Note: Descriptions are shown in the official language in which they were submitted.
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Description
Method and device for generating a carrier frequency
sequence
The present invention relates to a method and a
device for generating a sequence of carrier frequencies
fx which are used for a mobile radio transmission.
The so-called frequency hopping spread spectrum
is a known method for transmitting data on a plurality of
carrier frequencies. A frequency hopping spread spectrum
system is in this case to be understood as meaning a
system in which, for the purpose of radio transmission of
data, a multiplicity of carrier frequencies are provided
and the currently used carrier frequency is periodically
changed. In a time division multiplex (TDMA) system, in
particular, the carrier frequency may be changed after
each time slot or time frame of the time division multi-
plex transmission or an integral multiple thereof. Such
a frequency hopping spread spectrum system has advantages
to the extent that the energy of the entire radio trans-
mission is distributed over all the carrier frequencies.
This is important particularly when a generally available
frequency band, such as the 2.4 GHz ISM (Industrial,
Scientific, Medical) band, for example, is used. For the
use of this frequency band, an upper limit for the
maximum energy occurring per carrier frequency is defined
in accordance with the relevant specifications (in the
USA, the specification "FCC part 15" defined by the
Federal Communications Commission), in order to minimize
interference of other subscribers. Furthermore, it is
prescribed by the specification "FCC part 15" that 75
different carrier frequencies must be used.
A further advantage of the frequency hopping
spread spectrum system that may be mentioned is that the
system becomes more insensitive to interference as a
result of the provision of a large number of carrier ~~
frequencies. Furthermore, the security of the system
against eavesdropping by third parties is increased since
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the third party generally does not know the carrier
frequency to which a change is made after a certain
period of time.
The sequence of carrier frequencies which are
used one after the other for transmission is determined
by an algorithm. Such an algorithm is implemented in an
identical manner in the fixed station and also in each
mobile station of the mobile radio transmission.
Consequently, if a mobihe part is synchronized with the
associated fixed station with regard to the carrier
frequencies, the mobile part and the fixed station will
perform the carrier frequency change predeterminEd by the
sequence of the algorithm in synchronism with one
another.
A special operating mode is the interference-
source evasive mode. When the interference-source evasive
mode is switched on, a carrier frequency which is actual-
ly predetermined by the sequence and is detected as being
subjected to interference is not used, and a different
carrier frequency (not subjected to interference) is used
for this.
A problem arises with the interference-source
evasive mode when a mobile part is in the so-called idle
locked mode. The idle.locked mode is an operating mode in
which a mobile part, although ready to receive, is,
however, connected to the fixed station without any
active communication. For the purpose of saving energy,
in particular, a mobile part, which in other words is
ready to receive in a type of standby state in the idle
locked mode, resynchronizes its carrier frequencies only
after m carrier frequencies since, after all, each
resynchronization requires a connection to the fixed
station in order to receive at least one time slot and,
consequently, consumes energy. The mobile part in the
idle locked mode, which is therefore resynchronized with
the base station only in every mth frame, therefore does
not exchange any information with the base station in the
remaining frame time durations, i.e. during the first to
(m-1)-th frame. The problem that exists, therefore, is
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that a mobile part which is in the idle locked mode
cannot be informed by the base station about instances of
inhibiting and instances of enabling of certain fre-
quencies. Consequently, there is the risk that the mobile
part will completely lose synchronization with the base
station in the idle locked mode (multiframe mode).
The object of the present invention, therefore,
is to provide a method and a device for generating a
carrier frequency sequence which can ensure that a mobile
part which is synchronized with the base station only in
every mth frame remains reliably synchronized with the
base station.
