A method and apparatus for assigning codes

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

0001The 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.

DESCRIPTION OF THE RELATED ART

0002The 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.

0003The conventional CDMA system using tree structured orthogonal multi-spreading factor sequences is disclosed in, for example, a technical report entitled "<nplcit id="ncit0001" npl-type="s"><text>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</text></nplcit>. The conventional system will be explained with reference to <figref idref="f0011">Fig. 11</figref>.

0004<figref idref="f0011">Fig. 11</figref> shows a configuration of the conventional tree structured orthogonal multi-spreading factor sequences.

0005The 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 <figref idref="f0011">Fig. 11</figref> 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.

0006In <figref idref="f0011">Fig. 11</figref>, 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 <figref idref="f0011">Fig. 11</figref>. When C₂(1) is assigned to a user of four times the transmission rate of the lowest rate (C₄(x) level in <figref idref="f0011">Fig. 11</figref>), the codes C₄(1), C₄(2), C₄(3), C₄(4), C₃(1), and C₃(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 C₂(1) is the assigned code, all the lower codes C₄(1), C₄(2), C₄(3), C₄(4), C₃(1) and C₃(2) connected with the assigned code by branches are not-assignable, and the upper code C₁(1) connected with the assigned code by the shortest branch is not-assignable, either.

0007As stated above, in CDMA system using codes of tree structured orthogonal multi-spreading factor sequences, the code assignment for a mobile station is restricted because of the characteristic. Accordingly, in the case of <figref idref="f0011">Fig. 11</figref>, if C₄(5) and C₄(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 upper than level 3. However, on the supposition that C₄(6) is assigned to the call of level 4 instead of C₄(7), it is possible to assign C₃(4) to the call of level 3, consequently channels are effectively utilized.

0008<patcit id="pcit0001" dnum="EP0765096A"><text>EP-A 076 5 096</text></patcit> discloses a CDMA mobile communication system for code assignment by a base station in order to simultaneously communicate between the base station and a plurality of mobile stations in order to reduce collision between packets sent from different mobile stations. Therefore, each mobile channel may switch from access channel to message channel without switching from one transmission code to another transmission code for transmission between said mobile station and the base station. <patcit id="pcit0002" dnum="EP0814581A2"><text>EP 08 14 581 A2</text></patcit> discloses a CDMA communication method capable of providing a multi-rate user with high quality transmission. The spreading sequences are arranged in a hierarchical tree structure. Higher level sequences are selected for higher data transmission rate.

OBJECTS OF THE PRESENT INVENTION

0009In 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 upped 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.

0010In 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.

0011In 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.

0012In 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.

0013Namely, 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

0014These problems are solved by the radio communication system according to claim 1.

0015The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

0016In the drawings: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> shows a configuration of radio communication system applying a code assignment method according to the present invention;</li><li><figref idref="f0002">Fig. 2</figref> illustrates sequence of a code assignment procedure according to the present invention;</li><li><figref idref="f0003">Fig. 3</figref> shows a code structure used for the code assignment procedure of <figref idref="f0002">Fig. 2</figref>;</li><li><figref idref="f0004">Fig. 4</figref> is a flowchart showing a process of the code assignment procedure of <figref idref="f0002">Fig. 2</figref>;</li><li><figref idref="f0005">Fig. 5</figref> illustrates sequence of another code assignment procedure according to the present invention;</li><li><figref idref="f0006">Fig. 6</figref> shows a code structure used for a code switching procedure of <figref idref="f0005">Fig. 5</figref>;</li><li><figref idref="f0007">Fig. 7</figref> shows a code structure used for selecting a code-switching candidate code;</li><li><figref idref="f0008">Fig. 8</figref> is a flowchart showing a process of the code assignment procedure of <figref idref="f0005">Fig. 5</figref>;</li><li><figref idref="f0009">Fig. 9</figref> illustrates sequence of another code assignment procedure according to the present invention;</li><li><figref idref="f0010">Fig. 10</figref> illustrates sequence of another code switching procedure according to the present invention;</li><li><figref idref="f0011">Fig. 11</figref> shows a tree structure of codes having tree structured orthogonal multi-spreading factor sequence, used in a conventional art;</li><li><figref idref="f0012">Fig. 12</figref> shows a configuration of a code selecting unit in a base station controller according to the present invention;</li><li><figref idref="f0013">Fig. 13</figref> shows a level list according to the present invention;</li><li><figref idref="f0014">Fig. 14</figref> shows configurations of a code selecting unit and a code-switching/code-assignment processing unit, in a base station controller according to the present invention;</li><li><figref idref="f0015">Fig. 15</figref> shows a switching list according to the present invention;</li><li><figref idref="f0016">Fig. 16</figref> shows a system structure according to the present invention; and</li><li><figref idref="f0017">Fig. 17</figref> shows a system structure according to the present invention.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1.

