CA2300061A1 - Method and arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station - Google Patents
Method and arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station Download PDFInfo
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- CA2300061A1 CA2300061A1 CA002300061A CA2300061A CA2300061A1 CA 2300061 A1 CA2300061 A1 CA 2300061A1 CA 002300061 A CA002300061 A CA 002300061A CA 2300061 A CA2300061 A CA 2300061A CA 2300061 A1 CA2300061 A1 CA 2300061A1
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- mobile unit
- fixed station
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/713—Spread spectrum techniques using frequency hopping
- H04B1/7156—Arrangements for sequence synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/713—Spread spectrum techniques using frequency hopping
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- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Mobile Radio Communication Systems (AREA)
Abstract
The invention concerns a method and a device for synchronising carrier frquencies (fx) in a mobile telephone (11) on carrier frequencies (fx) of a fixed station (1), for radio data transmission, said data being transmitted on several carrier frequencies. The fixed station (1) comprises a device (4) for transmitting a control signal, the control signal carrier frequency (fx) being changed from time to time. In the mobile telephone (11) is provided a device (5) sensing the transmission conditions succeeding one another for each of the plurality of carrier frequencies (fx). In the mobile telephone is further provided a device (16) adjusting the mobile telephone on reception of the carrier frequency (fx) offering the most favourable receiving conditions, until the fixed station (1) transmitting device (4) shifts the control signal carrier frequency (fx) to the carrier frequency (fx) on which the mobile telephone (11) is adjusted. In the fixed station (1) or the mobile telephone (11), devices (12, 13) transmit synchronising data from the fixed station (1) to the mobile telephone (11) and/or vice-versa.
Description
aR s~ P 212 P
Description Method sad arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station The present invention relates to a device and a method for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station for a transmission of data by radio, in which transmission the data are transmitted oa a plurality of carrier frequencies, such as is the case in particular in systems which are based oa a so-called Frequency Hopping Spread Spectrum (FHSS) and a time division multiplex method (TDMA) .
In the majority of cordless telephones currently -- available on the market, it is possible to serve more than one mobile unit from a fixed station. For communica tion, synchronization sad logging oa must firstly take place. Often, a cordless telephone system is retrofitted by adding a further mobile unit to the already exist' mobile unit or units. For this purpose, the new mo unit must be logged oa in the already existing cordless telephone system, i.e. is particular at the fixed sta-tion. Logging on is therefore to be understood within the terms of the present description to mean that a mobile unit, in particular a further mobile unit, is logged on, in the sense of a subscription, at the fixed station, so that once logging on has taken place said mobile unit can transmit, in particular, voice information data to the fixed station and receive it from the fixed station.
Problems are experienced if a so-called frequency hopping spread spectrum system is to be used as radio interface and a mobile unit, in particular a further mobile unit, is to be integrated into such a system. A
frequency hopping spread spectrum system is to be under-stood here as a system in which a plurality of carrier frequencies is available for transmitting data by radio and the carrier frequency used is changed from time to time. In particular, in a time division multiplex system (TDMA), the carrier frequency can be changed after each time slot of the time division multiplex transmission.
However, the carrier frequency can also be changed after one frame of the transmission standard has been trans-mitted. Such a frequency hopping spread spectrum system has advantages to the extent that the energy of the entire radio transmission is distributed over all the carrier frequencies and thus one single carrier frequency less is loaded. This is important in particular if a generally available frequency band, such as the 2.4 GHz ISM (Industrial Scientific Medical) band is used in which an upper limit for the maximum energy occurring per carrier frequency is prescribed, in order to keep inter-ference with other subscribers as low as possible.
A further advantage of the frequency hopping spread spectrum system is that the provision of a large number of carrier frequencies makes the system less susceptible to interference. Furthermore, the protection of the system against listening in by third parties is increased, since the third party does not usually know which carrier frequency is being changed to after a certain time period.
Even if a frequency hopping spread spectrum system has the abovementioned advantages, there is still the problem of synchronizing the carrier frequencies and in particular of changing the carrier frequencies when logging on a new mobile unit at a fixed station. It is in fact a precondition of logging on that the mobile unit to be logged on is capable of communicating with the fixed station, i.e. can precisely perform the change of carrier frequency.
P'or this purpose, it is firstly necessary to reach the state in which the carrier frequency of the fixed station on which, for example, a check signal is broadcast, is equal to the carrier frequency to whose reception the mobile unit is set. That is to say the mobile unit which before the logging on is in a state which is asynchronous with respect to the carrier frequency of the check signal of the fixed station, must firstly find the, as it were, hopping transmitter, namely the carrier frequency of the fixed station, in order then to be able to synchronize with the fixed station. ..
~ ~~' .~
~'~a method and an arrangement for syacing the carrier frequencies of a mobile wait ,y~#~e carrier frequencies of a fixed station~~ method and arrangement permit the carrier f encies of a mobile unit to be syachron-ized w those of a fixed station, even if the carrier In order to solve the aforesaid object, according to the invention a method for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station is provided for a transmission of data by radio, is which transmission the data are transmitted on a plurality of carrier frequencies in the sense of a frequency hopping spread spectrum, the fixed station broadcasting a check signal which changes the carrier frequency whenever a specific time period has elapsed.
The mobile unit senses the reception conditions successively for each of the plurality of carrier frequencies. Than, the mobile unit is set to reception oa the carrier frequency which has the most favourable reception conditions, as was sensed previously, the mobile unit remaining on reception oa the aforesaid most favourable carrier frequency until the carrier frequency of the fixed station changes to the carrier frequency of the mobile unit. Then, synchronization data can be exchanged from the fixed station to the mobile unit and/or from the mobile unit to the fixed station. The mobile unit therefore scans all the carrier frequencies and picks out the one which provides the most favourable reception conditions.
The mobile unit can assess the reception condi-tions of a carrier frequency, for example by means of the energy content prevailing on this carrier frequency, i.e.
by means of the magnitude of the signal present on this carrier frequency, which can be taken as a measure of the A radio system with error-tolerant frequency hopping synchronization is known from EP 0 650 274 A2 in which header bits of a check signal are used for the signalling of carrier frequencies that takes place from a fixed station to a mobile station and/or vice versa.
A radio system according to the frequency hopping method is known from EP 0 650 304 A2 in which special synchronization bursts are inserted into the frequency hopping sequence of the control channels to improve the initial synchronization of control channels of the mobile stations of the radio system.
The object of the present invention is to provide a method and an arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station, which method and arrangement permit the carrier frequencies of a mobile unit to be synchronized with those of a fixed station, even if the carrier frequency of the fxied station changes from time to time.
interference prevailing oa this carrier frequency.
After the mobile unit has bean set to reception on the most favourable carrier frequency and a predeter mined time period has elapsed in which the fixed station has not changed to the carrier frequency of the mobile unit, said mobile unit can again sense the reception conditions successively for each of the plurality of carrier frequencies sad again be set to receptiba on the carrier frequency which now has the most favourable reception conditions.
The predetermined time period is therefore determined as a function of the number of carrier frequencies and of the frequency with which the frequency station changes the carrier frequency.
The fixed station can change the carrier fre-quency is particular in each case after the time period .
in accordance with a time slot in a time division multi-plex method.
The fixed station can change the carrier fre quency in accordance with a predetermined frequency, the check signal broadcast by the fixed station then indicat ing the position of the predetermined sequence to which the current carrier frequency corresponds. By means of the sequence, also knows here, and the transmitted position of the predetermined sequence, the mobile unit can then itself determine the next carrier frequency which the fixed station will change to.
As an alternative, the check signal can also specify the carrier frequency which the base station will "jump to" next.
As a further alternative, the check signal can specify which carrier frequency the base station will use in the m-th time slot or m-th frame. This is advantageous if a mobile unit ie is the so-called idle-locked or multiframe mode. In such a mode. a mobile unit resyn-chronizes with the base station only in every m-th time slot or frame if said mobile unit is not in the process of active voice communication with the base station.
The check signal does not have to be broadcast in every time slot or frame. If a mobile unit which would like to synchronize with a base station receives a time slot or frame which does not contain a check signal, it scans all the carrier frequencies again, this procedure being repeated until the mobile unit receives from the base station a time slot or frame which contains the check signal.
Time-slot and frame synchronization can be carried out as a result of the exchange of syachroniz ation data.
