The present invention relates to mobile radio systems. More specifically the invention relates to a digital, cellular, mobile radio system. The invention also relates to a method of transmitting information digitally to and from mobile stations in a cellular mobile radio system.
The mobile radio systems that were first taken into common use were of analog type, i.e. message information was transmitted in analog form to and from mobile stations by transmitting and receiving analog-modulated radio signals. In such systems it is known to have two or more base station transmitters at a distance from each other simultaneously transmitting radio signals within the same frequency range and modulated with the same message information to the mobile stations. Such mobile radio systems are described in EP 0040731 and EP 0072479, as well as in the two publications: NTG-Fachberichte, Bewegliche Funkdienste, Vortrxc3xa4ge der NTG-Fachtagung vom 25. bis Nov. 27, 1985 in Munich, xe2x80x9cGLEICHKANALFUNKSYSTEME Fxc3xcR DIE FREQUENZxc3x96KONISCHE VERSORGUNG GROSSER GEBIETExe2x80x9d Berndt Heynisch pp 41-46, VDE-VERLAG GmbH, Berlin, Elektrizitxc3xa4tswirtschaft, Jg. 80(1981), Heft 6, pp 187-198 xe2x80x9cQuasissynchroner Gleichwellenfunk-ein Gleichkanalfunk-Ver-fahren zur Erhxc3x6hung der Erreichbarkeit in Mobilfunknetzenxe2x80x9d.
In known systems of the kind in question here, it is known to transmit message information from a central station or exchange to the base station transmitters via either cables or radio signals. It is also known to have equalizers in the fixed part of the mobile radio system for equalizing differences in propagation time and attenuation in transmission from the exchange to the base station transmitters. The equalizers can be at the exchange and/or at the base station transmitters. The object of the equalisers is that irrespective of position in relation to the exchange the base station transmitters shall transmit the radio signals simultaneously, and modulated with the same message information.
Digital mobile radio systems in which message information is transmitted digitally to and from mobile stations by transmission and reception of digitally modulated signals have been proposed in U.S. Pat. No. 4,675,863 and xe2x80x9cDigital Mobile Telephone System Using TD/FDMA Schemexe2x80x9d, Kota Kinoshita, Masaharu Hata and Kenkichi Hirade, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. VT-31, NO. 4, NOVEMBER 1982, pp 153-157.
It has been proposed to have adaptive equalizers in mobile radio stations in digital radio systems, whereby multi-path propagation of radio signals can be used to improve signal quality, instead of the multipath propagation acting as noise. Among the publications on adaptive equalizers in digital mobile radio systems can be mentioned: xe2x80x9cMulti-path Equalization for Digital Cellular Radio Operation at 300 k. bits/sxe2x80x9d. K Raith, J-E Sjternvall and J Uddenfeldt, 36th IEEE Vehicular Technology Conference, pp 268-272, Dallas, Tex., U.S.A. May 1986. xe2x80x9cRadio Test Performance of a Narrowband TDMA Systemxe2x80x9d, J-E Stjernvall, B. Hedberg, and S Ekmark, IEEE Vehicular Conference. Tampa, Fla., U.S.A., June 1987, RADIO TEST PERFORMANCE OF A NARROWBAND TDMA SYSTEM-DMS 90, J-E Stjervall, B. Hedberg, K Raith, T Backstrom and R Lofdahl.
In mobile radio systems there are problems due to reflections and radio shadows from natural obstacles such as rocks and hills, as well as structures such as buildings. These problems are especially troublesome in transmitting information requiring great accessibility/reliability and high transmission speed. In particular the problems may become large in certain urban environments where the propagation conditions for radio signals can vary heavily within a small geographic area, while radio traffic is intensive at the same time. Up to now attempts have been made to solve these problems by having adaptive equalizers in the mobile stations and small cells with specially selected positioning of the base station transmitters. In areas with much traffic it is, however, a desire to be able to select the size of the cells and their positions in the mobile radio system cell plan in an optimum way with respect to the traffic handling capacity of the system. Reducing the cell size and selecting the positions of the small cells to avoid radio shadows thus involves a complication. Another complication resulting from the reduction of cell size to below what is necessary for reasons of capacity is that the number of handovers increases.
