Narrow-band interference rejecting spread spectrum radio system and method

#1. Field of the Invention

#The present invention relates to communications-related systems and methods as well as digital signal processing devices and methods. More particularly, the invention is directed to the field of spread spectrum (SS) communication that employs a SS transmitter and a SS receiver, or SS transceiver configured to convey a message in a transmitted SS signal by spectrally spreading the message signal on transmission and correlating on reception so as to “despread” the SS signal and recover the message signal.

#2. Description of the Background

#A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

#FIGS. 1A and 1B are diagrams of a spread spectrum radio transmitting and receiving system that rejects narrow-band interference.

#FIGS. 2, 13 and 14 are diagrams of synchronization and demodulation methods that reject narrow-band interference in a spread spectrum radio receiver.

#Referring now to the drawings, wherein the two least significant digits of reference numeral designations designate identical or corresponding parts throughout the several views and the third and fourth significant digits of the reference numeral designations refer to the respective drawing number, the present invention is next described.

#The present invention resolves both frequency difference between a receiver and a transmitter and the spreading code delay (time difference) between a receiver and a transmitter. A transmitter is shown in FIG. 1A where a carrier wave generated by a frequency source 102 is modulated 104 by a message 106. The resultant signal is then modulated 108 by a spreading code 110 to achieve the benefits of spread spectrum communications. This signal is then amplified at amplifier 112 prior to transmission via antenna 114. Frequency source 102 will generate the carrier wave as well as control the timing of both the message 106 and the timing of the spreading code generator 110. The spread spectrum transmitter that works in conjunction with the receiver in the preferred embodiment of the present invention has a crystal controlled frequency source. This invention enables an inexpensive transmitter frequency source having an accuracy of ±50 parts per million (ppm) in the preferred embodiment.

#The receiver of the present invention is shown in FIG. 1B where diversity antennas 120 and 122 (and, alternately, additional antennas) provide an RF signal to analog front-end 124. This front end may convert the RF signal either to an intermediate frequency (IF) signal or directly to baseband. The downconverted signal is digitized at the analog to digital converter (ADC) 126. The digitized signal is sent to the digital signal processing (DSP) section 128. Frequency source 130 provides one or more downconversion frequencies to the analog front end 124, a sampling clock to ADC 126, and a processor clock to DSP 128. The receiver in the preferred embodiment of the present invention has a crystal controlled frequency source with an accuracy of ±15 ppm. The total system frequency error is the sum of the errors from both the transmitter and the receiver. The overall frequency accuracy of the system in the preferred embodiment is about ±65 ppm. In the preferred embodiment, the center frequency of the transmitted signal is about 915 MHz. The receiver must be able to resolve a signal with about ±60 kHz of frequency error.