EP0760193A1

EP0760193A1 - Method and apparatus for rapid handover in a mobile radio system in the presence of abrupt radio signal attenuation

Info

Publication number: EP0760193A1
Application number: EP95920043A
Authority: EP
Inventors: Valentina Fidanza; Giacomo Premoli
Original assignee: Italtel SpA
Current assignee: Italtel SpA
Priority date: 1994-05-19
Filing date: 1995-05-16
Publication date: 1997-03-05
Also published as: ITMI941009A1; IT1269794B; ZA954052B; ITMI941009A0; AU2564595A; CN1152991A; WO1995032593A1

Abstract

Procedure and circuit for advance handover in mobile radio systems in the presence of abrupt radio signal attenuation and associated circuit particularly useful in urban microcellular environments. The procedure consists of an appropriate digital filtering of the short-term mean values (approximately 0.5 seconds) of level samples either measured downlink by a mobile apparatus or sent uplink to the serving station. Said filtering is implemented by applying a FIR methodology to the mean input values expressed in dBm. The filtered values are compared with a predetermined threshold, the exceeding of which brings advance emission of a level-caused handover request between the mobile and a preferred adjacent cell (target). When the threshold is not exceeded, the criteria normally provided for handover continue to apply.

Description

METHOD AND APPARATUS FOR RAPID HANDOVER IN A MOBILE RADIO SYSTEM IN THE PRESENCE OF ABRUPT RADIO SIGNAL ATTENUATION

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DESCRIPTION The present invention relates to the field of mobile radio systems and in particular to a procedure and circuit for advance handover in mobile radio systems in the presence of abrupt radio signal attenuation.

As known, in the connection between a mobile radio telephone handset and a base radio station, the radio signal is subject to attenuation for many different reasons. An exhaustive discussion of the problem concerning such attenuation is given in ETSI Recommendation GSM 03.30, version 4.1.0 dated January 1993. In appendix B of this specification are given empirical formulas for calculation of the path attenuation in different mobile radio zones such as for example rural, suburban or urban areas and also distinguishing between medium-large, small or very small cells. The above mentioned formulas are represented by algebraic sums of logarithmic terms, each of which allows for a respective cause to which the attenuation can be attributed. The main causes are e.g.. the distance between the antennas, their height above ground, the presence of obstacles on the ground which could mask the signal, the characteristics of the ground, the speed of the mobile unit and propagation by multiple paths, etc. The latter gives rise to an attenuation known as Rayleigh's fading which is essentially probabilistic in nature and appears and disappears very rapidly.

As a result of the above, it appears clear that in the course of a telephone conversation there can occur variations in the attenuation of signals for the reasons mentioned above. Sometimes these variations are very abrupt and persistent, e.g.. when in an urban microcell a mobile unit turns the comer of a building or when a user enters a building. In these cases the intelligibility of the conversation underway fails untill exhausting all possibility of recovery of the mobile radio system, and cutoff of the connection is then inevitable. As known, advanced mobile radio systems have different means for ensuring good quality of the radio connection between users during their moving about. Examples of these means are: the redundancy introduced in voice signal coding, gradual control of transmitter power, and lastly handover, i.e. the assignment of the mobile unit to another base station. It is also necessary to premise that, where it appears reasonable, in some mobile radio zones, is used a space diversity reception system within the base radio station, the primary purpose of which is to remedy Rayleigh's fading. However, even in the absence of diversity receivers, this type of fading tends to be eliminated from the radio signal by means of an observation of the signal for a time much longer than the most likely duration of similar attenuation. As concerns the GSM system, the observation time is approximately one-half second, during which is executed a "short-term" mean on 104 level samples. The affirmation that Rayleigh's fading tends to disappear in the 5 averaged signal is demonstrated e.g. in the article "A model of analyzing Handoff algorithms" by Holtzmann and Vjayn published in IEEE Transactions on Vehicular Technology, vol. 42, No. 3, August 1993.

In modern mobile radio systems the decision to perform a handover is well weighted, first, to perform a handover where it is actually needed and, second, to

10. avoid overloading the system with excessive signalling. In any case the emission of the commands for power control or for performance of a handover are the fruit of appropriate processing performed on the measured values of some physical magnitudes, which characterize the radio signal in both transmission directions and which are typically: transmission level, quality and delay.

