METHOD AND SYSTEM TO DIAGNOSE THE FUNCTIONAL STATE OF RADIO BASE STATIONS IN A DIGITAL TELECOMMUNICATION SYSTEM

Abstract: The invention relates to a method and a system to diagnose the functional state of a radio base station (RFP) in a system of a digital telecommunication system of the type known as DECT system (Digital Enhanced Cordless Telecommunications), that is a system in which the signals are arranged in frames, and the frame is divided in 2 half frames, during the first one of which the stations (RFP) are arranged to monitor the interference level of the radio channels, while they are arranged during the second half frame to pick up the access requests of the mobile stations (PP). According to the invention the method foresees the steps of: predisposing every radio base station (RFP) to tune up to the programmes transmitted by another radio base station (RFP) during said first half frame seeking a synchronism word/field (S) representing the configuration emitted by the radio base stations; if the tuning operation presents a positive result, decoding the field (D) reserved for the data transmission and sending of the decoded station identification data (RFPI) to the central control fixed part (CCFP) for further processing.

Description: "Method and System to diagnose the Functional State of Radio Base Stations in a Digital Telecommunication System" Technical Field The present invention relates to a method for the diagnosis of the functional state of radio base stations, Radio Fixed Parts (RFP) in a digital telecommunication system, and in particular of a access sub-network such as for example a sub-network in DECT technology organized in a plurality of cells. in particular, the proposed method makes it possible to verify the correct functioning of an RFP limited to its functionality at physical layer (PHL) and at protocol layer MAC (Medium Access Control layer). As already known the DECT technology comprises a plurality of fixed radio transceiver stations distributed in a way to cover the interested areas and connected to a switching network, that may be the public one or a switching network of the private type (PABX), and portable user sets adapted to be connected via radio to the fixed stations. Said portable user sets being therefore able to communicate among them and with other fixed users connected to the telephone network. The links may be of the voice type or adapted to exchange data occupying the equivalent of one or more voice "channels". A first structure of a DECT network is schematically illustrated by way of an example in Fig.1. It comprises groups of h (in the example of the figure h = 4) radio base stations RFP (Radio Fixed Part) preferably connected to coppers (in the example 3 coppers) set up by connecting lines L through drop/insert devices DI. Without departing from the field of the invention it turns out to be obviously possible to connect in a stellar way each RFP station to unity CCFP, eliminating the aforesaid connection lines and therefore obviously also the drop/insert device DI. Each fixed station RFP can manage via radio a certain number of PP (Portable Part) users, which are generally mobile. The coppers abut on a series of transcoders TRAS, and this latter is connected, for example, to a public network PSTN (Public Switching Telephone Network) by means of a CCFP (Central Control Fixed Part). Digital 64 Kbit/s channels typically set up the interface between the CCFP and the DECT network regarding the voice signal. To the CCFP unit is associated a series of transcoders TRAS which provide for the conversion of the coding PCM (Pulse Code Modulation) on 8 bits used on the PSTN side in a coding ADPCM (Adapted Differential Pulse Modulation) on 4 bits used on the DECT side. In this way on each 64 Kbit/s occur two 32 Kbit/s channels. The bi-direction connection line with 2,048 Mbit/s, that connects the CCFP to the different fixed stations RFP supports therefore 48 voice channels plus a predetermined number of synchronism and signalling channels. The area covered by a fixed station or cell is commonly rather reduced, arriving also at a coverage with a radius of tens or hundreds of metres. Typical applications of personal telephony systems are found in industrial plants, where they can replace the installation of a cable based network, in commercial or urban centres, where they can favourably replace public telephone boxes, and others. The telecommunications network for personal telephony is of digital type. The radio access belongs to the FDMA - TDMA - TDD category. The system is of TDD type (Time Division Duplex) as transmission and reception do not occur at the same time. More precisely, as illustrated with reference to Fig. 2 the basic frame of 10 milliseconds is divided in 2 halves: usually during the first 5 ms the radio base station transmits and the portables receive (TX DECT half frame for the radio base station and RX DECT for the portable), and during the following 5 ms occurs the contrary. Each half frame is set up by 12 time slots (from where derives that it is a TDMA system of the "full" type (see DECT ETS specification 300 175), each one of which is set up by 420 (or according to the cases 424) bits, the time slots are transmitted or received using 10 (or more or less) different frequencies (from where derives that it is an FDMA system) for a total of 120 (or more) channels available to the system, which are naturally assigned time by time in order not to create interference problems. In the traffic channels of the duplex type (used