AU1664400A

AU1664400A - Synchronisation method in a telecommunication system

Info

Publication number: AU1664400A
Application number: AU16644/00A
Authority: AU
Inventors: Frederic Boulnois; Philippe Desblancs; Frederic Lejay
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel Lucent SAS
Priority date: 1998-12-17
Filing date: 1999-12-16
Publication date: 2000-07-03
Also published as: JP2002533011A; CN1293843A; CA2320879A1; FR2787660A1; WO2000036766A1; FR2787660B1; EP1014601A1

Description

A TELECOMMUNICATIONS SYSTEM SYNCHRONIZATION METHOD The present invention relates to time-division multiple access (TDMA) telecommunications systems. These systems convey a data signal comprising a synchronization 5 part and a message part in a time window on a radio channel. In the air interface of the GSM cordless telephone system, it is beneficial to use a mechanism for shifting the time window within the frame on the broadcast control 10 channel to reduce interference with the GSM network and other fixed parts of the GSM cordless telephone system. The usual shifting rules entail cyclic, random or pseudo-random shifts. However, because the mobile station cannot tell which time slot in the frame was used 15 by the fixed part, cyclic, random or pseudo-random shifts cannot quickly determine the position of the start of the frame of the broadcast control channel without dedicated signaling or without knowing the frame number. In the GSM cordless telephone system, if the shift sequences do 20 not have any particular property the mobile station is unable to find out the frame number quickly by listening only to the broadcast control channel. A method of determining the start of the frame frequently used in the prior art adds additional 25 signaling to the signal. That solution enables the start of the frame to be found at the cost of overloading the signal, which makes it slower to use or requires a higher bit rate. The object of the invention is to enable the mobile 30 station to determine the position of the start of the frame of the broadcast control channel as quickly as possible knowing only the fixed part broadcast identifier (FPBI) that is broadcast on the broadcast control channel. The invention also makes it possible to 35 determine the position of the time window within any frame of the broadcast control channel if the frame number and the FPBI are known. In this context, this 2 minimizes the probability of collusion between the time windows of the broadcast control channel of two fixed parts. The invention provides a method of determining the 5 position of the start of the frame of a fixed part transmitting a time-division multiple access radio signal at a specific frequency in time slots in a local geographical area to communicate with a mobile station in the same geographical area, which method is characterized 10 in that it includes the following steps: - defining shifting rules; - defining sequences for shifting the position of the time window in the frame in accordance with the shifting rules, where each sequence defines different 15 shifts from an initial position of the time window and the fixed part transmits the signal in accordance with a specific time window shift sequence; - memorizing the time slot shifting rules and the time slot shift sequences in the mobile station; 20 - searching by the mobile station for a synchronization window transmitted by the fixed part; - searching by the mobile station for at least one subsequent synchronization window to determine the shifts for each window as a function of the preceding ones in 25 accordance with the shifting rules; and - determination by the mobile station of the position of the start of the frame of the fixed part from at least one shift determined from a knowledge of the sequences used by the fixed part. 30 The mobile station advantageously identifies the sequence used by the fixed part from signaling transmitted by the fixed part. The position of the start of the frame can preferably be found from a maximum of two shift 35 measurements. Alternatively, eight fixed parts can transmit in parallel in the same frame without collusion. In an advantageous embodiment the position of the 3 time window in the frame can be determined from the frame number. In some embodiments the sequences are divided into sub-sequences and consecutive shift sub-sequences are 5 strung together without repetition of a time slot. The invention also provides a fixed part transmitting a time-division multiple access (TDMA) radio signal at a specific frequency in different time slots in a local geographical area to communicate with a mobile 10 station in the same geographical area using the method according to the invention. It is characterized in that it comprises means for memorizing a time window shift sequence in accordance with the shifting rules, each sequence defines different shifts from an initial 15 position of the time window, and the fixed part transmits the signal in accordance with a specific time window shift sequence. In the same manner, a mobile station receiving time division multiple access (TDMA) radio signals to 20 communicate with a fixed part in the same geographical area using the same method is characterized in that it comprises means for memorizing shifting rules, means for memorizing a time window shift sequence in accordance with the shifting rules wherein a sequence defines 25 different shifts from an initial position of the time window and the fixed part transmits the signal in accordance with a time window shift sequence previously stored by said mobile station, means for searching for a synchronization window transmitted by the fixed part, 30 means for searching for a subsequent synchronization window to determine shifts for each slot as a function of the preceding ones in accordance with the shifting rules, and means for determining the position