Pulse or digital communications – Spread spectrum
Reexamination Certificate
1996-07-16
2001-06-26
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
C375S141000, C370S342000
Reexamination Certificate
active
06252898
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved spread spectrum communication method and communications system.
Referring first to
FIG. 2
, in a radio communications system utilizing spread spectrum modulation system, data to be transmitted applied at an input terminal
15
is multiplied by a spread code generated by a spread code generator
16
in a spread modulator
17
. Here, the multiplication of digital data means a logical exclusive OR operation.
Specifically, the value (1 or 0) of binary data to be transmitted is left as it is when the value of the spread code is zero, while the polarity of the data to be transmitted is inverted when the value of the spread code is one.
Explaining in detail with reference to
FIG. 3
, where data to be transmitted is represented by (A) and a spread code by (B), the data to be transmitted (A) is spread modulated on the spread code (B) to generate a signal as illustrated in FIG.
3
(C). As can be seen from FIGS.
3
(A)-(C), the modulated signal (C) has the polarity of data reverse to the polarity of the spread code (B) during a period in which the data to be transmitted (A) is at one level.
Here, the spread code may be random data having a data rate which is an integer multiple of that of the data to be transmitted, and an M-series code, for example, may be used therefor. The M-series code has a finite recursive period (for example, approximately 1,000 bits), and may be regarded as a pseudo random code.
As a result of the multiplication of the data to be transmitted (A) by the spread code (B), the data to be transmitted is converted into a random code having the data rate equal to that of the spread code, as illustrated in FIG.
3
(C).
Assuming that the data rate of the data to be transmitted is f
T
and the data rate of the spread code is f
D
, the latter is divided by the former to derive g (f
D
/f
T
=g) which is called the “spread gain”.
A bandwidth occupied by the data to be transmitted depends on the contents of the data (amount of information). When the data is random (including many high frequency components), it has a frequency bandwidth substantially corresponding to f
T
.
A bandwidth occupied by the data to be transmitted multiplied by the spread code does not depend on the contents of the data to be transmitted, in which case the resulting data has a frequency bandwidth substantially corresponding to f
D
.
Referring back to
FIG. 2
, an output of the spread modulator
17
is modulated, for example, in accordance with the BPSK (binary phase shift keying) scheme in a modulator
18
, and then converted into a high frequency signal in a frequency convertor
19
. In the radio transmission in accordance with the spread spectrum modulation scheme, a 2.4 GHz bandwidth in an ISM (industrial scientific and medical equipment) band is used as a frequency band for such high frequency signals. The high frequency signal is transmitted through a transmission antenna
20
.
The high frequency signal transmitted from the transmission antenna
20
is received by a reception antenna
21
at a receiving station, and converted into an intermediate frequency signal in a frequency convertor
22
. Then, the intermediate frequency signal is demodulated to random data having the data rate f
D
in a BPSK demodulator
23
.
Explaining in greater detail with reference again to
FIG. 2
, a spread code generator
24
generates the same spread code as that generated on the transmission side (called the “inverse spread code). The inverse spread code is multiplied by the demodulated random data in the inverse spread demodulator
25
. As a result of the multiplication in the inverse spread demodulator
25
, if the inverse spread code has the value of zero, the value of the demodulated data is left as it is. Conversely, if the inverse spread code has the value of one, the polarity of the demodulated data is inverted.
In other words, when received data, identical to the spread modulated data to be transmitted (C) as illustrated in
FIG. 3
, is multiplied by the inverse spread code identical to the spread code (B), the received data identical to the data to be transmitted (A) is derived.
Thus, if the recursive period and the phase of the spread code on the transmission side are maintained identical to those of the inverse spread code on the reception side, received data equal to transmitted data is delivered at an output terminal
26
.
The spread spectrum scheme features that an originally imparted bandwidth f
T
of data is expanded to a wider bandwidth f
D
for transmission, so that the S/N (signal to noise) ratio of a received signal is increased by a factor of g equal to the spread gain, thus enabling signals to be transmitted even on a transmission path exhibiting a low S/N ratio.
In the spread spectrum communications scheme, a C/N (carrier to noise) ratio is degraded in a receiver under communication when another spread spectrum transmitter approaches thereto. More specifically, assuming that the approaching spread spectrum transmitter has a carrier (a portion inputted to the receiver under communication) C
1
, noise N introducing into the receiver is increased to N+C
1
, whereby the C/N ratio is degraded to C/(N+C
1
). In other words, other nearby transmitters may interfere with a receiver under communication.
U.S. Pat. No. 5,056,109 discloses that the transmission power of a mobile station is adjusted in accordance with a communication distance from the mobile station to others in a cellular mobile telephone system to prevent the carrier of the mobile station from interfering with other nearby mobile stations, as mentioned above.
The transmission power may be controlled by a variety of schemes, such as open loop control, closed loop control, and so on. However, the addition of any control scheme would make the entire system more complicated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a spread spectrum communication method and communications system which are capable of performing high quality communications without employing a complicated configuration.
In the spread spectrum communication method according to the present invention, during a communication in accordance with the spread spectrum communications scheme between two stations remote from each other, a parameter indicative of a C/N value of a received signal is measured at one of the stations. If the measured C/N value presents a change equal to or larger than a predetermined value, the data rate of a signal to be transmitted from the other station is changed.
Also, a spread spectrum communications system according to the present invention has a first communications station including a unit for spread spectrum modulating an input signal with a spread code and a unit for converting a spread spectrum modulated signal into a radio frequency signal and transmitting the radio frequency signal, and a second communications station including a receiver unit for receiving a radio frequency signal from the first communications station and reproducing a spread spectrum modulated signal, a demodulator unit for inverse spread spectrum demodulating the spread spectrum modulated signal with an inverse spread code having the same period and phase as the spread code, a unit for detecting a parameter indicative of a C/N value of the received signal in the receiver unit, and a unit for communicating information on the detected C/N value to the first communications station. The first communications station also has a control unit for receiving the information on the C/N value and changing the data rate of a signal to be transmitted from the first communications station when the C/N value presents a change equal to or more than a predetermined value.
In the present invention, the data rate of a signal to be transmitted is reduced to maintain a predetermined C/N value even if the C/N value (or the S/N value) of a received signal is degraded. Alternatively, if the C/N value of a received signal is excessively high, the data rate of a signal to be tran
Asahina Takashi
Eto Yoshizumi
Antonelli Terry Stout & Kraus LLP
Chin Stephen
Fan Chieh M.
Hitachi Denshi Kabushiki Kaisha
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