15 This processing, as concerns the GSM, consists mainly of: calculation of the short term mean of the measured values; memorization of the 32 most recent values of these means, to obtain information on the average trend of the respective magnitudes; comparison with appropriate thresholds whose passing is subjected to a persistence of P times out of N before a power control or a handover request are 0 emitted; and lastly the preparation of a list of preferred cells, in order of priority, with which the mobile unit can make a handover. For all this are provided numerous engineerable system parameters, among which the values of P and N, which are loaded in the initialization phase of the base radio stations. P and N are drawn with experimental criteria having the objective of slowing the handover 5 requests without thereby worsening the quality of the radio connection.

Taken together, the above remarks lead to the conclusion that the mobile radio systems of advanced conception are essentially prudent - hence slow - in deciding a handover and therefore they are not capable of quickly recovering abrupt and persistent attenuation of the radio signal. 0 The nature of the shortcoming mentioned is better clarified by analyzing the behaviour of a traditional system faced with a fast attenuation lasting a few seconds. This type of attenuation is e.g. that caused by a sudden masking of the signal by an obstacle. When this happens the system notices it because the signal falls below a minimum reasonable intervention threshold. Since as mentioned the 5 attenuation is fast, it is also very probable that the signal will go beyond this threshold even in the first phases of the attenuation, and then continue to weaken more or less at the same speed before the system can take the necessary measures. Once taken, the measures are more often than not ineffective since the signal has in the meantime degraded so far that the command for performance of a handover is not deciphered by the mobile unit. In this case the connection is unavoidably destined to cutoff but not without having first subjected the users to severe listening inconvenience. In certain mobile radio zones, such as for example those of urban microcells, the probability of abrupt attenuation of the radio signal is greater because of masking by obstacles. Allowing for the fact that in urban microcells handovers occur with greater frequency than elsewhere, it can be seen how these zones are also the most severely penalized by the excessive slowness of the system in deciding handovers.

A first method allowing a mobile radio system to govern the handovers in an urban microcellular zone, while keeping unchanged its performance in a macrocellular zone, is described in International patent application PCT No. WO 91/19403. The method described therein operates on the measurements of magnitudes useful for obtaining handover criteria and obtaining two different handover indicators placed in mutual OR, to generate a single handover command. A first handover indicator is obtained by adding to a constant called "first hysteresis margin" a long-term average of the values measured by the mobile unit in the serving cell. The sum thus obtained is in turn compared with that of the same number of long-term averages of the values measured by the mobile unit in N adjacent cells, with the passage from positive to negative values in the outcome of the comparison being interpreted each time as a true logical value at the OR input for the first handover indicator. As concerns the second handover indicator, one proceeds by analogy as for the first. In this case however the average of the values measured by the mobile unit in the serving cell is calculated in the short-term, and the "second hysteresis margin" is relatively large if compared with the first. The comparisons are made with the short-term averages for the same N adjacent cells synchronously in relation to the preceding comparisons.

It can be immediately inferred that the method described does not produce a handover criterion, because of the excessively low level received by the mobile unit, but rather it chooses an adjacent cell capable of better serving the mobile unit. This function is comparable to that known as "power budget" in the GSM mobile radio system.

The greatest shortcoming imputable to this first method is that in particular circumstances it is entirely ineffective. If there occurrs a sudden drop in the level of the signal received by the mobile unit in the serving cell, and at the same time the signals coming from the adjacent cells are perceived by the mobile unit with levels lower than those of the attenuated signal, the handover is not commanded. Then when the mobile unit draws near to an adjacent cell and the level of the signal therefrom rises and causes a handover criterion, it could happen that in the meantime the signal of the serving cell is so attenuated that the mobile unit is no longer able to perceive the handover command. A second method allowing advance handover decision, before the quality of the connection in a mobile radio system between a mobile unit and its own serving cell becomes too poor, is described in European patent application No. 91850066.1. The method described therein generates a prediction of future measurement values of magnitudes useful for the handover decisions. It is characterized in that at discrete time instants, represented by integers n, some measurement values s(n) are introduced in a predictor comprising an adaptive filter F1, whose coefficients C(n) are used to produce estimates s(n+L|n) of future measurements s(n+L|n) in n+L instants, on the basis of the values effectively measured. A first serious shortcoming of this second method is due to the complexity of the calculations required for the predictor. Considering the already considerable complexity of the processing presently necessary for managing radio resources, the possible addition of a predictor for every voice channel could excessively penalize the real time of the processors. A second shortcoming is that in the presence of an abrupt attenuation of the signal followed by an equally rapid rise, the predictive filter could deceive the system and induce it to request an unnecessary handover.

