Abstract:
In a cellular radio system down-link diversity is provided. The system has means in a base station (BS) for receiving signals from a mobile station (MS) using a diversity reception arrangement, in particular an antenna diversity arrangement ( 207, 209 ). Information regarding the current reception conditions is extracted from the reception diversity system and is used for improving the transmission. Thereby the transmitted signals can have less signal power, which in turn makes it possible to provide smaller cluster sizes and thereby increase the channel capacity in the cellular radio system.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a system for radio communication. 
     DESCRIPTION OF RELATED ART 
     In cellular radio systems the number of cells used for covering a particular area should be as small as possible in order to minimize the costs. However, in order to maintain a sufficiently good signal to noise ratio (SNR) for a system having many mobile stations, the cell size should be as small as possible. 
     Thus, using a small cell size the same signals can be reused at other locations in the cellular radio system. In a system having M orthogonal signals or channels these are usually divided into K groups having the same or close to the same number of channels. In a hexagonal cell pattern with the base station located in the middle of the cell, see FIG. 1 a , K can be any integer written as 
     
       
           K =( i+j ) 2   −ij , 
       
     
     where i,j=0,1,2,3, . . . 
     Thus K can be K=3,4,7,9,12,13, . . . 
     A group of cells where each channel group is used once is usually termed cluster. K is therefore usually termed cluster size. 
     It is also common to use more advanced cell structures, where the base stations use directive antennas located in the corners of the cells, see FIG. 1 b . A cell structure having a cluster size of 12 and 4 base station locations per cluster is denoted a 4/12 cluster and same way a cell structure having cluster size of 21 and 7 base station locations per cluster is denoted a 7/21 cluster etc. Such arrangements have the advantage that several base stations can be located at the same location, which is cost saving. The maximal distance from the base station to a mobile station will be increased but the number of interfering closely located base stations will be fewer. 
     In order to optimize the system for handling as many calls per cell as possible it is desired to have access to as many channels as possible per cell. This can be termed the capacity of the system (n), where 
     n= M/K   
     For a certain system the number of channels M which can be used is usually predetermined. Hence, in order to increase the capacity of the cellular radio system the number of groups K should be as small as possible. How small the number of groups K can be is determined by how sensitive to noise and interfering signals the signals are. 
     In order to increase the performance in transmission on a fading radio-channel, diversity is employed. Diversity means that the same signal is transmitted via two or more transmission paths. There are many different types of diversity arrangement such as: frequency diversity, time diversity, polarisation diversity and spatial diversity etc. In spatial diversity or antenna diversity, several antennas are used when receiving the signal. It can be shown that if the distance between two antennas is sufficiently long the signals at the different antennas will be uncorrelated. 
     The distance required is mainly dependent on the wavelength used in the transmission. A distance of about the wavelength divided by 2 can be sufficient to reduce the correlation to acceptable levels. The longer the distance between the antennas, the less correlation. However, very long distances may not be feasible. A normal value can be about 12-20 times the wavelength used. 
     Antenna diversity has the advantage of not requiring a larger frequency range. Also the implementation of an antenna diversity arrangement is quite simple. 
     In cities and other areas having a large concentration of mobile stations (mobile telephones), the frequency range available is very limited in comparison to the heavy concentration of mobile stations. In this environment frequency diversity systems and other types of diversity system increasing the required frequency range are not possible to use, since the number of channels available would be to low to cope with the large number of mobile stations. Therefore, the use of antenna diversity systems is preferred in many cases. 
     The receiver can use or weigh the signals from the different antennas in a number of different ways. The most common methods are: 
     Selection diversity: The receiver chooses the signal having the highest signal to noise ration (SNR) and uses this signal for the signal detection. 
     Maximum ratio combining diversity: The receiver uses the information from all the reception branches all at the same time. The signals from the different branches are given a weight which is proportional to the SNR at the respective branch. When using the method the individual signals must have the same phase when the addition takes place. Thus, it is necessary to implement a phase compensation circuit. The phase compensated and optimal weighted signal is then used as the output signal. 
