WO1998031110A1

WO1998031110A1 - Method for improving system capacity in a cellular radio system, and a base station

Info

Publication number: WO1998031110A1
Application number: PCT/FI1997/000801
Authority: WO
Inventors: Marko Silventoinen; Harri Posti
Original assignee: Nokia Telecommunications Oy
Priority date: 1996-12-18
Filing date: 1997-12-17
Publication date: 1998-07-16
Also published as: FI965104L; FI965104A7; FI965104A0; AU5400198A

Abstract

The invention relates to a method for improving system capacity in a cellular radio system, and a base station. The cellular radio system comprises at least one base station in each cell, the base station having a connection with subscriber terminal located in its area. The method of the invention is characterized in that the near-by base stations using the same carrier for a broadcast do not send a broadcast time slot simultaneously. The method and the base station of the invention enable a considerable reduction in the BCCH carrier reuse distance, whereby the cellular radio system capacity is improved.

Description

METHOD FOR IMPROVING SYSTEM CAPACITY IN A CELLULAR RADIO SYSTEM, AND A BASE STATION

The invention relates to a method for improving system capacity in a cellular radio system, which system comprises at least one base station in each cell, the base station having a connection with a subscriber terminal located in its area, and which base station transmits by using at least one carrier, and in which system a signal to be transmitted at each frequency is divided on a time division basis into a frame composed of several time slots, and which base stations transmit base station related information as a broadcast to the subscriber terminal.

The invention also relates to a base station used in a cellular radio system, which system comprises at least one base station in each cell, the base station having a connection with a subscriber terminal located in its area, and which base station transmits by using at least one carrier, and in which system a signal to be transmitted at each frequency is divided on a time division basis into a frame composed of several time slots, and which base stations transmit base station related information as a broadcast to the subscriber terminal.

A user's speech and data information is transmitted in cellular radio systems by using a traffic channel between a base station and a subscriber terminal. In addition, various control messages and system information are required between the base station and the subscriber terminal, the control messages and system information being transmitted by means of broadcast control channels. The BCCH is used for example in the GSM system for transmitting call set-up information from the base station to the subscriber terminal.

The existing GSM system transmits a carrier including a BCCH signal, i.e. a BCCH carrier, uninterruptedly at a constant power level. The subscriber terminal continuously measures the power level in the BCCH carriers transmitted by near-by base stations and reports on measurement results to the base station serving the subscriber terminal. The system determines a suitable moment for a changeover to another base station on the basis of the measurement results. To be able to measure signal attenuations between the base station and the subscriber terminal on the basis of the measurement re- suits, the BCCH carriers must be transmitted at a constant power level. The subscriber terminal has a limited time available for measuring power levels in the BCCH carriers of the near-by base stations in the existing GSM system. If the subscriber terminal receives a signal from the base station in a time slot 0 of a frame, it then transmits the signal to the base station in a time slot 3 and measures the near-by base stations in time slots 5 and 6. Since the reception can take place in any time slot 0 to 7 of the frame, the transmission and even the measurement can take place in any time slot of the frame. Consequently, the base station BCCH carriers must be transmitted uninterruptedly. The requirement that the BCCH carrier must be transmitted uninterruptedly at a constant power level in the existing GSM system prevents the use of methods particularly designed to improve connection quality and system capacity in such low-capacity base stations which comprise only one radio unit. The problem is not so serious in base stations comprising several radio units, although the connection quality and system capacity is also impaired in these base stations. Methods such as frequency hopping, discontinuous transmission and transmission power adjustment are developed to improve the connection quality and system capacity in cellular radio systems.

The above described problem does not exist in such systems where solely the BCCH carrier time slot including a BCCH needs to be transmitted uninterruptedly at a constant power level. It is possible to use methods such as frequency hopping, discontinuous transmission and transmission power adjustment designed for the improvement of the cellular radio system quality and system capacity in other base station BCCH carrier time slots. The frames start exactly simultaneously in all the base stations in synchronized telecommunication networks, as in some GSM networks. Consequently, for example a first time slot of the frame, indicated by 0, starts exactly simultaneously in all the base stations. The BCCH is typically transmitted in the first BCCH carrier time slot. In one method, the constant power level used for transmitting a BCCH time slot of the BCCH carrier is higher than the power levels in BCCH carrier traffic channels on average. In such a cellular radio system, a BCCH carrier reuse distance is determined by the power level of the BCCH time slot. In consequence, the BCCH carrier reuse distance is then unnecessarily great considering the BCCH carrier traffic channels. This, in turn, decreases the cellular radio system capacity described above. The above described problem can occasionally also occur in non-synchronized telecommunication networks, and thus, such problems must also be taken into account in non-synchronized networks.

The object of the present invention is, thus, to improve the system capacity in such a cellular radio system described above. This object is achieved by the method described in the preamble, characterized in that the near-by base stations using the same carrier for the broadcast do not transmit the broadcast time slot substantially simultaneously.

