MXPA00001398A - Method and apparatus for standby state cell selection in a cellular telephone system - Google Patents

Abstract

In an effort to have a mobile station (230) choose a lowest level cell (LN,CN) to lock onto, a method and apparatus for standby state cell selection in a layered cellular telephone system (110) is proposed. Upon entering (300) a standby state, the mobile station (230) is provided (310) with parameters containing a value for a minimum threshold downlink signal strength level and information pertaining to hierarchical levels associated with a plurality of cells (LN,CN) within the cellular telephone system (110). The mobile station (230) measure (330) downlink signal strengths of a plurality of radio frequencies transmitted by the plurality of cells (NL,CN) and selects (360) a cell (LN,CN) having a lowest level in the hierarchy of cells (NL,CN) which transmits a radio frequency having a greatest downlink signal strength, the downlink signal further having a signal strength above the minimum threshold level.

Description

METHOD AND APPARATUS FOR THE SELECTION OF CELLS, IN STATE IN PAUSE, IN A CELLULAR TELEPHONE SYSTEM BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention pertains, in general, to the selection of a cell in a cellular telephone system and, more specifically, to a method and apparatus for the selection of a cell, in a state of pause, in a cell phone system having a cell architecture in layers.

Description of the Related Art The following descriptions use as an example the protocol of the Global System for Mobile communication (GSM, Global System for Mobile Communications). However, it should be understood that the problems, concepts and solutions provided by the present invention also apply to other systems and protocols for wireless communication. In cellular telephony systems a geographic service area is divided into a plurality of individual regions known as cells, each of which is provided with cellular telephone service over a plurality of separate communication channels, the frequencies' of which they are reused between different cells separated from each other so that there is a sufficiently low level of interference between them. When a mobile station operating within a cellular telephone system is in a paused state, for example immediately upon power up, it searches for a cell and its associated transmission control channel to lock onto it. To select a cell, the mobile station scans the radio frequencies in the cellular telephone system and measures the strength levels of the downlink signal for each of these. The mobile station is tuned to the strongest carrier frequency and determines whether this is a transmission control channel. If so, the mobile station is blocked in the transmission control channel associated with the cell. If it is determined that the carrier is not in a transmission control channel, the mobile station is tuned to the second strongest carrier and repeats the process. In such form, the mobile station is finally locked to the cell with the strongest downlink signal strength in the transmission control channel. As the demand for cell phone services increases over time, the capacity of existing systems has been severely stressed to serve all subscribers who want cell phone service, particularly in large metropolitan areas. For example, in the area in and around a convention center, the number of mobile stations can be so large that service demands can not be met by the entire channel capacity of the base station that serves the cell within of which the convention center is located. In these cases, additional "layers" of cellular telephone service are provided by additional base stations with lower energy located within a so-called "umbrella", or "macro" existing cell known as "microcells". These microcells can have an area of coverage or service in the order of a few hundred meters in contrast to some kilometers of coverage of the umbrella cell superimposed. A plurality of these microcells can be located adjacent to each other and form a contiguous coverage area of substantial width, all of which are within the total coverage area of the umbrella cell. Otherwise, they can be separated from each other within the umbrella cell. When a layer-cell structure is used, an improved level of serviceability is provided which can be configured for individual circumstances and which provide a guarantee that users can receive the service despite an extremely high demand within an area very small geographical further, additional layers of the cellular telephone service may be added, for example, by a plurality of contiguous or separate "picocells" placed within the service area of the individual microcells, each of which in turn are within the cell total umbrella. The base stations that provide the cell phone service within the picocells are of even lower power than the base stations serving the microcells and have a service area of, for example, a few hundred meters to provide coverage within a single building or a single floor inside a large convention center. In layered cell architectures many more options are available for a mobile station when a cell is selected to latch onto it. For example, the mobile station can receive cellular telephone service at any given time from any pico cell base station, a microcell base station or an umbrella cell base station. When the conventional criteria for cell selection, in the paused state, in a single-layer cellular telephony system architecture is applied to a layered architecture, the results are less than ideal. When the selection of cells, in the paused state, is performed in an architecture of a single-layer cellular telephone system, the main criterion used is the strength of the downlink signal received by the mobile station from the control channel of transmission of the respective base station. In a layered architecture, the strongest transmission control channel is most likely associated with the highest level layer or the umbrella cell, since the umbrella cell transmits with the largest power to service a relatively large geographic area. . However, in a layered architecture it is not convenient for a mobile station to latch onto a higher level layer cell since it is generally preferable to service the mobile station with the lowest possible layer cell for capacity reasons. In order to maximize the capacity of the cellular telephone system, mobile stations usually have cell phone system instructions for placing and receiving telephone calls in the lowest possible cell. Thus, if there is sufficient signal quality from a lower level cell, such as a microcell or pico cell, it is preferred to service the mobile station from this cell instead of the umbrella cell having fewer total channels available to service comparison with the lowest level cells according to the geographical size of their service area. However, the use of the conventional selection of cells, in the paused state, of a single layer, the mobile station is hooked into a cell of a higher level than is desirable to carry a telephone call. Thus, when the mobile station places or receives the telephone call using the traditional criteria for the selection of cells, in the paused state, for a single-layer cellular telephony architecture in a cellular telephony architecture in layers, the telephone system Cellular performs a transfer from a higher level cell that has selected the mobile station to a lower level cell. The transfer increases the capacity requirements of the processor and slows down the call set-up procedure. Due to the above problems associated with the use of the cell selection method, in the paused state, existing within the cellular telephony system architectures in layers, it is desirable to introduce a method and apparatus for the selection of cells, in a state of pause, to maximize the efficient utilization of channel availability within a layered cellular telephone system. The system of the present invention offers such a method and apparatus.

