DE4241618C2 - Process for the transmission and backup of data on disturbed channels - Google Patents

Description

The invention relates to a method for transmission and Si backup of data on disturbed channels from a transmitter a receiver over a medium with one relative to Bitra slow fluctuating damping.

When transferring digital data in networks or on Point-to-point connections are often not aler properties of the transmission channel single or meh other information symbols falsified. To the integrity of Securing data must therefore take measures to detect corrective action and to correct errors. Soon the transfer of a certain amount of data should be possible in time take up as little time as possible because the channel U. with represents shared resource to other participants. As fixed Boundary conditions are the maximum in most cases possible transmission power (or the peak amplitude), the Emp catcher quality, the bit rate and the acceptable error rate give.  

The quality of a transmission can be determined by the following size be described. Reliability is the probability is a measure of that from the recipient incorrect data can also be recognized as such. Of the Throughput gives the ratio of the average number of success richly transmitted information bits in a certain time to the number of bits that are in the same time without Channel interference could have been transmitted. Here a fixed bit rate is required. The data throughput time is the absolute time from the start of a transmission of a particular section of data passes until that data are accepted as correct by the recipient.

One of the known solutions is the so-called forward mistake correction. With this method, the transferred information mation coded so that it is possible at the receiving end, errors to recognize and correct. Block- or convolutional codes used. Through the additional use a spread (interleaving) is also possible, esp to correct error bursts, especially for convolutional codes. Each however, despite the provision of substantial redundancy, the percentage of correctable errors per block rela tiv low. For example, he is with Reed-Solomon Rate 1/2 codes on the order of 20%. For be agreed news channels and applications this is not sufficient.

With the forward correction method, therefore, not very much great ability to be corrected, though great Redundancy must be created. Especially with an Ab Whole data packets can be lost on radio channels go. For heavily disturbed news channels are therefore the Forward correction methods alone are not sufficient.

In the so-called diversity process, the Emp catch in different time, space or frequency ranges the influence of statistically occurring disturbances is reduced.  

Are particularly important for a packet-oriented transmission the automatic repeat processes (Automatic Repeat Request). Here is a high-rate code with gro ß security determines whether the data packet is correct or Not. In the event of an error, the transmitter will be prompted transmitted for repetition; then this happens often until the package has been received correctly. To do this used different strategies (such as Go-Back-N, Selective Repeat, a use of a positive / negative receipt, timeout etc.).

The repeat procedures require an acknowledgment channel, which is often also disturbed, and therefore we the multiple round trip time to long data run times. For example, satellite channels pass between one First and one retransmission are often more than one half a second. Often several repetitions are necessary so that runtimes of several seconds are quite possible can.

The so-called hybrid processes represent a combination of the two methods listed above. Only when the ability to correct the code is not sufficient Repeated package (type I). In another variant (type II) always becomes a block after the detection of a block error transmitted with redundancy, so that a code with corrective ability always increases. For every type This repeat process is a number of variations and Improvements have been suggested.

Due to the hybrid process, some shortcomings are pure Repetition procedure eliminated; in particular, a height throughput and better adaptability to the Channel reached. Because the adjustment of the broadcast strategy, however can only take place after a round trip period, above all for channels with a long round-trip time, like for satellites channels, not an ideal adaptation to the current channel  was standing. The long data runtimes, the need for one Acknowledgment channel and the large data buffer remain however receive. Therefore, the complexity and the effort increase Receiver compared to the pure repeat procedure pregnant.

The object of the invention is therefore a method for transmission and backup of data on disturbed channels create, with the help of a relatively simple executed setup with short data runtimes considerably safer than with the hybrid processes previously used data throughput increased considerably can be.

According to the invention, therefore, a method for transmission and backup of data on disturbed channels from a transmitter to a receiver via a medium with a damping slowly fluctuating relative to the bit rate due to the Features achieved in claim 1. Advantageous further training are the subject of claim 2.

