WO1995034180A2

WO1995034180A2 - A method of selecting the transmission to be received in a mobile radio system, and a mobile station

Info

Publication number: WO1995034180A2
Application number: PCT/FI1995/000310
Authority: WO
Inventors: Mika Heiskari; Mika Lehmusto
Original assignee: Nokia Telecommunications Oy
Priority date: 1994-06-02
Filing date: 1995-06-01
Publication date: 1995-12-14
Also published as: FI95974C; WO1995034180A3; FI95974B; FI942601A0; AU2569495A

Abstract

A method and a mobile station for selecting the transmission (10, 11) to be received in a mobile radio system. In the method, the base station (BS1) transmits a first transmission (10) at a first frequency on its control channel, the first transmission comprising an address codeword and a control channel system codeword, and the control channel system codeword comprising a codeword completion sequence and an identity code of the mobile radio system and/or the first base station, and a second transmission (11) is transmitted at a second frequency. To select the transmission to be received, the mobile station (MS1) receives the address codeword of the first transmission (10) transmitted by the base station (BS1) at the first frequency, tunes to temporarily receive a second transmission (11) at a second frequency simultaneously as the base station (BS1) transmits the codewood completion sequence of the first transmission (10) and the identity code of the mobile radio system and/or the base station on the control channel, measures the quality of the second transmission (11), and if the quality of the transmission is higher than the threshold value, the mobile station (MS1) tunes to receive the second transmission (11).

Description

A method of selecting the transmission to be received in a mobile radio system, and a mobile station

Field of the Invention The invention relates to a method of selecting the transmission to be received in a mobile radio system that comprises a base station and at least one mobile station receiving a transmission from the base station; in the method, the base station transmits a first transmission at a first frequency on its control channel, the first transmission comprising an address codeword and a control channel system codeword, and the control channel system codeword comprising a codeword completion sequence and an identity code of the mobile radio system and/or the first base station, and a second transmission is transmitted at a second frequency.

The invention relates to radio tele¬ communications systems. A subscriber of a radio telecommunications system, i.e. a subscriber station, such as a mobile phone or some other means of communication, may be registered in a radio network or radio system through system or traffic channels maintained by the base stations of the radio network.

The invention can be applied e.g. in trunking networks, i.e. divided radio networks, which are dedicated networks offering - through joint use of radio channels - several groups of users the operations and services of a dedicated network employed by a single organization. The invention can be applied in radio tele¬ communications systems with either digital or analogue radio paths. Analogue radio systems are described e.g. in MPT 1327, January 1988, revised and reprinted November 1991, 'A Signalling Standard for Trunked Private Land Mobile Radio Systems, ' Radiocommunications Agency, and MPT 1343, January 1988, revised and reprinted September 1991, 'Performance Specification, ' Radiocommunications Agency, published by the British Ministry of Trade and Industry. The invention is particularly well-suited for use in the radio telecommunications systems described in these two MPT publications.

Background of the Invention In a cellular radio telecommunications system, the coverage areas, i.e. radio cells, of adjacent base stations often overlap to a large degree. The mobile phones located at the area where the coverage areas of two base stations overlap are then able to use either one of the base stations.

A common technical problem -in mobile radio systems is thus to settle a situation where a mobile station receives transmissions from two base stations or other sources, such as a mobile station operating on a direct mode channel, and has to select the transmission with the better quality, which it will then start to listen to. Another problem solved in the prior art systems is a situation where a mobile station is located within the coverage area of two transmitters sending transmissions in a different order of importance, and the mobile station must listen to one channel continuously and listen to or monitor the other channel occasionally in order to be able to decide which channel is more important to listen to continuously. It should be noted that transmissions sent by base stations can be transmitted on both channels, or that one of the channels listened to may be a direct mode channel on which another mobile station is transmitting. The possibility of listening to and monitoring a direct mode channel is very important in trunking networks utilizing direct mode channels, and especially so in authority networks.

To be able to perform the monitoring and handover operations, the mobile stations, such as mobile phones, must know the field strengths of the other radio channels or must at least identify the carrier wave of another radio channel.