COMPENDIUM OF THE INVENTION The present invention consists of a method and apparatus for the selection of cells, in a state of pause, in a cellular telephony system in layers. Upon entering a paused state, a mobile station is provided with parameters that contain a value for a downlink signal strength level with a minimum threshold and information pertaining to the hierarchical levels associated with a plurality of cells within the system. cell phone. The mobile station measures the forces of the downlink signals of a plurality of radio frequencies transmitted by the base stations associated with the plurality of cells and selects a cell with a lower level in the hierarchy of the cells, which transmits a radio frequency having a larger downlink signal strength, the downlink signal also having a signal strength above the minimum threshold level.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention reference is made to the following detailed description taken in conjunction with the accompanying drawings, wherein: FIGURE 1 is a representation of a service area of a cellular telephone system in layers; FIGURE 2 is a block diagram, functional, of a plurality of base stations and a mobile station for performing the selection of the cell, in a paused state, in a manner consistent with the present invention; and FIGURE 3 is a flow diagram of a method for performing cell selection, in the paused state, in a manner consistent with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Now with relation to Figure 1, a pictorial representation of a service area of a cellular telephone system in layers is illustrated. A service area 100 of a cellular telephony system with a layered or hierarchical architecture 110 comprises a plurality of Cl-CN cells having a plurality of hierarchical levels Ll-LN. The highest hierarchical level of the cells (Ll, Cl-CN) are known as umbrella cells or macrocells. The umbrella cells (Ll, Cl-CN) are served by base stations (not shown) that usually transmit with the largest power, cover the widest service area and have the lowest number of communication channels per area geographical service compared to cells of lower hierarchical level (L2-N, Cl-CN). In the next lower hierarchical level are the microcells (L2, Cl-CN). The microcells ((L2, Cl-CN) are served by base stations (not shown) that usually transmit at a lower power level than the parasol cells (Ll, Cl-CN) but higher than the lower level cells (L3-N, Cl-CN), they cover a smaller service area than the umbrella cells (Ll, Cl-CN) but larger "than the lower level cells (L3-N, Cl-CN), and have a greater number of communication channels per geographic service area than the umbrella cells (Ll, Cl-CN) but smaller than the lowest level cells (L3-N, Cl-CN), although any number of hierarchical levels is possible , the cells of lower hierarchical level are usually known as picocells (LN, Cl-CN) .Peak cells (LN, Cl-CN) are served by base stations (not shown) that usually transmit with the power weaker compared to the higher level cells (Ll- (Nl), Cl-CN), they cover the smallest service area in comparison ation with the higher level cells (Ll- (Nl), Cl-CN), and have the largest number of communication channels per geographical service area compared to higher level cells (Ll- (Nl), Cl- CN). In addition, now with reference to Figure 2, a functional block diagram of a plurality of base stations of a mobile station for performing the selection of the cells, in a state of pause, in a manner consistent with the present invention is illustrated. . The cellular telephone system 110 stores information pertaining to the hierarchical levels associated with the plurality of cells (Ll-N, Cl-N) in a memory 200. Each of the cells (Ll-N, Cl-N) is served by a base station 210 having a memory 220. Each of the base stations 210 communicates with the telephone system cellular 110 and receives the operating parameters from the cellular telephone system 110. A first parameter contains information pertaining to the hierarchical levels associated with the plurality of cells (Ll-N, Cl-CN). In a preferred embodiment of the present invention, each cell (Ll-N, Cl-N) contains hierarchical information pertaining to this particular cell while in an alternative modality, each cell (Ll-N, Cl-N) is provided with hierarchical information belonging to each cell (Ll-N, Cl- N) The base station 210 receives a second parameter containing a value representing a minimum, threshold, downlink signal strength level. The threshold value represents a minimum downlink signal strength of a radio frequency transmitted by the base station 210 of a cell, which is necessary for a mobile station to latch onto the cell and its associated transmission control channel. Figure 2 further illustrates a mobile station 230 subscribing a cellular telephone service provided by the cellular telephone system 110. The mobile station 230 comprises the functionality that is normally present in a mobile station including a transceiver 240. The mobile station 230 also it includes a memory 250, a processor 260 and a device 270 for measuring the strength of the downlink signal of the signals transmitted by the base stations 210 serving the plurality of cells (Ll-N, Cl-N). When the mobile station 230 goes into a paused state, for example after power-up, the mobile station 230 searches for a cell and its associated transmission control channel to latch onto it. The mobile station 230 is provided with the first parameter and the second parameter. In a preferred embodiment, the parameters are transmitted from the cellular telephone system 110 to the mobile station 230 by the radio frequencies transmitted by the base station 210. Although the parameters can be transmitted in any communication channel of the cellular telephone system 110, in In the preferred embodiment, the parameters are transmitted in a control channel of each cell (Ll-N, Cl-N). In particular, in a cellular telephone system based on the Global System for Mobile communication (GSM), the parameters are transmitted in a transmission control channel of each cell (Ll-N, Cl-N).

