WO1996009733A2

WO1996009733A2 - Base station

Info

Publication number: WO1996009733A2
Application number: PCT/FI1995/000512
Authority: WO
Inventors: Arto JÄNTTI
Original assignee: Nokia Telecommunications Oy
Priority date: 1994-09-19
Filing date: 1995-09-19
Publication date: 1996-03-28
Also published as: WO1996009733A3; FI944346A; FI944346A0; AU3390095A

Abstract

The invention relates to a base station in a cellular radio system, comprising one or more transceiver groups (22, 24, 26), each group comprising two or more transceiver units (23a, 23b, 25a, 25b, 27a, 27b), said groups being connected each to an individual antenna (20a-20c). For enabling frequency hopping advantageously in the base station of the invention, the coverage areas of at least some of the antennas (20a-20c) of the base station are substantially overlapping.

Description

Base station

The invention relates to a base station in a cellular radio system, comprising one or more transcei- ver groups, each group comprising two or more transcei¬ ver units, said groups being connected, in the direction of the transmitter each via an individual wideband tran¬ smitter combiner, and in the direction of the receiver each via an individual combiner, each to an individual antenna.

In cellular radio networks, the quality of the radio channel varies as a function of time and location. A transmitting and a receiving antenna often have no visual contact, but there are obstructions within the line of sight distance, caused by the terrain or build¬ ings, and the signal appearing in the receiver is a sum of several beams propagated along different routes and reflected from obstructions, said beams having diff¬ erent phases. The sum of several signals having differ- ent phases follows the Rayleigh-distribution when the phases are randomly distributed.

Fading of the signal also depends on the fre¬ quency of the signal. Thus, if the frequency difference between two signals is sufficient, their fadings do not correlate. For instance, a difference of 1 MHz is suffi¬ cient, so that the fadings of the signals will be mut¬ ually independent.

The above-described frequency-selective fading of the signal is one reason for that a frequency hopping technique has been introduced in digital cellular radio networks. Frequency hopping refers to changing the tran¬ smission frequency used cn a connection at predetermined intervals. Due to frequency hopping, the quality of the transmission may be improved in particular in such cases in which a terminal equipment is moving very slowly or is stationary, which is often the case when one is call¬ ing with e.g. hand-portable phones.

In addition to frequency diversity caused by frequency hopping, the method is useful in spreading interference caused by the radio connection onto several frequencies, whereby momentary interference on one spe¬ cific frequency remains low.

So that a great advantage would be obtained from frequency hopping in digital GSM and DCS systems, the number of the frequencies used should be more than four. Nowadays, narrowband transmitter combiners are generally used in base stations. Narrowband combiners, the task of which is thus to combine several transmit¬ ters into one antenna, limit the number of the fre- quencies used in frequency hopping to the number of actual transceiver units and combiner filters. Thus, in a base station, for instance, in which three transceiver units and three combiner filters are in use, it is poss¬ ible to use three frequencies in frequency hopping. Narrowband transmitter combiners are large and complex components, which results from the tuning possibility required by them. Wideband combiners that do not require similar tuning have also been developed. Wideband transmitter combiners together with frequency- hopping synthesizers do not restrict the number of the frequencies used. A problem of wideband combiners, is, however, that combination attenuation values become remarkably high compared with narrowband combiners, when more than two transmitters are combined with the same wideband combiner. Thus, it has not been possible to use wideband combiners in base stations the traffic capacity required of which is high, but it has been necessary to use narrowband combiners. Therefore, implementation of frequency hopping at these base stations has been inad- equate. The object of the present invention is thus to enable effective implementation of frequency hopping at base stations with a high capacity, as well. Another object of the invention is to implement a base station for which, when the need for capacity further increases, the change from an omnidirectional cell into an sectored cell may be carried out in a simple manner without changes to the equipment configuration in the base sta¬ tion itself. Another object of the invention is to imp- lement a base station in which an advantageous antenna filtering unit may be used, in which case implementing a separate transceiver combiner is avoided.

This is achieved with a base station of the type set forth in the introduction, which is character- ized in that the coverage areas of at least some of the antennas of the base station are substantially overlap¬ ping.

Due to the solution in accordance with the invention, synthesized frequency hopping may thus also be applied in a high-capacity base station, as the high attenuation of the wideband combiners may be avoided by utilizing overlapping coverage areas. Furthermore, in the solution of the invention, the use of a separate transceiver combiner is avoided, as an advantageous antenna filtering unit (Antenna Filtering Unit, AFU) may be used in the base station.

