Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2000-06-20
2004-10-12
Ho, Duc (Department: 2665)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C455S562100, C714S790000
Reexamination Certificate
active
06804216
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a code division multiple access communication method using dynamic code assignment. More particularly, the invention pertains to a code division multiple access communication method using dynamic code assignment in which a spectral spread code is dynamically changed to reduce possible interference during communication between a base station and a mobile terminal in an application such as cellular radio communication, and a base station facility for implementing the code division multiple access communication method using dynamic code assignment.
2. Description of the Related Art
In code division multiple access (CDMA) communication, an information signal band is spread out over a wider band for transmission using a code independent of information signals. Since the CDMA provides many advantages such as excellent security of privacy and secrecy of communication, high resistance to interference, adaptability to poor transmission path condition, asynchronous multiple accessibility based on code division, etc., it is expected to be employed extensively as a signaling scheme suitable for mobile communication, e.g., cellular telephone communication.
As a technique of CDMA communication, there is disclosed a DS-CDMA transmission system in Japanese Unexamined Patent Publication No. H10-145282, for example.
In mobile communication which is the principal application field of CDMA communication, it is common practice to divide a geographical control region into cells for implementing cellular communication. For making features of CDMA communication apparent, a multi-sector system which is one of mobile communication cell models will first be explained here.
Referring to
FIG. 7
, there is shown a schematic diagram for explaining the multi-sector system in cellular radio communication.
A control area called a cell is a geographical area in which communication is set up between a base station and a mobile terminal currently located in the coverage thereof. A multiplicity of cells are arranged in a service area so that each mobile terminal can communicate with at least one of base stations in the service area.
In
FIG. 7
, each area enclosed in a circle is a cell
42
which is covered by a base station
40
.
By using a plurality of antennas each having directivity at the base station
40
, the cell
42
is spatially divided into control sub-areas called sectors
41
, one of which is shown as a fan-shaped section (hatched) in FIG.
7
. Since a directional antenna is used for each of the sectors
41
, it is possible to preclude external interference due to other sectors and increase the capacity of communication.
In the cell-sector scheme mentioned above, each mobile terminal such as a cellular telephone belongs to any cell-sector and a call termination service is provided to each mobile terminal upon receipt of a terminating call signal from a base station which covers the cell-sector scheme. In this situation, the mobile terminal periodically checks a common control signal issued by an adjacent base station to always attain base station conditions in the vicinity thereof. When a cell-sector changeover becomes necessary due to movement of the mobile terminal, the current position of the mobile terminal is re-registered in a relevant base station for carrying out a hand-over procedure (which is a link changeover procedure to be performed for switching to another control area).
Then, the following explains codes to be used for spectral spreading in CDMA communication.
In a CDMA system represented by the IS-95 or W-CDMA, the bitwise product or exclusive-OR result of long and short codes in synchronization is used for spread-spectrum coding for downlink transmission from a base station to a mobile terminal.
The functions of the long and short codes are as follows:
In short code assignment, an orthogonal code having a single-symbol cycle is used for identifying each of plural communication channels in a sector and for reducing possible interference between communication channels.
In long code assignment, a long-cycle code having a different phase or a different code sequence per sector or cell is used for identifying each sector or cell.
Table 1 presents a summary of the functional features of these two kinds of codes.
TABLE 1
Long code
Short code
Function
Identification of
Identification of
cell/sector
channel
Period
long
short
Code number in a period
many
few
Orthogonality
It does not always have
It has
Examples of IS-95,
PN code
Walsh code
W-CDMA, etc.
When correlation operation is performed on different short codes for the same long code, orthogonality is attained in coding. By way of contrast, when correlation operation is performed on different short codes for different long codes, orthogonality is not attained in coding. Orthogonalization in coding signifies that mutual interference does not occur in spread-spectrum communications, and non-orthogonalization in coding signifies that mutual interference may take place in spread-spectrum communications.
In CDMA communication, an improvement in its capacity can be made by reducing possible interference. As methods for significantly increasing the capacity of CDMA communication, advanced techniques such as an interference canceller technique and an adaptive array antenna technique have been suggested in publications. The introduction of these interference reduction techniques makes it possible to simultaneously connect more mobile terminals than ever before.
With reference to
FIG. 8
, a communication system using adaptive array antennas will then be briefly explained.
FIG. 8
is a block diagram of a base station in a communication system wherein adaptive array antennas are employed.
The base station is provided with a plurality of antennas. A signal received by the plural antennas is down-converted through an RF circuit
8
and then fed to a beam former
20
, in which a weight value is added so that a signal-to-interference power ratio of the received signal will be maximized. The beam former
20
acts to provide a spatial filter.
More specifically, a main beam is oriented toward a desired user, and a null position of an antenna radiation is formed in the direction of a major interference potential. In this manner of operation, the adaptive array antenna can vary directivity thereof using electrical circuits without having to change physical positioning thereof. The weight-added composite signal is demodulated by a receiver
10
to provide user information.
In downlink transmission, an antenna beam can also be formed in the same manner. For a signal produced by a data generator
6
, a beam adapted to the direction of each mobile terminal is formed according to uplink channel (propagation path) information. Thus, a signal is transmitted in a limited range of space. In the 1997 Conference Proceedings of the Institute of Electronics, Information and Communication Engineers B-5-88, for example, there is reported a method for estimating a weight value for downlink transmission according to a correlation matrix attained in uplink transmission. Through formation of a beam by this method, it is possible to substantially suppress interference power in downlink transmission. Therefore, the use of the adaptive array antenna having an advantageous capability of interference suppression allows a substantial increase in the number of simultaneous mobile terminal connections, making it possible to simultaneously connect more mobile terminals than those permissible at a spectral spread rate in a conventional arrangement.
The technique of adaptive array antennas is found in NTT DoCoMo Technical Journal Vol. 5—No. 4, p. 25, for example. Through the use of adaptive array antennas in cellular radio communication, the number of mobile terminals to be simultaneously connected with a base station can be increased in a cell covered by the base station.
As described above, in CDMA communication, an information signal spectrum is spread using a spread code to enable multiple access
Doi Nobukazu
Kuwahara Mikio
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