A mobile telephone system intended for indoor and outdoor subscriber use.

(54) NAME Mobile telephony systems intended for use by subscribers indoors and outdoors (56) PUBLISHING PUBLICATIONS: --- (57) SUMMARY:

A mobile telephone system comprises an inner subsystem (6) in a building (1) and an external subsystem (7). The internal subsystem has internal base stations (BSI) which are connected to a mobile switching center (MSC), to which also external base stations (BSI, BS2) are connected. The mobile telephony system is assigned a frequency band with a number of channels. The outer subsystem is cellular (C1, C2, C3) and has fixed channel assignment with a few channels to each outer base station (BSI, BS2). The internal subsystem is adaptive and the internal base stations (BSI) have access to all the system's channels. To determine whether a mobile station (MSI) belongs to the internal or external subsystem, an internal identification word is sent from the internal base stations and an external identification word from the external base stations. If the signal strength of the internal identification word exceeds a threshold, the mobile station sends an internal identification word to the internal base stations (BSI). If the threshold is lower than the mobile station (HSI) identifies itself for the external subsystem (7).

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The numbers in brackets indicate the international identification code. INID code Letters within clamps indicate international document code

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TECHNICAL FIELD

The present invention includes a mobile telephony system intended for use by subscribers indoors and outdoors, comprising, on the one hand, an outdoor subsystem for outdoor use with relatively signal-rich external base stations directly connected to a mobile switching center, and an internal subsystem for indoor use with relatively signal-poor internal base stations. the mobile switching center.

BACKGROUND OF THE ART

A known mobile telephone system comprises geographic cells with each base station, which are connected to a switching center. Each base station has access to a certain number of channels that can be both frequency-shared and time-shared. The cells are grouped into larger cell groups, in which adjacent cells have been assigned different frequencies so-called fixed channel assignment. Within a cell group, all frequencies of the system are present. Several cell groups are placed next to each other to cover larger geographical areas and the frequency assignment in the cells is chosen so that different groups should not interfere with each other. A detailed description of the above-mentioned system can be found in CMS 88, Cellular Mobile Telephone System, Ericsson Telecom AB, 1988. Adaptive mobile telephone systems are also known where each base station has access to all the system's channels. A mobile subscriber is assigned a channel after a measurement of the occurrence of disturbances, called dynamic channel allocation. A system as above is intended for outdoor use by usually vehicle-borne subscribers. Both the base station and the subscriber's mobile station have relatively high transmitter power. For indoor use in, for example, a factory or office, the corresponding mobile telephone systems are known, for example, as described in the 39th IEEE Vehicular Technology Conference, Volume 1, May 1-3, 1989, Dag Åkerberg: Properties of a TDMA Pico Cellular Office Communication System. This report describes an adaptive telephony system with wireless, portable subscriber devices, which can be connected to a subscriber switch through base stations. It is a requirement that such an indoor system must have great capacity to connect. The system therefore has a relatively large number of small cells

465 992 with each base station and both the base stations and the portable subscriber devices have relatively low signal strength. It is thus possible to use the same channels repeatedly within a building without the inconvenient disturbance occurring.

In mobile telephony, it is a problem to offer the subscribers a sufficient number of channels within a limited frequency range. This frequency range is allocated by authorities and must not be exceeded, so that adjacent frequency ranges are disturbed. The number of channels within a frequency range can be substantially increased by time-sharing of a frequency, but the number of channels remains limited. It is therefore difficult to have an indoor system and an outdoor system for mobile telephony with large capacity in the same geographical area. Special difficulties exist in introducing an indoor system in an area with an already established outdoor system. It is also difficult for a subscriber to use both systems with one and the same mobile station.

DISCLOSURE OF THE INVENTION

The above-mentioned problems are solved according to the invention by a combination of an outdoor system and an indoor system, both of which have access to substantially the entire allocated frequency range. The indoor system has adaptive channel assignment and relatively low signal strength. A subscriber's mobile station can distinguish a base station in the outdoor system from a base station in the indoor system and make a choice between these base stations. The mobile station can identify itself for the selected base station.

The invention has features which appear from the appended claims.

