Method for allocating Walsh codes by group in a CDMA cellular system

[0002] 1. Field of the Invention

[0003] The present invention relates to a CDMA (Code Division Multiple Access) cellular system, and more particularly it relates to a method for allocating Walsh codes by group in a CDMA cellular system.

[0004] 2. Description of the Related Art

[0005] In a CDMA communication system, a plurality of subscribers utilize the same frequency concurrently. Therefore, each communication channel using the same frequency must be distinguished from every other communication channel. This is generally accomplished by utilizing a spreading code for separating the data signals from one another in a CDMA communication system. Two common spreading codes which are used are the PN (pseudo noise) code and the Walsh code. Both of these codes use 64-bit binary sequences.

[0006] The PN code, in common with all the signals in a cell or a sector in a CDMA mobile communication system, is utilized by the mobile station to distinguish the signals transmitting from multi-paths, different cells, and different sectors. The Walsh code is utilized to distinguish the signals corresponding to each mobile station in the cell or sector signals after PN coding. A Walsh code corresponds to a single row or column of the Hadamard matrix. For example, in a 64 channel CDMA spread spectrum system, particularly mutually orthogonal Walsh codes can be selected from the set of 64 Walsh codes within a 64 by 64 Hadamard matrix. Also, a particular data signal can be separated from the other data signals using a particular Walsh code to spread the particular data signal.

[0007] The bit pattern of above-noted 64 codes are arrayed according to the characteristics of the Walsh function. During the modulation, ‘0’ is converted to ‘+V’, and ‘1’ to ‘−V’, to have the orthogonality that, for random Walsh codes W_aand W_b, the following equations apply: W_a⊕W_a=1, W_b⊕W_b=1, W_a⊕W_b=0, and W_b⊕W_a=0 for transmitting to the mobile station.

[0008] The 64 Walsh codes are distinguished as two types: one type being assigned to the overhead channels, and the other type being assigned to the traffic channels (see FIG. 1). The 0^thcode is assigned to the pilot channel; the 1^st-7^thcodes are assigned to the paging channels; the 32^ndcode is assigned to the synchronization channel; and the remaining codes, i.e., the 8^th-31^stand the 33^rd-63^rdcodes, are assigned to the traffics. In the case where the 1^stcode is only utilized for the paging channel, the 2^nd-7^thcodes (i.e., the remaining six codes) can be used as additional traffics.

[0009] In the existing CDMA DCS (Digital Cellular System) and CDMA PCS (Personal Communication Services), the Walsh codes are serially allocated according to the generated calls or data being transmitted. That is, referring to FIG. 1, if “n” calls occurred in a cell or a sector, the corresponding BCP (Base station Call control Processor) allocates respective “n” Walsh codes to the available codes code-by-code, except for the Walsh codes used for the overhead channels, i.e., the Walsh codes used for the pilot, the paging, and the synchronization channels.

[0010] Referring to FIG. 1, in the first state (S1), the Walsh codes for the pilot, the paging, and the synchronization are all BUSY (B), and all the Walsh codes for the traffics are IDLE (I). In the second state (S2), calls occurred corresponding to each of the following Walsh codes: W8, W9, W10, and W11. In the third state (S3), a plurality of calls are allocated serially corresponding to the following Walsh codes: W12, W13, W14, and W15. In the fourth state (S4), the calls corresponding to the 12^thand 14^thWalsh codes of the third state are removed making these Walsh codes IDLE (I). In the fifth state (S5), other calls are allocated to the 12^thand 14^thWalsh codes.

[0011] In the CDMA DCS and CDMA PCS system according to the prior art, when a call occurs, the Walsh codes are searched serially to determine an IDLE Walsh code, i.e., a Walsh code which does not have a call assigned thereto. This typically involves a long processing time, and, in case of concurrent calls, a caller may have to wait a certain time before his call is assigned to an IDLE Walsh code, thereby, increasing the time required to set up the caller's call.

