Cellular digital mobile radio system with plural base station transmitters, and method of transmitting information in such a system

Cellular digital mobile radio system and method for transmitting information in a digital cellular mobile radio system

Technical area

The present invention relates to mobile radio systems. More particularly, the invention relates to a digital cellular mobile radio system. The invention also relates to a method for transmitting message information digitally to and from mobile stations in a cellular mobile radio system.

The state of the art

The mobile radio systems that were first introduced into general use were of an analog type, ie message information was transmitted analogously to and from mobile statons by transmitting and receiving analog modulated radio signals. In analog mobile radio systems, it is known to have two or more base station transmitters spaced from each other simultaneously transmitting radio signals within the same frequency range modulated with the same message information to mobile stations. Such mobile radio systems are described in EP 0040731 and EP 0072479 and the following two publications: NTGFachberichte, Bewegliche Funkdienste, Vorträge der NTGFachtagung vom 25, until 27 November in Munich,

FUNCTIONAL FUNCTION SYSTEMS FOR THE FREQUENCY ECONOMIC

VERSORBUNG GROSSER BEBIETE, Berndt Heynisch, pages 41-46, VDE-VERLAG GmbH, Berlin, Electricity Management, Jg. 80 (1981), Heft 6, pp 187-198, Quasisynchroner Gleichwellenfunktein Gleichkanalfunk-Verfahren zur

Increase accessibility in modifunction networks.

In known mobile radio systems of the current kind, it is known to transmit message information from a central to the base station transmitters either via wires or radio signals. In this connection, it is known to have equalizers in the fixed part of the mobile radio system to equalize differences in duration and attenuation during transmission from the central station to the base station transmitters. The equalizers may be provided at the central station and / or at the base station transmitters. The purpose of the equalizers is that the base station transmitters, irrespective of the position relative to the control panel, 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 through the transmission and reception of digitally modulated radio signals have been proposed in US 4675863 and the Digital Mobile Telephone System Using TD / FDMA Scheme, Kota Kinoshita and Masaharu Hata and Kenkichi Hirade, IEEE TRANSACULS ON TECHNOLOGY , VOL. VT-31, nro 4, November 1982, pp 153-157.

It has been proposed to have adaptive equalizers in mobile stations in digital mobile radio systems, whereby multipath propagation of radio signals can be utilized to improve signal quality rather than multipath propagation acting as a disturbance. Publications on adaptive equalizers in digital mobile radio systems include: Multi-path equalization for digital cellular radio operation at 300 kbits / s. K. Raith, JE. Stjernvall and J. Uddenfield, 36th IEEE vehicular technology cenference, pp

268-272, Dallas, Texas, USA, May 1986. Radio Test Performance of a Narrowband TDMA System, JE. Stjernvall, B. Hedberg and S. Ekmark, IEEE Vehicular Conference, Tampa, Florida, USA, June 1987, RADIO TEST PERFORMANCE OF A NARROWBAND TDMA SYSTEM-DMS 90, JE. Stjernvall, B. Hed30 berg, K. Raith, T. Bäckström and R. Lofdahl.

Disclosure of the Invention

In mobile radio systems, reflections and radio shadows from natural obstacles such as mountains and constructed obstacles such as buildings pose problems, in particular for the transmission of information requiring high availability / reliability and high transmission speed. The problems can be particularly severe in certain urban environments where the distribution conditions for radio signals can vary very greatly within a small geographical area, at the same time as radio traffic is intense. So far, these problems have been attempted by having adaptive equalizers in the mobile stations and having small cells with specially selected location of the base station transmitters. However, in high-traffic areas, it is desirable to be able to select the size and location of the cells in the cellular plan of the cellular radio system in a manner that is optimal with respect to the mobile radio system's traffic harvesting capacity. Reducing the cell size and choosing the location of the small cells to avoid radial shadows means a complication. Another complication of reducing the cell size more than is justified for capacity reasons is that the number of handovers is increasing.

The invention aims to solve the aforementioned problems and complications and to provide a method and a cellular digital mobile radio system which is also suitable for transmitting information requiring high availability / reliability and high transmission speed.

