WO1995029562A2

WO1995029562A2 - Method for facilitating call establishment in a cellular network, and a base station

Info

Publication number: WO1995029562A2
Application number: PCT/FI1995/000223
Authority: WO
Inventors: Risto Uola; Harri Jokinen; Timo Ali-Vehmas; Matti Jokimies; Harri Posti
Original assignee: Nokia Telecommunications Oy
Priority date: 1994-04-20
Filing date: 1995-04-19
Publication date: 1995-11-02
Also published as: WO1995029562A3; FI97933B; AU2260695A; FI941841A0; FI941841A; FI97933C

Abstract

The invention relates to a method for facilitating call establishment in a digital cellular radio network, in which there is in each cell at least one base station (BTS) to serve subscriber terminal equipments (MS) in the area, and in which the base station transmits by using more than one carrier wave frequency, and in which a signal transmitted on each frequency is divided by time division into frames containing several time slots, and which base stations transmit information concerning themselves about frame structure to terminal equipments in a previously known location for the purpose of call establishment. In order to improve the quality and speed of call establishment, call establishment information regarding the base station is transmitted on all frequencies used in the cell, on which frequencies said previously known location is not used for another transmission.

Description

Method for facilitating call establishment in a cellular network, and a base station

The invention relates to a method for facilitating call establishment in a digital cellular radio network, in which there is in each cell at least one base station to serve subscriber terminal equipments in the area, and in which the base station transmits by using more than one carrier wave frequency, and in which a signal transmitted on each frequency is divided by time division into frames containing several time slots, and which base stations transmit information concerning themselves about frame structure to terminal equipments in a previously known location for the purpose of call establishment.

Subscriber terminal equipments of a cellular radio network need for operating plenty of information about the base station in which area they are. In the first place, terminal equipments have to be synchronized for the transmission of the base station to be able to communicate with the base station. In addition to this, they need information, for example, about the network the transmitting base station belongs to, about frequencies used in the base station and about traffic and control channels so that the terminal equipments would be able to send a call establishment message.

In current digital cellular radio systems, base stations transmit the kind of information described above regularly in each cell. In known arrangements, such as the European digital cellular radio network GSM, a base station transmits information needed in call establishment on one frequency in the first time slot of the frame structure, which information is called a BCCH control channel (Broadcast Control Channel) . In the idle state (switched to the network, but not in a call) , terminal equipments scan frequencies and search for BCCH channels transmitted by different base stations so that they could send a call establishment message to a base station in the opposite direction of transmission of a corresponding channel.

In arrangements of the prior art in which a base station transmits call establishment information on one frequency, a drawback is that because of the propagation conditions of the radio path, for example, said signal can attenuate too much for the terminal equipment to be able to receive it every moment in time. In current arrangements a terminal equipment may have to search and wait for call establishment information even a relatively long time, especially if fading is of a long duration. A situation of this kind can occur especially if the terminal equipment is stationary, in which case changes in the propagation conditions of the radio channel are slow and signal fadings can last even longer than the duration of a normal call. If a terminal equipment does not receive call establishment information from a base station of its own, it may search for a corresponding transmission of a neighbouring cell. If the cells are large, as often is the case in scarcely populated areas, such a connection established to a neighbouring cell will naturally be weaker in quality because the distance is longer.

The object of the present invention is to improve the speed and quality of call establishment in a digital cellular radio network and eliminate the problems described above.

This is achieved with a type of method shown in the preamble that is characterized in that call establishment information regarding a base station is transmitted on all frequencies used in the cell, on which frequencies said previously known location is not used for another transmission.

The invention also relates to a base station in a digital cellular radio network in which there is in each cell of the network at least one base station to serve subscriber terminal equipments in the area, and which base station has means for transmitting by using more than one carrier wave frequency and means for dividing a signal transmitted on each frequency by time division into frames containing several time slots, and means for transmitting information concerning themselves about frame structure to terminal equipments in a previously known location for the purpose of call establishment. The base station of the invention is characterized in that the base station has means for transmitting call establishment information on all frequencies used in the cell, on which frequencies said previously known location is not used for another transmission. The call establishment of subscriber terminal equipments in a cellular radio network can be improved with the method of invention. The method is especially well suited for use in systems in which the radio network replaces the last junction line of a fixed telephone network to a subscriber, in so-called LL (Wireless Local Loop) arrangements. In these arrangements terminal equipments are fixed in place near the users, replacing an ordinary telephone. In arrangements of this kind the sizes of cells may be large and the antennas of the terminal equipments may even be antennas directed towards a base station of its own. In this case if the frequency of a base station of its own on which a BCCH signal is transmitted fades, the terminal equipment has to wait until fading ceases because in practice, there is no contact with other base stations. The arrangement of the invention makes use of frequency diversity in transmitting a BCCH on every frequency in use in the cell . Because fading is frequency-dependent, it is probable that not all frequencies fade at the same time and call establishment information is almost continuously available to the terminal equipment.

