TRANSMISSION METHOD AND RADIO SYSTEM

Abstract: The invention relates to a transmission method and a radio system comprising at least one transmitter (200) and one receiver (100) which employ TDD duplexing when setting up a connection. The transmitter (200) receives signals propagated on different signal paths by means of antennas (201). The radio system comprises transmission means (220) which receive a signal from several polarization directions, measuring means (210) which measure signals received by the transmitter (200), said signals being combined into one signal on the basis of the measurement. The transmission means (220) polarize the signal and transmit the polarized signal to the receiver (100) in several polarization directions on the basis of the measurement performed by the measuring means (210).

Description: TRANSMISSION METHOD AND RADIO SYSTEM FIELD OF THE INVENTION The invention relates to a transmission method to be employed in a radio system comprising at least one transmitter and one receiver which em- ploy TDD duplexing when setting up a connection with each other, and in which signals propagated on different signal paths are received by means of antennas. DESCRIPTION OF THE PRIOR ART In a typical mobile telephone environment, signals between a base station and a mobile station propagate on several paths between a transmitter and a receiver. This multipath propagation is mainly caused by the signal being reflected from surrounding surfaces. The signals propagated on different paths are received by the receiver at different times on account of different transit delays. In signal reception multipath propagation can be utilized in the same way as diversity. In the receiver solution, a multi-branch receiver structure is commonly employed in which each branch is synchronized to a signal component propagated on a different path. The signals of different receivers are combined in the receiver, preferably coherently or incoherently. A high quality signal can be generated by the combining manners. A commonly employed method in radio systems is Frequency Division Duplex (FDD) in which a signal is transmitted and received in different frequency bands. If the FDD method is employed in a radio system in conjunction with distributed antennas, downlink channel estimation is particularly problematic in advance. Since the downlink channel is unknown in the trans- mitter of a FDD base station, it is impossible for the base station to transmit a downlink signal as optimally as to receive an uplink signal. Of course, the problem becomes even more difficult when downlink traffic is heavier than uplink traffic. The aforementioned situation occurs when browsing through WWW (World Wide Web) pages, for instance. BRIEF DESCRIPTION OF THE INVENTION It is therefore an object of the present invention to provide a transmission method in which a signal is transmitted to a receiver on the basis of signals received by transmitters. This is achieved with a transmission method of the type described in the introduction, characterized in that signals from several polarization directions are received by the transmitter, the signals received by the transmitter are measured, the received signals are combined into one signal on the basis of the measurement, the signal is transmitted to the receiver in several polari- zation directions by means of the measurements performed on the received signals. The invention further relates to a radio system comprising at least one transmitter and one receiver which employ TDD duplexing when setting up a connection, said transmitter receiving signals propagated on different signal paths by means of antennas. The radio system of the invention is characterized in that it comprises transmission means which receive a signal from several polarization directions, measuring means which measure the signals received by the transmitter, which are combined into one signal on the basis of the measure- ment, and in which the transmission means polarize the signal and transmit the polarized signal to the receiver in several polarization directions on the basis of the measurement performed by the measuring means. Several advantages are achieved by the invention. The invention is particularly suited to radio systems employing TDD duplexing. The invention enables downlink and/or uplink channels to be estimated in advance. The method of the invention is particularly well suited to systems in which the data transmission need in the uplink and downlink directions is different. In the radio system of the invention, multipath components and antennas are combined in the transmitter. Furthermore, linear intersymbol interference is elimi- nated in the transmitter. Intersymbol interference refers to interference caused by crosstalk between successive symbols propagating in the multipath channel. The above means that the invention enables tasks demanding much capacity and processing to be transferred from the receiver to the transmitter. The invention further enables part of the tasks of the transmitter to be trans- ferred to the receiver. The invention further enables transmission to occur in different polarization directions which are, if necessary, differently weighted. In transmission, the polarization directions are weighted on the basis of the signals received from the above polarization directions. The signal is polarized in the transmitter and the receiver. Polarization enables extremely different channels to be generated. