DE102013215829B3 - Method for transmitting data - Google Patents

Abstract

Method for transmitting data, wherein a random access transmission channel to a receiver is shared by a plurality of users, the time axis being divided into frames, a user sending his data using the following method steps: generating an overall codeword the user data to be transmitted, - generation of a large number of coded packets from the overall codeword, - transmission of the coded packets by a user at different times within a frame via the transmission channel, the receiver processing the received data in accordance with the following method steps: Attempt to decode the received coded packets by the receiver using the code known to him, - if at least one coded packet was successfully decoded: reading out the information about the position of the other d-1 coded packets from the same user within the frame from the successfully decoded packet t, - reconstruct the symbol sequence in each of the other d-1 encoded packets of the same user within the frame, - removing the interference caused by the d encoded packets in packets of other users using a successful interference cancellation method.

Description

The invention relates to a method for transmitting data, wherein a transmission channel to a receiver is shared by a plurality of users.

In communication networks, the transmission of data is often done by random access methods, namely without coordination and often without synchronization and user coordination. An example of this is the ALOHA protocol, in which each user sends his data directly when there is data to be transmitted. If a collision occurs with another user's message while the message is being transmitted, both collided messages will be lost. Due to this fact, the maximum possible throughput of the ALOHA process is limited to approx. 18% of the channel capacity. The main advantage of the ALOHA method over other methods, such as Slotted-ALOHA, CSMA or CRDSA, lies in its simplicity, since user coordination is not necessary.

Such a need for random access methods in which the individual users can transmit uncoordinated and unsynchronized, for example, in the satellite communication.

While with the pure ALOHA protocol (pure ALOHA) each user can start its transmission at any time, slotted ALOHA divides the time into time intervals so that a transmission can only be started at the beginning of a time interval. This requires a temporal synchronization of the users.

The slotted ALOHA method increases the efficiency compared to the pure ALOHA method to approximately 37%. The slotted ALOHA method was further developed by the use of Successful Interference Cancellation (CRC) techniques (CRDSA: Contention Resolution Diversity Slotted Aloha and IRSA: Irregular Repetition Diversity Slotted Aloha). Here, the time is not only in time slots, but also in so-called frames, namely groups of time slots, divided. A data transmission (data packet) is not performed only once. On the contrary, within a time frame (frames), one or more replicas of a data packet are sent, which have references to the positions of the other replicas. If one of the replicas is received successfully, the interference caused by them can be removed from the respective overall signal for all copies. As a result, possibly other packets that were previously subject to interference, can be decoded. The decoding takes place by iterative removal of interference, so this technique is referred to as Successive Interference Cancellation.

In the context of IRSA, the number of replicas is not determined statically, but determined on the basis of a probability distribution.

Non-slot based systems require a less complex synchronization of the frames and allow a data unit to be transmitted at any time, so that partial interference of one data packet with another data packet is possible.

• [1] E. Casini, R. D. Gandenzi, O. d. Rio Herrero, ”Contention Resolution Diversity Slotted Aloha (CRDSA): an enhanced random access scheme for satellite access packet networks”, Wireless Communications, IEEE Trans. Commun., vol. 6, num. 4, Seiten 1408–1419, April 2007
• [2] De Gaudenzi, R.; del Rio Herrero, O.; ”Advances in Random Access protocols for satellite networks, ”Satellite and Space Communications, 2009. IWSSC 2009. International Workshop on, Seiten 331–336, 9–11 September 2009
• [3] G. Liva, ”Graph-based Analysis and Optimization of Contention Resolution Diversity Slotted ALOHA”, Communications IEEE Transactions on, vol. 59, no. 2, Seiten 477–487, Februar 2011
• [4] C. Kissling, ”Performance Enhancements for Asynchronous Random Access Protocols over Satellite”, IEEE International Conference on Communications, 5–9 Jun. 2011
• [5] F. Clazzer, C. Kissling, ”Method for improving the throughput in an unslotted Aloha scheme with Contention Resolution via successive interference cancellation and replica combining”, DE 10 2012 219 468.1 (noch nicht veröffentlicht).

