DIVERSITY METHOD AND BASE STATION EQUIPMENT

Abstract: The invention relates to a base station equipment and a diversity method in a cellular radio system comprising transceivers (100 to 106) communicating with other transceivers by means of radiation patterns (110 to 116) generated by one or more antennae. To achieve good diversity gain, in the method of the invention the transceiver antennae are preferably used to generate two or more radiation patterns with identical power radiation patterns and dissimilar phase radiation patterns.

Description: DIVERSITY METHOD AND BASE STATION EQUIPMENT FIELD OF THE INVENTION The invention relates to a diversity method in a radio system wherein transceivers communicate with other transceivers by means of radiation patterns generated by one or more antennae. PRIOR ART Signal quality on connections between different transceivers varies constantly in radio systems. This is true for all radio systems, but particularly for the ones wherein either of the transceivers is mobile. A typical system suf- fering from varying signal quality is a cellular radio system. In a cellular radio environment the signals between a base station and user terminal are subjected to different fading and distortion which are the reason for developing ways to alleviate the effects of interference on a connection. Different diversity methods are commonly used in radio systems particularly for eliminating Rayleigh distributed interference. Diversity methods refer to methods wherein more than one signal components are utilized in signal transmission. The component offering the best quality at each particular moment can be selected from these signal components, or alternatively the components can be preferably combined, resulting in maximal signal quality. Various diversity methods are being used. Cellular radio systems generally employ space diversity wherein signal transmission and reception take place by the use of more than one antenna. Other known methods include polarization diversity, 'jitter', 'switched pattern' and beam diversity. These diversity methods are described in more detail in references [1] - [5]. A list of references is at the end of the specification. Several of these diversity methods ('jitter', switched pattern' and beam diversity) are based on modifying the direction of the antenna radiation pattern or beam. This makes their performance dependent on antenna beam- width. In addition, particularly in microcells, the average signal power received by an antenna is substantially dependent on the direction of the beam. When receiving a signal on, for example, two beams, and the average power level of the signal received by one beam being considerably higher than the average power level of a signal received by the other beam, then the diversity gain is lost. Diversity methods based on modifying the antenna beam are thus de- pendent on circumstances, particularly in microcells. Polarization diversity, which is based on the use of two different polarizations, i.e. horizontal and vertical polarization, is also very sensitive to the environment, as reference [5] shows. The average received power levels of different polarizations are different particularly owing to the lack of reflec- tions. The direction of a mobile station antenna with respect to the vertical direction is also very significant as far as the performance of polarization diversity is concerned. The major disadvantage of space diversity is that to operate it needs more than one antenna, increasing the costs and making the position- ing of antennae cumbersome particularly in microcell environments. CHARACTERISTICS OF THE INVENTION It is the object of the present invention to provide a diversity method with which a good diversity improvement is achieved and which is not dependent on the environment as are known methods. It is a further object of the invention to provide an equipment with which diversity improvement can be inexpensively achieved and which is easy to position. This is achieved by a method of the kind described in the preamble, characterized in that the antennae of the transceiver are used to generate two or more radiation patterns with identical power radiation patterns and dissimilar phase radiation patterns. The invention further relates to a base station equipment comprising one or more antennae for generating radiation patterns by means of which the base station communicates with subscriber terminals within its coverage area. The base station equipment of the invention is characterized in that the base station comprises means for using the antennae to generate two or more radiation patterns with identical power radiation patterns and dissimilar phase radiation patterns. The solution of the invention provides a plurality of advantages. The solution of the invention avoids the different average power levels of different antenna beams. Thus the solution of the invention is not dependent on the environment as are prior methods. In a preferred embodiment of the invention, antenna beams are achieved by the use of one compact antenna. Neither is diversity implementation dependent on antenna parameters, such as beam- width or directions, and accordingly these factors can be optimized in accor- dance with coverage considerations. DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail with reference to examples according to the accompanying drawings, in which Figure 1 illustrates an example of a radio system to which the in- vention can be applied, Figure 2 illustrates an example of phase radiation patterns, Figure 3 illustrates a first example of a transceiver implementing the invention, and Figure 4 illustrates a second example of a transceiver implementing the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The solution of the invention for achieving diversity can be applied to any radio system where transceivers communicate with each other over the radio path. One such radio system is a cellular radio system, typically consist- ing of a plurality of cells, each comprising