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in closer detail in the following with reference to the examples in accordance with the accompanying drawings, in which Figure 1 shows the radio system of the invention, Figure 2 shows a schematic block diagram of the transceiver employed in the radio system of the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 displays a radio system comprising a base station 200 and a subscriber terminal 100 which in practise is a mobile telephone, for example. The subscriber terminal 100 and the base station 200 operate as transceivers. The subscriber terminal 100 comprises an antenna 101 , and the base station 200 comprises a number of antennas 201. The subscriber terminal 100 can also comprise several antennas 101. In practise, the antennas 101, 201 oper- ate as transceiver antennas. Figure 2 displays the schematic structure of the transceiver employed in the radio system of the invention. The transceiver comprises an antenna 201 which in practise operates as a transceiver antenna. The transceiver further comprises radio frequency components 112, 124, a modulator 123, a demodulator 113 and a control block 102. The control block 102 typically controls other transceiver blocks. The radio frequency components 112 transmit the radio frequency signal supplied from the antenna 201 to an intermediate frequency. The radio frequency components 112 are connected to the demodulator 113 which restores a broadband signal to a narrowband data signal, if a CDMA signal is involved. The invention is not, however, restricted to the CDMA system, but the system can be TDMA, for instance, or a combination of said systems. The transceiver further comprises a coder 122 and a decoder 114. The data signal is supplied from the demodulator 112 to the decoder 114 which decodes the data signal in a suitable manner. The signal supplied to the decoder 114 is convolution coded, for example. The operation of the decoder 114 can be based on the Viterbi algorithm, for example. The decoder 114 typically decodes the encryption and interleaving of the signal. The coder 122 receives a signal and transmits the encoded signal to the modulator 123. In the encoding, the coder 122 employs convolution coding, for instance. The coder 122 further encrypts the signal, for example. The coder 122 further interleaves the bits or bit groups of the signal. Subsequently, the convolution coded signal is supplied to the modulator 123 which in practise operates as a symbol modulator. When the transceiver is of CDMA type, the signal received from the modulator 123 is pseudonoise encoded to be a broadband spread spectrum signal. Subsequently, the spread spectrum signal is converted into a radio frequency signal in a known manner in the radio frequency components 124. The radio frequency components 124 transmit the signal to a radio path via the antenna 201. The transceiver further comprises measuring means 210 and transmission means 220. The measuring means 210 measure the signals received by the antenna 201. The transmission means 220 and the measuring means 210 are operatively connected with each other. The transmission means 220 transmit signals to the radio path on the basis of the measurement data obtained from the measuring means 210. The measuring means 210 can be located in the radio frequency components 112, for example. The transmission means 220 are in practise located in the radio frequency components 124, for example. While setting up a connection, a subscriber terminal 100 transmits the signal to a base station 200 which routes forward the received signal to a PSTN network, for example. The radio system employs the TDD (Time Division Duplex) method. In the TDD method, the uplink and downlink directions operate at the same frequency, time-divisionally separated. The advantages of the TDD method employed in the described radio system can be seen particularly in the downlink data transfer. The radio system further comprises measuring means 210 and transmission means 220. In the solution of the figure the measuring means 210 and the transmission means 220 are operatively connected with the base station. When the subscriber terminal 100 transmits a signal to the base station 200, the signals propagate on different paths to the base station. The transmitted signal is reflected from possible obstacles on the radio path, whereby the base station 200 receives multipath components. Time delay and amplitude and phase changes occur between the multipath components. The amplitude changes are caused by multipath propagation which is largely dependent on the frequency employed. For example, the fadings of adjoining channels can be very different. The base station receives the multipath components preferably by means of the distributed antennas 201. From the antennas the signals are supplied to the measuring means 210 which weight the signals, if necessary. The different antennas 201 receive signals whose characteristics deviate from each other. Consequently, it is possible to weight the signals in a different manner. A strong signal is usually more weighted than a weak signal. After the weighting, the measuring means 210 combine the weighted signals. In the solution of the figure the measuring means 210 estimate the uplink channel. The measuring means 210 measure, for instance, delay, amplitude and phase from the signals received by the subscriber terminal 100 and the