The described data transmission methods are described inter alia in the following publications:

• [1] E. Casini, RD Gandenzi, O. d. Rio Herrero, "Contention Resolution Diversity Slotted Aloha (CRDSA): An Enhanced Random Access Scheme for Satellite Access Packet Networks", Wireless Communications, IEEE Trans. Commun., Vol. 6, num. 4, pages 1408-1419, April 2007
• [2] De Gaudenzi, R .; del Rio Herrero, O .; "Advances in Random Access Protocols for satellite networks," Satellite and Space Communications, 2009. IWSSC 2009. International Workshop on, pp. 331-336, 9-11 September 2009
• [3] G. Liva, "Graph-based Analysis and Optimization of Contention Resolution Diversity Slotted ALOHA", Communications IEEE Transactions on, vol. 59, no. 2, pages 477-487, February 2011
• [4] C. Kissling, "Performance Enhancements for Asynchronous Random Access Protocols over Satellite," IEEE International Conference on Communications, 5-9 Jun. 2011
• [5] F. Clazzer, C. Kissling, "Method for improving the throughput in an unselected Aloha scheme with contention resolution via successive interference cancellation and replica combining", DE 10 2012 219 468.1 (not published yet).

Among the non-slot-based systems, contention resolution aloha (CRA) according to publication [4] and the extended version (E-CRA) according to [5] use the idea of successive interference cancellation (SIC), so that a higher performance is achieved can be. The operating principle of E-CRA is in the 1a and 1b shown. An exemplary use case is shown in which three users access ABC on the common transmission channel and send two replicas in each frame. In the figure, interference-free portions of the packets are shown in white while the hatched areas indicate the presence of interference or collisions. In the illustrated scenario, the information content of the user A can be obtained by the collision-free replica A1. After successful decoding, the receiver removes the Replica A2 caused interference affecting user B's replica B1. This leads to the in 1b illustrated situation. At this point, methods such as CRDSA and CRA would not produce the desired result because no further collision-free replica can be decoded.

The E-CRA method makes use of the fact that possibly different and possibly complementary sections of the replicas of a particular data packet may be subject to interference and thus tries to combine the non-interference parts of the data packets in a suitable manner to enable decoding. In the illustrated example, the overall information content of user B can be reconstructed by using the initial interference-free packets of replica B2 and the subsequent non-interference packets of replica B1. Finally, if the data packet B can be recovered, the interference caused by B1 and B2 is removed from the packets of the user C, so that they too can be decoded.

The use of the Joint Successive Interference Cancellation method and the replica approach recombines with E-CRA for a maximum throughput of 0.42 (with 2 replicas per frame) compared to 0.35 for CRA and 0.18 for Pure Unslotted Aloha.

The described methods have some limitations that affect their performance and applicability in certain application scenarios. Slot-based systems place high demands on the hardware used, so hardware that does not have high computing power (eg Wireless Sensor Networks) can not be used. Furthermore, a high overhead is caused, for example, by sending a timing signal. Furthermore, the fact that users can only send packets at the beginning of a slot leads to an increase in the number of collisions leading to data loss, since data units that are sent simultaneously completely overlap each other at the receiver.

E-CRA overcomes these two drawbacks by requiring only frame-based synchronization and the fact that different sections of interference-prone packets are present and can be used to increase performance. However, the possibility of successful decoding of interference-prone packets depends on which portions of these packets are interference-prone, that is, whether enough of the missing symbols in the original copy of the data packet can be recovered from the further replicas of that data packet in the frame. This aspect is in 2 shown. In this situation, even with the E-CRA method, restoring the data is not possible. Two users send their data in one frame. Although parts of each replica are interference-free, none of the packets can be recovered because the last icons at the end of user A's packets and the middle icons of user B are always interference-prone. Further, the application of such a repetition coding method requires that all replicas have the same length and thus limits the freedom to send smaller packets.

DE 10 2011 011 397 B3 describes a method for transmitting data, wherein a transmission channel to a receiver is shared by multiple users. The data is transmitted to the recipient by the users using the Pure ALOHA method.

A multi-slot coded ALOHA system is known from the following: BUI, H.-C .; LACAN, J .; BOUCHERET, M.-L .: Multi-slot Coded ALOHA with Irregular Degree Distribution. In. IEEE First AESS European Conference to Satellite Telecommunications, Oct. 2012, pp. 1-6 - ISBN 978-1-4673-4687-0.

It is an object of the invention to provide a method for transmitting data in which a plurality of users share a transmission channel, wherein the data throughput can be increased.

