Pulse or digital communications – Testing – Phase error or phase jitter
Reexamination Certificate
1998-11-03
2001-06-12
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Testing
Phase error or phase jitter
C375S326000
Reexamination Certificate
active
06246717
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the measurement of transmission system parameters, and more particularly to a measurement test set and method for in-service measurement of carrier phase noise or carrier phase jitter.
Phase noise is a common phase distortion existing in transmitters and receivers of transmission systems. It is caused by phase jitter in the transmitter's and/or the receiver's local oscillators. The existence of phase noise can severely degrade the signal quality of a digitally transmitted RF signal. Typically the distortion present in the received signal are a mixture of linear and nonlinear magnitude errors, linear and nonlinear phase errors, additive noise, and phase noise. To monitor the quality of the transmitted signal and to trouble-shoot a degraded transmission system, accurate measurement of phase noise is very useful. However, the phase noise in the received signal is often combined with above mentioned linear and nonlinear phase errors, phase-induced additive noise errors. Due to the difficulty of separating the various phase errors, phase noise is traditionally measured in an out-of-service mode. An out-of-service mode not only requires removing the carrier's modulation, but also causes the possible loss of the phase noise characteristics caused by the presence of the digital signal as well as other distortions.
In an 8 level Vestigial Side Band (8-VSB) digital modulation system, for example, the overall system response of the combined transmitter and receiver corresponds to a raised cosine filter to avoid system generated intersymbol interference. The system response is implemented with nominally identical root raised cosine filters in the transmitter and in the receiver.
The information bearing digital data stream is randomized for spectrum spreading over the bandwidth of the frequency channel. The randomized data is forward-error-corrected (FEC) coded and interleaved. The data is trellis encoded as an 8-level (3-bit) one dimensional constellation with the outputs of the trellis coder referred to as symbols that are one of eight symmetric odd-valued integer levels from −7 to +7 units. To aid synchronization in low signal to noise and/or high multipath situations, segment and field syncs are inserted in the 10.76 Msymbols/sec symbol stream. A small pilot tone is added as well at the carrier frequency generated by offsetting the real or I channel of the complex signal containing the data and the sync pulses by 1.25 units. The offset causes the pilot tone to be in-phase with the I channel carrier frequency. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. Alternately, the composite signal may be used to directly modulate the RF carrier.
Synchronous demodulation may be used to detect the eight constellation decision levels. However, the constellation of the transmitted symbols may change due to the previously mentioned distortions. For example, the constellation may bend or stretch to form certain curvatures due to the nonlinear distortions in the system. Therefore, for measurement purposes, attempting to determine the transmitted symbols from the received signal in the constellation space would not be reliable using the conventional slicing method.
What is needed is a method and apparatus for accurate in-service measurement of transmitter phase noise of a received signal, where the signal has a mixture of linear distortions, nonlinear distortions, phase noise and additive noise present, that uses reliable constellation decision levels to estimate transmitted digital symbols and preserves original spectral information of the received signal.
SUMMARY OF THE INVENTION
In accordance with the illustrated preferred embodiment of the invention, an apparatus and method is disclosed which provides for in-service measurement of transmitter phase noise of a received signal that has a mixture of linear distortions, nonlinear distortions, phase noise and additive noise present, while using reliable estimation of constellation decision levels and preserving useful spectral information. Prior to the phase noise measurement, also referred to as measurement of carrier phase jitter, the received signal is time synchronized, carrier recovered and corrected for linear distortion. It includes the processes of first demodulating the received signal using the transmission system receiver filter to acquire timing and carrier information, equalizing the demodulated signal to derive the equalizer coefficients, and then using these attained parameters to perform timing synchronization, carrier recovering and equalization directly on the received signal without processing it through the transmission system receiver filter to produce unfiltered signal samples. Also prior to the nonlinear measurement, the received signal is compensated for instrument front-end linear distortion by applying a compensation filter.
The transmitted digital symbols, which are 8-VSB symbols in the preferred embodiment but may be other digital formats, are estimated for the purpose of generating reference signal samples representing the transmitted symbols. This is performed by applying the receiver filter to the unfiltered signal samples, dynamically estimating constellation decision levels from the filtered samples, and then slicing the signal samples accordingly. Reference signal samples are generated from the estimated digital symbols. Phase nonlinearities are then measured by comparing the unfiltered signal samples with the generated reference signal samples. While doing so, nonlinear distortions are distinguished from other noise-like distortions by using the systematic nature of transmitter nonlinearities and the random nature of other distortions by running a weighted, least-square based polynomial regression on the phase errors measured from the unfiltered signal samples and the reference signal samples. Phase noise caused by carrier phase jitter and additive noise is determined by removing the phase nonlinearity from the phase error. The variances of the phase noises by additive noise and by carrier jitter are estimated and the additive noise induced phase noise is suppressed by establishing a threshold based on the derived variances and replacing the phase noise outside the threshold with random values having a distribution equivalent to the carrier phase jitter or with interpolated values from adjacent within-threshold values.
The objects, advantages, and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.
REFERENCES:
patent: 4317206 (1982-02-01), Nossen
patent: 4953186 (1990-08-01), Levy et al.
patent: 4985900 (1991-01-01), Rhind et al.
patent: 5987069 (1999-11-01), Furukawa et al.
“VSB Modulation Used for Terrestrial and Cable Broadcasts” by Gary Sgrignoli, Wayne Bretl, Richard Citta, Zenith Electronics Corporation, Jun. 1995.
Chen Xiaofen
Gumm Linley F.
Lockwood Larry R.
Whitlow Dana E.
Bocure Tesfaldet
Bucher William K.
Tektronix Inc.
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