CN1223761A - Process and arrangement for operation of time division multiple access (TDMA) radio systems - Google Patents

Abstract

The invention relates to a process and an arrangement for interference-free operation of TDMA radio systems, in particular for cordless communication systems in accordance with the DECT standard. An interference occurring in a first TDMA radio system which is caused by a lack of synchronism of the time bases of the first and at least one other TDMA radio system, is initially detected. The interference is subsequently suppressed according to the invention in that the time bases of the asynchronous TDMA systems affecting each other mutually are automatically synchronised. For this purpose, the frequency of the time base of at least one of the two TDMA radio systems contributing to the interference is adapted in such a manner that the time bases of the TDMA radio systems operate in synchronism. Firstly, the time overlap of the time zones associated with each established connection is eliminated by the single shifting of the TDMA frames (F1, F2), relative to one another, associated with the frequence adaptation, and consequently the resulting interference suppressed. Secondly, a further time drift of the associated TDMA frames (F1, F2) towards each other is eliminated by the automatic synchronisation and consequently the cause of possible further interference removed from the start. Said process is also advantageous in that changing the channel frequently, and disadvantages associated therewith, can be avoided.

Description

Method and device for operation of TDMA wireless communication system

The invention relates to a method and a device for interference-free operation of a TDMA radio communication system, in particular a radio communication system according to the DECT standard.

The requirement in modern information transmission systems is to economically transmit the maximum amount of speech and data information with the minimum bandwidth required. As a wireless communication system, such requirements are of particular significance. For such requirements their width must be limited and the frequency spectrum can be used particularly economically technically. In many cases, so-called multiplexing methods are therefore used, with which it is possible to transmit multiple telephone calls and multiple data streams together via a common transmission channel.

For this purpose, a distinction is made in principle between two multiplexing methods, frequency-division multiplexing and time-division multiplexing.

Frequency Division Multiplexing (FDM) is based on subdividing an existing frequency band into multiple identical frequency bands (channels). Thus, each frequency band can be used, for example, as a telephone transmission. The FDM method is mainly used to transmit analog signals.

The TDM method of time division multiplexing, in contrast, is only applicable to digital signals. For this, combining multiple single channels as transmissions such as via a cable or a wireless communication line into a common transmission channel, at which point the digital data stream in each single channel is subdivided into individual data packets (bitrahmen), and the bitrahmen of these different single-channels are each merged into a transmission frame (uebertragungsrahmen), which are then serialized, ie one after the other, on the transmission line. For this purpose, the individual channels are each permanently assigned a time slot in each transmission frame, in which the bit frames of the corresponding individual channels are transmitted using the entire available frequency bandwidth. In the receiver, the data are again selected from the time slots of the transmission frame and assigned to the individual receive channels.

The use of these two so-called multiplexing techniques as transmission lines allows the number of communication channels to be multiplied and thus their economy to be improved. Furthermore, in radio communication systems it is generally also required that as many subscribers as possible can be used via a channel using the multiplexing method. For this, the so-called "multi-access" is used, which can be in FDM as well as in TDM. The corresponding systems are referred to as FDMA systems and TDMA systems (Frequency Division Multiple Access and Time Division Multiple Access). The following refers only to TDMA systems or to corresponding TDMA stages in complex radio communication systems consisting of multiple stages with different multiplexing techniques (for example, in a DECT radio communication system).

For example, if a TDMA wireless communication system operates in the vicinity of another TDMA system using the same frequency band in a cellular communication system composed of multiple such systems, mutual interference will occur. In order to avoid the damage of such transmission quality, it is necessary to continuously select a TDMA channel in a subsystem when a new communication is established, and its time slot in the TDMA transmission frame is different in time from another The TDMA subsystem uses a single channel whose time slots are staggered and therefore completely non-interfering.

However, asynchronous TDMA wireless communication systems, for example based on the DECT standard, have the disadvantage that the time base of a single TDMA system as clock signal generator has a permissible error (in DECT, + /-5ppm). Consequently, a relative time drift of the TDMA frame structures of the two systems relative to each other usually results. In practice such a shift would lead to the fact that a communication initially established in an independent, ie undisturbed time zone of the TDMA frame would gradually arrive at a time zone already used by another TDMA system, and would thus cause Interference with corresponding communications in the system. Depending on the circumstances, the interference may also affect other wireless communication channels in other wireless communication systems.

The conventional approach (as in DECT) is to bypass the TDMA system, if possible, by changing the radio communication channel ("handover") when interference is detected. For this, however, it is necessary for an unused and undisturbed wireless communication channel to be continuously available. In addition, channel change has the further disadvantage that two wireless communication channels are required at the same time for channel change in a short period of time if it is not interrupted so that the user is not aware of the channel change. Therefore, when the wireless communication channel is occupied together by frequently changing channels, a load peak will be quickly caused, and the channel will be blocked. However, there is the possibility that a disturbed communication may not be followed by an undisturbed channel, which may even lead to a communication interruption in this case.

