireline 150. In practice,
this may be achieved by increasing the amplitude of the signal to be transmitted with
the corresponding attenuation value A
j.
[0041] Looking to Fig. 7a to 7d, an example of how a gain curve, associated with the wireline
frequency response function H(f) shown in Fig. 6b, can be used will be explained.
From Fig. 6b and using Eqn. 2, the attenuation A
j at each of the single tone characterisation signals 610 can be calculated. Starting
with n single tone characterisation signals 610 of the same transmit amplitude P
Tx, a gain function G(f) can be estimated. Such a gain function G(f) is shown in Fig.
7b. This gain function can be implemented as a digital filter and the filter can be
applied to the single tone characterisation signals 610 in Fig. 7a. This will result
in the signal of Fig. 7c. Passing the single tone characterisation signals 610 to
the frequency response function H(f) that was the basis for the gain function G(f)
will result in a substantially level response at a power of P
Rx, as shown in Fig. 7d. Note that the gain in Fig. 7b is marked with a peak at 0dB,
this is of course just an example to simplify the explanation. The gain may be any
number, positive or negative and the skilled person will know how to dimension the
gain to optimise transmitter linearity and minimise noise.
[0042] The transmission loss L
T of a wireline may be characterised as the transmitted power P
Tx minus the received power P
Rx. The transmission loss L
T may, as has been explained together with the single tone characterisation signals
610, be used frequency dependent. The wireline transceivers 320 used in the downhole
communications system 200 typically have a limited dynamic range. The dynamic range
is characterised by the minimum received power P
Rx:min necessary to, with sufficiently low BER, receive, demodulate and decode data, this
is called the sensitivity. Analogously, the transmit part of the transceiver has a
maximum output power P
Tx:max at which it, with e.g. sufficient linearity and spectral efficiency, transmits data.
There are corresponding limits in maximum received power P
Rx:max and minimum transmitted power P
Tx:min and their impact can be clearly derived from the reasoning of the other levels. The
specified power may be different depending on what modulation and modulation speed
is used. For instance, the minimum received power P
Rx:min necessary for successful decoding is lower for e.g. GFSK than for 16QAM. As has been
explained in previous sections, a lower symbol-rate will increase the energy per symbol
E
S and reduce the minimum received power P
Rx:min. The maximum dynamic range of the downhole communication system 200 is calculated
as P
Tx:max - P
Rx:min.
[0043] The transmitting wireline transceiver 320 of the downhole communication system 200
is naturally aware of what modulation and bitrate (and consequently symbol-rate) to
use. Further to this, the dynamic range of the system is known and from this, the
maximum allowable compensation or shaping of the transmitted signal can be estimated.
If the frequency response function H(f) requires a compensation outside of the dynamic
range of the downhole communication system, the bitrate may be decreased and/or the
modulation changed.
[0044] With reference to Fig. 8a and Fig. 8b, a method 800 performed by a wireline communication
equipment 210 in a downhole communication system 200 will be explained. The method
comprises the steps of determining 810 the characteristics of a wireline 150. Based
on these characteristics, a wireline frequency response function H(f) is estimated
and this, and/or the characteristics of the wireline 150, is used to adjust 830 the
bitrate such that the highest speed is achieved with required reliability.
[0045] The step of determining 810 may be done in many different ways and the following
section will give an overview of how the step may be performed. The order in which
things are done, and which device is configured to do what may be varied and the skilled
person understands that such modifications of the description are well within the
scope of the disclosure.
[0046] In one embodiment of the method 800, the step of determining 810 comprises transmitting
at least one single tone characterisation signal 610 with a transmit power PTx configured
such that it is possible for a receiving wireline communication equipment 210 to estimate
e.g. the attenuation of the wireline 150 and/or other wireline 150 characteristics
from the received single tone characterisation signal 610.