The object is achieved according to the invention by the features of claim 1.

In the method according to the invention, several users share a transmission channel with a receiver. This may be, for example, a scenario in satellite communication in which multiple users send data directly or indirectly to a satellite, which forwards it to a receiver.

According to the invention, the time axis is divided into frames. As access method to the common transmission channel, a non-slot-based random access scheme is used.

According to the invention, a user sends his data using the following method steps:
First, an overall code word is generated from the payload data to be transmitted. Various codes known from the prior art can be used for this purpose.

Subsequently, a plurality d of coded packets are generated from the overall codeword. This is done by a so-called puncturing method. The individual coded packets may differ from each other. The receiver is aware of this strategy by which the coded packets are generated from the overall codeword. It is also aware of the code used to encode the packets. The number d of the coded packets that are generated from the overall code word can be set as a parameter of the method according to the invention in such a way that the performance of the system is optimized, for example, with regard to the data throughput. In principle, it is possible that each node accessing the common transmission medium transmits a different number d of coded packets. This means that d can change over time or from node to node (similar to the principle of IRSA).

• [A] J. Hagenauer, ”Rate-Compatible Punctured Convolutional Codes (RCPC Codes) and Their Applications”; IEEE Trans. Commun., vol. 36, no. 4, pp. 389–400 1988
• [B] Jalil Seifali Harsini, Farshad Lahouti, Marco Levorato, Michele Zorzi: Analysis of Non-Cooperative and Cooperative Type II Hybrid ARQ Protocols with AMC over Correlated Fading Channels. IEEE Transactions on Wireless Communications 10 (3): 877–889 (2011)

The generation of a plurality d of coded packets from the overall codeword is known in the art by the term "puncturing". This technique is used in channel coding to generate one or more encoded packets from an output packet. The idea behind "puncturing" is to first generate a large code word with a certain code rate from the original information bits representing the payload to be transmitted. This codeword is then "punctured", ie some of the coded bits are removed, resulting in one or more higher rate code packets. Different strategies can be used to puncture the mother codeword. Such can, for example, be taken from the following publications:

• [A] J. Hagenauer, "Rate Compatible Punctured Convolutional Codes (RCPC Codes) and Their Applications"; IEEE Trans. Commun., Vol. 36, no. 4, pp. 389-400, 1988
• Jalil Seifali Harsini, Farshad Lahouti, Marco Levorato, Michele Zorzi: Analysis of Non-Cooperative and Cooperative Type II Hybrid ARQ Protocols with AMC over Correlated Fading Channels. IEEE Transactions on Wireless Communications 10 (3): 877-889 (2011)

The generated packets can also be constructed such that each of them contains all the information so that upon successful decoding of one of the packets, all of the original payload data can be recovered.

The coded packets are transmitted by a user within a frame over the transmission channel. In contrast to the prior art, non-identical replicas of the same data packet are now transmitted within one frame. Rather, the individual packets may differ from one another, so that a larger or smaller proportion of redundant data is transmitted by each coded packet. The basic step of transmitting the encoded packets by a user within a frame via the transmission channel corresponds to the CRA method known from the prior art. Each coded packet contains information about the position of the other coded packets of the same user within the current frame.

The receiver processes the received data according to the following method steps: The receiver attempts to decode the received encoded packets using the code known to it.

Upon successful decoding of at least one encoded packet, the information about the position of the other d-1 encoded packets of the same user within the frame is read from the successfully decoded packet.

Subsequently, the symbol sequence in each of the other d-1 encoded packets of the same user is reconstructed within the frame. This is done using the recovered payload of the decoded packet, the strategy by which the encoded packets are generated from the overall codeword (puncturing strategy) and the code used to encode the packets.

Subsequently, the interference coded by the d coded in packets of other users is removed using a successive interference cancellation method.

Since it is not necessary according to the invention that each encoded packet contains the entire user data, it may happen that one of the encoded packets is successfully decoded and nevertheless not the entire user data can be obtained. In this case, further method steps can be carried out in order to successfully restore the user data by using the information of further coded packets. Such process steps are described in detail in the further course of the present application.