The object of the present invention is to provide a method for the interference-free operation of a TDMA radio communication system which overcomes the disadvantages mentioned above.

This task is achieved by a method comprising the features stated in claim 1 . The device for carrying out the method is as claimed in claim 8 .

Unlike in traditional TDMA wireless communication systems, in that system, endure the above-mentioned shortcoming, by continuing to suffer interference communication on another wireless communication channel, avoid channel interference, be used for TDMA wireless communication according to the present invention The method of system operation is characterized by the elimination of the cause of the disturbance.

For this purpose, disturbances occurring in the first TDMA radio communication system caused by a lack of synchronization of the time bases of the first and at least one other TDMA radio communication system are initially detected. In the DECT system, for example, the identification of such a so-called "sliding collision" is standard-dependent, and the transmitted bit frames in a TDMA time slot each have a specific field by which different The interleaving drift of the system TDMA frame.

Interference is then eliminated according to the invention, in which the time bases of mutually influencing non-synchronized TDMA systems are forced to be synchronized. For this purpose, the frequency of at least one of every two interfering TDMA systems is adapted in such a way that the time bases of the TDMA radio communication systems are synchronized. Firstly, the individual shifts of the TDMA frames of the system relative to one another, which are relevant to the frequency adaptation, eliminate the temporal overlap of the time slots allocated to the respective communications and thus eliminate the interference that has occurred. Secondly, the forced synchronization also eliminates further time drifts of the associated relative TDMA frames and thus eliminates said possible other sources of interference from the start.

Furthermore, the method has the advantage that the said disadvantages associated with channel replacement are overcome.

According to claim 8, the TDMA system for implementing the method of the invention comprises a time transmitter with a variable frequency as a time base and a TDMA frame structure for determining the system relative to another TDMA wireless communication system A detection device for frame-structured time drift. Interference in the TDMA radio communication system is detected by means of the detection device, which interference is caused by a time shift of a TDMA frame of the TDMA radio communication system relative to a second TDMA radio communication system. The system is synchronized by corresponding frequency changes of the time generators of the system, that is to say the corresponding time base frequencies are adapted to each other. In this case, the synchronization is carried out automatically in a suitable manner upon detection of the mutual time drift of the systems involved in the interference. And can be achieved by changing the frequency of the time base of at least one of every two systems.

Further structures and arrangements which are preferred according to the invention are described in the dependent claims.

According to a first configuration of the invention - claim 2 - in at least one such TDMA radio communication system, the identification is made by a first Interference occurs due to lack of synchronization with a time base of at least one other TDMA wireless communication system. Thus, it is possible to identify "sliding conflicts" early on, in which a time slot of a TDMA frame structure of one system that happens to be used is pushed forward on a time slot of a TDMA frame structure of another system that is also occupied, because, Firstly, in "slower" systems the last bit of the transmitted bit frame in the disturbed time slot will always be corrupted.

Correspondingly, a further arrangement according to the invention - claim 3 - is expedient in that each bit frame transmitted in a TDMA time slot comprises at its end a field which only contains redundant information and/or is useful for effective data communication Unnecessary information. For example, this is already specified in the standard for the DECT system.

In a further arrangement of the invention - claim 4 - the adaptation of the time base frequency of the TDMA wireless communication system is repeated in small steps with respect to each other. Among them, by repeatedly adjusting the frequency change in this way, on the one hand, the frequency difference of the time base is eliminated; on the other hand, what the system involved realizes is not transition compensation but a great degree of synchronization. In addition, the stepwise frequency adaptation prevents inadvertent loss of synchronization of the transmitter and receiver in TDMA radio communication systems and thus prevents interruptions, short interruptions or at least channel changes of the corresponding communication. For example, in the DECT system, on the premise of continuous communication between the base station and the mobile device, repeated frequency changes can be realized, and only when a large frequency jump occurs, the mobile device must establish a connection with a base station for continued communication. of new contacts.

Likewise, in order to prevent loss of information, it is expedient—according to claim 5—to always carry out a frequency change in a time window in which no data transmission takes place in the TDMA system concerned. Generally, this time window is given by the so-called guard time (guard space) between the individual channels of the TDMA frame.

According to still another preferred structure of the present invention - claim 6 - if the TDMA wireless communication systems interfere with each other due to the overlap of the used TDMA wireless communication channels, when the time base synchronization of the interfered TDMA wireless communication system requires a frequency change value When the specified limit value is exceeded, then at least in one of the involved systems, a transfer is made from the disturbed wireless communication channel to another sufficient undisturbed wireless communication channel. Therefore, the aforementioned frequency adaptation is limited to the desired, achievable and specified frequency range at the specified frequency and prevents, for example, the impact on the TDMA system caused by any one external disturbance from causing a runaway frequency change without hope of elimination. interference.

