Cell prioritising in a cellular radio system

The invention relates generally to the routing of radio communications between base stations and terminals in a cellular radio system. The invention relates in particular to a method and equipment with which the terminals can be controlled individually to give priority to certain base stations.

A cellular radio system includes stationary base stations, each with a certain coverage area, and terminals that can move relative to the base stations and their coverage areas. The coverage areas are also called cells. In this patent application, a mobile phone is treated as an illustrative terminal. When a certain mobile phone is switched on, it somehow tries to find the best reception signal at a base station and tries to register with the so-called location area (LA) that this base station represents. Registration means that the mobile telephone informs the mobile network via the base station that it can receive calls via the location area with which the base station is connected. In idle mode, a mobile telephone regularly receives messages transmitted by the base station to detect paging messages, which represent an incoming telephone call, and other messages intended for this mobile telephone. At the same time, the mobile phone monitors the power of signals transmitted by other, adjacent base stations, so that it can quickly change base station when required.

The rest mode drive of a mobile telephone according to the GSM system (Global System for Mobile telecommunications) and its extension DCS1800 (Digital Communications System at 1800 MHz) are described below to explain the background of the invention. These functions are described in more detail in the EBU (European Broadcasting Union) and ETSI (European Telecommunications Standards Institute) standards ETS 300 535 (GSM 03.22) and ETS 300 578 (GSM 05.08). It is obvious to a person skilled in the art that as a background to the invention most of these studies can

524 554 are generalized so that they become applicable to all digital cellular radio systems.

There are four requirements for a cell, so that a mobile phone can usually be in it:

the cell must belong to the network of the selected operator;

the cell must not be blocked by the network;

- the location area represented by the cell must not be included in the list of prohibited location areas defined for each mobile phone; and

the attenuation of the radio path between the mobile telephone and the base station must be lower than a certain threshold value defined by the operator (this requirement is briefly called road loss condition).

A cell that meets the requirements listed above is called a suitable cell. When a mobile phone is switched on, it receives the so-called BCCH (Broadcast Control CHannel) signals and goes through them in order of strength and begins to operate in a suitable cell with the strongest signal. The BCCH signal may also contain a recommendation value associated with the cell that says whether the cell is recommended by the system or not. The mobile phone starts working in a non-recommended cell only if no suitable recommended cells are available. This step is called cell selection.

The mobile phone regularly checks whether there is a suitable cell nearby that is more suitable in terms of radio communication, and if necessary, the mobile phone performs new cell selection. The mobile phone can select a new cell for three different reasons:

- according to a certain cell selection condition, the new cell is better than the current cell;

- certain characteristics of the cell in question change, so that this cell is no longer suitable but the new cell is suitable; or

the mobile phone detects that the downlink signaling connection has been interrupted in the cell in question.

Cell selection and new cell selection are based on two parameters calculated by the mobile phone, the so-called C1 and C2 parameters defined in the standard ETS 300 578 (GSM 05.08). Of these, the first, the C1 parameter, describes the power level received by the mobile telephone from the examined base station in relation to the minimum value of the received power level defined by the system and the maximum permissible transmission power of the mobile telephone. The value of the C2 parameter is affected by the value of the Cl parameter and two correction factors, the first of which is an offset parameter transmitted by the base station and the second is a timed clearing, which aims to prevent fast, consecutive cell selection of the mobile phone.

Successful management of radio communication or optimal routing of connections between the terminals and base stations has a significant effect on the level of service that the radio system can provide users. Especially in areas with very heavy traffic, the cells can partially or completely overlap, whereby the mobile phones and the other terminals must be able to be controlled to use certain cells and avoid certain other cells to guarantee an even level of service. As an example, we can consider an office building that is located within the coverage area of ​​a public cellular radio system, but which also has an internal wireless communication system that functions as an extension of the public system, the wireless system being based on so-called nanocells or picocells with a size of one or two rooms. For a mobile phone belonging to an employee working in the building, it is often more advantageous to work in a cell of the building's internal system than in a cell of the public cellular radio system. The operator who handles the cellular radio system can for each mobile phone also define a so-called home area which comprises a single cell or a pair of cells of the public cellular radio system, where the mobile phone is offered lower rates or other benefits in the home area. On the other hand, it is advantageous to define certain cells as handover cells only, in which case it is not desired that any mobile telephones operate in such a cell for a longer period than is required by the handover function.