In the method for transmission and Si Saving data on disturbed channels is an estimate a reception signal to be expected at the receiver made the transmitter continuously during a broadcast. For this purpose, the transmitter transmits immediately for each information symbol as much redundancy as with the current state of a transfer supply channel is necessary so that a predetermined ratio from bit energy to noise power (Eb / No) is achieved. Here it is assumed that the transmitter and receiver are on measurable in the same medium in a similar measuring range Send signals. However, the receiver does not have to do this A corresponding signal is emitted specifically for the transmitter but it can be a very unspecific signal like for example a signal intended for other services  to be used. Such a signal is, for example, at data transmission with the help of satellites almost always where, for example, a pilot tone, a beacon signal, a time code, etc. are transmitted. The transmitter points here to a receiving part, for example, via a circus lator is connected to a common antenna, although in following is only ever spoken by sender and receiver.

The transmitter measures the Ka during a transmission of data quality and decides after each symbol whether the recipient enough signal power is obtained or whether the same Symbol must be repeated again. This can be done in time grid of information symbols a momentary reception line measured and added up. By comparing accumulated power values with a setpoint are the Determines when a new information symbol is transmitted. The threshold can also be changed over time to protect parts of a message; ever but the recipient must know about it. Depending on the type Other methods for Estimation of a channel state can be used.

Redundancy is transmitted after each symbol in the unchanged bit map and lasts un depending on the channel status different lengths. The redundancy can be different transmitted in the same way. In the simplest case there is it from the constant repetition of the information sent mation symbols. However, it is more advantageous at this time to start transmitting a code sequence and this to the extent that it is estimated by the channel is required by the transmitter. The recipient must this sequence, which is called the signature sequence below know. The information can follow the signature sequence according to the type of "Direct Sequence Spread Spectrum" process by multipli cation can be impressed. In the borderline case, the signature exists follow from all "1" values, the process goes into a pure one Repetition about. Please note that the signal  follow again and again with each new information symbol is transmitted starting from the beginning.

In the receiver, a received signal becomes the same Like a current performance measured at the transmitter sen, and until a predetermined threshold is reached mulated. Reaching the threshold shows the beginning of a new information symbol and the end of redundancy of the previous one. Certainly at these times to be able to receive incoming signals le compared with the signature sequence and thus can by Correlation, d. H. through one to sync "Spread spectrum" signals analog process, the times be be agreed when the signature sequence be again starts.

For everyone belonging to a certain information symbol, redundant symbols in the receiver again the informa reconstruction symbol; for example, by a Integration over all belonging to an information symbol Repetitions so that the signature sequence is constantly "one" ("1"), receive a correspondingly higher signal energy. An inevitably integrated noise grows but slower in performance than the signal. Is this Noise correlated from data symbol to data symbol, see above the noise power only grows linearly with the integration duration while the signal power increases quadratically. The means that with M repetitions an M times larger Sig nal / noise ratio than achieved with a single symbol becomes. If a signature sequence not equal to "1" is selected, detection of the transmitted information symbol more agile processes, such as the type of Viterbi Algorithm to be improved.

The method according to the invention thus has the following advantages on:

• a) A separate acknowledgment channel is not required.  
• b) A transmitted data packet is available in the receiver as soon as the criterion bit energy to noise power (Eb / No) allowed.
• c) One or more additional orbital periods between sen the and recipient are eliminated.
• d) Compared to the conventional hybrid repeat procedure throughput is increased because only the moms are always used redundancy is transferred.
• e) Furthermore, the method according to the invention reacts immediately a current channel status.
• f) On the sending and receiving side only puf fer that do not take up more than one data packet have to.
• g) The facilities required in the transmitter and receiver are much easier than for advanced hybrid Repeat procedure.

The invention based on preferred Aus management examples with reference to the attached Drawings explained in detail. It shows

Figure 1a is a schematic representation of a data transmission system between a mobile transmitter and a quasi-geostationary satellite.

FIG. 1b shows a simplified block diagram of a data tragungssystems for carrying out the method erfindungsge MAESSEN;

FIG. 2 shows a simplified block diagram of a transmitting part of a transmitter according to FIG. 1b;

Fig. 3 shows an example of signals for example, data sequence, and

Fig. 4 is a simplified block diagram of a receiver partly a receiver according to Fig. 1b.