In yet another configuration of the present invention - claim 7 - after each wireless communication in the TDMA wireless communication system, the changed frequency of its time base returns to its original value before frequency adaptation. This precludes the continuous selection of frequencies for a TDMA system such as a TDMA system adjusted with the maximum licensed frequency correction, which would have the consequence that the scope for further frequency adaptation in adjacent systems would be considerably restricted and thus make the selection The possibility of synchronizing multiple systems within a fixed-frequency system is also greatly restricted.

Below in conjunction with accompanying drawing, take DECT system as an example to further illustrate the present invention.

Fig. 1 structure of a bit frame in a time slot of a TDMA frame according to the DECT standard;

Fig. 2 briefly shows the formation of "sliding conflict";

FIG. 3 is a flow chart for handling "sliding conflicts" according to the method of the present invention.

In the DECT standard, by subdividing the available frequency band, 10 sub-frequency bands are defined by means of FDMA, and the sub-frequency bands are subdivided again, each defining 24 channels by means of TDMA, and a total of 240 channels are defined. Thus, each TDMA frame (transmission frame) consists of 24 time slots. For two-way communication, every two channels are combined into one full-duplex channel, resulting in a total of 120 full-duplex channels. The superior structure formed by arranging sequentially transmitted TDMA frames into rows in time is called DECT multiframe.

In FIG. 1, the structure of a bit frame transmitted in the time slots of a TDMA frame is depicted as a DECT system in the so-called full-slot mode, the most common operating mode of a DECT system. A bit frame is transmitted for 416 μs and has a length of 480 bits. The synchronization field S contained in the first 32 bits is used for clock recovery and synchronization of the receiver for each frame. The following header field A of 64 bits in length contains signaling data and control data. The actual payload data is transmitted in the payload field B of length 320 bits. As an invention, it first makes sense to add fields X and Z each having a length of 4 bits. The end of each frame is also followed by a guard time GS, which is 50 μs in DECT. The X field is determined by means of a special parity, called a "cyclic redundancy check", obtained from selected bits of the B field, and thus serves as an indicator of defective transmissions to the B field. The Z field is formed by doubling the X field. It contains redundant data as previously described and is used to identify surrounding out-of-sync DECT systems. The content of the Z field is therefore compared with the content of the X field of the foreground in the receiver and this is checked by repeated statistics of deviations. If a systematic deviation occurs, a "sliding conflict" is thus considered identified and measures to eliminate the disturbance are initiated. The invention described in this connection in connection with FIGS. 2 and 3 will be referred to further below. In addition to the X field and Z field, which are used to detect interference at the end of the bit frame, the S field at the beginning of each bit frame can also be used to detect interference at the beginning of the frame.

FIG. 2 briefly describes the creation of a "swipe conflict" and its removal according to the invention.

Part (a) in Fig. 2 shows a DECT transmission frame F1 consisting of 24 time slots. The highlighted time slot 10 is just used for wireless communication between the base station and the mobile in the first DECT system (as the mobile in the opposite direction - the base station uses channel 22, which together with channel 10 forms a full-duplex channel) .

Part (b) of FIG. 2 depicts, as instant t ₁ , a fragment of a transmission frame F ₂ of a second DECT system around the first existing DECT system, where exactly time slot 12 (together with time slot 24) Occupied by calls. The respective occupied channels 10 and 12 (and 22 and 24) of the two DECT systems do not overlap in time and therefore do not interfere.

In general, the internal clock pulses of the time base of the DECT system deviate from a standardized set value within a defined tolerance range. This causes a drift between the time bases of the two DECT systems, which causes the TDMA frame structures (DECT multiframes) of the two systems to shift relative to each other. As a result, the time slots of the two systems that are originally mutually misaligned in time lead to at least partial time coverage, thereby causing mutual interference.

This situation is depicted in sub-panel (C) of FIG. 2 . The clock pulses of the second system occur at a slightly higher frequency than the clock pulses of the first system, so that a temporal overlap of channels 10 and 12 occurs up to a certain instant t ₂ >t ₁ . In the first system, "sliding collisions" are recognized on the basis of the last bit corruption of each bit frame transmitted in the channel 10 and also on the basis of a comparison of the X and Z fields.

Part (d) and part (c) in Fig. 2 show the situation after the forced synchronization of the two systems by means of the frequency adaptation of the present invention. Here, for example, the frequency boosting of the first, ie "slower" system alone can be used to generate synchronization. The closest bit frame of the first system following the synchronization (and for this, the entire DECT multiframe) starts earlier than the synchronization violation as stated before, then the overlap and thus mutual interference between the channels 10 and 12 of the two systems is will be ruled out. Because now the two time bases are running synchronously to the greatest extent, repelling other overlaps, at least significantly delaying them.

