A method of locating a mobile station in a mobile telephone system

TECHNICAL FIELD

The present invention relates to a method of a mobile telephone system comprising locating a mobile station (MS), which mobile telephone system comprises base stations (BS1, MT) which are connected to a mobile switching center (MSC) and which have been assigned a desired number of frequencies.

STATE OF THE ART

Known mobile telephony systems using TDMA, i.e. Time Division Multiple Access, and MAHO, i.e. Mobile Assisted Handoff, have in each base station a control channel whose signal strength the mobile stations can measure and whose frequency allows identification within a certain geographical area of ​​the transmitting base station. Examples of such systems are the European GSM system and the American system which is specified according to a standard designated EIA / TIA IS-54. The land system can command the mobile station to measure and report the signal strength at specified frequencies, which makes it possible to identify the base stations closest to the mobile station.

In the Swedish patent application number 9001312-9, a mobile phone system has been proposed intended to be used by subscribers both indoors and outdoors. The system comprises an external subsystem and an internal subsystem. In the internal subsystem, adaptive channel assignment is applied, which gives users indoor access to all traffic channels in the system. The transmitter effects are small compared to the transmitter effects in the external subsystem, which makes it possible to use the same traffic channels in several cells within a building. The indoor control channel associated with each cell is not adaptive but fixed and in the same way as for the outdoor cells it identifies the base station for the mobile station. The number of indoor base stations is large in this mobile system. This is a disadvantage of the fixed control channel because it can easily be exposed to disturbances from the outdoor system and then not be replaced adaptively. In addition, it is difficult for the land system to unambiguously identify an indoor base station for handoff based on the frequency of the control channels when a subscriber during calls reports measurements made on the control channels of the surrounding base stations. This difficulty can

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036 occur when the subscriber from a street in a certain outdoor cell chooses to enter one of several possible buildings. The indoor base stations in these buildings must then be unambiguously defined in the reported measurements from the mobile phone, which requires that the control channel frequencies in all these buildings are different.

DESCRIPTION OF THE INVENTION

The present invention is based partly on the insight that fixed control channels for indoor base stations in the above-mentioned adaptive indoor system are exposed to disturbances, and partly on the insight that the indoor base stations can be identified with certainty based on the mobile station's reported measurements.

It is an object of the present invention to provide indoor base stations, hereinafter referred to as substations, which have only traffic channels but no control channels.

It is further an object to solve the identification problem of the substations by reserving a number of frequencies of the available frequencies of the mobile system for this purpose. The substations within a geographical area are each assigned a unique selection of these reserved frequencies, which are constantly radiated. The assignment can be fixed but is preferably semi-permanent and is ordered by the system's mobile switching center. In this way, local disturbance conditions and the need for changes in connection with the expansion of the mobile telephone network can be taken into account. The identification frequencies in a cell can be used not only for identification purposes but also to carry traffic in that cell.

The invention has the features which appear from the appended claims.

DESCRIPTION OF FIGURES

An embodiment of the invention will be described in more detail in connection with drawing figures of which

Figure 1 schematically shows a cellular mobile telephone system, Figure 2 shows time slots for time division radio transmission,

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Figure 3 shows a signal sequence which is transmitted in a time slot, Figure 4 shows a base station and substations in a building and Figure 5 shows a flow chart of the method according to the invention.

PREFERRED EMBODIMENT

Figure 1 shows a part of a cellular mobile telephony system with cells C1, C2 and C3. The cells each have base stations BS1, BS2 and BS3, respectively, which are connected to a mobile switching center MSC. The cell C1 houses a building 1 which has substations commonly referred to as MT and which are connected to the mobile switching center MSC by a cable 2. A subscriber who can make calls via the mobile telephone system with a mobile station MS moves in the cell C1.

It is a wish that the subscriber should be able to make his call both outdoors and inside the building 1. The call should also be able to be made when the mobile station is moved into or out of the building without the subscriber having to manually switch the call. The mobile telephony system shall have a low level of interference even on control channels and shall before handoff provide unambiguous location of mobile stations both indoors and outdoors. These requirements are met by a mobile telephony system according to the present invention.