[0047] It should be noted that the receiving and the transmitting wireline communication
equipment 210 may be one and the same. This may be done by having the wireline 150
comprise different signals paths for transmit data and receive data and connect these
paths together in one end of the wireline 150 and connect the other end to the wireline
communication equipment 210. By having the transceiver 320 of the wireline communication
equipment 210 simultaneously transmitting and receiving the single tone characterisation
signal 610, it is possible to determine characteristics of the wireline 150 with one
single wireline communication equipment 210. These characteristics may comprise e.g.
loss and phase shift of the wireline 150. The phase shift may be determined by comparing
the phase of the received single tone characterisation signal 610 with the transmitted
single tone characterisation signal 610. The loss is, as described earlier, achieved
by comparing amplitudes of received and transmitted single tone characterisation signal
610. It goes without saying that the characterisation using a single wireline communication
equipment 210 will result in double the phase shift and loss since the wireline 150
is characterised both in transmit and receive at the same time and consequently this
needs to be compensated. It should be pointed out that phase shift along a wireline
150 may occur both directly as a function of the electrical length of the wireline,
i.e. the length as a factor of the wavelength λ at the frequency of the single tone
characterisation signal, and also due to parasitic effects and resonances occurring
along the wireline 150. If the wireline 150, in the single wireline communication
equipment 210, is arranged such that the total phase shift of the signal round trip
is more than 360° it will not be possible to differentiate e.g. 380° phase shift from
20° phase shift which would result in different phase shift characteristics of 190°
and 10° respectively, i.e. a possibly erroneous phase shift of 180°. This phase shift
error is not relevant for most types of communication but there are modulations where
it is important to have all signals in phase e.g. adjacent subcarriers in OFDM where,
if high bandwidth channels are used, there may phase shifts on certain channels that
needs to be accurately determined. This potential problem may be solved by transmitting
the single tone characterisation signal 610 at low frequencies stepping the frequency
of the single tone characterisation signal 610 while keeping track of the accumulation
of the phase shift to determine when a full 360° occurs and compensate accordingly.
A similar solution is presented below when dual wireline communication equipment 210
is used to determine the wireline characteristics.
[0048] If characterisation if done with a pair of wireline communication equipment 210,
the receiving wireline communication equipment 210 will know the reference power used
to transmit the single tone characterisation signal 610 and will thus be able to determine
the loss characteristics of the wireline 150 at the frequency of the single tone characterisation
signal 610. The phase shift may be determined in a number of ways, one way to determine
the relative frequency shift across a frequency range is to sweep the frequency of
the single tone characterisation signal 610 at a defined pace and measure the frequency
and phase of the received signal to. Any difference in phase, once the pace of the
frequency sweep has been compensated for, is due to phase shift in the wireline 150.
In a dual path wireline 150, i.e. a wireline 150 comprising separate transmit and
receive paths, the determining of wireline 150 characteristics may be done simultaneously
in both transmit and receive. If a wireline with a single path is used, it may be
possible to only characterise the communication in one direction and share the wireline
150 characteristics with the other wireline communication equipment 210. It may also,
in any scenario, be possible to only have one wireline communication equipment 210
knowing the wireline 150 characteristics; this may be the case if, e.g. data in one
direction is comparably slow and neither speed nor reliability is a factor in that
direction.
[0049] Sending a series of single tone characterisation signals 610 on different frequencies
will make it possible to determine the characteristics of the wireline on multiple
frequencies. If a multi-carrier communications protocol, such as e.g. OFDM or any
FDM system for that matter, is used it may be beneficial to characterise the wireline
on the frequencies of all, or at least a subset of the carriers to be used.
[0050] In another embodiment of the method 800 in Fig. 8a and Fig. 8b, the step of determining
810 comprises sending at least two single tone characterisation signals 610 at at
least two different frequencies. In a further embodiment, the downhole communication
system is a channelised system comprising at least two carriers at at least two different
frequencies, and the step of determining 810 comprises sending a single tone characterisation
signal 610 on at least two of the at least two different frequencies.
[0051] On the topic of determining phase and amplitude characteristics of the wireline,
it should be mentioned that in the scenario with a pair of wireline communication,
the characteristics will not only comprise the wireline 150 but also the associated
path of the wireline transceiver 320 used when determining the wireline 150 characteristics.
