CDMA cellular communication system using frame offset distribution of all base stations to avoid traffic peak

Abstract: In a CDMA communication system, wherein mobile stations transmit frames at timing offset from each other according to a frame offset value, a base station controller determines a distribution of frame offset values according to usage counts of the frame offset values simultaneously used by all wireless base stations and transmits a message to a requesting base station, containing the offset distribution. The requesting base station is responsive to the transmitted message for selecting a frame offset value according to the offset distribution contained in the message.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to code division multiple access (CDMA) communication systems, and more specifically to a CDMA cellular mobile communication system in which frame offsets are used to avoid traffic peak at base station controllers. Description of the Related Art According to the EIA/TIA Interim Standard IS-95 for code division multiple access communication systems, mobile stations are assigned unique timing offsets, known as frame offsets, by a base station to transmit their frames at such times that there is no concentration of voice traffic between the base station and the base station controller even when that base station receives calls from a number of mobile stations at the same time. Since a number of base stations are served by the base station controller and their frame offsets are uncorrelated with each other, their voice packets may still cause a traffic peak at the base station controller if their frame offsets are not uniformly distributed over time. This is particularly true of CDMA base station controllers where conversion is necessary between the QCELP coding format of mobile stations and the PCM format of the network. WO 98/44669 discloses a communication system where frame boundaries for the discrete frames for different users may be offset relative to one another, and where assignment of a remote unit to a particular frame offset occurs by a processor determining a frame offset belonging to a group that is the least busy group of all possible groups. US-A-5,363,404 discloses a direct sequence code-division multiple access communication system which schedules the conveyance of blocks of information such that the blocks arrive at a base site just in time for transmission during a discrete frame of an air interface. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a CDMA cellular communication system in which traffic peak is avoided. According to the present invention, there is provided a code division multiple access cellular communication system, as defined in claim 1. The dependent claims define particular embodiments of the invention. According to an embodiment of the invention wherein mobile stations transmit frames at timing offset from each other according to a frame offset value. The system comprises a base station controller and a plurality of wireless base stations connected by respective communication links to the base station controller. In order to avoid traffic peak, the base station controller determines a distribution of frame offset values according to usage counts of the frame offset values simultaneously used by all of the wireless base stations and transmitting a message to a requesting base station, containing the distribution. The requesting base station is responsive to the transmitted message for selecting a frame offset value according to the distribution contained in the message. In a specific aspect, the base station is arranged to notify the base station controller of the selected frame offset value, and wherein the base station controller includes a frame offset memory and is arranged to store the notified frame offset value in the frame offset memory, determine a distribution of frame offset values stored in the frame offset memory according to usage counts each indicating the number of same frame offset values, identify frame offset values of low usage counts as high priority frame offset values and frame offset values of high usage counts as low priority frame offset values, and transmits an offset distribution message to the requesting base station, containing the identified frame offset values. The base station includes a frame offset memory and is arranged to store the selected frame offset value in the frame offset memory, determine a distribution of frame offset values stored in the frame offset memory according to usage counts each indicating the number of same frame offset values, select frame offset values of low usage counts as candidate offset values, and determine a frame offset value as the selected frame offset value, using the identified offset values and the candidate offset values. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in further detail with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a typical CDMA cellular mobile communication system; and Fig. 2 is a flowchart of the operation of a base station controller according to the present invention; and Fig. 3 is a flowchart of the operation of a wireless base station according to the present invention. DETAILED DESCRIPTION As illustrated in Fig. 1, a CDMA cellular mobile communication system designed to the EIA/TIA Interim Standard IS-95 comprises a mobile switching center 10 to which a plurality of CDMA base station controllers are connected. For simplicity, only one base station controller 11 is illustrated. Base station controller 11 is a control station serving a plurality of wireless base stations 12 each covering respective cells to establish a wireless link with a user's mobile station 13. Forward channel signals from the base stations 12 to the base station controller 11 are transported on respective wired links 15 and multiplexed on a wired link 14 to the mobile switching center 10. Reverse channel signals from the mobile switching center 10 are multiplexed on the wired link 14 and demultiplexed at the base station controller 11 onto the respective links 15. According to the present invention, the base station controller 11 cooperates with each of the base stations 12 to distribute their frame offsets so that all of their voice packets distribute over time at the base station controller. For this purpose, a frame offset memory is provided in the base station controller 11 for storing frame offset values simultaneously used by all base stations. As will be described, when the base station controller 11 receives a traffic channel assignment request from a base station, it reads the contents of the frame offset memory and determines the distribution of stored frame offsets according to their usage counts, and identifies those offset values whose counts are smaller than a predetermined threshold as "recommendable" offset values and those offset values whose counts arc greater than the threshold as "unrecommendable" offset values. Base station controller 11 sends a bit map to the requesting base station, containing a recommendable/unrecommendable indication for each frame offset value. Each base station 12 is also provided with a frame offset memory for storing frame offset values simultaneously used by the base station. The base station reads the contents of the frame offset memory when it sends a traffic channel assignment request to the base station controller, determines the distribution of the frame offsets according to their usage counts and selects those offset values whose usage counts are smaller than a predetermined threshold as BS candidate values. In Fig. 2, the operation of the base station controller 11 begins at step 20 when it receives a call request from a mobile station via a requesting base station. At step 21, the base station controller reads contents of its own frame offset memory and determines the distribution of stored frame offsets according to their usage counts, and identifies frame offset values of low usage counts smaller than a predetermined threshold as "recommendable" (high priority) values and frame offset values of high usage counts greater than the threshold as "unrecommendable" (low priority) values. Base station controller 11 formulates an offset distribution message containing a bit map of recommendable/unrecommendable indications for use of each frame offset value by base station. Referring to Fig. 3, when the requesting base station 12 receives the offset distribution message from the base station controller (step 30), it proceeds to step 31 to read contents of its frame offset memory and determine frame offset values of different usage counts and select those offset values whose usage counts are smaller than a threshold value as BS candidate offset values. At step 32, the base station removes the unrecommendable frame offset values contained in the bit map data from the candidate offset values and identifies the remaining values as usable frame offset values. Flow proceeds to step 33 to check to see if the number of usable offset values is equal to zero. If there is at least one usable offset value, the decision at step 33 is negative and flow proceeds to step 34 to select one of the usable offset values and proceeds to step 36. If there is no usable frame offset value, the decision at step 33 is affirmative and flow proceeds to step 35 to select one of the candidate offset values and proceeds to step 36. In this way, a frame offset value is determined for the requesting mobile station. At step 36, the requesting base station updates its frame offset memory with the determined frame offset value and notifies the base station controller of the determined frame offset value. When the base station controller 11 receives a frame offset value from a requesting base station (step 22, Fig. 2), it updates its frame offset memory with the received frame offset value and sends an acknowledgment message to the mobile station via the requesting base station (step 23), containing the received frame offset value. Upon receipt of the acknowledgment message (step 37, Fig. 3), the requesting base station repeats transmission of the message co the calling mobile station (step 38). Upon receipt of the acknowledgment message, the calling mobile station uses the notified frame offset value as timing for the transmission its messages. When the call is terminated, the base station updates its frame offset memory by erasing the frame offset value of the call. At the same time, the base station transmits an end-of-call message to the base station controller to crase the frame offset value of the call to update its frame offset memory. Since the frame offset value selected by the requesting base station is of low usage count, the rate of all voice traffic at the base station controller 11 is maintained at a level below its format conversion performance.