According to the invention, then, a method for
generating a sequence of carrier frequencies fx for a
mobile radio transmission is provided. In this case, a
first algorithm predetermines the value of the carrier
frequencies fx with the operating parameter (n-1) xm +1 to
(nxm)-1. n is in this case a whole number greater than or
equal to 1 and m is a predetermined whole number greater
than 1, which specifies to a certain extent the
periodicity of the idle locked mode. This means that, for
example if m is equal to 16, the first algorithm prede-
termines the carrier frequencies for the frames or time
slots with the operating parameter 1 to 15, 17 to 31,
etc., that is to say, for example for the frames 1 to 15,
17 to 31, etc. According to the invention, furthermore,
a second algorithm which is independent of the first
algorithm is provided, the second algorithm
predetermining the value of the carrier frequencies fx~Xm.
In the example cited, the second algorithm therefore
predetermines the value for the carrier frequencies of
each 16th frame.
Only the first algorithm can have an opportunity
for inhibiting certain values and for replacing the
inhibited values by another value. Consequently, it is
possible to realize an interference-source evasive mode
by means of the first algorithm for the operating parame-
ters (n-1)xm +1 to (nxm)-1.
The second algorithm can be defined by the base
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station when a mobile part logs onto a base station.
The first algorithm can be defined at the begin-
ning of a set-up of a connection between a mobile part
and a base station.
The second algorithm can be realized by a random
number generator.
The second algorithm can alternatively be gener-
aced by a frequency table that is to be processed sequen-
tially. This frequency table can be determined in par-
ticular using the PIN code number of the relevant mobile
part.
According to the invention, furthermore, a device
for generating a sequence of carrier frequencies fx is
provided, the carrier frequencies being used in mobile
radio transmission. In this case, a first calculation
device for a first algorithm is provided, the first
algorithm predetermining the value of the carrier fre-
quencies fx,n_l,xm .1 to fX,nxm)-1. As already explained, n is
in this case a whole number as operating parameter and m
is a predetermined whole number greater than 1. Fur-
thermore, a second calculation device for a second
algorithm which is independent of the first algorithm is
provided. The second calculation device predetermines the
value for the carrier frequencies fxnxm, in other words
every mth carrier frequency.
The first calculation device may have a device
for inhibiting certain values and a device for replacing
the inhibited values by another value, as a result of
which it is possible to realize a so-called interference
source evasive mode.
The second calculation device may be provided in
a base station and define the algorithm when a mobile
part logs onto the base station.
The first calculation device may be provided in
a base station and define the first algorithm at the
beginning of a set-up of a connection between a mobile
part and the base station.
The second calculation device may have a random
number generator.
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The second calculation device may alternatively
or .additionally have a frequency table that is to be
processed sequentially, the frequency table being able to
be determined using the PIN code number of the mobile
part, for example.
According to the invention, furthermore, a base
station for mobile radio applications is provided, which
has a device for generating a sequence of carrier fre-
quencies fx of the abovementioned type.
Furthermore, according to the invention, a mobile
radio system having a base station and at least one
mobile part is provided, the base station having a device
for generating a sequence of carrier frequencies fx of
the abovementioned type.
The invention will now be explained in more
detail using an exemplary embodiment and with reference
to the accompanying drawings, in which:
Figure 1 shows a mobile radio transmission system
having a fixed station according to the inven
tion,
Figure 2 shows a time frame of a data trans-
mission standard of the kind that can be employed
in the case of the present invention,
Figure 3 shows, in detail, the internal structure
of a fixed station (base station) according to
the invention,
Figure 4 shows a diagrammatic illustration of a
frequency hopping spread spectrum system, in
particular also for the case of an inteference
source evasive mode.
Referring to Figure 1, it is intended first of
all to give an explanation of the general structure of a
mobile radio transmission. As is generally customary, the
arrangement for the radio transmission of data has a
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fixed station 1 and a plurality of mobile parts (mobile
stations'), cordless telephones 2, 3 ... The fixed station
1 is connected by a terminal line 10 to the fixed net-
work. For the purpose of communication, it is possible to
provide an interface device (not illustrated) between the
fixed station 1 and the terminal line 10. The fixed
station 1 has an antenna 6 by means of which, for
example, a communication with the mobile part 2 takes
place via a first radio transmission path 8 or a communi-
cation with the mobile part 3 takes place via a second
radio transmission path 9. The mobile parts 2, 3 ... each
have an antenna 7 for receiving and/or for transmitting
data. Figure l diagrammatically illustrates the state in
which the fixed station 1 is actively communicating with
the mobile part 2 and, consequently, is exchanging data.