As implied above, apart from system channels of radio telecommunications systems, direct mode channels can also be used, i.e. direct mode operation is applied. Subscriber stations using direct mode operation do not communicate directly with a radio network or its base stations. Direct mode channels are frequencies at which the mobile phones or other means of communication are able to communicate directly with one other without the system.

A direct mode channel is also called a direct or simplex channel. A direct mode channel is a channel that is typically not at all used by the system. It may be, for example, a channel that is of the same breadth as the channels of the system, e.g. 12.5 kHz or 25 kHz. Of the mobile phones operating on the direct mode channel, the transmitting station has tuned its transmitter to the channel and transmits speech or data information. The other mobile phones set to direct mode operation have tuned their receivers to the same channel, whereby they are able to directly hear the transmission.

Thus, on the one hand, a mobile phone has to be able to estimate the quality of the transmissions sent by those transmitters in whose coverage area the mobile phone is located, and on the other hand, it has to be able to select the transmission that is the most important from its point of view. The mobile phone thus has to be able to monitor a channel in the above- described manner, i.e. tune for a short time to a channel that it does not listen to continuously. When the mobile phone has tuned its transceiver to the desired monitored channel, it determines the quality of the transmission that it receives on that channel or estimates the importance of the transmission. If the transmission is of better quality than the one received on the original channel, or if the transmission received on the monitored channel is more important to the mobile phone or the user than the one received on the original channel, the mobile phone switches to receive transmissions sent on that new channel. To perform the above operations, the mobile phone naturally has to reserve time for measuring the quality and estimating the importance of the transmissions sent e.g. by a neighbouring station or neighbouring stations.

According to MPT 1327, a radio tele¬ communications system can give mobile stations, typically mobile phones, time to measure other channels by sending a vote-now message from the base station to the mobile station. The message advises the mobile stations to measure other MPT control channels and gives them the identity code of the channel to be measured. The vote-now message is described in MPT 1327, 5.5.4.5 Broadcast message, BCAST, item (f).

In the following, we shall discuss the drawbacks involved in the prior art solutions. When a vote-now technique is employed, the mobile phone system carefully controls what and when the mobile phones measure and on which channel. A method based on a vote- now message is thus not suited for implementing monitoring that is programmed in the mobile phones. The problem, however, can be overcome by modifying the mobile phones such that they are authorized to measure some other channel than the one named by the system. The major drawback in measurements based on vote-now messages is, however, undue loading of the control channel. Namely, in order that the monitoring would be sufficiently rapid, i.e. that the mobile phone would respond sufficiently quickly to a speech transmission starting on the other, monitored channel, the monitored channel must be measured 3-5 times a second. According to the MPT specification, however, one vote-now message takes about 100 ms on the control channel. Maintenance of a suitable level of service thus requires that the intervals between the measuring or monitoring operations should not be too long, so that the speech transmissions on the neighbouring channels are detected as soon after their start as possible. This means that 3-5 measuring or monitoring operations have to be performed per second, and since according to the MPT specification one measurement takes about 100 ms, the 3-5 measurements a second take up 30-50% of the capacity of the control channel. This large a drop in the capacity from the point of view of the other control channel operations also makes it practically impossible to use vote-now messages for providing monitoring times for mobile phones.

Description of the Invention

The object of the present invention is to solve the problem of finding the time needed for measuring the field strength or identifying the carrier wave in the frame structure of the base station of the mobile radio system when the mobile phone is on a (MPT) control channel. The problems encountered in the prior art solutions are thereby to be avoided. The object of the invention is especially to overcome the problem that occurs when vote-now messages are used, i.e. that the vote-now messages take up too much of the capacity of the control channel in question.

This new kind of method of selecting the transmission to be received in a mobile radio system is characterized in that the mobile station receives at a first frequency the address codeword of the first transmission transmitted by the base station; the mobile station temporarily tunes to receive a second transmission at a second frequency simultaneously as the base station transmits the codeword completion sequence of the first transmission and the identity code of the mobile radio system and/or the base station on its control channel; the mobile station measures the quality of the second transmission; the mobile station compares the quality of the second transmission with a predetermined threshold value, and if the quality of the transmission is higher than the threshold value, the mobile station switches to receive the second transmission, but if the quality of the transmission is lower than the threshold value, the mobile station switches back to the first frequency to receive the first transmission.

