Pulse or digital communications – Spread spectrum
Reexamination Certificate
1996-09-26
2001-03-06
Luther, William (Department: 2664)
Pulse or digital communications
Spread spectrum
C375S147000
Reexamination Certificate
active
06198763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transmitter and a receiver for spread-spectrum communication employing a direct spreading method, and also relates to a spread-spectrum communication system.
2. Description of the Related Art
In a spread spectrum communication system employing a direct spreading method in which digital information to be transmitted is directly spread with a digital code sequence, when one period of the code sequence is allotted to one bit of information, the bandwidth used expands as the information rate to be transmitted increases, as long as the bit length in one period of the code sequence is fixed. On the other hand, if the information rate is to be increased within a predetermined bandwidth, the expansion ratio must be lowered by reducing the bit length of the code sequence. In other words, in spread-spectrum communication employing the conventional direct spreading method, with a given bandwidth and expansion ratio (bit length of spreading code) the information rate cannot be increased above a certain value.
Japanese Unexamined Patent Publication No. 7-46222 discloses a communication system wherein the code phase of a code sequence for directly spreading the information to be transmitted is modulated in accordance with another information or the code sequence itself is phase-modulated for transmission, thereby achieving the transmission of larger volumes of information than is possible in spread-spectrum communication employing the conventional direct spreading method under conditions of a predetermined bandwidth and spreading code length. In this system, since the code phase in the received signal varies due to the modulation, synchronization cannot be maintained by establishing code synchronization and chip synchronization by computing the correlation between the received signal and the despreading code at the receiving end, as is done in the conventional spread-spectrum communication. Tn the above patent publication, therefore, synchronization is established by transmitting a special bit pattern in both directions before commencing a communication, and after the synchronization is established, the synchronization is maintained by synchronizing the code sequence at the receiver with the code sequence at the transmitter in the opposite direction.
The above prior art has a problem in that a bit pattern for establishing synchronization needs to be transmitted every time a communication is commenced. Another problem is an inability to cope with phase shifts caused by minute frequency variations after the establishment of synchronization.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a spread-spectrum communication system that can transmit larger volumes of information with a given bandwidth and code sequence length and that can establish and maintain synchronization for a received signal.
According to the present invention, there is provided a transmitter for spread-spectrum communication comprising: a first signal generating section for generating a first signal containing a first code sequence; a second signal generating section for generating a second signal containing a second code sequence phase-modulated with a first information signal; and a modulating section for modulating a carrier with the first and second signals.
According to the present invention, there is also provided a receiver, for spread-spectrum communication, for accepting a received wave consisting of a first signal containing a first code sequence and a second signal containing a second code sequence phase-modulated with a first information signal, and for recovering the information signal, comprising: a demodulating section for demodulating the received wave; a synchronizing section for determining a phase of the first signal by establishing synchronization with the first code sequence contained in the first signal demodulated in the demodulating section; and a first information recovering section for recovering the first information signal by identifying, based on the phase of the first signal, a phase of the second code sequence contained in the second signal demodulated in the demodulating section.
According to the present invention, there is also provided a spread-spectrum communication system comprising: a transmitter including a first signal generating section for generating a first signal containing a first code sequence, a second signal generating section for generating a second signal containing a second code sequence phase-modulated with a first information signal, and a modulating section for modulating a carrier with the first and second signals; and a receiver for accepting a received signal from the transmitter and for recovering an information signal, including a demodulating section for demodulating the received wave, a synchronizing section for determining a phase of the first signal by establishing synchronization with the first code sequence contained in the first signal demodulated in the demodulating section, and a first information recovering section for recovering a first information signal by identifying, based on the phase of the first signal, a phase of the second code sequence contained in the second signal demodulated in the demodulating section.
In the above system, the second code sequence contained in the second signal is phase-modulated with the information signal, whereas the first code sequence contained in the first signal is not phase-modulated; accordingly, synchronization can be established and maintained.
REFERENCES:
patent: 5687162 (1997-11-01), Yoshida et al.
patent: 5689526 (1997-11-01), Slonneger et al.
patent: 5715239 (1998-02-01), Gilhousen et al.
patent: 7-46222 (1995-02-01), None
Inoue Takeshi
Matsuyama Koji
Fujitsu Limited
Helfgott & Karas P.C.
Luther William
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