The mobile part 3, on the other hand, is in the so-called
idle locked mode, in which it waits in standby-like
fashion for a call from the fixed station 1. In this
state, the mobile part 3 does not communicate continually
with the fixed station 1, but rather receives the data
for example of a time slot only at periodic intervals,
which data are necessary for resynchronization of the
carrier frequencies fx.
The internal structure of the fixed station 1 is
illustrated diagrammatically in Figure 1. The voice
information data are fed to an RF module 4, which is
driven by a carrier frequency sequence unit. The exact
structure of a fixed station 1 according to the invention
will be described later.
Referring to Figure 2, it is now intended to give
an explanation of a transmission standard of the kind
that can be used in the case of the present invention. As
is evident from Figure 2, data are transmitted chrono-
logically successively on a plurality of carrier fre-
quencies fx, ten of which are illustrated, in a plurality
of time slots, 24 time slots Zx in the case illustrated,
using a time division multiplex method TDMA (Time Divi-
sion Multiple Access). In the case illustrated, operation
is in the duplex mode, that is to say after the first
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twelve time slots Zx have been transmitted, a switch is
made to reception, and the second twelve time slots (Z13
to Z24) are received by the fixed station in the opposite
direction.
If the so-called DECT standard is used for the
transmission, the time duration of a time frame is 10 ms,
and 24 time slots Zx are provided, namely twelve time
slots for transmission from the fixed station to mobile
parts and a further twelve time slots Zx for transmission
from the mobile parts to the fixed station. According to
the DECT standard, ten carrier frequencies fx between
1.88 GHz and 1.90 GHz are provided.
Of course, other frame structures, for example
with half the number of time slots compared with the DECT
standard, are just as suitable for the present invention.
The present invention is used particularly for
transmissions in the so-called 2.4 GHz ISM (Industrial,
Scientific, Medical) frequency band. The generally
accessible ISM frequency band has a bandwidth of
83.5 MHz. In accordance with the specification "FCC part
15", at least 75 carrier frequencies fx must be distrib-
uted over these 83 . 5 MHz . What is particularly advantage-
ous is a division of the bandwidth of 83.5 MHz between 96
carrier frequencies, i.e. a channel spacing of 864 kHz.
The abovementioned frequency bands and standards are
cited purely as an example. The only fundamental precon-
dition for applicability of the present invention is that
a so-called frequency hopping spread spectrum is used,
i.e. that a plurality of carrier frequencies are avail-
able, and that the carrier frequency selected for trans-
mission is periodically changed. For such a change, it is
advantageous if the data are transmitted in time slots Zx
(time division multiplex method). The DECT standard is
therefore suitable, for example, as well as any other
modified standard based on this DECT standard.
Referring to Figure 3, it is now intended to give _
a more detailed explanation of the internal structure of
a fixed station 1 according to the invention. As can be
seen in Figure 3, information data are fed to the RF
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module 4 when transmission is to be effected from the
fixed station 1 to a mobile part 2, 3 ... by means of the
antenna 6, and information data are output from the RF
module 4 when data from mobile parts are received. The RF
moduel 4 modulates the digital encoded information data
onto a carrier frequency fx. The carrier frequency fx
that is currently to be used is in this case predeter-
mined by a carrier frequency sequence unit, which is
designated in general by 20.
As illustrated in Figure 3, the main components
of the carrier frequency sequence unit 20 are a first
calculation device, which is designated in general by 25,
and also a second calculation device 26. A switching
device 27 is furthermore provided. This switching device
27 is driven by the processor 23 in the manner shown and
selects whether the first calculation device 25 or the
second calculation device 26 is to predetermine the
current value for the carrier frequency fx.