The mobile telephony system in Figure 1 has been assigned by authorities a frequency range which is divided into a number of frequencies, or actually frequency pairs, one frequency of which is used for communication from the base stations or substations to the mobile station MS, so-called downlink, and whose other frequency is used in reverse. Each frequency is in a known manner divided into a number of time slots for different calls, which is usually called TDMA, Time Division Multiple Access. In the present application, the term traffic channel refers to such a time slot on a certain frequency. Figure 2 shows the regularly recurring time slots 1-N for one of

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036

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the frequency pair. The designation T indicates the time and T1 indicates the duration of a time slot, according to the example time slot number 10. T2 indicates a time interval between two occasions when the time slot 10 was transmitted. For calls and connection connections, the mobile telephone system has control channels CC which lack time slots and which occupy a whole frequency, for example as in the above-mentioned American standard EIA / TIA IS-54.

It should be noted that the term channel in a radio context is sometimes used to denote a space between a transmitter and a receiver. This meaning of the word channel is not used in the present application.

Figure 3 shows a known signal sequence which is transmitted in the time slots. The signal sequence is divided into a start sequence GR, a synchronization sequence SYNC, two data sequences D1 and D2 and an associated control sequence SACCH, which is a reserved signal space for control signals.

Figure 4 shows in more detail the base station BS1 and the building 1 in the cell C1. The building has three rooms 3, each with substations MT1, MT2 and MT3 respectively, which are connected to the mobile switching center MSC via cable 2. The mobile station MS is shown in three different positions designated A, B and C respectively. The base station BS1 has access to all the system's traffic channels, which are assigned adaptively, as well as an entire frequency which is the control channel CC. Using the control channel CC, the mobile station MS calls the base station BS1 and the mobile switching center MSC assigns a call channel by means of the control channel. When changing channels, orders for a new call channel are placed on the old call channel. These functions are performed in a known manner, for example according to the above-mentioned American mobile phone standard, which, however, only indicates three time slots instead of the number N shown in Figure 2. The number of N time slots is used to describe the invention, which is not limited to the American mobile phone standard.

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According to the invention, the substations MT1, MT2 and MT3 are simplified base stations with a few transmitters / receivers. The substations do not have a control channel but can transmit and receive on the mobile phone system's various traffic channels. Of all the frequencies for traffic channels that the mobile system uses and that the base station BS1 has access to, a certain number is reserved for identification purposes. According to the example, thirty frequencies designated Nol to No30 are reserved. The substations MT1, MT2 and MT3 are each assigned a unique selection among these thirty frequencies, each selection comprising five frequencies. Each of the substations constantly transmits its five assigned frequencies and because these frequencies for each substation constitute a unique frequency selection, the substations close to the mobile station MS can be identified by the mobile switching center MSC. Broadly speaking, this identification can be described so that the mobile station MS measures signal strengths for the thirty reserved frequencies and reports the five strongest to the mobile switching center MSC. Hereby the base station BS1 can order the mobile station MS when it is indoors to make handoff to the nearest of the substations. Such a handoff will be described in more detail below.

In addition to their unique frequency selection, the substations MT1, MT2 and MT3 have access to traffic channels for calls. These channels can be permanently assigned. According to the present exemplary embodiment, however, the traffic channels are assigned adaptively afterwards by the mobile switching center MSC. It should be noted that, in the case of adaptive channel assignment, the frequencies belonging to a substation's unique frequency selection for identification can also be used by this substation as the frequency for traffic channels.

According to the exemplary embodiment, the following unique frequency selection has been made for the substations.

MT1: frequency Nol, No2, No3, No4, No5.

MT2: frequency Nol, No3, No4, No5, No6.

MT3: frequency Nol, No4, No5, No6, No7.

It should be noted that the substations have frequency Zero in common. Indoor signal strengths from substations MT1, MT2

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and ΜΤ3 is generally greater than the signal strengths outdoors from the base station BS1 due to the relatively small distances indoors. By comparing the strength of the frequency Nol with a threshold value, the mobile switching center MSC can determine whether the mobile station MS is outdoors or inside the building, as will be shown in more detail below.

The frequency assignment allows a large number of substations in the mobile phone system to be identified. According to the example, 29! / 25! -4! = 23751 unique frequency selections are made. The exclamation mark denotes the usual faculty.