Accordingly the purpose of the present invention is to overcome the above mentioned shortcomings and indicate a procedure for advance handover in mobile radio systems in the presence of abrupt and persistent attenuation of the radio signals and particularly useful in urban microcellular zones and others.

To achieve this purpose the subject matter of the present invention is a procedure for advance handover in mobile radio systems in the presence of abrupt attenuation of the radio signal in the connection between a mobile unit and a base radio station of a serving cell. This procedure consists of appropriate digital filtering designed to identify strong and persistent attenuation gradients in the middle and short term mean values of radio signal level samples.

The digitally filtered values are then compared with a predetermined threshold, exceeding which produces advance emission of a handover request, because of the excessively low level of the received signal, called hereinafter also level-caused handover, to be performed between the mobile unit and an adjacent preferred cell. If the threshold is not exceeded, the criteria normally provided for handover performance remain in force as better described in claims 1 to 9. Another subject matter of the present invention is a circuit for advance handover in mobile radio systems in the presence of abrupt attenuation of the radio signal as described in claims 10 to 19.

The above mentioned digital filtering is preferably performed in the base radio station of the serving cell on the level values measured by the mobile unit and concerning the signal sent downlink to the mobile unit by the serving station. This is justified by the fact that the handover command reaches the mobile along that link. Any filtering also performed on the level mean values of samples, of the signal sent uplink by the mobile unit , could be advantageous to reduce the performance times of the handover if the attenuation should appear predominantly on this slope.

Introduction in a mobile radio system of the procedure or circuit which is the subject matter of the present invention, brings several advantages, a first of which is that of allowing the system to drastically reduce the time necessary for reaching a level-caused handover decision, in the presence of strong and persistent attenuation of the radio signal. This is due to the fact that the comparison with a threshold does not take place when the level has fallen to a minimum value, contrary to what occurs in the prior art, but takes place in the initial phase of the attenuation, and this allows the system to act promptly before the signal is too degraded. In the GSM system for example, the transmitter power control phase would be completely omitted as well as the subsequent multiple comparison P/N with a low threshold for emission of a handover request. This first advantage is obtained without causing handover requests, where in reality there would be no need thereof in the absence of the procedure which is the subject matter of the present invention.

A second advantage is that of avoiding in advance the cutoff of calls due to failure in performance of the handovers, upon occurrence of the above mentioned attenuation. This is all the more appreciable in urban microcells, where as mentioned above attenuation due to masking of the signal is more probable and handover frequency is greater.

A third advantage (for the user) is that of always ensuring good intelligibility of the conversation, regardless of the particular mobile radio zone traversed.

The advantages as a whole introduced do not at all affect the normal ability of the system to manage handovers under more normal attenuation conditions. Further purposes and advantages of the present invention are clarified in the detailed description of an embodiment thereof, given below by way of nonlimiting example with reference to the annexed drawing wherein: the only figure (FIG. 1) shows a block diagram of the advance handover circuit which is the subject matter of the present invention, useful also for understanding the procedure which is also the subject matter of the present invention.

With reference to FIG. 1, FLT indicates a digital filter of the fifth order having finite pulse response (FIR), in which aO to a5 are constant coefficients, T are identical 480ms delay elements, P0 to P5 are multipliers, and Σ is a 6-input summation block. The structure and design procedures of this class of filters has been long known and well described in the specialized literature to which reference is made. At the input of the filter FLT arrives a digital signal C(nT) which traverses the filter and is found at the output as a filtered signal CF(nT). The latter reaches the input of a comparator CMP where it is compared with a threshold value Δ. The decision whether or not to advance a handover request for a mobile apparatus in question depends on the outcome of this comparison.

The effects resulting from application of the procedure or circuit which are the subject matter of the present invention are now discussed with reference to the non-limiting case of application in a GSM system. As concerns generation of the digital signal C(nT), for the sake of simplicity, illustration of the architecture and operation of the GSM, to which reference is made, is not given here. It is recalled however that the mobile units and base radio stations are equipped with respective means of: reception and transmission, modulation and demodulation of a radio earner, synchronization, sampling and codification of the voice signal, generation or extraction of voice channels and signalling, decoding of voice samples, mixing of the level of the decoded voice samples and other physical magnitudes necessary for control of the handovers and operation in general, as well as microprocessors for processing said measurements, and for generation and/or interpretation of the signalling messages such as e.g. MEASUREMENT_RESULT, CHANGE_MS/BS_POWER,

CONDITION_FOR_INTER/INTRA_CELL_HANDOVER, HANDOVER_COMMAND, HANDOVER_COMPLETE, etc.