[0012] It is an object of the present invention to provide a method for grouping the Walsh codes to reduce the load requirements when checking whether the Walsh codes are idle or busy.

[0013] It is another object of the present invention to provide a method for detecting the Walsh group having the minimum traffic group to minimize the time for allocating a call or data to be transmitted to an idle Walsh code, thereby, improving the performance of the system and reducing the time required to set up the call.

[0014] A preferred embodiment for grouping the Walsh codes in a CDMA DCS and CDMA PCS according to the present invention comprises the steps of initialization, allocation of the Walsh codes, and removal of the Walsh codes. The step of initialization includes the step of dividing the Walsh codes into a plurality of Walsh groups by assigning the number table of the Walsh codes to correspond to these groups. The step of allocation of the Walsh codes includes the steps of searching the plurality of Walsh groups to determine a Walsh group having the minimum traffic, and allocating an idle Walsh code within the minimum traffic group to a call or data to be transmitted. The step of removal of the Walsh code, after it is determined that the call has been completed, includes the steps of searching the Walsh group; selecting the Walsh code which had the call allocated thereto, selecting the Walsh code for removing, reducing the allocation counts of the selected Walsh group, and removing the selected Walsh code.

[0015] In the step of initializing, the number of groups the Walsh codes are divided into is determined according to the number of Walsh codes which can be allocated for the purpose of traffics. For example, the number of groups for the most efficient structure is determined by either of two numbers, where one number has the minimum value of the group x (the Walsh code number per group), and the remaining number is assigned as the number of Walsh codes per group. The number table is made to include a starting number of the Walsh code per group according to the determined number of groups.

[0016] In the step of allocating an idle Walsh code to a call, the BCP (Base station Call control Processor) of the CDMA base station iteratively checks the numbers of the allocated Walsh codes per group by utilizing the Walsh Group Allocation Count (Wg_Alloc_Cnt) table. This table stores the allocation counts for each of the plurality of Walsh groups. The group which has the minimum allocated Walsh codes is selected as the minimum traffic group. Therefore, the step of allocating the identified idle Walsh code to the call includes the steps of: identifying a starting Walsh code of the minimum traffic group as a starting point and identifying a starting Walsh code of a next group as an ending point using a Walsh group start table (this table stores the starting numbers of the Walsh codes for each of the plurality of Walsh groups); iteratively checking each Walsh code within the minimum traffic group until the idle Walsh code is identified; allocating the call or transmitted data to the identified idle Walsh code in the minimum traffic group; and changing the state of the Walsh code where the call or transmitted data was allocated to in a Walsh code state table from the “idle” to the “busy” state.

[0017] In the step of removing the Walsh code, when the removal of the allocated Walsh code is requested, the BCP of the CDMA base station searches the Walsh code group to locate the Walsh code to be removed by utilizing the Walsh Group Start Table. This table stores the number of the starting Walsh code of the Walsh code group which contains the Walsh code to be removed. After selecting the Walsh code to be removed, the allocation counts of the Walsh code is reduced by reducing the counter of the Walsh code's respective Walsh group in the Walsh Group Allocation Count table. The selected Walsh code is then removed by converting the “busy” state of the Walsh code to be removed into the “idle” state.

[0018] In the step of allocating the Walsh code to the minimum traffic group, if an “idle” Walsh code is not identified within the minimum traffic group, then this step terminates. Further, in the step of searching for the Walsh code to be removed, if the Walsh code group which contains the Walsh code to be removed is not selected, then the step of removing the selected Walsh code is terminated.

[0019] FIG. 1 illustrates a table for allocating Walsh codes according to the prior art;

[0020] FIG. 2 illustrates a table for allocating Walsh codes according to an embodiment of the present invention;

[0021] FIG. 3 illustrates a flow chart for allocating an idle Walsh code to a call according to an embodiment of the present invention;

[0022] FIG. 4 illustrates a flow chart for removing a Walsh code allocated to a call according to an embodiment of the present invention;

[0023] FIG. 5 illustrates a table comparing the call numbers in the prior art and the present invention in the context of Walsh code allocation; and

[0024] FIG. 6 illustrates a chart for comparing the amount of time required in allocating the Walsh codes to calls between the prior art and the present invention.

