Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-04-30
2001-01-16
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S298000, C375S340000
Reexamination Certificate
active
06175599
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to systems and methods for the transmission of digital data over transmission lines. In particular, a well-logging application is disclosed where data gathered by a downhole logging tool is transmitted to the surface.
BACKGROUND OF THE INVENTION
In many domains it is desirable to take measurements of physical phenomena and transmit the digital data acquired over a transmission line. Measuring the characteristics of earth formations is a good example. Measurements of the characteristics of different earth formations traversed by a borehole are generally carried out by lowering into the borehole a “tool” containing various types of sensing instrumentation. The tool is attached to a logging cable which is used both for holding the equipment and as an electrical medium for the transmission of data signals from the tool to a data receiver on the surface.
Most downhole data acquisition systems currently in use process and store the information thus gathered in digital form. A digital signal carrying that information has frequency components ranging from very high to very low (or d.c.) frequencies and is referred to as a baseband signal. Baseband signals cannot normally be transmitted over bandpass channels (i.e., those channels which transmit only a limited range of frequencies) such as logging cable due to pulse-shape and frequency distortion of the signal. Therefore, it is necessary to resort to modulation methods whereby the transmitter modulates a sinusoidal carrier waveform with the baseband signal, the modulated carrier being suitable for transmission over the bandpass channel. The uphole receiver then recovers the baseband signal by demodulation, the modulator-demodulator pair being referred to as a modem.
In order to reduce downtimes, it is typical to simultaneously lower into the borehole several tools in the same combination. The information gathered by the different tools must then be transmitted to the surface either by time or frequency multiplexing. When it is desired to increase the number of tools within a given combination and yet to have the same quantity of data transmitted per tool per unit of time, the data transmission rate must be increased. That rate, however, is limited by both the frequency characteristics of the logging cable as well as environmental constraints on the downhole transmitter.
The logging cable has a relatively narrow useable bandwidth of about 5 kHz to 90 kHz; however, it does have a high signal to noise ratio of about 30 dB. The downhole environment sometimes reaches temperatures of 175° C. These high temperatures restrict the selection of analog and digital components, which eliminates many standard modulation techniques. Thus specialized techniques much be employed for implementing high data rate digital transmission systems in such environments. There is a need, therefore, for a method and apparatus for transmitting digital data at high speeds over a bandpass channel. In particular, there is a need for transmitting log data of high data rates over logging cable.
SUMMARY OF THE INVENTION
The problems outlined above are solved by the method and apparatus for transmitting digital data of the present invention. The present invention is particularly advantageous for transmitting acquired digital information over bandpass channels at high speeds. In the preferred embodiment, the bandpass channel is a oil well-logging cable and the digital information is acquired from the downhole well-logging tools. The present invention uses combined amplitude and phase modulation—referred to as quadrature amplitude modulation (“QAM”)—to achieve high speed data transfer. The present invention further increases the data transfer rate by reducing the processing load during the modulation by advantageously selecting the data sampling rate, the carrier frequency, or the encoding symbol rate.
Broadly speaking, the method of transmitting acquired digital data over a bandpass channel of the present invention includes the steps of generating a carrier signal, mapping the acquired digital data into a series of symbols, modulating the carrier signal with the symbol streams representing the digital data, and converting the modulated carrier signal into an analog transmission waveform for passage over the bandpass channel. During mapping of the digital data (at symbol rate), certain sequences of digital bits are represented by unique symbols where each symbol represents the coordinates of a point in signal space. Preferably each symbol represents the x, y coordinates of a point in signal space and the symbols are output at a sample rate as two symbol coordinate streams—x coordinate and y coordinate.
Functionally, digital samples of the carrier signal (at sample rate) are modulated with the one or more symbol streams to produce a digital sampled waveform. Preferably, the carrier signal comprises two orthogonal sinusoids with the carrier phase being an integer multiple of &pgr;/2 during each sample time. The symbol streams are passed through a digital filter to restrict the bandwidth and provide appropriate pulse shaping prior to modulating the carrier signal. During modulation each symbol stream is multiplied by a digital sample of a respective carrier signal and the two resulting products are combined in phase quadrature to produce the sampled waveform. The resulting quadrature amplitude modulated sampled waveform is converted from a digital sampled waveform into an analog QAM waveform and transmitted over the bandpass channel.
While the use of quadrature amplitude modulation is a significant advance in transmitting acquired data digitally at high speeds (e.g. greater than 350 kilobits per second) over bandpass channels, careful selection of the sample rate, symbol rate, and carrier frequency further enhance the data transmission rate by reducing the computational load during modulation (and demodulation). For example, choosing the sample rate to be four times the frequency of the carrier signal and adjusting the carrier phase to always be an integer of &pgr;/2 during each sample time, results in amplitude samples for each of the orthogonal sinusoids of +1, 0, −1, or 0. During modulation this selection of the sample rate and carrier frequency obviates the need for true multiplication reducing the computational overhead.
Additionally, selecting the sample rate to be an integer multiple of the symbol rate further reduces computational overhead. The sample rate (e.g. 210 kHz) and symbol rate (e.g. 70 kHz) can be made equal by inserting 0 values in between actual symbol values in the x and y coordinate streams (e.g. two 0 values for every actual value). This selection also simplifies the low pass digital filtering of each of the x and y coordinate symbol streams through a transversal-type filter since no multiplications between the zero values and the tap weights, nor additions of these products, need to be performed. In the method of the preferred embodiment digitally acquired logging data is modulated using quadrature amplitude modulation, converted to analog, and transmitted over a bandpass channel such as a logging cable. The sample rate is chosen as 210 kHz, the symbol rate 70 kHz, and the carrier frequency 52.5 kHz to significantly reduce computational overhead. With a bit packing of 6 bits per symbol, high transmission rates are obtained (e.g. greater than 350 kilobits per second).
The modem receiver essentially performs the inverse operation of the transmitter. That is, the analog QAM waveform received over the logging cable is passed through an analog to digital converter. The resulting regenerated sampled waveform is fed to a demodulator where the x and y coordinate streams are multiplied by sampled orthogonal sinusoids (and fed through low pass filters as appropriate) to recreate x and y coordinate symbol streams. Here again, the demodulator takes advantage of the sample rate being an integral multiple of the carrier frequency (preferably four times) and the carrier phase being adjusted to
Lyon David Lewis
Montgomery Michael Andrew
Neeley Jimmy Everett
Poonpol Chanchai
Emmanuel Bayard
Jansson Pehr B.
Maseles Danita J. M.
Pham Chi H.
Schlumberger Technology Corporation
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