BASE STATION IDENTIFICATION

Abstract: In identifying base stations in cellular telecommunication, system information is normally sent out on the Broadcast Control Channel, BCCH, taking up the first time slot (B) of a TDMA frame. In case of interference from other base station transmitting on the same frequency or when neighbouring base station use the same identity code, BSIC, another BCCH (b) is transmitted on another time slot. The double BCCH, according to the invention, makes it possible to identify base stations, for example suitable as handover candidates in urban environments, even if they have sent their regular control information on the same frequency or with the same BSIC.

Description: BASE STATION IDENTIFICATION TECHNICAL FIELD OF THE INVENTION The present invention relates to the cellular mobile communication area and in particular how to identify base stations in the radio network environment. BACKGROUND A cellular telephone system is, as the name implies, built up by cells each covering a certain smaller area of a greater territory assigned to an operator giving mobile telecommunication service to its subscribers. An example of such a cell configuration is shown in figure 1. Each cell has a base station communicating with mobile stations occupying the area of the cell. A certain number of radio frequencies are allocated to the operator and a task for him is to optimise his resources in serving his subscribers. As the number of frequencies is limited they must be used efficiently. Depending on the services one or several frequencies are used in a particular cell. None of these frequencies may be used in neighbouring cells because of interference. The reuse distance, that is the distance to another cell area where the same frequency can be used again without interference, is at least the cross-distance of the neighbouring cell, assuming that the radio coverage area of a base station is its own cell area and partly into the neighbouring ones but not beyond that. The sizes of the cells depend on the radio propagation distance and the number of users in the cells. In urban environments, where the number of users can be quite high, the cells tend to get small. Especially with these small cells and also with a complicated topography, big houses, indoor and underground users, etc, the ideal picture of the cell configuration as shown in figure 1 is certainly not true. A more likely situation is depicted in figure 2. As can be seen in the figure the coverage of a base station is not just the area surrounding it. Spots of coverage could be found even far away from the base station. On the other hand there are places close to a base station, which are shadowed for example by a building. In that case transceivers situated further away have to be used for the system to work even in those shadowed places. Such a telephone system is configured, to start with, by making a rough estimation of coverage and placing out the base stations. Thereafter the system is refined by using test mobiles moving around in the area and reporting back measurement results to the system controller. Based on these results the system may be reconfigured. Also the normal users of the system report measurement results. In fact, the whole thing is an ongoing process taking accounts of the dynamic behaviour of the users and the changes in the environment. The goal is to have an optimised system at all times. Another object of the measurements is for the mobile to know which base station to turn to when using the system. Depending on where the mobile is located, a list of suitable frequencies is sent out to the mobile for it to listen to and choose from when it wants to contact the system and making handovers as the mobile, for example, moves into other coverage areas. In a Time Division Multiple Access, TDMA, cellular system like the GSM-system, the information is physically sent out in the form of bursts in short time slots. In GSM eight time slots form a TDMA-frame, which continuously repeats itself during transmission. The type of message sent out on these physical channels categorises the channels as Logical Channels. There are two basic types of logical channels, Control Channels, CCH, for control purposes and Traffic Channels, TCH, which carry the actual payload. One of the standardised control channels is the Broadcasting Control Channel, BCCH, which broadcasts information about the base stations, e.g. its transmitting frequency and the so called Base Station Identity Code, BSIC. The BSIC is composed of just a few bits and may discriminate between the closest neighbouring base stations. The BCCH is sent out at regular intervals in a certain time slot, normally the first (i.e time slot number zero) and with no power reduction. The mobiles are programmed to listen to the BCCHs sent out by neighbouring base stations and report back to the system measurement results concerning signal strength and quality concerning the identified BCCHs. The purpose of the measurements is, as mentioned above, to help refining the system configuration and to facilitate handovers. SUMMARY OF THE INVENTION In urban environment with a great number of base stations in the vicinity of the active mobiles it happens that BCCH-information from different base stations collide, they interfere with each other. If such information from several stations is sent out at the same time and with the same carrier frequency and received on comparable power levels, no useful information may be collected by the mobile receiver. Another problem is when different base stations, not that close to each other, have the same BSIC while sending on the same frequency. In these cases the system is unable to identify the base stations properly and the mobile cannot report any relevant measurement results. According to