a base station communicating with subscriber terminals within its area. In the following the invention will be described by using a cellular radio system as an example, and more specifically, the base station equipment of the system, without, however, being restricted to it. Figure 1 illustrates an example of a cellular radio system to which the invention can be applied. The Figure shows a base station 100, and a number of subscriber terminals 102 to 106 with which the base station communicates by means of radiation patterns 110 to 116 to be generated with one or more antennae 108. A known manner of implementing the antenna 108 is to compose the antenna of several antenna elements to which a signal to be transmitted is applied. Each element is weighted as desired, and as a result the desired radiation pattern is achieved. The same takes place in the reception direction. By placing different weights on the elements, different radiation patterns are thus generated. This way the same antenna can be used to generate different antenna beams in desired directions or to implement an omnidirectional antenna pattern. An antenna consisting of elements may also be called an antenna group, although a number of different antennae are not involved, but instead one antenna consisting of elements. It is the basic idea of the inventive solution that the antennae of the transceiver are used to generate two or more radiation patterns with identical power radiation patterns but with dissimilar phase radiation patterns. The generated radiation patterns may be either omnidirectional 116 or beam-like 110 to 114, according to the application. The radiation patterns may be uniform in either the horizontal or vertical directions, or in both. The antenna used in a preferred embodiment of the invention is a group of antennae, i.e. it consists of a group of antenna elements which can be controlled using weighting coefficients, as described above. The weighting coefficients for the antenna group are selected such that two or more radiation patterns with identical power radiation patterns but dissimilar phase radiation patterns are generated. Let us have a look at Figure 2 which illustrates the phase radiation patterns of two omnidirectional antenna patterns with identical power radiation patterns. The Figure shows on the horizontal axis a horizontal angle and on the vertical axis the phase, and two phase radiation patterns 150, 152. The phase patterns of the Figure can be generated by e.g. an eight-element antenna group. As can be seen, the phases are not uniform except at a few individual angles. Let us study next diversity utilization according to the invention in the reception direction. There are several alternatives for the utilization of diversity. The first alternative uses one receiver. Let us assume for the sake of clarity that two radiation patterns are implemented with a uniform power pattern but dissimilar phase patterns. Upon reception, the transceiver momentarily receives a signal by one phase radiation pattern and measures signal qual- ity, and momentarily by the other phase radiation pattern and measures signal quality, and then selects the radiation pattern that provides better quality. The control of the antenna elements can be implemented e.g. by means of digital signal processing. In another alternative, upon reception the transceiver receives con- tinuously by several different phase radiation patterns and advantageously combines the received signals. In this case the receiver equipment has to comprise two different receivers whose received signals are combined. Let us next study the structure of the base station equipment implementing the solution of the invention. Figure 3 illustrates the receiver side of a first example of the equipment implementing the invention. The equipment comprises an antenna group 200, comprising a plurality of antenna elements 202 to 206. The antenna elements are connected to weighting means 208, wherein the signals received by the antenna elements are subjected to the desired weighting. The weighting means can be implemented by methods known per se to those skilled in the art. From the weighting means the signal is conveyed to radio frequency parts 210, wherein the signal is converted to a base or intermediate frequency. From the radio frequency parts the signal is further conveyed to base frequency parts 212, wherein the signal is subjected to other processing, such as channel decoding, deinterleaving and speech decoding. These measures are not essential to the present invention. Let it be pointed out here that the location of the weighting means 208 and the radio frequency parts 210 can be different, i.e. the weighting can be performed after the radio frequency parts. Thus the weighting can be both digital and analogical, as is obvious to those skilled in the art. The moment of performing the weighing is not essential to the present invention. The equipment further comprises control means 214, controlling the operation of the other equipment parts. The control means can be implemented by means of a general or signal processor into which the steps required by the invention are programmed. The control means can alternatively be implemented by separate components or as an ASIC circuit. The control means 214 also control the weighting means 208 and the radio frequency means 210 in such a way that a signal is momentarily received by one phase radiation pattern and signal quality is measured, and momentarily by the other phase radiation pattern and again signal quality is measured. The control means 214 select the radiation pattern providing better quality to the reception on the basis of the measurements. The measurements can be effected rapidly in succession without propagation circumstances having time to change in the radio channel. Let us next study the structure of a base station equipment implementing a second embodiment of the solution of the invention. Figure 4 illus- trates the receiver side of another example of the equipment implementing