FR2752659A1 - SWITCHING MANAGEMENT SYSTEM FOR CELLULAR NETWORKS - Google Patents

Abstract

Switching management system for a hierarchical cellular network in which switches between different levels are controlled according to the rate of change of candidate identification data reported to a base station in service by a mobile unit. Rapid changes identify a mobile station which is moving relatively quickly and is therefore assigned to a higher level base station rather than a lower level station from which a new switch would be required quickly.

Description

Title Switching management system for cellular network Domain

  The present invention relates generally to cellular radio networks and more specifically to the management of communications switching involving a mobile station, from one base station to another in a network of this type. Background of the Invention In a conventional cellular radio network, the area in which service is offered is divided into a number of smaller areas called cells, each of which is served by its own base station. Each cell has its own antenna (s) for transmitting to and receiving from a user station, normally a mobile station. When a user station moves outside the area covered by a cell carrying a particular call, this call is switched to a cell among a certain number of

neighboring cells.

  The switching process can be divided into three phases. First, the identification of the need for switching, then the identification of a suitable candidate for base station switching and finally, the switching of the call from a base station.

base on another.

  Traditionally, determining whether or not to switch is done by monitoring the signal quality, comparing that quality to predetermined thresholds, and initiating switching when the monitored quality falls below a predetermined threshold. The method can also take into account measurements of the quality of the signals from neighboring cells and switching can be initiated as a function of the relative level of the signal received from the cell in service and of the

neighboring cells.

  The general principles which govern switching management in a conventional network operating under the GSM (Global System for Mobile Communications) protocol are described in the publication entitled "European digital cellular communications system (Phase 2): Radio subsystem link control", GSM 05.08 version 4.9.0, dated April 15, 1994 and published by the European Telecommunications Standards Institute

(AND IF).

  The capacity of a cell is determined by the number of channels, i.e. frequencies, made available by the service provider. Since this number is limited, the cell size can be reduced in order to increase the capacity since this allows available frequencies to be reused without intolerable interference being introduced into the network. For example, in rural areas where traffic density is low, cells tend to be large, while in urban areas, traffic density tends to be higher, which requires the use of cells.

smaller.

  For areas of high density traffic, it has been proposed to develop a multilayer cell structure composed of an upper layer of conventional cells defined as macrocells, a lower layer of smaller cells, defined as being microcells, serving small areas, for example blocks from a neighborhood. Another layer of even smaller cells, defined as picocells, may further be provided to serve

  individual buildings or floors of buildings.

  In a network of this type, called a hierarchical network, a number of factors such as the displacement of a mobile station in a cell make the switching process unsatisfactory according to the conventional techniques which have been referred to above. In particular, if a mobile station moves at a speed such that it remains inside a cell of the lower layer only for a very short time, the number of switching operations required then becomes too high to be managed correctly. . In addition, problems may arise with regard to the identification of candidate stations suitable for switching to quickly move the mobile stations because the updating of the identification codes of the candidate stations cannot be carried out quickly enough. This confirms the need to have, in hierarchical systems,

  an improved switching management system.

  The invention takes advantage of the fact that cellular networks generally provide data, often gathered by the mobile station, which identify the neighboring stations which are most likely to be able to ensure a reliable link with a mobile station.

  given. These data are updated regularly.

  For example, in a GSM network, a base station supplies a mobile station currently served with a list of other neighboring base stations (the BA list) which the mobile station must monitor and on which it must perform periodic reports. regarding the quality of the signals to provide them to the

base station.

  Summary of the Invention According to the invention, there is provided a switching management system for a hierarchical cellular radio network having a plurality of base stations each capable of collecting data from mobile stations in its service area, identifying the stations base potential candidates for switching on the basis of the potential quality of the signals of the moment, comprising a rhythm measuring means making it possible to measure the rate of change of the potential candidate stations thus identified, a means of comparison making it possible to compare this rhythm with a predetermined threshold value and switching control means responsive to said comparing means for maintaining communication between a mobile station and a higher level base station in the hierarchy or for enabling switching to a lower layer on the base

of this comparison.