base station 200. The signals are received by combining multipath components received by several antenna branches or antennas. Measurement results obtained from the measuring means 210 are utilized in transmitting a signal to the subscriber terminal 100. It is possible to employ the measurement results since the uplink and downlink directions use the same frequency. The transmission means 220 transmit signals in such a manner as to enable the subscriber terminal 100 to receive preferably only one strong signal component by its antenna 101. It is possible to simplify the structure of the receiver 100 on account of the above mentioned. The invention enables the antenna beam formed by the distributed antennas together to be directed towards the subscriber terminal. It is also possible to transfer part of the structure of the base station to the subscriber terminal, whereby the structure of the base station becomes less complex. Part of the operations of the base station 200 can be transferred to the subscriber terminal 100, particularly when the base station 200 employs an elaborate interference elimination. The distributed antennas 201 direct their beams towards the subscriber terminal 100 on the basis of channel measurement, enabling the base station to receive an optimal signal. The subscriber terminal 100 can comprise several antennas 101. The subscriber terminal 100 can also comprise transmission means 220 and measuring means 210 which measure the downlink channel. In the above situation the subscriber terminal 100 transmits a signal to the base station 200 on the basis of the downlink channel estimate. The transmission means 220 regulate their transmission power on the basis of the measurement performed by the measuring means 210. The transmission power regulation results in a decreased need for power regulation signalling and fewer signalling errors. If the channel between the sub- scriber terminal 100 and the base station 200 is weak on account of fadings, the transmission power is not necessarily raised. In the above situation the transmission of the signal, a frame for example, is inhibited. Transmission power raised too high can cause interference in the radio system. The trans- mission means 220 also eliminate linear intersymbol interference from the signal to be transmitted on the basis of the signals received by the transmitter 200. The receiver 100 is able to receive a higher-quality signal owing to the interference elimination. The same frequency employed in the uplink and downlink channels allows the structure of receiver to be less complex. The transmitter can be of the Pre-RAKE type. In a Pre-RAKE transmitter, it is possible to combine the multipath components and antenna signals on the basis of the channel estimated by the receiver. Furthermore, the structure of the receiver is simplified in such a manner that elimination of the linear intersymbol interference is per- formed in the transmitter. The subscriber terminal 100 and the base station 200 preferably employ polarization transmission and polarization reception. The polarization is implemented by horizontal and vertical polarization. The use of different polarization directions allows different channels to be implemented at least in- doors. Polarization branches are made use of in the polarization transmission and reception. The measuring means 210 weight the signals supplied through the polarization branches in such a manner as to achieve the best possible signal/noise ratio. The different polarization directions can fade almost independently of each other in the channel between the base station and the sub- scriber terminal. The faded polarization components can be combined in the reception in the same manner as the multipath propagation signals. The measuring means 210 estimate the multipath components and signals supplied through the polarization branches. Subsequently, the transmission means 220 transmit their signal on the basis of the estimate of the measuring means 210. In practise the signal can be transmitted and received in two different polarization directions, for instance. In the system of the invention the transmitter antennas transmit the same data from each antenna. It is therefore possible to improve the downlink performance by the method of the invention. The method is extremely well suited to be employed in systems in which the last junction line of a fixed telephone network to a subscriber is replaced by a radio network, in other words in so-called WLL (Wireless Local Loop) applications. The subscriber terminal 100 is stationary or moves slowly in the WLL applications. The WLL technique allows the use of the SDMA (Space Division Multiple Access) technique. In the SDMA technique the users are distinguished by means of location. This is performed by regu- lating the receiver antenna beams in the base station in the desired directions in accordance with the mobile station locations. For this purpose, adaptive antenna arrays, in other words phased antennas, are used in the radio system. Furthermore, the received signal is processed in such a manner as to enable the mobile stations to be monitored. Even though the invention has been described in the above with reference to the examples in accordance with the accompanying drawings, it is to be understood that the invention is not restricted to them, but can be modified in many ways within the scope of the inventive idea disclosed in the appended claims.