In accordance with a further aspect of the invention, in order to perform the abovemeationed task an arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station is provided for a transmission of data by radio, in which transmission the data are transmitted on a plurality of carrier frequencies. The arrangement here has a device in the fixed station for broadcasting a check signal, the check signal changing the carrier frequency whenever a specific time period has elapsed. A
device which senses the reception conditions successively for each of the plurality of carrier frequencies is provided is the mobile unit. In addition, a device is provided in the mobile unit which sets the mobile unit to reception on the carrier frequency which has the most favourable reception conditions determined by the device for sensing the reception conditions, the mobile unit remaining on the aforesaid carrier frequency until the broadcasting device of the fixed station changes the carrier frequency of the check signal to the set carrier frequency of the mobile unit. In the fixed station and is the mobile unit devices are provided which enable the synchronization data to be exchanged between the fixed station and the mobile wait and/or in the opposite direction.
In particular, the sensing device in the mobile unit may be provided in such a way that it senses the reception conditions on a carrier frequency by means of the energy content prevailing on this carrier frequency or the prevailing interference.
After the reception device in the mobile unit has set the mobile unit to reception on the most favourable carrier frequency and a predetermined time period has elapsed in which the broadcasting device in the fixed station has not changed the carrier frequency of the check signal to the carrier frequency of the mobile unit, said reception device can again sense the reception conditions successively for each of the plurality of carrier frequencies. Then, the reception device again sets the mobile unit to reception on the carrier fre-quency which then has the most favourable reception conditions.
The predetermined time period can be defined here as a function of the number of carrier frequencies and of the frequency with which .the fixed station changes the carrier frequency.
The broadcasting device in the fixed station can change the carrier frequency of the check signal in accordance with a predetermined sequence. The check signal broadcast by the fixed station can have data here which indicate the position of the current carrier frequency in an algorithm or frequency table, by means of which algorithm or table the predetermined sequence is determined.
The exchange devices can carry out time-slot and frame synchronization by exchanging the synchronization data.
The invention will now be explained in more detail by means of an exemplary embodiment and with reference to the accompanying figures, in which:
Fig. 1 shows an arrangement according to the invention for transmitting data in a wire-free fashion, Fig. 2 shows a time frame of a data transmission standard such as is used in the present invention, Fig. 3 shows a detailed illustration of a time frame according to the invention, for a carrier frequency, and Fig. 4 shows a schematic representation of a frequency hopping spread spectrum system to illustrate the embodiment.
With reference to Fig. 1, the general design of the arrangement according to the invention for radio transmission will be explained first. As is generally customary, the arrangement for the transmission of data by radio has a fixed station 1 and a plurality of mobile units (mobile stations, cable-free telephones) 2, 3, 11. The fixed station 1 is connected here to the landline network with a terminal line 10. The ffixed station 1 has an antenna 6 by means of which it is possible to communicate, for example, with the mobile unit 2 via a radio transmission path 8 or with the mobile unit 3 via a radio transmission path 9. The mobile units 2, 3, 11 each have an antenna 7 for receiving and transmitting data.
' CA 02300061 2000-02-11 GR 97 P 2127 P - 7(~-frame according to the invention, for a caier frequency, and ~, ~'' Fig. 4 shows a schematic represtation of a frequency hopping spread spect~system.
With reference to Fig. 1,,~.,~~~the general design of the arrangement according to .~t,~he invention for radio transmission will be explai~id firstly. As is generally customary, the arrangeme~r~l for the transmission of data E..
by radio has a fixed ,~, atioa l and a plurality of mobile units (mobile stags, cable-free telephones) 2, 3, 11.
The fixed stat,pi~ti 1 is connected here to the landline network with ,rsi~r~ terminal line 10 . The fixed station 1 has an antenaa°r6 by means of which it is possible to communi-cate. iii' example, with the mobile unit 2 via a radio tra~~ission path 8 or with the mobile unit 3 via a radio tnsmission path 9. The mobile units 2, 3, 11 each have The internal design of a fixed station 1, insofar as it is of significance for the present invention. will now be explained in more detail. A processor 15 which determines. a predetermined sequence by means of a predetermined algorithm (hop algorithm) is provided in the fixed station 1. As an alternative, a plurality of different algorithms may be provided in the processor 15, so that the processor 15 can determine different sequences in accordance with the respectively used algorithm. The sequences determined by the processor l5 are then transmitted to a storage and output device 13.
The stoxage and output device 13 transmits to as RF
module 4 either the sequence which is continuously determined by the processor 15 or a sequence which has been previously permanently stored in it. The RF module 4 receives and transmits data on a carrier frequency fx which depends oa the current value of the sequence transmitted from the storage and output device 13.
Therefore, a radio transmission takes place on a carrier frequency f,~. the currently used carrier frequency either being determined indirectly by the processor 15 by mean' of an algorithm or, alternatively. being determined directly from the value of a sequence which has been permanently stored is the storage and output device 13.
The internal design of a mobile unit, insofar as it is relevant to the present invention, will now be described in more detail. In this respect, the design of a mobile unit 2, 3, 11 is essentially symmetrical to the internal design of the fixed station 1 described above.
That is to say each mobile unit 2, 3, 11 has, as illus-trated is the invention only for the mobile unite 2 and 11, a processor 16. This processor 16 determines, by means of one. or alternatively by means of a plurality of available hop algorithms, a sequence which it transmits to a storage and output device 12. The storage and output device l2 transmits to an RF module 5 either the values of the sequence based on the algorithm which are determined continuously by the processor 16 or, alternatively, values of a sequence which has been permanently stored in it. The RF module 5 transmits or receives data on a carrier frequency fx whose level depends on the value of the sequence transmitted to it by the storage and outputting device 12. A mobile unit 2. 3, 11 therefore receives or transmits data on a carrier frequency fx whose level depends either on the current value of the sequence determined by the processor 16 or on the value of a sequence which has been permanently stored in the storage and output device 12.
It is to be noted here that the processor 15 in the fixed station l and the processors 16 in the mobile units 2, 3, 11 are based on the same algorithm for determining sequences, or is the event that a plurality of algorithms are available. have the same selection of algorithms. In the event that the sequence is not deter-mined continuously by the processor 15, 16 but rather permanently prescribed in the storage and output devices 12, 13. the sequence which is stored in the storage and output device 13 of the fixed station 1 is of course identical to the sequences which are respectively stored in the storage and output devices 12 of the mobile units 2, 3, 11.
A transmission standard such as is used in the present invention will now be explained with reference to Fig. 2. As is clear in Fig. 2, data are transmitted in chronological succession is a plurality of time slots, 24 time slots Zx in the case illustrated, using the time division multiplex method TDMA (Time Division Multiple Access) on a plurality of carrier frequencies fx, of which tea are illustrated. Ia the case illustrated, duplex mode is used on the carrier frequencies. This means that after the first twelve time slots Zx have been transmitted, reception is switched to and the second twelve time slots (13 - 24) are received is the opposing direction. ..
In the event that the so-called DECT Standard is used for transmission, the chronological duration of a time frame is 10 milliseconds, and 24 time slots Zx are provided. namely twelve time slots for the transmission from the fixed station to mobile units and a further twelve time slots Zx for the transmission from the mobile units to the fixed station. In the DECT Standard, 10 carrier frequencies fx between 1.88 GHz and 1.90 GHz are provided.
However, the present invention is also used in particular for transmission in the so-called 2.4 GHz ISM
(Industrial Scientific Medical) frequency band. The ISM
frequency band has a bandwidth of 83.5 MHz. In accordance with the specification "FCC Part 15" (Federal Communica-tions Commission), at least 75 carrier frequencies fx must be distributed over these 83.5 MHz. Distributing the 83.5 Mfiz bandwidth over 96 carrier frequencies, i.e. a channel spacing of 864 kHz, is particularly advantageous.
The abovementioaed frequency bands and standards are mentioned purely by way of example. The only funda-mental precondition for the invention is that a so-called frequency hopping spread spectrum is used, i.e. a plural-ity of carrier frequencies are available, and that the aR 97 P 2127 P - 10 -carrier frequency fx selected for the transmission is changed from time to time. A precondition of such a change is that the data are transmitted is time slots Zx (time division multiplex method). The so-called DECT
Standard, for example, as well as any other modified standard based on this DECT Standard, is therefore suitable. The modification can comprise, for example, reducing (for example halving) the time slots per frame.
A time slot or a time frame of the transmission, for example, is suitable as the time period after which the carrier frequency is changed.
How the selection of a carrier frequency fx for a specific time slot Zx is carried out will now be explained with reference to Fig. 4. It will be assumed that, at the time of the time slot Zl, the processor 15 of the fixed station 1 determines, oa the basis of an algorithm. a value which the RF module 4 of the fixed station 1 converts indirectly into a carrier frequency fl. In Fig. 4, the hatching shows that the carrier fre-quency fi is selected at the time of the time slot Zl. At the transition from the time slot Zl to the following time slot Z2, the carrier frequency fx is inevitably changed. As is illustrated by an arrow in Fig. 4, it is possible, for example, for the processor 15 of the fixed station 1 to determine by means of its algorithm a value which is converted by the RF module 4 into a carrier frequency f3. In the same way, a carrier frequency fz can then be selected for the time slot Z3, which is illus-trated by hatching and by an arrow.