the present invention the problem is solved by duplicating the BCCH- information and sending it out on another physical channel, i.e. another time slot, and in case of the second problem with another BSIC. Preferably, the old information is still sent out not to disturb the overall traffic plan symmetry set up for the system. The new information could be sent out temporarily as soon as the problems occur or on a more or less regular basis if the problems tend to persist. As the new information occupies an own time slot, thus burdening the accessibility to the system, it should be used to a limited extent. The invention is further defined in the appended claims related to a method and an arrangement. DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic overview of cellular mobile communication system. Figure 2 illustrates how the coverage of radio base stations could look like in an urban environment. Figure 3 describes three problem scenarios. Figure 4 shows a base station with its transceiving equipment. Figure 5 demonstrates how the invention is realised in connection with a time slot diagram. DETAILED DESCRIPTION OF THE FIGURES AND OF PREFERRED EMBODIMENT Figure 1 illustrates a part of a cellular telephone system. A conventional schematic hexagonal ideal structure is shown with a number of equally sized cells C1-C6. Base station transceivers, BS1-BS6, serve mobile users M1-M4 moving around in the area. The base station transceivers, BS1-BS6 are connected to a Mobile Switching Centre, MSC, via Base Station Controllers BSC1 and BSC2. An Operations and Maintenance Centre, OMC, is connected to the MSC and an Operation and Support System, OSS, controls the activities of the OMC. The MSC is connected to a public telephone network PSTN. By way of an example, the coverage of BS5 is shown by the dotted circle 10. The coverage extends slightly into the neighbouring cells C3 and C6 but not into Cl, C2 or C4. The same transmitting carrier frequency as used in C5 can thus be used again in, for example, Cl and C4 without interference. A more realistic picture of the coverage structure is shown in figure 2. Base station mounted on roofs of buildings, have corresponding coverage areas represented by different patterns in the figure. As shown the picture is quite chaotic. Coverage areas are scattered around overlapping each other more or less significantly. At least when the number of base stations increases it is impossible to allocate different frequencies to all of them. An interference problem is thus a fact. Figure 3 describes three different problem scenarios. In fig 3a a mobile M has established a connection with base station BS31 and the mobile regularly reports measurement results relating to signal strength and quality of the neighbouring base stations, BS32-34, via BS31 to the system controller. Assume that the mobile has measured and reported the BSIC/BCCH combination 07/98 from BS 32 (07 is the BSIC-number and 98 the number of the carrier frequency). BS 32 is the closest of the neighbouring base stations and is assumed to have the highest signal strength. However, another neighbouring base station, BS33, has the same identity, 07/98, and the system will not know which base station the information belongs to (BS32 or BS33?). Fig 3b shows another situation. The mobile reported an unknown BSIC x on a certain frequency number 98. In fact it has not reported anything but that, on listening to a certain frequency (number 98), it could not decode any BCCH-information. The explanation is that BS32 and BS33 has sent its BCCH information on the same physical radio channel, which leads to a serious interference. A corresponding situation, shown in figure 3c is when the mobile M tries to listen to a base station, BS32, broadcasting its BCCH on the same frequency as the one, BS31 , it is connected to. This also leads to useless measurement report. In this case, BS32 will also interfere with other communications between the mobile M and its own base station BS31. Figure 4 is a schematic illustration of a base station transceiver, BS. It consists of a cabinet C with the necessary radio equipment inside, including Transceiving Units, TRU 1 and TRU 2. The radio equipment is connected to an antenna A. According to the invention a second BCCH, in addition to the usual one, is transmitted from base stations when there is a risk for problems of the mentioned kind e.g. if the system has registered interference or ambiguous BSICs. A couple of examples are illustrated in figure 5. In fig 5a two frames, 51 and 52, of the TDMA-type are used for broadcasting radio signals on two different carrier frequencies, f 1 and f2. The two tranceiver units, TRU1 and TRU2 are used as means for the transmitting. The' frames are built up by eight time slots each. The time slots are assigned to a number of logical channels and in the first time slot of the first frame the normal BCCH information, B, with belonging BSIC is sent out. In the example the second slot is used for other control information, S, and the last five slots are occupied by traffic channels T. Frame 52 is only used for traffic channels. Suppose the system has received confusing reports regarding the BSIC from a mobile listening to the BCCH in time slot B of frame 51. Then a second BCCH, b, with another BSIC is sent out, according to the example, in the third time slot. The mobile is then ordered to listen to this third time slot of frame 51 of TRU1. After that the mobile will be able to report correct results relating to the base station listened to. By changing the BSIC of the particular base station the system will be able to discriminate between base stations otherwise having the same BSIC. The second BSIC may be a unique code or the combination of the first and second BSICs maybe unique. The described solution