If the user data of a user can not be restored because z. If, for example, the encoded packets of this user could not be restored because of not yet removed interferences with other users, or because the packets successfully decoded for this user do not contain the entire payload data, the following method steps can be carried out: An attempt is made of one of the encoded packets Find packages containing the information about the position the other coded packets of this user in the current frame is decodable. For example. For this, the package with the highest SNIR can be used.

Subsequently, a joint decoding step is performed, in which the data of all encoded packets of this user are used in the current frame to restore the user data of this user. Again, the strategy used by the transmitter to generate the encoded packets from the overall codeword is considered. Furthermore, the encoding procedures known to the receiver of the code used to generate the overall code word are taken into account. With successful joint decoding, the user data of the respective user is available.

If the joint decoding step was successful, the following process steps are performed:
The symbol sequence in all d coded packets of the relevant user in the current frame is reconstructed on the basis of the decoded user data. Again, the strategy used by the transmitter to generate the coded packets from the overall codeword and, further, the code used to generate the overall codeword is taken into account.

Subsequently, according to the last method step already described above, the interference caused by the d coded packets in packets of other users is removed using a Successive Interference Cancellation method.

The method according to the invention offers the following advantages over the prior art and in particular over the E-CRA method:
First, the described joint decoding step does not force the receiver to reestablish the payload from different identical replicas of a data unit. This simplifies the decoding procedures because, for example, it is avoided that the individual replicas have to be examined symbol by symbol in order to find out the respective interference level and to decide which replicas are to be used for decoding. On the contrary, in the present method all transmitted coded packets can be used for decoding since each of their symbols contains relevant information (and non-duplicated information).

Furthermore, by intelligently selecting the encoding method used, it can be ensured that recovery of the payload data is possible even if the same or a similar portion of bits in different coded packets of a user is affected by interferences (as described, for example, in US Pat 2 for the E-CRA method). As discussed earlier, the E-CRA process would not be able to decode the packets under these conditions, as some of the original symbols in all replicas are corrupted and therefore not available for restoring the payload.

Advantages compared to slot-based methods such. B CRDSA arise in the following way: First of all, much less synchronization requirements are imposed on individual users. It is therefore no longer required such a precise timing. In addition, partial interference between data packets is allowed. Furthermore, the use of intelligent coding techniques allows data packets of different lengths to be sent.

Preferably, it is possible for each of the d-coded packets to contain all the payload data and in particular parity data.

Alternatively, it is possible for at least one of the d coded packets to contain the entire payload data, and in particular parity data, whereas the remaining coded packets do not contain all or even no payload data. These packets can thus increase the redundancy, while the interference caused by these packets is less due to their shorter length. In this embodiment, a joint decoding method is preferably used. It is preferred that each of the packets contain a pointer in its header that references the other d-1 encoded packets. It is thus sufficient to decode one of the headers to find out where the other d-1 encoded packets are and then to begin joint decoding.

It is preferred that a non-clutter code, LDPC code, turbo code, or convolution code be used to generate the overall code word. The use of other suitable codes is also possible.

Furthermore, it is preferable that the header of the coded packets is coded separately and at a lower code rate. This can ensure that the position of the other encoded packets of this user in the current frame can be successfully decoded.

In the following, preferred embodiments of the invention will be explained with reference to figures.

Show it:

1a and 1b : the operation of the known from the prior art ECRA method,

2 a situation where the E-CRA procedure does not succeed,

3 an exemplary embodiment of the method according to the invention,

4 a graphical representation of the data throughput of the method according to the invention.

The 1a and 1b , as well as the 2 have already been described in connection with the prior art.

In 3 the mode of operation of the method according to the invention is shown by way of example. Users A, B and C access the common transmission channel and transmit d = 2 encoded packets within the frame shown. Again, interference-prone portions of the encoded packets are hatched while the white portions are interference-free.

The receiver may decode user A's payload data from coded packet A1 and remove the interference caused by these packets from other data packets. Subsequently, the two encoded packets of the user C are decoded together as part of a joint decoding step. The interference-free sections present here are sufficient to obtain the payload. Subsequently, the interference caused by the packets of the user C in the packets of the user B can be removed, so that these packets can also be decoded.

