Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-10-08
2002-02-05
Ghayour, Mohammad H. (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S134000, C375S136000, C375S137000, C375S145000, C375S147000, C375S149000
Reexamination Certificate
active
06345073
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of Communication systems and more particularly to Direct Sequence Spread Spectrum communication systems.
BACKGROUND OF THE INVENTION
Direct Sequence Spread Spectrum (DSSS) is a commonly used method for protecting communication and navigation signals against both jamming and unintentional interference, and for providing code division multiple access (CDMA), in which multiple DSSS signals may share the same bandwidth. DSSS signals are widely used to provide protection against narrowband interference and reduce multipath signals within predetermined bandwidths. Spread spectrum systems provide signal security, frequency management, and jamming and noise immunity.
A DSSS signal includes a pseudo-random sequence, that is, a direct sequence, spreading code modulated by a data stream. A CDMA channel bandwidth may be used for transmitting a plurality of superimposed DSSS signals each comprising a respective unique data stream modulating a respective unique spreading code signal. The DSSS modulation requires the generation of a pseudo-random sequence of chips, that is, spreading code bits of zeros and ones, as the spreading code. The chips are communicated as digital symbols. There is a predetermined number of chips per each data bit, with the data bits and code chips combined in synchronism. A remote transmitter code generator generates the spreading code at a chipping rate typically much higher than the data bit rate of the data stream so that the direct sequence spreading code can be modulated by the lower bit rate of the digital data stream so as to spread the bandwidth of the data stream by the chipping rate over CDMA bandwidth. The modulation of the spreading code by the data stream effectively spreads bandwidth of the data stream over the CDMA bandwidth. The symbol or chipping rate of the spreading code must be higher than the data rate of the underlying data stream, for example, ten thousand times faster, for spectrum spreading with high processing gain equal in dB to ten times the log of the ratio of the bandwidth of the chipping rate divided to the bandwidth of data modulated by the chips. The digital chips of the spreading code are added modulo two to the respective underlying data stream. The resulting sequence of chips has a symbol rate equal to the chipping rate of the spreading code. The spreading code is used to spread the data stream bandwidth. Several DSSS signals have respective unique spreading codes that are typically transmitted by superposition within the CDMA bandwidth communicating the plurality of DSSS signals.
The spreading codes are chosen so that the cross correlation between different codes, of respective DSSS signals, is low. Low cross correlation between different spreading codes ensures that the correlation process upon reception will isolate only the DSSS signal and the data bit stream of interest that corresponds to the selected spreading code while rejecting the other DSSS signals that share the CDMA bandwidth. Typically, the spreading codes are linear recursive sequences in which each chip is the modulo two sum of a fixed set of preceding chips. Sequences of this type are usually generated as the output of a linear feed back shift register. The register is tapped at the locations corresponding to the chips that are to be summed and the modulo two sum is fed back into the first stage of the shift register. To begin the process, a vector must be loaded into the shift register. The vector must be a non-zero vector to avoid a constant zero sequence. This vector is referred to as the initial fill word or initial state. For DSSS applications, it is desirable that the sequences be pseudo-random and long. Only certain choices of taps in the shift register produce sequences of desirable cross correlation properties and maximum length. The shift register taps correspond to the non-zero coefficients of a generating polynomial function for the DSSS code sequence. An identical polynomial function may be implemented using the same shift register feedback structure in the receiver to locally generate the same code sequence for despreading with the same initial fill word.
Upon reception, the spreading code is used to despread the CDMA channel to isolate and acquire the particular DSSS signal of interest and to then recover the underlying digital data of the digital data stream. The DSSS signal has a bandwidth larger than the digital bit stream by virtue of modulating the data stream at the higher chipping rate, that is, at the frequency of the spreading code. The spreading code is generated at a higher frequency and is modulated by the digital data bit stream having a lower frequency, and hence the digital data bit stream is spread from a narrow data rate bandwidth across the larger bandwidth defined by the higher chipping rate, that is, the symbol rate.
The receiver of a DSSS signal requires the generation of the same spreading code for correlation detection of the respective particular modulated data stream of interest. The receiver typically locally generates the same spreading code. The data stream is recovered from the DSSS signal by correlating the locally generated replica of the spreading code with the received DSSS signal. The correlation process removes the spreading code by despreading the DSSS signal back into the underlying data stream and increases the strength of the received signal by a factor equal to the processing gain equal to the bandwidth of the DSSS signal divided by the bandwidth of the underlying data stream. The DSSS bandwidth is thereby effectively compressed back into the bandwidth of the underlying data stream.
In practice, a DSSS signal or a plurality of DSSS signals may further modulate a high frequency carrier signal suitable for radio frequency transmission. There are many types of carrier modulation methods used, such as Binary Phase Shifted Keyed, (BPSK), Quadrature Phase Shift Keyed (QPSK) and Gaussian Minimum Shift Keyed (GMSK) carrier modulation methods. Further still, the data bits of the data stream may be formatted before producing a modulated data stream. There are many types of data stream data modulation methods, such as, non-return to zero (NRZ), and Manchester (Bi-Phase-L) data stream modulation methods. Upon reception, the carrier signal is typically demodulated using tracking, Costas loop or squaring loop carrier demodulators, to produce the digital stream. The data of the data stream is then demodulated using data bit synchronizers to detect data bits at the data rate to recover the data bits from the data stream. Carrier modulation and data stream modulation during transmission, and, carrier demodulation and data stream demodulation during reception typically occur in addition to the spreading and despreading of the DSSS signal over the CDMA bandwidth. Carrier and data stream modulation and demodulation are well known to those skilled in the art of spread spectrum systems.
A DSSS signal is received by a DSSS receiver that recovers the data of the spread spectrum data stream. The DSSS receiver must not only generate a replica of the chip sequence of the spreading code for DSSS despreading, but must also time shift the locally generated replica to be aligned in time with the remotely generated spreading code in the received DSSS signal to within a fraction of a chip interval so that the correlation produces a despread signal. The time shift between the locally generated spreading code and the remotely received spreading code in the received DSSS signal is related to a specific number of chips in the spreading code at the chipping rate. Hence, the time shift offset between the received spreading code and the replica despreading code establishes a code phase. Determining this time shift code phase for correlation alignment is necessary to recover the data stream and is known as code phase acquisition, or more simply code acquisition. When the locally generated despreading code is in time alignment, then the despreading correlation will isolate only that DSSS sign
Collins John F.
Curry Samuel J.
Schwartz David M.
Ghayour Mohammad H.
Reid Derrick Michael
The Aerospace Corporation
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