Even though the case of the carrier frequency fx being changed after each time slot Zx has been illus-trated. other change intervals are also conceivable. For example. the carrier frequency fx can be set in each case after one frame of the transmission has elapsed.
The fixed station 1 therefore changes the carrier frequency fx from the carrier frequency fi to the carrier frequency f3 and then to the carrier frequency fs.on the basis of the sequence determined by the processor 15. If communication is to take place between the fixed station 1 and a mobile unit 11, it is necessary to ensure that the mobile unit 11 can follow synchronously the sequence of carrier frequency fx changes carried Qut by the fixed station 1. This is a problem in particular when a mobile unit 11 is to be first integrated into a radio trans-mission system. i.e. has to be logged on and signed on at the fixed station 1. During unsynchronized operation of the new mobile unit 11 after it has been switched on, the mobile unit 11 will change the carrier frequencies fx used, in the way prescribed by its sequence. The sequence as such is identical here with the sequence l, which is prescribed in the fixed station 1 and explained above.
However, this does not ensure that the sequence of the mobile unit 11 is synchronized temporally with the sequence of the fixed station 1 after said mobile unit 11 has been switched on.
As an alternative, the check signal can also specify the carrier frequency which the base station will "jump to° next.
As a further alternative, the check signal can specify which carrier frequency the base station will use in the m-th time slot or m-th frame. This is advantageous if a mobile unit is in the so-called idle-locked or multiframe mode. In such a mode, a mobile unit resynchronizes with the base station only in every m-th time slot or frame if said mobile unit is not in the process of active voice communication with the base station.
The check signal does not have to be broadcast in every time slot or frame. If a mobile unit which would like to synchronize with a base station receives a time slot or frame which does not contain a check signal, it scans all the carrier frequencies again, this procedure being repeated until the mobile unit receives from the base station a time slot or frame which contains the check signal.
After it has been switched on, the mobile unit 11 scans the available range of carrier frequencies fx until it senses the carrier frequency fx currently being used by the fixed station 1. It will now be explained in detail below how the searching for the continuously changing carrier frequency is performed according to the invention.
Directly after it has been switched on, the mobile unit 11 is in a state which is asynchronous with respect to the carrier frequency fx of the fixed station 1. Even if the fixed station 1 is still not carrying out aoa a voice data transmission, it transmits, oa the carrier ~
frequency fx which changes after a predetermined time ~~
~r'~lss~n period, a check signal (check chaanel) in order to permit mobile units to be logged on. This check signal can also be retained when voice transmission takes place.
The mobile unit 11 to be logged on then scans all the available carrier frequencies fx. In the process, it senses for each carrier frequency fx the energy content prevailing on it, said content being taken ae a measure of the presence of interference. For this purpose, the mobile unit 11 sets itself to reception on a carrier frequency and senses the level of the signal modulated onto this carrier frequency (fx). If there is no signal modulated onto the carrier frequency, this means that on this carrier frequency there is neither another, inter-fering mobile radio system nor a genuine noise source 25~ (microwave or the like).
The mobile unit 11 thus senses, by scanning all the carrier frequencies, the carrier frequency fx which provides the best reception properties. After the sens-ing, the mobile unit 11 sets itself to reception on the carrier frequency fX which is determined as being the most favourable. The mobile unit 1I then waits, ready to receive on the carrier frequency fx sensed to be the most favourable, for the carrier frequency of the check signal of the fixed statioa~' to be changed precisely to this carrier frequency. As soon as the carrier frequency fx of the check signal of the fixed station ~ changes to the ~ ;
V ~
.
carrier frequency fx on which the mobile unit 11 is waiting ready to receive, time-slot and frame synchronization can take place by means of a transmission of data from the mobile unit 11 to the fixed station and in the opposite direction.
Fig. 3 illustrates how it is ensured according to the invention that the new mobile unit 11 carries out carrier frequency changes which are synchronous with the fixed station 1. As ie clear in Fig. 3, the data trans-mitted in a time slot (channel) Zx are. for the most part, information data. i.e. for example data which represent an item of voice information data of a telephone call. Before the range of the information data there is then a check range which is referred to as A
field in the DECT Standard. In this check range, data are provided for synchronizing the operation of a mobile unit 11 to be logged on with the operation of the fixed station 1. If no data are transmitted in the information range, only the data of the check range are transmitted.
Transmission only of data of the check range therefore constitutes a check signal.
If a plurality of algorithms are available to the processor 15 in the fixed station 1 for determining the sequence which directly prescribes the changes of the carrier frequency fx of the fixed station l, the check range contains data which identify the algorithm cur rently in use.
If. as explained above, a so-called multiframe mode is used, the check range can contain data which indicate directly or indirectly which carrier frequency the base station will use in every m-th frame or time slot. This can be carried out indirectly by, for example, the data of the check range specifying which algorithm is used in the respective m-th frame or time slot if the algorithm in these frames or time slots is a different one from that in the other frames or time slots.
Further synchronization data contained in the check region are data which indicate which position in the predetermined sequence corresponds to the carrier frequency fx used for the current time slot Zx. The data of the check range which are illustrated in Fig. 3.
namely data which refer to the algorithm used and which refer to the current position of the sequence of the current algorithm, are broadcast by the fixed station 1 to the mobile unit 11.
During the sensing of the carrier frequency fx which is currently being used by the fixed station l, the mobile unit 11 senses the data of the check signal which is broadcast by the fixed station 1.
If every frame or time slot which is broadcast by the base station does not contain the check data, and a mobile unit which would like to synchronize with this _base station receives such a frame or time slot, the mobile unit does sot receive any information which would enable it in any way to infer the carrier frequency which is to be used next by the base station. In such a case, the mobile unit continues to scan all the available carrier frequencies until it receives a frame or time slot which contains the frequencies in the check data.
At first, the mobile unit 11 can therefore determine which algorithm is currently being used by the processor 15 in the fixed station 1, which algorithm of course directly prescribes the change of the carrier frequencies fx of the fixed station 1. In addition, the mobile unit 11 can sense, from the position data of the check range, which position in the predetermined fre-quency corresponds to the broadcast carrier frequency.
The mobile unit 11 is therefore then aware of the __ algorithm in use and of the position in the sequence. The mobile unit 11 can therefore then determine independently by means of the position in the sequence, which is knows here, as well as the sequence stored in it, which carrier frequency fx will be used by the fixed station 1 in the following time slot Zx. From the information fed to it, the mobile unit 11 can therefore generate information for the carrier frequencies to be used in the following time slots Zx. Thus, it is possible to communicate with the fixed station l, as is necessary for a signing on or logging-on procedure. As a result of the information _ supplied relating to the future carrier frequency change, the mobile unit 11 is therefore then synchronized with the fixed station 1.
The fixed station 1 can have a switching device 14 which can be switched between two positions, namely a position in the logging on mode R and a position corresponding to the normal transmission mode. Only if the switching device 14 is switched to logging on mode R
does the fixed station 1 automatically broadcast the check range data necessary for synchronization with a mobile unit to be newly logged on, this data being namely the information relating to the algorithm in use and the information relating to the position in the predetermined frequency on the basis of the algorithm. If the switching device 14 is switched to the normal transmission mode N, the aforesaid synchronization data are normally not broadcast, or only broadcast on request.
~s~~m~b~ __ d'11 can result from a so-called noise source fall- ck mode. Firstly, it will be explained what action fixed station 1 takes in accordance with this ise source fall-back mode with regard to the crier frequency selection. With reference to Fig. 4~ is clear that at the time of the time slot Z3 the~arrier frequency f, is indicated by the predetermine frequency. It will now be assumed that the predate ned sequence for the time of the time slot Z4 in Gates a change to the carrier frequency f,. In a ition, it will be assumed that, for example in the eceding time frame of the transmission, the fixed ation 1 has determined that interference occurre uring a transmission on the carrier frequency f,. s interference may result, for example, from the f~t that another radio transmission arrangement is If the fixed station 1 is in the so-called noise source fall-back mode, when selecting the carrier fre-quency fx for the time slot Z4 it will not select the carrier frequency f4 which is, of course, actually pre-scribed by the predetermined frequency. The carrier frequency f, which is sensed as being subject to interference is instead passed over and another carrier GR 97 P 2127 P ~~'~~w A problem when logging on a further mobile unit 11 can result from a so-called noise source fall-back mode. First, it will be explained for the embodiment of the invention according to Fig. 4 what action the fixed station 1 takes in accordance with this noise source fall-back mode with regard to the carrier frequency selection. With reference to Fig. 4, it is clear that at the time of the time slot Z3 the carrier frequency f2 is indicated by the predetermined frequency. It will now be assumed that the predetermined sequence for the time of the time slot Z4 indicates a change to the carrier frequency f4. In addition, it will be assumed that, for example in the preceding time frame of the transmission, the fixed station 1 has determined that interference occurred during a transmission on the carrier frequency f4. This interference may result, for example, from the fact that another radio transmission arrangement is adversely affecting this carrier frequency f4.
frequency fx, for example the carrier frequency fx which follows in the predetermined frequency, is selected for the time slot Z4 (as illustrated by the arrow Pl). In the case illustrated in Fig. 4, the carrier frequency selected for the time slot Z4 is therefore not the carrier frequency fs which is sensed as being subject to interference but instead the carrier frequency fl which is sensed as being free of interference.