What is characteristic of a method and a digital cellular cellular radio system according to the invention and particularly preferred embodiments thereof are made by the independent and respective autonomous resp. independent claims. Simply put, it can be said that according to the invention at least two base station transmitters are used for each of a number of cells, which base station transmitters spaced from each other at least partially simultaneously transmit radio signals within the same frequency range digitally modulated with the same message information to mobile stations in the cell. The digital modulation changes at modulation time intervals adapted to the largest transmitter distance between two base station transmitters serving the same cell in one area. The mobile stations have adaptive equalizers for reconstructing the digital modulation of the transmitted radio signals from radio signals received during a reception time interval, which is also adapted to the largest transmitter distance between two base station transmitters serving the same cell in one area.

In a preferred embodiment of a method according to the invention, the digitally modulated radio signals with coherent message information are transmitted to a certain mobile station with some mutual transmission time offset from the various base stations. In this way, the transmission time offset is selected to counteract the differences in arrival time of the radio signals from the different base station transmitters to this mobile station.

Preferably, at each mobile station, the arrival time offset between the digitally modulated radio signals with aggregated message information from the various base station transmitters is estimated. Information on the estimated arrival time offset at each mobile station is transmitted from the mobile station to at least one base station. At the base station, the estimated arrival time offset is used by transmit time offset for at least one base station transmitter. Hereby, individually for each affected mobile station, a transmission time offset which is adapted precisely to the location of this mobile station is obtained relative to the base station transmitters. The transmission time shift can thereby vary from mobile station to mobile station.

In a somewhat different preferred embodiment, at different base station receivers, the reception time 30 is estimated to be the shift between corresponding radio signals with coherent message information from the mobile station. The estimated receive time offset is utilized to determine the transmission time offset between the corresponding base station transmitters. As a result, a transmission time offset that is adapted to the position of this mobile station in relation to the base station transmitters can also be obtained individually for each mobile station concerned. The transmission time shift can thereby vary from mobile station to mobile station.

A preferred embodiment of a mobile radio system according to the invention has timing means for estimating mutual reception time offset at a mobile station between, on the one hand, digitally modulated radio signals transmitted to the mobile station via a base station transmitter for the cell where the mobile station is located and on the other corresponding digitally transmitted radio signals to the mobile station via another base station transmitter for the cell. In this embodiment, the mobile radio system has transmit timing offsets to mutually time-shift the transmit times for the same cell's base station transmitters so that they transmit corresponding digitally modulated radio signals to the mobile station with greater or less mutual transmission time offset, depending on the estimated reception time offset.

Preferably, the timing means include arrival 20 time comparison means in at least some mobile stations for comparing the arrival times of corresponding digitally modulated radio signals transmitted from different base station transmitters for the same cell.

In a somewhat different preferred embodiment, the timing means comprise arrival time comparison means in the stationary portion of the mobile radio system for comparing the arrival times of digitally modulated radio signals transmitted from a mobile station in a cell and received at different base station transmitters of the cell.

A method and a digital cellular cellular radio system according to the invention offer significant advantages. The coverage ratio can be made higher, which means better opportunities to establish new connections and to maintain already established connections. The degree of coverage can be increased without the need for cells to be reduced, which gives • »· •« greater freedom when using cell planes and a smaller number of handovers. The individually selectable transmit time offset in preferred embodiments reduces the risk of interruptions in interrupting connections when a mobile station transfers from receiving radio signals mainly from one base station transmitter to receiving radio signals mainly from another base station transmitter. When a mobile station simultaneously receives radio signals from at least two base station transmitters for the same cell, the variable transmission time offset means that the received radio signals together more closely resemble the radio signals received from a single base station transmitter in connection with reflexes.

Figure Description

Figure 1 illustrates cell division and placement of transmitters in base stations in one embodiment of a mobile radio system according to the invention.

Figure 2 illustrates parts of a mobile radio system according to the invention.

Embodiments

A cellular mobile radio system according to the invention has mobile stations and base stations with transmitters and receivers for radio signals. Message information is transmitted digitally to and from the mobile stations by transmitting and receiving radio signals with digital modulation corresponding to the message information. The radio signals are transmitted on someone by a radio channel. On the same radio channel, radio signals can be transmitted to and from multiple mobile stations in time multiplexes.

The mobile stations are mobile within and between the cells of the mobile radio system. The transmitters of the base stations are assigned to the cells so that for each cell there is at least one base station transmitter for transmitting radio signals to the cell's mobile stations.

