DATA TRANSMISSION METHOD AND RADIO SYSTEM

Abstract: The invention relates to a data transmission method and a radio system comprising a number of transceivers (110, 120, 130, 140) and at least one subscriber terminal (400) which transmits a number of access bursts on its traffic channel during handover. In the radio system, a connection between a transceiver and a subscriber terminal is set up when the transceiver receives from its random access channel an access burst transmitted by the subscriber terminal (400), the reception of said access burst activating the allocation of a channel used for the connection. The radio system comprises means (260) for measuring the bursts received by the transceiver from the random access channel, means (250) for generating a handover reference signal which deviates from the bit pattern of the random access burst and which is transmitted to the treansceiver during handover, whereupon it is possible for the means (260) to filter off the handover reference signal received in order to prevent the allocation of a channel.

Description: DATA TRANSMISSION METHOD AND RADIO SYSTEM FIELD OF THE INVENTION The invention relates to a data transmission method to be used during handover in a radio system comprising a number of transceivers and at least one subscriber terminal which transmits a number of access bursts on its traffic channel during handover, and in which radio system a connection between a transceiver and a subscriber terminal is set up when the transceiver receives from its random access channel an access burst transmitted by the subscriber terminal, the reception of said access burst activating the allocation of a channel to be used for the connection. The invention further relates to a radio system comprising a number of transceivers and at least one subscriber terminal which transmits a number of access bursts on its traffic channel during handover, and in which radio system a connection between a transceiver and a subscriber terminal is set up when the transceiver receives from its random access channel an access burst transmitted by the subscriber terminal, the reception of said access burst activating the allocation of a channel to be used for the connection. DESCRIPTION OF THE PRIOR ART A special random access channel (RACH) is used in radio systems for setting up a connection between a terminal and a base station. When the terminals desire to set up a radio connection, they send a message of setting up the connection, in other words a random access burst, to the base station which forwards it to the system in which resources are allocated for the connection. This means that a particular time slot which enables the connection setup message to be sent by the terminals to the base station is allocated for the message. The system can by no means know when the terminals desire to communicate, so the first message of the terminal to the base station can not be coordinated. The terminals also lack information about the length of the propagation delay of the signal, thus the messages are randomly supplied within a given time slot. In a typical cellular radio system, the subscriber terminal communicates with only one base station at a time, although particularly in the CDMA system, for example, the subscriber terminal can also communicate with several base stations simultaneously. When the terminal moves in the area of the cellular radio system, it becomes necessary to perform a handover from time to time. In a known, soft handover the connection to the base station network remains uninterrupted regardless of the handover. The base station is typically changed in this kind of handover. A softer handover where the base station is not changed but the sector of the base station used is changed is also known. A hard handover, which is a break-before-make-type handover, is also used in cellular radio systems. This means that a new connection from the subscriber terminal to the base station will not be set up until the previous base station connection has been interrupted. Although the radio system has been designed to operate on soft and softer handover, there are several reasons for performing the hard handover in cellular radio systems. These reasons include a need to modify the connection parameters, the frequency channel used, the radio system, etc. During handover, the target base station participating in the handover receives handover access bursts transmitted by the subscriber terminal. The subscriber terminal transmits the handover access bursts on a traffic channel (TCH). After the transmission of the handover access bursts the sub- scriber terminal sends an acknowledgement of the successful handover. When a subscriber terminal sets up a connection to a base station it transmits access bursts to the base station. The base station receives the access bursts from its random access channel, i.e. RACH channel. After the reception of the access bursts the base station controller controlling the base station in the radio system transmits a channel-activating signal to the base station. In practice, the access bursts used for performing a handover and setting up a speech connection are similar as far as the base station receiver is concerned. An access burst transmitted during handover can sometimes be forwarded to a base station which is not involved in the handover. If the base station which is not involved in the handover receives from its RACH channel a burst which was originally transmitted to the traffic channel of another base station by the subscriber terminal, the base station system allocates a channel unnecessarily. In handover, the subscriber terminal generally transmits several access bursts which can, disadvantageously, allocate all