Base station and method for channelling transmit power in the desired direction
The invention relates to a base station which communicates with subscriber terminals within its coverage area, said coverage area being divided into one or several sectors, and which comprises one or several transceiver units.
During the construction of a cellular system, the aim is to obtain the desired coverage area with the lowest possible costs. When the locations of the system base stations are considered, both the required traffic capacity and the cell size to be obtained are taken into account. In areas where a high capacity is required the cell sizes are small, whereas in sparsely populated regions with a low traffic capacity requirement the aim is to provide as large cells as possible.
One of the central factors limiting the cell size in a cellular system is the path attenuation of the radio signal. Especially on higher frequencies, such as 1800 and 1900 MHz, the path attenuation is considerable. Since the transmit power used in the transmitters cannot be increased endlessly, there have been efforts to enlarge the cell size by increasing the antenna gain. A known method for lengthening the radius of the base station coverage area is to increase the number of sectors used at the base station. Sectorization means that the antennas of the base station are oriented radially so that the coverage area of the base station consists of several, typically three, sectors, each of which is serviced by a group of transceiver units. Each transceiver unit is thus connected to an antenna servicing one sector. If the number of the sectors is increased beyond the usual three, the beams of the antennas used can be made narrower so that their gain, and therefore the radius of the coverage area, increases. In such a case, the number of the base station transceiver units must be correspondingly increased, however, and the capacity of the base station must be unnecessarily increased, whereupon the network planning also becomes more difficult.
Another known method is the use of an adaptive antenna group. In this arrangement, the base station coverage area is realized either with several narrow beams of which only the beams having subscriber terminals in their area are active, or with freely controlled beams which follow the mobile subscriber terminals. However, adaptive antenna groups are expensive and complicated. Furthermore, problems will occur in the signalling and power measurement in such systems. In cellular systems, the base station generally transmits a specific call channel which is continuously listened to by the subscriber terminals and by means of which a call is established and other information concerning the base station is transmitted. In the GSM system, for example, this channel is called a BCCH channel. All times slots of the BCCH channel must be continuously audible in the entire coverage area of the cell, wherefore this determines the cell size. Furthermore, the transmission of the BCCH channel must be on the same power level as the transmission of the beamed traffic channels, so that the call establishment and the power measurement of the neighbouring channels would not be disturbed. The use of diversity is also difficult in connection with adaptive antenna groups.
The purpose of the present invention is to implement the antenna arrangements of the base station in such a way that a greater coverage area is obtained than what has been advantageous with the conventional methods. The purpose of the invention is to enlarge the coverage area of the base station with low costs and by means of simple technical changes made in the present equipment.
This is achieved with the base station of the type described in the preamble, characterized in that means to which four antennas or an antenna group forming four beams are connected, are attached to one or several receivers of the base station, the means combining the signals of the subscriber terminals in the receiver direction by utilizing said four antennas or antenna group.
The invention also relates to a method for channelling the transmit power in the desired direction at a base station which has a coverage area that is divided into one or several sectors and which comprises one or several transceiver units, each of which is connected to one or several antennas. The method according to the invention is characterized in that four antennas or an antenna group forming four beams are connected to at least one of the base station transceiver units by means of one four-port diversity unit, and that said four antennas are directed in such a way that the sectors formed by their beams differ essentially from one another, and that it is measured in the receiver direction which sector has the servicing antenna that receives best the signal from the subscriber terminal, and that in the direction of transmission, an integral part of the signal power to be transmitted to the subscriber terminal from the base station is directed, by means of a switch, to the transmitting antenna servicing the corresponding sector.
In the base station according to the invention, narrow-beam high-gain antennas can be used so that the cell radius can be considerably increased from the present one. On the other hand, it is possible to apply in the base station more various diversity arrangements than previously in order to improve the quality of the received signal. For example vertical, horizontal, depth and even polarization diversity may be used at the base station if there is no need for antennas with a narrow beam. The simultaneous use of several types of diversity is especially advantageous in urban areas where the rapid fading of the signal constitutes a problem.
In the following, the invention will be described in greater detail with reference to the examples according to the accompanying drawings, in which
Figure 1 shows a conventional cell with three sectors, Figure 2 illustrates the structure of the base station according to the invention with respect to the antenna coupling,
Figure 3 shows the coverage area of the base station according to the invention, realized by means of twelve sectors,
Figure 4 shows the coverage area of the base station according to the invention, realized by means of twelve sectors and an omnidirectional antenna,
Figure 5 illustrates a possible manner of arranging antennas when vertical diversity is used,
Figure 6 illustrates a possible manner of arranging antennas when horizontal diversity is used.
Figure 7 illustrates the structure of the base station according to the invention with respect to the antenna coupling of the receiver and transmitter directions.
Figure 1 illustrates the coverage area of a conventional base station 10. The coverage area is typically implemented by means of three sectors 11, 12, 13, which have an angle of typically 120 degrees. In the example of the figure, transceiver antennas, the beams of which form an angle of about 120 degrees, are directed to each sector. Conventionally, the transmitter antenna and the receiver antenna are separate. In a conventional base station, each sector is serviced by one or several transceiver units. In the GSM network, each sector has its own BCCH frequency. In the example of Figure 1, a mobile subscriber terminal 14 is in the area of sector 13 and it communicates with a transceiver unit servicing the sector.
Figure 2 illustrates the structure of the base station according to the invention with respect to the antenna coupling of the receiver branch. At least one receiver unit of the base station according to the invention comprises means 20 which comprise a control unit 21 and four antenna ports 22a to 22d, a receiver antenna 25a to 25d being connected to each of these antenna ports by means of a converter unit 23a to 23d and a radio-frequency unit 24a to 24d. In the base station according to the invention, the control unit 21 can combine signals that are received by the antennas 25a to 25d, that are converted to an intermediate frequency by the radio-frequency means 24a to 24d, that are transformed into digital form by the converter means 23a to 23d, and that are supplied to the aforementioned antenna ports 22a to 22d.
