Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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.
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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
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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
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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,
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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.
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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
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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
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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
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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
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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
GR 97 P 2127 P - 12 -
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
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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
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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
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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
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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.
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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
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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
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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.
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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)
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