[0025] The present invention will now be described in detail with reference to the accompanying drawings. Even though the present invention is described with reference to allocating Walsh codes to calls, it is contemplated that Walsh codes can be allocated to other data capable of transmission in a CDMA cellular system, such as facsimile, video, and audio-visual transmissions, within the context of the present invention. It is further contemplated that the method of the present invention is preferably performed by the BCP (Base station Call control Processor) of a CDMA base station.

[0026] A method of the present invention includes the step of dividing the Walsh codes into a plurality of groups. The number of groups the Walsh codes are divided into is determined according to the number of Walsh codes which have been allocated or are available for the purpose of traffics. For example, the number of groups for the most efficient structure is preferably determined by either of two numbers, where one number has the minimum value of the group x (the Walsh code number per group) and the remaining number of the two numbers is assigned as the number of Walsh codes per group. For example, if the number of the Walsh codes available for traffics is 56, the most efficient number for grouping is either 7 or 8. For illustrative purposes herein, the number 8 is selected as the number of groups and the number 7 as the number of Walsh codes per Walsh code group.

[0027] After the Walsh codes are divided into a plurality of groups, a Walsh group start (WG_START) table is configured to store the starting numbers of the Walsh codes for each group. In the example presented above which provides 8 Walsh code groups, the numbers stored within the WG_START table are 8, 15, 25, 29, 36, 43, 50, and 57.

[0028] With reference to FIG. 2, there is illustrated a table having the Walsh codes allocated into 8 groups according to the methodology of the present invention. In the first state (S1), the Walsh codes for the pilot, the paging, and the synchronization channels are all BUSY, and all the Walsh codes for the traffics are IDLE (I). In the second state (S2), the occurred calls are allocated serially to the first Walsh code (W8, W15, W22, . . . , W57) of each group. In the third state (S3), the call of the second group is removed. In the fourth state (S4), the 9^thcall is allocated to the second group of the above-noted third state. In the fifth state (S5), each call of the 1^stand 8^thgroup corresponding to the above-noted fourth state is removed. In the sixth state (S6), other calls are allocated to the first Walsh code of the 1^stand 8^thgroups.

[0029] FIG. 3 illustrates a flow chart for allocating Walsh codes according to a preferred method of the present invention where “s” denotes a step. In FIG. 3, steps 300 to 305 show a method for finding the Walsh code group which has the minimum traffic; steps 310 to 319 show a method for allocating calls to Walsh codes within the Walsh group identified in steps 300 to 305.

[0030] With continued reference to FIG. 3, if a call is initiated, a request is made for a Walsh code to be allocated to the call (WCODE) in step 300. In step 301, the group index “i”, which indexes one of the Walsh groups, and the maximum code channel number (Max_Code_Ch=64) are stored in the minimum traffic count (min_traffic_cnt). In step 302, the group index “i” is compared with the maximum Walsh code group (Max_Walsh_Group=8). If the group index is smaller than the maximum Walsh code group, the BCP of the CDMA base station refers to the Walsh Group Allocation Count (WG_Alloc_Cnt) table which stores the Walsh group allocation counts for each Walsh group (step 303). It is noted that the group index “i” is one number less than the Walsh code group it is indexing, since the group index “i” is originally initialized to equal 0 (i=0). If the group index is greater than the Max_Walsh_group, the process proceeds to step 310.

[0031] In step 303, if the Walsh group allocation count for the group being indexed by the BCP, is greater than the minimum traffic count (min_traffic_cnt), the process returns to step 305 where the group index “i” is incremented by one. If the Walsh group allocation count is determined by the BCP to be less than the minimum traffic count, the process proceeds to step 304. In step 304, the BCP selects the Walsh code group having the minimum Walsh code allocation counts as the minimum traffic group (min_traffic_group), i.e., the minimum traffic group is identified as the current Walsh group being referenced by the group index “i” (min_traffic_group=i).

