Telecommunications – Transmitter and receiver at same station – With frequency stabilization
Reexamination Certificate
1998-10-13
2001-05-08
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter and receiver at same station
With frequency stabilization
C455S063300, C455S136000, C455S226200, C455S076000
Reexamination Certificate
active
06229991
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication devices employing automatic frequency control (AFC) and more particularly to extending the range of AFC in those communication devices.
BACKGROUND OF THE INVENTION
In an effort to alleviate frequency spectrum crowding and expand services, high frequency bandwidth has been allocated for subscriber communication systems. One such high frequency subscriber communication system is the Digital Cellular System (DCS) 1800 radiotelephone system, which has a transmit frequency band of 1710 MHz-1785 MHz and a receive frequency band of 1805 MHz-1880 MHz. The DCS 1800 frequencies are roughly two times larger than that of conventional radiotelephone systems, such as the Global Standard for Mobile (GSM) system whose transmit frequency band is 890 MHz-915 MHz and receive frequency band is 935 MHz-960 MHz.
In order to communicate with the DCS 1800 radiotelephone system, a DCS 1800 radiotelephone performs AFC to synchronize a receiver of the radiotelephone to a base station transmitter of the system. AFC includes automatically correcting, to within an acceptable degree of error, frequency discrepancies between the carrier frequency of the signal to be received from the base station transmitter and the frequency of a crystal reference oscillator of the radiotelephone from which receiver reception is set. The DCS 1800 radiotelephone initiates AFC by receiving and processing a 67 kHz tone sent by a DCS 1800 base station transmitter on a control channel. However, if the initial frequency error of the DCS 1800 radiotelephone is too great, the tone can become too attenuated and fall outside of the bandwidth of the receiver. To prevent this from happening, the DCS 1800 radiotelephone uses a crystal reference oscillator having a crystal with a high accuracy of +/−16 ppm. Unfortunately, the +/−16 ppm crystal is expensive. For example, the +/−16 ppm crystal is about 20% greater in cost than the +/−25 ppm crystal traditionally employed in GSM radiotelephones. However, use of the +/−25 ppm crystal in place of the +/−16 ppm crystal in the DCS 1800 radiotelephone described above is not acceptable as such substitution would result in initial frequency errors of about +/−50 kHz, which are large enough to prevent correct reception by the radiotelephone.
Therefore, what is needed is a way to avoid use of expensive, high accuracy crystals in a high frequency communication device, such as a DCS 1800 radiotelephone, without sacrificing AFC.
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Arora Arvind S.
Hietala Alexander W.
Cong Le Thanh
Motorola Inc.
Nguyen Thuan T.
Patrick Mark D.
Watanabe Hisashi D.
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