0017This embodiment is not an embodiment of the invention, but helpful to understand certain aspects of the invention.

0018<figref idref="f0001">Fig. 1</figref> shows a configuration of radio communication system applying a code assignment method according to the present embodiment. <figref idref="f0002">Fig. 2</figref> illustrates sequence of a code assignment procedure according to the present embodiment. <figref idref="f0003">Fig. 3</figref> shows a code structure example of the code assignment procedure of <figref idref="f0002">Fig. 2</figref>. <figref idref="f0004">Fig. 4</figref> is a flowchart showing a process of the code assignment procedure of <figref idref="f0002">Fig. 2</figref>. <figref idref="f0012">Fig. 12</figref> shows a configuration of a base station controller 1. Referring to <figref idref="f0001">Figs. 1</figref>, <figref idref="f0002">2</figref>, <figref idref="f0003">3</figref>, <figref idref="f0004">4</figref>, and <figref idref="f0012">12</figref>, one embodiment of code assignment performed in a base station 2 and the base station controller 1 according to the present invention will be described.

0019<figref idref="f0001">Fig. 1</figref>, showing a configuration of radio communication system of the present embodiment and other embodiments, is composed of plural mobile stations 3a, 3b and 3c, plural base stations 2a, and 2b, and the base station controller 1 which controls the plural base stations 2a and 2b. The mobile stations 3a, 3b and 3c are just called as a "mobile station 3" in the case of describing each mobile station being unnecessary. The base stations 2a and 2b are also called as a "base station 2" in the case of describing each base station being unnecessary. The mobile station 3 is connected to the base station 2 via a radio channel: The radio channel utilizes the Code Division Multiple Access (CDMA) system as a radio access system, and uses spreading codes having tree structured orthogonal multi-spreading factor sequences. According to the radio communication system of the present embodiment, it is possible to set up multiple rates. Namely, the base station 2 and the mobile station 3 can set up channels of plural-kinds rates for each call, using the radio channel. As orthogonal multi spreading codes are used, a channel having a double transmission rate can be set up in proportion as a code level goes up one by one.

0020<figref idref="f0002">Fig. 2</figref> illustrates sequence of a code assignment procedure in responsive to a call from the mobile station 3. Referring to <figref idref="f0001">Figs. 1</figref> and <figref idref="f0002">2</figref>, the code assignment procedure regarding the base station controller 1, the base station 2 and the mobile station 3 will now be explained. First, a radio channel set-up request message is transmitted from the mobile station 3 to the base station controller 1 via the base station 2. The base station controller 1 analyzes the message in order to detect a transmission rate requested by the mobile station 3. The base station controller 1 determines a pair of codes for transmission to be used by the mobile station 3 and the base station 2, based on the code assignment process stated later. Then, the base station controller 1 transmits the radio channel set-up request message to the base station 2.

0021The radio channel set-up request message includes information of the code pair. Receiving the message, the base station 2 begins to prepare for reception/transmission based on an appointed code, and transmits a radio channel set-up complete message to the base station controller 1. On receiving the radio channel set-up complete message, the base station controller 1 transmits the radio channel set-up complete message appointing the pair of codes to the mobile station 3. After receiving the message, the mobile station 3 starts communication with the base station 2, based on the appointed code.