In connection with the flow chart in Figure 5, it will be described how the mobile telephone system according to the invention works. As mentioned above, the mobile telephone system is assigned its frequencies according to a block 20 and the identification frequencies Nol - No30 are designated, block 15. The substations MT1, MT2 and MT3 are assigned according to block 22 'in a unique frequency selection and the substations transmit these frequencies as shown in a block 23. When the mobile station MS is in the cell C1 and dials or is called itself, the mobile switching center MSC assigns to the base station BS1 a vacancy of its traffic channels, for example the first time slot on a frequency

No36. This channel assignment, which is shown in a block 24, takes place in an edge manner and the mobile station MS begins to carry its call on the assigned traffic channel. The mobile station MS is ordered by the mobile switching center MSC via the base station BS1 to listen to the frequencies of nearby base stations and substations according to a block 25. The mobile station reports, on the associated control sequence SACCH of the assigned traffic channel, the signal strength of the five strongest frequencies to the base station BS1, block 26. In a block 27 it is shown that the mobile switching center 30 MSC receives measurements regarding these frequencies and compares the reported signal strengths with a threshold Th. If the signal strength of the common frequency Nol is below the threshold value Th or if the frequency Nol is not found at all, an alternative "No, the mobile station MS is probably outdoors, for example in position A. The mobile switching center MSC locates the mobile station in this way by signal strength. at the common frequency Nol. According to the “No470 036 alternative, the connection to the base station BS1 may continue on the assigned traffic channel, the first time slot on the frequency No36. If instead the signal strength of the common frequency Nol exceeds the threshold value Th according to an alternative Ja ”in block 27, the mobile station MS is probably indoors according to a block 29, for example in position B. The mobile station MS hereby reports the frequencies Nol, No3, No4, No5 and No6 on the associated control sequence SACCH of the assigned traffic channel, the first time slot on the frequency 36. The mobile switching center MSC thus first locates the mobile station MS indoors by means of the signal strength N of the common frequency Nol. Then, according to a block 30, the mobile switching center MSC examines whether the reported frequencies match any of the unique frequency selections. In an alternative Yes, the mobile switching center MSC locates the mobile station MS to the substation MT2 according to a block 31. MT2 is thereby identified by means of the reported frequencies No3, No4, No5 and No6. The mobile switching center MSC allocates, according to a block 32, a free channel to a connection 4 between the mobile station MS and the substation MT2. This is done by pointing out, for example, the time slot 10 of a frequency No35. The mobile switching center then orders handoff according to a block 33. This handoff is performed from the previously assigned traffic channel, the first time slot on the frequency No36, to the designated channel, the time slot 10 of the frequency No35. When the connection 4 is connected, the substation MT2 communicates with the mobile switching center MSC over the cable 2. In an alternative No in block 30, the mobile station MS continues to communicate via the base station BS1 according to block 24 and intercept the identification frequencies according to block 25.

During the call, the mobile station MS can be moved through a door 5 to the position C in the adjacent room 3. The mobile station MS continues to intercept the frequencies Nol-No30 according to block 25 even during the call on the connection 4. This interception takes place during the time interval T2 10 returns according to Figure 2. The mobile station MS reports, according to block 26, to the mobile switching center MSC via the substation MT2 the five

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strongest frequencies of the associated control sequence SACCH of the assigned traffic channel, time slot 10 of the frequency No35. At some point when the mobile station MS passes the door 5, it reports the strongest frequencies Nol, No4, No5, No6 and No7 to the mobile switching center MSC. This frequency range does not belong to the MT2 substation. The mobile switching center orders, according to the option "No" in block 30, the base station BS1 and the mobile station MS to make a handoff to a free traffic channel in the cell C1 of BS1, for example the second time slot of a frequency No49. The mobile station MS still reports the strongest frequencies Nol, No4, No5, No6 and No7, which now takes place on the associated control sequence SACCH at the second time slot, the frequency No49. If the signal strength of the frequency Nol exceeds the threshold value Th, block 27, the mobile switching center locates the mobile station MS indoors, block 29. MS is also located near the substation MT3 by means of its unique frequency selection as described above in connection with blocks 30 and 31. A connection 6 between the substation MT3 and the mobile station MS is assigned to one of the free traffic channels of this substation, block 32, for example the third time slot of the frequency No37, and the mobile station MS makes a handoff to this traffic channel according to block 33.