Claims: CLAIMS 1. A transmission method to be employed in a radio system comprising at least one transmitter (200) and one receiver (100) which employ the TDD duplexing when setting up a connection with each other, and in which signals propagated on different signal paths are received by means of antennas, characterized in that signals from several polarization directions are received by the transmitter (200), the signals received by the transmitter (200) are measured, the received signals are combined to form one signal on the basis of the measurement, the signal is transmitted to the receiver (100) in several polarization directions by means of the measurements performed on the received signals. 2. A method as claimed in claim 1, characterized in that the signals are transmitted by distributed antennas (201) which enable the capacity of the signal received by the receiver (100) to be increased. 3. A method as claimed in claim ^characterized in that the method employs distributed antennas (201) whose beams are directed towards the receiver on the basis of the measurement. 4. A method as claimed in claim 1, characterized in that the connection between the transmitter (200) and the receiver (100) is estimated on the basis of the measurement, the estimation providing an estimate result which is utilized in transmitting the signal to the receiver (100). 5. A method as claimed in claim 1, characterized in that the transmission power of the transmitter (200) is regulated on the basis the received signal, said regulation enabling power regulation signalling to be decreased. 6. A method as claimed in claim ^characterized in that linear interference is eliminated from signals received from different signal paths in order to improve the quality of the signal received by the receiver (100). 7. A method as climed in claim ^ characterized in that the transmitter (100), on the basis of the signals it has received, transmits a signal which, when propagating on one signal path to the receiver, is coherently combined in the receiver. 8. A method as claimed in claim ^characterized in that the transmitter (200) transmits signals, on the basis of the signals it has received, in such a manner that the transmitted signals are combined shortly before they arrive at the receiver (100). 9. A method as claimed in claim ^characterized in that the method is employed particularly when the receiver (100) is stationary or the receiver (100) is moving slowly. 10. A method as claimed in claim 1, characterized in that the method is employed in cellular radio systems implemented by the SDMA technique. 11. A method as claimed in claim ^ characterized in that a signal is transmitted in at least two polarization directions by weighting the signal before the transmission on the basis of said measurement. 12. A method as claimed in claim 1, characterized in that when the received signal comprises polarization components, each polariza- tion component is weighted, whereupon the weighted polarization components are combined. 13. A radio system comprising at least one transmitter (200) and one receiver (100) which employ TDD duplexing when setting up a connection, said transmitter (200) receiving signals propagated on different signal paths by means of antennas (201), characterized in that the radio system comprises transmission means (220) which receive a signal from several polarization directions, measuring means (210) which measure the signals received by the transmitter (200), said signals being combined into one signal on the basis of the measurement, and in which the transmission means (220) polarize the signal and transmit the polarized signal to the receiver (100) in several polarization directions on the basis of the measurement performed by the measuring means (210). 14. A radio system as claimed in claim 13, characterized in that the transmission means (220) transmit the signals by means of the distributed antennas (201) whose beams are directed towards the receiver (100) on the basis of the signals measured by the measuring means (210). 15. A radio system as claimed in claim 13, characterized in that the measuring means (210) estimate the connection in the direction of the receiver (100) and the transmitter (200). 16. A radio system as claimed in claim 13, characterized in that the transmission means (220) regulate their transmission power on the basis of the signal received by the transmitter (200), the estimation enabling power regulation signalling to be decreased. 17. A radio system as claimed in claim 13, characterized in that the transmission means eliminate linear intersymbol interference from a signal to be transmitted on the basis of the signals received by the transmitter (200), which enables the receiver (100) to receive an optimal signal. 18. A radio system as claimed in claim 13, characterized in that the transmission means (220) transmit the signals on the basis of the signals received by the transmitter (200) in such a manner that the transmitted signals are combined shortly before they arrive at the receiver. 19. A radio system as claimed in claim 13, characterized in that the transmission means (220) transmit the signals in such a manner as to enable the receiver (100) to receive the transmitted signals coherently. 20. A radio system as claimed in claim 13, characterized in that the transmitter (200) is a base station, and the receiver (100) is a subscriber terminal. 21. A radio system as claimed in claim 13, characterized in that the transmitter (200) is a subsriber terminal, and the receiver (100) is a base station. 22. A radio system as claimed in claim 13, characterized in that the radio system is a cellular radio system implemented by the SDMA technique. 23. A radio system as claimed in claim 13, characterized in that the transmission means (220) polarize the signal and transmit the polarized signal in at least two polarization directions. 24. A radio system as claimed in claim 13, characterized in that the transmission means (220) polarize the signal by means of polarization branches and weight the polarization branches on the basis of the signals measured by the measuring means (210). 25. A radio system as claimed in claim 13, characterized in that when the measuring means (210) measure the signal comprising polarization components, the measuring means (210) weight each polarization component, whereupon the measuring means (210) combine the weighted polarization components.

Inventors: HAEKKINEN HANNU, HOLMA HARRI, HAEKKINEN, HANNU, HOLMA, HARRI
Applicants: HAEKKINEN HANNU, HOLMA HARRI, NOKIA TELECOMMUNICATIONS OY, HAEKKINEN, HANNU, HOLMA, HARRI
Application year: 1998
Main classification: H04B7
IPC codes: H04B7