524 554

In a system according to known technology as described above, there are no possibilities for realizing priority cells regarding individual mobile phones. The first correction factor or offset parameter associated with the C2 parameter definition can be used for general prioritization, so that a certain value of the offset parameter transmitted by a base station causes all mobile phones to generate a C2 parameter value indicating a poor cell selection. However, a priority of this type does not differ for different mobile phones but is identical for all mobile phones.

U.S. Patent No. 4,916,728 (Blair) discloses a method in which a mobile telephone can operate in networks operated by several different operators. To be able to select the network with the most advantageous operator, the mobile phone goes through several reception frequencies, decodes the SID codes (System IDentification) from the signals transmitted by the base stations and tunes to the frequency at which the received SID code indicates the most advantageous operator. The information about the advantages of different operators is stored in the memory of the mobile phones, so that in this arrangement different mobile phones react differently to the information transmitted by the base stations. In this way, however, it is not possible to make mobile phones work in different ways apart from the choice of operator, since all base stations in the network for a certain operator send the same SID code.

PCT application publication WO 95/24809 (Motorola Inc.) deals with a system in which the central equipment checks, based on the identity transmitted by the mobile station, whether that mobile station is authorized for a particular service in a particular area. If specific regional restrictions and / or restrictions regarding individual mobile phones are defined for the service, the central equipment may either refuse to provide any services to a particular mobile phone in said area or to allow the use of only one service, e.g. data communication. To change the services offered, however, the mobile phone must move, as the restrictions are always the same in a certain area.

In this way, it is thus not possible to influence cell selection

524 554 or new cell selection when the mobile telephone or another terminal in the cellular radio system is stationary.

Finnish Patent Application No. Fl 952965 and the corresponding European Patent Publication No. EP 749 254 A1 (Nokia Mobile Phones Oy), a multi-level home area pricing is known for a mobile phone in a cellular radio system, in which a certain binary character string is stored in the mobile phone. Each base station then transmits its own binary identity at regular intervals and the mobile telephone uses the binary character string stored therein as a mask, with which it selects certain bits from the character string transmitted by the base station as the subject of a logical comparison operation. If the said logical comparison operation generates correct results, the mobile phone considers itself to be in the home area or in another area where a certain regional service is available. By using different logical comparison operations, it becomes possible to form a number of individual areas or areas placed in a mutual hierarchy in which the mobile telephone can receive different services from the cellular radio system. Nor can this be applied for actual cell prioritization because the services are regional and the services offered only change when the mobile phone is moving.

In addition to the above-mentioned known methods, there are a number of known methods and systems in which a mobile telephone or other terminal in a cellular radio system can detect whether it is operating in a priority cell associated with this device and provide information about this to the user. In these ways, however, a user or a terminal cannot contribute to the cell selection and determine whether a particular cell selection is maintained even when the terminal is stationary.

The object of this invention is to provide a method and a system which, for each terminal, can effectively influence in which cell of the cellular radio system the terminal of the cellular radio system begins to operate when it is switched on and which cell it selects in connection with new cell selection. An object of the invention is also that the method and system according to the invention

524 554 is flexible and can handle changes made to the priority definitions.

The objects of the invention are achieved at a terminal in the cellular radio system by adding, to the calculation operation which controls the cell selection and the new selection of cell, a step and / or a factor which depends on the contents of the list of priority cells given to the terminal.