In the base station according to the invention, the receiver unit can thus combine signals received by four different antennas. Therefore, it is possible to utilize either more versatile diversity or several sectors in the receiver. In practice, the base station receiver according to the invention can be realized in such a way that in the prior art diversity combiner two antenna ports are added to the previous two and the calculation capacity and software of the control unit 21 are increased so that the control unit 21 can process four signals instead of two. The arrangement according to the invention can thus be realized with relatively small changes in the present base stations too. Figure 3 illustrates a preferred embodiment of the coverage area of the base station according to the invention, in which embodiment the base station 10 of Figure 1 that utilizes three sectors is implemented with twelve sectors in the receiver direction. The previous sector 11 can now be realized with four antennas 25a to 25d that are directed adjacent to each other, so that the antenna beams constitute four adjacent sectors 30a to 30d having the combined angle of 120 degrees. Correspondingly, the previous sector 12 can now be realized with four antennas 25a to 25d that are oriented adjacent to each other so that the antenna beams form four adjacent sectors 31a to 31d. Further, the previous sector 13 can now be implemented with four antennas 25a to 25d that are directed adjacent to each other so that the antenna beams constitute four adjacent sectors 32a to 32d.
The beams shown in Figure 3 are only indicative, and in reality the beams have a wider shape. Since the arrangement according to the invention may utilize antennas having a narrower beam, i.e. a higher gain, the cell radius can be increased in the direction of reception without any need for power increase in the mobile subscriber terminals. In the transmission direction, the transmit power can be correspondingly increased in the base station without any problems, so that the same coverage area is also obtained in that direction.
In the downlink direction, the antenna beam arrangement of Figure 1 is utilized, in which arrangement the terminal equipment 14 receives a signal from the base station via the beam 13. In the uplink direction, the orientation of the antenna beams in the manner of Figure 3, made possible by the four-port diversity unit, is utilized in such a way that a corresponding area is covered by means of the different beams 32a to 32d. The terminal equipment 14 transmits a signal to the base station 10, and in the example of the figure the base station receiver receives the signal from the area covered by the antenna beam 32a. The coverage area of the base station can also be realized in such a way that in addition to the sector beams, the coverage area 40 that is provided by means of the omnidirectional antenna and that is serviced by one or several transceiver units is utilized in the manner of Figure 4. By means of this omnidirectional antenna, the handovers occurring near the base station from one sector to another can be decreased, and the shadow region of the adjacent areas of the high-gain antennas having vertical narrow beams can be covered. If there is no need to increase the coverage area, the base station according to the invention can utilize diversity methods that are more versatile than previously. Since one receiver unit can utilize four antennas, the antennas 25a to 25d can be simultaneously exploited for example in such a way that horizontal, vertical, polarization and depth diversity are used. Therefore, the effect of fadings can be efficiently compensated for in the received signal. The antenna beams can be directed in such a way that they are overlapping or partly interlaced.
Figure 5 illustrates a possible manner of arranging antennas when vertical diversity is used. The antennas can be placed in an antenna tower 50 in such a way that the omnidirectional antenna 51 is uppermost so that it has unlimited coverage in every direction. Sectorized receiver antennas 52, transmitting antennas 53 and reception diversity antennas 54 are attached to the antenna tower one below the other. The antennas form three hexagonal groups of six antennas around the tower. The antennas can be positioned near the tower, so that the installation will be compact and the wind load will be small.
Figure 6 illustrates a possible manner of arranging antennas when horizontal diversity is used. The transmitting antennas can be attached around the tower in the manner of Figure 5. In the case of horizontal diversity, the fastening of the receiver antennas can be realized by building a sufficiently great platform on top of the tower, and the antennas can be attached to the edges of the platform possibly by utilizing brackets. Figure 6 illustrates the placement of the receiver antennas on the edges of the platform. The figure shows the receiver antennas 60a to 65a and the corresponding diversity antennas 60b to 65b. Figure 7 illustrates the structure of a base station according to the invention with respect to the antenna coupling of the receiver and transmitter directions. One or several transceiver units 73 of the base station according to the invention comprise, in the direction of reception, means 21 for detecting which diversity antenna receives best the signal transmitted by a subscriber terminal 76a. In the beam arrangement of Figure 2, the antenna beam that covers the area where the subscriber terminal 76a is situated is detected. The transmitter unit of the base station correspondingly comprises beamed transmitting antennas 70a to 70d having coverage areas that correspond to the coverage areas of the receiver antennas 25a to 25d. The transceiver unit of the base station further comprises switching means 71 with which the signal of the transmitter 75 can be guided, on the basis of control obtained from the measuring means 21, to the transmitting antenna 70c covering the coverage area in question. It should be noted that the subscriber terminals 76a and 76b shown in Figure 7 constitute in reality one and the same terminal, but for the sake of clarity the subscriber terminal is depicted twice in the figure, with respect to transmission and reception.
The invention thus also relates to a method for channelling transmit power to the desired sector in a sectorized base station. Utilizing the data that is received from the diversity combiner 20 of the receiver and that concerns the antenna beam providing the best result, an integral part of the signal power of the transmitter can be directed to the desired coverage area, so that better connection quality is obtained and interference can be decreased in the directions that differ from the direction of location of the terminal equipment when viewed from the base station. It should be noted that the method according to the invention cannot be applied on a frequency transmitting the BCCH signal.
Even though the invention is described above with reference to the example according to the accompanying drawings, it is clear that the invention is not restricted thereto, but it can be modified in many ways within the scope of the inventive idea disclosed in the appended claims.