for example for voice calls), the fixed unit and the mobile unit interact transmitting at the same frequency on 2 time slots spaced out by 5 ms (locally positioned in the first and in the second half frame, respectively). For further details on the DECT system please refer to the specifications published by the European Telecommunications Standards Institute, briefly ETSI. In the DECT systems structured as above specified, there is obviously the technical problem to check the correct functioning of the radio base stations Background Art In the DECT systems of the known type such test operation is realised by means of specific test devices. An example of a dedicated test device is feasible associating an alarm device, both for example to the transmitter and to the receiver of the RFP, and such devices emit an alarm signal whenever they find a bad functioning of the transmitter and/or the receiver. The problem of such solution consists in the fact that such alarm devices have a not negligible cost. Moreover such devices do not provide any solution to problem of monitoring the correct synchronisation of the RFP in question compared to the other RFPs of system. In order to remedy to these drawbacks, devices called "test mobile" have been introduced in the GSM systems as they are associated to each station and they are provided with appliances for the access via radio to the respective station according modalities which are completely coinciding with the access appliances mobile sets are provided with. Each test mobile reaches periodically via radio the respective station to carry out specific tests. If the test result is negative, a message is dispatched to a central unit, which receives in this way an information that there is a bad functioning. This solution, even if it is directly applicable to a DECT system and even if it provides a solution to the problem of testing the correct synchronisation of the station, implies the adding of a substantial hardware to each station increasing in a not negligible way the cost of the entire system. Finally it is necessary to point out that a reliable test element has to be checked at its turn. Object of the invention The object of the present invention is to overcome the above-mentioned drawbacks and limitations, and in particular, to realise a method to check the state of functioning of radio base stations without introducing elements of dedicated test which could weigh in a sensible way on the cost of the entire system. Summary of the invention The invention achieves these objects by means of a method presenting the characteristics pointed out in claim 1 and by means of a system with the characteristics mentioned in claim 9. Further advantageous characteristics are the object of the appended claims. The method according to the invention makes it possible to monitor the state of a radio base station without introducing dedicated test elements, but exploiting stations already installed for service and their operational modality. This makes it possible to improve the quality of service without introducing complications and without additional costs. In particular the proposed method enables the control of the correct functioning of a RFP as far as regards its Physical Layer functionality (PHL) and of the protocol layer MAC (Medium Access Control layer) exploiting the electromagnetic contiguity with the others stations of the same operator. The proposed method, further to result useful for control operations as mentioned before can also be favourably applied by the operator also to obtain a series of useful information for the management of the system, such as: ■ to obtain radio quality indications (as the Bit Error Rate = BER) and the Frame Error Rate = (FER) of the single station; • detect possible asynchronisms among radio base stations at air interface; ■ setting up a map of electromagnetic coverage of the useful system to point out the rising of possible unforeseen/existing obstacles during the installation. Brief description of drawings The invention will now be described in more detailed way with reference to a preferred but not limiting embodiment, illustrated with reference to the enclosed drawings, in which: • Fig. 1 (already described) illustrates the architecture of a DECT system; • Fig. 2 (partially already described) illustrates the structure of the DECT frame and more one in details the information fields of a time slot of the full type; • Fig. 3 it illustrates areas of electromagnetic coverage of a muiticellular system, which is connected in a star-shaped way to the CCFP unit. Detailed Description of a preferred embodiment of the invention Referring to Fig. 2, the structure of a DECT time slot will be synthetically reminded. The time slot comprises a field S reserved to the transmission of synchronism information and a field D reserved to data transmission in a broad sense. Field S is used in particular for the recovery of the phase, and comprises 2 words, in particular a first word set up by a series of bits alternatively with the logical value one or with of the logical value zero for recovery of the clock, and a second word characterised by fact that it has maximum correlation properties. The data field includes at its turn a field A and a field B. Field A is normally used for signalling that is for the transmission of the signalling oriented to the construction, the maintenance and the closing down of the connections. Field B is instead normally assigned to the transmission of real information, that is of the so-called