FIGURES

An embodiment of the invention will be described below with reference to figures in which Figure 1 shows an internal mobile telephone system in a building which is in an area of ​​an external mobile telephone system, Figure 2 shows schematically signal sequences for the internal mobile phone3.

465 992 the telephony system, Figure 3 schematically shows signal sequences for the external mobile telephony system and Fig. 4 shows an alternative embodiment of the building with the inner and outer mobile telephony system.

PREFERRED EMBODIMENT

Figure 1 shows a building 1 with office space 2, laboratory rooms 3, corridors 4 and a foyer 5. The building has a system 6 with wireless telephones, which is an internal subsystem of a mobile telephone system. This has an external subsystem 7 which is a cellular cellular telephone system. In the corridors 4, the internal subsystem has internal base stations BSI which are connected to an internal switching center MXI via lines 8. This is connected to an external mobile switching center MSC, which is connected to a switch PX in the public wired telephone network. Each internal base station covers an area within the building 1, which is usually referred to as a microcell. The outer subsystem 7 is cellular, with cells C1, C2 and C3 schematically shown in the figure and has a base station in each cell, BSI and BS2 respectively. These base stations are connected to the external mobile switching center MSC and the external subsystem has a so-called fixed channel assignment. The entire external subsystem has access to a frequency band for mobile telephony, which is divided into a number of bidirectional channels. Each cell has access to only some of these channels and the channels are distributed among the cells so that adjacent cells do not interfere with each other. One of the channels in each cell is a control channel that is used for, for example, connecting connections and changing channels, so-called hand over. The outer subsystem is described in more detail in the above reference CMS 88.

The internal subsystem 6 also has access to a frequency band which is divided into a number of bidirectional channels. Also in this system, one of the channels is selected to be a control channel. In an office, it is generally required that the telephone system has great capacity. Many users MSI within the limited area of ​​construction 1 must be able to connect at the same time without disturbing each other and the users

465 992 must be able to move freely within the building and always be connected to one of the base stations BSI. All the base stations of the internal subsystem BSI therefore have access to all the channels and the channel assignment is adaptive. In addition to the desired high capacity, this adaptive system also offers purely practical installation advantages. An adaptive wireless telephone system for indoor use is described in more detail in the above-mentioned reference IEEE Vehicular Technology Conference.

The allocation of frequency bands for, for example, mobile telephony, takes place j.0 through the authorities' provision. The frequency range available at all is subdivided into frequency bands assigned to users. It is essential that users do not intrude on each other's frequency band so that, for example, a mobile radio system does not interfere with an air landing system. The overall frequency range available is limited and the number of users is large and it is essential for each user to utilize their limited frequency band effectively. In order to enable this efficient frequency utilization, both the internal subsystem 6 and the external subsystem 7 have access to the same frequency band.

There is an obvious risk that the inner subsystem 6 and the outer subsystem 7 interfere with each other. The mobile user MSI in the internal subsystem connects to one of its base stations BSI on a channel with a fixed frequency. A vehicle-borne user MS, which is in a position 8 in the outer subsystem 7, establishes connection with its base station BSI on the same channel. The transmitter effects of the internal subsystem are very small compared to the transmitter effects of the external subsystem and the risk that the connection to MS is disturbed by the internal subsystem is small. In contrast, the connection to the internal mobile station MSI 30 can be disturbed by the established outdoor connection and this risk is relatively large if the vehicle-borne mobile station MS moves to a position 9 near the building 1. The internal subsystem detects this disturbance, the disturbed channel in the internal subsystem 6 is disconnected and an undisturbed channel is connected.

Because the internal subsystem is adaptive and all the internal base stations BSI have access to all the channels, it is a

465 992 has a relatively low probability of all the channels being occupied or disturbed, so that this connection of the new channel is prevented.

It is essential that the mobile station MSI can determine whether it belongs to the internal subsystem 6 or the external subsystem 7. The mobile station MSI must be able to determine this if the subscriber wants to be able to call or be called by another subscriber. The signals of the outer subsystem 7 are so strong that they often reach into the building 1, so that it is possible to make a call x0 indoors via the base subsystem BS1 of the external subsystem. In contrast, the signals of the internal subsystem are weak and it is usually impossible to make a call outdoors via the internal base stations BSI. Thus, if the subscriber is outdoors, connection must be made via the external base station BSI. If the subscriber is indoors, it is usually an advantage to set up a call via the internal base stations BSI, among other things, depending on the channel supply and for cost reasons. This is also an advantage because the internal base stations BSI each have all the channels of the system, so that the risk of non- connection of the connection is small.