of the start of the frame of the fixed part from at least one shift 35 determined in this way. It is possible to identify the position of the start of the frame by looking at the relative position of the 4 time window of the broadcast control channel in successive frames. Specific sequences are used to obtain optimum performance (fast identification and small observation window). What is more, these shift sequences 5 are constructed from a family of basic sub-sequences in accordance with certain parameters, chosen for each fixed part. The number of parameters for each fixed part is usually limited. The time window shift sequences are such that it is 10 possible to identify the position of time slot 0 by observing a limited number of successive sequences, for example three successive sequences, without knowing the frame number. The chaining of the shift sub-sequences is regularly 15 changed in a deterministic way so that it is possible to find the time slot (TN) that will be used within a given frame knowing the fixed part broadcast identifier (FPBI). A plurality of fixed parts can work in parallel on different time slots without collusion at any time. It 20 is possible to use as many fixed parts as there are time slots in a frame. A frame usually contains eight time slots. In a preferred embodiment, the system relates to a fixed part (FP) and mobile stations (MS) of a GSM 25 cordless telephone system (CTS). The mobile stations can be in various states: either they are not registered and have no information on the fixed part apart from the fixed part broadcast identifier (FPBI), or they are registered and should know all the parameters of the 30 fixed part, including the current frame number. Shifting the time window within the frame on the broadcast control channel reduces interference between fixed parts and with the GSM network. It can also be beneficial if the broadcast control channel is not 35 transmitted in a fixed time slot to facilitate its observation by the mobile station. The shift is defined so that a mobile station that 5 is not registered can find the position of time slot 0 of the fixed part knowing only the FPBI. A registered mobile station can predict in which time slot the fixed part transmits its broadcast control signal from the 5 frame number and the FPBI. The shifts advantageously do not exceed four time slots, the risk of collusion between the two fixed parts advantageously has a low probability, and the time slots are advantageously used in an equiprobable manner. 10 Incorrect determination of the time window is not possible during the change from one shift sub-sequence to another. Sixteen families.of sub-sequences of eight time slot shifts (TSS) are preferably used, each sub-sequence 15 running through the eight possible time slots of a frame. These are used to determine the position of the start of the frame from only three successive observations. The TSS used are also regularly changed in accordance with the frame number and the FPBI. 20 In an advantageous first embodiment, a zero shift is prohibited in a sub-sequence and on changing from one sub-sequence to another the first time slot of a new sub sequence is the last time slot of the preceding sub sequence (it is therefore repeated). This prevents 25 errors in determining the position of the time window, as a zero shift characterizes the sub-sequence change. If it is not registered, the mobile station searches for a frequency time window transmitted every 52 frames on the broadcast control frequency by the fixed part. 30 When the time window is found, the synchronization time window is decoded 26 frames later and the FPBI extracted from it. The FPBI is needed to identify the TSS family used. The mobile station then searches for the next frequency time window 52 frames later in a window of nine 35 time slots (from -4 to +4 around the time slot of the frequency time window previously found). The time shift with the time slot of the first frequency time window is 6 saved (si). The process is repeated to obtain the second shift value (s2). Knowing s1, s2 and the TSS family, it is possible to find which time slot is used from the table of sub 5 sequences. The position of slot 0, and therefore the start of the frame, can then be found. If si or s2 is zero, the fixed part was in the process of changing sub sequence and the procedure must be started over. If it is registered, a mobile station can seek to 10 predict the broadcast control channel time slot. The family of four sub-sequences used by the fixed part and their order are extracted from the FPBI. The series TSS(0), TSS(1), TSS(2), TSS(3) can be deduced from it. For example, if the choice is made to define 16 families 15 for the CTS, four bits in the FPBI designate which- one is used by the fixed part. A series of four sub-sequences is then constructed from this family. The FPBI can indicate, for example: TSS(0) = TSS#l, TSS(1) = TSS#1, TSS(2) = TSS#3, TSS(3) = TSS#l. This requires 4 x 2 bits 20 in the FPBI to designate the four sub-sequences. The initial time slot (TNI) is also extracted from the FPBI. Multiframes each comprising 52 frames, subsets, sets and supersets, all of which are defined below, are used to describe the shifts: 25 - A multiframe contains 52 frames. The broadcast control channel has a multiframe structure. The time slot is the same for the frequency time window and the subsequent synchronization window. - A subset contains 25 multiframes. The shift is 30 effected in accordance with a particular shift sub sequence. The last time slot is the same as the first: TN(24) = TN(16) = TN(8) = TN(0). - A set is made up of four subsets and two multiframes. The four subsets, numbered from 0 to 3, use 35 four respective shift sub-sequences TSS(0) to TSS(3) specific to the fixed part. Each set terminates with the two multiframes using the last time slot of subset 3.