payload. As still illustrated in Fig. 2, field A is at its turn composed of 3 subfield, the first of which is indicated by H and is assigned to the transportation a "header" H, the second, indicated by T or «Tail» is dedicated to the transport of the information content, and the third one is a subfield useful for the definition of the errors known as CRC or cyclic redundancy code. Header H contains a codification expressed by 3 bits, that identifies a respective logical channel, that is concretely carried into the second subfield T, that is to the subfield relative to the content. At last, field CRC has the function to point out if the time by time transmitted logical channel is correct. Between the logical channels carried into the content subfield T it can be mentioned for example channel QT related to the paging information, etc. One of these such logical channels is in particular channel Cs which has a transmission capacity of 2 Kbit s. According to the DECT standard, this channel Cs is assigned to the transmission of high level signalling. In absence of traffic channels (that is of connections set up on one or more PPs) the fixed unit has to activate the transmission of at least one channel called dummy bearer to guarantee dispatching in broadcast of system information necessary for the mobile units to reach the network services. The channel of the dummy bearer type uses a radio carrier in the band of the system and a time slot of the full type in the first half frame. Therefore, a radio base station qualified for the service keeps always at least one radio channel active in transmission. The information transmitted in a radio channel (the traffic one as well as the one of the dummy bearer type) are therefore composed by 3 adjacent fields, respectively: a synchronisation field or field S, a field used for the conveyance of signalling or field A, and a field assigned to the transport of user data or field B. The fields S and A are always present, while field B could miss, as for example in the dummy bearer. Field S transmitted by the fixed unit differs from that transmitted by mobile unit as one is the conjugated one of the other. Among the information transmitted in field A, there is an information called system identity or RFPI (Radio Fixed Part Identity). This information is carried into a logical channel called NT, and it is set up by two parts: an identity portion of the access right or ARI (Access Rights Identity), and an RPN number, the identification of the specific station transmitting this specific information, (Radio Fixed Part Number). During the normal operational functioning, an RFP scans frequency by frequency the time slot of the frame that has free ones (that is which are not yet active in transmission or reception), and this is to identify the possible access requests and to monitor the interference level of the channels. In a geographic area served by an operator, each RFP identifies a cell, that is an area of electromagnetic covering inside of which it becomes visible and accessible for the PPs. A wireless system is defined "muiticellular", if it is composed by more than one RFP (exactly of several cells) which transmit into the logical channel NT identity information identifying the system or the operator, apart from themselves (by means of the parameter RPN). In FIG. 3 the areas of electromagnetic covering of a muiticellular system are illustrated. For asymmetric connections it is possible, that a radio unit transmits (or receives) a useful signal into both half frames. Besides this it is possible to suppose that, for the quality of the service, the contiguous cells present at the limit areas of an electromagnetic coverage excess (that is that there are areas covered/served by more than one station), and that one radio base station is always in electromagnetic visibility with least one confining station. According to the invention, the above-mentioned modality of scanning is used to achieve the mentioned scope, that is the invention foresees a first starting step consisting in predisposing every RFP to receive in the first half frame a signal of the DECT type transmitted by an other RFP of the same system/operator. In fact returning to the examination of Fig. 2, it turns out to be evident, that in the functioning of the system according to the traditional modalities which does not foresee the access of one RFP to other RFPs, every station predisposes itself for the listening of field S presenting the configuration only transmitted by the radiomobile units during the second half frame to identify possible access requests of the mobile parts PP. In the first half frame every station, always according to the traditional operation modalities of the DECT system is limited to the monitoring the degree of interference of the different channels to seek for example the best channel for the transmission of the aforesaid dummy bearer. The invention foresees instead that each one of the RFPs is structured so that it predisposes for the listening of the aforesaid field S (always with the configuration transmitted by radio base stations) also in the first half frame and then to codify field A, and in particular to seek the channel NT, in order to detect and memorise the identity of the RFP station or stations with which it is in electromagnetic visibility. According to the invention the RFP therefore synchronises itself on the logical channel NT emitted by another radio base station and analyses the content. If the identified identity information identifies the