The method according to the invention is described below using the example of a satellite-based mobile radio connection for the transmission of binary data. Here, the transmission of the binary data takes place from a transmitter 1 accommodated in a vehicle 1 , ie from a mobile transmitter via a transmission link I to a schematically illustrated quasi-geostationary satellite 2 serving as a receiver, while on a transmission path II from as a transmitter the nending satellites 2 preferably transmit completely unspecific signals to the transmitter accommodated in the vehicle 1 .

This is shown in principle in FIG. 1b in the form of a simplified block diagram. Here, a message to be transmitted is entered into a transmitting part 11 of a transmitter 10 and sent on the transmission path I via a schematically indicated transmission channel 3 to a receiving part 21 of a receiver 20 . A pre-receiver 20 in the transmit portion 22 are overlooked on a dot-dash line angedeu ended transmission path II over the same transmission channel 3, a preferably specific signal to a receiving part 12 in the transmitter 10 from.

For the sake of clarification and illustration, only the case of a simple correlation receiver is considered below, of which a simplified block diagram of a baseband transmission part 11 is shown in FIG. 2.

In FIG. 2, incoming data {a _i } are stored in a buffer 111 in a simplified block diagram of the transmitting part 11 of the transmitter 10 ( FIG. 1b). In this case, the incoming data a _i , the information to be transmitted itself or a sequence 110, which is protected with a code and is represented in a dash-dotted line, can be a code. When packet data is transmitted, an entire data packet can be buffered in the buffer 111 . The buffer 111 is then read out at a predetermined data rate R that corresponds to the requirements. In this case, a signal present at the buffer 111 and designated "buffer control" causes the correct information signal to be read out.

The signal referred to as "buffer control" is obtained in that a signal applied by the receiving part 12 of the transmitter 10 via a receiving stage 120 designed as an HF stage, in which a signal coming from the transmitting part 11 of the receiver 12 is processed to a signal Estimation unit 121 is created. The output signal of the estimation unit 121 is applied to a unit 122 which is decisive for redundancy and to which, in addition to the data rate R, a desired setpoint value is also applied. This setpoint corresponds to a so-called "design" value of the connection from transmitter 10 to receiver 20 . If this setpoint of damping is exceeded, the method becomes effective. The output signal of the decision unit 122 is then the signal used to control the buffer 111 , which is therefore referred to above as "buffer control" for this reason.

The signal designated "buffer control" also ensures that a unit 113 generating a signature sequence {b _k } is correctly read out. Here, depending on the level of the signal present at the receiving part 12 of the transmitter, it is determined when the signature sequence {b _k } is set to its beginning or whether it continues. The signals read out by means of the "buffer control" signal from the buffer 111 and from the unit 112 generating the signature sequence {b _k } are multiplied in a multiplier 114 . The signal b (t) obtained in the multiplier 114 is applied to a modulator 112 ; the signal correspondingly prepared with respect to an intermediate frequency applied to the modulator 112 is then applied to a high-frequency part, not shown.

In Fig. 3, an emerging data sequence is shown in detail when the received signal power S (see signal curve in the first "line" of Fig. 3) is used as a criterion. Here, the method according to the invention works correctly if the estimate of the respective bit energy in the transmitter 10 and in the receiver 20 match (see second “line” from above in FIG. 3). If the reception level is low and the estimate on the reception side is therefore less certain, the boundary between the individual information symbols can be determined using the signature sequence {b _k } generated by the unit 113 . In the case of high channel attenuation, a lot of redundancy has to be transmitted, and accordingly a corresponding number of bits of the signature sequence b _{k are} transmitted.

In any bit interval jT (where the bit interval T equals l / R) the multiplier output signal b (t) can be described as follows:

b (t) = a _i b _k g (t - jT) (1).

Here, a _{i is} an i-th information bit, b _{k is} a k-th bit of the signature sequence {b _k } and g (t) is a pulse shape - shown rectangular in FIG. 3. The i-th bit is repeated M (i) times, or the size M (i) is the length up to which a signal sequence {b _k } is transmitted in the i-th bit. The mean value <M (i) <over all i is equal to the quotient of the so-called link power, the receiving power if the link has no additional damping, and the expected value of the receiving power.

