Pulse or digital communications – Repeaters – Testing
Patent
1991-07-29
1993-05-25
Barron, Jr., Gilberto
Pulse or digital communications
Repeaters
Testing
H04L 700
Patent
active
052146693
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a code acquisition process and circuit for a spread-spectrum signal receiver, i.e. a receiver for demodulating a signal generated by modulation of an electrical signal with a noisy binary code.
A binary code modulated signal is used for example in radio-communication systems and in pulse-compression radar systems. A receiver adapted for receiving and demodulating a binary code PSK (Phase-Shift-Keying modulation) modulated signal must include a circuit organised to perform an initial search for the single correct epoch of the received code among a wide range of possible epochs, with the code binary sequence being assumed to be known a priori. The search process is referred to as "code acquisition" and the circuit provided for carrying out the search is referred to as code acquisition circuit.
Most of the known code acquisition circuits are based on the implementation of a signal processing method comprising a correlation process (despreading) followed by a quadratic detection process. After amplification and down-conversion into the appropriate frequency band (intermediate frequency or baseband), the received signal is mixed with a reference code set at a given code epoch, then the mixed signal is filtered and squared, and thereafter it is passed through an integrate circuit. The output from the integrator is compared with a threshold and the integrated signal level crossing said threshold level indicates in all likelihood that the received signal is a spread-spectrum signal and that the code epoch thereof matches the epoch of the reference code.
If the epoch of the reference code matches the epoch of the received noisy signal, then the signal at the mixer output is a narrow-band carrier. As to the wideband noise, it is further spread by mixing with the reference code and the narrow-band filter following the mixer rejects the wideband noise and thus improves the carrier-to-noise ratio.
However, if the epoch of the reference code does not match the epoch of the received noisy signal, then both carrier and noise at the mixer output are wide-band signals and the narrow-band filter rejects them both. The carrier-to-noise ratio then remains unchanged.
The bandwidth of the narrow-band filter determines the carrier receiving band. It must be large enough to accommodate a narrow-band carrier affected by link Doppler and by frequency uncertainty pertaining to the local oscillators. Conversely, the filter bandwidth must be small enough as to reject most of the wideband noise and the spread carrier when the epoch of the reference code is not correct.
A quadratic detection process follows the correlation process described hereabove, whereby the former aims at detecting the presence of a carrier in additive white gaussian noise. The integration time and the threshold are set to detect a despread carrier (equivalent case to a correct code epoch) and to not detect a spread carrier (equivalent case to an incorrect code epoch), both with a high degree of confidence. Typically, the desired probability of detection is greater than 0.9 and the desired probability of false acquisition per trial is less than 0.001.
To obtain a fast rejection of the wrong code epochs, the output from the integrator may be evaluated during the integration, instead of at the end of the operation as previously described. This modified process, called sequential acquisition, reduces the acquisition time by a factor of approximately two to three.
In known acquisition circuits, the correlation process is performed on analog signals and the filtering is thus carried out by means of an analog filter. Subsequently, the despread signal is sampled and quantized by means of a multi-bit analog-to-digital converter. The subsequent processing is performed digitally.
This known process has the following drawbacks:
a) performing partly analogical processing results in a lack of flexibility, complex non-reproductible construction and, in certain applications, the need for circuit tuning;
b) need for using at least one complex and c
REFERENCES:
patent: 4007330 (1977-02-01), Winters
patent: 4203071 (1980-05-01), Bowles et al.
patent: 4550414 (1985-10-01), Guinon et al.
patent: 4774715 (1988-09-01), Messenger
patent: 5022047 (1991-06-01), Dixon et al.
Proceedings of Melecon '87 Mediterranean Electrotechnical Conference, Mar. 24-26, 1987, Rome, Italy, M. K. Sust et al. entitled "All Digital Signal Processing in a Spread Spectrum Communication System", pp. 157-161.
Agence Spatiale Europeenne
Barron Jr. Gilberto
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