Frequency acquisition method for direct sequence spread...

Pulse or digital communications – Spread spectrum – Direct sequence

Reexamination Certificate

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Details

C375S344000, C370S514000

Reexamination Certificate

active

06222874

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to improvements in digital spread spectrum systems and, more particularly, pertains to a new and improved device and method for frequency acquisition and compensation for digital cordless direct sequence spread spectrum telephones.
2. Description of Related Art
Cordless telephones currently exist which provide good speech quality using analog frequency modulation in the 46/49 MHz band. These analog cordless phones are inexpensive and have become a common replacement for the traditional corded phones. However, analog cordless phones have a limited useful range, requiring the handset to be operated in close proximity to the base station. Also, analog cordless phones are susceptible to interference and lack adequate security for protecting the privacy of conversations.
One solution to improve cordless telephone performance is to use digital modulation and coding techniques. Digital modulation and coding offer more robust voice communication over a radio channel than analog modulation. Digital modulation also provides a more noise-free conversation by suppressing co-channel and adjacent channel interference. Additionally, with digital modulation and coding, effective scrambling codes can be added to improve phone security.
Another solution is to use the 902-928 MHz Industrial, Medical and Scientific (ISM) band for radio transmission and reception. The Federal Communications Commission (FCC) allows transmitters in this band to have increased power levels, as compared to the 46/49 MHz band, which increases the useful operating range. At the highest transmit power levels, which are needed by a digital phone to overcome interference, the FCC requires that the phone system implement spread spectrum modulation.
In Direct Sequence Spread Spectrum (DSSS) systems, such as digital cordless telephones operating in the ISM band, it is desirable to use very inexpensive crystal oscillator references in both the handset and base station in order to reduce overall system cost. However, the cheaper the crystals, the greater the inaccuracy in the oscillator frequency.
SUMMARY OF THE INVENTION
Thus, it is an object of the present invention to provide an improved frequency acquisition and compensation device and method which allows the use of inexpensive and inaccurate crystal references on the order of ±100 ppm, such that frequency errors three times the normal limits can be tolerated without any loss of performance.
An Identification (ID) sequence is used for frequency acquisition and is included in every Acquisition frame (A-frame). The dot product, cross product and inverted cross product of the ID sequence is determined and compared with an ID sequence stored in the receiver. This allows for simple detection of the incoming signal with ±135 degrees of phase change at the demodulator due to frequency errors, with less than 3 db loss. In addition, by knowing whether the cross, inverted cross, or dot product detector found a match with the incoming ID sequence, the estimate of the frequency error can be obtained and used to update the frequency compensation during the rest of the message. Thus, there is no need to track out frequency errors while the phone is sleeping with up to 30 ppm errors in the disclosed system.


REFERENCES:
patent: 4896336 (1990-01-01), Henely et al.
patent: 5042050 (1991-08-01), Owen
patent: 5732111 (1998-03-01), Walley
patent: 5799034 (1998-08-01), Walley et al.
patent: 5892792 (1999-04-01), Walley

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