Neglecting other signal distortions and non-ideal effects, a signal e (t) is formed in the receiver 2 in the simple case from the signal b ′ (t) by integration over time M ′ (i) T:

where b _{k is} the receiving side known and b _{k 'is} a received signature sequence, while n (t) denotes a noise signal. The time t _i and the duration M '(i) are estimated by the receiver 20 on the basis of the received signals. The time t _i is determined from the measured received power; the current reception power is added from bit to bit until the same threshold value is reached with which the transmitter 10 has transmitted.

The signal energy which is obtained with an undisturbed signal after the transmission of a single bit in the receiver is achieved with additional attenuation by the multiple transmission after M bit times, ie the threshold value is reached, which is shown in Fig. 3 ("second Line ") is represented by the signal" Integrale + Dump ".

The next line labeled Bit (Information) shows, for example, the bit numbering that begins randomly with "1". For bit "1", the required energy is only reached after five (5) bit periods, for bit 2 only after eight (8), for bit 3 after three (3) etc. Also, due to the consecutive numbering of k in the last Line indicated under the rectangular pulse labeled "transmitted sequence", which in turn are the signal s (t) at the input to the modulator 112 in FIG. 2.

The next bit is considered a new bit and the Accumulated reception power is reset to zero. All signals still received are now repeated interpretation of the first symbol after reset, until again the threshold of the accumulated reception power is reached.

The scatter of the noise signal n (t) increases by integration proportional to the root of M ′ (i), while the signal increases linearly if the signature {b _k } known at the receiving end and the actually received signature sequence {b _{k ′} } are the same, ie {b _k } = {b _{k ′} }. This applies to non-corrected noise values n (t) at times jT and lT for all values l j. Therefore, filters on the receiving side must not be dimensioned too narrowly. (The autocorrelated noise signal n (t) must have almost decayed within the bit interval T.)

In Fig. 4, a simplified block diagram of the receiver 20 ( Fig. 1b) is shown. A cumulative signal energy is estimated by means of an estimation unit 211 . If the desired value required for a bit decision has been reached, a decision-making unit 212 sends the corresponding signal to the decision controller 213 . The decision control 213 stops the signal sequence (block 215 ), causes the "Integrale + Dump" - (I + D-) unit 217 to deliver the previously integrated value to the unit "bit decision" 218 and a reset of the I + D unit 217 . Both the clock of the individual bits being transmitted and the clock of the information bits, which may be recovered from a plurality of transmission bits, are required. The latter always appears at the end of an integration interval, also at the output. The clocks are recovered based on received signals in the clock recovery unit 214 .

Errors occur here if the number of redundant bits which the receiver accepts on the basis of its estimate of the received power in the estimating unit 211 does not match the actual received power. The following action can be taken against this: The values after the demodulator are saved. However, decision controller 213 only accepts a value as correct if the received power estimate was within a predetermined tolerance range. If this was not the case, a search is made as to whether there is a better value for this case, namely that the decision is made a transmission time earlier. If this is not the case, a further bit interval must be received.

The method according to the invention can, depending on the Boundary conditions for different applications at Message transmission are used, for example for time-critical processes with a long signal runtime, a typical example of this is the transmission digita language via satellites.

Claims (2)

1. A method for transmitting and securing data on ge disrupted channels from a transmitter to a receiver via a medium with a slowly fluctuating attenuation relative to the bit rate, wherein
an instantaneous channel attenuation is measured by the transmitter ( 10 ) during a transmission of data, and an estimate is continuously made from the measured channel attenuation with regard to the expected receiving power at the receiver ( 20 ), for which purpose any signal is constantly available from the remote receiver ( 20 ) to be provided;
the receiver ( 20 ) continuously measures the instantaneous reception power during the reception of data, and
the transmitter ( 10 ) repeats a bit that has just been sent until it estimates that the signal energy at the receiver ( 20 ) is sufficiently large, and
the receiver ( 20 ) integrates the signal according to the measured received power until the total energy is sufficiently large. 2. The method according to claim 1, characterized in that
that instead of repeating bits from the transmitter ( 10 ), transmission of a certain predetermined signature sequence {b _k } begins in a bit pattern and continues until, based on the estimate in the transmitter ( 10 ), a sufficient total energy is to be expected in the receiver , and
that a correlation is carried out piece by piece in the receiver ( 20 ) with the signature sequence {b _k } known there.