Even if this noise source fall-back mode has, of course, large advantages during the radio transmission mode with mobile units 2, 3 which have already been integrated, it is clear that this noise source fall-back mode simultaneously causes large problems for the logging on of a new mobile unit 11. The mobile unit 11 will, in fact, determine. on the basis of the algorithm stored in it and the position of the carrier frequency, known to it from the check region of the data transmitted from the fixed station, in the predetermined sequence in accord-ance with the algorithm at the time of the time slot Z3, that a transmission on the carrier frequency f~ will take place starting from the next value of the sequence at the time of the time slot Z4. However, if, owing to the noise source fall-back mode, the fixedstation 1 selects the carrier frequency fl at the time of the time slot Z4 in order to avoid the carrier frequency f4 which is subject to interference, and at the same time the mobile unit 11 to be logged on selects, on the basis of the information available to it, the carrier frequency f~ at the time of the time slot Z4, synchronization of the operation of the fixed station 1 with that of the mobile unit 11 fails.
If, for this reason, the logging on mode R is selected by the switching device 14 in the fixed station 1, the noise source fall-back mode of the fixed station 1 can simultaneously be switched off. This means that. in contrast with the normal mode in which, as stated above, the fixed station 1 will in order to avoid the carrier frequency f, which has been recognized as being subject to interference, switch, in a position of the switching device 14 in logging on mode R, to the carrier frequency f, at the time of the time slot Z4 as is prescribed by the sequence on the basis of the algorithm of the processor 15, although the fixed station 1 is aware that the carrier frequency f, is subject to interference. The change of the carrier frequency fx from time slot Z3 to time slot Z4 is illustrated in Fig. 4 by the unbroken arrow Ps. As a result of the fact that the noise source fall-back mode of the fixed station 1 is simultaneously switched off when the switching device 14 is positioned in logging on mode R, it is therefore ensured that a synchronization of the operation of the. mobile unit 11 with that of the fixed station 1 can take place. After the signing-on procedure or logging on of the mobile unit 11 at the fixed station 1 has been completed, the switching device 14 is then switched back from the logging on mode R to the normal transmission mode N, which can take place in as automated way, and the noise ' source fall-back mode can thus be switched on again automatically.
Alternatively, in order to synchronize a mobile unit 11 with the fixed station~~,~the noise source fall-back mode may also remain switched on. However, in this case the state may occur in which the mobile unit 2 also senses a carrier frequency fx as the most favourable one in the asynchronous state and sets itself ready to receive on this carrier frequency fx, to which the fixed station changes owing to the noise source fall-back mode.
In this case. the mobile unit 11 is set in such a way that it waits a predetermined time period for the carrier , frequency fx of the check signal of the fixed station to change to the carrier frequency fx selected by the mobile unit 11. If.the predetermined time period has elapsed without the carrier frequency fx of the check signal of the fixed station ~ having changed to the carrier frequency fx selected by the mobile unit 11, the mobile unit 11 repeats the abovementioned search pro-cedure (scanning of all the carrier frequencies fx, _ sensing of the carrier frequency fX with the smallest energy content, setting of the receive capability to the carrier frequency fx sensed as the most favourable one, waiting).
The length of the predetermined waiting time period is determined here as a function of the number of carrier frequencies and of the change frequency f" with which the carrier frequency of the check signal is changed.
If, for example, a hundred carrier frequencies are available, the carrier frequency is changed in each case after one frame and a frame is 10 ms long, the predetermined time period can be selected as 100 * 10 ms = 1 s.
List of reference symbols l: Fixed station (base) 2: Mobile unit (cable-free telephone) 3: Mobile unit 4: RF module (in the fixed station) 5: RF module (in the mobile unit) 6: Antenna (fixed station) 7: Antenna (mobile uait) 8: First radio transmission path 9: Second radio transmission path 10: Terminal line 11: Mobile unit 12: Output device (mobile unit) 13: Output device (fixed station) 14: Switching device 15: Processor (fixed station) 16: Processor (mobile unit) fx: Carrier frequency fw: Frequency of the carrier frequency changed by the fixed station Zx: Time slot Pl: Frequency jump (noise source fall-back mode on) P,: Frequency jump (noise source fall-back mode off)
Description Method sad arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station The present invention relates to a device and a method for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station for a transmission of data by radio, in which transmission the data are transmitted oa a plurality of carrier frequencies, such as is the case in particular in systems which are based oa a so-called Frequency Hopping Spread Spectrum (FHSS) and a time division multiplex method (TDMA) .
In the majority of cordless telephones currently -- available on the market, it is possible to serve more than one mobile unit from a fixed station. For communica tion, synchronization sad logging oa must firstly take place. Often, a cordless telephone system is retrofitted by adding a further mobile unit to the already exist' mobile unit or units. For this purpose, the new mo unit must be logged oa in the already existing cordless telephone system, i.e. is particular at the fixed sta-tion. Logging on is therefore to be understood within the terms of the present description to mean that a mobile unit, in particular a further mobile unit, is logged on, in the sense of a subscription, at the fixed station, so that once logging on has taken place said mobile unit can transmit, in particular, voice information data to the fixed station and receive it from the fixed station.
Problems are experienced if a so-called frequency hopping spread spectrum system is to be used as radio interface and a mobile unit, in particular a further mobile unit, is to be integrated into such a system. A
frequency hopping spread spectrum system is to be under-stood here as a system in which a plurality of carrier frequencies is available for transmitting data by radio and the carrier frequency used is changed from time to time. In particular, in a time division multiplex system (TDMA), the carrier frequency can be changed after each time slot of the time division multiplex transmission.
However, the carrier frequency can also be changed after one frame of the transmission standard has been trans-mitted. Such a frequency hopping spread spectrum system has advantages to the extent that the energy of the entire radio transmission is distributed over all the carrier frequencies and thus one single carrier frequency less is loaded. This is important in particular if a generally available frequency band, such as the 2.4 GHz ISM (Industrial Scientific Medical) band is used in which an upper limit for the maximum energy occurring per carrier frequency is prescribed, in order to keep inter-ference with other subscribers as low as possible.
A further advantage of the frequency hopping spread spectrum system is that the provision of a large number of carrier frequencies makes the system less susceptible to interference. Furthermore, the protection of the system against listening in by third parties is increased, since the third party does not usually know which carrier frequency is being changed to after a certain time period.
Even if a frequency hopping spread spectrum system has the abovementioned advantages, there is still the problem of synchronizing the carrier frequencies and in particular of changing the carrier frequencies when logging on a new mobile unit at a fixed station. It is in fact a precondition of logging on that the mobile unit to be logged on is capable of communicating with the fixed station, i.e. can precisely perform the change of carrier frequency.
P'or this purpose, it is firstly necessary to reach the state in which the carrier frequency of the fixed station on which, for example, a check signal is broadcast, is equal to the carrier frequency to whose reception the mobile unit is set. That is to say the mobile unit which before the logging on is in a state which is asynchronous with respect to the carrier frequency of the check signal of the fixed station, must firstly find the, as it were, hopping transmitter, namely the carrier frequency of the fixed station, in order then to be able to synchronize with the fixed station. ..
~ ~~' .~
~'~a method and an arrangement for syacing the carrier frequencies of a mobile wait ,y~#~e carrier frequencies of a fixed station~~ method and arrangement permit the carrier f encies of a mobile unit to be syachron-ized w those of a fixed station, even if the carrier In order to solve the aforesaid object, according to the invention a method for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station is provided for a transmission of data by radio, is which transmission the data are transmitted on a plurality of carrier frequencies in the sense of a frequency hopping spread spectrum, the fixed station broadcasting a check signal which changes the carrier frequency whenever a specific time period has elapsed.