This means that amplitude shifts and phase shifts associated with the transmit and
receive paths of the wireline transceiver 320 may also be characterised with regards
to phase and amplitude. With this knowledge, it may be considered to use different
power levels as well as different frequencies for the single tone characterisation
signals 610. Such a configuration with different power levels will enable further
shaping of the transmitted signal such that non-linarites of the signal chain are
compensated for.
[0052] In one embodiment of the method 800, the step of determining 810 comprises determining
one or more wireline characterisation parameters. In a further embodiment, the step
of determining 810 further comprises sending at least two single tone characterisation
signals 610 with at least two different power levels.
[0053] The method 800 may be initiated for several reasons, and depending on arrangement
and configuration a characterisation trigger of the method may be different. In, for
instance, one embodiment, the method 800 is initiated at the installation of a wireline
150 to a downhole tool 110, e.g. when presence of a wireline is detected by the wireline
communication equipment 210. Depending on e.g. if there is a connection between the
receive path and the transmit path in a wireline comprising separate paths for transmit
and receive, the determining step 810 associated with one single wireline communication
equipment 210 may be initiated. If not, the determining step 810 associated with dual
wireline communication equipment 210 may be attempted by a first wireline communication
equipment 210 detecting the presence of the wireline, if no suitable acknowledgement
is received from a second wireline communication equipment 210 it is likely that only
the first wireline communication equipment 210 is connected and the determining step
810 has to wait until the second wireline communication equipment 210 is connected.
Once the second wireline communication equipment 210 detects the presence of the wireline
150, it may attempt the determining step 810 and the first wireline communication
equipment 210 will acknowledge in a suitable manner.
[0054] In another embodiment, that may very well be additional to any other embodiment,
the determining step 810 is initiated at the start-up of the wireline communication
equipment 210.
[0055] Additionally, in another embodiment, the determining step 810 is initiated upon detection
of a change in one or more environmental parameters. These environmental parameters
may be any measurable parameter e.g. acidic concentration, air pressure, humidity,
temperature etc. It may be that many of these parameters are not directly correlated
to the frequency response H(f) of the wireline 150, but they may very well affect
the performance of the wireline transceiver 320. Take temperature as an example, where
a temperature shift of 20° has little or no effect on passive cabling but may greatly
impact e.g. the linearity and noise of the wireline transceiver 320.
[0056] In a further embodiment, the determining step 810 may be initiated by the detection
of an increase in bit error rate of the received signal and/or a decrease of the signal
strength of the received signal.
[0057] In yet another embodiment, the determining step 810 may be started at configurable
time intervals and/or manually by control commands communicated to the wireline communication
equipment 210.
[0058] With reference to Fig. 8b, having determined the wireline characteristics the wireline
transfer function H(f) may be estimated 820. The wireline characteristics may comprise
one or more attenuations A
j and/or one or more phase shifts each associated with one or more frequencies and/or
transmit amplitude P
Tx. The wireline transfer function H(f) may, in any embodiment, be one single, or a
series of discrete characteristics rather than a continuous function. From the estimated
wireline transfer function H(f), an inverse transfer function H
-1(f) may be estimated simply by e.g. changing positive wireline 150 characteristic
values to negative values and/or calculating the inverse wireline 150 characteristic
factors. Note that the estimated wireline characteristics may be separate for both
e.g. different frequencies and power levels but also for e.g. different environmental
conditions, further detailed below. Each of the different series or value of characteristic
of the wireline 150 may be stored and accessed as the appropriate situation arises.
For instance, if wireline characteristics are estimated for a number of environmental
situations, a change in environmental conditions may not have to trigger a restart
of the method 800 but could simply result in the applicable wireline characterisation
being retrieved from storage.
[0059] In one embodiment of the method 800 in Fig. 8b, the estimate step 820 comprises estimating
one or more wireline 150 attenuation values. In another embodiment, estimating 820
comprises estimating one or more wireline 150 phase shift values and in yet another
embodiment the step of estimating 820 comprises estimating both phase shift and attenuation
values of the wireline 150. In a further embodiment, the step of estimating is done
for different power levels of the single tone characterisation signal 610.
