AU7770598A

AU7770598A - Data transmission method and radio system

Info

Publication number: AU7770598A
Application number: AU77705/98A
Authority: AU
Inventors: Kari Haataja; Kari Huttunen; Jari Parkkinen
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1997-06-16
Filing date: 1998-06-16
Publication date: 1999-01-19
Also published as: EP0995329A2; WO1999001002A2; FI972561A; FI972561A0; CN1265815A; WO1999001002A3

Description

WO 99/01002 PCT/FI98/00520 1 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 5 least one subscriber terminal which transmits a number of access bursts on its traffic channel during handover, and in which radio system a connection be tween 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 10 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 15 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 20 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 con nection. This means that a particular time slot which enables the connection 25 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 30 within a given time slot. In a typical cellular radio system, the subscriber terminal communi cates with only one base station at a time, although particularly in the CDMA system, for example, the subscriber terminal can also communicate with sev eral base stations simultaneously. When the terminal moves in the area of the WO 99/01002 PCT/FI98/00520 2 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 net work remains uninterrupted regardless of the handover. The base station is 5 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 10 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 rea sons for performing the hard handover in cellular radio systems. These rea sons include a need to modify the connection parameters, the frequency 15 channel used, the radio system, etc. During handover, the target base station participating in the hando ver 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 20 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 ac cess bursts from its random access channel, i.e. RACH channel. After the re ception of the access bursts the base station controller controlling the base 25 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 set ting up a speech connection are similar as far as the base station receiver is concerned. An access burst transmitted during handover can sometimes be 30 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 35 access bursts which can, disadvantageously, allocate all channels from the WO 99/01002 PCT/F198/00520 3 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 5 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 plan ning is constantly becoming more complex. In practice, it is not possible to prevent all unnecessary channel allocations by means of network planning. 10 Fl 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, how ever, disclose any criterion by which it would be possible to perform a filtering. 15 The described solution is, however, used for deciding whether to change base stations and not for preventing channel allocation. EP 0615392 Al discloses a method in which the parameters lo cated in a signal transmitted between a base station and a mobile station are measured. The measurement results obtained can be used to decide whether 20 to change base stations. The solution disclosed in the publication is not, how ever, suitable for filtering unnecessary channel allocation requests. DE 19510256 Al discloses a method in which the parameters lo cated 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 25 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 re ceived signals are measured. The measurement results are, however, used to decide whether to change base stations and not for filtering channel allocation 30 requests. WO 95/22876 discloses a method in which time slots are measured and in which some parameters are picked from the measurement results ob tained. The method is, however, used in handover and not for filtering channel allocation requests. 35 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 WO 99/01002 PCT/FI98/00520 4 parameters are picked from the measurement results obtained. The methods are, however, used in handover and not for filtering channel allocation re quests. BRIEF DESCRIPTION OF THE INVENTION 5 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 10 scribed in the introduction, the method being characterized in that bursts re ceived 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 15 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 20 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 ref erence 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 25 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 30 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 WO 99/01002 PCT/FI98/00520 5 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, 5 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 10 ating as transceivers, a base station controller 300, and at least one sub scriber 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. 15 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 sta tion utilizing a traffic channel (TCH). In practice, one time slot forms a TCH channel. 20 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 25 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 30 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 lo cated at point 1. Let us further assume that point I is located within the cover age area of the base station 110. The setup of a connection is initiated in such 35 a manner that the subscriber terminal 400 transmits an access burst to a radio WO 99/01002 PCT/F198/00520 6 path. The signal is received at the base station 110 since the subscriber termi nal 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 5 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 sub scriber terminal 400 is commanded to use the channel that was previously al 10 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 sta tion by the access burst it transmitted. Figure 3 shows an access burst com 15 prising 88 bits. The burst comprises eight extended T-bits (Tail-bits). In addi tion, the burst comprises a synchronization sequence comprising 41 synchro nization 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 20 whose length is 68.25 bits. When the subscriber terminal 400 moves in the radio system, it be comes 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 de scribed in the following in connection with Figure 1 where the subscriber ter 25 minal 400 moves from point 1 towards point 3. The handover is thus per formed 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 30 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 35 ter receiving the handover command, the subscriber terminal transmits access bursts on its traffic channel to the base station 140 operating as the target WO 99/01002 PCT/F198/00520 7 base station. After the handover the subscriber terminal 400 transmits an ac knowledgement of the successful handover to the base station controller 300, the acknowledgement passing via the base station 140. The subscriber termi nal 400 and the base station 140 use the traffic channel (TCH) in the above 5 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 com prises means 250 which are preferably operatively connected to the base sta 10 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 sub scriber terminal 400 can, however, transfer in accordance with a signal 450 in 15 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 hando ver reference signal which is forwarded to the base station 110. The base sta 20 tion controller 300 commands the subscriber terminal 400 to perform the han dover, whereupon the base station 110 transmits the handover reference sig nal 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 25 mission method of the invention, the means 250 select a bit pattern which de viates 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 011 XXXX bit pattern as the handover reference signal. The 30 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 ac cess 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 35 cess bursts received from the RACH channel to be detected and to be filtered.