In the base station of the invention, increasing the capacity may be carried out easily if necessary. Changing the base station from one using omnidirectional antennas into one using sectored antennas takes place by replacing t: * antennas with sectored antennas, and making a change co the software in the base station. Previously, a similar change has required changes to the equipment in the base station. In the base station of the invention, the cove¬ rage areas of different antennas may also be implemented so that there is a sector, larger than the other ones, in the base station, handling call establishment, and there are other, smaller sectors within the area of the larger sector.

In the following, the invention will be dis¬ closed in greater detail with reference to the examples in the attached drawings, in which Figure 1 shows a cellular radio system in which the base station of the invention may be used,

Figure 2 is a block diagram illustrating the structure of a base station of the invention,

Figure 3 is a block diagram illustrating the structure of the base station of the invention in greater detail,

Figure 4 illustrates changing the base station of the invention into a sectored base station, and

Figure 5 illustrates a possible coverage area arrangement of the antennas of the base station of the invention.

The cellular radio system in accordance with Figure 1, in which the base station of the invention may be used, comprises in each cell at least one base sta- tion 10, which is communicating with subscriber terminal equipments 11-13 within its area. The base station is also connected to a base station controller 14, which controls the operation of the base station and connects the calls of the subscriber terminal equipments further to a fixed network or to other parts of the system. The base station of the invention may advantageously be used e.g. in digital GSM and DCS cellular radio systems.

Figure 2 is a block diagram illustrating the structure of a base station in accordance with a pre- ferred embodiment of the invention. In the base station of the invention, a transceiver group 22 is connected to an omnidirectional antenna 20a via one antenna fil¬ tering unit 21a, said transceiver group advantageously comprising two transceiver units 23a and 23b, which thus comprise a transmitter and a receiver for establishing one bidirectional connection between a terminal equip¬ ment and the base station. The structure of the antenna filtering unit will be described below. The base station of the invention may comprise, depending on the required capacity, a number of antennas 20a-20c, each connected via an individual antenna filter unit 21a-21c to a tran¬ sceiver group 22, 24, 26, each advantageously comprising two transceiver units 23a, 23b, 25a, 25b, and 27a, 27b, respectively. One transceiver group may also comprise more than two transceiver units if necessary, but in that case, the combination attenuation increases on the transmitter side . Groups connected to different antenna filtering units may also have different numbers of units. The capacity of the base station may also be increased later by adding antennas, antenna filtering units and transceiver units .

All omnidirectional antennas substantially serve the same coverage area. As far as the GSM system is concerned, a BCCH control channel must be transmitted on one frequency, said channel being used in call estab¬ lishment. In the base station of the invention, in the example of Figure 2, a transceiver unit 22a transmits the BCCH control channel, but the other transceiver units do not transmit it . Figure 3 describes the structure of one antenna filtering unit of the base station of the invention in greater detail. The figure shows an antenna filtering unit 31 connected to an antenna 30, two transceiver units 36, 37 being connected to said antenna filtering unit. The base station of the invention may comprise several units in accordance with Figure 3 , depending on the need for capacity of the base station. The antenna filtering unit 31 comprises a duplex filter 32 connected to the antenna 30, which duplex filter is needed when the same physical antenna is used as the transmitter and the receiver antenna. If the transmitter and the receiver antennas are separate, no duplex filter is required. A signal received with the antenna 30 is applied from the duplex filter 32 via a pre-amplifier 33 to a receiver combiner 34, which distributes the received signal to receivers RX1, RX2 of different tran¬ sceiver units 36, 37. To one antenna filtering unit 31, two transceiver units 36, 37 are advantageously connect¬ ed, but there may also be more of them. In the direction of the transmitter, the output signals of transmitters TX1, TX2 of different transcei¬ ver units 36, 37 are applied to a wideband transmitter combiner 35, in which the combined signal is further applied via the duplex filter 32 to the antenna 30. As the combiner used now has a wide band, it is possible to apply in the transmitters synthesized frequency hopp¬ ing, in which the number of frequencies does not depend on the number of the transmitters and the narrowband transmitter filters available. In the base station of the invention, the increase of the capacity may be carried out easily if necessary. The base station may be converted from one using omnidirectional antennas into one using direc¬ tional antennas, whereby the capacity may be increased by means of sectors. In the following, let us study the example in accordance with Figure 4, illustrating a base station in which sectored coverage areas are applied. The base station of the invention, equipped with omnidirectional antennas, and illustrated in Figure 2 may be converted into a sectored base station by replac- ing the antennas with another type and making a change to the software in the base station. Changes to the equipment or new linkages are not required. The base station in accordance with Figure 3 comprises three antennas 40a-40c, which are directed radially, so that the coverage area of the base station is covered with three sectored coverage areas. Each antenna is connected via its individual antenna filtering unit 21a-21c to a transceiver group 22, 24, 26 of the base station, each group comprising two transceiver units 23a-23b, 25a-25b, 27a-27b. As the coverage areas of all the antennas now do not overlap, one of the transceiver units connected to each antenna must transmit the BCCH channel for the call establishment information of the terminal equip- ments. In the example in accordance with the figure, units 23a, 25a and 27a transmit the BCCH channel. Upon converting an omnidirectional base station into a sectored one, a change must be made to the software in the units of the base stations, in addition to replacing the antennas for transmitting the BCCH channel. No changes to the equipment are required for transmitting the BCCH channel .