0022As stated above, the communication channel between the mobile station 3 and the base station 2 is set up based on the request from the mobile station 3. A code selection process, performed in the base station controller 1 for the mobile station 3 and the base station 2 is explained with reference to <figref idref="f0003">Figs. 3</figref>, <figref idref="f0004">4</figref> and <figref idref="f0012">12</figref>. <figref idref="f0003">Fig. 3</figref> shows a tree structure of codes having orthogonal multi-spreading factor sequence. In <figref idref="f0003">Fig. 3</figref>, 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 channel using the lowest level (level 4) code. The channel transmission rate using level 2 code is four times the rate of channel using level 4 code. The channel transmission rate using level 3 code is twice the rate of channel using level 4 code.

0023Cx(y) in <figref idref="f0003">Fig. 3</figref> 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 the tree structured orthogonal multi spreading sequence codes, codes lower than a specific code being used (that is, larger number level codes connected with the specific code by branches in <figref idref="f0003">Fig. 3</figref>) can not be used because of keeping the orthogonality characteristic.

0024In addition, codes upper than the specific code being used (that is, smaller number level codes connected with the specific code through only upward branches) also can not be utilized. When C₃(2) is being used in <figref idref="f0003">Fig. 3</figref>, C₁(1), C₂(1), C₄(3) and C₄(4) can not be used. Therefore, it is necessary to appropriately assign a code to a new call from the mobile station 3.

0025<figref idref="f0012">Fig. 12</figref> shows a configuration of a code selecting unit 10 in the base station controller 1. <figref idref="f0004">Fig. 4</figref> is a flowchart showing code assignment processes, for the mobile station 3 and the base station 2, performed in the code selecting unit 10. When a first selector 11 of the base station controller 1 receives the radio channel set-up request message from the mobile station 3 (S0101), the first selector 11 analyses rate information in the message (S0102), and determines a code level needed for the mobile station 3 and the base station 2 (S0103). In <figref idref="f0003">Fig. 3</figref>, for instance, the code level requested by the mobile station 3 is level 3. Secondly, a second selector 12 of the base station controller 1 selects candidate codes which can be used in the determined level (S0104). In <figref idref="f0003">Fig. 3</figref>, C₃(1), C₃(3) and C₃(4) are selected as the candidate codes.

0026A level detector 13 of the base controller 1 selects a code out of the candidate codes (S0105), selects another code in a level one more upper than the selected code (S0106), and judges whether or not the one more upper level code can be assigned (S0107). In <figref idref="f0003">Fig. 3</figref>, if the candidate code is C₃(1), the one more upper level code is C₂(1)-Then, when the one more upper level code is able to be assigned, the judging procedure is repeated upwards along the branches until it comes to a code which can not be assigned (S 1018, S0109). If a code that is not assignable is detected, the code and its level are memorized in a level list 16 for each of the candidate codes (S0107, S0110). When C₃(1) in <figref idref="f0003">Fig.3</figref> is the candidate code, one more upper code C₂(1) is not assignable. Then, C₃(1), C₂(1) and the level 2 are stored in the level list 16 as shown in <figref idref="f0013">Fig. 13</figref>. This procedure is performed for each candidate code to complete the level list 16 shown in <figref idref="f0013">Fig. 13</figref> (S0111).

0027A third selector 14 selects a candidate code whose upper not-assignable code has the lowest level, in the level list 16 (S0112). Then, the candidate code is selected to be assigned to the mobile station 3 (S0113). If there are several candidate codes whose upper not-assignable codes have the same code level, any of the candidate codes can be selected. If a candidate code has no upper not-assignable code, the top level code is regarded as the upper not-assignable code. As shown in <figref idref="f0013">Fig. 13</figref>, C₂(1) being the upper not-assignable code and level 2 are selected for the candidate code C₃(1) in <figref idref="f0003">Fig. 3</figref>. C₁(1) and level 1 are for the candidate code C₃(3), and C₁(1) and level 1 are for the candidate code C₃(4). Therefore, C₃(1), whose upper not-assignable code is C₂(1), is selected as a code to be assigned. According to this assigning method, C₃(3) and C₃(4) in <figref idref="f0003">Fig. 3</figref> are not selected. The code C₃(1) is assigned to the mobile station 3 by a code assigning unit 15. If a new call corresponding to level 2 is requested after the call corresponding to level 3 has been in use as shown in <figref idref="f0003">Fig. 3</figref>, C₂(2) can be assigned to the new call.