The invention also relates to a mobile station comprising a memory means, a transceiver, a controller for controlling the operation of the subscriber station, and measuring means for measuring the quality of the transmissions received by the mobile station. The mobile station of the invention is characterized by comprising a detecting means for detecting the end of the address codeword of the first transmission received by the transceiver at the first frequency and for generating a signal indicating the end; a computing means, responsive to the signal generated by the detecting means, for determining the time for which the transceiver is temporarily tuned to receive a second transmission at a second frequency when the base station is transmitting the codeword completion sequence of the first transmission and the identity code of the mobile radio system and/or the base station on its control channel, and for returning the transceiver to receive at the first frequency after the specified time; and a comparison means, responsive to the measuring means; the transceiver, in response to the comparison means, tuning to receive a second transmission if the quality of the second transmission is higher than the threshold value and switching back to the first frequency to receive the first transmission if the quality of the second transmission is lower than the threshold value.

The basic idea of the invention is that the mobile station tunes its transceiver to a second frequency to temporarily receive the transmission of the base station or the traffic on the direct mode channel when the base station transmits the codeword completion sequence of the first transmission and the identity code of the mobile radio system and/or the base station on its control channel. When the transceiver of the mobile station is tuned to the second channel, the mobile station receives the second transmission and measures the quality of the transmission. By comparing the quality with the threshold value, the mobile station is able to conclude whether it is worthwhile to switch entirely to receive the second channel or to switch back to the original channel.

The advantage of this kind of method for selecting the transmission to be received in a mobile radio system and of this kind of mobile station is that the problems of the prior art solutions are avoided. The new invention makes it possible for the mobile station to measure the quality of the transmissions received and to make a choice between the transmissions sufficiently quickly so that the control channel of the base station is not loaded to excess.

Brief Description of the figures In the following the invention will be described in greater detail with reference to the attached drawings, wherein fig. 1 is a diagram illustrating the operation of a base station, mobile station and transmitter of a mobile radio system according to the invention, fig. 2 is a diagram showing the frame structure of a control channel of a base station according to standard MPT 1327, fig. 3 is a diagram showing the control channel system codeword in the frame structure of a control channel of a base station according to standard MPT 1327, fig. 4 is a block diagram showing a mobile station according to the invention.

Detailed Description of the Invention

Fig. 1 is a diagram illustrating the operation of a base station, mobile station and transmitter of a mobile radio system according to the invention. The figure shows a trunking system controller TSC that controls a base station BSl communicating with a mobile station MSI over a radio path. The base station BSl of the mobile radio system sends a transmission 10 at a certain frequency on its control channel, the transmission being a control channel transmission defined in MPT 1327. The mobile station MSI receives from the base station BSl a transmission 10 containing the address codeword of the transmission sent on the control channel. When the mobile station MSI has received the address codeword, the mobile station tunes its transceiver to receive at a second frequency, where a second transmitter Tl is sending a second transmission 11. The transmitter may be either another base station or another mobile station, such as a subscriber station, i.e. a mobile station or radio phone, communicating on a direct mode channel.

Fig. 2 is a diagram showing the frame structure of a control channel transmitted by a base station according to standard MPT 1327. The figure shows the 1st and 2nd time slots, which comprise a control channel system codeword 21, 23, i.e. a CCSC field. The field contains information concerning the synchronization and the identification of the control channel. The structure of the CCSC is described in MPT 1327, 5.1, p. 5-3. The time slots also comprise an address codeword 22, 24, which contains the actual signalling messages sent on the control channel. The address codeword 22, 24 is described in MPT 1327, 5.2, p. 5-4. Fig. 3 is a diagram showing the control channel system codeword CCSC of the frame structure according to MPT 1327 of the control channel transmitted by the base station, the CCSC being indicated in fig. 2 by reference numbers 21 and 23. The first one of the fields in the figure, field 31, is a '0' bit. The second field, field 32, is a SYS, which comprises the identity code of the base station that has sent the transmission and/or of the whole mobile radio system. The third field, field 33, is a codeword completion sequence CCS. The fourth field, field 34, is a bit synchronization field, i.e. a preamble, PR. The preamble PR 34 is used for bit synchronization and it always ends with a '0' bit. The fifth field, field 35, is a parity field P, which is used for codeword completion and frame synchronization. The length of each field, measured in bits, is stated below the figure.

