FI116355B - Channel switching system in a cellular network - Google Patents

Description

116355

The present invention relates generally to cellular networks, and in particular to controlling the handover of communication traffic 5 from one base station to another by a mobile station in such a network.

In a typical cellular network, the area in which the service is provided is divided into a number of smaller areas, called cells, each served from its own base station. Each cell has its own antenna or antennas for transmitting and receiving from a user station, normally a mobile station. As the user station leaves the area covered by a particular call transferring cell 15, the call is switched to one of a plurality of neighboring cells.

The handover procedure can be divided into three steps. First, it is recognized that a handover may be required, second, a suitable base station is identified as a handover candidate, and finally, the call is switched from one base station to another.

·· Usually, the decision as to whether a handover is needed is made by monitoring the signal quality, compare: ·. 25 clubs of this quality to predetermined thresholds and '· · _ starting handoff if the observed quality is below a predetermined threshold. The procedure may also take into account the signal quality measurements of the neighboring cells, and the handover may be initiated on the basis of the relative level of signals received from the serving cell and the neighboring cells.

·. ·. Handoff in a typical GSM protocol (GSM, Global

The general principles governing a ver-35 operating under System for Mobile Communications are disclosed in 116355 2, entitled "European digital cellular communications system (Phase 2): Radio subsystem link control", GSM 05.08, version 4.9.0, dated 15 April. 1994, published by the European Telecommunications Standards Institute (ETSI).

5

The number of channels, i.e. frequencies, available to the service owner determines the cell capacity. Because this number is limited, the cell ranges can be reduced to increase capacity, since this allows reuse of the available frequencies without any intolerable interference to the network. Thus, in rural areas with low traffic density, the cells tend to be large, whereas in urban areas, the density of traffic tends to be higher, requiring the use of smaller cells.

It has been proposed that such high-density areas use a multilevel cellular structure comprising an overlay of conventional 20 cells defined as macrocells, together with a substrate of smaller cells defined as microcells that serve small areas such as blocks of flats or blocks. An additional layer of even smaller cells, defined as pico-cells, can be further used to serve individual buildings or layers of a building.

In such a network, called a hierarchical network, a number of factors, such as the mobile station's movement within the cell 30, make the aforementioned conventional handover processing unsatisfactory. In particular, if the mobile station moves at such a rate that it stays in the lower layer cell for a very short time, the number of handovers required becomes uncontrolled. In addition, high speed mobile stations may have problems identifying suitable handover candidates because updating of candidate station ID codes cannot be performed quickly enough, thus creating the need for hierarchical systems to provide an improved handover management system.

The present invention takes advantage of the fact that cellular networks typically utilize data collected by a mobile station, which identifies neighboring stations with the highest potential for establishing a reliable connection to a particular mobile station. This information is updated regularly. For example, in a GSM network, a base station provides a list of other neighboring base stations (BA list) to the mobile station being served, and the mobile station is commanded to monitor it and provides the base station with periodic signal quality reports.

According to the present invention, there is provided a handover control system for a hierarchical cellular network having: • a plurality of base stations, all of which are capable of collecting data from mobile stations within their service area; which identify potential base station candidates for the channel! '·; for switching, based on the respective potential signal quality, the management system comprising rate measuring means for measuring the rate of change of any candidate candidates thus identified, comparing means for comparing this rate with a predetermined threshold, and handover control means for responding and comparing between a base station at a higher level or a lower level to allow handover based on comparison.

In a network operating under the GSM protocol, each mobile station is required to report the identities of the six best potential handover candidates and to send the base station identification code (BSIC) to the base station for each such candidate. In such a system, the reported identifiers can be continuously appropriately monitored at the base station, and if the number of cell identifier changes in cells classified as microcells is less than a predetermined threshold, the handoff control means may change to a lower level in the mobile hierarchy.

The handover control system performing the present invention is typically included in a base station forming part of the network.

In addition, there is provided a method for controlling handover in a hierarchical cellular network having a plurality of base stations, each of which is capable of collecting information from a plurality of mobile stations located in front of the service area · silencing any candidate base stations for switching ·: A method comprising measuring the switching rate of the potential base station candidates thus identified, comparing this rate to a predetermined threshold value, and maintaining a connection between the mobile station and the higher level base station, or allowing handover to a lower level based on comparison.

30

In order that the invention may be well understood, the preferred embodiment thereof will be described below with reference to the accompanying drawings.

116355 5

Figure 1 shows a simplified form of a hierarchical cellular network.

FIG. 2 is a flowchart illustrating an embodiment of the present invention 5 in a GSM cellular network.