7 For all the subsets of the same set, the first and last time slots are the same. This property is then transmitted to the whole.. In a set, the change from one subset to another does not modify the time slot. 5 - A superset comprises eight consecutive sets having the following property: the first time slot is shifted in accordance with TSS(0). After the eight sets, the initial configuration returns. The general shift structure has an overall period of 10 one superset, i.e. 16 x 51 x 52 frames. A second embodiment using the same method but with a different implementation of the sequences and sub sequences is described below. It has the advantage of not having to repeat a time slot on changing from one 15 sub-sequence to the other. This is made possible by using sub-sequences having additional properties compared to the previous ones. This accelerates the determination of the frame start by a mobile station that is not registered because there is no instance of failure. 20 For a fixed part, an ordered pair of shift sub sequences is chosen from N predefined pairs. The pair used by the fixed part concerned is indicated in the FPBI (four bits for 16 pairs, for example) and the order within the pair (1 sub-sequence, 2nd sub-sequence) is 25 determined by one bit of-the FPBI. The pair is denoted (TSS#l, TSS#2). The N pairs are constructed so that it is possible to determine the start of the frame unequivocally from three successive observations of the broadcast control channel, including a change from one 30 sub-sequence of the pair to the other. Example of a pair having the required properties: TSS#1: slot sequence 0 2 1 4 7 6 5 3 0 shift sequence 2 -1 3 3 -1 -1 -2 -3 TSS#2: slot sequence 0 1 5 6 3 7 4 2 0 35 shift sequence 1 4 1 -3 4 -3 -2 -2 A law governing use of the sub-sequences TSS#1 and TSS#2 is then defined so that the time slot to be used 8 can easily be determined from the frame number. For this purpose, subsets of 17 multiframes are defined, with the following law: - sub-sequence TSS#1 on x multiframes, 5 - sub-sequence TSS#2 on 8 multiframes, - sub-sequence TSS#1 on 9 - x multiframes, where x is an integer from 1 to 8. A set is then defined as the string of three subsets, i.e. 51 multiframes. The law governing use of 10 the TSS of a set is: - sub-sequence TSS#1 on x1 multiframes, - sub-sequence TSS#2 on 8 multiframes, - sub-sequence TSS#1 on (9 - x1) + x2 multiframes, - sub-sequence TSS#2 on 8 multiframes, 15 - sub-sequence TSS#1 on (9 - x2) + x3 multiframes, - sub-sequence TSS#2 on 8 multiframes, - sub-sequence TSS#1 on 9 - x3 multiframes, where x1, x2, x3 are integers from 1 to 8. The numbers x1, x2 and x3 are specific to each fixed 20 part and are determined from the FPBI (3 x 3 = 9 bits in total). Example for x1 = 2, x2 = 3, x3 = 5 and with a first slot equal to 0: 1 subset: 25 TSS used . 1 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 slot 0 2 0 1 5 6 3 7 4 2 1 4 7 6 5 3 0 2 n subset: TSS used 1 1 1 2 2 2 2 2 2 2 2 1 1 1 1 1 1 slot 2 1 4 2 0 1 5 6 3 7 4 7 6 5 3 0 2 30 3 rd subset: TSS used 1 1 1 1 1 2 2 2 2 2 2 2 2 1 1 1 1 slot 1 4 7 6 5 6 3 7 4 2 0 1 5 3 0 2 1 In any subset, the sub-sequence TSS#2 is always used for eight successive shifts, and the time slot after the 35 last shift is therefore equal to that before the first shift. The sub-sequence TSS#1 is used for nine shifts in each subset, which creates a shift of 1 in each subset.