In the following, the invention will be described in more detail with reference to the examples of the accompanying drawings, in which

Figure 1 shows a cellular network in which the method of the invention can be applied,

Figure 2 illustrates a frame structure of frequencies transmitted by a base station, Figures 3a-3c illustrate a call establishment event on a frame structure level, and

Figure 4 illustrates the structure of the base station of the invention.

The invention can be applied in any digital cellular radio system in which a frame structure with time division is used. In the following, the invention will be explained in more detail when applied to the European cellular radio system GSM, without restricting thereto in any way. Figure 1 illustrates a cellular radio system in which the method of the invention can be applied. A base station BTS serves subscriber terminal equipments MS in the area in a way that more than one frequency is in use, depending on the size of the cell and the number of the subscriber terminal equipments. Subscriber terminal equipments MS can be ordinary mobile phones or data terminal equipments. In case it is a question of a WLL arrangement, terminal equipments are fixedly installed inside buildings so that the antenna of the terminal equipment is possibly outside the building. Even in an arrangement of this kind part of the terminal equipments can be mobile stations.

It is assumed by way of example that said cell has for the use of the base station four frequencies fl - f4 on which it can communicate with terminal equipments. According to the GSM specifications, on each frequency the transmission is divided in time domain into a frame structure in which each frame has eight time slots. The direction of transmission from the base station to the terminal equipment will in the following be referred to as a downlink direction. For each downlink frequency fl - f4, there is naturally a corresponding uplink frequency fl' - f4', that is, a frequency to the direction of transmission from the terminal equipment to the base station, the frame structure of which corresponds to that of the downlink direction.

In the traditional GSM system, BCCH information is transmitted cyclically in the first time slot of the frame structure so that 4 frames out of 52 successive frames contain BCCH information. In the corresponding position in other frames other control information is transmitted, such as acknowledgement messages of a call establishment message, i.e. AGCH (Access Grant Channel) , so-called paging information PCH (Paging Channel) and a synchronisation channel SCH and a frequency correction channel, i.e. FCCH. The frame structure of the GSM system is described in more detail for example in The GSM System for Mobile Communications by M. Mouly and M. Pautet, published by the authors in 1992, which is used as reference.

Figure 2 illustrates the transmission of the base station BTS on frequencies fl - f4 of the downlink direction. Figure shows the frame structure of each frequency. On the first frequency, marked fl, call establishment information BCCH and other control information are transmitted in the first time slot. The time slots with traffic are indicated with letters TCH. In the method of the invention, BCCH information is thus transmitted on all downlink frequencies used in the cell in the corresponding positions of the frame structure to those on frequency fl. If the number of connections momentarily grows large on some frequency, all time slots are needed for transmitting traffic to the terminal equipments and then call establishment information is not transmitted on said frequency until a time slot is free again for this purpose. However, in the cell there is all the time at least one frequency on which a BCCH signal is transmitted, but this frequency may change from time to time.

Figure 2 illustrates a situation of this kind. The BCCH signal is always transmitted in 4 frames out of 52 successive frames on frequency fl and other control information is also transmitted cyclically. As in a momentary situation shown in Figure 2, of frequencies in the use of the base station, BCCH information is also transmitted on frequencies f2 and f3 in the corresponding positions of the frame structure to those on frequency fl. In addition to the BCCH signal, AGCH and PCH channels are also transmitted on these frequencies.

On frequency f3 all the time slots are in the use of different connections with terminal equipments so that call establishment information cannot be transmitted on said frequency until the first time slot is free again.

It is assumed that in the idle state, a terminal equipment in the area of a base station transmitting the frame structure above listens to the transmission of the base station. The terminal equipment scans the frequencies transmitted by the base station and receives call establishment information from channels fl, f2 and f4. The terminal equipment measures the strength of said signals. It is assumed that the strongest frequency in this example would be f2 as shown in Figure 3a. When intending to establish a call to a base station, for example as a user wants to make a call, the terminal equipment transmits a call establishment message to the base station on the frequency of the uplink direction of transmission which corresponds to the downlink frequency on which the received BCCH information was strongest. In this case the message RACH is transmitted on frequency f2' as shown in Figure 3b. After transmitting the message, the terminal equipment listens to the corresponding downlink frequency f2 and waits for the acknowledgement, as in Figure 3c, on the AGCH channel, which is transmitted with BCCH information on the same frequency. If the call establishment fails, the terminal equipment may attempt a call establishment on the uplink frequency which corresponds to the second strongest BCCH signal .

