Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-09-18
2002-11-05
Ngo, Ricky (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C370S441000, C370S479000, C370S320000
Reexamination Certificate
active
06477158
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to code assignment in multi-rate communications of the radio communication system employing Code Division Multiple Access (CDMA) system, where codes having tree structured orthogonal multi-spreading factor sequence are used as spreading codes.
2. Description of the Related Art
The system utilizing coherent multi code and the system utilizing tree structured multi-spreading factor sequence code have been introduced as a multi-rate transmission system in the radio communication system employing DS-CDMA (Direct Sequence-CDMA). In the multi-rate transmission utilizing the coherent multi code, plural codes are simultaneously used to perform multi-code-multiplexing. In the multi-rate transmission utilizing the tree structured multi-spreading factor sequence code, orthogonal codes having equivalent spreading factors are multiplexed in parallel. Receivers for receiving plural codes are needed for a mobile station used for the multi-rate transmission utilizing the coherent multi code. On the other hand, only one receiver is needed for the system utilizing the tree structured multi-spreading factor sequence code.
The conventional CDMA system using tree structured orthogonal multi-spreading factor sequences is disclosed in, for example, a technical report entitled “Multi-rate Wideband DS-CDMA Radio Access For Next Generation Mobile Radio Systems”, the Institute of Electronics, Information and Communication Engineers, RCS-97-86, 1997. The conventional system will be explained with reference to FIG.
11
.
FIG. 11
shows a configuration of the conventional tree structured orthogonal multi-spreading factor sequences.
The tree structure is composed of four levels: level
1
at the top through level
4
at the bottom, defined just for convenience. As there are four levels in this case, the channel transmission rate using the top level (level
1
) code is eight times as fast as that of the channel using the lowest level (level
4
) code. The channel transmission rate using level
2
code is four times the rate of channel using the level
4
code. The channel transmission rate using level
3
code is twice the rate of channel using the level
4
code. Cx(y) in
FIG. 11
represents a kind of codes, x stands for a code level, y stands for an identification number in the same code level, and ◯ stands for a code.
In
FIG. 11
, a code in the level
1
can be used for communication of twice the transmission rate of level
2
. Now, a concrete example will be explained referring to the case of tree structure as shown in FIG.
11
. When C
2
(
1
) is assigned to a user of four times the transmission rate of the lowest rate (C
4
(x) level in FIG.
11
), the codes C
4
(
1
), C
4
(
2
), C
4
(
3
), C
4
(
4
), C
3
(
1
), and C
3
(
2
) can not be assigned to other users because of keeping the code orthogonality. Due to this characteristic, all the lower level codes connected with an assigned code through branches and all the upper level codes connected with the assigned code through only upward branches are not-assignable. When C
2
(
1
) is the assigned code, all the lower codes C
4
(
1
), C
4
(
2
), C
4
(
3
), C
4
(
4
), C
3
(
1
) and C
3
(
2
) connected with the assigned code by branches are not-assignable, and the upper code C
1
(
1
) connected with the assigned code by the shortest branch is not-assignable, either.
As stated above, in a CDMA system using codes of tree structured orthogonal multi-spreading factor sequences, the code assignment for a mobile station is restricted because of the tree structure characteristic. Accordingly, in the case of
FIG. 11
, if C
4
(
5
) and C
4
(
7
) are assigned to calls of the lowest rate (level
4
), it is impossible to assign a call having the rate of level
3
or higher than level
3
. However, on the supposition that C
4
(
6
) is assigned to the call of level
4
instead of C
4
(
7
), it is possible to assign C
3
(
4
) to the call of level
3
, and consequently, channels are effectively utilized.
OBJECTS OF THE PRESENT INVENTION
In the multi-rate transmission of the conventional radio communication system employing Code Division Multiple Access (CDMA) system where codes having tree structured orthogonal multi-spreading factor sequence are used, when a code has been already assigned (in use), it is impossible to assign all the codes upper than the code. This causes a problem that the channel utilization is deteriorated. It is an object of the present invention to utilize all the system (all the branches) of the tree structure for assigning a code to be responsive to a new call, in order to enhance the channel utilization.
In the conventional art, a channel mismatch would occur at the call ending, even if a channel match was kept at the code assigning time of a new call starting. This sometimes causes a problem that selecting a code to be assigned to a new call is impossible. It is another object of the present invention is to avoid the channel mismatch by switching a code being used in communication. Consequently, call loss rate can be reduced.
In the code assignment of the conventional radio communication system employing CDMA system, it is necessary for a base station and a mobile station to simultaneously switch codes in order not to stop the communication under way. However, as the base station does not have means for judging a code switch timing over the mobile station, the base station is troublesomely needed to simultaneously perform reception and transmission using both the codes used before and after switching. It is another object of the present invention to reduce the stop time caused by code switching and the hardware needed for reception/transmission using both the codes between the base station and the mobile station, by means of sending the code switching timing from the base station.
In the code assignment of the conventional radio communication system employing CDMA system, there is a problem that much process is needed for synchronizing the code switch timings in the base station and the mobile station. It is another object of the present invention to reduce the code switching process by means of depending on mobile station's own timing.
Namely, it is a general object of the present invention to provide an apparatus and a method for effectively utilizing the channel, comprising means in the base station to judge a code switch timing over the mobile station, and reducing the synchronization process.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a code assigning apparatus used in a radio communication system including a base station controlling apparatus, a plurality of base stations, and a plurality of mobile stations, employing CDMA (Code Division Multiple Access) method, which uses codes of tree structured orthogonal multi-spreading factor sequences as spreading codes, for radio access between the plurality of base stations and the plurality of mobile stations, and providing multi-rate transmission between the plurality of mobile stations and the plurality of base stations, the code assigning apparatus comprises a code selecting unit for selecting an assignable code which suits specifications requested by a new starting call and exists in a location as close to a not-assignable code as possible and in a level lower than the not-assignable code in the tree structure.
According to another aspect of the present invention, the code selecting unit of the code assigning apparatus comprises
a first selecting unit for selecting one or more than one codes corresponding to rate information included in a message from one of the plurality of mobile stations, as first codes,
a second selecting unit for selecting one or more than one first codes if they are assignable to the one of the plurality of mobile stations, as second codes,
a level detecting unit for retrieving codes upper than the second codes one by one, detecting not-assignable codes, which are assigned to another of the plurality of mobile stations, out of the codes upper than
Ngo Ricky
Phan Tri H.
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