In the invention, when the base station BSl sends a transmission, i.e. a frame structured as described in connection with figs. 2 and 3 above, the mobile station MSI listens to the control channel of the base station and receives the address codeword 22 of the 1st time slot. The mobile station MSI then receives the '0' bit 31 and the SYS 32, which indicates the mobile station the identity code of the transmitting base station BSl, and optionally also the identity code of the whole mobile radio system. It is sufficient that the mobile station MSI, i.e. the radio phone, receives the SYS 32 from the base station BSl in time slot 1 once; after a successful reception, the mobile phone need not go on receiving the SYS again and again. The time that is saved when the mobile station need not receive the SYS 32 in time slot 2 is used in the invention for listening to other channels, and for measuring the quality of the transmissions and estimating their urgency. The other channels can be transmitted by other base stations or mobile stations, typically mobile stations communicating on a direct mode channel.

In a first embodiment of the invention, immediately as the mobile phone has received an address codeword 22, it switches to receive and measure another channel. It tunes its transceiver to the desired channel, which is other than the one on which the address codeword 22 transmitted by the base station BSl was received. The mobile phone returns to the control channel transmitted by the base station BSl at the beginning of the bit synchronization field PR, i.e. the preamble 34. In so doing, the receiving mobile station, i.e. mobile phone, can conduct measurements for the duration of 32 bits. The bit rate of an MPT control channel is 1200 bit/s, whereby 32 bits correspond to 32 bit/1200 bit/s = 26.7 ms of time. The time is sufficiently long for the mobile station to temporarily tune to a second frequency, receive a transmission at that frequency, and if necessary, measure the field strength of the received transmission and thereby estimate the quality of the received transmission. Another way of measuring the quality of the transmission is to measure its bit error ratio. It should be noted that at the same time as the mobile station receives a transmission from a second source, the base station BSl sends the codeword completion sequence CCS 33 of the first transmission and the identity code, i.e. SYS 32, of the mobile radio system and/or the base station on its control channel. In any case, after the measurement the mobile station MSI compares the quality of the second transmission with a suitable threshold value, and if the quality of the received transmission is higher than the threshold value, the mobile station MSI switches to receive the second transmission 11. If, on the other hand, the quality of the received transmission 11 is lower than the threshold value, the mobile station MSI switches back to the first frequency to receive the first transmission 10. In one embodiment of the invention, the threshold value is determined on the basis of the quality of the first transmission 10 received by the mobile station MSI. The qualities of the first and second transmissions received by the mobile station can here be compared with each other to choose the better one so that the mobile station can switch to listen to the radio transmitter whose transmissions it hears better.

In a second embodiment of the invention, depending on the performance of the mobile station MSI, such as a mobile phone, the mobile phone does not necessarily need all the bits of the bit synchronization field 34, i.e. the preamble, for bit synchronization, and so the extra bits can be used for receiving other transmissions from other sources. If, for example, the mobile phone needs only 4 bits for bit synchronization, (32+12) bit/1200 bit/s = 36.7 ms of measuring time remains. In this embodiment of the invention, the mobile station MS 1 thus temporarily tunes to receive a second transmission 11 at a second frequency simultaneously as the base station sends the bit synchronization field PR 34 of the control channel system codeword CCSC 21 of the first transmission 10 on the control channel.

Below is a description of a third embodiment of the invention. If the measuring time achieved in the first two embodiments is not sufficiently long for the mobile phone, i.e. if the mobile phone needs even longer for the measuring and/or monitoring, more measuring time can be taken from the preceding address codeword 22, 24, but it must be ensured that no information is lost thereby. The address codeword is described in MPT 1327, 5.2, p. 5-4.