After receiving a call establishment message from the subscriber terminal equipment, the base station transmits an acknowledgement to the terminal equipment in an AGCH message which is transmitted on the same frequency as the original BCCH message was transmitted. The arrangement selects the final traffic channel and traffic frequency for the terminal equipment, and the measurement results of the BCCH channel can be utilized in this. If the uplink frequency corresponding to the best BCCH frequency is of a weak quality, some other frequency can also be selected to be the frequency of the traffic channel .

In the method of the invention the same information, which can also be interleaved, is thus transmitted piece by piece on all frequencies . When scanning different frequencies, the terminal equipment can gather the necessary information piece by piece from different frequencies on the basis of the transmissions it has received.

According to the invention, the information transmitting on various frequencies can thus momentarily be different in different time slots, depending on the loading of time slots by other traffic and possible interleaving. The terminal equipment can receive the same signal from several different frequencies and thus it can be made sure that the message will reach its destination.

The method of the invention is especially advantageous when applied to an arrangement in which the uplink and downlink are on the same frequency separated by time division. An arrangement of this kind is said to use the TDD method (Time Division Duplex) . In this case the call establishment message can be transmitted on the frequency that is strongest. In a GSM type of arrangement most advantage is gained in the downlink direction of transmission.

The structure of a base station accomplishing the method of the invention is illustrated on a block diagram level in Figure 4. The base station of the invention comprises means 40 for developing a base band signal, the output signal of which is transferred to modulation means 41, from which the base band signal is transferred via an output amplifier 42 to be transmitted by an antenna 43. The base station also has means (44) with which the above-mentioned components are controlled. The base station to be accomplished naturally contains other components, too, such as filters and converters, but as they are not essential for the invention, they are not shown in the figure for the sake of simplicity.

The base station of the invention comprises means (41) for transmitting information concerning itself to subscriber terminal equipments on all frequencies used in the cell for the purpose of call establishment . The base station further comprises means (44) for directing the use of the time slots of the frame structure so that if on some frequency all time slots are needed for the use of traffic, call establishment information is not transmitted on said frequency. However, call establishment information is always transmitted on the frequency on which other control information is also transmitted to subscriber terminal equipments.

Even though the invention is explained above with reference to the example of the accompanying drawings, it is evident that the invention is not restricted thereto, but it can be modified in many ways within the scope of the inventive idea shown in the appended claims.

Claims

Claims :

1. A method for facilitating call establishment in a digital cellular radio network, in which there is in each cell at least one base station (BTS) to serve subscriber terminal equipments (MS) in the area, and in which the base station transmits by using more than one carrier wave frequency, and in 'which a signal transmitted on each frequency is divided by time division into frames containing several time slots, and which base stations transmit information concerning themselves about frame structure to terminal equipments in a previously known location for the purpose of call establishment, c h a r a c t e r i z e d in that call establishment information regarding a base station is transmitted on all frequencies used in the cell, on which frequencies said previously known location is not used for another transmission.

2. A method according to claim 1, c h a r a c- t e r i z e d in that terminal equipments can transmit a call establishment request to a base station using the corresponding location and frequency in the opposite direction of transmission as were used by the base station for transmitting information concerning itself.

3. A method according to claim 1, c h a r a c- t e r i z e d in that call establishment information transmitted on each frequency concerning a base station is of the same kind.

4. A method according to any of the preceding claims, c h a r a c t e r i z e d in that the subscriber terminal equipment receives information concerning call establishment from the base station on more than one frequency.

5. A method according to claim 4, c h a r a c- t e r i z e d in that the subscriber terminal equipment measures the quality of signals it has received on more than one frequency and stores in memory the frequencies on which the quality of the connection is best.

6. A method according to claim 5, c h a r a c- t e r i z e d in that the subscriber terminal equipment sends a call establishment message to the base station using the channel on which the received signal from the base station is strongest.

7. A method according to claim 2, c h a r a c- t e r i z e d in that the base station sends an acknowledgement to the call establishment message sent by the subscriber terminal equipment using the same channel as the call establishment message was sent on.

8. A base station in a digital cellular radio network in which there is in each cell of the network at least one base station (BTS) to serve subscriber terminal equipments (MS) in the area, and which base station has means (41, 42) for transmitting by using more than one carrier wave frequency and means (41) for dividing a signal transmitted on each frequency by time division into frames containing several time slots, and means (41) for transmitting information concerning themselves about frame structure to terminal equipments in a previously known location for the purpose of call establishment, c h a r a c t e r i z e d in that the base station has means (41, 44) for transmitting call establishment information on all frequencies used in the cell, on which frequencies said previously known location is not used for another transmission.