The mobile unit senses the reception conditions successively for each of the plurality of carrier frequencies. Than, the mobile unit is set to reception oa the carrier frequency which has the most favourable reception conditions, as was sensed previously, the mobile unit remaining on reception oa the aforesaid most favourable carrier frequency until the carrier frequency of the fixed station changes to the carrier frequency of the mobile unit. Then, synchronization data can be exchanged from the fixed station to the mobile unit and/or from the mobile unit to the fixed station. The mobile unit therefore scans all the carrier frequencies and picks out the one which provides the most favourable reception conditions.
The mobile unit can assess the reception condi-tions of a carrier frequency, for example by means of the energy content prevailing on this carrier frequency, i.e.
by means of the magnitude of the signal present on this carrier frequency, which can be taken as a measure of the A radio system with error-tolerant frequency hopping synchronization is known from EP 0 650 274 A2 in which header bits of a check signal are used for the signalling of carrier frequencies that takes place from a fixed station to a mobile station and/or vice versa.
A radio system according to the frequency hopping method is known from EP 0 650 304 A2 in which special synchronization bursts are inserted into the frequency hopping sequence of the control channels to improve the initial synchronization of control channels of the mobile stations of the radio system.
The object of the present invention is to provide a method and an arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station, which method and arrangement permit the carrier frequencies of a mobile unit to be synchronized with those of a fixed station, even if the carrier frequency of the fxied station changes from time to time.
interference prevailing oa this carrier frequency.
After the mobile unit has bean set to reception on the most favourable carrier frequency and a predeter mined time period has elapsed in which the fixed station has not changed to the carrier frequency of the mobile unit, said mobile unit can again sense the reception conditions successively for each of the plurality of carrier frequencies sad again be set to receptiba on the carrier frequency which now has the most favourable reception conditions.
The predetermined time period is therefore determined as a function of the number of carrier frequencies and of the frequency with which the frequency station changes the carrier frequency.
The fixed station can change the carrier fre-quency is particular in each case after the time period .
in accordance with a time slot in a time division multi-plex method.
The fixed station can change the carrier fre quency in accordance with a predetermined frequency, the check signal broadcast by the fixed station then indicat ing the position of the predetermined sequence to which the current carrier frequency corresponds. By means of the sequence, also knows here, and the transmitted position of the predetermined sequence, the mobile unit can then itself determine the next carrier frequency which the fixed station will change to.
As an alternative, the check signal can also specify the carrier frequency which the base station will "jump to" next.
As a further alternative, the check signal can specify which carrier frequency the base station will use in the m-th time slot or m-th frame. This is advantageous if a mobile unit ie is the so-called idle-locked or multiframe mode. In such a mode. a mobile unit resyn-chronizes with the base station only in every m-th time slot or frame if said mobile unit is not in the process of active voice communication with the base station.
The check signal does not have to be broadcast in every time slot or frame. If a mobile unit which would like to synchronize with a base station receives a time slot or frame which does not contain a check signal, it scans all the carrier frequencies again, this procedure being repeated until the mobile unit receives from the base station a time slot or frame which contains the check signal.
Time-slot and frame synchronization can be carried out as a result of the exchange of syachroniz ation data.
In accordance with a further aspect of the invention, in order to perform the abovemeationed task an arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station is provided for a transmission of data by radio, in which transmission the data are transmitted on a plurality of carrier frequencies. The arrangement here has a device in the fixed station for broadcasting a check signal, the check signal changing the carrier frequency whenever a specific time period has elapsed. A
device which senses the reception conditions successively for each of the plurality of carrier frequencies is provided is the mobile unit. In addition, a device is provided in the mobile unit which sets the mobile unit to reception on the carrier frequency which has the most favourable reception conditions determined by the device for sensing the reception conditions, the mobile unit remaining on the aforesaid carrier frequency until the broadcasting device of the fixed station changes the carrier frequency of the check signal to the set carrier frequency of the mobile unit. In the fixed station and is the mobile unit devices are provided which enable the synchronization data to be exchanged between the fixed station and the mobile wait and/or in the opposite direction.
In particular, the sensing device in the mobile unit may be provided in such a way that it senses the reception conditions on a carrier frequency by means of the energy content prevailing on this carrier frequency or the prevailing interference.
After the reception device in the mobile unit has set the mobile unit to reception on the most favourable carrier frequency and a predetermined time period has elapsed in which the broadcasting device in the fixed station has not changed the carrier frequency of the check signal to the carrier frequency of the mobile unit, said reception device can again sense the reception conditions successively for each of the plurality of carrier frequencies. Then, the reception device again sets the mobile unit to reception on the carrier fre-quency which then has the most favourable reception conditions.
The predetermined time period can be defined here as a function of the number of carrier frequencies and of the frequency with which .the fixed station changes the carrier frequency.
The broadcasting device in the fixed station can change the carrier frequency of the check signal in accordance with a predetermined sequence. The check signal broadcast by the fixed station can have data here which indicate the position of the current carrier frequency in an algorithm or frequency table, by means of which algorithm or table the predetermined sequence is determined.
The exchange devices can carry out time-slot and frame synchronization by exchanging the synchronization data.
The invention will now be explained in more detail by means of an exemplary embodiment and with reference to the accompanying figures, in which:
Fig. 1 shows an arrangement according to the invention for transmitting data in a wire-free fashion, Fig. 2 shows a time frame of a data transmission standard such as is used in the present invention, Fig. 3 shows a detailed illustration of a time frame according to the invention, for a carrier frequency, and Fig. 4 shows a schematic representation of a frequency hopping spread spectrum system to illustrate the embodiment.
With reference to Fig. 1, the general design of the arrangement according to the invention for radio transmission will be explained first. As is generally customary, the arrangement for the transmission of data by radio has a fixed station 1 and a plurality of mobile units (mobile stations, cable-free telephones) 2, 3, 11. The fixed station 1 is connected here to the landline network with a terminal line 10. The ffixed station 1 has an antenna 6 by means of which it is possible to communicate, for example, with the mobile unit 2 via a radio transmission path 8 or with the mobile unit 3 via a radio transmission path 9. The mobile units 2, 3, 11 each have an antenna 7 for receiving and transmitting data.
' CA 02300061 2000-02-11 GR 97 P 2127 P - 7(~-frame according to the invention, for a caier frequency, and ~, ~'' Fig. 4 shows a schematic represtation of a frequency hopping spread spect~system.
With reference to Fig. 1,,~.,~~~the general design of the arrangement according to .~t,~he invention for radio transmission will be explai~id firstly. As is generally customary, the arrangeme~r~l for the transmission of data E..
by radio has a fixed ,~, atioa l and a plurality of mobile units (mobile stags, cable-free telephones) 2, 3, 11.
The fixed stat,pi~ti 1 is connected here to the landline network with ,rsi~r~ terminal line 10 . The fixed station 1 has an antenaa°r6 by means of which it is possible to communi-cate. iii' example, with the mobile unit 2 via a radio tra~~ission path 8 or with the mobile unit 3 via a radio tnsmission path 9. The mobile units 2, 3, 11 each have The internal design of a fixed station 1, insofar as it is of significance for the present invention. will now be explained in more detail. A processor 15 which determines. a predetermined sequence by means of a predetermined algorithm (hop algorithm) is provided in the fixed station 1. As an alternative, a plurality of different algorithms may be provided in the processor 15, so that the processor 15 can determine different sequences in accordance with the respectively used algorithm. The sequences determined by the processor l5 are then transmitted to a storage and output device 13.
The stoxage and output device 13 transmits to as RF
module 4 either the sequence which is continuously determined by the processor 15 or a sequence which has been previously permanently stored in it. The RF module 4 receives and transmits data on a carrier frequency fx which depends oa the current value of the sequence transmitted from the storage and output device 13.
Therefore, a radio transmission takes place on a carrier frequency f,~. the currently used carrier frequency either being determined indirectly by the processor 15 by mean' of an algorithm or, alternatively. being determined directly from the value of a sequence which has been permanently stored is the storage and output device 13.
The internal design of a mobile unit, insofar as it is relevant to the present invention, will now be described in more detail. In this respect, the design of a mobile unit 2, 3, 11 is essentially symmetrical to the internal design of the fixed station 1 described above.
That is to say each mobile unit 2, 3, 11 has, as illus-trated is the invention only for the mobile unite 2 and 11, a processor 16. This processor 16 determines, by means of one. or alternatively by means of a plurality of available hop algorithms, a sequence which it transmits to a storage and output device 12. The storage and output device l2 transmits to an RF module 5 either the values of the sequence based on the algorithm which are determined continuously by the processor 16 or, alternatively, values of a sequence which has been permanently stored in it. The RF module 5 transmits or receives data on a carrier frequency fx whose level depends on the value of the sequence transmitted to it by the storage and outputting device 12. A mobile unit 2. 3, 11 therefore receives or transmits data on a carrier frequency fx whose level depends either on the current value of the sequence determined by the processor 16 or on the value of a sequence which has been permanently stored in the storage and output device 12.
