WO1999009668A1

WO1999009668A1 - Method and device for producing a random number sequence for carrier frequencies in a mobile radiotelephone system

Info

Publication number: WO1999009668A1
Application number: PCT/DE1997/001734
Authority: WO
Inventors: Jürgen KOCKMANN; Uwe Sydon
Original assignee: Siemens Aktiengesellschaft
Priority date: 1997-08-14
Filing date: 1997-08-14
Publication date: 1999-02-25
Also published as: EP1002375A1; CA2300064A1

Abstract

The invention concerns a method and a device for producing a sequence of random numbers, for transformation into carrier frequencies (fx) for a mobile radiotelephone transmission, using an algorithm, based on a shifting register (25) with feedback (27)with n bits. A bit of the shifting register (25) with feedback (27) is added by modulo2 addition to another bit of the shifting register (25). The shifting register (25) content is shifted, time pulsed, by a bit to the right. The modulo2 addition result is input in the site of the left bit cleared by the shift. Then, the shifting register (25) content is transformed (23) into a carrier frequency (fx) for a mobile radiotelephone transmission. For said transformation (23) an algorithm in the form of an endless loop is used.

Description

description

Method and device for generating a random number sequence for carrier frequencies of a mobile radio transmission

The present invention relates to a method and a device for generating a random number sequence which is converted into carrier frequencies for a mobile radio transmission.

The so-called frequency hopping spread spectrum system is known as a method for transmitting data on a plurality of carrier frequencies. A frequency hopping spread spectrum system is understood to mean a system in which a large number of carrier frequencies are provided for radio transmission of data and the carrier frequency currently used is changed periodically. In a time division multiplex (TDMA) system in particular, the carrier frequency can be changed after each time slot or time frame of the time division multiplex transmission (or multiples thereof). Such a frequency hopping spread spectrum system has advantages in that the energy of the entire radio transmission is distributed over all carrier frequencies. This is particularly important if a generally available frequency band, such as the 2.4 GHz ISM (Industrial, Scientific, Medical) band, is used. In accordance with the relevant regulations ("FCC part 15", Federal Communications Commission), an upper limit for the maximum energy occurring per carrier frequency is set for the use of this frequency band in order to keep interference to other participants as low as possible. Furthermore, the regulation "FCC part 15 "that at least 75 different carrier frequencies must be provided.

A further advantage of the frequency hopping spread spectrum system is that the system is made less sensitive by providing a large number of carrier frequencies. against disturbances. In addition, the system is more secure against eavesdropping from third parties, since the third party generally does not know which carrier frequency is to be changed after a certain period of time.

The sequence of carrier frequencies that are used for transmission in succession is determined by an algorithm. Such an algorithm is implemented in an identical manner in the base station and in each mobile station of the mobile radio transmission. If a handset is thus synchronized with the associated base station, the handset and the base station will synchronously carry out the carrier frequency changes specified by the sequence of the (identical) algorithm.

The algorithm should ensure that each carrier frequency is used the same number of times and for the same length of time over a period of time.

The object of the present invention is therefore to create a method and a device for generating a random number sequence for carrier frequencies of a mobile radio transmission, which enable a random number algorithm to be implemented in a simple manner.

This object is solved by the features of the independent claims. The dependent claims further develop the central idea of the invention in a particularly advantageous manner.

According to the invention, a method for generating a random number sequence is thus provided, the random number sequence being converted into carrier frequencies for a mobile radio transmission. First, a shift register with n bits is provided. This is followed by a feedback modulo2

Add a bit of the shift register with one or more other bits of the shift register. Then the register content of the shift register is shifted clockwise by one bit. The result of the modulo2 addition is then inserted into the bit position freed up by the shift. The content of the shift register is cyclically converted into a carrier frequency fx for a mobile radio transmission. The feedback of the shift register thus creates in a particularly simple manner a random number sequence with a maximum periodicity t (depending on the type of feedback) of 2 ^n- 1, where n is the number of bits in the shift register.