Provided in the first calculation device 25 is a
detection device 24, to which the demodulated signal from
the RF module 4 is fed. Inteference in this context means
that either interference in the actual sense or seizure
by another transmitter is present. Interference in the
sense of the present description can therefore be
detected by demodulating a received signal on a carrier
frequency and detecting whether or not a signal level is
present on this carrier frequency.
Inteference in the actual sense can be detected
using CRC errors or burst losses.
The detection device 24 therefore uses the
demodulated signal from the RF module 4 to determine how
high the signal component modulated onto a specific
carrier frequency fx is or whether or a burst or CRC
error has occurred. If the signal component detected lies
above a predetermined limit value or if one of the above-
mentioned errors has occurred, the detection device 24
passes an interference detection signal to an
inhibit/enable unit 21. Depending on the interference-
source detection signal from the detection device 24, the
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inhibit/enable unit 21 passes an inhibit/enable informa-
tion item to a processor 23. This inhibit/enable informa-
tion item indicates which of the carrier frequencies fx
are inhibited or enabled again on account of the detec-
tion of interference by the detection device 24, as will
be explained later.
In other words, the detection device 24 and the
inhibit/enable device 21 provide an independent procedure
by means-~of which frequencies subjected to interference
can be inhibited and enabled again. In addtion to the
inhibit/enable information items from the inhibit/enable
unit 21, a sequence from a random number generator 22 is
fed to the processor 23. On the basis of a random
algorithm implicit in it, the random number generator 22
generates a randomly distributed sequence of carrier
frequency values within the predetermined frequency band.
The random number generator 22 consequently carries out
a procedure which is independent of the procedure of
frequency inhibiting for the case of interference.
The second calculation device 26 is provided for
the purpose of implementing a second algorithm, which is
independent of the first algorithm, realized in the first
calculation device 25. As is evident, there is no poss-
ibility for frequency inhibiting in the case of the
second algorithm realized in the second calculation
device 26. The second algorithm can be determined by the
base station 1 for example when a mobile part logs onto
the base station 1, with the result that after the
logging on, there is no need for any further information
exchange between the base station and the mobile part
with regard to this second algorithm.
The second calculation device 26 can generate the
second algorithm by means of a random number generator
contained in it, for example. As an alternative or in
addition, a frequency table may also be provided in the
second calculation device 26, which table is processed
sequentially by the second calculation device 26. The
frequency table therefore contains the carrier frequency
values to be used for the carrier frequencies with the
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operating parameter nxm, that is to say in the event that
the~carrier frequency is changed after the duration of a
frame, for every mth frame. The frequency table may, for
example, be derived from the PIN code number of the
mobile part 2 that has logged on, as a result of which
mutually independent mobile radio systems each comprising
a base station and at least one mobile part use different
tables.
As is indicated in Figure 3 by arrows from the
processor 23 to the random number generator 22 and to the
second calculation device 26, the processor 23 outputs
different information items to these components. The
random number generator 22, for example, receives the
information item regarding how many different values it
is supposed to generate.
In a mobile part, in particular, the processor 23
can furthermore predetermine for the random number
generator 22 a start value for the algorithm thereof.
This start value is communicated to the mobile part
during synchronization, which can be achieved by the
mobile part and fixed station using the same start value
and the same algorithm.
The second calculation device 26 receives from
the processor 23 information items regarding the period
icity of the idle locked mode, that is to say the value
of m.
Referring to Figure 4, it is now intended to give
a more detailed explanation of the method of operation of
a fixed station 1 according to the invention and of the
method according to the invention. As is illustrated in
Figure 4, a carrier frequency fl is used, for example,
during a frame Rx of a mobile radio transmission, as is
illustrated in hatched fashion in Figure 4. This
frequency fl is therefore the first value of the sequence
which is generated by the random number generator 22 and
is fed to the processor 23, which, in turn, drives the RF
module 4 accordingly. For the frame R2, let it be assumed
that the random number generator 22 prescribes, on the
basis of its calculated frequency, a frequency hop P1 to
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a carrier frequency f3.