The cellular radio system according to the invention is characterized in that, with respect to the establishment and maintenance of radio communication, at least one terminal is arranged to favor at least one cell in relation to other cells in a manner independent of other terminals.

A further object of the invention is a terminal, which is characterized in that, with regard to the establishment and maintenance of radio communication, it is arranged to favor at least one cell in relation to other cells in a manner independent of other terminals.

Furthermore, the invention relates to a method of realizing priority cells. The method according to the invention is characterized in that, with regard to the establishment and maintenance of radio communication, it uses priority data regarding individual terminals in order to favor at least one cell in relation to other cells in a way independent of other terminals.

In the device according to the invention it is possible to define, for each terminal in the cellular radio system, a cell or several cells in which the terminal is to try to operate to the extent that the quality of the radio communication allows it. A list of priority cells for individual terminals is stored in a certain database in the system, from which it is read and transmitted often enough to the terminal, preferably always when the terminal registers or when it changes location area or when the list of priority cells changes. As a result, the terminal always has an updated list of the priority cells.

524 554

When selecting a cell, the terminal can be made to favor the priority cells, in the simplest way by programming it so that when the priority cell's C2 parameter is calculated, the offset parameter and the delay factor are given the value zero or the other such values ​​are generated which represent a C2 parameter value. selection of a cell. Via the base stations, the system can send to the terminal a message, in which special flag bits or other information pieces enable the terminals to apply cell prioritization or prevent it. The message is preferably the same as the message that includes the list of priority cells.

The invention is described in more detail below with reference to preferred embodiments given by way of example and to the accompanying figures, in which:

Fig. 1 schematically shows a message transmitted by the cellular radio system;

Fig. 2 schematically shows a cellular radio system applying cell prioritization, and

Fig. 3 illustrates an embodiment of a method according to the invention.

Fig. 1 shows a summary of a so-called priority information message (PI message), with which a cellular radio system according to the preferred embodiment of the invention controls cell priorities in individual terminals. It has the following fields:

Heading

The heading defines that this is a message that transmits priority information from the cellular radio system to a specific terminal. The invention does not otherwise limit the content or structure of the title.

524 554

CI format

Using this field, the systems talk about whether the terminal should base its function only on the cell identity code (CI code), on a combination of the cell identity and location area code (LAC) or only on the base station LAC. The system may have cells which have the same cell identity but which are located in different location areas, whereby also the said LAC is required to obtain an unambiguous identification of the cell. For the user, the terminal can also display information on a single cell (Cl or CI + LAC) or multiple cells (multiple cell identities or LAC + multiple cell identities) or the entire location area code (LAC) on a display.

Show txt

Using this field, the systems define whether the terminal for the user displays the messages mentioned in connection with the previous field only in idle mode or even during a call.

Txt format

Using this field, the systems define whether the terminal for the user displays a text that is common to all cells or only text that is unique to the cell.

Text-CI

Using this field, the systems define how the short text messages and priority cells described below relate to each other (see cells 20 and 22).

Delay?

The description of the prior art showed the time delay (the so-called penalty time) used in the calculation of the C2 parameter, during which the delay of a certain cell only recently included in the list of suitable cells is given as a C2 parameter a value which shows a poor choice. In this field, the system can give an instruction to the terminal, according to which the terminal does not apply the time delay in the calculation of the C2 parameter for the priority cells.

524 554

Offset?

The description of the prior art showed the offset parameter used in the calculation of the C2 parameter, with which it becomes possible to have priorities regarding a base station. Within this field, the system can give the terminal an instruction, according to which it does not apply the offset parameter when calculating the C2 parameter for the priority cells.