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Figure 1 illustrates somewhat simplified cell division and assignment of base station transmitters to the cells of a mobile telephone system according to the invention. For the sake of simplicity, in Figure 1, all cells C1 through C24 have been illustrated as equal Large regular hexagons with side L. In practice, cells will often be of somewhat different size and shape. In addition, the dependence on traffic conditions will often be appropriate with some overlap in the boundary areas between the cells. The base station transmitters can to some extent relieve each other by taking care of transmission to mobile stations which, from a purely geographical point of view, should be performed by neighboring cell base station transmitters.

For each cell C1 through C24 there is an ordinary base station transmitter BS1 through BS24. The usual base station transmitters for neighboring cells are in a known manner co-located in groups with three base station transmitters in each group. For example, the base station transmitter BS1 for the cell C1 is co-located with the base station transmitter BS3 for the cell C3 and with the base station transmitter BS5 for the cell C5. Correspondingly, the base station transmitter BS14 for cell C14 is co-located with the base station transmitter BS16 for cell C16 and with the base station transmitter BS18 for cell C18. The co-located regular base station transmitters are located in the boundary areas between the cells to which they belong. For example, the regular base station transmitters BS2, BS4 and BS6 are co-located at the boundary region between cells C2, C4 and C6.

In addition to the regular base station transmitters BS1 even

BS24, the mobile radio system includes a number of additional base station transmitters for some of the cells. For cells C6, C7, C10, C11, C13, X14, C19 and C20, each has its own base station transmitter. For cells C15, C18 and C22, each has two additional base station transmitters. In the additional base station transmitters are XS6, XS7, XS10, XS11, XS13, XS15A, XS15B,

XS18A, XS19, XS20 and XS22A co-located in groups with three additional base station transmitters in each group in a similar manner to the regular base station transmitters. Thus, for example, the auxiliary base station transmitter is XS15B for the cell

C15 co-located with the auxiliary base station transmitter XS19 for cell C19 and the auxiliary base station transmitter XS18A for cell C18. In contrast, neither the auxiliary base station transmitter XS14 for the cell C14 nor the auxiliary base station transmitter XS22B for the cell C22 are co-located with any other base station transmitter without located downstream of the cell to which it belongs.

An additional base station transmitter need not differ from an ordinary base station transmitter in technical terms. An additional base station transmitter for a particular cell can thus have the same equipment of the same type as an ordinary base station transmitter for the same cell. It can also work in basically the same way as the regular one. If there are two identical base station transmitters for a particular cell, then, in some cases, which heist of them can be regarded as ordinary extra 20.

The auxiliary base station transmitter (s) of a particular cell transmits substantially the same radio signals as the regular base station of the cell. The radio signals are digitally modulated with digital message information to the mobile stations in the cell. Therefore, a mobile station in a cell for which there is one or more auxiliary base station transmitters may, at least in the displayed case, receive corresponding radio signals from more than one base station transmitter approximately simultaneously in the same frequency range. Depending on the positions of the mobile station and the base station in the cell and the radio signals broadcast times and propagation paths from the base stations to the mobile stations, corresponding radio signals from different base stations can be received without or with some time offset at the mobile station. The greater the distance between the base station transmitters of the cell, the greater the time delay can generally be. Where the regular base station transmitters and the extra base station transmitters are located according to Figure 1, the mutual distance between two base station transmitters for the same cell varies between L and 2L, ie between the side of the regular hexagons and the diameter of the regular hexagons. If, for simplicity's sake, one ignores reflexes and assumes that the base station transmitters transmit each other time offset, then the time offset at the mobile station could then be maximally equal to 2L / c, where c is the propagation speed of the radio signals.

Figure 2 illustrates parts of a mobile radio system according to the invention. A mobile radio switch MSC is via cables

L ,, L _..., L, L, connected to a number of base stations of 1 '2' m 'n' which are two. B and B are illustrated in Figure 2.

mn ^A

The base station B has a central unit which via cables m

L_ma and L_mg is the antecedent of two transmitter-receiver units

B_ma respectively. B ^ located at a distance from the central unit.