It is to be noted here that the processor 15 in the fixed station l and the processors 16 in the mobile units 2, 3, 11 are based on the same algorithm for determining sequences, or is the event that a plurality of algorithms are available. have the same selection of algorithms. In the event that the sequence is not deter-mined continuously by the processor 15, 16 but rather permanently prescribed in the storage and output devices 12, 13. the sequence which is stored in the storage and output device 13 of the fixed station 1 is of course identical to the sequences which are respectively stored in the storage and output devices 12 of the mobile units 2, 3, 11.
A transmission standard such as is used in the present invention will now be explained with reference to Fig. 2. As is clear in Fig. 2, data are transmitted in chronological succession is a plurality of time slots, 24 time slots Zx in the case illustrated, using the time division multiplex method TDMA (Time Division Multiple Access) on a plurality of carrier frequencies fx, of which tea are illustrated. Ia the case illustrated, duplex mode is used on the carrier frequencies. This means that after the first twelve time slots Zx have been transmitted, reception is switched to and the second twelve time slots (13 - 24) are received is the opposing direction. ..
In the event that the so-called DECT Standard is used for transmission, the chronological duration of a time frame is 10 milliseconds, and 24 time slots Zx are provided. namely twelve time slots for the transmission from the fixed station to mobile units and a further twelve time slots Zx for the transmission from the mobile units to the fixed station. In the DECT Standard, 10 carrier frequencies fx between 1.88 GHz and 1.90 GHz are provided.
However, the present invention is also used in particular for transmission in the so-called 2.4 GHz ISM
(Industrial Scientific Medical) frequency band. The ISM
frequency band has a bandwidth of 83.5 MHz. In accordance with the specification "FCC Part 15" (Federal Communica-tions Commission), at least 75 carrier frequencies fx must be distributed over these 83.5 MHz. Distributing the 83.5 Mfiz bandwidth over 96 carrier frequencies, i.e. a channel spacing of 864 kHz, is particularly advantageous.
The abovementioaed frequency bands and standards are mentioned purely by way of example. The only funda-mental precondition for the invention is that a so-called frequency hopping spread spectrum is used, i.e. a plural-ity of carrier frequencies are available, and that the aR 97 P 2127 P - 10 -carrier frequency fx selected for the transmission is changed from time to time. A precondition of such a change is that the data are transmitted is time slots Zx (time division multiplex method). The so-called DECT
Standard, for example, as well as any other modified standard based on this DECT Standard, is therefore suitable. The modification can comprise, for example, reducing (for example halving) the time slots per frame.
A time slot or a time frame of the transmission, for example, is suitable as the time period after which the carrier frequency is changed.
How the selection of a carrier frequency fx for a specific time slot Zx is carried out will now be explained with reference to Fig. 4. It will be assumed that, at the time of the time slot Zl, the processor 15 of the fixed station 1 determines, oa the basis of an algorithm. a value which the RF module 4 of the fixed station 1 converts indirectly into a carrier frequency fl. In Fig. 4, the hatching shows that the carrier fre-quency fi is selected at the time of the time slot Zl. At the transition from the time slot Zl to the following time slot Z2, the carrier frequency fx is inevitably changed. As is illustrated by an arrow in Fig. 4, it is possible, for example, for the processor 15 of the fixed station 1 to determine by means of its algorithm a value which is converted by the RF module 4 into a carrier frequency f3. In the same way, a carrier frequency fz can then be selected for the time slot Z3, which is illus-trated by hatching and by an arrow.
Even though the case of the carrier frequency fx being changed after each time slot Zx has been illus-trated. other change intervals are also conceivable. For example. the carrier frequency fx can be set in each case after one frame of the transmission has elapsed.
The fixed station 1 therefore changes the carrier frequency fx from the carrier frequency fi to the carrier frequency f3 and then to the carrier frequency fs.on the basis of the sequence determined by the processor 15. If communication is to take place between the fixed station 1 and a mobile unit 11, it is necessary to ensure that the mobile unit 11 can follow synchronously the sequence of carrier frequency fx changes carried Qut by the fixed station 1. This is a problem in particular when a mobile unit 11 is to be first integrated into a radio trans-mission system. i.e. has to be logged on and signed on at the fixed station 1. During unsynchronized operation of the new mobile unit 11 after it has been switched on, the mobile unit 11 will change the carrier frequencies fx used, in the way prescribed by its sequence. The sequence as such is identical here with the sequence l, which is prescribed in the fixed station 1 and explained above.
However, this does not ensure that the sequence of the mobile unit 11 is synchronized temporally with the sequence of the fixed station 1 after said mobile unit 11 has been switched on.
As an alternative, the check signal can also specify the carrier frequency which the base station will "jump to° next.
As a further alternative, the check signal can specify which carrier frequency the base station will use in the m-th time slot or m-th frame. This is advantageous if a mobile unit is in the so-called idle-locked or multiframe mode. In such a mode, a mobile unit resynchronizes with the base station only in every m-th time slot or frame if said mobile unit is not in the process of active voice communication with the base station.
The check signal does not have to be broadcast in every time slot or frame. If a mobile unit which would like to synchronize with a base station receives a time slot or frame which does not contain a check signal, it scans all the carrier frequencies again, this procedure being repeated until the mobile unit receives from the base station a time slot or frame which contains the check signal.
After it has been switched on, the mobile unit 11 scans the available range of carrier frequencies fx until it senses the carrier frequency fx currently being used by the fixed station 1. It will now be explained in detail below how the searching for the continuously changing carrier frequency is performed according to the invention.
Directly after it has been switched on, the mobile unit 11 is in a state which is asynchronous with respect to the carrier frequency fx of the fixed station 1. Even if the fixed station 1 is still not carrying out aoa a voice data transmission, it transmits, oa the carrier ~
frequency fx which changes after a predetermined time ~~
~r'~lss~n period, a check signal (check chaanel) in order to permit mobile units to be logged on. This check signal can also be retained when voice transmission takes place.
The mobile unit 11 to be logged on then scans all the available carrier frequencies fx. In the process, it senses for each carrier frequency fx the energy content prevailing on it, said content being taken ae a measure of the presence of interference. For this purpose, the mobile unit 11 sets itself to reception on a carrier frequency and senses the level of the signal modulated onto this carrier frequency (fx). If there is no signal modulated onto the carrier frequency, this means that on this carrier frequency there is neither another, inter-fering mobile radio system nor a genuine noise source 25~ (microwave or the like).
The mobile unit 11 thus senses, by scanning all the carrier frequencies, the carrier frequency fx which provides the best reception properties. After the sens-ing, the mobile unit 11 sets itself to reception on the carrier frequency fX which is determined as being the most favourable. The mobile unit 1I then waits, ready to receive on the carrier frequency fx sensed to be the most favourable, for the carrier frequency of the check signal of the fixed statioa~' to be changed precisely to this carrier frequency. As soon as the carrier frequency fx of the check signal of the fixed station ~ changes to the ~ ;
V ~
.
carrier frequency fx on which the mobile unit 11 is waiting ready to receive, time-slot and frame synchronization can take place by means of a transmission of data from the mobile unit 11 to the fixed station and in the opposite direction.
Fig. 3 illustrates how it is ensured according to the invention that the new mobile unit 11 carries out carrier frequency changes which are synchronous with the fixed station 1. As ie clear in Fig. 3, the data trans-mitted in a time slot (channel) Zx are. for the most part, information data. i.e. for example data which represent an item of voice information data of a telephone call. Before the range of the information data there is then a check range which is referred to as A
field in the DECT Standard. In this check range, data are provided for synchronizing the operation of a mobile unit 11 to be logged on with the operation of the fixed station 1. If no data are transmitted in the information range, only the data of the check range are transmitted.
Transmission only of data of the check range therefore constitutes a check signal.
If a plurality of algorithms are available to the processor 15 in the fixed station 1 for determining the sequence which directly prescribes the changes of the carrier frequency fx of the fixed station l, the check range contains data which identify the algorithm cur rently in use.
If. as explained above, a so-called multiframe mode is used, the check range can contain data which indicate directly or indirectly which carrier frequency the base station will use in every m-th frame or time slot. This can be carried out indirectly by, for example, the data of the check range specifying which algorithm is used in the respective m-th frame or time slot if the algorithm in these frames or time slots is a different one from that in the other frames or time slots.
Further synchronization data contained in the check region are data which indicate which position in the predetermined sequence corresponds to the carrier frequency fx used for the current time slot Zx. The data of the check range which are illustrated in Fig. 3.
namely data which refer to the algorithm used and which refer to the current position of the sequence of the current algorithm, are broadcast by the fixed station 1 to the mobile unit 11.