Hysteresis

CRH (Cell Reselection Hysteresis) means that the terminal that changed cells and base stations cannot immediately switch back to its previous cell. The purpose of CRH is to reduce the number of new cell choices between location areas. If the new cell is located in a different location area than the current cell, the C2 parameter is not applied per se, but with the addition of the value specified in CRH. With this field, the system can give the terminal an instruction according to which it does not apply hysteresis to the priority cells, whereby the terminal easily switches to a priority cell, even if this cell would be in a different location area. The hysteresis prevents a terminal, which has switched to a priority cell, from immediately switching back to a non-priority cell located in another location area.

Cl

This field contains the identities of all priority cells. These may be in a sequence, the next field containing the respective LAC codes in a sequence, or the fields may alternate so that the cell identity (C1) and the LÄC are displayed in sequence for each priority cell. The preferred length for a cell identity (C1) is, for example, 2 octets (16 bits).

LAC

For each priority cell, this field contains the respective location area code (LAC) and the length of the field is preferably 2 octets (16 bits). The mutual order options for the C1 and LAC codes were described above.

524 554

Text

This field contains short text messages (eg 16 octets or eight alphanumeric characters per message) which are intended to be displayed to the user on the terminal display when the terminal is operating in the respective cell (see fields 13, 15 and 20). The text messages can refer to a cell, a cell group or a location area (see box 12).

The invention does not otherwise limit the content of the PI message 10, so it may also contain fields other than those indicated above.

Fig. 2 shows a cellular radio system 30 having base stations 31, 32, 33 and 34 with their coverage areas or cells 31a, 32a, 33a and 34a. Block 36 simply represents the other stationary parts of the cellular radio network, such as controllers for base stations, switching equipment, connections to other communication networks, and so on. A database 37 is also linked to this block. The operation of a mobile telephone 35 in the cellular radio system 30 is discussed below.

Let us assume that the priority cells 32a and 33a are defined for the mobile telephone 35, as indicated by the dash in these cells. The cell identities representing these cells and the other parameters that control the prioritization of the mobile telephone are stored in the database 37, which may be physically located at a mobile service station (MSC, not shown in the figure) or in any other place where the operator handling the cellular radio system generates so-called intelligent network services (IN services) for the network. When the mobile telephone 35 is switched on or otherwise arrives at the area of ​​the cellular radio system 30, it establishes a connection to the base station in a manner known per se and then sends, according to prior art, a so-called IMSI Attach request to the network 36, whereby it is registered to operate in the network area and its position are updated in the network position databases (not shown in the figure). In an arrangement according to a preferred embodiment of the invention, the network 36 then sends to the mobile telephone a PI message (priority 524 554 information) according to Fig. 1, which contains a list of priority cell identities read from the database 37, the message being a USSD message (Unstructured Supplementary Service Data) or SMS message (Short Message Service) according to prior art.

The list of priority cell identities and the other parameters relating to the priority use could of course be permanently stored in the memory of the mobile telephone, but the above-described use of the database associated with the network offers certain specific advantages. The most important of these is the automatic updating of the information in the mobile phone. If the priority data changes, e.g. when a new base station is installed or as a result of a changed agreement between the operator and the user, the operator or someone else making the changes registers these changes in the database 37, whereby the mobile phone 35 receives updated information the next time it registers without having to visit a qualified sales representative. to update the software. It is also possible to provide an embodiment in which changed information in the database 37 automatically generates an update message from the network to the mobile telephone 35 without the need to register again. When the priority information is stored in a database in the network, a dishonest user cannot change the priority settings as easily as if the data were permanently stored in the mobile phone. The priority data can also be defined as being identical for a certain user group, whereby all mobile telephones belonging to the group receive PI messages which are substantially identical. The group settings are changed simply by changing the data in the database 37.