The central unit of the base station B comprises a central m line and control unit 1, a transmission time offset 2A and 2A respectively. 2B for each transmitter-receiver unit, a reception time offset 3A and 2A, respectively. 3B for each transmitter-receiver unit as well as a line unit 4A and 1A, respectively. 4B for each transmitter-receiver unit.

The two transmitter-receiver units of the base station

B_m are equal among themselves. Each such transmitter-receiver unit comprises a line and control unit 5A and 1A, respectively. 5B, a transmitter means 6A and 6A, respectively. 6B, a receiver means 7A and 6A, respectively. 7B, a transmit-receive filter 8A and 8A, respectively. 8B and an antenna 9A and 8A, respectively. 9B.

The base station B ^ differs in part from the base station B_m mainly because its central line and control unit 10 is located adjacent to its one transmitter receiver unit B. Any cable with associated line units corresponding to L, L. , 4A-5B, is therefore not needed to mb one transmitter-receiver unit B_na but only to the second transmitter-receiver unit B Furthermore, no transmission time offsets or reception time offsets in any central unit of the base station B_n without corresponding means 2A, 2B, 3A and 3B are included in the respective transmitter-receiver units B and 2 respectively. B.

^A na nb

The mobile stations MS 2 and MS₂ are equal among themselves. Each mobile station includes audio recording means 11, coding means 12, transmitting means 13, transmitting-receiving switch 10, receiving means 15, equalizer and decoding means 16, sound reproducing means 17, control means 18 and means 19 for input and output or digital presentation.

Provided that the mobile stations have two transmitter-receiver units spaced apart from each other and that the base stations have controllable transmit and receive time offsets, the mobile radio system of Figure 2 functions in most respects in ways well known in mobile radio systems. Therefore, a complete description of how the mobile radio system works in various respects should not be necessary for those skilled in the art but should be required to describe only that which is unique or unusual in the mobile radio system of Figure 2. For those who are not skilled in the field of mobile radio system, reference is made to the literature and the publications mentioned above under the prior art section.

Message information that the mobile radio switch transmits to a mobile station in cell C, e.g. the mobile station MS 2, is transmitted from the mobile radio switch via the cable

L_m to the line and control unit 1. From the line and control unit 1, the message information is transmitted via the transmission time offset 2A, the line unit 4A, the cable L_A and the line and control unit 5A of the transmitter means 6A. Via the transmitter-receiver filter 8A and the antenna 9A, the transmitter means emits • · ·

6A radio signals with digital modulation corresponding to the message information from the mobile radio exchange.

The message information from the line and control unit 1 also via the transmission time offset 2B, the line unit

4B, the cable and line and the controller 5B of the transmitter means 6B of the transmitter-receiver unit B_m^. Via the transmitter-receiver filter 8B and the antenna 9B, the transmitter means 6B transmits radio signals with digital modulation corresponding to the message information from the mobile radio exchange.

Depending on the delay of the message information in the transmission to the transmitter means 6A and the corresponding delay in the transmission to the transmitter means 6B, the radio signals can be transmitted from one of the transmitter-receiver unit B_ma antenna 9A substantially without time offset or time offset in relation to the corresponding radio signal transmission from the other transmitter-receiver unit B. antenna 9B. mb

The radio signals from antenna 9A in B_ma arrives at a particular mobile station in cell C, e.g., the mobile station MS MS, with or without time delay relative to corresponding radio signals from the antenna 9B in B B. The possible time delay on arrival at the mobile station is partly due to any time delay in the transmission from the antennas and partly to any difference in the propagation time of the radio waves from the antennas. The transmission time offsets 2A and 2B have a variable delay and can be controlled by the line and control unit 1 so that the radio signals are transmitted from the antenna 9A in B_ma time offset more or less before or after the corresponding radio signals from the antenna 9B in B_mt>. In the preferred embodiment of Figure 2, the line and control unit 1 control the variable delays in the transmission time offsets 2A and 2B such that the differences in delay in the cables L_maand L as well as the differences in the propagation time of the radio signals are counteracted. It can also be expressed that line and • · • · · · · · · · ·