By changing the feedback, ie by changing the bits of the shift register used for the modulo2 addition, different sequences can be generated. With a shift register with 16 bits, for example, 2048 different sequences with a maximum length (2 ⁿ -l) can be generated.

The step of converting the content of the shift register into carrier frequencies for a mobile radio transmission comprises adding the content of the shift register to the current carrier frequency. It is then decided whether the result of the addition is greater than the number of carrier frequencies present. In the event that the decision is positive, the addition result is then reduced by the number of available carrier frequencies until the decision is negative, i.e. until the result of the addition is no longer greater than the number of available carrier frequencies. In the event that the decision is negative, the addition result is used as the next carrier frequency. The shift register is then shifted by one bit and the process begins again.

According to the invention, a device for converting a random number sequence into carrier frequencies fx for ne cellular transmission provided. This device has a shift register with n bits. Furthermore, an addition device for modulo2 addition of one bit of the shift register with one or more other bits of the shift register is provided in the manner of a feedback. After the addition, the register content is shifted clockwise by one bit, preferably to the right. The result of the adder is then inserted into the bit position that has been freed by the shift. Furthermore, a device for converting the content of the shift register into a carrier frequency for a mobile radio transmission is provided.

The type of feedback, i.e. the bits used for the modulo2 addition and their number can be changed in order to generate different sequences.

The device for converting the content of the shift register into carrier frequencies also has means for adding the content of the shift register to the current carrier frequency. Means are also provided for deciding whether the result of the addition is greater than the number of available carrier frequencies. In the event that the decision is positive, the addition result is reduced by the number of available carrier frequencies until the decision is negative, i.e. until the result of the addition is no longer greater than the number of carrier frequencies present. In the event that the decision is negative, the addition result is used as the next carrier frequency and then the shift register is shifted clockwise by one position.

The shift register preferably has 16 bits.

The shift register can be implemented in an 8 bit or 16 bit processor. The invention further relates to a mobile radio device which has a device of the type mentioned above.

The invention will now be explained in more detail using an exemplary embodiment and with reference to the accompanying drawings. Show it:

1 shows a mobile radio transmission system with a base station according to the invention,

2 shows a time frame of a data transmission standard as can be used in the present invention,

3 shows in detail the internal structure of a base station according to the invention, and

4a shows a shift register as used in the present invention.

Fig. 4b the content of the shift register for the different clocks of a period, and

5 shows a flowchart for converting the random number sequence into carrier frequencies.

1, the general structure of a mobile radio transmission will be explained first. As is generally customary, the arrangement for radio transmission of data has a base station 1 and several mobile parts (mobile stations), wireless telephones 2, 3 .... The base station 1 is connected to the fixed network by a terminal line 10. An interface device, which is not shown, can be provided for communication between the base station 1 and the terminal line 10. The base station 1 has an antenna 6, by means of which, for example, a first radio transmission communication path 8 with the handset 2 or via a second radio transmission path 9, communication with the handset 3 takes place. The handsets 2, 3 ... each have an antenna 7 for receiving or transmitting data. In Fig. 1 the state is schematically shown in which the base station 1 actively communicates with the mobile part 2 and thus exchanges data. The handset 3, on the other hand, is in the so-called idle locked mode, in which it waits stand-by for a call from the base station 1. In this state, the mobile part 3 does not communicate with the base station 1, but rather only periodically receives the data, for example of a time slot, from the base station in order to be able to psychonize itself on the carrier frequencies fx.

The internal structure of base station 1 is shown schematically in FIG. 1. The voice information data are supplied to an RF module 4, which is controlled by a carrier frequency sequence unit. The exact structure of a base station 1 according to the invention will be described later.