In order to determine whether the mobile station MSI belongs to the inner or outer subsystem, the control channels in the respective subsystems are utilized. The control channel is composed of a channel from the base station to the mobile station, often referred to as the FOCC (Forward Control Channel) and a channel from the mobile to the base station, often referred to as the RECC (Reverse Control Channel). The MSI mobile station constantly listens to the FOCC to be prepared to receive a call. When the mobile phone first hears FOCC, it sends a signal to identify itself to the base station, so-called login. This allows the call to be connected to the correct base station of the mobile switching center MSC. A more detailed description of the control channel for a digital mobile telephone system can be found in, for example, the above-mentioned reference CMS 88, Chapter 3.

The mobile station MSI can determine if it belongs to the inner or outer subsystem as follows. From the 6 base stations of the internal subsystem BSI, FOCC signal sequences are transmitted on the control channel

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SSI with data words A and B for various purposes in known manner, as well as an internal identification word IDI, as shown in Figure 2. In this figure, T means the time. IDI is unique to the internal subsystem. From the base station B1, B2 of the outer subsystem 7, FOCC transmits signal sequences SSO with data words C and E for various purposes in known manner on the control channel as shown in Figure 3. The signal sequence SSO also contains an external identification word IDO which is unique to the external subsystem. The mobile station MSI constantly senses the control channels in both subsystems. As soon as the signal strength of the identification word IDI in the internal subsystem exceeds a threshold value, the mobile station MSI for the internal base stations BSI identifies the above-mentioned login. This is done through a signal IDIR on the control channel RECC as shown in Figure 2. The mobile station registers the external identification word IDO and as soon as the signal strength of the internal identification word IDI falls below the threshold, the mobile station MSI identifies itself for one of the external base stations. This is done by an external identification signal IDOR as shown in Figure 3. As a result, the mobile station MSI can itself request call connection or be connected to a calling subscriber via one of the indoor base stations BSI as long as the mobile station MSI is within the building 1 and the signal strength of IDI exceeds the threshold. If the mobile station MSI moves out of building 1, the signal strength of the identification word IDI will fall below the threshold, whereby the mobile station MSI identifies for the external base station BSI. The mobile station MSI can accordingly request connection or be connected via the external base station BSI.

In the embodiment, the signal strength of the identification word IDI in the control subsystem's control channel determines which of the two subsystems the mobile logs on. It is also possible to utilize the signal strength of the entire signal sequence SSI for the internal control channel or parts thereof, for example parts A and B. It is also possible to use, for example, a threshold value for signal strength, for example, bit error content as a measure of the signal quality on the channel.

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The invention has been described above in connection with an exemplary embodiment in which the internal base stations BSI and the outer base stations BS1, BS2 are connected directly to the mobile switching center MSC. This embodiment has the advantage that it is relatively easy to maintain an ongoing call connection to the mobile station MSI as this mobile station moves between the inner and outer subsystem.

Figure 4 shows an alternative embodiment of the invention which differs from the embodiment of Figure 1 in that the two subsystems are connected to each other via a wired public telephone network 10. The internal subsystem 6 is connected to a subscriber switch AX which is connected via a line with a public switch PX in the public telecommunications network 10. The mobile switching center MSC is also wired to the public switch PX. Wired telephone sets H may be connected to the subscriber exchange AX.

The outer subsystem 7 of the described embodiment has fixed channel assignment to cells C1, C2 and C3. However, it is possible according to the invention that the external subsystem is also adaptive and the base stations BSI and BS2 have access to all the channels of the system. It is also conceivable that most of the traffic channels in the outer subsystem are permanently assigned to the external base stations according to a predetermined frequency plan, but a smaller number of the traffic channels are allocated adaptively, for example according to the Swedish patent number 8801555-7. In the embodiment, the internal subsystem 6 has access to all the channels of the system. It is within the scope of the invention to limit this channel access.

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