System and method for digital data communication

The invention relates to digital transmission systems, wherein where structured to transmit digital data in context are. In particular, the invention relates to such Transmission systems in which erroneously transmitted frame can be sent repeatedly. The invention relates In addition to a transmission method, which in the implementation such a transmission system is determined, as well as on a Transmitter and a receiver for such a transmission system.

It is known in the transmission of digital data, the data in arranging frame having a specific structure. Among digital Data are from the general character or series of characters understood a finite character set. Most of them are In this case, binary data in which the character set only from logic zero and logic one is. Digital data z. B. from digital data processing equipment are produced or produced during the digitization of audiovisual speech signals. User data are those digital data the transmission in a Communication is paramount. Typically, in the context not only user data but also additional data Spaces provided. This additional information is used for. B. the Aufund Degradation or the monitoring of the connection between the communicating parties. What data these are in detail, depends inter alia on the modulation used and Coding method, the number of allowable transmission error and call processing requirements.

A concrete example of such a frame structure is described in "The GSM System for Mobile Communications ", page 281 et seq., By M. Mouly and M.-B. Pautet explained. The frame structure described therein is in mobile radio systems according to the GSM Standard ( "Global System for Mobile Communications ") use. GSM mobile systems Time multiplex systems (TDMA) system, that is, each frame is in a number of equally long sub-frames (time slots, "timeslots") divided. It is here, as is often the case, for reasons of Clarity, such a sub-frame simplifies further than "Frame" are referred to. The defined in the GSM standard Radio link protocol (RLP, "Radio Link Protocol") provides the framework before, each of which has a length of 240 bits and, inter alia, a 16 contains bits comprehensive test sequence. The recipient of such Framework evaluates this test sequence and can thus determine whether the frame has been transmitted correctly as a whole. Each RLP frame also includes a frame number values ​​between 0 and 63 may take. To specify the frame number are in RLP frame 8 bits available.

If the GSM transmission system in confirmation mode ( "acknowledged mode "), the receiver sends the transmitter a confirmation of each frame received without error. This is confirmation the transmitter is not the number of correctly received but the informed of the expected as the next frame. If the sender no confirmation of the error-free reception of a frame is received, it repeats the transmission of the frame. The number of possible repetitions is limited so that at zusammengebrochenem transmission no endless loop. In heavily disturbed channels, however, the number of repetitions be very high with the result that the effective User data transfer rate drops significantly.

To this drop in the user data transfer rate to counteract, can be made of a method in which Design a majority comparison is performed. The basic idea this procedure is described in published international Patent application WO 92/21085 describes: If the recipient a recognizes framework as incorrect, so he does not reject this framework, but saves him. Additionally, the receiver asks the Station a repetition of the incorrectly received frame. If the store a certain number of erroneous frames contains, so these frames are read out and a Majority comparison subject. In this majority Comparison the frames are compared bit by bit with each other, as shown in Fig. is shown 3rd From the erroneously transmitted frame 302, 303, 304 a new frame is 305 determined by each single bit is carried out a majority decision. at disagrees large error rate, as in this first Example, the new frame 305 with the transmitted, not by Error impaired frame 301 match. The in Fig. 3 below Frames 306, 307, 308 illustrated contain more errors than the Frames 302, 303, 304. Thereby, in this second example, not all bits of the new frame 309 correctly determined. If he has led majority compared to a faultless new framework or not is determined by the same error checking algorithm stated how he also immediately received frame is applied. If the error checking that determined the new Frame is error-free, this is communicated to the transmitter; a repeated transmission of the frame is omitted in this case. On Thus, it is even possible in extreme cases, an error-free to ensure data transmission, without causing a single Framework had actually been transmitted without errors.

The above mentioned international patent application refers to a relatively simple Datenflußsteuerungsprotokoll, not which z. B. allowing said one frame is transmitted before the successful Reception of the previous frame is confirmed. such a Approach is only useful if the requirements for the Data throughput is low. In modern digital Radio transmission systems, such as. For example, according to the GSM standard, the Requirements for the data throughput, as well as to data protection significantly higher. Therefore come here more complex Datenflußsteuerungsprotokolle used. If the above method explained z. B. applied to data frame as determined by the RLP protocol are defined, what is decisive, in the Carrying out the majority of comparison only frames with the same Frame number to compare. This presupposes that Also the frame numbers of incorrectly transmitted frames can be determined reliably. The frame number of a faulty However, the frame itself may be faulty. It is conceivable that Frame number of error-detected frame of the sequence the correctly received frame to determine. This requires, however, if at all possible, a considerable computing effort on the Receiver side ahead. In the GSM standard, this is partly due to the temporal offset between transmitter and receiver: The sender, the sent a frame with the frame number n, awaiting the Transmission of the frame n + 1 is not on the confirmation that the has been sent first frame n received without error. Much more the transmitter sends to the arrival of the confirmation of a larger Zdhi further frame. In GSM transmission with full Transfer Rate ( "full rate") this is about 9 frames at Transfer half-rate ( "half rate") about 14 Frame. An additional difficulty also that this displacement does not is constant but temporal fluctuations. With strong disturbed reception is the recipient thus u. U. reliant from the start of transmission to the entire sequence of frame numbers and track confirmations, to store and process to the frame numbers of erroneous frames to determine reliably. This requires the recipient a significant computational and Memory performance. If no such processing power and memory is made available or when the frequency of disturbances, especially on the feedback channel from receiver to transmitter, a exceeds certain level, it is a reliable determination of Numbers of erroneous frames are not possible.