Let it now be assumed that the detection device
24 has detected for example during a previous trans-
mission that the carrier frequency f2 is subjected to
interference, in other words the detection device 24 has
passed a corresponding interference signal to the
inhibit/enable unit 21 which, in turn, has indicated
inhibition of the frequency f2 to the processor 23. Let
it furthermore be assumed that the random number gener- -
ator 22, using its determined sequence, prescribes for
the frame R3 the carrier frequency f2 which was previ-
ously detected as being subjected to interference.
Proceeding from the coincidence between the prescribed
carrier frequency f2 according to the sequence of the
random number generator 22 and, at the same time, the
inhibit signal from the inhibit/enable unit 21 for the
same carrier frequency f2, the processor 23 now replaces
the carrier frequency f2, which was actually prescribed
but has been detected as being subjected to interference,
for the frame R3 by a carrier frequency that has not been
detected as being subjected to interference by the
detection device 24, for example the carrier frequency
f4, as is indicated by the frequency hop arrow P3.
Therefore, instead of the carrier frequency f2 actually
prescribed by the sequence, the RF module 4 is driven to
the backup carrier frequency f4. Replacing the carrier
frequency which was detected as being subjected to
interference therefore creates a modified sequence of
carrier frequencies. The modified sequence has in such a
case only carrier frequencies that are not subjected to
interference. By virtue of the fact that a carrier
frequency which has been detected as being subjected to
interference is replaced, and not skipped, as a result of
the transition to the following carrier frequency, the
positions of the carrier frequencies that are not
subjected to interference in the modified sequence are
not altered in comparison with the original sequence.
The basis of this modified sequence comprising
only carrier frequencies fx that are not subjected to
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interference is, therefore, two superposed, mutually
independent procedures (random number generator 22 and
inhibit/enable unit 21). The first procedure comprises
the random number generator 22, which generates values
between 0 and N, where N is the number of possible
carrier frequencies. The second procedure inhibits
frequencies that are subjected to interference as
explained above. This inhibiting can be cancelled again
by the inhibit/enable unit 21 as soon as a recent detec-
tion by the detection device 24 indicates that the
carrier frequency that was formerly subjected to inter-
ference is now no longer subjected to interference. For
this case, the inhibit/enable unit 21 passes to the
processor 23 an enable signal, which indicates that the
processor 23 now no longer has to replace the carrier
frequency that was formerly subjected to interference by
a different carrier frequency.
As an alternative, the inhibit/enable unit 21 can
automatically output an enable signal to the processor
23, without recent detection by the detection device 24,
as soon as a predetermined period of time has elapsed.
Each of the said procedures therefore ensures per se that
the entire predetermined frequency spectrum is utilized
with uniform distribution. Consequently, by adapting the
times in the procedure for the inhibiting of frequencies,
... it is possible to comply with standards such as, for
example, the US specification "FCC part 15", which impose
upper limits on energy transmitted on a carrier fre
quency.
The random number generator 22 is constructed in
a known manner and is therefore not explained any further
in the course of the present description. It is import-
ant, however, that the random number generator be oper-
ated independently of the inhibit/enable procedure. An
identical random number generator is, moreover, imple-
mented in each mobile part 2, 3.
The fixed station 1 is the master during fre-
quency allocation, i.e. at the beginning of a connection
set-up, the random number generator in a mobile part is
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initialized with the state of the random number generator
22 .of the fixed station 1. The random number generators
in the mobile part 2, 3 ... and in the fixed station 1
then generate the same carrier frequency values
synchronously in timing and autonomously of one another.
The procedure for frequency inhibition which is
carried out by the detection device 24 and the
inhibit/enable unit 21 uses a unidirectional protocol on
the radio interface during the entire connection time
between the fixed station 1 and a mobile part 2, 3 ... If
the detection device 24 finds one of the n possible
frequencies fx from the fixed station 1 to be subjected
to interference, then the fixed station 1 therefore
communicates to all mobile parts with which it is operat-
ing connections that this frequency which is subjected to
interference, if it is generated by the sequence of the
random number generator, must be replaced by a different
carrier frequency that is not detected as being subjected
to interference. The random number generator 22 is not
influenced by the frequency inhibition. This frequency
inhibition is cancelled again by the inhibit/enable unit
21 when the inhibited carrier frequency is again suitable
for transmission, or when it has been inhibited for
longer than a previously defined time.
