Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-10-08
2001-03-06
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Reexamination Certificate
active
06198764
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for the transfer of a digital data signal from a transmitter to a receiver using the spread spectrum technique, in which the data signal is modulo-2 added with a PN code sequence, the bit rate of which is very much higher than the bit rate of the data signal, then a carrier is modulated with the thereby resulting spread data signal, the modulated carrier is transmitted, in the receiver the modulated carrier is demodulated and the demodulated signal, to obtain the digital data signal, is multiplied with a PN code sequence produced in the receiver, coinciding and synchonized with the PN code sequence of the transmitter.
DESCRIPTION OF THE RELATED ART
Data transfer methods which operate using the spread spectrum technique, are used especially when a high suppression of spurious signals is to be achieved under unfavourable transfer conditions. The principles of the spread spectrum technique are described for example in the “Taschenbuch der Hochfrequenztechnik”, 5th edition 1992, publ. Springer-Verlag, Chapter 0 46 ff. It is also indicated there that this technique was used hitherto in particular in the field of air and space travel for the transfer of information under particularly difficult conditions. Also in fields in which a particular protection against interception is involved, for example in radar technology and navigation, this spread spectrum technique was frequently used. A type of spread spectrum technique is the so-called direct sequence method (DS method), in which the signal which is to be transferred, which is present in digital form, is modulated with a PN code sequence (in which PN stands for “Pseudo Noise”), which is provided so that a spreading of the signal which is to be transferred is achieved. Here, the possibility exists of modulating the data signal to be transferred before spreading onto a carrier and then spreading the modulated carrier or else only spreading the data signal to be transferred and then modulating the spread signal onto the carrier. On the receiver side, the data signal is recovered by the spread signal being unspread again by mixing in the PN code sequence. This unspreading process can be carried out using an autocorrelation stage. The PN code sequences used in the spread spectrum transfer methods hitherto have a very great length and are generally produced with binary shift registers. For this, a relatively great circuitry expenditure is necessary, so that the spread spectrum technique has not been used in data transfer systems which are to be realized expending as little cost as possible. As a result of the use of the very long PN code sequences, the autocorrelation process also takes up a relatively great deal of time, because firstly the PN code produced on the receiver side has to be synchonized with the PN code sequence contained in the received signal, so that the unspreading process can be carried out.
In applications in which a very quick synchronisation of the PN code sequence must be achieved on the receiver side, transfer methods are less suitable in which the synchronisation is achieved by autocorrelation. Transfer methods have already been used in which in addition to the information to be transferred from the transmitter, a reference code is transmitted which is used on the receiver side for synchronisation of the PN code sequence. Such a method is described for example in “UKW-Berichte”, 2/84, pages 90 ff. This method is unsuitable for applications in which low costs are important on the transmitter side.
BRIEF SUMMARY OF THE INVENTION
The invention is based on the problem of providing a method of the type initially mentioned, in which on the receiver side a very quick synchronisation can be achieved of the PN code sequence produced in the receiver with the PN code sequence contained in the received signal, using simple circuit means without great expenditure.
This is achieved according to the invention in that in the transmitter, the carrier is modulated by FSK- or BPSK modulation with the spread data signal and in addition by means of the respectively other of these two modulation types is modulated with a synchronizing signal representing the synchronizing information of the PN code sequence, and that in the receiver the modulated carrier is demodulated by means of a quadrature demodulator, from the output signal of which both the spread data signal and also the synchronizing signal can be derived by filtering.
In the method according to the invention, in addition to the information which represents the data to be transferred, information is also transferred which represents the synchronizing status of the PN code sequence. For the transfer of these two pieces of information, the FSK modulation and the BPSK modulation are used, it being immaterial which of these modulation types is used for respectively one of the two pieces of information to be transferred. Since with the use of this method, on the receiver side immediately after reception of the transmitted signal, a piece of information is available concerning the synchronizing status of the PN code sequence used on the transmitter side for spreading the data signal, on the receiver side a synchonized PN code sequence can be produced very quickly, so that the unspreading process can be carried out correspondingly quickly.
An advantageous further development of the method according to the invention consists in that the PN code sequence is selected so that the associated synchronizing signal has a substantially lower bit rate than the spread data signal, that for modulating the carrier for the spread data signal the BPSK modulation is used and for the synchronizing signal the FSK modulation is used, and that in the receiver the output signal of the quadrature demodulator is transferred through a low pass filter to obtain the synchronizing signal and through a high pass filter to obtain the spread data signal.
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patent: 5960040 (1999-09-01), Cai et al.
Mayer Jurgen Anton
Schuermann Josef H.
Brady III W. James
Chin Stephen
Hernandez Pedro P.
Jiang Lenny
Telecky , Jr. Frederick J.
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