0028According to the present assigning method, a channel mismatch at the code assigning time of a new call starting, caused by the tree structure characteristic, is avoided. Therefore, a code assignment for multi- rate transmission can be performed without deteriorating the channel utilization efficiency. It is also acceptable to provide the code selecting unit 10 of <figref idref="f0012">Fig. 12</figref> in the base station 2 instead of the base station controller 1.

Embodiment 2.

0029This embodiment is not an embodiment of the invention, but helpful to understand certain aspects of the invention.

0030<figref idref="f0005">Fig. 5</figref> illustrates a code assignment sequence showing a code switching procedure, according to the present embodiment. <figref idref="f0006">Fig. 6</figref> shows a code structure applied for the code switching procedure of <figref idref="f0005">Fig. 5</figref>. <figref idref="f0007">Fig. 7</figref> shows a code structure used in selecting a candidate code for the code switching. <figref idref="f0008">Fig. 8</figref> is a procedure flowchart for the code assignment sequence of <figref idref="f0005">Fig. 5</figref>. <figref idref="f0014">Fig. 14</figref> shows a configuration of the base station controller according to the present embodiment. Referring to <figref idref="f0001">Figs. 1</figref>, <figref idref="f0005">5</figref>, <figref idref="f0006">6</figref>, <figref idref="f0007">7</figref>, <figref idref="f0008">8</figref> and <figref idref="f0014">14</figref>, the code assignment according to the present embodiment will now be described.

0031The code switching is explained with reference to <figref idref="f0006">Fig. 6</figref> showing a code assignment state change. The same code definition as <figref idref="f0003">Fig. 3</figref> is used in <figref idref="f0006">Fig. 6</figref>. As stated in Embodiment 1, when a code has been already assigned to the mobile station 3, it is impossible to newly assign a code in upper level than the assigned code to another mobile station 3, because of the characteristic of spreading codes having tree structured orthogonal multi-spreading factor. In the left of <figref idref="f0006">Fig. 6</figref>, as C₄(1) and C₄(3) have been already assigned, it is impossible to assign C₃(1) and C₃(2) to another mobile station 3. Accordingly, if there is a call requesting a transmission rate corresponding to level 3 at this time, the call is to be lost. By switching the code from C₄(3) to C₄(2), however, C₃(2) can be assigned, so that the call requesting the transmission rate corresponding to level 3 is accepted.

0032Referring to <figref idref="f0001">Figs. 1</figref> and <figref idref="f0005">5</figref>, the code assignment procedure and the code switching procedure regarding the base station controller 1, the base station 2, and the mobile station 3 will now be explained. In <figref idref="f0001">Fig. 1</figref>, a mobile station #1 (3a) has been already used, and a mobile station #2 (3b) newly makes a call. As shown in <figref idref="f0005">Fig. 5</figref>, the mobile station #2 (3b) transmits a radio channel set-up request message indicating a transmission rate, to the base station controller 1 as stated in Embodiment 1.

0033When there is no assignable code as shown in the left of <figref idref="f0006">Fig. 6</figref>, it is necessary to perform code switching. Therefore, the mobile station #1 (3a) and a code are selected for the code switching, based on a method stated later. The radio channel set-up request message is transmitted to a base station 2a in order to assign the selected code to the mobile station #2 (3b).

0034The base station 2a judges whether the code appointed in the radio channel set-up request message can be used or not. In this case, as the code is being used by the mobile station #1 (3a), it is judged to be not usable. Then, a code switching request is transmitted to the base station controller 1. On receiving the code switching request from the base station 2a, the base station controller 1 transmits a code switching request message including information of code after switching to the mobile station #1 (3a). In the case of <figref idref="f0006">Fig. 6</figref>, the base station controller 1 defines C₄(2) as the code after-switching in the code switching request message.