The invention consequently affords a number of
advantages. In the idle locked mode, mobile parts cannot
acknowledge frequency inhibition to the fixed station 1
since, after all, they can only receive, but not trans-
mit, in this special operating mode. However, if the
frame with an information item regarding the frequency
inhibition is subjected to such interference during the
transmission from the fixed station 1 to the mobile part
(unidirectional protocol) that the mobile part does not
actually receive this information item regarding the
frequency inhibition, the synchronously running random
number generators in the fixed station 1 and in the
mobile parts 2, 3 ensure that in the case of the non-
inhibited carrier frequencies in the frames after the
frames of an inhibited carrier frequency, the fixed
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station 1 and all of the active mobile parts use the same
carrier frequency.
According to the invention, a so-called multi
frame mode can be realized in a particularly favourable
and simple manner. A multiframe has a length of m frames.
m may be 16, for example. According to the invention, the
value for the carrier frequency fx which is output by the
algorithm of the second calculation device 26 and is
completely independent of the first algorithm realized by
the first calculation device 25 is used in every mth
frame (multiframe frame). In other words, the first
algorithm in the first calculation device 25 is used for
the carrier frequencies having the frame number (n-1) xm+1
to (nxm)-1, where n is an operating parameter a 1 and m
is a predetermined whole number > 1. The value of the
second algorithm of the second calculation device 26 is
then used for the carrier frequencies having the frame
number nxm, which is illustrated symbolically in Figure
3 by a changeover device 27. If, for example, the multi-
frame has a length of 16 frames, the carrier frequencies
are determined by the first algorithm of the first
calculation device 25 in the frames having the numbers 1
to 15, while the carrier frequency is determined by the
second algorithm in the multiframe frame having the
number 16. Consequently, a mobile part which is in the
multiframe mode can always remain synchronized with the
base station since the second algorithm used for the
multiframe mode never changes. In the case of an actual
connection set-up, an information exchange regarding the
first algorithm is carried out between the base station
and the mobile part to be connected. After the informa-
tion exchange regarding the first algorithm, all of the
frames can then be utilized.
According to the invention, it is therefore
ensured that mobile parts which are in the so-called
multiframe mode and are resynchronized only every m
frames, and therefore cannot receive the signalling of a
frequency inhibition in the idle locked mode, are not
adversely affected by frequency inhibitions of the fixed
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station 1 in the sense that their synchronization with
the fixed station 1 is altogether lost.
Figure 4 illustrates the example of a multiframe
comprising 5 frames. As illustrated, the carrier fre
quencies for the frames having the numbers 1 to 4 are in
this case predetermined by the first algorithm. For the
frame having the number 5 (multiframe frame) , the value
on the basis of the second algorithm of the second
calculation device 26 is used. As illustrated in Figure
4 this value may be for example a value (carrier fre-
quency f2) which was actually detected as being subjected
to interference by the first calculation device. The
second calculation unit 26, however, never carries out
inhibitions of carrier frequency values and therefore
even uses, completely independently of the detection in
the first calculation device 25, carrier frequencies
which are subjected to interference.
According to the invention, it is thus possible
to realize a multiframe mode by providing two mutually
independent algorithms.
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List of reference symbols
l: Fixed station
2: Mobile part
3: Mobile part
4: RF module
6: Antenna fixed station
7: Antenna mobile part
8: First radio transmission path
9: Second radio transmission path
10: Terminal line
20: Carrier frequency sequence unit
21: Inhibit/enable unit
22: Random number generator
23: Processor
24: Detection device
25: First calculation device
26: Second calculation device
27: Switching device
fx: Carrier frequency
Rx: Frame
Zx: Time slot
