GB2236458A

GB2236458A - Mobile radio system synchronisation

Info

Publication number: GB2236458A
Application number: GB9017473A
Authority: GB
Inventors: Jan Erik Ake Steinar Dahlin; Walter Ghisler; Bengt Yngve Persson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1989-09-12
Filing date: 1990-08-09
Publication date: 1991-04-03
Anticipated expiration: 2010-08-09
Also published as: CN1050294A; CA2039699C; FR2652468B1; MY106831A; DE4026698C2; KR970002747B1; FR2652468A1; JP2978243B2; AU6353590A; DE4026698A1; SE8902994D0; WO1991004620A1; GB9017473D0; SE464553B; HK58695A; KR920702120A; NZ234720A; JPH04501944A; GB2236458B; CA2039699A1

Abstract

A synchronizing method, in a mobile radio system which operates with Time Division Multiple Access (TDMA) but which lacks global time reference for all base stations and mobiles in the system, is intended for ciphered transmission of data D and speech S in accordance with a given ciphering key (E2). When handing-over a call, the ciphering synchronism is lost. in accordance with the method, subsequent to hand-off (the time t2), the takeover base station (BS2) signals non- ciphered information (S1) to the mobile (MS) disclosing the number of frames (R1-R13), Fig. 4, (not shown) which shall pass until ciphering can again be commenced, calculated from a given frame (R1). Call information is transmitted in the following frame (R2), this information now being ciphered with the aid of a ciphering key (E1) different to the original key. This other ciphering key consists of a periodic bit sequence having a period which is equal to one frame interval. <IMAGE>

Description

SYNCHRONIZING METHOD IN A MOBILE RADIO SYSTEM

TECHNICAL FIELD

The present invention relates to a synchronizing method in a mobile radio system which lacks a global time reference, i.e. a system which lacks a time reference that is common to all mobiles and base stations in the system. More specifically, although not exclusively, the present invention relates to a method of synchronizing a random bit flow -which is superimposed cryptically on the normal traf f ic flow during call transmission or data transmission between the base stations and the mobiles. The proposed method can be applied solely to a so-called TDMA-system, i.e. a mobile radio system in which calls are transferred in frames and time slots by means of time multiplex.

BACKGROUND ART

It is desirable in mobile radio systems, and in TDMA-systems in particular, to be able to maintain the security of calls so as to prevent unauthorized persons from setting-up and making calls over the network. In order to meet this requirement, it has been suggested that the calls are ciphered; see for instance 20 EP-A-273289.

Calls between a base station and a mobile are cipheredby processing the speech message in a scrambler, which functions to convert the speech signals into a random sequence in accordance with a given key. For instance, the speech signals can be superimposed with a random sequence of bits of relatively long duration (several minutes). In this case, the cipher key consists in the knowledge of the total bit pattern of the sequence and also knowledge of the time at which the sequence was commenced. An authorized subscriber can plug into a circuit module in the apparatus which stores this sequence and said starting timepoint, and is thus able to decode incoming calls.

2 DISCLOSURE OF THE INVENTION

Some TDMA mobile radio systems lack a global time-reference, i.e.

means in the mobile telephone exchange which contains a clock common to the overall system and operative to produce the same time reference for all base stations and mobiles in the system.

The purpose of such a common time reference is to enable the mobiles and the base stations to be synchronized with one another in certain frames and time slots when synchronizing from signal ling or data/speech signals falls away for some reason or other.

When transferring (hand-off) a call from one radio channel to another radio channel, synchronization between the mobile and its base station may be lost, since a brief interruption will occur during the actual transmission or swithching of the call. If the call is also ciphered, a further problem can occur as a result of fall-of f of the synchronism of the actual ciphering key concerned, thereby rendering deciphering impossible.

These problems also occur when setting up ciphered calls, although said problems are most pronounced during "hand-off".

The present invention is based on the time sharing in different frames of the traf f ic flow (TDMA-principle) and also on the access to a given ciphering sequence of relatively long duration (about 3 minutes) in relation to the time during which coupling or "hand of f 11 shall take place. The invention is also based on ciphering by superimposing a bit sequence on the normal traf f ic flow (data or speech and signalling). Prior to handing-off a call or during a given time interval during hand-of f where synchronization of the ciphering sequence has ceased, non-ciphered signalling is ef f ected and thereafter the traffic flow is ciphered with a periodic bit sequency whose period is equal to a frame time interval. This enables synchronization data to be transmitted to a mobile from a base station simultaneously with a ciphered traf f ic f low and discloses when normal ciphering shall commence.