It is recalled that the MEASUREMENT_RESULT messages include the fields containing the values of the transmission messages performed by the mobile units and by the base radio station of the serving cell, in relation to every connected mobile unit, in the course of the respective originated or terminated calls. The cadence of these messages is 480ms. In particular the entirety of the fields D, TEI and CHANNEL_NBR constitute an identification of a mobile unit to which is referred a sequence of fields included between said identification and the immediately following one. Within each sequence, the field RXLEV_FULL_DL contains a mean value expressed in dBm and calculated on 104 signal level samples measured by the mobile unit. The field RXLEV_FULL_UL includes an equivalent average value of samples of the signal measured by the base radio station. The message CONDITION_FOR_INTER_CELL_HANDOVER is a handover request message for a given mobile unit generated in the base radio station by a microprocessor, which carries out on the various fields of the message of MEASUREMENT_RESULT the specific processing mentioned in the introduction. This message is sent to a processor also included in the base radio station and which manages transmission resources on the basis of user mobility. The message CONDITION_FOR_INTER_CELL_HANDOVER includes a command which the above mentioned processor must send to the various entities involved, so that the mobile unit in question can execute an intercell handover with a preferred cell, indicated in a list updated every 480ms, and also included in the message. The message HANDOVER_COMMAND is the one which is sent through the base radio station to the mobile unit to initiate a handover.

With the above clarifications, the condition which must occur to anticipate a handover request caused by an excessively low level of the signal received from the mobile unit, is given by the following expression: m CF(nT) = Σ aj.C((n-j)T) < Δ (1) j=0 where C(nT), by j=0, is the content of the field RXLEV_FULL_DL of the present message MEASUREMENT_RESULT, while for the other values of the index j is an equivalent content referred to a respective j^,h MEASUREMENT_RESULT message preceding by j periods T the present one. The value of CF(nT) is expressed in dBm. As concerns the form of the filter FLT, it would be possible in a first analysis to model it as an ideal derivator because a slope must be discriminated in the behaviour of the digital signal at the filter input. As known, in the presence of a constant slope of the input signal, the output of the ideal derivator is a constant whose value is a measure of said slope. Since what it is sought to discriminate is attenuation, the value of the threshold Δ must necessarily be negative. For the filter FLT, as explained below, a form similar to that of a pseudo-derivator would be a better choice, since it reacts with not excessively steep edges after abrupt changes of slope in the input signal. As concerns the values to be assigned to m, aj and Δ in the expression (1), a rather beneficial way is certainly that of taking them with experimental criteria, while always remembering that the mutual relationships between the coefficients aj are established by the peculiarities of the pseudo-derivator. It is also preferable to take different series of m, aj and Δ, each being suitable for a particular mobile radio zone, as engineerable parameters of the system. In particular the value of m, on which depends the response time of the filter FLT, grows smaller as the slope, it is desired to discriminate, grows steeper. In the different mobile radio zones there can be found, in the medium and short term mean values, attenuation slopes which range approximately from a minimum of 15dB in 4 seconds to a maximum of 30dB in 1.5 seconds. A series found suitable for an urban microcell in which the strong attenuation slopes more likely have values of at least 30dB in 3 second is, for example, the following: m = 5; aO = 3; a1 = 2; a2 = 1; a3 = -1; a4 = -2; a5 = -3; Δ = -120dBm. From expression (1) comes out the rather original fact of having activated a digital filtering of the FIR type on a sequence of level logarithmic samples. The filtering FIR is a typically linear operation which then assumes a linearity in the input samples. However, the introduction in the filter FLT of logarithmic samples brings a drastic compression in the dynamics of the input levels, to the benefit of greater simplicity of the filter and enhanced capability to discriminate strong attenuation gradients. This constitutes a doubtless advantage, because it allows detecting the rise of an abrupt attenuation already in its initial phase, i.e. before the level is lowered too far.