same operator/system of belonging, the RFP memorises the identity parameter (RFPI) of the radio base station that it has intercepted. The identity parameters RFPI memorised in this way are transmitted to the control centre CCFP (Central Control Fixed Part), periodically or on request, to be analysed by it. Starting from the analysis of this information the CCFP is able to examine every RFP that is to carry out the tests specified hereafter. Testing if the specific station is suitable to receive correctly. This test/control operation is realisable in an indirect way applying the method of the invention, because if an RFP has synchronised and has decoded the transmitted signal at least from another station it is reasonable to conclude that such type of test has had a positive result. Test if the specific station is suitable to transmit correctly Also this test control operation is realisable in a indirect way by applying the method of the invention, because, if its identity parameters or RFPI has been detected/stored by at least an additional radio fixed part of the system it is reasonable to conclude that such type of test has had a positive result. Test if the specific station is suitable to transmit correctly into the air synchronous frames with the other stations with which it has entered in visibility. Also this test/control operation is realisable in an indirect way applying the method of the invention, because, in a dual way to what said before, it is possible to diagnose a bad functioning of transmitting and receiving or an asynchronism in the air, if the specific station is never in visibility with at least one other RFP and/or is never taken by any RFP of the system. In addition to a diagnosis of the breakdown, the invention makes it also possible to monitor/check the quality parameters of the network and the electromagnetic configuration of a geographic area. Relatively to the quality of the network, the single station can indeed: • estimate its own quality of receiving calculating the bit error probability, Bit Error Rate (BER) and the frame error probability, Frame Error Rate (FER), as the transmitted signal is known (i.e. NT) by the intercepted RFP and the received field level, Received Signal Strength Intensity (RSSI). Premised in fact that it is a priori possible to establish which is the bit or frame error probability when the received field level amounts to a predetermined entity, if the detected BER or the FER has increased in a significant way compared to the predetermined one, it is reasonable to conclude that the proper quality of receiving has considerably degraded, due to the arising for example of a some bad functioning; • value the synchronism level in air with the other RFPs, modifying for example the amplitude of the correlation window (that is the time interval within which the word/ the synchronism field has to be researched). Every RFP on the reception side seeks the correlation word (that is the aforesaid field S) within a time interval said correlation window» which is slightly ampler than the duration of field S proper to compensate phenomena such as the jitter, time instabilities, and propagation delays, that is the shiftiness of the real signal with respect to its ideal characteristics. In optimum conditions, the aforesaid field S is perfectly reduced in the aforesaid correlation window, while in presence of a some synchronisation problems the aforesaid time window may turn out to be to narrow, therefore it is in some cases sufficient to introduce artificially a slight delay/advance of said time window (equal for example to the duration of N bits) to be no more able to correlate with the received signal. . In conclusion the synchronism level in air can be tested according to the invention introducing time by time delays/advances with a predetermined entity and detecting for example how many are the introduced delay bits to define the synchronisation loss. If almost the totality of the RFPs inform the central unit CCFP that they are perfectly centred with respect to the correlation window, while a single RFP communicates that it is synchronous to ±N bits (that it is sufficient to introduce a delay of N bits so that the correlation impulse comes out of the correlation window, it is reasonable to conclude that this last RFP has some synchronism problems in air with the other RFPs due for example to any bad functioning. Relatively to the analysis of a geographic area, the invention makes it possible to put the CCFP data-collecting centre in conditions to: • draw the map of the radio base stations in electromagnetic visibility, supervising for example if obstacles have come up after the installation (for example detecting that two stations previously in visibility have ceased to be in visibility); • draw the map of the stations, which are synchronous among them. The above-mentioned advantages are achieved by using the normal operational modality of the stations, without the additional introduction of specific test points and without the supplementary employment of the radio spectre (in fact the already allocated channels are used to offer the service). Although the invention has been described with particular reference to a preferred embodiment, it should be understood that the present invention is not limited thereto, since other embodiments may be made by those skilled in the art without departing from the scope thereof. It is thus contemplated that the present invention encompasses any and all such embodiments covered by the following claims.