channels from the base station which is not involved in the handover. The unnecessary channel allocations reduce the capacity available for the radio system. Insufficient network planning is the main reason why a base station receives a signal which is not originally intended to the base station. In prac- tice, however, it is not possible to plan a radio network in such a manner that all the above problems could be eliminated. It is increasingly difficult to take account of said problems in advance in network planning, since network planning is constantly becoming more complex. In practice, it is not possible to prevent all unnecessary channel allocations by means of network planning. FI 100077 B discloses a mobile communication system in which a mobile station and a base station measure the power of a received signal, whereupon it is possible for the base station to use the measurement results to decide whether to change base stations. This publication does not, however, disclose any criterion by which it would be possible to perform a filtering. The described solution is, however, used for deciding whether to change base stations and not for preventing channel allocation. EP 0615392 A1 discloses a method in which the parameters located in a signal transmitted between a base station and a mobile station are measured. The measurement results obtained can be used to decide whether to change base stations. The solution disclosed in the publication is not, however, suitable for filtering unnecessary channel allocation requests. DE 19510256 A1 discloses a method in which the parameters located in a signal transmitted between a base station and a mobile station are measured and the values of the parameters are compared with threshold val- ues. The method seems to be suitable for deciding whether to change base stations and not for filtering channel allocation requests. WO 97/15169 discloses a method in which the time slots of the received signals are measured. The measurement results are, however, used to decide whether to change base stations and not for filtering channel allocation requests. WO 95/22876 discloses a method in which time slots are measured and in which some parameters are picked from the measurement results obtained. The method is, however, used in handover and not for filtering channel allocation requests. FI 934731 , GB 2280335 A, GB 296628 A and WO 96/166524 A3 each discloses a method in which time slots are measured and in which some parameters are picked from the measurement results obtained. The methods are, however, used in handover and not for filtering channel allocation requests. BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is thus to prevent a radio system from each time allocating a channel on the basis of a signal received even though a base station receives a signal which activates the allocation of a channel. This is achieved with the data transmission method of the type de- scribed in the introduction, the method being characterized in that bursts received from the random access channel by the transceiver are measured, a handover reference signal which deviates from the random access burst is transmitted to the transceiver during handover, and the received handover reference signal is filtered off on the basis of a measurement, whereby the allocation of a channel can be prevented. This can be achieved with the radio system of the invention, which is characterized in that the radio system comprises means for measuring the bursts received from the random access channel by the transceiver, means for generating a handover reference signal which deviates from the bit pattern of the random access burst and which is transmitted to the transceiver during handover, whereupon it is possible for the means to filter off the handover reference signal they received in order to prevent the allocation of a channel. Considerable advantages can be achieved by the data transmission method of the invention. The method enables optimum channel allocation in a radio system, whereby unnecessary channel allocations can be avoided. Since it is possible to restrict channel allocations, the radio system can serve subscriber terminals attempting to set up a connection faster and in a more flexible way. The method is extremely well suited for radio systems where there are many connections and many handovers occur in relation to the channel capacity of the radio system. 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 a radio system in which the method of the invention is used Figure 2 is a signal flow diagram of a connection setup, Figure 3 shows an access burst, Figure 4 is a signal flow diagram of a handover, Figure 5 shows the radio system of the invention in closer detail. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a radio system in which the method of the invention is used. The radio system comprises base stations 110, 120, 130, 140 oper- ating as transceivers, a base station controller 300, and at least one subscriber terminal 400. The base station controller 300 and the base stations are interconnected by a digital transmission link 500 in the solution of the figure. The base station controller 300 controls the operation of the base stations 110, 120, 130, 140. When the subscriber terminal 400 moves from the coverage area of a base station to the coverage area of another base station, a handover is performed. The subscriber terminal 400 sets up a connection to the base station utilizing a traffic channel (TCH). In practice, one time slot forms a TCH channel. Figure 1 shows a dotted line 10 to describe the boundary region between the base stations 110 and 140. The boundary region separates the coverage areas of the base stations from each other. At point 1 the