9 For each set, the first time slot is obtained by 3 x 1 shifts in accordance with the sub-sequence TSS#1 relative to the first time slot of the preceding set. A superset is defined as a string of eight sets. 5 8 x 3 shifts according to the sub-sequence TSS#1 will therefore have been effected, which amounts to a zero shift. The time slot after the last shift of a superset is then identical to the first time slot of the superset. The initial time slot TNI of the supersets is specific to 10 each fixed part and is identified by three bits of the FPBI. A pattern is therefore formed with a length of 8 x 51 x 52 frames. The number of different patterns is equal to (16 x 2) x (8 x 8 x 8) x 8 = 217, which 15 corresponds to 17 flagged bits in the FPBI. In a final embodiment it is also possible to define other laws governing use of sub-sequences TSS#1 and TSS#2 that eliminate the need to define subsets in a set. The resulting embodiment has the advantage of not using the 20 sub-sequence TSS#2 for only eight successive shifts. The following law can be defined for a set of 51 multiframes, for example: - sub-sequence TSS#1 on x5 multiframes, - sub-sequence TSS#2 on x4 multiframes, 25 - sub-sequence TSS#1 on x3 multiframes, - sub-sequence TSS#2 on x2 multiframes, - sub-sequence TSS#1 on xl multiframes, - sub-sequence TSS#2 on 8 multiframes, - sub-sequence TSS#1 on 8 - x1 multiframes, 30 - sub-sequence TSS#2 on 8 - x2 multiframes, - sub-sequence TSS#1 on 8 - x3 multiframes, - sub-sequence TSS#2 on 8 - x4 multiframes, - sub-sequence TSS#1 on 11 - x5 multiframes, where x1, x2, x3, x4 are integers from 1 to 7 and x5 is 35 an integer from 1 to 10. The numbers x1, x2, x3, x4 and x5 are specific to each fixed part and are determined from the FPBI 10 ((4 x 3) + (1 x 4) = 16 bits in total). Example for x1 = 2, x2 = 3, x3 = 5, x4 = 4, x5 = 7 and with a first slot (TNI) equal to 0: 5 TSS used . 1 1 1 1 1 1 2 2 2 2 1 1 1 1 1 2 2 2 1 1 slot 0 2 1 4 7 6 5 6 3 7 4 7 6 5 3 0 1 5 6 5 3 TSS used 2 2 2 2 2 2 2 2 1 1 1 1 1 1 2 2 2 2 2 1 1 slot 7 4 2 0 1 5 6 3 0 2 1 4 7 6 3 7 4 2 0 2 1 10 TSS used 1 2 2 2 2 1 1 1 1 slot 4 2 0 1 5 3 0 2 1 The property of the set is preserved: for each set, the first time slot is obtained by three shifts in 15 accordance with sub-sequence TSS#1 relative to the first time slot of the preceding set. As in the previous embodiment, the superset is defined as a string of eight sets. The property that the time slot after the last shift of a superset is identical 20 to the first time slot of that superset is therefore preserved. A pattern is therefore produced with a length of 8 x 51 x 52 frames. The number of different patterns is equal to (16 x 2) x (7 x 7 x 7 x 7 x 10) x 8, which 25 corresponds to 23 flagged bits in the FPBI.

Claims

1. A method of determining the position of the start of the frame of a fixed part transmitting a time-division multiple access (TDMA) radio signal at a specific 5 frequency in time slots in a local geographical area to communicate with a mobile station in the same geographical area, which method is characterized in that it includes the following steps: - defining shifting rules; 10 - defining sequences for shifting the position of the time window in the frame in accordance with the shifting rules, where each sequence defines different shifts from an initial position of the time window and the fixed part transmits the signal in accordance with a 15 specific time window shift sequence; - memorizing the time slot shifting rules and the time slot shift sequences in the mobile station; - searching by the mobile station for a synchronization window transmitted by the fixed part; 20 - searching by the mobile station for at least one subsequent synchronization window to determine the shifts for each window as a function of the preceding ones in accordance with the shifting rules; and - determination by the mobile station of the 25 position of the start of the frame of the fixed part from at least one shift determined from a knowledge of the sequences used by the fixed part.

2. A fixed part frame start position determination method according to claim 1, characterized in that the 30 sequence used by the fixed part is identified by the mobile station from signaling transmitted by the fixed part.

3. A fixed part frame start position determination method according to either preceding claim, characterized 35 in that the start of the frame can be found from a 12 maximum of two shift measurements.

4. A fixed part frame start pos-ition determination method according to any preceding claim, characterized in that eight fixed parts can transmit in parallel in the 5 same frame without collusion.

5. A fixed part frame start position determination method according to any preceding claim, characterized in that the position of the time window in the frame can be determined from the frame number. 10

6. A fixed part frame start position determination method according to any preceding claim, characterized in that the sequences are divided into sub-sequences..

7. A fixed part frame start position determination method according to any preceding claim, characterized in 15 that consecutive shift sub-sequences are strung together without repeating a time slot.

8. A fixed part transmitting a time-division multiple access (TDMA) radio signal at a specific frequency in different time slots in a local geographical area to 20 communicate with a mobile station in the same geographical area using the method according to claim 1, characterized in that it comprises means for memorizing a time window shift sequence in accordance with the shifting rules, each sequence defines different shifts 25 from an initial position of the time window, and the fixed part transmits the signal in accordance with a specific time window shift sequence.

9. A mobile station receiving time-division multiple access (TDMA) radio signals to communicate with a fixed 30 part in the same geographical area using the method according to claim 1, characterized in that it comprises 13 means for memorizing shifting rules, means for memorizing a time window shift sequence in accordance with the shifting rules where a sequence defines different shifts from an initial position of the time window and the fixed 5 part transmits the signal in accordance with a time window shift sequence previously stored by said mobile station, means for searching for a synchronization window transmitted by the fixed part, means for searching for a subsequent synchronization window to determine shifts for 10 each slot as a function of the preceding ones in accordance with the shifting rules, and means for determining the position of the start of the frame of the fixed part from at least one shift determined in this way.