Abstract: The invention pertains to methods, apparatus, and systems for controlling the sub-carrier frequencies in an optical frequency division multiplex communication system by using a metric available from the sub-carrier modem's digital receive block as an indicator of cross-talk between sub-carriers and adjusting the sub-carrier frequency separation as a function of that metric.

Abstract: The invention pertains to methods, apparatus, and systems for controlling the sub-carrier frequencies in an optical frequency division multiplex communication system by using a metric available from the sub-carrier modem's digital receive block as an indicator of cross-talk between sub-carriers and adjusting the sub-carrier frequency separation as a function of that metric.

Abstract: The invention pertains to methods, apparatus, and systems for controlling the sub-carrier frequencies in an optical frequency division multiplex communication system by using a metric available from the sub-carrier modem's digital receive block as an indicator of cross-talk between sub-carriers and adjusting the sub-carrier frequency separation as a function of that metric.

Abstract: The invention pertains to methods, apparatus, and systems for controlling the sub-carrier frequencies in an optical frequency division multiplex communication system by using a metric available from the sub-carrier modem's digital receive block as an indicator of cross-talk between sub-carriers and adjusting the sub-carrier frequency separation as a function of that metric.

Abstract: A method and system for initializing a coherent optical receiver. Upon detection of an optical signal, a multi-bit digital sample stream of the optical signal is digitally processed to initialize each one of a plurality of adaptive control blocks of the coherent optical receiver. The adaptive control blocks include at least a dispersion compensation block and a clock recovery block. The dispersion compensation block is initialized before initializing the clock recovery block.

Abstract: In method of controlling an optical amplifier dynamically adapts to both configuration and performance changes of a communications system. An error vector is calculated to indicate a difference between respective detected values and target values of a parameter of a light beam downstream of the optical amplifier. A sensitivity matrix indicative of a sensitivity of the detected parameter value to incremental changes in a control variable of the optical amplifier is calculated. A predicted optimum value of the control variable is then calculated using the error vector and the sensitivity matrix. Calculation of the predicted optimum control variable value can be iterative, with the sensitivity matrix calculated either during each iteration, or at the beginning of each optimization run. As a result, optimization of the amplifier control variables is performed based on a sensitivity matrix that accurately reflects the performance of the amplifier.

Abstract: A blind convergence process for an adaptive decision feedback equalizer that includes an quadrature amplitude modulation (QAM) slicer having a index n, a forward filter defined by a plurality of forward tap coefficients and a feedback filter defined by a plurality of feedback tap coefficients. The blind convergence process includes initializing the forward tap coefficients of the forward filter and the feedback tap coefficients of the feedback filters with predetermined values. The slicer operates in two modes: a clustering mode and a decision directed mode. In the clustering mode the following steps are performed: (1) update the forward tap coefficients of the forward filter using an error estimate (e.g. Sato error), and (2) update the feedback tap coefficients of the feedback filter using an error estimate (e.g. Sato error).

Abstract: An automatic gain control method and apparatus for modem receivers. The automatic gain control circuit includes a programmable loop gain for scaling a digital signal Y to a first prescribed level during a first mode of operation and to a second prescribed level during a second mode of operation; and filters and converters for converting the scaled signal Y into an analog gain control signal for input to the analog AGC. The gain control circuit and method of operation provides control over the parameters of the programmable loop gain such that during start-up initialization the signal Y is scaled to the first prescribed value and during steady state operation the signal Y is scaled to the second prescribed value. This is accomplished by changing the gain parameter of the programmable loop gain.

Abstract: A system of energy detection for detecting a valid signal in a modem to enable power savings in receiver and transmitter circuits. The method of energy detection includes the steps of: (a) processing a prescribed number of signal samples (e.g. typically between 512 and 2048); (b) determining an average energy level for the prescribed number of signal samples; (c) determining a gain of an amplifier based on the processed signal samples; (d) comparing the average energy level with a prescribed energy level threshold; and (e) comparing the gain of the amplifier with a prescribed amplifier gain threshold. If the average energy level is greater than the prescribed energy level threshold and if the gain of the amplifier is less than the prescribed amplifier gain threshold then a valid signal is declared and the transmitter of the modem is activated.

Abstract: A method and apparatus for timing recovery in modem receivers. The timing recovery circuit includes a voltage controlled oscillator for controlling the voltage controlled sampling frequency of an analog-to-digital converter. The oscillator generates a timing clock that is dependent on an average phase error signal calculated from Nyquist signals of the input signal. A phase detector circuit is used for generating an instantaneous phase error signal of the in-phase and quadrature-phase signals. A digital loop filter receives the instantaneous phase error signal over time to generate the average phase error signal. The average phase error signal is conditioned further (after conversion to analog) by an analog loop filter such that the average phase error signal adjusts the timing clock generated by the oscillator. The low pass filter provides control of the acquisition and steady state operations by changing the gain and pole parameters of the filter.