It is therefore an object of the invention, the reliable determination of to improve frame number erroneously received frame without the Receiver with elaborate computing and storage device having to equip.

This object is achieved in that the invention in the Transmission system frames are transmitted, the in claim 1 having specified structure. An invention for the Transmission system suitable transmitter is subject of the claim 4, a suitable receiver is the subject of claim 6, a inventive transmission method subject of the claim 12. Advantageous further developments are respectively the dependent claims removable.

Essential for the invention is the division of the error check in an error check for the frame number and a separate Error checking for other data, in particular payload. In order it is possible, even when incorrectly received payload data frame in evaluate the in most cases a majority comparison.

The frame structure of the invention is also advantageous used when no majority comparison is to be performed. If the frame number is correct, the receiver can the Transmitter state precisely what context it is to repeat. Thereby can be prevented, that the transmitter frame more than necessary repeated. This will be explained later in detail.

The invention is described in detail with reference to the drawings described. Show it:<dl tsize="7"><dt>Fig. 1:</dt><dd>A transmission system and a frame structure in accordance with the present invention;</dd><dt>Fig. 2:</dt><dd>A frame structure according to the prior art; </dd><dt>Fig. 3:</dt><dd>A schematic representation of how the Majority comparison same from erroneous frames Frame number, a new frame is detected;</dd><dt>Fig. 4:</dt><dd>A flow chart of an inventive Transmission method.</dd></dl>

In FIG. 1 is at the top schematically an inventive Transmission system shown. In the data source 101 digital data generated. As already mentioned, it may in itself this data source z. B. a binary generating electronic data processing system or a facsimile machine act. are in a transmitter 102, depending on application, devices the source and / or channel coding and for line coding available. In the transmitter 102, devices can also be present, the performing of one or more of the known Multiplex method used. By reference numeral 103 is the Transmission symbolizes the in this embodiment, a non-line transmission of electromagnetic Waves includes. In a receiver 104, the transmitted Signal back into digital data and corresponding Facilities decoded. Normally, integrated in the receiver, but shown separately here for clarity, is the Error control unit 105. is there to processes taking place discussed in more detail below. User data, which in the Error control unit have been found free of errors, enter to the data sink 106th

are on the transmission path between the transmitter 102 and receiver 104 the data in frame 107 is structured, as is already in the Introductory part has been described. The frame 107 in FIG. 1 are each marked with a frame number. among them shown is the structure of used in the transmission Frame. In this frame structure, the field 110a for user data provided. Maybe in another field 110b for bits contain additional data, such. as for synchronization or the connection and disconnection. These bits of additional data are hereinafter referred to as signaling bits. The field 111 consists of a test sequence, with which can be checked whether the bits contained in the areas 110a and 110b without error have been received. The field 108 contains bits for the Frame number, the field 109 a test sequence, are tested with the may, if the frame number has been received without error. in the -Described embodiment, the length of the frame 240 Bits, the frame number field 108 is 8 bits long, for Test fields 109 and 111 are each 16 bits is provided. The remaining bits are payload 110a and signaling data 110b reserved.

The advantages of the invention are in a frame structure Comparison with a frame structure according to the prior art significantly. In FIG. 2, such a conventional frame structure sketched. User data and signaling bits in the fields 202a and 202b contain, for the frame number, the field 201 provided. The test sequence in field 203 enables testing whether one or more of the fields 201, 202a and 202b Bits contained is faulty. The error-checking extends So arranged in a frame in the prior art at all Bits. A distinction whether bits in field 201 or field 202 have been transferred 202b faulty or in the field, is not there possible. If an error is found, in any of the Setting bits contained are believed to be error-free has been received. Thus, not the frame number reliably determined from the corresponding bits in the field two hundred and first This has inter alia the result that as in the conventional GSM transmission systems, recovery of a faulty frame for the purpose of a majority comparison is not possible.