Another advantage, which derives from the fact that the filter FLT is a pseudo-derivator, is that of making the circuit which is the subject matter of the present invention immune from generating false advances of the handover request especially in specific situations, so that, in the absence of the filter FLT, there would be no need thereof. The above mentioned situations occur for example when in the radio signal there enter strong but short-lived attenuations, i.e. of a duration such as to not allow the level to fall below the threshold, so that, in the absence of the filter FLT, the system would have commanded a handover.

A reasonable extension of the circuit described is that of introducing in the filter FLT linear input signals. In this case it is necessary to find new values for m, aj and Δ of expression (1), while, as concerns the transfer function, that of a pseudo-derivator can still be used advantageously.

The emission of a command to execute a handover, in response to a level-caused handover request, is generally granted by the system in a timely manner so that the command is received by the mobile unit before the radio signal has become too weak.

In the case of the non-limiting example, calculation of the expression (1) for all the mobile units connected to a base radio station and advance emission of handover request messages are duties entrusted to a microprocessor belonging to the radio base station, specialized in the performance of arithmetic operations in real time, and already used for processing the MEASUREMENT_RESULT messages. The procedures for use of said microprocessor, before those introduced additionally by the present invention, are described in Italian patent application Ml 93 A 001267 dated 15 JUNE 1993 in the name of the same applicant. It is recalled here that said use provides that upon reception of a new MEASUREMENT_RESULT message, i.e. every 480ms, there are memorized all the fields regarding transmission measurements for all the connected users and that there is completed a processing step on each of said fields. The additional programmes necessary for implementation of the invention which is the subject matter of the present invention consist essentially of a routine which cyclically calculates the expression (1) at each new MEASUREMENT_RESULT message in relation to all the mobile units of the serving cell for which a call is under way. Upon occurrence, for a given mobile unit, of the condition CF (nT) < Δ there is provided in the above mentioned routine a call to a pre-existing routine which emits the

COND!TION_FORJNTER_CELL_HANDOVER message for that mobile unit. Calculation of the expression (1) is not especially complicated, and therefore it can be readily performed in a base station of any mobile radio system upon arrival of the signal carrying the transmission measurements of all the mobile units connected, without thereby saturating the real time of the assigned processors.

Summarizing, in the example described there is used a microprocessor belonging to the base station to implement a group of digital filters FLT and respective comparators CMP capable of detecting cases of attenuation for which the handover request messages for the connected mobile units have to be anticipated. The mobile units send to the base radio station calculated mean values C(nT) inserted in the respective RXLEV_FULL_DL fields of the MEASUREMENT_RESULT message, on the basis of signal samples measured by themselves.

Explained below are two variations in the circuit which is the subject matter of the present invention, and from the explanation of which are obtainable by analogy as many variations suitable for the procedure which is also the subject matter of the present invention.

A first variation consists of the fact that the above mentioned microprocessor belonging to the base station is used as follows: - to implement first digital filters FLT and first comparators CMP designed to supply first criteria for anticipating as many handover request messages for connected mobile units, which send to the base station calculated mean values C(nT), inserted in respective RXLEV_FULL_DL fields of the MEASUREMENT_RESULT message, taking signal samples measured by themselves; to implement second digital filters FLT and second comparators CMP designed to supply second criteria for anticipating as many handover request messages for connected mobile units, the mean values C(nT) reaching the input of the second filters being calculated by said base radio station and inserted in the RXLEV_FULL_UL fields of the MEASUREMENT_RESULT message on the basis of signal samples measured by itself; to complete logical OR operations between the above mentioned first and second advance criteria, to obtain additional criteria suitable for anticipating in the above said manner respective handover request messages. A second variation consists of using a microprocessor belonging to a generic mobile unit to implement the digital filter FLT and the comparator CMP. The mean values C(nT) reaching the input of the filter are calculated by the mobile unit and inserted by it in a respective RXLEV_FULL_DL field of the MEASUREMENT_RESULT message on the basis of signal samples measured by itself. At the output of the comparator can be read a criterion for anticipating a handover request message which is sent to the base station by means of a special field of the MEASUREMENT_RESULT message.

As may be seen from the description of the embodiment and its variations, and for each connected mobile unit, the filter FLT and the comparator CMP act "in parallel" with the normal processing carried out by the system on the various fields of the MEASUREMENT_RESULT message, because for their implementation there were arranged additional programmes without modifying significantly the pre-existing ones. This means that, until a handover request is anticipated, the system continues to complete the processing normally provided for execution of the handovers, as though filter and comparator did not exist.