Abstract: A method of timing recovery convergence monitoring in modems using an average phase error signal. The method involves continuously tracking the peak of the average phase error. The average phase error is compared to a dynamic threshold (i.e. a threshold that can change over time based on changes to the detected peak at a given time). Convergence is declared when the average phase error remains less than the threshold over a given length of time (i.e. after processing a prescribed number of consecutive samples).

Abstract: A method of detecting misconvergence in an equalizer having a quadrature amplitude modulation (QAM) slicer having an index n. The method includes the steps of initializing a plurality of signal counters then processing random symbols (for example between 500 and 20000 symbols) through the equalizer where the symbols are quantized into an appropriate constellation point in an n-QAM constellation map, where each signal counter corresponds to a respective one of the constellation points in the constellation map. The signal counters are incremented when one of the symbols is quantized to the corresponding constellation point. After the signals have been processed into the constellation map the number of non-zero counters are detected. If the number of non-zero counters is less than a prescribed number (for example between approximately 0.5n and approximately 0.

Abstract: A method and apparatus for filtering interference (e.g. transmitter feed-through and RF) in a modem receiver. The method of reducing transmitter feed-through (Tx-FT) interference at a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator includes attenuating the Tx-FT interference and passing the Rx signal at the carrier frequency using an analog filter. The output signal from the analog filter is converted to a digital representation by a analog-to-digital (ADC) converter. A digital band pass filter is used to pass the Rx signal at the carrier frequency and to attenuate the Tx-FT interference further. A mixer is then used to shift the carrier frequency of the spectrum of the Rx signal to an intermediate frequency. The mixer introduces at least one spectral transmitter feed-through image Tx-FT(I) and a receive signal image Rx(I).

Abstract: A method of estimating signal-to-noise ratio at an output of an adaptive decision feedback equalizer receiving an input signal. The method of SNR estimation includes (a) calculating an average output error of the equalizer; and (b) estimating the signal-to-noise ratio (SNRest=kn−20 log10(Eavg)) where kn is a constant based on a QAM index n and Eavg is the average output error of the equalizer. The estimated SNR can be used to detect proper equalizer convergence by comparing the estimated SNR to an SNR threshold (e.g. lowest SNR for a 4QAM signal), wherein the equalizer is not properly converged when the estimated SNR is lower than the SNR threshold. The estimated SNR can also be used to adjust the operating QAM index of the equalizer by comparing the estimated SNR to an ideal SNR based on an objective bit error rate for a prescribed QAM index, and reconverging the equalizer at a QAM index lower than the prescribed QAM index when the estimated SNR is lower than the ideal SNR.

Abstract: A blind convergence process for an adaptive decision feedback equalizer having an quadrature amplitude modulation (QAM) slicer, a forward filter defined by a plurality of forward tap coefficients and a feedback filter defined by a plurality of feedback tap coefficients. The blind convergence process includes the step of initializing the forward tap coefficients of the forward filter and the feedback tap coefficients of the feedback filters with predetermined values. The QAM slicer operates in two modes: a clustering mode and a decision directed mode. The clustering mode includes the step of updating only the forward tap coefficients of the forward filter for a prescribed start-up QAM index (such a 4 QAM). The decision directed mode is predefined for set of QAM indexes having values n1, n2, . . .

Abstract: A method of reducing leakage noise introduced to an equalizer in a modem during steady state operation. The equalizer includes forward and feedback filters represented by tap coefficients. The tap coefficients of the filters are updated using least mean squares adaptation equations at a symbol rate during steady state operation. The method of the invention includes the steps of: (a) reducing the absolute value of the updated tap coefficients of the filters at a prescribed rate that is less than the symbol rate. Long term convergence stability is improved and leakage noise is reduced by leaking the filter coefficients less frequently than the symbol rate.

Abstract: The invention provides a method and apparatus for scaling transmitter power to a prescribed level (e.g. at or below the maximum average transmitter power supported by the communication system) for a full range of QAM index levels. In particular, in an n-level quadrature amplitude modulation (QAM) communication system having a digital-to-analog converter and an n-QAM signal having an average transmitter power, for transmission on a communication channel, the method of adjusting the average transmitter power of the signal includes selecting a reference QAM index from a prescribed set of QAM index levels (e.g. 4, 16, 64, 256) where the composite signal is processed with a maximum average transmitter power supported by the communication system. The next step involves generating a power adjustment coefficient for each of the QAM index levels from the prescribed set of QAM index levels based on the reference QAM index level.