FIG. 3 describes in detail the method for handling "sliding conflicts" according to the present invention according to the flow chart.

In a TDMA system, in step 1, interference is detected by means of "sliding collisions" based on the destruction of individual bits or entire fields of a bit frame transmitted in a TDMA time slot, eg as in the case of the DECT system already described above. In order to distinguish "sliding collisions" which are disturbed by other causes, for example, a statistical check of the repeated statistics of bit errors occurring in the bit frames of several consecutive time slots is used. A frequency change of the system time base is then performed in a first correction step 2 . Depending on the case, whether a corruption occurs at the beginning or end of the bit frame, for which the corresponding frequency reduction or frequency increase described in the figure also results in at least partially adapted frequencies to the interfering system. Here, all frequency changes that occur during synchronization are always instantaneous. No data is transmitted in the system at this instant, also expediently within the guard time at the end of each time slot. In a next step 3 it is checked whether the frequency correction value DF accumulated over all correction steps exceeds a defined maximum value DF _max . It is more appropriate to select the first correction amount to be less than DF _max (in DECT, for the output frequency, the appropriate order of magnitude is about 1ppm), so that step 4 can be performed. Here it is checked on the basis of the next bit frame received in its single channel after correction whether the number m of erroneous bits in the bit field, in DECT systems also in the S field and the Z field, does not exceed the established threshold value S ₁ (eg select S ₁ =2). If it is not already, synchronization has not been reached, and a correction iteration is started at another frequency by returning to step 2. In another case, in step 5, by means of a comparison, a further second threshold S ₂ (with S ₂ > S ₁ ) is used to test the number of remaining bit errors, whether there is filter compensation of the original frequency difference. Such a transient compensation is accepted, for example, when a bit error no longer occurs (S ₂ =1). Then, at step 6 cancel some of the last revisions, and go back to step 4 for a new check. However, the number m of bit errors remaining is within the tolerance range set by the thresholds _S1 and _S2 , which at least maximizes the desired synchronization. In step 7, all trimming performed at the end of each communication in the system is cancelled, so as to avoid continuous frequency dominance of the trimming system for adjacent systems. If the result of the correction method in step 3 proves that the frequency correction carried out, such as synchronization, exceeds the determined maximum value Dfmax, then in step 8, under the condition that all frequency corrections carried out so far are cancelled, the channel is changed to an undisturbed channel ("handover").

Claims (8)

1. be used for the TDMA wireless communication system, especially according to the method for the wireless communication system of dect standard operation, wherein, so be identified in and occur in the TDMA wireless communication system, and disturb by the wireless communication that the time drift with respect to the TDMA hardwood (F1) of the TDMA wireless communication system of the TDMA hardwood (F2) of the 2nd TDMA wireless communication system causes, it is characterized in that, eliminate interference by forced synchronism, wherein, timebase frequency to major general's one in two TDMA wireless communication systems so changes, promptly make two TDMA wireless communication systems the time base synchronously.

2. according to the method for claim 1, it is characterized in that according to last/last several destruction of a position hardwood that transmits, identification is disturbed in tdma slot (1 to 24).

3. according to the method for above-mentioned arbitrary claim, it is characterized in that, in the TDMA wireless communication system, the position hardwood that each transmits in tdma slot (1 to 24), at its end a field (Z) is arranged, it only contains redundancy and/or to the valid data unwanted information of communicating by letter.

4. according to the method for above-mentioned arbitrary claim, it is characterized in that the adaptation of TDMA wireless communication system timebase frequency repeats with small step mutually.

5. according to the method for above-mentioned arbitrary claim, it is characterized in that, carry out the frequency change, wherein, in related tdma system, do not carry out transfer of data at time window.

6. according to the method for above-mentioned arbitrary claim, it is characterized in that, when for make two TDMA wireless communication systems time base synchronously, when needed frequency change (DF) value surpasses the limiting value (DFmax) of regulation, in a TDMA wireless communication system, from disturbed wireless communication (10), transfer to another enough not disturbed wireless communication at least.

7. according to the method for above-mentioned arbitrary claim, it is characterized in that, in the TDMA wireless communication system, finish each wireless connections after, with at that time the base the frequency that changes be returned to its original value again.

8. be used to implement above-mentioned arbitrary claim method, especially follow the tdma system of dect standard, it is characterized in that,

-this TDMA wireless communication system comprise one as the time base timer, its frequency is at least in a restricted portion, and is variable with the frequency values of regulation;

-this TDMA wireless communication system can pass through a checkout gear, is used for determining the time drift of its TDMA hardwood structure with respect to the TDMA hardwood structure of another tdma system;

-the interference that causes by such drift by means of checkout gear identification, and by means of two systems synchronously when corresponding the mutual frequency adaptation of base eliminate this interference.

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