The base station of the invention is characterized in that the base station comprises means for synchronizing the transmission of a broadcast time slot of a broadcast carrier in such a way that the near-by base stations using the same broadcast carrier do not transmit the broadcast time slot substantially simultaneously.

The method and the base station of the invention provide many advantages compared with the known art. The method and the base station of the invention enable a significant reduction in the BCCH carrier reuse distance, whereby the cellular radio system capacity is improved. Naturally, the method and the base station of the invention can also be applied in such cellular radio systems where the BCCH time slot of the BCCH carrier is transmitted at the same power level than other BCCH carrier time slots. In the following, the invention will be described in more detail with reference to the examples in the accompanying drawings, in which

Figure 1 shows a cellular radio system wherein the method and the base station of the invention can be applied,

Figure 2 shows synchronization of base stations and time slots in the synchronized GSM system,

Figure 3 shows synchronization of frames and time slots in the method and the base station of the invention,

Figure 4 illustrates determination of a current BCCH carrier reuse distance, Figure 5 illustrates determination of a BCCH carrier reuse distance in the method and the base station of the invention, and

Figure 6 shows the structure of the base station transmitter of the invention by means of a block diagram.

Figure 1 illustrates a cellular radio system wherein the method and the base station of the invention can be applied. The cellular radio system comprises at least one base station BS in each cell, the base station having a connection with- subscriber terminal MS located in its area. A user's speech and data information is transmitted in the cellular radio system by using a traffic channel between the base station and the subscriber terminal. In addition, various control messages and system information are required between the base station and the subscriber terminal, the control messages and the system information being transmitted by means of broadcast control channels. The method of the invention can be applied in any cellular radio system utilizing the time division multiple access system TDMA. In the following, the method and the base station of the invention are described with reference to the GSM system, although they can also be applied in other connections.

The effective utilization of a frequency spectrum is one of the greatest challenges in cellular radio systems. Most channel allocation methods are based on the reuse of the same frequencies behind a specific signal-to-noise ratio. Conventional analogue cellular radio systems and the existing GSM system are implemented by using a fixed channel allocation. In this method, frequencies used by the system are allocated into groups. The frequency groups, in turn, are allocated fixedly to different cells. The frequency groups can be reused in the cells locating at a sufficient distance from each other. The reuse distance is determined by a co-channel interference level tolerated by the system. Figure 1 shows by way of example cells A, B, C and D, wherein the same BCCH carrier can be used because said cells are located at a sufficient distance from each other.

Figure 2 illustrates synchronization of base station frames and time slots in the synchronized GSM system. Both the frames and the time slots start exactly simultaneously in all the base stations in the synchronized GSM system. Figure 2 shows by way of example the synchronization of two successive frames and time slots included therein in eight base stations BS1 to BS8. It can be assumed by way of example that said base stations BS1 to BS8 are located at a sufficient distance from each other to be able to use the same BCCH carrier. The BCCH carrier frames drawn in Figure 2 comprise eight time slots each, numbered from 0 to 7. A broadcast B, i.e. a BCCH time slot, is transmitted in the first time slot of each frame, the time slot being indicated by 0. Figure 2 shows clearly that the broadcast- B, i.e. the BCCH time slot, is transmitted exactly simultaneously in all the base stations in the synchronized GSM system. The above described situation can occasionally also occur in non-synchronized cellular radio systems, and thus, such situations must also be taken into account in the non-synchronized systems.

