GB2052217A - Data transmission system - Google Patents

Abstract

In systems for transmitting digital data through transmission channels in which bit errors occur, especially in bursts, and which systems are capable of detecting and correcting bit errors and yet maintain the telegram synchronization an excellent telegram acceptance is achieved by providing in said system means for repeating each word at least once, means for Exclusive OR-ing each word or word repetition with each individual part of a predetermined bit pattern having a large autocorrelation coefficient, means for Exclusive OR-ing each word or word repetition with an individual part of said bit pattern during decoding, means for comparing each word with each repetition of said same word until the comparison exhibits identity, and means for accepting such identity as a correctly received telegram and further means for additionally employing recognized identity as a synchronization information.

Description

SPECIFICATION Data transmission system When transmitting digital data through transmission medias which gives rise to bit errors, it is known to employ transmission equipment capable of detecting and correcting bit errors In some transmission medias bit errors occur substantially evenly distributed in time, and means have been elaborated, which means are often called codes, which are particularly suited to detect and correct said distribution of bit errors. In other transmission medias, for instance in connection with mobile radio communication services or systems, there is most probability of bit errors occurring in groups, the so-called bursts, which is often due to fading. The transmission equipment of the present invention offers special advantages when utilized in connection with transmission medias in which bit errors especially occur in bursts.

Prior art transmission equipment of the aforementioned kind additionally have means for securing synchronism between transmitter and receiver so that the receiver gets the information of when a telegram or a certain part of it starts. To this end use can be made of special bit sequences in which of course errors may occur too during transmission. It is known to provide means in the decoding equipment which is adapted to ignore a number of bit errors occurring during transmission of the sync characters.

The error detecting method most commonly used imply the transmission of so-called check bits, the value of which depend on the bit-values of the word to be protected by these check bits.

Among these methods the best known one is that in which the parity check is involved, but more complex polynomial. checks are also known, which are derived from a polynomium and which are decoded under the same polynomium.

For the purpose cf error detection and correction it is also known to employ a word repetition procedure. By this the decoding scheme consists in comparing the repeated transmissions of the very same word bit by bit, and that bit is considered to be correctly received which occur most frequently. The method is called error correction by voting and has further been proposed when comparing bit-combinations. The practical value of the different methods may be calculated on the one hand, and on the other it may be measured in terms of the so-called telegram acceptance defined by the ratio, measured in per cent, between the number of correctly received telegrams and the number of transmitted telegrams.

The present invention relates to a digital transmission system comprising encoding and decoding means as well as synchronizing means, and an object is to provide an improved telegram acceptance also by the use of transmission medias in which bit errors occur frequently especially in bursts.

The above mentioned object is achieved by the features stated in the characterizing part of claim 1.

The increased telegram acceptance thus obtained appears from Figure 4 of the drawings as it will be described below in connection with the detailed part of the specification taken in conjunction with the drawings, in which: FIG. 1 is a block diagram of an embodiment of a radio station having transmission equipment according to the present invention; FIG. 2a-2b show prior art telegrams; FIG. 3a-3f show a number of telegrams transmitted by the transmission equipment according to the invention; and FIG. 4 shows findings of measurement verified as graphs of the telegram acceptance versus an average HF-signal for two prior art digital transmission equipments and for two embodiments of digital transmission equipment according to the invention.

The radio station illustrated in Figure 1 comprises an aerial 1 which via an aerial feeder 2 is connected to an aerial switch 3 which has an electronically controlled relay 4 capable of connecting the aerial 1 to either a transmitter 5 or a receiver 6. A second relay 7 also being controlled electronically has switching contact means 8 capable of connecting the transmitter 5 to either a modulator 9 or a microphone amplifier 10 including a microphone 11 and switching contact means 12 capable of connecting the receiver 6 to either a demodulator 1 3 or an amplifier 14 including a sound reproducing apparatus 1 5.

The relays 4, 7 and 12, the transmitter 5, the receiver 6, the modulator 9 and the demodulator 1 3 respectively are via conductors connected to an input/output unit 1 6 of a microcomputer which is based on a microprocessor 1 7. The microprocessor is in a well known fashion connected to a timer 18, a ROM-unit or a PROMunit 19, a RAM-unit 20 and a CLOCK-unit 21, respectively. The microcomputer may further be connected via a number of lines generally indicated by reference numeral 22 to various control appliances such as lamps, bells, displays, push-buttons, switches, keyboards, printers or the like.

It will be understood that the transmitter/receiver described above is disposed to communicate binary telegrams as well as analogue signals such as speech-signals, and the embodiment in question is capable of switching between digital and analogue transmission and vice versa.

