Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-09-13
2003-06-17
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06580748
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a spread spectrum communication method and a spread spectrum communication apparatus suitable for application to mobile radio communication or the like.
Along with an increase in radio communication stations, the spread spectrum communication system, which is relatively immune from noise and interference, is attracting extensive interest. The spread spectrum communication system is a communication formula whereby the spectrum bandwidth is intentionally expanded by modulating signals, which have undergone usual data modulation, such as PSK or QAM, with a high speed sequence of codes known as spreading codes, and the coding rate of these spreading codes is called the chip rate fc. The chip rate fc has a relationship to the coding rate of the transmit data to be spread, i.e. the bit rate fb, of BW=fc/fb (BW is an integer), and this integral value BW is called the bandwidth expansion factor.
In recent years, techniques to make the aforementioned chip rate fc for communication systems using spread spectrum communication have been proposed. For instance, the Japanese Patent Laid-open No. 8-065264 discloses a method by which, although a plurality of receiving stations use the same spreading codes, each receiving station is enabled, by making the chip rate fc variable, to extract only desired signals by detecting correlations and performing despreading at the same chip rate fc as the counterpart transmitting station.
FIG. 12
is a block diagram illustrating the configuration of the receiving section of the spread spectrum communication apparatus proposed in the Japanese Patent Laid-open No. 8-065264. In this system, the mutually opposite transmitting and receiving stations can choose the type of code from a selective spreading code generator
65
and the chip rate from a selective clock generator
66
.
Therefore, by predetermining the code type and the chip rate between the transmitting and receiving stations, only signals from the counterpart in the communication can be extracted at the time of despreading even if the code type of an interfering wave coincides because the chip rate is different.
Further, the Japanese Patent Laid-open No. 6-276176 proposes a method by which, with a view to solving the relative distance problem by reducing inter-signal interference at the time of demodulation due to an imbalance in reception field strength of signals from remote stations at a base station, a lower chip rate fc is given to the transmitting side when receive signals of a high field strength are received by the base station and a higher chip rate fc is given to the transmitting side when receive signals of a low field strength are received so as to achieve the best possible uniformization of reception field strength at the base station.
FIG. 13
is a block diagram illustrating the configuration of the CDMA communication system disclosed in the Japanese Patent Laid-open No.
6-276176
. Signals from remote stations
71
and
72
are subjected to despread-demodulation by a despread-spectrum demodulating section
74
in a base station
73
and to receive power determination by a receive power detecting section
75
. On the basis of the detected receive power, a chip rate determining section
76
and a chip rate notifying section
77
carry out chip rate control over the aforementioned remote stations.
Whereas the benefit of a variable chip rate fc is as described above, making the chip rate fc variable means making the bandwidth expansion factor BW variable, and making the bandwidth expansion factor BW variable provides the following benefits.
(a) By raising the bandwidth expansion factor BW, the S/N ratio of the desired wave after despreading on the receiving side can be improved.
(b) Raising the bandwidth expansion factor BW results in an expanded bandwidth and a corresponding reduction in transmit peak power, which makes possible suppression of interference with other stations.
(c) The bandwidth expansion factor BW can be so set as to optimize the efficiency of frequency utilization of the whole system.
On the other hand, since the S/N ratio of the largest correlated values obtained by a correlator in a receiver is proportional to the code length (the number of chips per period) L [chips] of spreading codes, the correlation detecting performance of the receiver can be improved by extending the code length L.
Now, in the conventional spread spectrum communication system described with reference to FIG.
12
and
FIG. 13
, 1 period equivalent of spreading codes is always accommodated within 1 information bit, and the following equation holds.
L=BW=fc/fb
(1)
In order to improve the correlation detecting performance of a receiver and make it relatively immune from noise and interference, a method to raise the bandwidth expansion factor BW and another to extend the code length L of spreading codes is conceivable. However, where the code length L and the bandwidth expansion factor BW are always kept equal as in the spread spectrum communication systems according to the prior art, it is impossible to make these factors independently variable.
Especially, the code length L of spreading codes cannot be made independently variable in disregard of the bandwidth expansion factor BW, and in almost every case it is limited by the bandwidth expansion factor BW.
The reason is that the bandwidth expansion factor BW is prevented from being raised beyond a certain level by the need to optimize the efficiency of frequency utilization by the whole system in consideration of the environment of use, spread processing and the limitation of the operating speed of a device performing analog-to-digital (A/D) conversion at a later stage, both on the transmitting side, and despread processing and the limitation of the operating speed of a device performing digital-to-analog (D/A) conversion, both on the receiving side.
Therefore, in the conventional spread spectrum communication systems, where the environment of use or the limitation of devices prevents the bandwidth expansion factor BW from being raised substantially, the code length L of spreading codes is kept short, resulting in poor correlation detecting performance of the receiver.
In order to solve this problem, it is necessary to enable 1 period of spreading codes to span a plurality of information bits. Where 1 period of spreading codes spans N information bits, the following equation holds.
N=L/BW
(2)
What poses a problem here is that the value of N bits is not fixed. Since this value of N bits constitutes an information bit, it is not in a fixed pattern, such as being always “1” or the like. Correlation detection at this time is accomplished as represented by the following equation.
C
(
j
)
=
1
L
∑
k
=
0
L
-
1
R
(
j
-
k
+
1
)
·
pn
(
k
)
(
3
)
In Equation (3), C(j) represents the correlated value at a time j; R(j), the spread receive signal entered into the correlator at the time j; and pn(k), a despreading code.
If transmit and receive codes are identical in timing, and the values of all of N information bits before the spread are either “1” or “−1”, Equation (3) will give a value of “1”or “−1”, respectively.
However, if N information bits before the spread randomly include “1” and “−1”, the result will vary with the ratio between “1” and “−1” at a given time. If, for instance, “1” and “−1” are included in equal proportions, the result will be “0”. As a correlator usually recognizes the peak of correlated values as the coincidence of transmit and receive codes in timing, the correlator is unable to correctly detect coincidence in code timing in such a case.
As described above, spread spectrum communication systems according to the prior art involve the problem that they do not allow the code length L of spreading codes and the bandwidth expansion factor BW to vary independently and, if this problem is to be solved, there will arise another prob
Dickstein , Shapiro, Morin & Oshinsky, LLP
NEC Corporation
Nguyen Dung X.
Pham Chi
LandOfFree
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