subscriber terminal 400 communicates with the base station 110. When the subscriber terminal 400 moves on in the base station network, it arrives at point 2 where it is on the edge of the coverage areas of the base stations 110 and 140. In such a case, a handover is performed to the subscriber terminal 400, which means that the base station 110 is changed to the base station 140. At point 3 the subscriber terminal communicates only with the base station 140. Figure 2 is a signal flow diagram of a process of setting up a con- nection to a base station by a subscriber terminal. The set-up connection is used, for example, for transmitting speech to another subscriber terminal. Let us assume, with reference to Figure 1 , that the subscriber terminal 400 is located at point 1. Let us further assume that point 1 is located within the coverage area of the base station 110. The setup of a connection is initiated in such a manner that the subscriber terminal 400 transmits an access burst to a radio path. The signal is received at the base station 110 since the subscriber terminal is located within the coverage area of the base station 110. The base station 110 receives from its RACH channel the access burst transmitted by the subscriber terminal 400, which is forwarded to the base station controller 300. The base station controller 300 sends the base station 110 a channel activation command on the basis of the access burst received. Next, the base station controller 300 transmits a channel allocation command to the subscriber terminal 400 via the base station 110. The subscriber terminal 400 is commanded to use the channel that was previously al- located from the base station on the basis of the allocation command. The subscriber terminal 400 subsequently transmits an acknowledgement signal of the set-up connection to the base station controller 300. The subscriber terminal 400 sets up the connection to the base station by the access burst it transmitted. Figure 3 shows an access burst com- prising 88 bits. The burst comprises eight extended T-bits (Tail-bits). In addition, the burst comprises a synchronization sequence comprising 41 synchronization bits. The synchronization sequence operates as a training sequence. Furthermore, the burst comprises 36 bits which are allocated to data. The burst further comprises, after the data bits, three T-bits and a guard period whose length is 68.25 bits. When the subscriber terminal 400 moves in the radio system, it becomes necessary to perform a handover at some stage. Figure 4 is a signal flow diagram showing a handover in closer detail. The handover will be described in the following in connection with Figure 1 where the subscriber ter- minal 400 moves from point 1 towards point 3. The handover is thus performed from the base station 110 which operates as a source base station to the base station 140. The base station 140 operates as a target base station in the situation of the figure. Let us further assume that in the initial situation the subscriber terminal 400 has already in advance set up the connection to the base station controller 300 via the base station 110. In the above situation the base station controller 300 transmits a signal to the base station 140 which activates a channel on the basis of the signal received. Next, the base station controller 300 transmits a handover command passing via the base station 110 to the subscriber terminal 400. Af- ter receiving the handover command, the subscriber terminal transmits access bursts on its traffic channel to the base station 140 operating as the target base station. After the handover the subscriber terminal 400 transmits an acknowledgement of the successful handover to the base station controller 300, the acknowledgement passing via the base station 140. The subscriber terminal 400 and the base station 140 use the traffic channel (TCH) in the above situation when they transmit the signals associated with the handover. Figure 5 shows the structure of the radio system of the invention in closer detail. The radio system comprises means 401 which are operatively connected to the subscriber terminal 400. In addition, the radio system comprises means 250 which are preferably operatively connected to the base sta- tion controller 300. In the radio system of the figure, the base station controller 300 comprises the means 250. Let us assume that the subscriber terminal 400 moves towards point 2 shown in Figure 1 , whereby the handover is performed as described above. The handover access bursts transmitted by the subscriber terminal 400 can, however, transfer in accordance with a signal 450 in such a manner that the base station 130 receives the bursts from its RACH channel. In the prior art radio systems, the reception of the access bursts thus causes the allocation of channels from the base station 130. In the radio system of the figure, the means 250 generate a handover reference signal which is forwarded to the base station 110. The base sta- tion controller 300 commands the subscriber terminal 400 to perform the handover, whereupon the base station 110 transmits the handover reference signal generated by the means 250 to the subscriber terminal 400. The means 401 echo the handover reference signal received by the subscriber terminal 400 back to the radio system in the handover access burst. In the data trans- mission method of the invention, the means 250 select a bit pattern which deviates from the regular bit pattern of the random access burst for the reference signal used in the handover command. The means 250 use an eight-bit signal which comprises a 01100XXX or a 0111XXXX bit pattern as the handover reference signal. The X-bits are 'don't care bits', in other words they can be given the values '0' or '1'. It is also possible to use rare bit patterns in the radio system as the bit pattern of the reference signal. It is possible to distinguish the handover access bursts and the random access bursts from each other on the basis of the bit pattern. The bit pattern of the reference signal enables the handover ac- cess bursts received from the RACH channel to be detected and to be filtered. In practice, the base station controller 300 commands the subscriber terminal to perform the handover. The means 250 generate the handover reference signal which is transmitted to a radio path. When the subscriber terminal 400 receives the reference signal the received handover reference signal is echoed back to the radio network in the handover access burst. The base stations 130, 140 receive the handover access burst comprising the handover reference signal, transmitted by the subscriber terminal 400. The radio system of the figure comprises means 260 which measure the RACH channel. The means 260 filter the handover access bursts detected on the RACH channel on the basis of the measurement. In practice the means 260 are located at the base station or the base station controller. The filtering of the bursts prevents the base station 130 from allocating its channels on the basis of the access bursts received from the RACH channel. The radio system further comprises means 270 for determining the base station which is to receive the handover access burst transmitted by the subscriber terminal 400. The means 270 control the base station to allocate a channel. In known solutions, such as in the European digital cellular radio network GSM, the base station transmits information required in setting up a connection at one frequency in the first time slot of the frame structure. The time slot forms a Broadcast Control Channel (BCCH). The means 260 also measure the BCCH channel. If the means 260 detect handover access bursts on the BCCH channel they measure, the means 270 prevent the base station from allocating a channel if the measured burst comprises a handover reference signal. When the means 260 detect a burst comprising a handover reference signal, the means 260 filter off the burst, whereby the allocation of a channel is prevented. Let us assume that the base station 130 receives a reference signal of, for example, the bit pattern 01100111. If the base station 130 receives said bit pattern from its RACH channel, the means 260 detect that the bit pattern meets the condition defined for the reference signal, whereby the means 260 filter off the signal, i.e. the burst. Another preferred data transmission method by means of which it is possible to detect handover access bursts and filter off the detected bursts will be described in the following. A handover is initiated by sending a handover command signalling message to the subscriber terminal 400. In the data transmission method of the invention the base station controller 300 generates the signalling message. The message is sent from the base station controller 300 to a base station which sends the message to the subscriber terminal 400. The message comprises information elements comprising a so-called handover reference value. The subscriber terminal 400 uses the handover reference value located in the signal it received when it transmits handover access bursts in handover. The method is based on the measurement of the transmission frequency of the random access bursts transmitted by the subscriber terminal 400. The subscriber terminal transmits random access bursts typically on the RACH channel at intervals of a few dozens of frames, for example. Random reference values which are randomly selected are used in the random access bursts. This means that each burst comprises a different random reference value, i.e. parameter. Dozens of access bursts comprising the same random reference value in successive RACH channel frames are detected in the event of a handover fault. If the base station 130, for example, receives a number of similar access bursts exceeding a predetermined limit from the successive RACH channel frames of the RACH channel, it is thus possible to infer, on the basis of the above, that the subscriber terminal 400 attempts a handover. The method thus enables to detect that the access bursts received from the RACH channel were intended for handover and not for setting up a connection. After detecting the bursts the means 260 filter off the access bursts comprising similar data, whereby the base station is prevented from allocating a channel. The means 260 measure the RACH channel uninterruptedly. Furthermore, in the method it is possible to use a timing advance parameter in addition to the random reference parameter. The timing advance parameter is used for correcting transit delay on a radio path. The timing advance parameter is used as a transmission advance parameter which describes the distance between the subscriber terminal 400 and the base station. In the radio system of the figure, the means 260 measure the random reference and timing advance parameters from the signal received by the base station, and the signal is filtered on the basis of these parameters. The filtering can also be based on parameters corresponding to the above parameters. The prevention of channel allocation can also be based on the filtering of a handover signal supplied to the RACH channel when the handover signal comprises a handover reference signal. Although the invention has been described in the above with reference to the examples in accordance with the accompanying drawings, it will be obvious that the invention is not restricted to them but it can be modified in many ways within the scope of the inventive idea disclosed in the appended claims.