The object of the present invention is to solve the above-mentioned problems and complications, and to provide a method and a cellular digital mobile radio system which are also suitable for transmitting information requiring great accessibility/reliability and high transmission speed.
What is distinguishing for a method and a digital cellular mobile radio system in accordance with the present invention, and particularly preferred embodiments thereof is disclosed in the independent and dependent claims. Somewhat simplified, it may be said that according to the present invention at least two base station transmitters are utilized for each of a plurality of cells, these transmitters being at a distance from each other and at least partially simultaneously transmitting radio signals within the same frequency range digitally modulated with the same message information to the mobile stations in the cell. The digital modulation is changed with a modulation time interval which is adapted to the greatest transmitting distance between two base station transmitters serving the same cell in an area. The mobile stations have adaptive equalizers for reconstructing the digital modulation in the transmitted signals from the signals received during a reception time interval, which is also adapted to the greatest transmitting distance between two base station transmitters serving the same cell in an area.
In a preferred embodiment of a method in accordance with the present invention, the digitally modulated signals are transmitted with the same message information to a given mobile station with a given, mutual transmission time shift from the different base stations. The transmission shift is then selected such that it counteracts the difference in arrival time for the signals from the different base station transmitters to this mobile station.
In each mobile station, there is preferably estimated the arrival time shift between the digitally modulated radio signals with the same message information from the different base station transmitters. Information about the estimated arrival time shift at the respective mobile station is transmitted from there to at least one base station transmitter. This estimated arrival time shift is utilized at the base station for selecting the transmission time shift for at least one base station transmitter. A transmission time shift is thus obtained individually for each affected mobile station, and which is adjusted for this particular mobile stations""s position in relation to the base station transmitters. The amount of transmissions time shift can thus vary from mobile station to mobile station.
According to a somewhat different, preferred embodiment, there is estimated in different base station receivers the reception time shift between corresponding radio signals with the same message information from the mobile station. This estimated reception time shift is utilized for selecting the transmission time shift between corresponding base station transmitters. A transmission time shift can thus be individually obtained for each affected mobile station such that it is adjusted to the position of this particular mobile station in relation to the base station transmitters. The amount of transmission time shift can thus vary from mobile station to mobile station.
A preferred embodiment of a mobile, radio system in accordance with the present invention has a time measurement unit for estimating the mutual reception time shift in a mobile station between, on one hand, digitally modulated radio signals transmitted to the mobile station via a base station transmitter for the cell where this mobile station is, and, on the other hand, corresponding digitally modulated signals transmitted to this station via another base station transmitter for the cell. In this embodiment the mobile radio system has a transmission time shifting unit for mutually time shifting the transmission times for base station transmitters of the same cell so that they transmit corresponding digitally modulated signals to the mobile station with greater or less mutual transmission time shifting in response to estimated reception time shift.
The time measurement unit preferably includes an arrival time comparison unit in at least certain mobile stations, for comparing the arrival times for corresponding digitally modulated signals transmitted from different base station transmitters for the same cell.
According to a somewhat different, preferred embodiment, the time measurement unit includes an arrival time comparison unit in the stationary part of the mobile radio system, for comparing the arrival times of digitally modulated signals transmitted from a mobile station in a cell and received at different base station transceivers of the cell.
A method and a cellular digital mobile radio system in accordance with the invention provide substantial advantages. The coverage degree can be made greater, signifying better opportunities for establishing new connections and maintaining those already established. The coverage degree can be made greater without the cell size needing to be reduced, which give greater freedom in the selection of cell plan and a lesser number of handovers. In preferred embodiments, the transmission time shift, which is individually selectable for each mobile station, reduces the risk of noise and interruption of calls in progress, when a mobile station goes from receiving signals chiefly from one base station transmitter to receiving signals chiefly from another base station transmitter. When a mobile station simultaneously receives signals from at least two base station transmitters of the same cell, the variable transmission time shift enables the received signals together to be more like the signals obtained from a single base station transmitter in conjunction with reflections.