In the inventive frame structure, however, is the Error control for commercial and signaling one hand and the frame number on the other hand performed separately. Even at faulty production or signaling can thereby be determined whether at least the frame number errors has been received. Such a determination is useful, since the Chances are that, despite errors in the payload and / or signaling data received the frame number errors has been. This follows from the fact that an error in the Frame number statistically occurs much less frequently than one Errors in the payload. If it is assumed that each bit in a frame has the same probability of error, then the Ratio between the probability that an error in the Frame number occurs, and the probability that an error occurs in the remaining bits, equal to the ratio between the Number of frame number bits to the number of remaining bits. For RLP frame, this ratio is at about 0033, or otherwise words, an error in the frame number occurs 30 times rarer to as an error in the remaining bits. From 30 frames faulty production or signaling data have therefore in Means 29 frames a faultlessly transmitted frame number. at GSM transmission method, the top of the conventional, described frame structure making use, none of these 30 Frames are supplied to a majority comparison. Using the frame structure of the invention, however, can on average 29 these frames are processed in a majority comparison.

The same considerations apply when the transmission system performing error-correcting measures provides. with the help such measures it is possible, even if faulty bits have been received, to correct these bits and a error-free frames to produce. The error correction requires in the frame structure an additional field for correction bits. at However predetermined number of correction bits can not arbitrarily many mistakes are corrected. In case of adverse Channel characteristics, it comes therefore also to the top described problem that errors occur in the context and the Calculate frame number of the erroneous frame is not reliable leaves. The frame structure of the invention provides here also in the manner described above remedy.

In the following, an embodiment of an inventive Transmission method explained. Fig. 4 shows in the form of a carried out the flow chart of the steps on the receiving end will. For the sake of clarity, in the flow chart not listed all the steps described below. In Bracketed numbers refer below to the each step in the flowchart. A transmitter structured to transmitting digital data in frames. Each frame contains Payload, possibly signaling data, a frame number, Correction bits, check bits for error checking of the useful and Signaling data and separate check bits for error checking the frame number. The receiver receives (401) the frame and leads using the correction bits of the error correction frame through (402). For this error correction, there are two options: On the one hand can, such as in current GSM systems, the Error correction on the entire frame extend. The Total number of errors in the frame then decides whether the Error correction to an error-free frame results or not. Alternatively, two independent Error corrections are carried out: a correction of the Majority Rates important frame number and a correction of the remaining bits contained in the frame. Although this requires Alternative solution additional correction bits for the frame number in the frame structure, but this may be the number of discardable Frames are reduced. Which of the two illustrated is preferable in error correction capability, must in Individual cases judged in the light of the overall system configuration will.

After the error correction, the receiver checks every single framework to make sure even errors in the production or the Signaling data is present (403). In a second Inspection stage the receiver checks whether the frame number has been received without errors (404, 405). There are now four cases to distinguish: <sl><li>a) Both the user and signaling data as well as the Frame number are correct. The receiver forwards the frame to the data sink (407) and so advise the sender to error-free reception of the frame (406).</li><li>b) Both the user and signaling data as well as the Frame number is wrong. In this case, the frame is discarded (409).</li><li>c) The user and signaling data is error free, the Frame number, however, is flawed. The frame is discarded (408). This occurs statistically only rarely.</li><li>d) The user and signaling data is incorrect, the Frame number is no problem though. In this more often occurring case the receiver stores the frames from (410) and asks the sender this erroneous frames again (415). For this purpose is achieved according to the frame number in question immediately transmitted to the sender and not, as in the state art, the expected as the next frame number. This facilitates the transmitter, the overview of the frame to transmitted and which are repeated. If the channel is strong disturbed, the probability increases that even when Repetition of frame errors. Thus it comes often prevents three transmitted frames with the same have frame number faulty production or signaling data.</li></sl>

If two such frames are stored, and a third erroneous frame is received the same frame number, so leads the receiver has a majority compared with (412), as in the above patent is shown in detail. alternative Also, more than three frame and stored together are compared. The result of this comparison is a majority new framework. The memory is cleared (413) and the new framework then checks whether the user and signaling data are error-free (414). With accuracy of this new framework will the data sink (407) supplied and this confirmed to the sender (416).

Is also the new framework defective, rejects the recipient to Frame (417) and calls for the sender to repeat the frame of the relevant frame number (in FIG. 4 not shown). The Procedure can now begin again (4 not shown in Fig.).

The frame structure of the invention may also be advantageous are used when performing a majority comparison is not provided. In this case, it is the receiver in many Cases possible frame with faulty agricultural or Signaling data directly indicating the frame number at Transmitter to repeat request. As already indicated above, resulting in complex transmission systems according to the prior art the time, often fluctuating offset between the sending a frame and the arrival of a repeat request to that the transmitter needs to monitor in an expensive manner, which frame were received correctly and which have to be repeated. ever on how this monitoring is carried out, there is even unnecessary repetition of already correctly received frame. With the aid of the frame structure according to the invention, it is possible that to reduce the transmitter required intelligence and unnecessary to avoid repetition.

Finally, it is noted that the use of Terms "transmitter" and "receiver" is not to imply that it necessarily is a simplex data transmission. Rather, the terms are any transceivers extend. A duplex data transmission, as described, for. Example, the GSM mobile phone standard describes therefore also for the Using the frame structure according to the invention suitable.