In accordance with the above remarks those skilled in the art will be capable of implementing the programme routines necessary for implementation of the present invention and its variations.

Claims

CLAIMS 1. Procedure for advance handover in mobile radio systems, in the presence of abrupt attenuation of the radio signal, in connections between mobile units and a base station of a serving cell, said mobile units and said base station being respectively equipped with: means of measurement of physical magnitudes characteristic of radio signals; means for calculation of mean values of voice sample level measurements within a period T of brief duration, indicatively but not limited to 480 ms in the GSM system, such as to neutralize abrupt but only slightly persistent attenuation; means of generation or interpretation of messages sent or received through signalling channels; and means of execution of the handovers; said base station also comprising processing means which, on the basis of said measurements, detect handover conditions for the mobile units connected thereto, and for which to emit respective handover request messages; characterized in that, for each connected mobile unit, the said mean values of the level measurements are subject to the above mentioned processing which detect said handover conditions, and in parallel to a digital filtering apt to detect strong and persistent slopes in the said abrupt radio signal attenuation, to obtain filtered mean values which are compared with a threshold, whose passing brings sudden emission of a respective handover request message for the connected mobile unit.

2. Procedure for advance handover in accordance with claim 1 , characterized in that said digital filtering and said comparison with a threshold are executed on the basis of the following expression: m CF(nT) = Σ aj.C((n-j)T) < Δ j=0 where C((n-j)T) is a mean value of the level measurements which precedes by j periods T the present one C(nT), aj are constant coefficients, CF(nT) is a present filtered mean value, Δ is a negative value constituting said threshold, m is a parameter which determines the response time of the filtering operations, said expression used for filtering being similar to that of a digital pseudo-derivator having finite pulse response (FIR).

3. Procedure for advance handover in accordance with claim 2, characterized in that said mean values C((n-j)T), CF(nT) and said threshold Δ are logarithmic values and are preferably expressed in dBm.

4. Procedure for advance handover in accordance with claim 2, characterized in that, by choosing appropriate values of said parameter m, said coefficients aj and said threshold Δ, said expression discriminates strong and persistent attenuation slopes which range indicatively from a minimum of 15dB in 4 seconds to a maximum of 30dB in 1.5 seconds.

5. Procedure for advance handover in accordance with claim 2, characterized in that, by choosing the following values: m = 5; aO = 3; a1 = 2; a2 = 1 ; a3 = -1 ; a4 = -2; a5 = -3; Δ = -120dBm and said mean values C((n-j)T), CF(nT) expressed in dBm, said expression discriminates strong attenuation slopes with constant values of 30dB in 3 seconds.

6. Procedure for advance handover in accordance with claim 2, characterized in that the values of said coefficients aj, said parameter m and said threshold Δ are system parameters memorized in the initialization phase.

7. Procedure for advance handover in accordance with any of claims 1 to 6, characterized in that for each of said connected mobile units, said digital filtering is executed within said base station on said mean values of the level measurements calculated by the mobile unit on the basis of samples measured by itself, said mean values calculated by the mobile unit being sent to said base radio station by means of a said signalling channel.

8. Procedure for advance handover in accordance with any of claims 1 to 6, characterized in that within said base station, for each said connected mobile unit, there are performed the following operations: - a first of said digital filterings on said mean values of the level measurements, calculated by the mobile unit on the basis of samples measured by itself, said mean values calculated by the mobile unit being sent to said base radio station by means of a said signalling channel, a second of said digital filterings on said mean values of the level measurements, calculated by said base station, on the basis of samples measured by itself, a comparison of the values obtained by means of said first filtering with a first of said thresholds, a comparison of the values obtained by means of said second filtering with a second of said thresholds, and a logical OR operation between the conditions of passing of said first and second threshold and obtaining a criterion for said sudden emission of said handover request messages.

9. Procedure for advance handover in accordance with any of claims 1 to 6, characterized in that said digital filtering is executed by the mobile unit on said mean values of the level measurements calculated thereby on the basis of samples measured by itself; and in that the passing of said threshold constitutes a suitable criterion for said sudden emission of a handover request message, said criterion being sent to said base station by means of a said signalling channel.