2, a transmission standard that can be used in the present invention will now be explained. As can be seen from FIG. 2, data are transmitted successively in a plurality of time slots, in the illustrated case 24 time slots Zx, on a plurality of carrier frequencies fx, ten of which are shown, in a time division multiplex method TDMA (Time Division Multiple Access). In the case shown, work is carried out in alternating mode (duplex), i. i.e., after the first twelve time slots Zx have been transmitted by the base station 1, the system switches to reception and in the opposite direction it receives the second twelve time slots (Z13 to Z24) from one or more mobile stations.

In the event that the so-called DECT standard is used for transmission, the duration of a time frame is 10 ms, and there are 24 time slots Zx, namely twelve time slots for the transmission from the base station to handsets and a further twelve time slots Zx for transmission from the handsets to the base 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 can also be used in the present invention, for example those with a time slot number that is halved compared to the DECT standard.

However, the present invention also finds particular application 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. According to FCC part 15, at least 75 carrier frequencies fx must be distributed over this 83.5 MHz. A division of the bandwidth from 83.5 MHz to 96 carrier frequencies is particularly advantageous, i. H. a channel spacing of 864 kHz. The frequency bands and standards mentioned above are given purely as an example. The basic requirement for applicability in the present invention is only that a frequency hopping spread spectrum is used, i. H. that several carrier frequencies are available and that the carrier frequency selected for transmission is changed periodically. For such a change, it is advantageous if the data are transmitted in time slots Zx (time division multiplex method). For example, the DECT standard and any other modified standard based on this DECT standard are suitable.

The inner structure of a base station 1 according to the invention will now be explained in more detail with reference to FIG. 3. As can be seen in FIG. 3, the RF module 4 is supplied with information data if the base station 1 is to transmit to a mobile unit 2, 3... By means of the antenna 6 and is transmitted by the HF Module 4 outputs information data when data is received from handsets. The RF module 4 modulates the digitally coded information data onto a carrier frequency fx. The carrier frequency fx currently to be used is predetermined by a carrier frequency sequence unit, which is generally designated 20. A detection device 24 is provided in the carrier frequency sequence unit 20, to which the demodulated signal is supplied by the RF module 4. Interference means that there is either an interference in the proper sense or an assignment by another transmitter. A disturbance in the sense of the present description can thus be detected, for example, by demodulating a received signal on a carrier frequency and detecting whether a signal level is present on this carrier frequency or not. In this case, a disturbed carrier frequency is a carrier frequency onto which a signal is modulated that exceeds a certain threshold value.

Faults in the actual sense can be detected by the occurrence of CRC errors or burst losses.

The detection device 24 thus uses the demodulated signal from the RF module 4 to determine how high the signal component modulated onto a specific carrier frequency fx is.

If the detected signal component is above a predetermined limit value or one of the abovementioned errors has occurred, the detection device 24 sends a fault detection signal to a blocking / releasing unit 21. / Release unit 21 a blocking / release information to a processor 23. This blocking / release information indicates which of the carrier frequencies fx are blocked or released again due to the detection of a fault by the detection device 24, as will be explained later. An independent procedure is thus created by means of the detection device 24 and the blocking / enabling device 21, by means of which disturbed frequencies can be blocked and released again. In addition to the lock release information from the lock / release unit 21, the processor 23 is supplied with a sequence from a random number generator 22. On the basis of a random algorithm implied in the random generator 22 generates a randomly distributed sequence of carrier frequency values within the predetermined frequency band. The random number generator 22 thus executes a procedure which is independent of the procedure for frequency blocking in the event of a fault. The processor 23 finally sends a control signal to the RF module 4, which specifies the carrier frequency value to be used for the RF module 4.

As shown in FIG. 3 by an arrow from the processor 23 to the random number generator 22, the processor 23 specifies how many different values it is to generate. This number of values to be generated corresponds to the number of carrier frequencies to be generated, which must be at least 75, for example in accordance with the US specification “FCC part 152.

In a mobile part in particular, the processor 23 also provides the random number generator 22 with a starting value for its algorithm. The mobile station receives this start value from the base station for synchronization, which is achieved by using the same start value and the same algorithm. With the same starting value and the same algorithm, the same sequences are forcibly generated by the base station and the handset.