0035When the mobile station #1 (3a) receives the code switching request message, the mobile station #1 (3a) switches the original code to the target code (code after switching) (C₄(2) in <figref idref="f0006">Fig. 6</figref>), and transmits a code switching complete message to the base station controller 1. The base station controller 1 judges, by receiving the code switching complete message, that the code has been switched in the mobile station #1 (3a). (The state has been changed from the left side to the right side in <figref idref="f0006">Fig. 6</figref>.) Then, the base station controller 1 transmits the code switching complete message to the base station 2a. After receiving the code switching complete message, the base station 2a transmits a radio channel set-up complete message to the base station controller 1. The base station controller 1 transmits the radio channel set-up complete message to the mobile station #2 (3b) in order to assign an assignable and requested-level code whose level is corresponding to the transmission rate requested by the mobile station #2 (3b) (that is, C₃(2) in <figref idref="f0006">Fig. 6</figref>).

0036By performing the above procedures, a new radio channel requested by the mobile station 3 has been set up between the mobile station 3 and the base station 2. Now, the code switching procedure, for the mobile station #1 (3a) and the base station 2a, performed in the base station controller 1, and the code assigning procedure, for the mobile station #2 (3b) and the base station 2a, performed in the controller 1 are explained with reference to <figref idref="f0007">Figs. 7</figref>, <figref idref="f0008">8</figref> and <figref idref="f0014">14</figref>. Similar to <figref idref="f0003">Fig. 3</figref>, <figref idref="f0007">Fig. 7</figref> shows a tree structure of codes having orthogonal multi-spreading factor sequence. The candidate code/not-assignable in the figure represents an unused code which is corresponding to a requested transmission rate and whose lower level code has been already assigned. <figref idref="f0014">Fig. 14</figref> shows a configuration of a code-switching/code-assignment processing unit 20 in the base station controller 1. <figref idref="f0008">Fig. 8</figref> is a flowchart showing a code switching and a code assigning process performed in the code-switching/code-assignment processing unit 20.

0037Similar to Embodiment 1, when the first selector 11 of the base station controller 1 receives the radio channel set-up request message from the mobile station#2 (3b) (S0101), the first selector 11 analyzes rate information in the message (S0102) and determines a code level in the tree structure corresponding to the rate information(S0103). In <figref idref="f0007">Fig. 7</figref>, for instance, the code level corresponding to the rate information is level 3. The second selector 12 of the base station controller 1 retrieves an assignable code of the determined level (S0104). When there is no assignable candidate code of the determined level (S0201), a fourth selector 21 selects a code which is not assignable and has not been itself assigned, as a candidate code (S0204). In <figref idref="f0007">Fig. 7</figref>, C₃(1) and C₃(2) are such codes. In the case of there being no code which is not assignable and has not been itself assigned yet (S0202 : No), the original call becomes lost (S0203).

0038An assigned code detector 22 selects a code out of candidate codes ( C₃(2) in <figref idref="f0007">Fig. 7</figref>) (S0205), and detects an assigned code in lower level than the selected candidate code. (In <figref idref="f0007">Fig. 7</figref> case, C₄(3)) (S0206). A switchable code detector 23 retrieves codes to see whether there is a switchable code at the same level as the above assigned code or not. (S0208). When there is a switchable code in the same level as the assigned code (C₄(2) in <figref idref="f0007">Fig. 7</figref>), the candidate code (C₃(2)), the assigned code (C₄(3)) and the switchable code (C₄(2)) are stored in a switching list 26 as shown in <figref idref="f0015">Fig. 15</figref> (S0210). This procedure is performed for every candidate code and every assigned code at the lower level than the candidate code. (S0207, S0209, S0211). In the case of there being candidate codes, the switchable code detector 23 selects one candidate code having the least number of assigned codes at its lower level, as an assignment target code (S0213).