Let us assume that the mobile telephone 35 has sent an IMSI Attach request via a non-priority base station 31 and via the same base station has received information about the priority cell identities. The mobile telephone 35 begins to receive SI messages (System Information) which are transmitted on the BCCH channels by other base stations in a manner known per se, whereby it obtains the cell identities of the other cells. For new cell selection, the mobile phone generates a list of possible new cells in addition to the current cell,

<img file="SE524554C2_D0001.tif" />

wherein the C2 parameters calculated for the cells are the determining factors that determine the order of the list. In the situation shown in Fig. 2, the C2 parameter calculated for the cell 32a shows that it is more advantageous than the cell 31a, whereby the mobile telephone selects the cell 32a as a new cell. A way to generate a beneficial value for a priority cell's C2 parameter is shown in more detail below as an example.

When the C2 parameter is calculated, it is advantageous that the mobile telephone 35 in the PI message is instructed not to take into account the time delays of the priority cells (field 16 in Fig. 1), whereby the priority cell 32a immediately appears as a very advantageous cell on the cell new list. If the mobile telephone 35 is located at the boundary of the cell 32a which is close to the base station 31 of the non-priority cell 31a, it is likely to receive the signal transmitted by the latter base station as a stronger signal than that transmitted by the base station 32. In order to have the mobile station select the priority cell 32a even in this situation, the offset parameters and PI messages transmitted by the base stations 31 and 32 must be arranged so that the PI message instructs the mobile phone to calculate the priority cell 32a C2 parameter without the offset parameter but to calculate the non-priority cell 31a C2 parameter with the offset parameter (field 17 in fig. 1), whereby the C2 parameter representing the priority cell 32a becomes as advantageous as possible even at the edge of the cell 32a.

According to Fig. 1, the PI message contains information on whether the hysteresis should also be applied in the case of priority cells (field 18 in Fig. 1). The hysteresis may be the above-mentioned CRH hysteresis or the time hysteresis according to section 6.2.2 of the GSM 05.08 standard. In Fig. 2, the priority cell 32a is entirely in the area of ​​the non-priority cell 31a, so that in some situations it may happen that both cells appear to the mobile telephone 35 as being almost equal in terms of cell selection. If the application of hysteresis is prevented as cell re-selection would involve switching from a non-priority cell to a priority cell, the mobile phone can always be made to change quickly

524 554 from cell 31a back to cell 32a but to delay a change in the opposite direction.

We will now show in more detail an example given way to calculate the C2 parameter so that the functions described above are obtained. For non-priority cells, the mobile phone calculates the C2 parameters in a manner previously known per se, i.e. by using the formulas

C2 (T) = Cl + CELL_RESELECT_OFFSET - TEMPORARY_OFFSET *

H (PENALTY_TIME -T), then PENALTY_TIME &lt;&gt; 11111, and

C2 = Cl - CELL_RESELECT_OFFSET, when PENALTY_TIME = 11111, where the step function H (x) is defined as

H (x) = 0, when x &lt;0, and H (x) = 1, when x&gt; = 0.

The above-mentioned offset parameter for a base station is shown here with the name CELL_RESELECT_OFFSET according to the GSM standards and the above-mentioned delay factor is the product of the temporary offset parameter TEMPORARY_OFFSET and the step function H, the value of the step function H being dependent on the relationship between The value of the correction factors for priority cells depends on the PI message received by the mobile phone. If the PI message field delay? contains a certain value (eg 1), the mobile phone TEMPORARY_OFFSET gives a value of zero when the priority cell's C2 parameter is calculated. Similarly, if the SI message field is offset? contains a certain value (eg 1), the mobile phone CELL_RESELECT_OFFSET gives a value of zero when calculating the C2 parameter of the priority cell when PENALTY_TIME is 11111.