The address codeword 22, 24 comprises a 48-bit data part and a 16-bit parity part. With the help of the 16 bits of the parity part, one erroneous bit of the data part can be corrected, and the residual error can then be identified.

In the third embodiment of the invention, the mobile station MSI thus temporarily tunes to receive a second transmission at a second frequency simultaneously as the base station BSl transmits the address codeword

22, 24 of the first transmission on the control channel.

Fig. 4 is a block diagram showing a mobile station according to the invention. Fig. 4 shows a typical mobile station, i.e. a subscriber station. The function of a transceiver (TX/RX) 501 is to tune to the channel used. With the transceiver 501 is connected an antenna 502, which is connected with the radio path RP. Radio frequencies in the range of 60 to 1000 MHz (VHF and UHF ranges) are usually used. On the radio path RP, analogue modulation can be used, the modulation being then usually phase modulation. Other kinds of modulation can also be used. Signalling can be transmitted e.g. by a voice-frequency subcarrier wave (FFSK). Transmission on a radio path can also be digital.

A subscriber unit 505 may comprise electro- acoustic transducers, typically a headphone 506 and a microphone 507, and optionally buttons in a user inter¬ face 508 for starting and ending a call, and for dialling. Since in a trunking system, transmission on a radio path RP is advantageously unidirectional, the subscriber station usually also has a push-to-talk button that must be depressed during the transmission.

The function of a controller 503 is to control the operation of the subscriber station. The controller 503 is connected with the subscriber unit 505, from which it receives signals e.g. for starting and ending a call. The controller 503 may also give the user, via the subscriber unit 505 or the user interface 507, voice or visual signals that relate to the operation of the mobile station and/or the radio telecommunications system.

The controller 503 is connected with the trans¬ ceiver TX/RX 501. The channel employed by the trans- ceiver is determined by the controller 503, i.e. the transceiver 501 tunes to the channel, i.e. radio fre¬ quency, determined by the controller 503. The trans¬ ceiver 501 is also switched on by the controller 503. The controller 503 receives and transmits signalling messages via the transceiver 501. The controller can also receive information about the field strength of the received signal. In addition, the controller determines the time slot listened to at the radio frequency used, if the system is a time division system. With the controller 503 is connected a memory means 504, which contains both permanent and changing information. The permanent information includes e.g. the subscriber number of the subscriber station, the numbers of the groups to which the subscriber station belongs, and the radio channels belonging to the system. The changing information includes the mode of the subscriber station, such as idle mode, call set-up mode, and call mode, and e.g. information indicating the channel to which the station is tuned. A mobile station operating in accordance with the invention also comprises measuring means 509 for determining the quality of the transmissions received by the mobile station. The quality of the received transmissions is typically determined by measuring the field strength or determining the bit error ratio.

In addition to the above-mentioned units, the mobile station of the invention comprises a detecting means 510 for detecting the end of the address codeword of the first transmission received by the transceiver 501 at a first frequency and for generating a signal indicating the end.

The mobile station of the invention also comprises a computing means 511, responsive to the signal generated by the detecting means 510, for determining the time for which the transceiver 501 is temporarily tuned to receive a second transmission at a second frequency when the base station BSl is transmitting the codeword completion sequence of the first transmission and the identity code of the mobile radio system and/or the base station on its control channel, and for returning the transceiver 501 to receive at the first frequency after the specified time. The mobile station of the invention also comprises comparison means 512, responsive to the measuring means 509; the transceiver 501, in response to the comparison means, tuning to receive a second transmission if the quality of the second transmission is higher than the threshold value, and the transceiver 501, in response to the comparison means, switching back to the first frequency to receive the first transmission if the quality of the second transmission is lower than the threshold value.

The drawings and the description thereof are only intended to illustrate the idea of the invention. The method of the invention for selecting the transmission to be received in a mobile radio system and the mobile station of the invention may vary in their details within the scope of the claims. Although the invention is described above mainly in connection with trunking radio systems, it can also be used in other kinds of mobile radio systems.