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the control unit controls the variable delays in the transmission time offset means 2A and 2B so that the time delay of the radio paths on arrival at the mobile station is reduced compared to if the time offset means 2A and 2B had the same fixed delay. It would be thought that the ideal case would be that the line and control unit controlled the delays in the time offset means 2A and 2B such that the digitally modulated radio signals from the antenna in B arrived at the antenna in the MS ^ exactly simultaneously in phase with the corresponding radio signals from the antenna in B ^. In practice, this is neither achieved nor sought. Reflections occur when the radio signals propagate between the antennas and the mobile station have an adaptive equalizer. Therefore, it is not necessary that the radio signals from the different transmit-receive purities arrive exactly at the same time to the mobile station. On the contrary, a small time delay is preferably sought to achieve diversity towards Rayleigh fading. Those who are not skilled in the art and want further information on this can find the seed in the publications mentioned above under the prior art, for example RADIO TEST PERFORMANCE OF A NARROWBAND TDMA SYSTEM-DMS 90.

In principle, there are two conceivable methods for determining how the line and control unit 1 controls the delay in transmission time offsets 2A and 2B. One method is to estimate in the fixed part of the mobile radio system the time offset between the mobile station's radio signals at one of the transmitter-receiver unit B_ma and corresponding radio signals at the second transmitter-receiver unit ^Bm ^ · This gives an estimate of the differences in distribution time to the mobile station, which differences vary with the location of the mobile station. Other differences in delay relate to the fixed part of the mobile radio system, for example differences in the length of the cables L_ma and ^Lmb ^and t&gt;^eras not by the location of the mobile station. In the embodiment of Figure 2, this method can be applied in practice so that the delays in the means 3A and 3B are adjusted so that message B received from the mobile station MS 3 arrives at the line and control unit 1 at the same time as at B_mb received message information from the mobile station MS MS arrives at the line and control unit 1. Thereafter, the delays in the send time offsets 2A and 2B are adjusted in accordance with the optimal delays in the receive time offsets 3A and 3B.

The second method is to estimate in the mobile station the differences in arrival time or time offset between the digitally modulated radio signals from one transmitter-receiver unit B_ma and the corresponding digitally modulated radio signals from the second transmitter-receiver unit B. For this, the required form of encoding of the radio signals indicating from which transmitter unit they are transmitted is required. It is known to send out special synchronization words in TDMA systems. These can be utilized if they are designed or complemented so that two base station transmitters for the same cell do not have identical synchronous words only. Alternatively, special synchronization words can be sent from the base station transmitters only for the mobile station to be able to estimate the differences in time of arrival or time delays. The mobile station transmits information about the estimated arrival time difference or time offset via radio signals to the fixed part of the mobile radio system, where it is used to control the transmission time offsets 2A and 2B. The line and control unit 1 receives information about the estimated arrival time difference from the mobile station via the line units 5A and 4A and 5B and 4B, respectively, in the same way as the line and control unit receives message information from the mobile station.

In itself, it is conceivable but hardly preferable to combine the two methods for controlling the transmission time shift in a mobile radio system according to Figure 2.

Measurement of difference in arrival time or time offset for corresponding radio signals can be done in known manner, for example by means of correlation. Where the radio signals in a manner known per se comprise predetermined synchronization patterns (words), the time difference between these in different signals can be measured by methods known per se. A mobile station controller 18 and / or a base station line and controller 1.9 optionally in combination with the transmitter-receiver units line units 5A, 5B may include timing means for estimating reception time offset or arrival time comparison means for comparing arrival times.

Preferably, in a known manner, a base station utilizes the same transmitter means and the same antenna to transmit radio signals digitally modulated with message information to different mobile stations belonging to the same cell within the same frequency range in time multiplex. Radio signals with message information for a particular mobile station are then transmitted from different base station transmitters with a possible transmission time delay specially adapted to the particular location of this mobile station. It may occur that a base station in a mobile radio system needs to transmit a radio diode signal with information intended to be received by several or all mobile stations in the cell, e.g., information about the base station / cell identity. Such radio signals are preferably transmitted simultaneously without any mutual time delay from the base stations transmitter-receiver units B_ma and

B_mb respectively. in a mobile radio system according to Figure 2.

The transmission time offsets are then controlled to a balancing state where the message information delay from the line and control unit 1 to the antenna of one transmitter-receiver unit is a B_A is as large as the message information delay from line and control unit 1 to the antenna of the second transmitter-receiver unit B_mb· The same may apply when a base station listens to available combinations of time slot and frequency range after calls from mobile stations with unknown location relative to the base stations transmitter-receiver units. the reception time offset means 3A and 3B can then be controlled to a balancing state where the delay of the message information from the antenna in one transmitter-receiver unit B_na to the line and control unit 9 is as large as the delay of the message information from the antenna of the second transmitter-receiver unit B ^ to the line and control unit 9.