[0060] In Fig. 8a and Fig. 8b, the step of adjusting 830 comprises changing, if necessary,
the bitrate/symbol-rate of transmissions. The wireline characteristics are known from
the step of determining 820 and these are used to find a suitable bitrate. If the
wireline characteristics comprise loss characteristics, the loss may be used together
with the known system factors such as the sensitivity and maximum transmit power of
the wireline transceiver 320 at different modulation parameters, e.g. type, speed
etc. If the loss characteristics is higher than the link budget allows, i.e. the sensitivity
subtracted from the maximum transmit power, the bitrate may be reduced. At the reduced
bitrate, the receiver will have a lower sensitivity (lower sensitivity means more
sensitive, i.e. better) and the link budget may hold with the determined loss characteristics.
It may be that there is head room in the link budget, and in that case the bitrate
may be increased without significant loss in reliability. If the bitrate is at a maximum
speed and the link budget still has significant head room, the transmit power of the
wireline transceiver 320 may be reduced. It may be that each of the supported bitrates
and modulations has a first threshold for the estimated wireline transfer function
H(f) such that if the estimated wireline transfer function is above the first threshold,
the bitrate may be increased. Further to this, each of the supported bitrates and
modulations may have a second threshold for the estimated wireline transfer function
H(f) such that if the estimated wireline transfer function is below the second threshold,
the bitrate may be decreased.
[0061] In FDM systems, or any system utilising carriers on different frequencies, where
wireline characterisation has revealed one or more carriers and/or channels to be
too poor to use, these carriers may be omitted or barred from communication. The decision
to remove a frequency may be based on a third threshold that is below or the same
as the second threshold as introduced above. It may be that there are transmissions
of different bitrates at different channels depending on the estimated wireline transfer
function H(f), i.e. all channels do not necessarily have to have the same bitrate
and/or modulation. Alternatively, if flat bitrate across the frequency band is desired,
the carrier exhibiting the worst bitrate may be used to set the bitrate for all carriers
or, the worst channel may be removed (omitted or barred) as mentioned above, and the
bitrate of the other carriers may be raised.
[0062] The discussion above regarding limits and their relation to change of bitrate is
of exemplary nature. There may be any number of limits, thresholds or intervals with
or without hysteresis relating to the estimated wireline frequency response function
(H(f)). Each interval may be associated with a particular bitrate and/or modulation.
There may be different sets of limits or intervals associated e.g. with different
environmental conditions or power levels. All mentioned limits, thresholds and intervals
may be configurable limits, thresholds or intervals. It is of course possible to make
each limit, threshold or interval individually configurable, i.e. one threshold may
be configurable and another threshold may be fixed.
[0063] In one embodiment of the method 800 in Fig. 8a and Fig. 8b, the adjusting 830 step
comprises comparing the estimated wireline frequency response function (H(f)) to a
first limit and a second limit and if the estimated wireline frequency response function
(H(f)) is above the first limit, increasing the bitrate and if it is below the second
limit, decreasing the bitrate. In further embodiments, if any value of the estimated
wireline frequency response function (H(f)) generates a response that is below a third
limit, bar the frequencies being associated with such values from use
[0064] The inverse transfer function H
-1(f) may be used as a shaping function and the corresponding discrete values may be
used as shaping parameters. An optional shaping step 840 may be comprised in the method
800 of Fig. 8b. The shaping step 840 is performed after the step of estimating 820
and may be done either before or after the step of adjusting 830. An example using
amplitude shaping will be used to explain this step and the skilled reader understands
that a similar approach can be used when applying phase pre-distortion. The estimated
wireline frequency response function (H(f)) comprises, in this example, losses at
frequencies. In order to have, from a power perspective, a substantially flat transmission
across all relevant frequencies used in the downhole communication system 200, the
frequency resulting in the highest loss from the estimated wireline frequency response
function (H(f)) is identified. This frequency will be the baseline, the 0dB, and the
losses at the other frequencies are relative to this frequency. These losses will
all be below the baseline since the baseline was the maximum. The relative losses
calculated are used to attenuate all the channels prior to transmission thus enabling
a, power wise, substantially flat transmission across all frequencies. Shaping is
very beneficial on e.g. communication system using sub-carriers where one burst comprises
several sub-carriers. In many of these applications, there is a limit as to how much
the power is allowed to vary across the burst. In a similar manner, shaping may be
used within the same channel to have a linear power response all the way to saturation.