Claims

1. A method of selecting the transmission to be received (10, 11) in a mobile radio system that comprises a base station (BSl) and at least one mobile station (MSI) receiving a transmission from the base station; in the method, the base station (BSl) transmits a first transmission ( 10) at a first frequency on its control channel, the first transmission comprising an address codeword (22, 24) and a control channel system codeword (CCSC, 21, 23), and the control channel system codeword comprising a codeword completion sequence (CCS, 33) and an identity code (SYS, 32) of the mobile radio system and/or the first base station, and a second transmission ( 11 ) is transmitted at a second frequency, the method being c h a r a c t e r i z e d in that the mobile station (MSI) receives at a first frequency the address codeword (22) of the first transmission (10) transmitted by the base station (BSl); the mobile station (MSI) temporarily tunes to receive a second transmission (11) at a second frequency simultaneously as the base station (BSl) transmits the codeword completion sequence (CCS, 33) of the first transmission (10) and the identity code (SYS, 32) of the mobile radio system and/or the base station on its control channel; the mobile station measures the quality of the second transmission (11); the mobile station compares the quality of the second transmission with a predetermined threshold value, and if the quality of the transmission is higher than the threshold value, the mobile station (MSI) switches to receive the second transmission (11), but if the quality of the transmission (11) is lower than the threshold value, the mobile station (MSI) switches back to the first frequency to receive the first transmission (10).

2. The method of claim 1, c h a r a c t e r ¬ i z e d in that the threshold value is determined on the basis of the quality of the first transmission (10) received by the mobile station (MSI).

3. The method of claim 1 or 2, c h a r a c ¬ t e r i z e d in that the mobile station (MSI) stays tuned, receiving the second transmission (11) at the second frequency simultaneously as the base station transmits the bit synchronization field (PR, 34) of the control channel system codeword (CCSC, 21) of the first transmission (10) on the control channel.

4. The method of claim 1, 2 or 3, c h a r a c - t e r i z e d in that the mobile station (MSI) tunes to temporarily receive a second transmission (11) from a second frequency (11) simultaneously as the base station (BSl) transmits the address codeword (22, 24) of the first transmission on the control channel.

5. The method of claim 1, 2, 3 or 4, c h a r - a c t e r i z e d in that the mobile station (MSI) measures the quality of the received transmission (11) by measuring the field strength of the transmission.

6. The method of claim 1, 2, 3 or 4, c h a r ¬ a c t e r i z e d in that the mobile station (MSI) measures the quality of the received transmission (11) by measuring the bit error ratio of the transmission.

7. The method of claim 1, 2, 3, 4, 5 or 6, c h a r a c t e r i z e d in that the second transmission (11) is transmitted by a second base station (Tl).

8. The method of claim 1, 2, 3, 4, 5 or 6, c h a r a c t e r i z e d in that the second transmission (11) is transmitted by a second mobile station (Tl) transmitting on a direct mode channel. .

9. A mobile station (fig. 4, MSI) comprising a memory means (504), a transceiver (501), a controller (503) for controlling the operation of the subscriber station, measuring means (509) for determining the quality of the transmissions received by the mobile station, c h a r a c t e r i z e d by comprising a detecting means (510) for detecting the end of the address codeword (22, 24) of the first transmission (10) received by the transceiver (501) at the first frequency and for generating a signal indicating the end; a computing means (511), responsive to the signal generated by the detecting means (510), for determining the time for which the transceiver (501) is temporarily tuned to receive a second transmission (11) at a second frequency when the base station (BSl) is transmitting the codeword completion sequence (CCS, 33) of the first transmission (10) and the identity code

(SYS, 32) of the mobile radio system and/or the base station on its control channel, and for returning the transceiver (501) to receive at the first frequency after the specified time; and a comparison means (512), responsive to the measuring means (509); the transceiver (501), in response to the comparison means, tuning to receive a second transmission (11) if the quality of the second transmission (11) is higher than the threshold value, and the transceiver (501), in response to the comparison means, switching back to the first frequency to receive the first transmission (10) if the quality of the second transmission (11) is lower than the threshold value.