The mobile stations MS. And MS2 have adaptive equalizers whereby the digital modulation during a modulation time interval of the transmitted radio signals from a base station transmitter can be reconstructed from radio signals received during a reception time interval. In known cellular digital mobile radio systems with only one base station transmitter per cell, the reception time intervals of the equalizers are dimensioned after the dispersion on the radio channel, ie. expected time offsets between corresponding radio signals from a single base station transmitter due to reflexes. Thanks to the equalizer, not only can the radio signal having the greatest amplitude or arriving first to the mobile station be utilized for the reconstruction of the digital modulation, but also other radio signals arriving with a time offset within the equalization time interval can be utilized; The devices of a mobile radio system according to the invention have equalizers which are dimensioned to be mobile stationed against. . ·. 30 taking intervals during the reconstruction of the digital modulation is greater than the time it takes for radio signals to spread a distance that is equal to the greatest distance between two base station transmitters belonging to the same cell within a limited geographical area. With the base station transmitters positioned as shown in Figure 1, the mobile station equalizers would be dimensionloned for a reception interval at the reconstruction greater than 2L / c. Given that dispersion can occur and that reflexes can extend the propagation time from one base station transmitter to the mobile station more than the extension of propagation time from another base station transmitter for the same cell, the reception time intervals of the mobile stations are preferably substantially larger than the propagation time range which is sufficient to extend the propagation time. as long as the largest distance between two base station transmitters assigned to the same cell within it current geographical area.

In mobile radio systems of the invention, the modulation time intervals of the digital modulation of the radio signals may be of the same order of magnitude as the propagation time it takes for the radio signals to extend a distance equal to the greatest transmitter distance between two base station transmitters serving the same cell within a limited range. Although the invention brings greater benefits, the smaller the modulation time interval is relative to this propagation time and the invention may be of greatest importance since the modulation time interval is as large as or less than said propagation time, the invention can bring significant advantages even where the modulation time interval is less fast as said propagation time.

It is conceivable to use different types of digital modulation in a mobile radio system whereby slightly different connections can save between information transfer rate and modulation time interval. In a digital modulation of the radio signals which means that one symbol is transmitted at a time by a succession of uncorrelated symbols, the modulation time interval becomes the time during which a symbol alone is decisive for the digital modulation. If, for example, a sequence of binary symbols is individually and one at a time be97759, the modulation is correct then the modulation time interval becomes the time during which a symbol determines the digital modulation. This can also be expressed so that the modulation time interval becomes the inverse value of the transmission rate in bits. In a digital modulation of the radio signals, which means that two or more at a time of a succession of digital symbols is crucial for the digital modulation, all or part of the overlapping times can be modulated, the modulation time interval may be the time when a preceding but not the next symbol affects the modulation. For example, in a digital modulation according to SE 8102802-9, Figures 1-2, a symbol influences the change of phase of a carrier path during a time interval 3T. The preceding symbol also affects the change and also the change in the phase of the carrier path during a first part of 2T of the time interval 3T. The next symbol also influences the change of phase of the carrier path during a final portion of 2T of the time interval 3T. In this case, the modulation time interval becomes T, which corresponds to what is called the symbol time interval in SE 810202-9. In general, it could be said that the modulation time interval is the time interval between two successive changes of the digital modulation.

A method and a digital cellular mobile radio system according to the invention are not limited to the described embodiments but can be modified within the scope of the claims. For example, it is conceivable to co-locate at least parts of the equipment in an ordinary base station transmitter with parts of the equipment in the additional base station transmitter for the same cell, provided the antennas are spaced apart. In an extreme case, it is conceivable that in principle all equipment other than the antennas is co-located to a location, for example near one antenna, and that the antennas are measured with radio frequency signals via cable from that location.

Therefore, with base station transmitters, at least one transmitter antenna for radio signals and preferably more or less of the other means required in a base station must be understood. Preferably, in a base station transmitter, there are at least tone means corresponding to the means contained in a transmitter-receiver unit B or B. in Figure 2.