3 The object of the present invention is thus to achieve synchronization of a cryptosequence when setting up or "handing-of f 11 calls in a mobile telephone system which lacks a common time reference.

The invention is characterized by the features set forth in the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a schematic illustration of two base stations and a mobile unit; Figure 2 is a time diagram for transmitting and receiving in accordance with the TDMA-principle; Figure 3 is a time diagram for ciphering in accordance with the proposed method; and Figure 4 illustrates, in more detail, signalling during a given time interval in accordance with Figure 3. K1 BEST MODE OF CARRYING OUT THE INVENTION Figure 1 illustrates schematically two base stations BSI and BS2 and a mobile MS which is assumed to move from the base station BSI towards the base station BS2. The base station BS 1 serves traf f ic within the cell Cl and the base station BS2 serves traf f ic within the cell C2. The cells Cl and C2 have a common border G. When the mobile M approaches the border G, the speech quality for a coupled call served by the base station BSI over a given radio channel K1 will fall-off. Switching of a new radio channel K2 to MS from BSI is effected, by measuring the field strength of the radio signal and making calculations in accordance with known principles. This new channel K2 is served by BS2. During the actual switching sequence, which may have a duration of about 100 ms, MS will not receive and the receiving circuits in MS can loose the synchronism of the traf f ic f low, i. e. the time position of frames and time slots from base station BS1.

4 Figure 2 illustrates a transmitting frame and a receiving frame (RX and TX respectively) for the mobile MS.

It is assumed that prior to "hand-of f 11, the traf f ic f low is ciphered with a certain key E2. This key consists of a bit sequence or a section of a longer, random bit sequence E (e.g. about 3 minutes long), which is superimposed on the traf f ic f low through modulo 2-addition bit for bit. As will be understood, the key E is known to the mobile MS when this is authorized to transmit and receive calls, and the base station BS1 transmit data concer- ning the starting time point, i.e. the place in the sequence E at which the bit flow shall commence, i.e. E2 is'known to the mobile MS. This ciphering technique is known in the art.

Figure 3 is a time diagram which illustrates application of the method during "hand-of f 11. It is assumed that the base station BS 1 communicates with the mobile MS and that the communication (the speech) is ciphered by means of a crypto key E2 up to the time point tV Hand-of f takes place at the time t 1 Upon termination of the "hand-ofC process at time t 21 MS has synchronized with the new base station BS2. There is namely transmitted from this base station on a given control channel (SY in Figure 2) during the time t i_t 2 a synchronizing sequence which denotes the time position of the frames and the allotted time slot for the continued call communication between BS2 and MS. This is effected in a known manner by means of a correlation process in MS.

Thus, at the time t=t 21 there is frame synchronization for speech/data transmission between BS2 and MS, but not for the ciphering. At t=t 21 the base station BS2 transmits a non-ciphered signal which denotes when ciphering in accordance with the key E2 shall recommence, i. e. BS2 announces the time point t 3 The traffic flow (speech/data) is ciphered in subsequent frames with a key E1 which consists of a periodic, random bit sequence having a period equal to the frame interval. This enables the mobile MS to count the number of frame intervals (announced by the nonciphered signal) until ciphering in accordance with the key E2 is commenced by BS2. Possibly the ciphering key E1 can consist of zeros only, i. e. the traf f ic f low in subsequent f rames is trans mitted unciphered (interval t 2-t 3). The mobile MS can anyway count the number of frame intervals in spite of absence-of the periodic bit sequence in the key El, due to the presence of frame synchroni zation in the control channel SY from the time point t 2- Figure 4 illustrates the sequence during the time interval t2-t 3 more clearly.