In a first alternative mode, the parameters of all the cells (Ll-N, Cl-N) are transmitted to all mobile stations 230 that are in the paused state in a single radio frequency. In a second alternative mode, the parameters are received by the mobile station 230 from the cellular telephone system 110 and stored by the processor 260 of the mobile station 230, in the memory 250, while the mobile station 230 is in a paused state initial. The parameters are then used during the initial and subsequent pause states. In a third alternative mode, the memory 250 of the mobile station 230 is preprogrammed with the parameters. In any case, the mobile station 230 is provided with the parameters and the transceiver 240 is tuned to a plurality of radio frequencies transmitted by the base stations 210 of the plurality of cells (Ll-N, Cl-N). The processor 260 uses the information pertaining to the hierarchical level of the cells (Ll-N, Cl-N) within the cellular telephone system 110 to identify the lowest level cells and instructs the device 270 to measure the strength of the signals of the cells. downlink of the plurality of radio frequencies transmitted by the base stations 210 of the lowest level cells. The processor 260 compares the measurements with each other and against the signal strength level of the minimum threshold and instructs the mobile station 230 to latch onto the cell and its associated transmission control channel having the largest downlink signal strength, measure that is also greater than or equal to the strength level of the minimum threshold signal. If no cell fulfills this criterion, the processor 260 repeats the process for the cells in successively higher hierarchical levels until a cell is selected. Now in relation to Figure 3, a flow chart of a method for selecting cells, in a paused state, in a cellular telephone system consistent with the present invention is illustrated. A mobile station is put in a paused state (step 300), for example by turning on the mobile station. The parameters are provided to the mobile station (step 310) containing information pertaining to the hierarchical levels of a plurality of cells comprising the cellular telephone system and a minimum threshold downlink signal strength necessary for the selection that the station will make. mobile and stored in the mobile station (step 320). In a preferred embodiment, the parameters are transmitted from the cellular telephone system to the mobile station by the radio frequencies transmitted by the base stations associated with the plurality of cells. Although the parameters can be transmitted in any communication channel of the cellular telephone system, in the preferred embodiment, the parameters for a single cell are transmitted in a control channel of this cell. In particular, in a cellular telephone system based on the Global System for Mobile communication (GSM), the parameters are transmitted in the transmission control channel of each cell. In a first alternative mode, the parameters for all the cells are transmitted to all mobile stations, in a single radiofrequency, which are in the paused state. In a second alternative mode, the parameters are received at the mobile stations from the cellular telephone system and stored in a memory of the mobile station while the mobile station is in an initial paused state. The parameters are then used during the initial and subsequent pause states. In a third alternative mode, the mobile station is preprogrammed with the parameters. In any case, the mobile station uses the information pertaining to the hierarchical levels of the cells within the cellular telephone system to identify the lowest level cells and measures the strength of the downlink signal of a plurality of radio frequencies transmitted by the cells. base stations of the lowest level cells (step 330). A determination is made to identify if any of the radio frequencies having a measured downlink signal strength level greater than or equal to the minimum threshold level is a control channel (step 340). If none of the radio frequencies meets these criteria, the mobile station uses the information pertaining to the hierarchical level of the cells within the cellular telephone system to identify the cells at the next higher level and measures the downlink signal strength of a cell. plurality of radio frequencies transmitted by the base stations of these cells (step 350) and a new determination is made in step 340. Once in step 340 a radio frequency is identified having a measured downlink signal strength level greater or equal to the minimum threshold level which is also a control channel, the mobile station selects the cell associated with the radiofrequency and is hooked to the cell and its associated control channel (step 360). Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying drawings and described in the aforementioned detailed description, it will be understood that the invention is not limited to the described modes, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as established and defined by the following clauses

