Raytheon Company

Robert G Curtis

Richard G Berard

Randall L Baumbach

Nathan A Liskov

Caesar J Favaloro

Wesley G Brodsky

Gregory L Pence

Gary H Hochstadt

A truncated maximum likelihood sequence estimator that provides a best estimate of a transmitted symbol sequence includes a lookup table and a comparator. The lookup table may be responsive to at least a next sample of a received RF signal, a first decision made on a first previous sample of the RF received signal and a second decision made on the second previous sample of the RF received signal. The lookup table provides at an output at least one threshold value stored in the lookup table that is addressed by the next sample of the received signal, the first decision made on the first previous sample and the second decision made on the second previous sample. The comparator is coupled to the output of a lookup table and is responsive to the at least one threshold value and a current sample of the received RF signal. The comparator compares the current sample of the received RF signal with at least one threshold value and provides a best estimate of the transmitted symbol sequence. The truncated maximum likelihood sequence estimator can be used for example to determine a best estimate of a transmitted symbol sequence of a current sample of 4-ary frequency shift keyed signal or a 2-ary frequency shift keyed received signal. In addition, the truncated maximum likelihood estimator can be used within a wireless radio that transmits and receives an RF waveform including a modulated symbol stream to provide the best estimate of the received modulated symbol stream. Further, the truncated maximum likelihood estimator can be used within a wireless local area network that includes at least one computer interfaced to a wireless radio containing the truncated maximum likelihood sequence estimator to provide wireless communications between either computers or between a computer and a computer network.

Truncated maximum likelihood sequence estimator

A timing recovery apparatus for a burst mode communication receiver. The apparatus provides for optimum sampling and digitizing of received data at a plurality of data rates. In particular, a VCO is phase-locked to a local frequency reference prior to data being received. A reference timing preamble transmitted prior to the data is filtered and fed to the VCO causing it to injection lock such that the VCO becomes phase aligned with the preamble. The VCO is then permitted to 'free run' during data transmission and continues to operate at substantially the same frequency. A synchronous divider and multiplexer, responsive to the VCO, allows selection of sampling clocks for the plurality of data rates. The divider is forced to a known state during VCO injection locking, to assure that the sampling clocks have maintained the proper phase for optimal sampling at the corresponding data rate. Further, means is provided to monitor the frequency of the VCO. Should the VCO frequency drift more than a predetermined amount an indication of such is produced.

Timing recovery scheme for burst communication systems having a VCO with

Timing recovery scheme for burst communication systems

A receiver including two optimizers for demodulation, sampling and quantization of received QPSK digital data. One optimizer adjusts the phase of a reference signal used for demodulating the QPSK data into baseband in-phase and quadrature channels and another optimizer adjusts a sampling clock used to sample and quantize the baseband channels. The reference signal and the sampling clock are dithered in quadrature. Variations in the amplitude of the quantized data extracted by an eye quality monitor. Each optimizer multiplies the eye quality monitor output by a corresponding dither clock and integrates to form an error signal. Phase shifters, responsive to corresponding error signals and dithered clocks, adjust the phase of the demodulation reference signal and sampling clock. The data quality signal monitor translates the magnitude of the quantized data signals to a common set of signals for each state of the data signal. These common signals are translated into a voltage representing the degree to which the data signal amplitude corresponds to a predetermined value. Additionally, a phase shifter is shown which splits its input signal into two signals in quadrature. Theses two signals, when multiplied by two corresponding values and the products summed, form a phase shifted version of the input signal. The two values correspond to a nominal phase shift and vary to impart a linear phase change to the input signal proportional to a control voltage.

QPSK demodulator

A receiver including two optimizers for demodulation, sampling and quantization of received QPSK digital data. One optimizer adjusts the phase of a reference signal used for demodulating the QPSK data into baseband in-phase and quadrature channels and another optimizer adjusts a sampling clock used to sample and quantize the baseband channels. The reference signal and the sampling clock are dithered in quadrature. Variations in the amplitude of the quantized data extracted by an eye quality monitor. Each optimizer multiplies the eye quality monitor output by a corresponding dither clock and integrates to form an error signal. Phase shifters, responsive to corresponding error signals and dithered clocks, adjust the phase of the demodulation reference signal and sampling clock. The data quality signal monitor translates the magnitude of the quantized data signals to a common set of signals for each state of the data signal. These common signals are translated into a voltage representing the degree to which the data signal amplitude corresponds to a predetermined value. Additionally, a phase shifter is shown which splits its input signal into two signals in quadrature. These two signals, when multiplied by two corresponding values and the products summed, form a phase shifted version of the input signal. The two values correspond to a nominal phase shift and vary to impart a linear phase change to the input signal proportional to a control voltage.

Qpsk demodulator

An eye opening monitor for a binary waveform such as provided as the video output of a demodulator. At the decision sample time, the video is compared to -V.sub.th and +V.sub.th reference voltages to determine if it falls in the range defined therebetween. If it does, a counter is reset. Because the carry-out pulses of the counter are used to produce -V.sub.th and +V.sub.th, the range defined therebetween is decreased by the additional time required to count to the carry-out after the counter is reset. The loop stabilizes with the range from -V.sub.th and +V.sub.th being quantitatively indicative of the eye opening. A voltage derived from V.sub.th is visually displayed.

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Sudbury, MA, United States of America

Robert G Curtis