0039In the above case, all the assigned codes have their switchable codes in the same level. In <figref idref="f0007">Fig. 7</figref>, there are two candidate codes: C₃(1) and C₃(2). As shown in <figref idref="f0015">Fig. 15</figref>, either of C₃(1) and C₃(2) has one assigned code (C₄(1) is for C₃(1), and C₄(3s) is for C₃(2)), and both the candidate codes C₃(1) and C₃(2) have their switchable codes (C₄(4) for C₃(1) and C₄(2) for C₃(2)) in the same level (level 4). Therefore, either of C₃(1) and C₃(2) can be selected as the assignment target code.

0040A code switch directing unit 24 switches the assigned code at the lower level of the selected assignment target code to the stored switchable code (S0214). In the case of selecting C₃(2) as the assignment target code in <figref idref="f0007">Fig. 7</figref>, C₄(3) is switched to C₄(2). The code used by the mobile station #1 (3a) in <figref idref="f0005">Fig. 5</figref> is corresponding to the assigned code. An assigning unit 25 assigns the selected assignment target code to the mobile station #2 (3b) (S0215).

0041The switching possibility is checked for all the assigned codes at the lower level of all the candidate codes, in the present embodiment. The checking process, however, can be completed at the first time of detecting a candidate code which has an assigned code to be switched at the lower level.

0042Even when a channel mismatch exists, according to the present embodiment, the call loss rate is reduced because a new high rate call can be achieved by switching a code being used in the mismatch channel. It is also acceptable to provide the code selecting unit 10 and the code-switching/code-assignment processing unit 20 of <figref idref="f0014">Fig. 14</figref> in the base station 2 instead of the base station controller 1.

Embodiment 3.

0043<figref idref="f0009">Fig. 9</figref> illustrates a code assignment sequence showing a code switching procedure, according to the present embodiment. <figref idref="f0016">Fig. 16</figref> shows a system structure. Referring to <figref idref="f0001">Figs. 1</figref>, <figref idref="f0009">9</figref>, and <figref idref="f0016">16</figref>, the code switching according to the present embodiment will be explained below.

0044As stated in Embodiment 2, it is sometimes necessary to switch codes during the communication. The code switching during the communication should be simultaneously performed at the base station 2 and the mobile station 3 in order not to stop the communication and so as to reduce interfering in other communication. The simultaneous switching is performed based on code switch timing information sent from a timing information sending unit 33 in the base station 2 to the mobile station 3. <figref idref="f0009">Fig. 9</figref> illustrates procedures of sending the code switch timing information from the base station 2 to the mobile station 3.

0045The procedures, from transmitting a radio channel set-up request message by the mobile station #2 (3b) up to transmitting a code switching request message to the mobile station #1 (3a) by the base station controller 1, are the same as Embodiment 2 shown in <figref idref="f0005">Fig. 5</figref>. Therefore, procedures after the above will now be described in the present embodiment. A code switch informing unit 31 of the base station controller 1 transmits a code switching request message indicating a target code (code after switching) to the mobile station #1 (3a). On receiving the code switching request message, a code switching unit 35 of the mobile station #1 (3a) starts preparing for the code switching and transmits a code switching response message to the code switch informing unit 31 of the base station controller 1.

0046After receiving the code switching response message, the code switch informing unit 31 transmits the message indicating that the code of the mobile station #1 (3a) can be switched, to the base station 2a. Receiving the code switching response message, the timing information sending unit 33 of the base station 2a transmits a code switching start message to the mobile station #1 (3a). This code switching start message includes start timing information for the code switching. Then, if the radio channel is composed of frames, the code switching start message can be time information per frame.

0047It is acceptable to switch at the time of frame N (N is equal to 1 or an integer more than 1) after receiving the code switching start message. It is also acceptable for the timing information to use a code period of the spreading code. Thus, a switching unit 34 is synchronized with the code switching unit 35 in order to switch the code.