Claims: A code division multiple access cellular communication system, wherein mobile stations transmit frames at timing offset from each other according to a frame offset value, comprising: a base station controller (11); and a plurality of wireless base stations (12) connected by respective communication links (15) to the base station controller (11) for establishing wireless links to the mobile stations, the base station controller (11) determining a distribution of frame offset values according to usage counts of the frame offset values simultaneously used by all of the base stations and transmitting a message containing the distribution to a requesting base station, requesting the assignment of a traffic channel the requesting base station being responsive to the message for selecting a frame offset value for said traffic channel according to the distribution contained in the message. A code division multiple access cellular communication system as claimed in claim 1, wherein the base station is arranged to notify the base station controller of the selected frame offset value, and wherein the base station controller includes a frame offset memory and is arranged to: store the notified frame offset value in the frame offset memory, determine a distribution of frame offset values stored in the frame offset memory according to usage counts each indicating the number of same frame offset values, identify frame offset values of low usage counts as high priority frame offset values and frame offset values of high usage counts as low priority frame offset values, and transmits an offset distribution message co the requesting base station, containing the identified frame offset values. A code division multiple access cellular communication system as claimed in claim 2, wherein the base station includes a frame offset memory and is arranged to: store the selected frame offset value in the frame offset memory, determine a distribution of frame offset values stored in the frame offset memory according to usage counts each indicating the number of same frame offset values, select frame offset values of low usage counts as candidate offset values, and determine a frame offset value as the selected frame offset value, using the identified offset values and the candidate offset values. A code division multiple access cellular communication system as claimed in claim 2, wherein the base station includes a frame offset memory and is arranged to: store the selected frame offset value in the frame offset memory; determine a distribution of frame offset values stored in the frame offset memory according to usage counts each indicating the number of same frame offset values, select frame offset values of low usage counts as candidate offset values, remove the low priority frame offset vales from the candidate offset values and determine whecher remaining frame offset values are present, and select one of the remaining offset values if at least one remaining frame offset value is present and select one of the candidate frame offset values if the remaining frame offset values are not present. A method of determining a frame offset value for a mobile station served by one of a plurality of wireless base stations connected to a CDMA base station controller via communication links, the method comprising the steps of: a) determining, at the base station controller (11), a distribution of frame offset values according to usage counts of the frame offset values simultaneously used by all of the base stations and transmitting a message containing the distribution to a requesting wireless base station (I2), and requesting the assignment of a traffic channel b) selecting, at the requesting base station, a frame offset value for said traffic channel according to the distribution contained in the message. The method of claim 5, wherein the step (b) comprises the step of notifying the base station controller of the selected frame offset value, and wherein the step (a) comprises the steps of: storing, at the base station controller, the notified frame offset value in a memory; determining a distribution of frame offset values stored in a memory according to usage counts each indicating the number of same frame offset values; identifying frame offset values of low usage counts as high priority frame offset values and frame offset values of high usage counts as low priority frame offset values; and transmitting an offset distribution message to the requesting base station, containing the identified frame offset values. The method of claim 5, wherein the step (b) comprises the steps of: determining, at the requesting base station, a distribution of frame offset values stored in a memory according to usage counts each indicating the number of same frame offset values; selecting frame offset values of low usage counts as candidate offset values; determining a frame offset value as the selected frame offset value, using the identified offset values and the candidate offset values; and storing the determined frame offset value in the memory. The method of claim 5, wherein the step (b) comprises the steps of: determining, at the requesting base station, a distribution of frame offset values stored in a memory according to usage counts each indicating the number of same frame offset values; selecting frame offset values of low usage counts as candidate offset values; removing the low priority frame offset values from the candidate offset values and determining whether remaining frame offset values are present; selecting one of the remaining offset values if at least one remaining frame offset value is present and selecting one of the candidate frame offset values if the remaining frame offset values are not present; and storing the selected frame offset value in the memory.

Inventors: KAMATA MASAKI, KAMATA, MASAKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1998
Main classification: H04B7
IPC codes: H04Q7, H04B7, H04B1