Claims (12)

1. A method for selecting cells, in the paused state, in a layered cellular telephone system comprises the steps of: placing a mobile station in a paused state; providing the mobile station with parameters containing a minimum threshold downlink signal strength level and information pertaining to the hierarchical levels associated with a plurality of cells within the cellular telephone system; measuring the forces of the downlink signals of a plurality of radio frequencies transmitted by the plurality of cells; and looking for a plurality of hierarchical levels for the lowest of the hierarchical levels having in it a cell transmitting a radio frequency whose measured downlink signal strength is higher than the minimum threshold level and is greater among all the cells of the hierarchical level lower, and select the cell to service the mobile station.

2. The method mentioned in claim 1, wherein the radiofrequency is a carrier frequency for a control channel of the cell.

3. The method mentioned in claim 2, wherein the radiofrequency is a transmission control channel. The method mentioned in claim 1, wherein the step of setting the mobile station to a paused state comprises the step of turning on the mobile station. The method mentioned in claim 1, wherein the step of providing the mobile station with the parameters comprises the step of preprogramming the parameters in a memory of the mobile station. The method recited in claim 1, wherein the step of providing the mobile station with the parameters comprises the step of transmitting the parameters associated with each of the respective cells in the radio frequencies transmitted by the respective cells. The method mentioned in claim 6, further includes the steps of: receiving the parameters when the mobile station is in an initial pause state; and storing the parameters within the mobile station for use within the subsequent pause states. The method recited in claim 1, wherein the step of providing the mobile station with the parameters comprises the step of transmitting the parameters to a plurality of mobile stations in the paused state. 9. An apparatus for cell selection, in the paused state, in a layered cellular telephone system, consists of: a measuring device within a mobile station for measuring the signal strengths of the downlink signals transmitted by a plurality of cells in the cellular telephone system; a processor within the mobile station to receive information pertaining to the hierarchical levels associated with the plurality of cells within the cellular telephone system, the processor furthermore to receive from the cellular telephony system a minimum threshold value of the signal strength of downlink, the processor also to search the lowest of the hierarchical levels in a plurality of hierarchical levels, having in it a cell that transmits a radiofrequency whose measured downlink signal strength is above the minimum threshold level and is the largest between all the cells of the lowest hierarchical level, and to select the cell to service the mobile station. 10. The apparatus mentioned in claim 9 further comprises a memory within the mobile station for storing the hierarchical information pertaining to the plurality of cells and the minimum threshold strength of the downlink signal received by the processor. The apparatus of claim 9, wherein the processor receives the information prior to the selection of an initial cell for the mobile station, and wherein the selected cell is the initial cell for the mobile station. The method of claim 1, wherein the step of providing is performed while the mobile station is in a paused state and before the selection of an initial cell for the mobile station, and wherein the selection step includes selecting the cell as the initial cell for the mobile station.