Information of in which way tho telgrams which are to be transmitted via the modulator 9 and the transmitter 5 to a remote receiver are to be formulated and in which way telegrams received from a remote transmitter are to be decoded, is stored in the ROM-unit 1 9.

The informative content of the telegrams is arbitrary but may for instance be related to a selective call to another subscriber or may be information on a condition which is recorded locally and which is incoming through one of the lines 22 in an analogue or digital form.

In Figure 2 is shown a telgram in a prior art format involving error detection but not error correction. The length of the telegram is of 52 bits, the first 12 of which are used for synchronization of the telegram, the following 32 bits representing data bits and the last 8 bits being cyclical redundance check bits (CRC) which can be computed by the transmitter as well as by the receiver on the basis of said 32 data bits. If the receiver recognizes the check bits as being irrelevant to the data bits received this is interpreted as a detected transmission error and the telegram is rejected.

Formats of this type are commonly used in several countries because of their simple structure, but because of the fact that only one single bit error in the telegram effects its rejection the telegram acceptance is relatively bad as illustrated by graph 41 in Figure 4.

Another format which, in order to give a fair comparison, comprises 32 data bits and besides that is structured in accord to known principles provides error detection as well as error correction.

This telegram format, which is illustrated in Figure 2b, contains 124 bits in total and begins with 32 bits arranged in a predetermined pattern with an inherent high autocorrelation coefficient solely serving as a telegram synchronization pattern. The error correction in the sync part of the telegram follows from the fact that only 28 of the 32 bits in total are required to be correctly received in order to accept the telegram synchronization pattern.

This criterion is an appropriate compromise between the toleration of a number of errors as large as possible without the number of false telegram synchronizations becoming disturbing.

The telegram synchronizing section is followed by 94 bits in total. Of these bits 32 are data bits, 8 are CRC-bits and the remaining 52 bits are redundance bits.

The error correction in this part of the telegram is carried out by coding said 32 data bits and 8 CRC-bits after a so-called Hagelbarger code described by D.W. Hagelbarger in Bell System Tech. Journal, vol 38, 1969, p.p. 969-984.

This code has the capability of correcting up to and including 6 consecutive bit errors. The following 1 9 bits have to be correct and then it is capable of correcting again the following 6 consecutive bit errors (in maximum), and so forth.

The telegram acceptance of the telegram format described so far is shown as graph 42 in Figure 4.

An embodiment of a telegram code having a telegram length of 96 bits and provided by means of a transmission equipment of the present invention is illustrated in Figure 3a in which the references B1, B2, B3 and B4 designate data byte number 1, 2, 3 and 4, respectively; and it should be noted that 1 byte equals 8 bits. Here, a data byte means a "word" and it appears that the first word is transmitted followed by two repeats after which the same pattern is repeated for the next three words of the telegram. The decoding is carried out as specified in claim 1, subsection g).

It is seen that a byte in each of the four groups each of which contains three bytes may comprise an error without the detection being disturbed and this demonstrates the error correction. If more than one of the three bytes in a group are erroneous then none of the comparsons will show identity, and this will cause a rejection of the telegram the error detection thus being demonstrated. Byte voting inherently provides error detection in contrast to bit voting which provides error correction only because 2-out-of-3 bytes, at least, have to be identical, whereas, by voting at bit level at least 2-out-of-3 bits have to be identical and this of course will always be the case.

Voting on byte level, especially, implies an advantage when the input signal to the decoder is random (noise). In this situation voting on bit level will always result in an erroneous message, whereas, by voting on byte level, the individual bits in at least two bytes have to be in accordance.

It may happen, however, that two bytes in the same group are subjected to an error in identical bit positions, which results in a false detection.

This possibility is always present in any code whatsoever. If one wishes a greater security than implicitly elaborated so far the telegram may be extended by a further byte, which serves as a cyclical redundance control (CRC). This byte then have to be repeated like the four data bytes and to be decoded in the same manner.

In what has been described so far, the telegram synchronization has not been dealt with for the sake of clarity. The bit pattern which actually is transmitted is not as shown in Figure 3a but as shown in Figure 3b. It should at this point be noted that the encircled ±sign stands for an Exclusive OR operation. Several means for carrying out this operation are known. The bytes Cl-Cl 2 represent 12 bytes which have been predetermined as sync words and have been selected from a bit pattern having a large autocorrelation coefficient, i.e., the bit pattern makes a pseudo-random binary sequence having even numbers of O-bits 1-bits which for instance may be generated in a shift register having one or more feedback loops.