  In a GSM network, each mobile station must report the identifiers of the six best potential switching candidates and transmit a base station identification code (BSIC) to the base station for each of these candidates. In such a system, the reported identifiers can be conveniently and continuously monitored at the base station and if the number of changes in the cell identifier, for cells classified as microcells, drops below a predetermined threshold, the switching control means can switch the mobile station to a lower level

in the hierarchy.

  A switching management system embodying the invention will generally be integrated into a monitoring station.

base part of the network.

  Furthermore, a switching management method is proposed in a hierarchical cellular radio network having a plurality of base stations each capable of collecting data from the mobile stations in its service area, identifying the potential candidate base stations for switching over the basis of the potential signal quality at the time, the method comprising measuring the rate of change of the potential candidate stations thus identified, comparing this rate to a predetermined threshold value and maintaining communication between a mobile station and a higher-level base station in the hierarchy or for switching to a layer

  lower based on this comparison.

  In order for the invention to be properly understood, a preferred embodiment of it will now be

  described with reference to the accompanying drawings.

Brief description of the drawings

  Figure 1 illustrates, in simplified form, a

hierarchical cellular network.

  Figure 2 is a flowchart showing the implementation of the present invention in a network

GSM cell.

  Detailed description of the preferred embodiment

  Referring firstly to FIG. 1, two conventional cells of a cellular network, the

  macrocell 1 and macrocell 2, are shown.

  In macrocell 1, a lower layer of microcells is provided and generally represents low power base stations having antennas located for example below the average height of the roof. The numbers inside the microcells represent the frequencies used by the respective base stations; in the example shown, sixteen microcells share four frequencies. Since these microcells generally extend over two hundred to five hundred meters, the power used can be kept at a very low level in order to reach

  specific levels of penetration into buildings.

  If the traffic density is particularly high, one can provide a lower level of even smaller cells of even lower power, called picocells, serving particular buildings or even storeys of buildings. Since radio propagation is effectively demarcated by buildings bordering a street or by the walls and partitions of individual buildings, it will be understood that the distance between stations which can cause interference can often be reduced in such a situation. However, if the cell size is reduced, the effects of moving a mobile station inside a cell increases by

considerably.

  In addition, since the overlap of cell coverage is much greater in a hierarchical system than in a non-hierarchical system, it frequently happens that a macrocell has for neighbors microcells or picocells which use the same frequencies as it. In such a situation, the neighbors can only be distinguished by coded identifications such as the BSIC code used in the GSM system to which it has been

refers above.

  FIG. 2 represents an embodiment of the invention in a base station of a hierarchical system which implements the GSM protocol. A base station of this type includes a processor operating under the control of program modules to provide signal processing and switching control operations required by the GSM standard. Although specialized processing means can be provided to perform the operations described below relating to the invention, sufficient capacity will normally be available in the existing processor. In the preferred embodiment, additional program modules are provided, and control the existing processor to perform the steps described with reference to the

figure 2.

  In a GSM system, each mobile unit must control a certain number of carrier frequencies determined by information broadcast by the base station which is serving that unit at that time. This information includes a frequency list (the BA list) but, as will be understood from the

  description of a hierarchical system mentioned above

  above with reference to Figure 1, a frequency list alone does not only determine which

  cell the mobile unit is checking.

  The mobile unit must therefore decode the cell identification code (BSIC) for each signal checked and check that it complies with the information in the list of neighbors broadcast by the

  base station serving the unit at that time.

  After the initial decoding, at update intervals which cannot exceed ten seconds according to the GSM protocol, the mobile unit must decode the identification codes of the controlling cell in order to verify that these cells are still on the list of neighbors distributed. In addition, the mobile unit must, according to the GSM protocol, report which are the six most powerful frequencies among the frequencies checked as well as their identification codes when the periodic update takes place. Referring to Figure 2, the process of

  control described above occurs in step (1).