Base station 1 is the master in frequency allocation, ie at the start of a connection establishment, the random number generator in a mobile part is initialized with the state of random number generator 22 of base station 1. On- finally, the random number generators in the handset 2, 3 ... and in the fixed station 1 generate the same carrier frequency values synchronously and independently of each other.

The procedure for frequency blocking, which is carried out by the detection device 24 and the blocking / releasing unit 21, uses a unidirectional protocol on the air interface during the entire connection time between the base station 1 and a handset 2, 3. If the detection device 24 finds one of the possible frequencies fx as disturbed by the base station 1, the base station 1 thus informs all the mobile parts with which it operates connections that this disturbed frequency, if it is generated by the frequency of the random number generator is to be replaced by another carrier frequency which is not detected as being disturbed. The random number generator 22 is not influenced by the frequency blocking. This frequency blocking is withdrawn by the blocking / releasing unit 21 when the blocked carrier frequency is again suitable for transmission or when it was blocked for longer than a previously defined time.

4a and 4b, it will now be explained how the random numbers can be generated by an algorithm that is simple to implement in a processor and at the same time the required computing time can be kept low.

As can be seen in FIG. 4a, the basis of the algorithm is a feedback shift register 25 with the length x, the length x being 4 in the example shown. The feedback structure of the register is I _r = {l, 4}. This means that, according to the example shown, the first bit is linked to the fourth bit by a modulo2 addition 26 in the sense of a feedback and the result of this modulo2 addition 26 is put in the place of the most significant bit, whereby this position is cleared by shifting the register content by one bit to the right.

At the beginning, the shift register 25 is loaded with the value 0001 as shown. For each new value, the contents of the shift register 25 are shifted to the right by one bit, the left bit being recalculated as shown. The type of feedback, ie in the present example the modulo2 addition of the left bit with the rightmost bit of the shift register 25 can be changed. By changing the type of feedback 27 and the number of feedback bits, different sequences with different lengths can thus be generated. Depending on the feedback, the sequence length, ie the periodicity after which the generated sequence is repeated periodically, is a maximum of 2 ^n- 1, where n is the number of bits in the shift register 25. In the arrangement shown (n = 4, I _r {l, 4), the sequence length is therefore 15 (and therefore maximum for a four-bit register), ie after 15 generated values, the generated values are repeated periodically. The value 0 is not generated with feedback shift registers. FIG. 4b shows how the content of the shift register 25 for the example shown in FIG. 4a is for the corresponding clocks of a period.

4a is to be understood in particular as an example for the generation of random numbers by means of a feedback shift register. In practice, for example, a 16-bit shift register can be used. Such a shift register can be easily implemented in 8- and 16-bit processors. Due to the different possibilities of the feedback 27, different sequences can be generated with a 16-bit shift register 2048. The sequence length for a 16-bit shift register is a maximum of 2 ¹⁶ -1 = 65535. Thus, if a carrier frequency corresponding to a value of the generated random sequence is maintained for the duration of a frame of, for example, 10 ms, the duration of the period is 65535 x 10 ms «10.9 min. This means that a maximum length sequence in a 16-bit register is only repeated every 10.9 minutes.

The use of a random generator with the algorithm shown has the further advantage that different carrier frequency hop sequences can be generated by simple-to-define feedback.

In the case of a shift register with 16 bits, the number of possible values of the sequence of the carrier frequencies is 65535 as explained. In the meantime, the number of actually used carrier frequencies can be considerably smaller and moreover variable. Thus the carrier frequency cannot be obtained directly by converting the values of the random sequence.

