Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-05-28
2001-12-18
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
C375S140000, C375S367000, C375S150000
Reexamination Certificate
active
06331996
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to data communications over a channel and, more particularly, to obtaining channel impulse response in the presence of frequency offset between the receiver and transmitter.
BACKGROUND OF THE INVENTION
Before a channel carrying digital information can be equalized by ascertaining its impulse response, it is necessary to synchronize the receiver to the frequency of the signal being transmitted. If the local oscillator at the receiver is not precisely tuned to the transmitted frequency a form of impairment, called frequency offset, occurs. Since channel equalization is different for different frequencies, the channel cannot be accurately equalized because of such offset. In order to achieve synchronization, it is customary for the transmitter to send a pseudo-random sequence until the receiver's local oscillator achieves lock-in to the transmitted frequency. Once precise synchronization has been achieved, a further pseudo-random sequence is sent in order for the receiver to ascertain the impulse response of the channel so that the channel can be equalized. However the need to use two transmission sequences, one for channel synchronization and another for ascertaining the channel's impulse response increases receiver overhead processing. It would be advantageous to be able to obtain receiver synchronization and channel response with the use of a single transmitted sequence.
SUMMARY OF THE INVENTION
In accordance with the principles of our invention, the same training sequence can be used to achieve synchronization with the transmitted symbols and also to ascertain the channel's impulse response despite an initial frequency offset. A predetermined binary sequence of pseudo random signals (PN) is transmitted over the channel and is correlated at the receiver with a modified version of the sequence (PN
−1
) in which the zeros of the known sequence PN have been mapped to −1. The use of these sequences has the important property that the product of training sequence A(ƒ) and impulse response of the composite channel and correlator B(ƒ) will be constant over the transmission band so that the result is an exact copy of the channel impulse response. In addition, a Fourier or other suitable transform of the correlator output will exhibit strong tones that indicate the degree of frequency offset initially present so that the signal may be accurately synchronized at the receiver. After the received signal has been compensated for frequency offset, the received signal is used to ascertain the channel's impulse response. Once the sounded channel impulse response is obtained at the receiver, a sub-optimal coefficient set for a decision feedback equalizer is computed. The sub-optimal solution is then modified to obtain a final solution for the equalizer which, with the proper set of coefficients installed, may be used to recover user data.
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patent: 4759034 (1988-07-01), Nagazumi
patent: 5359625 (1994-10-01), Vander Mey et al.
patent: 5467367 (1995-11-01), Izumi et al.
patent: 5675608 (1997-10-01), Kim et al.
patent: 5999561 (1999-12-01), Naden et al.
patent: 6055265 (2000-04-01), Kim
Lin Jenn-Shing
Rogoszewicz Richard
Wang Jin-Der
Al-Beshrawi Tony
Chin Stephen
Lucent Technologies - Inc.
Popper Howard R.
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