In the base station of the invention, the cove¬ rage areas of different antennas may vary in size. In the example in accordance with Figure 5, the coverage area 51 of an antenna 50 is remarkably larger than those of the other antennas 52, 53, the coverage areas 54, 55 of which are within a large coverage area. For example, call establishment may be carried out via the antenna serving a large coverage area. Stronger amplification '".ay be used in antennas serving smaller coverage areas, which amplification enables lower transmission powers during the actual connection. Thus, the power consumpt¬ ion of portable terminal equipments is reduced compared with the case in which a large coverage area is used. Although the invention has been disclosed above with reference to the example in accordance with the attached drawings, it is obvious that the invention is not limited thereto, but it may be modified in various ways within the scope of the inventive idea set forth in the attached claims.

Claims

Claims :

1. A base station in a cellular radio system, comprising one or more transceiver groups (22, 24, 26) , each group comprising two or more transceiver units (23a, 23b, 25a, 25b, 27a, 27b) , said groups being con¬ nected, in the direction of the transmitter each via an individual wideband transmitter combiner (35) , and in the direction of the receiver each via an individual combiner (34) , each to an individual antenna (20a-20c) , c h a r a c t e r i z e d in that the coverage areas of at least some of the antennas (20a-20c) of the base station are substantially overlapping.

2. A base station as claimed in claim l, c h - a r a c t e r i z e d in that the substantially over¬ lapping coverage areas have one common control channel in use, on which channel the base station transmits information on itself.

3. A base station as claimed in claim 1, c h - a r a c t e r i z e d in that said groups (22, 24, 26) are connected to an antenna (20a-20c, 40a, 40c) each by means of an individual antenna filtering unit (21a-21c) , said unit (21a-21c) comprising in the direction of the transmitter a wideband transmitter combiner (35) , and in the direction of the receiver a combiner (36) and a pre-amplifier (33) , as well as a duplex filter (32) .

4. A base station as claimed in claim ^ c h a¬ r a c t e r i z e d in that the directional patterns of said antennas (21a-21c) are omnidirectional.

5. A base station as claimed in claim ^ c h ¬ a r a c t e r i z e d in that the directional patterns of said antennas (40a-40c) form sector beams in the area of the base station.

6. A base station as claimed in claim 1, c h a- r a c t e r i z e d in that the base station comprises at least one antenna (50) the coverage area (51) of which is substantially larger than those of the other antennas, and a number of other antennas (52-53) , the coverage areas (54-55) of which each cover a part within said large coverage area.

7. A base station as claimed in claim 1, c h a¬ r a c t e r i z e d by comprising means (23a, 23b, 25a, 25b, 27a, 27b) which may be configurated by means of software to serve either overlapping or separate cover- age areas.

8. A base station as claimed in claim 7, c h a- r a c t e r i z e d in that the configuration comprises adjustment of the means (23a, 23b, 25a, 25b, 27a, 27b) , so that in each separate coverage area, one common con- trol channel is transmitted, on which channel the base station transmits information on itself to the terminal equipments .

9. A base station as claimed in claim ^ c h a¬ r a c t e r i z e d in that the cellular radio system is the GSM system.