WO1991004620A1

WO1991004620A1 - Synchronizing method in a mobile radio system

Info

Publication number: WO1991004620A1
Application number: PCT/SE1990/000497
Authority: WO
Inventors: Jan Erik Åke Steinar DHALIN; Walter Ghisler; Bengt Yngve Persson
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1989-09-12
Filing date: 1990-07-19
Publication date: 1991-04-04
Also published as: CN1050294A; AU625469B2; JP2978243B2; GB9017473D0; FR2652468A1; GB2236458A; JPH04501944A; DE4026698C2; MY106831A; GB2236458B; FR2652468B1; SE8902994L; NZ234720A; CN1023283C; KR970002747B1; SE464553B; DE4026698A1; AU6353590A; KR920702120A; SE8902994D0

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. The inventive method is intended for ciphered transmission of data and speech 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 take-over base station (BS2) signals non-ciphered information (S1) to the mobile (MS) disclosing the number of frames (R1-R13) 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.

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, thepresent invention relates to amethod of synchro¬ nizing a random bit flow which is superimposed cryptically on the normal traffic 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 EP-A-273289.

Calls between a base station and a mobile are cipheredby proces¬ sing 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 time- point, and is thus able to decode incoming calls. 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-off of the synchronism of the actual ciphering key concerned, thereby rendering deciphering impossible.

Theseproblems also occurwhen 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 traffic 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- off" shall take place. The invention is also based on ciphering by superimposing a bit sequence on the normal traffic flow (data or speech and signalling) . Prior to handing-off a call or during a given time interval during hand-off where synchronization of the ciphering sequence has ceased, non-ciphered signalling is effec¬ ted 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 traffic flow and discloses when normal ciphering shall commence. The object of the present invention is thus to achieve synchroni¬ zation of a cryptosequence when setting up or "handing-off" 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 BSl and BS2 and a mobile MS which is assumed to move from the base station BSl towards the base station BS2. The base station BSl serves traffic within the cell Cl and the base station BS2 serves traffic 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 BSl over a given radio channel Kl will fall-off. Switching of a new radio channel K2 to MS from BSl 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 se¬ quence, which may have a duration of about 100 ms, MS will not receive and the receiving circuits in MS can loose the synchron¬ ism of the traffic flow, i.e. the time position of frames and time slots from base station BSl. 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-off", the traffic flow is cipheredwith 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 traffic flow 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 BSl 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-off". It is assumed that the base station BSl 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 t_. Hand-off takes place at the time t..

Upon termination of the "hand-off" process at time t , 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.-t- 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 knownmanner bymeans of a correlation process inMS. Thus, at the time t=t_, there is frame synchronization for speech/data transmission between BS2 and MS, but not for the ciphering. At t=t_, 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 . The traffic flow (speech/data) is ciphered in subsequent frames with a key El 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 non- ciphered signal) until ciphering in accordance with the key E2 is commenced by BS2. Possibly the ciphering key El can consist of zeros only, i.e. the traffic flow in subsequent frames is trans¬ mitted unciphered (interval t₂-t_) . 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 .

Figure 4 illustrates the sequence during the time interval t_?-t, more clearly.

According to Figure 3, at time t_ the mobile MS is synchronized to the new base station BS2 and the normal traffic flow (data, speech, synchronization) shouldhave commenced if no cipheringhas been employed. Synchronization with respect to the bit flow in the ciphering key E2 up to the time t_, however, has been lost. The base station therefore sends to MS a signal SI 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 Rl. During the frames R2, R3, ... 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_) 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 confirma¬ tion 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 signal S2 is again transmitted in frame R7 and a new confirmation signal A2 is awaited. The signal Al can fall away because, e.g., of fading or difficult surrounding conditions at precisely the interval in which the signal Al is transmitted. Thus, the number of frames (=R13) from the base station BS2 should be sufficien¬ tly 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_ will preferably be chosen so that t_-t₃ > 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.

Claims

C LA I M S

1. A synchronizing method in a mobile radio system which lacks a global time reference in which the data messages (D) and the signalling messages (S) between a first base station (BSl) and a mobile station (MS) are transmitted in frames having time slots (TDMA) ciphered by superimposing on said message (D, S) a randomly selected bit flow, wherein upon handing over the communication of the mobile having the first base station (BSl) to a second base station (BS2) , ciphering (E2) is interrupted for a given period of time (t -t_) 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 (SI) is effected from the second base station (BS2) to the mobile station (MS) which discloses the time point (t.) counted in numbers of frame intervals (R2-R13) which shall pass until said ciphering (E2) is recommenced.

2. Amethod according to claim l, ch a r a c t e r i z e din that the messages (D,S) up to said time point (t₃) are transmitted by means of a ciphering sequence (El) which consists of a periodic bit sequence whose period is equal to a frame 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 (SI) is effected during a frame (Rl) 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 (SI) transmits a confirmation signal (Al) 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 (Al) subsequent to a given time lapse, further non-ciphered signalling (S2) is effectedby a further base station having the same information as the first mentioned signalling (SI) so as to obtain a confirmation signal (A2) .