The modified C2 parameter could also be called C2_PRIORITY_CELL. It is advantageous if the mobile phone uses modified C2 parameter calculation only when its current cell is a non-priority cell. The function of the mobile phone can

524 554 is formulated as an algorithm in pseudo-language as follows, each step being followed by a corresponding reference reference in Fig. 3:

IF (current cell is a priority cell) 301

THEN calculate C1 and C2 for N cells with the highest power 302

IF (at least 1 priority cell, including current cell, with Cl&gt; 0) 303

THEN select the cell with the highest priority in order determined by C2 304

ELSE selects the best non-priority cell in the order determined by C2 305

ELSE

IF (any of adjacent cells, other than the current cell, is a priority cell) 306

THEN {select N cells with maximum power 307, calculate C2_PRIORITY_CELL for the priority cells within the N cells, calculate C2 for non-priority cells among the N cells, 308 select best cell in the order determined by C2 / C2_PRIORITY_CELL} 309

ELSE {calculate C1 and C2 for N cells with the highest level, 310 select the best cell in the order determined by C2} 311

The effect of the hysteresis, whether used or not, is not apparent from the algorithm described above, but taking into account what is shown above, it is easy to add the hysteresis as part of the comparison of the values ​​for C2 or C2_PRIORITY_CELL.

For the operator operating a cellular radio system, the invention provides many different possibilities for controlling cell priorities regarding a device. This is a great advantage because from the point of view of the network not all terminals behave identically, so that the use of a single standard pattern for all r, ο / r ς /)

J ί- Q Ο J ύ devices would inevitably cause disadvantageous functions in some devices. As an example, we can consider a situation in which a user moves daily through or past a priority cell without staying too long in the cell (for example, the cell may be located in a building, which the user with his terminal passes daily at high speed in his car along the highway). Since the cell is generally a priority cell, it is not worthwhile for the mobile telephone to use this cell during such a fast passage, since a short visit to a cell only prevents the synchronization of the terminal in the network and entails extra signaling traffic. The operator can program the computer that monitors the system operations so that it detects corresponding cases. In the detected cases, each terminal can be instructed to use a delay factor in the calculation of the C2 parameter for said priority cell, whereby, during a fast passage, the cell's C2 parameter does not have to rise to a level which would indicate advantageous cell number. By testing and simulating, it is possible to find other corresponding situations, in which the system operations can be optimized by selecting parameters relating to individual devices.

In known systems, a terminal in the cellular radio system comprises a certain amount of cells in a list, from which it performs cell new selection. The length of a commonly used list is six cells. To better find the priority cells, the length of the list can be extended to include e.g. as many cells as there are cells in the BA (BCCH) or BA (SACCH) messages (BCCH Allocation - Broadcast Control Channel /

Slow Associated Control Channel) transmitted by the base station of the current cell. It is worth expanding the list, especially as the terminal does not otherwise detect priority cells in the vicinity but assumes, based on the stored handover history or based on the received LAC codes and / or cell identities, that there may be priority cells in the vicinity that do not appear in the short term. the list. The difference from the prior art is then that the terminal does not have to control all possible BCCH frequencies but it can simply increase the number of cell identities that it holds in memory. The scanning of all frequencies would require more power and take more time and opportunity to

524 554 receiving a paging message representing an incoming call would be reduced. If there are no priority cells in the list, an alternative to an extended list would be for the terminal to continuously replace the last cell in the list, so that it would have at least one priority cell on the list.

We have described above a priority arrangement with cells with only two levels: priority cells and non-priority cells. A user can also have several geographically different home areas, such as the area at home and at work. The system can consider these areas to be of equal value, so that if they are very close to each other or even touch each other, they form a priority home area. However, since the cells have individual different cell identities and text associated therewith (see field 23 in Fig. 1) and they may have different LAC codes, the terminal may display different text messages to the user depending on in which priority cell of the home area it operates. Furthermore, the invention does not limit the use of priorities at different levels, which can be realized with appropriate offset parameters and control instructions sent via the PI message.

The terminal must receive SI messages of so-called type 3, which are transmitted by other base stations, so that it can detect the cell identities of adjacent cells. This can result in the terminal not receiving a simultaneously sent paging message representing an incoming call. In order to make this happen as infrequently as possible, the terminal must receive SI messages of type 3 relatively infrequently, e.g. only once in 30 minutes.