FIG. 5 shows how the frequencies actually used are obtained in an endless loop from the content of the shift register 25. As can be seen in FIG. 5, the frequency fx _n -ι is first set to 0. Then the current content of the shift register 25 is added to this value (28). Then it is calculated and a decision is made as to whether the addition result calculated in step 28 is greater than the number y of the usable carrier frequencies fx. If the result of this decision 29 is positive, ie if the addition result in step 28 is greater than the number y of the actually usable carrier frequencies fx, then this value is reduced by the value of the number y of the usable carrier frequencies (30). This reduction in the addition result 28 by the number y of the usable carrier frequencies fx is carried out until the decision in step 29 results in a negative result, ie until the value fx _n -ι is used as the carrier frequency of the next time slot or time frame fx. The shift register 25 is then incremented by one bit. for example shifted further to the right (32). The process is thus ended and the process returns to step 28.

By changing the variable y, the maximum number of frequencies used can be adjusted.

The invention thus enables the generation of random numbers by means of an algorithm that is simple to implement in a processor. At the same time, the computing time required is kept to a minimum.

Reference list

1: base station

2: handset 3: handset

4: RF module

6: Antenna base station

7: Antenna handset

8: first radio transmission path 9: second radio transmission path

10: Terminal line

20: carrier frequency sequence unit

21: Lock / release unit

22: random generator 23: processor

24: detection device

25: shift register

26: Adders

27: Feedback 28: Addition

29: Decision

30: reduction

31: Implementation

32: Shift fx: carrier frequency

Rx: frame

Zx: time slot

Claims

claims

1. Method for generating a random number sequence which is converted into carrier frequencies fx for a mobile radio transmission (8), characterized by the following steps:

Provision of a shift register (25) with n bits, feedback modulo2 addition (26) of a bit of the shift register (25) with at least one other bit of the shift register (25), clockwise shifting of the register content by one bit, insertion of the result of the modulo2 addition (26) into the bit position released by the shift, and converting (23) the content of the shift register (25) into a carrier frequency fx for a mobile radio transmission.

2. The method according to claim 1, characterized in that the feedback (27) is changed in order to generate different sequences.

3. The method according to any one of the preceding claims, characterized in that the step (23) of converting the content of the shift register into y carrier frequencies fx comprises the following steps: a) adding (28) the content of the shift register to a current carrier frequency fx _n _ι, b) decision (29) whether the result of the addition (28) is greater than the number y of the carrier frequencies fx, c) in the event that the decision (29) is positive, lower (30) the addition result by Number y until the decision (29) is negative, d) in the event that the decision (29) is negative, using (31) the addition result of a) as the next carrier frequency fx _n , and e) shifting (32) the shift register (25).

4. A device for generating a random number sequence for conversion into carrier frequencies fx for a mobile radio transmission, characterized by: a shift register (25) with n bits, an addition device (26) for modulo2 addition of one bit of the shift register (25) with at least one other Bit of the shift register (25) in the manner of a feedback (27), the register content being shifted clockwise by one bit, the result of the addition device (26) is inserted into the bit position freed by the shift, and - a device (23 ) for converting the content of the shift register (25) into a carrier frequency fx for a mobile radio transmission (8) is provided.

5. Device according to claim 4, characterized in that the feedback (27) is variable in order to generate different sequences.

6. Device according to one of claims 4 or 5, characterized in that the device (23) for converting the content of the shift register (25) into y carrier frequencies fx further comprises: a) means (28) for adding the content of the shift register

(25) to the current carrier frequency fx _n _ι, b) means (29) for deciding whether the result of the addition

(28) is greater than the number y of the carrier frequencies, with c) if the decision (29) is positive, the

The addition result is reduced by the number y until the decision (29) is negative, d) in the event that the decision (29) is negative, the addition result (28) from a) is used (30) as the next carrier frequency fx _n , and then e) the shift register (25) is shifted (31).

7. Device according to one of claims 4 to 6, characterized in that the shift register (25) has 16 bits.

8. Device according to one of claims 4 to 7, characterized in that the shift register (25) is implemented in an 8-bit or 16-bit processor.

9. Mobile radio device, characterized in that it has a device (22, 23) according to one of claims 4 to 8.