In the embodiment example above, the hand change is performed handoff from the sub-state MT2 to the base station BS1, which then makes a handoff to the sub-station MT3. This reduces the risk of losing the call when changing the substation. According to an alternative embodiment of the invention, however, it is possible to make handoff directly from one of the substations to another. This method can be used when the reported signal strengths for the identification frequencies of the new substation MT3 exceed the signal strengths of the old substation MT2. It is appropriate here that the unique frequency selections differ from each other by more than one, frequency number.

The mobile station MS can move out of the housing 1 through the door 5 and doors 7 and 8. In this case, the connection is gradually switched

470 036 between the base station BS1 and the substations MT3, MT2 and MT1 as described above. When the mobile station MS passes the door 8, the base station BS1 takes over the connection on one of its own traffic channels, for example the second time slot of a frequency No42. The mobile station MS reports the five strongest frequencies on the associated control sequence SACCH of this channel. The strength of the common frequency Nol will in this case be below the threshold value Th, the alternative "No" in block 27. The base station BS1 hereby locates the mobile station MS outdoors and the connection with the base station BS1 continues on the second time slot of the frequency No42 according to block 24.

The frequency assignment to the substations can be performed so that only a subset of the reported frequencies need to coincide with a subset of the unique frequency selections for the substations. These frequency selections can be made in, for example, the following ways:

MT1: frequency Nol, No2, No3, No4, No5 MT2: frequency Nol, No6, No7, No8, No9 MT3: frequency Nol, NolO, Noll, Nol2, Nol3 At one point, the mobile station MS reports the frequencies Nol, No9, NolO, Zero , Nol2. The mobile switching center MSC can locate the mobile station MS indoors through the frequency Nol. By a majority decision on the frequencies NolO, Noll and Nol2, the mobile switching center MSC can locate the mobile station MS to the substation MT3 and assign the connection to one of this station's channels.

If a relatively small number of frequencies have been reserved for location purposes, the location of the mobile station MS can be divided into several periods. This enables a large number of substations to be identified using their unique frequency range according to the following example. Cell C2 has been assigned eleven numbered frequencies for localization purposes. The unique frequency selections for the substations in this cell consist of a common frequency as well as two frequencies transmitted during a first localization period and two frequencies transmitted during a second localization period. Through this procedure, one can

470 036 number (101/2! · 8Ι}² = 2025 different substations are identified, despite the small number of reserved frequencies. The unique frequency sample is thus divided into two sub-samples, which are transmitted during each of the localization periods.

In the embodiment above, the substations MT1, MT2 and MT3 lack a control channel, while the base station BS1 alone has the control channel CC to identify itself for the mobile station MS. However, the invention can also be used for substations which have access to a control channel and the base stations can radiate a unique frequency selection according to the invention for identification purposes.

In the example in Figure 2, the substations are located indoors, but sub-stations may also be located outdoors, for example within a factory area. Even in cells other than the cell C1, for example in the cells C2 and C3, the above-mentioned thirty reserved frequencies Nol to No30 can be used to locate substations. In the exemplary embodiment, the base stations and the substations have adaptive assignment of traffic channels, but also fixed channel assignment is within the scope of the invention.

In adaptive channel assignment, when the level of interference on possible traffic channels is measured, the designated identification frequencies Nol - No30 can also be used for traffic by one of the substations that uses the frequencies for identification. This is possible if the interference level of an identification frequency is below a determined maximum value. In the exemplary embodiment in connection with Figure 5, all reported signal strengths from the mobile station MS were compared with the threshold value Th. However, it is possible to use several threshold values, for example a threshold value for the common identification frequency Nol and another * threshold value for the other identification frequencies.

The invention has a number of advantages. As already mentioned, the sub-stations MT are relatively simple and the invention enables a secure location of a mobile station. A great advantage is that the invention uses a function called Mobile Assisted

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Handoff ”in some existing mobile phone systems. This function includes the mobile listening to a number of control channels from different base stations and reporting some of the strongest to the mobile switching center. This can thereby designate the most suitable base station for the connection with the mobile station. According to the exemplary embodiment of the invention, the function Mobile Assisted Handoff is used to listen to the reserved frequencies Nol - No30.

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