Figure 1 shows two conventional cellular network cells, macrocell 1 and macrocell 2. Macrocell 1 is provided with a microcell substrate, which typically represents lower power base stations whose antennas are located, for example, below the average ridge height of buildings. The numbers in the microcells represent the frequencies used by the respective base stations; the sixteen microcells shown in the example use four common 15 frequencies. Since the range of these microcells is typically two hundred to five hundred meters, the power used can be kept at a very low level provided that the penetration reaches certain values in the buildings. If traffic density is particularly high, a lower level of smaller cells, even tiny cells, serving single-storey buildings, or even • floors of a building, called t picocells, can still be used. Because ·. ·· street-side buildings or walls and partitions of individual buildings • »* * * '·' effectively limit the radios. 25 propagation, it is clear that the distances between potentially interfering stations can often be reduced in this situation. As the cell size becomes smaller, the effects of mobile station movement within the cell are greatly increased.

30

In addition, since cell coverage areas are more overlapping in a hierarchical system than in a non-hierarchical system, there may often be a situation where ** a macrocell may have the same frequencies i »> •; 35 microcell or picocell neighbors. In this situation, only the encoded identifiers, such as the BSIC codes used in the GSM system, for example, can be distinguished from each other.

Figure 2 illustrates an embodiment of the present invention at a base station of a hierarchical system implementing the GSM protocol. Such a base station comprises a processor which is controlled by program modules to perform the signal processing functions required by the GSM protocol and the handover control functions. While separate processing means may be used to perform the functions associated with the present invention, as described below, sufficient capacity is normally available in an existing processor. In the preferred embodiment 15, additional program modules are used which control the existing processor to perform the steps explained in connection with Figure 2.

In the GSM system, each mobile station is required to monitor a plurality of carrier frequencies, determined by the information transmitted by the base station that is currently serving the mobile unit. This information includes a frequency list (BA list), but as will be understood from the description of the hierarchical system presented in connection with Figure 1 above, the frequency list alone does not uniquely determine which cell the mobile station is currently in. Kailee. Thus, the mobile station is instructed to decode the cell identifier code (BSIC) for each signal being monitored and to verify that it is consistent with the neighbor list transmitted by the base station serving the unit. After initial decoding at refresh intervals which cannot exceed ten seconds according to the GSM-v protocol, the mobile station is required to decode the identifier codes of the cells it is monitoring to verify that these 35 cells are still in the transmitted neighbor list. In addition, the mobile station is required by the GSM protocol to report the six strongest frequencies, together with their identification codes, during a periodic update.

In Figure 2, the monitoring process described above occurs in step (1). According to the invention, in step (2), the number of changes of BSICs in the measurement interval T is determined. In the present embodiment, the BSIC samples are weighted according to sample timing and the samples are subjected to a progressive averaging process that takes into account leaky bucket '). This process generates a time interval T above the mean, which is updated as new samples are taken and old samples are weighted out of the process.

In step (3), a comparison is made between the average obtained in step (2) and the predetermined threshold value. If the average is above said predetermined threshold, the monitoring process is continued for an additional T interval. If, on the other hand, the average is below the predefined • threshold, the handover will start in step (4). Then, in step (j), an estimation of the potential performance of the available microcell: v handover candidates is performed based on the signal strength, as exemplified by the GSM parameter Rxlev. To obtain a reliable solution, this value is taken over a predetermined number of measurements n, and a handover candidate is selected. Assuming this candidate is available for 30 handoffs, the process ends in step (6).

The threshold number and measurement intervals can be set to take into account different traffic conditions. However, they are set to maintain a normally acceptable handover activity level in the case of user stations i * that move on the network according to rules set by the network operator.

116355 e

In order to determine handover candidates with a higher degree of accuracy, registers may be used to record the length of time each potential candidate has provided appropriate performance values, such as the number n of update intervals T over which such performance values have been measured. .

Although a signal strength based method is described in connection with step (5) of Figure 2, it is clear that it is possible to take into account other parameters such as signal quality expressed as the number of transmission bit errors (in GSM system parameter rxqual).

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Claims (5)

116355 A handover control system for a hierarchical cellular network having a plurality of base stations 5 each capable of collecting information from mobile stations within its service area identifying potential base station candidates for a handover based on the respective potential signal quality, comprising: measuring means, comparing means (3) for comparing this rate to a predetermined threshold, and handover control means (4, 5) responsive to the control provided by said comparing means for maintaining a communication link between the mobile station and a higher level base station or handover . A system for a cellular network operating according to the GSM protocol according to claim 1, characterized in that said rate measuring means are adapted to measure the rate of change of base station identity codes * (BSIC) for notifications made by the mobile to the serving base station. 25 sa. System according to claim 2, characterized in that said measuring means comprise a progressively averaging processor, * »·! 30 adapted to operate on the 'leaky buckets' principle. »4 *» · A system according to claim 3, characterized in that said speed measurement intervals. They are adapted to identify a base station which has remained a candidate for a longer period of time than the other candidates, and that said handover control means (5) responds to the control provided by said rate measurement means to select such a candidate for handover. 5 A base station for a hierarchical cellular network, characterized in that it comprises a handover control system according to any one of the preceding claims. »» · I * - '»<· ♦ • I · t I I I I> · * t) t * 116355