Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-02-21
2001-04-17
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S285000, C375S346000, C375S350000, C455S296000, C455S306000
Reexamination Certificate
active
06219376
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to receivers of broadband and/or spread-spectrum signals in communication systems, with particular application to receivers for the signals of satellite navigation systems (NAVSTAR and GLONASS).
BACKGROUND OF THE INVENTION
Broadband signals are used in digital communication systems and navigation systems. In particular, in the satellite navigation systems of GPS (NAVASTAR) and GLONASS (GLN), a receiver processes a great number of broadband signals, each of which is radiated by a corresponding satellite. The location of the receiver and the velocity of its movement are determined as a result of processing. Broadband signals in communication systems are used to increase noise-immunity, to raise secrecy of working, and to separate communication channels in multi-channel radio links.
The typical method of generating broadband signals is based on the use of pseudo-random codes (PR-codes), which modulate the carrier. Typically, the transmitter generates a finite sequence of +1 and −1 bits, or “chips”, which follow the pattern of a PR-code of finite length (called the code duration). This signal has a broad frequency spectrum due to the near random pattern of the code. The transmitter continually repeats the generation of this finite sequence, and may modulate the repeating sequence with an information signal. Such modulation may be done by multiplying a group of whole sequences by +1 or −1, as dictated by the information signal. In doing so, the relatively narrow frequency spectrum of the information signal is expanded, or “spread”, out to that of the repeating PR-code sequence. The modulated broadband signal is then used to modulate a carrier signal for transmission (i.e., up-converted).
If the repeating pseudo-random signal is not modulated by an information signal, it may nonetheless be useful to a receiver. For example, each GPS satellite transmits two repeating pseudo-random signals in two different frequency bands (L
1
and L
2
), respectively, at precise and synchronized times. Comparison of the transmission delays of the two signals enables the receiver to determine the effects of the ionosphere on the signal transmission. The comparison could be done with the L
1
signal being modulated by an information signal, and with the L
2
signal not being modulated by an information signal.
In a receiver, the broadband signal is down-converted and then compressed, or “de-spread”, by correlating the received signal with a locally generated version of the PR-code (which is often called the reference PR-code). The local version may be an identical replica of the PR-code used by the transmitter, or it may be a derivative of the PR-code, such as a strobed version. If a narrow-band interference signal is received at the input of such receiver along with the transmitter's signal, the interference signal will be suppressed because its frequency spectrum is much less than that of the transmitter's signal. The degree of suppression depends on the base of the PR-code, which in turn is determined by the ratio of the code duration to the duration of one chip element (bit) of the code.
In many cases such suppression turns out to be insufficient. In the systems of GPS and GLN, the signal power of a useful signal at a receiver input is usually considerably less than the power level of noise signals received by the receiver input and generated by the receiver's components, and the power level of a narrow-band interference signal often considerably exceeds the noise power level. As a result, even after the correlation processing, the effect of the narrow-band interference remains too strong to be able to properly demodulate the transmitter's signal.
The suppression of the narrow-band interference may be considerably increased by using the compensation method. According to this method, two signal-processing paths and a compensator are created in a receiver. The first processing path extracts the interference signal from the received input signal, using the following two characteristics which distinguish the interference signal from the useful signal and noise: the narrow bandwidth frequency of the interference signal and its higher power. A locally generated copy of the interference is then generated at the output of the first processing path. This copy is then subtracted from the received input signal by the compensator, and the resulting difference signal is provided to the second signal processing path. The signal provided to the second path has thereby been “compensated” by the subtraction of the interference copy signal, and the second path can then proceed with the usual steps of compressing and demodulating the transmitter's signal. In the difference signal generated by the compensator, the interference signal will be suppressed, and the degree of the suppression will depend upon how close the interference copy is to the original interference signal provided to the compensator. The steps of correlation processing, compressing the useful signal, and suppressing the uncompensated interference residue are realized after the above-described compensation.
There are different known methods of separating a narrow-band interference signal from the broadband useful signal and noise so that a copy of the interference signal can be generated for the compensator. These methods are briefly described below.
One of the known methods is based on application of a band limiter, as described by J. J. Spilker, et al., “Interference Effects and Mitigation Techniques,” Global Positioning System: Theory and Applications, Volume I (1996). The limiter is captured by a strong narrow-band interference signal (whose power considerably exceeds the total power of the useful signal and noise). A signal in which the interference essentially dominates is obtained after the limiter. The output signal of the limiter is considered to be a copy of the interference signal, and is subtracted from a copy of the input signal which has been passed through an amplitude regulator. As a result, interference suppression is observed with comparatively insignificant distortion of the broadband useful signal.
In another method described in U.S. Pat. No. 5,268,927, the copy of the narrow-band interference signal is obtained by means of an adaptive transversal filter. This method provides better interference suppression than the previously-described method.
This method described in U.S. Pat. No. 5,268,927 is based on the premise that the respective correlation intervals of the useful signal and of the noise in the receiver band are much less than the correlation interval of a narrow-band interference signal. Therefore, it is possible to sample the input signal at a sampling frequency which is selected to provide a weak correlation to the useful signal and noise but strongly correlated to the interference signal. This sampling enables one to predict the parameters (e.g., weight functions) of a programmable transversal filter (PTF) which can separate the interference signal from the useful signal (and noise) and generate at its output a copy of the interference signal, which in turn can be used by the compensator. The performance characteristics of the compensator are determined by the chosen algorithm of adapting the parameters of the PTF to track the changes in the interference signal. Both the algorithm and the resulting performance essentially depend upon the practical realization of the algorithm, the PTF, and the compensator on a concrete calculating device (e.g., digital signal processor). In practice, the adjustment algorithm for the adaptive programmable transversal filter turns out to be extremely complex and greatly taxes the processor of the digital receiver.
Another known method is based on the results from the Markov theory of the non-linear synthesis, and is described by G. I. Tuzov, “Statistical Theory of Reception of Compound Signals,” Soviet Radio, Moscow, 1977. According to these results, it is necessary to separate a narrow-band interf
Ashjaee Javad
Prasolov Victor A.
Vorobiev Michail Y.
Zhodzishsky Mark I.
Chin Stephen
Coudert Brothers
Ha Dac V.
Topcon Positioning Systems, Inc.
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