[0032] After the minimum traffic group is selected, the starting Walsh code of the selected Walsh code group is set as the starting point, and the starting Walsh code of the next Walsh code group as the ending point (step 310). The starting Walsh code of the selected Walsh code group is indexed by the Walsh code index “j” in step 311. If the Walsh code being indexed is the last Walsh code in the selected Walsh code group as determined in step 312 the process proceeds to steps 317, 318 and 319 where it terminates. If the Walsh code being indexed is not the last Walsh code in the selected Walsh code group, the process proceeds to step 313.

[0033] In step 313, if the Walsh code being indexed is not idle, the process proceeds to step 316 where the Walsh code index is incremented by one, i.e., steps 312 and 313 are repeated for the succeeding Walsh code in the selected Walsh code group. If the Walsh code being indexed in step 313 is idle, the Walsh code being indexed is allocated to the call. The state of the Walsh code in the Walsh code state table is then changed to the “busy” state (step 314) and the process proceeds to steps 315 and 319 where it terminates.

[0034] FIG. 4 illustrates a flow chart for removing the allocated Walsh code groups, where as with FIG. 3, “s” denotes a step. If the removal of a particular Walsh code is requested in step 400, because the call or data allocated to that Walsh code has been completed or transmitted, respectively, the Walsh code number (wcode) of the particular Walsh code is recognized and stored. In step 401, the group index “i” is initialized to 0. In step 402, the BCP of the CDMA base station compares the group index “i” with the maximum Walsh code group. If the group index “i” is greater than the maximum Walsh code group the process proceeds to steps 408, 409 and 410 where it terminates.

[0035] If the group index “i” is less than the maximum Walsh code group, the process proceeds to step 403 where the BCP determines if the indexed group contains the Walsh code which was requested to be removed in step 400. The BCP utilizes the Walsh group start table which contains the numbers of the starting Walsh codes to make this determination. If it is determined that the requested Walsh code is not within the indexed group, the process proceeds to step 407 where the group index “i” is incremented by one to index the next Walsh group.

[0036] However, if in step 403, it is determined that the requested Walsh code to be removed is within the indexed group, the process proceeds to step 404. Step 403 determines if the requested Walsh code to be removed is within the indexed group by comparing the Walsh code number (wcode) of the requested Walsh code and determining if it is greater than or equal to the starting Walsh code number of the indexed Walsh code group and less than the starting Walsh code number of the succeeding Walsh code group.

[0037] After selecting the Walsh code group to be removed in step 404, the allocation counts of the Walsh code to be removed is reduced by reducing the counter of its respective Walsh group in the Walsh group allocation table. In step 405, the requested Walsh code is removed by converting the “busy” state of the Walsh code to the “idle” state. The process then proceeds to steps 406 and 410 where it terminates.

[0038] FIG. 5 shows a table comparing the number of comparison iterations performed until an idle Walsh code is found for being allocated to a call, among the Walsh codes assigned for traffics between the prior art and the present invention. When the number of occurred calls exceeds the number of divided groups (in the illustration herein, the number of groups the Walsh codes are divided into is 8), the comparison iterations in the prior art method increases steeply. But, the comparison iterations according to the methodology of present invention increases at a much slower rate. That is, if the number of calls exceeds the number of groups, the present invention reduces the time for setting up the calls and improves the overall performance of a CDMA cellular system as compared to prior art methods.

[0039] FIG. 6 illustrates a chart for comparing the amount of time required for allocating the Walsh codes to calls between the prior art and the present invention where the number of groups is 8. When the number of calls increases, the processing time required in the prior art increases tremendously, but the processing time required in the present invention increases slightly.

[0040] Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.