  According to the invention, in step (2), the number of changes of BSIC identifications over a predetermined measurement interval T is determined. In the present embodiment, weights are associated with the BSIC samples according to the synchronization of the sample and a progressive averaging process is applied to the samples - taking into account the weightings of the principle known as "leaky bucket" (word to word: leaking bucket). At the end of this process, an average value is obtained over the interval T which is updated when new samples are taken and the old ones

  samples are removed from the process.

  In step (3), a comparison is made between the average obtained in step (2) and a predetermined threshold number. If the average is greater than the predetermined threshold, the control process continues for another interval T. However, if the average is less than the predetermined threshold, switching is initiated in step (4). At this stage, an estimation of the potential performance of the switching candidates on an available microcell is carried out in step (5) as a function of the signal strength, as indicated for example by the value of the GSM parameter Rxlev. In order to arrive at a reliable decision, this value is subjected to a predetermined number of measurements n and a choice of the switching candidate is made. Assuming that this candidate is available for the

  switching, the process ends in step (6).

  The threshold number and the measurement intervals can be set to cope with several traffic conditions. However, they will normally be set so as to maintain an acceptable level of switching activity for user stations moving through the network according to the

  rules established by the network operator.

  In order to determine the switching candidates with a higher degree of precision, registers can be provided to record the time during which each potential candidate cell has provided an appropriate performance, measured for example by the number n of update intervals T on which

  such performance has been measured.

  Although a control process based on signal strength has been described in connection with step (5) of FIG. 2, it will be understood that other parameters such as the quality of the signals, as indicated by the number of bit errors in the transmission (in a GSM system, the parameter

  rxqual), can be taken into account.

Claims (8)

  1. Switching management system for a hierarchical cellular radio network having a plurality of base stations each capable of collecting data from mobile stations in its service area identifying the potential candidate base stations for switching on the basis of the potential quality of the signals of the moment, comprising a means for measuring the rate (2) making it possible to measure the rate of change of the potential candidate stations thus identified, a means of comparison (3) making it possible to compare this rate with a predetermined threshold value and switching control means (4, 5) responsive to said comparing means for maintaining communication between a mobile station and a higher level base station in the hierarchy or for enabling switching to a   lower layer based on this comparison.   2. The system as claimed in claim 1, intended for a cellular radiotelephony network operating according to the GSM protocol, in which said rhythm measurement means is designed to measure the rate of change of the codes of base station identifiers (BSIC) in reports from a mobile station and   transmitted to a base station in service.   3. The system according to claim 2, wherein said rhythm measuring means is a progressive averaging processor designed to operate according to the   "leaky bucket" principle (word for word: leaking bucket).   4. The system as claimed in claim 3, in which said rhythm measuring means is designed to identify the base station which has remained a candidate for longer than the other candidates and in which said control means (5) reacts to said means of rhythm measurement to choose such a switching candidate. 5. Base station for a hierarchical cellular radiotelephony network comprising a switching management system according to one   any of the preceding claims.   6. Method for managing switching in a hierarchical cellular radio network having a plurality of base stations each capable of collecting data from mobile stations in its service area, identifying the potential candidate base stations for switching on the basis of the potential signal quality at the time, the method comprising measuring the rate of change of potential candidate stations thus identified, comparing this rate to a predetermined threshold value and maintaining communication between a mobile station and a base station higher level in the hierarchy or allowing switching to a lower layer based on this comparison.   7. The method as claimed in claim 6, intended for a cellular radio network operating according to the GSM protocol, in which the rate of change of the base station identifier codes (BSIC) reported by a mobile station to a base station according to said protocol is averaged over a predetermined period of time so as to produce a rhythm value making it possible to make a comparison with said predetermined threshold value.   8. The method of claim 7, wherein said rhythm value is established using the   "leaky bucket" principle (word for word: leaking bucket).