     Equal gain combining diversity: This is a special case of the Maximum ratio combining diversity system. Thus, in order not to have to measure the momentary SNR in each reception branch the weight is set constant and equal for all the branches. 
     Switched diversity: This is a simple method. Thus, one branch at a time is used and switching between different branches is only carried out when the signal envelop drops below a certain threshold value. 
     The use of diversity systems is a powerful tool in order to avoid long fading conditions. 
     However, the diversity systems of today cannot be used for significantly increasing the channel capacity for cellular radio systems. The limitation in channel capacity is an increasing problem, since the frequency range available is limited and the number of mobile terminals is constantly increasing. 
     SUMMARY 
     It is an object of the present invention to improve the channel capacity for cellular radio systems. This object is obtained by using a diversity arrangement in the base station, which makes use of the information obtained when receiving signals in an antenna diversity system for transmission purposes. 
     The radio transmission system is therefore equipped with means in the base station for extracting information regarding the current diversity conditions and the extracted information is used for controlling the transmission of the down-link signals according to a diversity (down-link) transmission scheme. 
     Thus, the same information used for the reception of the signal (up-link signal) is applied in a corresponding manner for transmission of the signal (down-link signal). This makes it possible to use less power in the transmitted signals whereby the cluster sizes can be reduced. This in turn results in a larger channel capacity according to the above. 
     The use of such a down-link diversity arrangement therefore provides a system operator with a tool by means of which he can easily increase the number of mobile stations without having to use a larger frequency range. For example it will be possible to reduce the co-channel interference so that a 4/12 cluster will have the same speech quality as for traditional base stations in a 7/21 cell cluster. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which: 
     FIGS. 1 a  and  1   b  show hexagonal cell structures 
     FIG. 2 shows a mobile station and a base station in a cellular radio system, where the base station has a combined receiver/transmitter using a diversity arrangement. 
     FIG. 3 shows a mobile station and a base station in a cellular radio system, where the base station has a combined receiver/transmitter using another diversity arrangement. 
     FIG. 4 is a flow chart illustrating different steps carried out when employing a down-link diversity scheme. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 2 a principal view of a cellular radio transmission system is shown. Thus, a mobile station is shown at  201 . The mobile station  201  has an antenna  203  by means of which it can communicate with a base station shown at  205 . The base station  205  has two antennas  207  and  209 . 
     The base station  205  shown in FIG. 2 is designed for maximum ratio combining diversity. Thus, the SNR is derived from the signals received by the antennas  207  and  209 . The SNRs are used for setting amplifiers  211  and  213  to values corresponding to the measured SNRs. In other words, the receiving branch having a better SNR will be more amplified than the receiving branch having a small SNR. 
     The same value as is set for the amplifier  211  is at the same time set for an amplifier  215  and the same value set for the amplifier  213  is set for an amplifier  217 . The amplifiers  215  and  217  form a part of the transmission circuit of the base station  205  and are also connected to the antennas  207  and  209 , which thus are used both for transmission and reception of signals to/from the mobile station  201 . 
     The output from the amplifiers  211  and  213  is fed to a circuit  219 . The circuit  219  is used for phase compensation in order to give the two received signals the same phase, before they are added, which also takes place in the circuit  219 . The information derived in the phase compensation circuit  219  is also fed to phase compensation circuits  221  and  223 , which are part of the transmission circuit of the base station  205 . The output from the circuit  219  is fed to a detector or demodulator circuit  225 , which provides the output from the base station  205 . 
     The base station is used for bi-directional communication with the mobile station  201 . Thus, when transmitting signals from the base station  205  to a mobile station  201  the signal to be transmitted is fed to a modulator  227 . The output from the modulator is fed on two different lines to the phase compensators  221  and  223 . The phase compensators  221  and  223  are set according to the current phase compensation performed in the circuit  219 . The output signals from the phase compensators  221  and  223  are then fed to the antennas  207  and  209 , respectively. 