0048Receiving the code switching start message, the code switching unit 35 of the mobile station #1 (3a) switches the original code (code before switching) to the target code (code after switching) based on the time prescription specified in advance, using the timing information in the message. Then, the code switching unit 35 transmits a code switching complete message to the timing information sending unit 33 in the base station 2a using the target code. By receiving the code switching complete message, the timing information sending unit 33 judges that the code switching has been properly completed, and transmits the code switching complete message to the base station controller 1. By receiving the code switching complete message, a code releasing unit 32 of the base station controller 1 judges that the code used by the mobile station #1 (3a) has been released, and transmits a radio channel set-up complete message for assigning the code used by the mobile station #1 (3a) to the mobile station #2 (3b).

0049As the code switch timing between the mobile station 3 and the base station 2 can be set up according to the present embodiment, it is not necessary for the base station 2 to simultaneously perform reception and transmission using both the original code and the target code. Consequently, interfering with other communications is reduced.

Embodiment 4.

0050This embodiment is not an embodiment of the invention, but helpful to understand certain aspects of the invention.

0051<figref idref="f0010">Fig. 10</figref> illustrates a code assignment sequence showing a code switching procedure using the maximum ratio combination of the mobile station 3, according to the present embodiment. <figref idref="f0017">Fig. 17</figref> shows a system structure. Referring to <figref idref="f0010">Figs. 10</figref> and <figref idref="f0017">17</figref>, the code switching according to the present embodiment will now be described.

0052The procedures, from transmitting the radio channel set-up request message by the mobile station #2 (3b) up to transmitting the code switching request message by the base station 2a to the base station controller 1, are the same as Embodiment 2 shown in <figref idref="f0005">Fig. 5</figref>. Therefore, procedures after the above will now be described in the present embodiment.

0053As shown in <figref idref="f0010">Fig. 10</figref>, after the base station 2a sends the code switching request message to the base station controller 1, a transmitting unit 43 of the base station 2a performs reception and transmission with a regenerating unit 44 of the mobile station #1 (3a) using both the original code (code before switching) and the target code (code after switching). When the base station controller 1 receives the code switching request message from the base station 2a, a first code switching unit 41 of the base station controller 1 transmits the message to the mobile station #1 (3a), and instructs the regenerating unit 44 of the mobile station #1 (3a) to switch the code. On receiving the code switching request message, the regenerating unit 44 begins to receive signals based on the maximum ratio combination, using both the original code (code just in use) and the target code appointed by the code switching request message.

0054Regarding the transmission, either of the following methods is acceptable. One is to independently switch the code by using a second code switching unit 45 of the mobile station #1 (3a). The other is to transmit signals from the mobile station #1 (3a) with using both the original code in use and the target code. In this case of transmitting with using both the codes, it is also acceptable to perform the maximum ratio combination at the base station 2.

0055When the second code switching unit 45 of the mobile station #1 (3a), in which the reception has been performed by the regenerating unit 44 based on the maximum ratio combination, begins to independently receive signals using the target code only, a switching complete informing unit 46 of the mobile station #1 (3a) transmits a code switching complete message to a code releasing unit 42 of the base station controller 1 with using the target code. Then, the code releasing unit 42 judges that the code having been used in the mobile station #1 (3a) until this time is released and the code switching is completed. The code releasing unit 42 transmits the code switching complete message to the base station 2a. On receiving the code switching complete message from the base station controller 1, the base station 2a stops transmitting signals to the mobile station #1 (3a) using the original code, releases the original code, prepares for the reception and the transmission with the mobile station #2 (3b) using the original code, and transmits a radio channel set-up complete message to the base station controller 1.

0056After receiving the radio channel set-up complete message from the base station 2a, the base station controller 1 transmits the message indicating to set up the original code, having been used in the mobile station #1 (3a), for the mobile station #2 (3b). As stated above, the code switching process using the maximum ratio combination has been performed.

0057According to the present embodiment procedures, the code switch timing can be achieved based on own timing of the mobile station 3 without deteriorating the channel quality. Namely, the time needed for code switching can be reduced.

0058In addition, it is also acceptable that the code assignment is controlled by the base station 2 instead of the base station controller 1.

0059Having thus described several particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only, and not intended to be limiting. The invention is limited only as defined in the following claims .