During the sensing of the carrier frequency fx which is currently being used by the fixed station l, the mobile unit 11 senses the data of the check signal which is broadcast by the fixed station 1.
If every frame or time slot which is broadcast by the base station does not contain the check data, and a mobile unit which would like to synchronize with this _base station receives such a frame or time slot, the mobile unit does sot receive any information which would enable it in any way to infer the carrier frequency which is to be used next by the base station. In such a case, the mobile unit continues to scan all the available carrier frequencies until it receives a frame or time slot which contains the frequencies in the check data.
At first, the mobile unit 11 can therefore determine which algorithm is currently being used by the processor 15 in the fixed station 1, which algorithm of course directly prescribes the change of the carrier frequencies fx of the fixed station 1. In addition, the mobile unit 11 can sense, from the position data of the check range, which position in the predetermined fre-quency corresponds to the broadcast carrier frequency.
The mobile unit 11 is therefore then aware of the __ algorithm in use and of the position in the sequence. The mobile unit 11 can therefore then determine independently by means of the position in the sequence, which is knows here, as well as the sequence stored in it, which carrier frequency fx will be used by the fixed station 1 in the following time slot Zx. From the information fed to it, the mobile unit 11 can therefore generate information for the carrier frequencies to be used in the following time slots Zx. Thus, it is possible to communicate with the fixed station l, as is necessary for a signing on or logging-on procedure. As a result of the information _ supplied relating to the future carrier frequency change, the mobile unit 11 is therefore then synchronized with the fixed station 1.
The fixed station 1 can have a switching device 14 which can be switched between two positions, namely a position in the logging on mode R and a position corresponding to the normal transmission mode. Only if the switching device 14 is switched to logging on mode R
does the fixed station 1 automatically broadcast the check range data necessary for synchronization with a mobile unit to be newly logged on, this data being namely the information relating to the algorithm in use and the information relating to the position in the predetermined frequency on the basis of the algorithm. If the switching device 14 is switched to the normal transmission mode N, the aforesaid synchronization data are normally not broadcast, or only broadcast on request.
~s~~m~b~ __ d'11 can result from a so-called noise source fall- ck mode. Firstly, it will be explained what action fixed station 1 takes in accordance with this ise source fall-back mode with regard to the crier frequency selection. With reference to Fig. 4~ is clear that at the time of the time slot Z3 the~arrier frequency f, is indicated by the predetermine frequency. It will now be assumed that the predate ned sequence for the time of the time slot Z4 in Gates a change to the carrier frequency f,. In a ition, it will be assumed that, for example in the eceding time frame of the transmission, the fixed ation 1 has determined that interference occurre uring a transmission on the carrier frequency f,. s interference may result, for example, from the f~t that another radio transmission arrangement is If the fixed station 1 is in the so-called noise source fall-back mode, when selecting the carrier fre-quency fx for the time slot Z4 it will not select the carrier frequency f4 which is, of course, actually pre-scribed by the predetermined frequency. The carrier frequency f, which is sensed as being subject to interference is instead passed over and another carrier GR 97 P 2127 P ~~'~~w A problem when logging on a further mobile unit 11 can result from a so-called noise source fall-back mode. First, it will be explained for the embodiment of the invention according to Fig. 4 what action the fixed station 1 takes in accordance with this noise source fall-back mode with regard to the carrier frequency selection. With reference to Fig. 4, it is clear that at the time of the time slot Z3 the carrier frequency f2 is indicated by the predetermined frequency. It will now be assumed that the predetermined sequence for the time of the time slot Z4 indicates a change to the carrier frequency f4. In addition, it will be assumed that, for example in the preceding time frame of the transmission, the fixed station 1 has determined that interference occurred during a transmission on the carrier frequency f4. This interference may result, for example, from the fact that another radio transmission arrangement is adversely affecting this carrier frequency f4.
frequency fx, for example the carrier frequency fx which follows in the predetermined frequency, is selected for the time slot Z4 (as illustrated by the arrow Pl). In the case illustrated in Fig. 4, the carrier frequency selected for the time slot Z4 is therefore not the carrier frequency fs which is sensed as being subject to interference but instead the carrier frequency fl which is sensed as being free of interference.
Even if this noise source fall-back mode has, of course, large advantages during the radio transmission mode with mobile units 2, 3 which have already been integrated, it is clear that this noise source fall-back mode simultaneously causes large problems for the logging on of a new mobile unit 11. The mobile unit 11 will, in fact, determine. on the basis of the algorithm stored in it and the position of the carrier frequency, known to it from the check region of the data transmitted from the fixed station, in the predetermined sequence in accord-ance with the algorithm at the time of the time slot Z3, that a transmission on the carrier frequency f~ will take place starting from the next value of the sequence at the time of the time slot Z4. However, if, owing to the noise source fall-back mode, the fixedstation 1 selects the carrier frequency fl at the time of the time slot Z4 in order to avoid the carrier frequency f4 which is subject to interference, and at the same time the mobile unit 11 to be logged on selects, on the basis of the information available to it, the carrier frequency f~ at the time of the time slot Z4, synchronization of the operation of the fixed station 1 with that of the mobile unit 11 fails.
If, for this reason, the logging on mode R is selected by the switching device 14 in the fixed station 1, the noise source fall-back mode of the fixed station 1 can simultaneously be switched off. This means that. in contrast with the normal mode in which, as stated above, the fixed station 1 will in order to avoid the carrier frequency f, which has been recognized as being subject to interference, switch, in a position of the switching device 14 in logging on mode R, to the carrier frequency f, at the time of the time slot Z4 as is prescribed by the sequence on the basis of the algorithm of the processor 15, although the fixed station 1 is aware that the carrier frequency f, is subject to interference. The change of the carrier frequency fx from time slot Z3 to time slot Z4 is illustrated in Fig. 4 by the unbroken arrow Ps. As a result of the fact that the noise source fall-back mode of the fixed station 1 is simultaneously switched off when the switching device 14 is positioned in logging on mode R, it is therefore ensured that a synchronization of the operation of the. mobile unit 11 with that of the fixed station 1 can take place. After the signing-on procedure or logging on of the mobile unit 11 at the fixed station 1 has been completed, the switching device 14 is then switched back from the logging on mode R to the normal transmission mode N, which can take place in as automated way, and the noise ' source fall-back mode can thus be switched on again automatically.
Alternatively, in order to synchronize a mobile unit 11 with the fixed station~~,~the noise source fall-back mode may also remain switched on. However, in this case the state may occur in which the mobile unit 2 also senses a carrier frequency fx as the most favourable one in the asynchronous state and sets itself ready to receive on this carrier frequency fx, to which the fixed station changes owing to the noise source fall-back mode.
In this case. the mobile unit 11 is set in such a way that it waits a predetermined time period for the carrier , frequency fx of the check signal of the fixed station to change to the carrier frequency fx selected by the mobile unit 11. If.the predetermined time period has elapsed without the carrier frequency fx of the check signal of the fixed station ~ having changed to the carrier frequency fx selected by the mobile unit 11, the mobile unit 11 repeats the abovementioned search pro-cedure (scanning of all the carrier frequencies fx, _ sensing of the carrier frequency fX with the smallest energy content, setting of the receive capability to the carrier frequency fx sensed as the most favourable one, waiting).
The length of the predetermined waiting time period is determined here as a function of the number of carrier frequencies and of the change frequency f" with which the carrier frequency of the check signal is changed.
If, for example, a hundred carrier frequencies are available, the carrier frequency is changed in each case after one frame and a frame is 10 ms long, the predetermined time period can be selected as 100 * 10 ms = 1 s.
List of reference symbols l: Fixed station (base) 2: Mobile unit (cable-free telephone) 3: Mobile unit 4: RF module (in the fixed station) 5: RF module (in the mobile unit) 6: Antenna (fixed station) 7: Antenna (mobile uait) 8: First radio transmission path 9: Second radio transmission path 10: Terminal line 11: Mobile unit 12: Output device (mobile unit) 13: Output device (fixed station) 14: Switching device 15: Processor (fixed station) 16: Processor (mobile unit) fx: Carrier frequency fw: Frequency of the carrier frequency changed by the fixed station Zx: Time slot Pl: Frequency jump (noise source fall-back mode on) P,: Frequency jump (noise source fall-back mode off)
Claims (16)
1. Method for synchronizing the carrier frequencies (f x) of a mobile unit (11) with the carrier frequencies (f x) of a fixed station (1) for a transmission of data by radio, in which transmission the data are transmitted on a plurality of carrier frequencies (f x) and in which a check signal whose carrier frequency (f x) is changed from time to time is broadcast by the fixed staton (1), characterized by the following steps:
- the reception conditions on each of the plurality of carrier frequencies (f x) are sensed successively by the mobile unit (11), - the mobile unit (11) is set to reception on the carrier frequency which has the most favourable reception conditions, until the carrier frequency (f x) of the check signal of the fixed station (1) changes to the carrier frequency (f x) to which the mobile unit (11) is set, and - synchronization data is subsequently exchanged between the fixed station (1) and the mobile unit (11) and/or in the opposite direction.