Claims: CLAIMS 1. Method to diagnose the functioning state of a radio base station (RFP) in a digital telecommunication system in which the signals are arranged in frames with a predetermined duration, and every frame is subdivided in a predetermined number of time intervals or time slots, in which each time slot is structured in a way that it comprises at least a field (S) reserved to the transmission of synchronisation information and at least one field (D) reserved to the transmission of data organised in logical channels and including at least one logical channel (NT) reserved to the transmission of system and/or station identity information, said system comprising a central control unit (CCFP), a plurality of mobile sets (PP) and at least two radio base stations (RFP), characterised by fact to foresee the steps of: I. defining the synchronisation of each radio base station (RFP) on the logical channel (NT) reserved for the transmission of system and/or station identity information (RFPI) emitted by another radio base station of the same system/operator; II. verifying if the system identity information identifies the same operator/ system of belonging; III. memorising the system and/or station identity parameter (RPN) of the other radio base station, if the aforesaid verification has positive result; IV. transmitting said memorised identity parameter to the above-mentioned central control unit (CCFP) to undergo further elaboration. 2. Method according to claim 1 , in which said digital system is a system operating according to the DECT standard, that is a system in which said frame is divided in 2 half frames, during the first of which the stations (RFP) predispose for the monitoring the interference level of the channels, while during the second half frame the stations (RFP) predispose to detect the access requests of the aforesaid mobile sets (PP), characterised in that it foresees the steps of: • predisposing every radio base station (RFP) to tune up to the transmissions emitted by another radio base station base (RFP) during said first half frame seeking a synchronism word/field (S) presenting the configuration emitted by the radio base stations; • if the tuning operation has got a positive result, decoding the aforesaid field reserved for data transmission; • repeating the cycle of operations starting from step II. 3. Method according to claims 1 and 2, characterised in that said transmitting operation described in step IV occurs periodically. 4. Method according claims 1 and 2, characterised in that said transmitting operation described in step IV happens on request of said control centre (CCFP). 5. Method according to the previous claims, characterised in that after having carried out the aforesaid step III a further step is carried out consisting in the calculation of its own probability of bit error (BER) and frame error (FER) analysing the data transmitted during said second field (D) by the intercepted radio base station (RFP), the transmitted signal and the level of the received field (RSSI) being known. 6. Method according to the claims from 1 to 4, characterised in that after having carried out the aforesaid step III, a further step is realised which consists in the modification of the time interval within which the aforesaid synchronism word/field (S) has to be sought in order to estimate the synchronisation level in air with the other radio base stations (RFP). 7. Method according to the claims from 1 to 4, characterised in that after having carried out the aforesaid step IV a further step is carried out by the aforesaid central control unit (CCFP), which consists in drawing a map of the radio base stations in electromagnetic visibility. 8. Method according to the claims from 1 to 6, characterised in that after having carried out the aforesaid step IV, a further step is realised by the central control unit (CCFP), which consists in drawing a map of the radio base station which are synchronous among them. 9. System to diagnose the functional state of a radio base station (RFP) in a digital telecommunication system in which the signals are arranged in frames with a predetermined duration, and each frame is subdivided in a predetermined number of time intervals or time slots, and every time slot being structured so that it comprises at least one field (S) reserved to the transmission of synchronisation information and at least one field (D) reserved to the transmission of data arranged in logical channels and including at least one logical channel (NT) reserved to the transmission of system and/or station identity information, said system comprising a central control unit (CCFP), a plurality of mobile sets (PP) and at least two radio base stations (RFP), characterised in that it is structured in such a way to implement the method defined by claim 1.

Inventors: DE BENEDITTIS ROSSELLA, MAGOTTI MATTEO, DE BENEDITTIS, ROSSELLA, MAGOTTI, MATTEO
Applicants: BENEDITTIS ROSSELLA DE, ITALTEL SPA, MAGOTTI MATTEO, DE BENEDITTIS, ROSSELLA, ITALTEL S.P.A., MAGOTTI, MATTEO
Application year: 1999
Main classification: H04Q7
IPC codes: H04B7, H04W24