the invention. The equipment comprises an antenna group 200 consisting of a plurality of antenna elements 202 to 206. The antenna elements are connected to weighting means 208 in which the signals received by the antenna elements are subjected to the desired weighting. From the weighting means the signal is conveyed to radio frequency parts 300, 302, in which the signal is converted to a base or intermediate frequency. From the radio frequency parts 300, 302 the signal is further conveyed to base frequency parts 304, in which other signal processing is carried out, such as channel decoding, deinterleav- ing and speech decoding. These measures are not essential to the present invention. As was pointed out before, the location of the weighting means and the radio frequency parts can be different, i.e. the weighting can be performed after the radio frequency parts. Thus the weighting can be both digital and analogical, as is obvious to those skilled in the art. The equipment further comprises control means 214 for controlling the operation of the other equipment parts. In this embodiment of the inven- tion, the control means 214 control the radio frequency means 300, 302 in such a manner that a signal can be continuously received by two different phase radiation patterns. Both signals can be processed in the base frequency parts 304, and the base frequency parts 304 comprise means 306 for advantageously combining the signals by utilizing known combination methods. Sig- nal combination can also be effected by the radio frequency parts, as is evident to those skilled in the art. Although this example describes the combination of two signals and phase patterns only, the invention is naturally not restricted to it, but there may be several phase patterns. Even though the invention has been described above with refer- ence to the example according to the accompanying drawings, it is evident that the invention is not restricted to it but can be modified in many ways within the scope of the inventive idea disclosed in the attached claims. REFERENCES [1] O. Norklit, J. Andersen: Mobile radio environments and adaptive arrays, Proc. IEEE PIMRC, Sept 1994, pp. 725 to 728. [2] O. Norklit, C. Eggers, J. Andersen: Jitter diversity in multipath environments, Proc. 45 th IEEE Veh. Tech. Conf., Chicago, 1993, pp. 853 to 857. [3] T. Aubrey, P. White: A comparison of switched pattern diversity antennas, Proc. 43 rd IEEE Veh. Tech. Conf., Secausus, pp. 89 to 92. [4] K. Nikoskinen, J. Lempiainen, K. Heiska: Beam diversity simulations in microcellular environments, Proc. AP-S/URSI International Symposium, Baltimore, 1996, vol. 1 , pp. 449 to 452. [5] A. Turkmani, A. Arowojolu, P. Jefford, C. Kellett: An experimental evaluation of the performance of two-branch space and polarization diversity schemes at 1800 MHz, IEEE Trans. Veh. Tech., vol. 44, No. 2, May 1995, pp. 318 to 326.

Claims: CLAIMS 1. A diversity method in a radio system wherein transceivers (100 to 106) communicate with other transceivers by means of radiation patterns (110 to 116) generated by one or more antennae, characterized in that the antennae of the transceiver are used to generate two or more radiation patterns with identical power radiation patterns and dissimilar phase radiation patterns. 2. A method as claimed in claim ^characterized in that the radio system comprises a number of base stations (100) and subscriber termi- nals (102 to 106), the base stations communicating with the subscriber terminals within their area, and that radiation patterns with dissimilar phase radiation patterns are used in the base station in both transmission and reception. 3. A method as claimed in claim 1 or 2, characterized in that the radiation patterns generated are omnidirectional (116). 4. A method as claimed in claim 1 or 2, characterized in that the radiation patterns generated are substantially beam-like (110 to 114). 5. A method as claimed in claim 1 or 2, characterized in that the antennae (200) used in the transceiver consist of a group of antenna elements (202 to 206), and that one antenna is used to generate two or more radiation patterns with identical power radiation patterns but dissimilar phase radiation patterns. 6. A method as claimed in claim 1 or 2, characterized in that upon reception the transceiver momentarily receives a signal by one phase radiation pattern and measures signal quality, and momentarily by the other phase radiation pattern and measures signal quality, and selects the radiation pattern that provides better quality. 7. A method as claimed in claim 1 or 2, characterized in that upon reception the transceiver receives continuously by several different phase radiation patterns and advantageously combines the received signals. 8. A method as claimed in claim 1 or 2, characterized in that the radiation patterns can be uniform in the horizontal or vertical directions or in both. 9. A base station equipment comprising one or more antennae (200) for generating radiation patterns (110 to 116) by means of which the base station (100) communicates with subscriber terminals (102 to 106) within its coverage area, characterized in that the base station comprises means (208, 214) for using the antennae to generate two or more radiation patterns with identical power radiation patterns and dissimilar phase radiation patterns. 10. A base station equipment as claimed in claim 9, characterized in that the antennae (200) employed in the base station comprise a number of antenna elements (202 to 206), and that the base station comprises means (208, 214) for using one antenna to generate two or more radiation patterns with identical power radiation patterns and dissimilar phase ra- diation patterns.

Inventors: JUNTUNEN JUHA, LEMPIAEINEN JUKKA, NIKOSKINEN KEIJO, JUNTUNEN, JUHA, LEMPIAEINEN, JUKKA, NIKOSKINEN, KEIJO
Applicants: JUNTUNEN JUHA, LEMPIAEINEN JUKKA, NIKOSKINEN KEIJO, NOKIA TELECOMMUNICATIONS OY, JUNTUNEN, JUHA, LEMPIAEINEN, JUKKA, NIKOSKINEN, KEIJO
Application year: 1997
Main classification: H04B7
IPC codes: H04B7