Claims: CLAIMS 1. A data transmission method to be used during handover in a radio system comprising a number of transceivers (110, 120, 130, 140) and at least one subscriber terminal (400) which transmits a number of access bursts on its traffic channel during handover, and in which radio system a connection between a transceiver and a subscriber terminal is set up when the transceiver receives from its random access channel an access burst transmitted by the subscriber terminal (400), the reception of said access burst activating the allocation of a channel to be used for the connection, characterized in that bursts received from the random access channel by the transceiver are measured, a handover reference signal which deviates from the random access burst is transmitted to the transceiver during handover, and the received handover reference signal is filtered off on the basis of a measurement, whereby the allocation of a channel can be prevented. 2. A data transmission method as claimed in claim 1, characterized in that during handover, the handover reference signal which deviates from the random access burst is transmitted to the transceiver on the traffic channel, and the received handover reference signal is filtered off if the handover reference signal is received from the random access channel. 3. A data transmission method as claimed in claim 1 , characterized in that the handover reference signal is filtered off when the base station receives the handover reference signal from the random access chan- nel. 4. A data transmission method as claimed in claim 1 , c aracterized in that the handover reference signal is transmitted to the subscriber terminal which echoes the handover reference signal received back to the transceiver. 5. A data transmission method as claimed in claim 1 , characterized in that the handover reference signal whose bit pattern deviates from the access burst used in allocating a channel on the basis of a different bit pattern is used in the handover. 6. A data transmission method as claimed in claim 1, charac- t e r i z e d in that the handover reference signal which comprises a 01100XXX or a 0111XXXX bit pattern where the X-bit is a 'don't care bit' is used in the method. 7. A data transmission method as claimed in claim 1 , characterized in that in practice the transceiver used in the method is a base sta- tion. 8. A data transmission method as claimed in claim 1, characterized in that the random access channel is measured uninterruptedly. 9. A data transmission method as claimed in claim 1 , characterized in that the signal is filtered at the base station or the base station controller in the radio system. 10. A radio system comprising a number of transceivers (110, 120, 130, 140) and at least one subscriber terminal (400) which transmits a number of access bursts on its traffic channel during handover, and in which radio system a connection between a transceiver and a subscriber terminal is set up when the transceiver receives from its random access channel an access burst transmitted by the subscriber terminal (400), the reception of said access burst activating the allocation of a channel to be used for the connection, characterized in that the radio system comprises means (260) for measuring the bursts received from the random access channel by the transceiver, means (250) for generating a handover reference signal which deviates from the bit pattern of the random access burst and which is transmitted to the transceiver during handover, whereupon it is possible for the means (260) to filter off the handover reference signal received in order to prevent the allocation of a channel. 11. A radio system as claimed in claim 10, characterized in that the radio system comprises the means (250) for generating the handover reference signal which deviates from the bit pattern of the random access burst and which is transmitted to the transceiver during handover, whereupon it is possible for the means (260) to filter off the handover reference signal received. 12. A radio system as claimed in claim 10, characterized in that during handover, the handover reference signal which deviates from the access burst is transmitted to the transceiver on the traffic channel, and the means (260) filter off the handover reference signal received if the transceiver receives the handover reference signal from its random access channel. 13. A radio system as claimed in claim 10, characterized in that the means (260) filter off the handover reference signal when the base station receives the handover reference signal from the random access channel. 14. A radio system as claimed in claim 10, c h a ra cte ri zed in that the handover reference signal is first transmitted to the subscriber terminal (400), and the radio system comprises means (401) for echoing the handover reference signal received by the subscriber terminal back to the transceiver. 15. A radio system as claimed in claim 10, characterized in that the bit pattern of the handover reference signal generated by the means (250) deviates from the random access burst used in allocating a channel on the basis of a different bit pattern. 16. A radio system as claimed in claim 10, characterized in that the handover reference signal generated by the means (250) comprises a 01100XXX or a 0111XXXX bit pattern where the X-bit is a 'don't care bit'. 17. A radio system as claimed in claim 10, characterized in that in practice the transceiver in the radio system is a base station. 18. A radio system as claimed in claim 10, characterized in that the means (260) measure the random access channel uninterruptedly.

Inventors: HAATAJA KARI, HUTTUNEN KARI, PARKKINEN JARI, HAATAJA, KARI, HUTTUNEN, KARI, PARKKINEN, JARI
Applicants: HAATAJA KARI, HUTTUNEN KARI, NOKIA TELECOMMUNICATIONS OY, PARKKINEN JARI, HAATAJA, KARI, HUTTUNEN, KARI, PARKKINEN, JARI
Application year: 1998
Main classification: H04Q7
IPC codes: H04W36, H04W74