Apparatus for communicating diversity signals over a...

Optical: systems and elements – Deflection using a moving element – Using a periodically moving element

Reexamination Certificate

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C359S199200, C359S199200, C359S199200, C455S103000, C455S093000, C455S202000, C455S303000, C375S219000, C375S259000

Reexamination Certificate

active

06268946

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains to simultaneously transmitting multiple signals, and in particular, to transmitting multiple signals at substantially the same center frequency over a single transmission medium.
BACKGROUND OF THE INVENTION
In some communications applications, it is desirable to receive a main signal and its diversity signal and to communicate both signals to a second location where one or the other signal is selected as the preferred signal for reception. In some applications it is desirable that the communication to the second location use an optical fiber as the communications medium.
For example, a microbase station in a cellular communication system might use a diversity antenna system located anywhere from several meters to several hundred meters away from the microbase station. The diversity antenna system would include an antenna element for receiving a main RF signal and another element for receiving a diversity RF signal corresponding to the main signal. The two RF signals would have the same center frequency. The two signals could be provided by, for example, a polarization diversity antenna system in which a horizontally polarized and a vertically polarized signal are both received and provided to the microbase station. The two signals could also be provided by a spatial diversity antenna system, located away from a microbase station, in which two antennas are physically separated, each receiving a signal traveling from the source by a different path, and both signals are to be provided to the microbase station.
With the diversity antenna system removed from the microbase station where the receiver components are located that select either the main signal or the diversity signal, both RF signals must be transmitted from the remote antenna system to the microbase station. It is often desirable to do this using optical fiber as the transmission medium; this of course requires that the RF signals modulate an optical carrier signal.
In propagating within the optical fiber, the main signal and diversity signal of either a spatial diversity antenna system or a polarization diversity antenna system are essentially indistinguishable, because the two signals have the same center frequency and the features that provide diveristy for propagation in air, polarization or path taken, are not available within the optical fiber. Therefore, measures must be taken to keep the two signals distinct. One way to keep the signals distinct is to shift one or both signals in RF frequency before modulating the optical carrier. Then after demodulating the optical carrier, the shifted RF signal or signals are converted back to their original RF frequency. In doing this back conversion, however, the back-converted signal is often distorted.
What is needed is a way to transmit a main signal and its diversity signal, at essentially the same center frequency as the main signal, along a single optical fiber in a way that does not cause distortion, and can be implemented without unduly complex hardware.
SUMMARY OF THE INVENTION
The present invention has as an object transmitting over a single optical fiber leading to a target destination, two RF signals f
1
and f
1
′ at a same center frequency f
1
, and recovering the two RF signals at the target destination so that the signals are provided reasonably free of distortion. It is a further object of the present invention to provide this reasonably distortion free communication using relatively little hardware.
The present invention achieves these objects by an apparatus that shifts in frequency one or both of the two RF signals f
1
and f
1
′ by mixing one or both of the signals with a signal f
LO
provided by a local oscillator, and, by modulating an optical carrier, transmitting along the optical fiber not merely the two RF signals f
1
and f
1
′, one shifted in frequency, but also the signal f
LO
provided by the local oscillator. Then, at the other end of the optical fiber, the optical carrier is demodulated and the result provided to three filters each tuned to a different one of the three frequencies of the signals transmitted on the optical fiber. One of these three filters extracts the signal f
LO
provided by the local oscillator. The extracted signal f
LO
is then used to down-convert, by mixing and filtering, the one or more frequency-shifted RF signals.
Thus, frequency errors are avoided because the receiving end filters provide, precisely, the same local oscillator signal used to shift the frequency of one or both of the two original RF signals f
1
and f
1
′.
If by mixing with the local oscillator signal, one of the original RF signals is down-shifted in frequency, instead of shifted up in frequency, subsequent filtering of the down-shifted shifted signal by a narrow band filter, such as a surface acoustic wave (SAW) filter, can be more selective because of filtering at a lower frequency.
The present invention is not restricted to transmitting only two same center frequency signals along a single optical fiber. The present invention can also be used to transmit multiple signals on different frequencies. For example, if the second signal is on frequency f
2
along with a corresponding diversity signal f
2
′, hardware can be provided including a second oscillator so that instead of multiplexing three signals, six signals will be multiplexed onto the optical fiber. At the receiving end of the optical fiber, instead of three filters, six filters would be provided to separate the signals.
The transmission medium need not be an optical fiber. The present invention instead could be used in communicating with a target destination by a radio link, coaxial cable, or other media suitable for handling RF signals.
In addition, the present invention can be used so that after mixing one of the two RF signals with the signal from a local oscillator frequency f
LO
, the signal at a frequency of f
1
+f
LO
is selected, instead of the down-shifted signal, f
1
−f
LO
. Moreover, the present invention can be used in support of a triversity antenna system by selecting both an upshifted signal and a downshifted signal after mixing two of three triversity signals with the signal from a local oscillator.


REFERENCES:
patent: 3860873 (1975-01-01), Ringstad
patent: 4225751 (1980-09-01), Hershberger
patent: 4289373 (1981-09-01), Sugimoto et al.
patent: 4323731 (1982-04-01), Hershberger
patent: 5067173 (1991-11-01), Gordon et al.
patent: 5457811 (1995-10-01), Lemson
patent: 5752180 (1998-05-01), Guo et al.
patent: 6018514 (2000-01-01), Bickham et al.
patent: 6075823 (2000-06-01), Sonoda

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