Figure 3 illustrates synchronization of frames and time slots in the method and the base station of the invention. The frames are synchronized in the method and the base stations of the invention in such a way that near-by base stations using the same BCCH carrier do not transmit a broadcast B, i.e. a BCCH time slot, simultaneously. Figure 3 shows by way of example the synchronization of two successive frames and time slots included therein in eight base stations BS1 to BS8 by applying the method of the invention. It can be assumed by way of example that said base stations BS1 to BS8 are located at a sufficient distance from each other to be able to use the same BCCH carrier. The BCCH carrier frames drawn in Figure 3 comprise eight time slots each, numbered from 0 to 7. The broadcast B, i.e. the BCCH time slot, is transmitted in the first time slot of each frame, the time slot being indicated by 0. The order of the time slots inside the frames is, thus, similar to that of the GSM system presented in Figure 2. The synchronization of the base station frames deviates from that of the GSM system described above in that the frames in the near-by base stations using the same BCCH carrier start at different times, the base stations not transmitting the BCCH time slot simultaneously. In accordance with the method and the base stations of the invention, the system is synchronized in such a way that the time slots start exactly simultaneously in different base stations provided however that the same time slot of the frame is not transmitted simultaneously in the near-by base stations using the same BCCH carrier. Figure 4 illustrates the current determination of a BCCH carrier reuse distance in such a cellular radio system where a base station BCCH time slot is transmitted at a higher power level than BCCH carrier traffic channel time slots on average. Three base stations BS1 to BS3 and their BCCH carrier reuse distances BCCH1 to BCCH3 and TCH1 to TCH3 are drawn by way of example in Figure 4. BCCH time slot reuse distances are denoted by BCCH1 to BCCH3 and traffic channel reuse distances are denoted by TCH1 to TCH3. Since the base station BCCH time slots are transmitted simultaneously in all the base stations in the synchronized system, the reuse distance of all the BCCH carrier time slots is determined on the basis of the BCCH time slot re- use distance. As can be detected from Figure 4, the reuse distance is unnecessarily great considering the traffic channels. Figure 5 illustrates the determination of a BCCH carrier reuse distance in the method and the base station of the invention in such a cellular radio system where a base station BCCH time slot is transmitted at a higher power level than BCCH carrier traffic channel time slots on average. Three base stations BS1 to BS3 and their BCCH carrier reuse distances BCCH1 to BCCH3 and TCH1 to TCH3 are drawn by way of example in Figure 5. BCCH time slot reuse distances are denoted by BCCH1 to BCCH3 and traffic channel reuse distances are denoted by TCH1 to TCH3. Since the frames in the method and the base station of the invention are synchronized in such a way that the near-by base stations using the same BCCH carrier do not transmit the BCCH time slot simultaneously, the BCCH carrier reuse distances are determined as described in Figure 5. The BCCH time slot reuse distances and the traffic channel time slot reuse distances in the near-by base stations using the same BCCH carrier determine the BCCH carrier reuse distance in a man- ner shown in Figure 5. When a comparison is made between the examples in Figures 4 and 5, it can be qualitatively detected that the BCCH carrier reuse distance can be reduced with the method and the base station of the invention, and the cellular radio system capacity can, thus, be improved.

Figure 6 shows by way of example the essential parts of the struc- ture of the base station transmitter of the invention by means of a block diagram. The base station transmitter comprises channel coding means 10, modulation means 11 , radio frequency means 12, an antenna 13, and control means 14. Digital data reaches the channel coding means 10 which add redundancy to the data flow for correcting errors caused by the channel more efficiently in a receiver. The signal is conveyed from the channel coding means 10 to the modulation means 11 which convert a discrete symbol flow developed in the channel coding means 10 into a time-dependent function, i.e. into a wave form. This wave form flow developed in the modulation means 11 is modulated by a carrier onto radio frequency in the radio frequency means 12 and is then filtered. Power adjustment of the traffic channel also takes place in the radio frequency means 12. The radio frequency signal is transmitted via the antenna 13. The transmitter operation is controlled by the control means 14 which can also be used for changing for example broadcast and data transmission frames and time slots in accordance with the inventive method. The base station of the invention comprises means for synchronizing the transmission of the BCCH time slot in the BCCH carrier for example in the GSM system in such a way that the near-by base stations using the same BBCH carrier do not transmit the BCCH time slot simultaneously. Naturally, the base station of the invention also comprises other components, as it is obvious for those skilled in the art. For the sake of clarity, they are, however, not described herein.

Although the invention is described above with reference to the example in the accompanying drawings, it is obvious that the invention is not restricted to the example only, but it can be modified in many ways in the scope of the inventive idea disclosed in the appended claims.

Claims

CLAIMS:

1. A method for improving system capacity in a cellular radio system, which system comprises at least one base station (BS) in each cell, the base station having a connection with subscriber terminal (MS) located in its area, and which base station transmits by using at least one carrier, and in which system a signal to be transmitted at each frequency is divided on a time division basis into a frame composed of several time slots, and which base stations transmit base station related information as a broadcast to the sub- scriber terminal, characterized in that the near-by base stations using the same carrier for the broadcast do not transmit a broadcast time slot substantially simultaneously.

2. A method as claimed in claim 1, characterized in that the base stations transmit the broadcast time slot at a broadcast carrier at a differ- ent power level than other time slots.

3. A method as claimed in claim 2, characterized in that the base stations transmit the broadcast time slot at the broadcast carrier at a higher power level than other time slots on average.

4. A base station employed in a cellular radio system, which system comprises at least one base station (BS) in each cell, the base station having a connection with subscriber terminal (MS) located in its area, and which base station transmits by using at least one carrier, and in which system a signal to be transmitted at each frequency is divided on a time division basis into a frame composed of several time slots, and which base stations transmit base station related information as a broadcast to the subscriber terminal, characterized in that the base station comprises means (14) for synchronizing the transmission of a broadcast time slot of a broadcast carrier in such a way that the near-by base stations using the same broadcast carrier do not transmit the broadcast time slot substantially simultaneously.

5. A base station as claimed in claim 4, characterized in that the base station comprises means (11, 12, 14) for transmitting the broadcast time slot at the broadcast carrier at a different power level than other time slots.

6. A base station as claimed in claim 5, characterized in that the base station comprises means (11, 12, 14) for transmitting the broad- cast time slot at the broadcast carrier at a higher power level than other time slots on average.