In the decoder the 12 bytes received are subjected to an Exclusive-OR process in conjunction with the selected sync bit pattern by which the initial words are provided. This is due to the fact that a word subjected twice to an Exclusive-OR operation reappears unchanged. By this procedure it is achieved that in the case that the decoder does not have in its store the proper bit "window" the probability will be very little that any of the comparisons show identity. Hence, the telegram synchronization has been demonstrated.

Thus, the 12 bytes C1C12 frames the message to be decoded, i.e., the decoder knows of the beginning and the end. Hence, telegram synchronization according to the invention has the effect that the provision of a telegram sync pattern before each message, such as illustrated in the Figures 2a and 2b, now is rendered unnecessary.

By selecting different bit patterns C 1412 each of which having a large autocorrelation coefficient but being mutually orthogonal the telegram code according to the invention may be used immediately for selective calls. Thus, it is possible to make calls to different units simply by changing the sync bit pattern, i.e., without using bits from the message itself.

It belongs to the general knowledge that in connection with data transmission via mobil radios the bit errors will often be present as a number of consecutive bit errors. Therefore, it may be advantageous to select, instead of the bit pattern shown in Figure 3a, the bit pattern shown in Figure 3c which resists up to 1 6 consecutive bit errors occurring up to two times. Two groups of bit errors each containing 9 bit errors and being mutually separated by 39 bits will be corrected with certainty.

Another possibility would be the selection of the sequence shown in Figure 3d which is capable of resisting up to 32 consecutive bit errors appearing only once. Groups of bit errors each containing 25 bit errors are corrected with certainty.

The selection of either of the formats shown in the Figures 3a, 3c or 3d, respectively, depends on weighing the demands on the error correction properties. The disadvantage of an increased resistance to consecutive bit errors is a lower resistance to evenly distributed bit errors.

In Figure 4 the graph 43 shows the telegram acceptance of the code represented by the Figure 3c.

Yet a larger telegram acceptance may be achieved if the telegram length is extended to say 128 bits. This allows each word to be transmitted four times as is illustrated in Figure 3e. This structure resists up to 1 6 consecutive bits appearing up to 4 times, and 4 groups of bit errors each containing up to 9 bit errors and mutually separated by at least 23 bits are corrected with certainty. The words may be interchanged as is shown in Figure 3f, so that, which is repeated, is a pair of words, and the comparison may then be executed between the individual words or between the word-pairs. A telegram as that shown in Figure 3f resists up to 32 consecutive bit errors as the matter of each half of the telegram is concerned, and groups of 25 consecutive bit errors separated by 39 bits are corrected with certainty. The telegram acceptance of a transmission coded in this way is illustrated by the graph 44 in Figure 4.

When the sequence of comparisons executed during the decoding procedure is adapted to the statistic distribution of bit errors of the transmission channel in question, then it is possible to improve the telegram acceptance of said channel. Although it has been shown in connection with Figure 2 that the decoding is executed by means of a microcomputer it should be understood that the comparison procedure may be carried out in circuits made up of standard IC components of say the TTL- or CMOS-type.

Claims (7)

1. Digital transmission system having: a) coding means including means for dividing an information string into telegrams and each telegram into words; b) decoding means; c) synchronization means, and d) means for repeating each word at least one time; characterized by the combination of the following: e)- means for executing an Exclusive-OR - operation between each word and word repetition and each part of a predetermined bit pattern having a large autocorrelation coefficient; f) means for Exclusive-OR-ing of each word or word repeat and each part of the same bit pattern during decoding; g) means for comparing each word with the repetition of the same word until the comparison exhibits identity; h) means for accepting such an identity as a correctly received word and for rejecting the word in the case of non-identity; and i) means for further employing the recognized identity as sync information.

2. Transmission system according to claim 1, characterized by j) means for repeating each word in the telegram a number of times before the next word in the telegram is transmitted.

3. Transmission system according to claim 1, characterized by k) means for structuring the telegram in sequences, which are repeated a number of times and each of which consisting of two or more words.

4. Transmission system according to claim 1, characterized by I) means for comparing systematically two words in a repetition sequence and for interrupting said comparison when recognizing two equal words.

5. Transmssion system according to claim 1, characterized by m) means for recovering a telegram by executing a number of times, said said number depending on the telegram length, systematic identity resulting comparisong between two or more words.

6. Transmission system according to claim 1, characterized by n) means for combining said comparison of the repetitively occurring words in a telegram in order to gain a reduction of the number of necessary comparisons.

7. Transmission system according to claim 1, characterized by o) means for rejecting a telegram as a false one, if only one of said systematically and repeatedly executed comparisons or combination of such comparisons do not result in identity.