10. Circuit for advance handover in mobile radio systems in the presence of abrupt attenuation of the radio signal in connections between mobile units and a base radio station of a serving cell, said mobile units and said station being respectively equipped with: means of measuring physical magnitudes characteristic of radio signals; means of calculating the mean values of the level measurements of voice samples within a period T of brief duration, indicatively but not limited to 480 ms in the GSM system, and such as to neutralize abrupt but only slightly persistent attenuation; means of generation or inteφretation of messages sent or received through signalling channels; means of execution of the handovers; said base station including also processing means which detect, on the basis of said measurements, handover conditions for the mobile units connected thereto, and for which emit respective handover request messages; characterized in that said mean values of the level measurements simultaneously reach said processing means included in the base radio station and means which discriminate strong and persistent slopes in said abrupt attenuations (FLT.CMP), and obtain therefor an equal number of discrimination criteria suitable for the sudden emission of said respective handover request messages for the connected mobile units.

11. Circuit for advance handover in accordance with claim 10, characterized in that said means designed to discriminate strong and persistent slopes in said abrupt attenuations comprise: digital filters (FLT) which act on the basis of the following first expression: m CF(nT) = Σ a_j.C((n-j)T) j=0 where C((n-j)T) is a said mean value of the level measurements which precedes by j periods T the present one C(nT), aj are constant coefficients, CF(nT) is a present filtered mean value, m is a parameter which determines the response time of the filter, said first expression being similar to that of a digital pseudo-derivator having finite impulse response(FIR), and comparators (CMP) which compare the outgoing values CF(nT) of said digital filters (FLT) with respective predetermined negative Δ thresholds on the basis of the following second expression: CF(nT) < Δ; a true logic value of said second expression discriminating a said strong attenuation slope.

12. Circuit for advance handover in accordance with claim 11 , characterized in that said mean values C((n-j)T), CF(nT) and the values of said thresholds Δ are logarithmic and preferably expressed in dBm.

13. Circuit for advance handover in accordance with claim 11 , characterized in that by choosing appropriate values of said coefficients aj, said parameter m and said threshold Δ, said digital filters (FLT) and said comparators (CMP) discriminate strong and persistent slopes of said abrupt attenuations which range indicatively from a minimum of 15dB in 4 seconds to a maximum of 30dB in 1.5 seconds.

14. Circuit for advance handover in accordance with claim 11 , characterized in that by choosing in said first expression the following values: m = 5; aO = 3; a1 = 2; a2 = 1; a3 = -1; a4 = -2; a5 = -3, with said mean values C((n-j)T), CF(nT) expressed in dBm, and in said second expression the threshold Δ = -120dBm, said digital filters (FLT) and said comparators (CMP) discriminate strong attenuation slopes with constant values of 30dB in 3 seconds.

15. Circuit for advance handover in accordance with claim 11 , characterized in that the values of said coefficients aj, said parameter m and said thresholds Δ are system parameters memorized in the initialization phase.

16. Circuit for advance handover in accordance with any one of claims 10 to

15, characterized in that said means designed to discriminate strong and persistent slopes in said abrupt attenuations (FLT.CMP) are placed within said base radio station, and thereto arrive said mean values of the level measurements (C(nT)) calculated by the mobile units on the basis of samples measured by themselves, said mean values calculated by the mobile units being sent to said base station by means of said signalling channels.

17. Circuit for advance handover in accordance with any one of claims 10 to 15, characterized in that: said means designed to discriminate strong and persistent slopes in said abrupt attenuation (FLT.CMP) are placed within said base station, to first of said discrimination means arrive said mean values of the level measurements calculated by the mobile units on the basis of samples measured by themselves, said mean values calculated by the mobile units being sent to said base radio station by means of said signalling channels, to second of said discrimination means arrive said mean values of the level measurements calculated by said base radio station on the basis of samples measured by itself, and said station includes also means which calculate logical OR operations between said discrimination criteria obtained from said first and second means respectively, to obtain additional criteria suitable for said sudden emission of said respective handover request messages.

18. Circuit for advance handover in accordance with any one of claims 10 to 15, characterized in that said means designed to discriminate strong and persistent slopes in said abrupt attenuations (FLT.CMP) are placed within said mobile units and act on the basis of said mean values of the level measurements calculated by the mobile units, by taking respective samples measured by themselves, the above mentioned discrimination criteria being transferred to said base station by means of said signalling channels and causing said sudden emission of said respective handover request messages.

19. Circuit for advance handover in accordance with any one of claims 10 to 18, characterized in that said means designed to discriminate strong and persistent slopes in said abrupt attenuation (FLT.CMP) are implemented by means of microprocessor units.