takes care of the problem scenario according to figure 3A. Fig 5b shows another example where two TDMA-frames 53 and 54 are sent out on two frequencies f 3 and f4 from the transceiver units TRU3 and TRU4 belonging to a base station. The extra BCCH, b, is here sent out, for example in the first time slot of the second frame, 54. Changing the frequency from f3 to f4 is a way out of the measurement problems caused by interference, c.f figure 3B and C. Base stations sending on different frequencies do not interfere any more. When the system has recorded measurement results from base stations using the same BSIC this code should be amended in the second BCCH b. It is of course possible to change the BSIC in the existing BCCH, but this may cause problems in other parts of the network. When the second BCCH is sent out on a new frequency as in the second example according to fig 5b it will not be necessary to amend the BSIC, because the new combination of frequency and BSIC will normally be enough to discriminate between otherwise confusing BSIC reports. Thus, transmitting the second BCCH on another frequency solves both problem scenarios above. In the examples with two base stations involved, it is sufficient to send out the second BCCH from one of the base stations. In case of more than two stations involved, the second BCCH must be transmitted from all but one of them. Of course, all base stations involved can use the double BCCH. An immediate purpose of the double BCCH method of the invention is to improve the cell planning instruments FOX and NOX to make unambiguous decisions and eliminate the human factor in the decision process. The invention will greatly improve the BCCH optimisation when separate BCCH bands are used. NOX (Neighbour Optimisation eXpert) is an OSS tool that records measurement reports sent out by mobiles. It uses the reports about measured frequencies and BSICs to decide which neighbour relations should be added. FOX (Frequency Optimisation eXpert) uses the measurement reports to calculate how much different cells interfere with each other and makes, on the basis of those calculations, suggestions for frequency changes. The second BCCH is primarily to be activated by the operator when needed. One such case can be right before or after the operator measures an inter cell dependency matrix (ICDM), which purpose is to show which cells can be heard where, but can be incomplete or even wrong if the situations with dubious BSIC/BCCH or strong interference on BCCH occur. It is also possible to do a more intelligent run with the double BCCH if celldata and/or ICDM are analyzed priorily. It is possible to automate this task. Dummy bursts can be sent on remaining timeslots on the extra BCCH-channel but are not required. The merits of the doubled BCCH method are: - It correctly identifies interfering cells. - It can distinguish more than one cell with the same BCCH frequency. - It is fast. - It does not need separate frequencies. - It is simple to implement.

Claims: CLAIMS 1. A method in a cellular mobile telecommunication system for identifying a base station transceiver (BS), said transceiver broadcasting on a first radio channel (BCCH) of a number of such channels, first control information (B) for identifying said transceiver, c h a r a c t e r i s e d in that second control information (b) for identifying said transceiver is sent on a second radio channel. 2. The method in claim 1 in which said first and second identifying control information (B; b) comprise the transmitting frequency number and a code (BSIC) which identifies the base station. 3. The method in claim 2, wherein the system has recognised interfering base stations sending said first control information (B) on the same radio frequency and/or base stations using the same identity code (BSIC), the system prepares at least one of the base stations to send out said second control information (b). 4. The method in claims 1-3 wherein the operator of the system activates transmitting of the second control information (b) when needed. 5. The method in claim 4 wherein the second control information (b) is transmitted when the system has recognised interference and/or colliding identity codes (BSIC) as a result of measurement reports made by the mobile stations (M). 6. An arrangement in a cellular mobile communication system of the Time Division Multiple Access (TDMA) type where radio information is transmitted in the form of bursts in short time slots and wherein consecutive time slots form a TDMA-frame (51- 54), including a system controller (BSC, MSC, OMC, OSS) and base stations (BS) serving mobile stations (M) using the system, comprising means (TRU1) for transmitting regular identity control information (B) in a certain time slot from the base stations (BS) to the mobile stations (M), c h a r a c t e r i s e d in that the arrangement further comprises means (TRU1; TRU2) for transmitting additional identity control information (b) in another time slot. 7. An arrangement of claim 6 wherein the identity information (B; b) comprises the transmitting frequency number and an identity code (BSIC) identifying the base station. 8. An arrangement of claims 6-7 wherein the additional identity information (b) is transmitted in another time slot in the same TDMA-frame (51). 9. An arrangement of claims 6-7 wherein the additional identity information (b) is transmitted in a time slot in another TDMA-frame (54). 10. An arrangement of claims 6-9 wherein the identity code (BSIC) of the additional information (b) is not the same as in the regular control information (B).

Inventors: JANSSON CHRISTIAN, JANSSON, CHRISTIAN
Applicants: ERICSSON TELEFON AB L M, JANSSON CHRISTIAN, JANSSON, CHRISTIAN, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2001
Main classification: H04Q7
IPC codes: H04W48