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Synchronizing method in a mobile radio system
| Details |
Inventors: Dahlin, Jan E. .ANG.. S.; Ghisler, Walter; Persson, Bengt Y.;
Assignee: Telefonaktiebolaget L M Ericsson (Stockholm, SE)
Primary Examiner: Gregory; Bernarr E.
Assistant Examiner:
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
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 t.sub.2), 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 transmit 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. |
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DETAILED DESCRIPTION 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 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 effected 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 synchronization 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.
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