According to Figure 3, at time t 2 the mobile MS is synchronized to the new base station BS2 and the normal traf f ic f low (data, speech, synchronization) should have commenced if no ciphering has been employed. Synchronization with respect to the bit f low in the ciphering key E2 up to the time tl, however, has been lost. The base station therefore sends to MS a signal S1 which indicates how many frames shall pass until ciphering according to key E2 is to be commenced. This frame number is assumed to be 13 in the Figure 4 illustration. This message can be transmitted in a non-ciphered form on the so-called FACCH (Fast Associated Control Channel) formed by a reserved time slot within a frame, see Figure 2. This time slot is thus located in frame RI. During the frames R2, R3, 20... R6 following the frame Rl, the ciphered call information is transmitted and ciphered with the key El. As before mentioned, this key is a periodic bit sequence whose period equals one frame interval and which can be superimposed on the call information by modulo-2 addition. Consequently, the mobile MS is aware of the time at which counting shall commence and also of the number of frame intervals to be counted. When this is understood by the mobile, a confirmation signal Al is transmitted back to the base station BS2 and received in frame R6. When the base station BS2 receives this signal, the station will wait for a period of time which equals the agreed number of frames, i.e. to frame R13 (t=t 3) where ciphering in accordance with the key E2 is commenced.

The base station BS2 thus waits a given length of time (in Figure 4 a time equalling 5 frame intervals) for receipt of the conf irmation signal Al. If, for some reason or other, this signal is not received by the base station BS2 within a given period of time, a 6 signal S2 is again transmitted in frame R7 and a new confirmation signal A2.is, awaited. The signal A1 can fall away because, e.g., of fading or difficult surrounding conditions at precisely the interval in which the signal A1 is transmitted. Thus, the number of frames (=R13) from the base station BS2 should be sufficiently large to enable repeated signalling according to the above to be carried out.

The signal delay between base station and mobile has been assumed to be about 2-3 frame intervals (-_15 ms) in the Figure 4 illustra- tion. This delay should also be taken into account. Thus, the ciphering time point t 3 will preferably be chosen so that t 2-t 3 > 4 times the maximum propagation time. This calculation offers no difficulties, since the mobile is located on the border between two cells Cl, C2 according to Figure 1, i.e. at a maximum distance from a base station.

1 1 1 1 7

Claims

1. A synchronizing method ina mobile radio systojm which lacks a global time reference in which the data messages (D) and the signalling messages (S) between a first base station (BS1) and a mobile station (MS) are transmitted in frames having time slots (TDMA) ciphered by superimposing on said message (D, S) a randowly selected bit flow, wherein upon handing over the communication of the mobile having the first base station (BS1) to a second base station (BS2), ciphering (E2) is interrupted for a given period of time (t i_t 3) from the time of transmitting said messages from the first to the second base stations when synchronization of the random bit flow is lost due to the absence of said time reference, c h a r a c t e r i z e d in that subsequent to completing the hand-over of said messages (D, S), non-ciphered signalling (S1) is effected from the second base station (BS2) to the mobile station (MS) which discloses the time point (t 3) counted in numbers of frame intervals (R2-R13) which shall pass until said ciphering (E2) is recommenced.

2. A method according to claim 1, c h a r a c t e r i z e d in that the messages (D, S) up to said time point (t 3) are transmitted by means of a ciphering sequence (E1) which consists of a periodic bit sequence whose period is equal to a f rame interval.

3. A method according to Claim 1, c h a r a c t e r i z e d in that said signalling (S1) is effected during a frame (R1) which immediately precedes the frames (R2-R6) which transmit said messages (D, S) %and in the time slot assigned to the mobile station (MS) in said frame; and in that the mobile station (MS) when receiving said signalling (S1) transmits a confirmation signal (A1) to the second base station (BS2).

4. A method according to Claim 3, c h a r a c t e r i z e d in that if the second base station (BS2) fails to receive said confirmation signal (A1) subsequent to a given time lapse, further 8 non-ciphered signalling (S2) is effected by a further base station having the same information as the f irst mentioned signalling (S1) so as to obtain a confirmation signal (A2).

5. A synchronizing method, substantially as herein described with reference to the accompanying drawings.

Published 1991 atThe Patent Office. State House. 66171 High Holborn. London WC1R41P. Further copies may be obtained from Sales Branch. Unit

6. Nine Mile Point. Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid, St Mary Cray. Kent.