     Thus, the antennas  207  and  209  are duplex antennas which are used both for reception of signals and for transmission thereof. The antennas  207  and  209  are fed with transmission signals having the same phase compensation and the same signal strength relation as the SNR of the currently received signals. 
     This arrangement will have the effect that the signals transmitted from the base station  205  can be transmitted using a smaller signal power, while maintaining the same SNR at the receiving mobile station  201 . 
     In a preferred embodiment the duplex antennas  207  and  209  are separated by a distance equal to half the wavelength used or an integer multiple thereof. It is also possible to use an arrangement having more than two antennas if that would turn out to be more useful in certain situations. For example, three or four antennas can be used in certain applications. 
     Furthermore, it is not necessary that the antenna diversity used in the base station is using a maximum ratio combining diversity algorithm for increasing the SNR. Other antenna diversity algorithms can also be used in the same manner. Thus, if selection diversity (as described above) is used the transmitting part of the base station will use the information provided by the receiving part and send information from the duplex antenna which is currently used for receiving signals from a mobile station. 
     This is shown in FIG.  3 . Thus, a mobile station  301  is transmitting and receiving signals via an antenna  303  to a base station  305 . The base station  305  has two duplex antennas  307  and  309 . The antennas  307  and  309  are connected to receivers  311  and  313 , respectively. The output from the two receivers  311  and  313  is connected to a logic unit  315 , which is arranged to choose the output signal from the two receivers having the highest SNR as output signal. 
     The logic unit  315  also controls two switches  317  and  319  so that only the antenna  307  or  309  which is currently used when receiving signals is used for transmission via a modulator  321 . 
     Thus, when the base station  305  transmits signals to the mobile station  301  the input signal modulated in the modulator  321  is only transmitted via the antenna  307  or  309  which currently has the highest received SNR. In this manner, as in the case described above in conjunction with FIG. 2, a lower signal power can be used while maintaining the same SNR at the receiver site, i.e. the mobile station. 
     The idea of letting the information obtained by an receiving antenna diversity system be used for transmission of signals is of course not limited to the antenna diversity systems described above, all other antenna diversity systems can be used for obtaining the effect. In fact, the method of letting the information provided by a receiver diversity system is not even limited to antenna diversity systems it can equally well be implemented in other diversity systems, such as a frequency diversity system. In that case the frequency information obtained by the diversity system is used for transmission purposes, e.g. the frequency which currently provides the best SNR is used for transmission. However, this would require that the receiver has means for receiving at the different transmission frequencies. 
     FIG. 4 is a flow chart illustrating the steps carried out in a base station when using a down-link transmission scheme. Thus, first in a block  401  an up-link signal is received using a suitable diversity arrangement, e.g. an antenna diversity arrangement. Next in a block  403  the information obtained from the diversity arrangement regarding the up-link signal is extracted. For example in the case when an antenna diversity system employing selection diversity is used, the information extracted is the which antenna/reception branch that currently receives the signal having the highest SNR. 
     Thereupon the information is passed to the transmission part of the base station in a block  405 , and finally the transmission part feeds the antenna/antennas with a down-link signal using the information obtained from the receiving diversity system in a block  407 . In the case when the information obtained is which antenna in an antenna diversity system that currently receives the signal having the highest SNR (selection diversity), the transmission part of the base station feeds that antenna with the down-link signal. 
     The method and system as described herein, which uses information derived from a diversity arrangement for receiving an up-link signal for transmission purposes, i.e. a kind of transmission diversity, enables use of a smaller signal power. 
     This is most valuable in today&#39;s cellular radio systems where frequency is a limited resource and reuse of frequencies at close distance is required in order to obtain a system capable of housing the increasing number of mobile stations, such as mobile telephones.