WO 99/01002 PCT/F198/00520 8 In practice, the base station controller 300 commands the sub scriber terminal to perform the handover. The means 250 generate the hando ver reference signal which is transmitted to a radio path. When the subscriber terminal 400 receives the reference signal the received handover reference 5 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 10 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 15 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 20 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 25 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 30 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 35 command signalling message to the subscriber terminal 400. In the data transmission method of the invention the base station controller 300 generates WO 99/01002 PCT/F198/00520 9 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 hando ver reference value. The subscriber terminal 400 uses the handover reference 5 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 fre quency of the random access bursts transmitted by the subscriber terminal 400. The subscriber terminal transmits random access bursts typically on the 10 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 15 value in successive RACH channel frames are detected in the event of a han dover 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 20 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. 25 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 ad vance parameter is used as a transmission advance parameter which de scribes the distance between the subscriber terminal 400 and the base station. 30 In the radio system of the figure, the means 260 measure the random refer ence 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 35 handover signal supplied to the RACH channel when the handover signal comprises a handover reference signal.

WO 99/01002 PCT/FI98/00520 10 Although the invention has been described in the above with refer ence 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 5 claims.

Claims

1. A data transmission method to be used during handover in a ra dio system comprising a number of transceivers (110, 120, 130, 140) and at least one subscriber terminal (400) which transmits a number of access bursts 5 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 al location of a channel to be used for the connection, c h a r a c t e r i z e d in 10 that bursts received from the random access channel by the transceiver are measured, a handover reference signal which deviates from the random ac cess burst is transmitted to the transceiver during handover, and 15 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, c h a r a c t e r i z e d in that during handover, the handover reference signal which devi ates from the random access burst is transmitted to the transceiver on the 20 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, c h a r a c t e r i z e d in that the handover reference signal is filtered off when the base station receives the handover reference signal from the random access chan 25 nel.

4. A data transmission method as claimed in claim 1, c h a r a c t e r i z e d in that the handover reference signal is transmitted to the sub scriber terminal which echoes the handover reference signal received back to the transceiver. 30

5. A data transmission method as claimed in claim 1, c h a r a c t e r i z e d 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, c h a r a c 35 t e r i z e d in that the handover reference signal which comprises a WO 99/01002 PCT/F198/00520 12 011OOXXX 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, c h a r a c t e r i z e d in that in practice the transceiver used in the method is a base sta 5 tion.

8. A data transmission method as claimed in claim 1, c h a r a c t e r i z e d in that the random access channel is measured uninterruptedly.

9. A data transmission method as claimed in claim 1, c h a r a c t e r i z e d in that the signal is filtered at the base station or the base station 10 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 15 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, c h a r a c t e r i z e d in that the radio system comprises means (260) for measuring the bursts received from the random 20 access channel by the transceiver, means (250) for generating a handover reference signal which de viates 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 25 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 30 it is possible for the means (260) to filter off the handover reference signal re ceived.

12. A radio system as claimed in claim 10, c h a r a c t e r i z e d 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 35 means (260) filter off the handover reference signal received if the transceiver receives the handover reference signal from its random access channel. WO 99/01002 PCT/FI98/00520 13

13. A radio system as claimed in claim 10, c h a r a c t e r i z e d in that the means (260) filter off the handover reference signal when the base station receives the handover reference signal from the random access chan nel. 5

14. A radio system as claimed in claim 10, c h a r a c t e r i z e d 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, c h a r a c t e r i z e d in 10 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, c h a r a c t e r i z e d in that the handover reference signal generated by the means (250) comprises a 15 011 OOXXX or a 011 1XXXX bit pattern where the X-bit is a 'don't care bit'.

17. A radio system as claimed in claim 10, c h a r a c t e r i z e d in that in practice the transceiver in the radio system is a base station.

18. A radio system as claimed in claim 10, c h a r a c t e r i z e d in that the means (260) measure the random access channel uninterruptedly.