This is beneficial in systems with an amplitude component in the modulation.
[0065] In one embodiment of the method 800 of Fig. 8b, the method comprises the step of
applying shaping 840 after the step of estimating 820.
[0066] It should be mentioned that the bitrate adaptation described above may very well
be used with in combination with other signalling protocols where for instance low
speed control channels are utilised. These control channels may be used to e.g. communicate
the start of a determining step 810, changes in environment, characterisation data
of the wireline 150, bitrates at different channels/frequencies etc.
[0067] Many of the embodiments have been described as utilising one or more single tone
characterisation signals 610. The skilled person understands that these signals may
be broadband signals of a certain bandwidth and that single tone does not necessarily
mean one absolute tone as noise by e.g. oscillators and phase locked loops will increase
the bandwidth of the signal. The single tone characterisation signal 610 may be understood
to mean any suitable characterisation signal and in many cases a single tone is the
most cost effective solution.
1. A method for downhole data communication in a downhole communications system (200)
performed by a communication equipment (210) configured to be arranged to transmit
and receive signals via an associated wireline (150) at a bitrate, the method comprising
the steps of:
determining (810), at one or more frequencies, one or more characteristics of the
wireline (150) associated with each of the one or more frequencies, and
adjusting (830) the bitrate based on the determined one or more characteristics.
2. The method according to claim 1, further comprising the step of estimating (820),
from the one or more characteristics, a wireline frequency response function (H(f))
associated with each of the one or more frequencies, and wherein the step of adjusting
(830) the bitrate is further based on the estimated wireline frequency response function
(H(f)).
3. The method according to claim 2, wherein the step of adjusting (830) comprises comparing
the estimated wireline frequency response function (H(f)) with a first threshold and
a second threshold and if the estimated wireline frequency response function is:
above the first threshold, increase the bitrate,
below the second threshold, decrease the bitrate.
4. The method according to claim 2 or 3, wherein the step of adjusting (830) comprises
comparing each of the values of the estimated wireline frequency response function
(H(f)) with a third threshold and for each value below the third threshold, bar the
associated frequency from use.
5. The method according to any of the preceding claims, wherein the one or more characteristics
of the wireline (150) comprises a loss characteristic.
6. The method according to any of the preceding claims, wherein the step of determining
(810) comprises transmitting and/or receiving at least one single tone characterisation
signal (610).
7. The method according to claim 6, wherein more than one single tone characterisation
signal (610) is sent, each single tone characterisation signal (610) having different
frequency and/or amplitude.
8. The method according to any of the claims 2 to 7, wherein the step of determining
(810) comprises receiving one or more single tone characterisation signals (610) and
the step of estimating (820) comprises comparing the one or more received single tone
characterization signals (610) to a reference characterisation signal.
9. The method according to any of claims 6 to 8, wherein the one or more single tone
characterisation signals (610) are more than one single tone characterisation signal
(610) and the single tone characterisation signal (610) spaced in frequency between
1 Hz and 10 Mhz, preferably between 10 Hz and 1 MHz.
10. The method according to any of the preceding claims, further comprising, after the
step of estimating (820), a step of shaping (840) the signal, wherein the step of
shaping (840) comprises calculating and applying one or more shaping parameters.
11. The method according to any of the preceding claims, wherein the method (800) is initiated
by the detection a characterisation trigger.
12. The method according to claim 11, wherein the characterisation trigger comprises the
detection of start-up of the wireline transceiver (320).
13. The method according to claim 11 or 12, wherein the characterisation trigger comprises
detecting a change in one or more environmental parameters.
14. The method according to claim 13, wherein the one or more environmental parameters
comprise(s) any or all of temperature, acidic concentration, air pressure, humidity
and cable changes.
15. A downhole data communications system (200), comprising at least one communication
equipment (210) configured to perform the method according to any one of claims 1
to 14.
16. A communication equipment (210) configured to be arranged to perform the method according
to any one of claims 1 to 14.