Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-06-09
2004-11-09
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06816539
ABSTRACT:
TECHNICAL FIELD
The present invention relates to systems for receiving digital radio signals and, more specifically, the invention relates to receivers of radio signals encoded by a pseudonoise sequence, which are used in the global positioning systems GPS (Global Positioning System) (USA) and GLONASS (Global Navigational Satellite System)(Russia).
BACKGROUND
Global positioning systems such as GPS (Global Positioning System) (USA) and GLONASS Global Navigational Satellite System) allow a user with a passive receiver of digital signals to generate an exact definition of the user's coordinates (longitude, latitude, altitude,) and time. (Cf. “Global Navigational Satellite System—GLOSNASS. The Interface Control Document.” KNITS VKS Russia, 1995. See also “Global Position System. Standard Positioning Service. Signal Specification”. USA, 1993). The navigational radio signal transmitted by the global positioning system satellite is a multicomponent phase-manipulated signal, in which the signal of a carrier frequency L
1
of about 1.6 GHz is modulated by a coherent pseudonoise binary sequence ±1 (phase manipulation on TL radian) having a length of 1023 characters (GPS) or 511 characters (GLOSSNAS). The pulse-repetition rate of the modulating sequence is equal to 1.023 MHz for the GPS and 0.511 MHz for the GLONASS, with the pulse-repetition period being 1 ms. Application of the method of digital reception and the correlation of a similar broadband digital signal allows for the successful reception and decoding of a very low amplitude signal located much below the level of natural thermal noise. Thus, in the case of the GPS C/A signal, its level is from −157 dBW up to −160 dBW so that at a standard density of thermal noise of −205.2 dBWILz and a minimum band of the radio-frequency channel of 2 MHz results in a signal-to-noise ratio of 14.8 dB to −17.8 dB.
In addition, the application of the method of reception and digital processing of broadband phase-manipulated signals allows one to reduce essentially the negative effect of the narrow-band interference which often results in a failure of reception of the narrow-band amplitude-modulated or frequency-modulated signals. Nevertheless, the suppression of the narrow-band (sinusoidal) interference for a digital receiver of a pseudonoise signal (PNS) is critical, especially in the case of high-power pseudonoise interference whose amplitude overcomes that of the thermal noise. Furthermore, the GLONASS is a system with a frequency division of the signals for the receiver based on the GLONASS system, while for the combined GPS/GLONASS receivers the width of the radio-frequency channel is broadened approximately to 10 MHz. The use of the “narrow correlator” technique also results in broadening the radio-frequency band of the receiver. (Cf. .J. Dierendonck, P. Fentor, N. Ford in Theory and Performance of Narrow Correlator Spacing in GPS Receiver”, Navigation: Journal of the Institute of Navigation vol. −39, No.3, Fall 92). The extension of the range of the radio-frequency channel results in an increase of probability of catching a high-power narrow-band interference, and, as a consequence, it is necessary to provide means for dealing with this interference.
Known in the art is a method of using the adaptive analog-to-digital converters enabling the narrow-band interference on the digital PNS receiver operation to be reduced. (Cf. Frank Amoroso, Jacob L. Bricker “Performance of the Adaptive ADC in Combined CW and Gaussian Interference, IEEE Transactions and Communications, vol. COM-J4, No.3, March 1986)[1]. Using a two-bit adaptive ADC as a digitizer with a variable quantization threshold &Dgr;, it is possible to reduce significantly the effect of the narrow-band interference on the operation of a digital correlator.
Also known in the art is a receiver for decoding a complex signal consisting of many PNS. The receiver comprises a reference generator, an automatic gain control (AGC) device having an input for complex PNS and an input for a signal controlling the amplification factor, a multilevel adaptive ADC converter whose input is connected to the AGC output and the clock input is connected to output of the reference generator. The converter produces at its output in-phase I and quadrature Q components of the complex signal. The receiver also has a set of digital counters, in which each counter calculates a value of digitized signals in one of the channels whose amplitude is within a preset quantization interval, and a control device reading the output values of the counters and producing a gain control signal on the basis of the analysis of the obtained data. See Patric Fenton, Kkwok-Ki K. Ng,. Thomas J. Ford in “Mulichannel Digital Receiver for Global Positioning System”, U.S. Pat. No. 5,101,416.
One of the feature of the present invention is that, given a multilevel ADC converter and calculating the percentage of the digitized signals appearing between two adjacent quantization thresholds, it is possible to evaluate how the distribution function of the digitized complex signal corresponds to the Gaussian. Thus, in an exemplary embodiment of the present invention, a 6-level complex quantizer is provided, at output of which the quadrature components can take values ±1, ±2, and ±3. By setting the value of the A-distances between quantization thresholds, one can achieve the necessary ratio of a value of digitized signals appearing in one or another quantization interval. In the present embodiment it is suggested to use a ratio of 49%, 32%, and 19% for the signals from the intervals +1, ±2, and ±3 corresponding to the quantization interval &Dgr;=.66&sgr;, where a is the square root from the dispersion of the Gaussian distribution.
The deviation from the given distribution points to the presence of narrow-band interference which can be compensated for by changing the gain of the AGC circuit and the quantizer output values. In this embodiment the presence of the narrow-band interference is recorded, when all quantized values are only in four of the six quantized intervals, i.e. the distribution will be 49%, 51%, and 0% for the signals from the intervals of ±1, ±2, and ±3. In this case the presence of narrow-band interference stated and the quantization threshold is changed so that the digitized values in the intervals corresponding ±1 is equal to 85%; ±2 is −15%; and ±3 is −0%. In so doing the numerical values of quantized values are changed: ±1 are replaced by 0, i.e. 85% of signals below the quantization thresholds ±&Dgr; are ignored; ±2 are replaced by ±1; and ±3 are replaced by 0. Thus, during further calculations only the approximately 15% of digitized signals occurring within &Dgr; <| signal amplitude |<2&Dgr;, are taken into account in correlator channels.
The disadvantages of the offered device include, first, the fact that it requires application of technically complex multilevel multibit analog-to-digital converters (AID), and, second, at some points of the distribution function it is possible to make only a rough estimate of the amplitude of the narrow-band interference, which, in turn, lead to a coarse gain control for changing the quantization thresholds. In the above example, the discrete adjustment: 49%, 51%, and 0% are replaced by 85%, 15%, and 0%. At each ratio V
si
/&sgr;, where A
si
is the sinusoidal interference amplitude and a is the Gaussian noise dispersion, it is possible to select the best value of the quantization thresholds to minimize the effect of the narrow-band interference on the useful signals.
DISCLOSURE
The basic object of the invention is the development of a digital PNS receiver compensating the effect of the narrow-band interference and allowing the removal of the above-said disadvantages due to the direct detection of the narrow-band interference, estimation of its amplitude, and installation of an optimal
Samsung Electronics Company Limited
Tran Khai
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