Claims
- the reception conditions on each of the plurality of carrier frequencies (f x) are sensed successively by the mobile unit (11), - the mobile unit (11) is set to reception on the carrier frequency which has the most favourable reception conditions, until the carrier frequency (f x) of the check signal of the fixed station (1) changes to the carrier frequency (f x) to which the mobile unit (11) is set, and - synchronization data is subsequently exchanged between the fixed station (1) and the mobile unit (11) and/or in the opposite direction.
Claims
2. Method according to Claim 1, characterized in that the mobile unit (11) senses the reception conditions on a carrier frequency (f x) by means of the magnitude of the signal modulated onto this carrier frequency (f x).
3. Method according to Claim 1 or 2, characterized in that, after the mobile unit (11) has been set to reception on the most favourable carrier frequency (f x) and a predetermined time period in which the carrier frequency (f x) of the check signal of the fixed station (1) has not changed to the carrier frequency (f x) of the mobile unit (11) has elapsed, said mobile unit (11) again senses the reception conditions successively for each of the plurality of carrier frequencies (f x) and is then set to reception on the carrier frequency (f x) which-has the most favourable reception conditions in accordance with the new sensing.
4. Method according to Claim 3, characterized in that the predetermined time period is determined as a function of the number of carrier frequencies (f x) and of the changing frequency (f w) with which the carrier frequency (f x) of the check signal is changed.
5. Method according to one of the preceding claims, characterized in that the carrier frequency (f x) of the check signal is changed after one or more time slots (Zx) of a time division multiplex transmission.
6. Method according to one of Claims 1 to 4, characterized in that the carrier frequency (f x) of the check signal is changed after one or more frames of a time division multiplex transmission.
7. Method according to one of the preceding claims, characterized in that the fixed station (1) changes the carrier frequency (f x) in accordance with a predetermined sequence and the check signal broadcast by the fixed station indicates the position of the predetermined sequence to which the current carrier frequency (f x) corresponds.
8. Method according to one of Claims 1 to 6, characterized in that the check signal broadcast by the fixed station indicates the carrier frequency which is to be changed to next.
9. Method according to one of Claims 1 to 6, characterized in that the check signal broadcast by the fixed station indicates which carrier frequency is used in each m-th frame.
10. Method according to one of the preceding claims, characterized in that time-slot (Z x) and frame synchronization is carried out by means of the exchange of synchronization data.
11. Arrangement for synchronizing the carrier frequencies (f x) of a mobile unit (11) with the carrier frequencies (f x) of a fixed station (1) for a transmission of data by radio, in which transmission the data are transmitted on a plurality of carrier frequencies (f x), having a device (4) in the fixed station (1) for broadcasting a check signal whose carrier frequency (f x) is changed from time to time, characterized by:
- a device (5) in the mobile unit (11) for sensing the reception conditions successively for each of the plurality of carrier frequencies (f x), - a device (16) in the mobile unit (11), which device (16) sets the mobile unit (11) to reception on the carrier frequency (f x) which has the most favourable reception conditions, until the broadcasting device (4) in the fixed station (1) changes the carrier frequency (f x) of the check signal to the carrier frequency (f x) to which the mobile unit (11) is set, and - devices (12, 13) in the fixed station (1) and the mobile unit (11) for subsequently outputting synchronization data from the fixed station (1) to the mobile unit (11) and/or in the opposite direction.
- a device (5) in the mobile unit (11) for sensing the reception conditions successively for each of the plurality of carrier frequencies (f x), - a device (16) in the mobile unit (11), which device (16) sets the mobile unit (11) to reception on the carrier frequency (f x) which has the most favourable reception conditions, until the broadcasting device (4) in the fixed station (1) changes the carrier frequency (f x) of the check signal to the carrier frequency (f x) to which the mobile unit (11) is set, and - devices (12, 13) in the fixed station (1) and the mobile unit (11) for subsequently outputting synchronization data from the fixed station (1) to the mobile unit (11) and/or in the opposite direction.
12. Arrangement according to Claim 9, characterized in that the sensing device (5) in the mobile unit (11) senses the reception conditions on a carrier frequency (f x) by means of the signal level modulated onto this carrier frequency (f x).
13. Arrangement according to Claim 11 or 12, characterized in that, after the sensing device (5) in the mobile unit (11) has set the mobile unit (11) to reception on the most favourable carrier frequency (f x) and a predetermined time period has elapsed in which the broadcasting device (4) in the fixed station (1) has not changed the carrier frequency (f x) of the check signal to the carrier frequency (f x) of the mobile unit (11), said mobile unit (11) again senses the reception conditions successively for each of the plurality of carrier frequencies (f x), and the reception device (16) then sets the mobile unit (11) to reception on the carrier frequency (f x) which then has the most favourable reception conditions in accordance with the new sensing.
14. Arrangement according to Claim 13, characterized in that the predetermined time period is determined as a function of the number of carrier frequencies (f x) and of the frequency (f w) with which the broadcasting device (4) in the fixed station (1) changes the carrier frequency (f x) of the check signal.
15. Arrangement according to one of Claims 11 to 14, characterized in that the broadcasting device (4) in the fixed station (1) changes the carrier frequency (f x) of the check signal in accordance with a predetermined sequence, and the check signal broadcast by the broad-casting device (4) has data which indicate the position of the predetermined sequence to which the current carrier frequency (f x) corresponds.
16. Arrangement according to one of Claims 11 to 15, characterized in that the output devices (12, 13) carry out time-slot (Z x) and frame synchronization by means of the outputting of the synchronization data.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/DE1997/001755 WO1999009673A1 (en) | 1997-08-14 | 1997-08-14 | Method and device for synchronising carrier frequencies in a mobile telephone on carrier frequencies of a fixed station |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2300061A1 true CA2300061A1 (en) | 1999-02-25 |
Family
ID=6918540
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002300061A Abandoned CA2300061A1 (en) | 1997-08-14 | 1997-08-14 | Method and arrangement for synchronizing the carrier frequencies of a mobile unit with the carrier frequencies of a fixed station |
Country Status (7)
| Country | Link |
|---|---|
| EP (1) | EP1002377B1 (en) |
| JP (1) | JP2001516168A (en) |
| BR (1) | BR9714817A (en) |
| CA (1) | CA2300061A1 (en) |
| DE (1) | DE59706415D1 (en) |
| ES (1) | ES2172808T3 (en) |
| WO (1) | WO1999009673A1 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1237300B1 (en) * | 2001-02-28 | 2008-05-07 | Nokia Corporation | Method for frequency update in a radio communications system |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5506863A (en) * | 1993-08-25 | 1996-04-09 | Motorola, Inc. | Method and apparatus for operating with a hopping control channel in a communication system |
| US5442659A (en) * | 1993-10-22 | 1995-08-15 | International Business Machines Corporation | Radio communications system with fault tolerant frequency hopping synchronization |
| US5537434A (en) * | 1993-10-25 | 1996-07-16 | Telefonaktiebolaget Lm Ericsson | Frequency hopping control channel in a radio communication system |
-
1997
- 1997-08-14 WO PCT/DE1997/001755 patent/WO1999009673A1/en active IP Right Grant
- 1997-08-14 CA CA002300061A patent/CA2300061A1/en not_active Abandoned
- 1997-08-14 DE DE59706415T patent/DE59706415D1/en not_active Expired - Fee Related
- 1997-08-14 EP EP97938775A patent/EP1002377B1/en not_active Expired - Lifetime
- 1997-08-14 JP JP2000510223A patent/JP2001516168A/en active Pending
- 1997-08-14 ES ES97938775T patent/ES2172808T3/en not_active Expired - Lifetime
- 1997-08-14 BR BR9714817-2A patent/BR9714817A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| BR9714817A (en) | 2000-10-03 |
| EP1002377B1 (en) | 2002-02-13 |
| JP2001516168A (en) | 2001-09-25 |
| ES2172808T3 (en) | 2002-10-01 |
| WO1999009673A1 (en) | 1999-02-25 |
| DE59706415D1 (en) | 2002-03-21 |
| EP1002377A1 (en) | 2000-05-24 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued | ||
| FZDE | Discontinued |
Effective date: 20040326 |