In 4 the performance of the method according to the invention is shown. It is assumed that the time axis is divided into frames of length T _f = 10 μs and there is a frame synchronization between the users. Each user generates and sends one data unit per frame, which is composed of L = 1000 information bits. The channel load is set by a variation of the number of users of the system. Transfers are performed using a 16-QAM modulation scheme with a symbol duration T _s = 1 μs and a code rate R = 1/2, so that packets sent over the channel consist of S = 500 coded symbols with a duration of T _p = 0.5 μs. The value d, which identifies the number of encoded packets within a frame, has been set to 2. Coded packets of equal duration are randomly distributed within the frame. A maximum of I _c = 10 iterations has been established to resolve user interference as part of the SIC procedure.

All transmitter-receiver links have an SNR of 6 dB. Fading or channel cancellations were not considered.

Three non-slot-based Aloha-based random access methods were compared, namely CRA according to the above-mentioned publication [4], E-CRA according to the publication [5] and the method according to the invention. If packets can not be restored after I _c iterations in the CRA method, they are discarded. In the E-CRA method, in this case, new versions of the data packet are assembled using the best version of a data packet that exists between replicas with respect to the SINR. Subsequently, an attempt is made to decode the reconstructed packet.

The cross-interference between the packets is calculated at symbol level and a data unit is marked successfully decoded as soon as the number of available information bits, taking into account the Shannon capacity in different portions of the original code word, exceeds the length of the payload data L. In the case of CRA and E-CRA, this applies to each of the available and identical replicas; H. it is assumed that a channel code was applied before transfer to the payload. In contrast, in the method according to the invention, all the d replicas of a data unit can be considered together for the decoding. This approach corresponds to a large class of codes z. B. LDPC or turbo codes, so that the expected performance of the code according to the invention can be derived accordingly.

In 4 is the average throughput (recoverable units of data per slot) as a function of the average load. It can be seen that the method according to the invention offers improved performance. For the parameters described, a maximum value for the efficiency of 0.96 data units / slot can be achieved, while known from the prior art methods such. B. CRA can not reach more than 0.33 data units / slot. E-CRA reaches a limit of 0.4 data units / slot.

The application of the method of the invention allows operation of the system under higher channel loads, so that more users can be assigned to the common transmission channel, while the throughput curve continues to increase linearly.

The presented invention can be used in terrestrial applications such. W-LAN, Wireless Sensor Networks, satellite systems, or space exchange procedures. In principle, an application is conceivable in every scenario in which several users access a common transmission channel via random access methods. The method according to the invention is of particular interest for applications in which few or small data packets are to be transmitted by a large number of users and a reduction of the overhead and a low complexity with respect to the time synchronization is required.

Claims (6)

Method for transmitting data, wherein a transmission channel to a receiver is shared by several users, where the timeline is divided into frames, wherein a non-slot-based random access scheme is used as access method to the common transmission channel, wherein a user sends his data using the following method steps: Generating a total codeword from the payload data to be transmitted using a code, Generating a plurality of coded packets from the overall codeword by a puncturing method, wherein the individual coded packets differ from one another, wherein the receiver is provided with the strategy by which the coded packets are generated from the overall codeword, and furthermore for the coding the packages used code are known Transmitting the coded packets by a user at different times within a frame over the transmission channel, each coded packet containing information about the position of the other coded packets of the same user within the current frame, wherein the receiver processes the received data according to the following method steps: Attempting to decode the received coded packets by the receiver using the code known to him, Upon the successful decoding of at least one encoded packet: reading out information about the position of the other d-1 encoded packets of the same user within the frame from the successfully decoded packet, Reconstructing the symbol sequence in each of the other d-1 encoded packets of the same user within the frame using the recovered payload of the decoded packet, the strategy by which the encoded packets are generated from the overall codeword, and further that used for encoding the packets codes Remove the interference caused by the d coded packets in packets of other users using a Successive Interference Cancellation method. A method according to claim 1 characterized by the following method steps, if the user data of a user could not be recovered due to not yet removed interference with other users or due to the fact that the packets successfully decoded for this user do not contain all the user data: Finding one of these coded packets, in which the information about the position over the other coded packets of this user is decodable in the current frame, Performing a joint decoding step using the data of all encoded packets of this user in the current frame, taking into account the strategy of generating the encoded packets from the overall code word and the known encoding procedures of the codes used, so that, upon successful joint decoding, the user data of the available to the user concerned, Performing the following process steps, if the joint decoding step was successful, Reconstructing the symbol sequence in all d coded packets of the relevant user in the current frame from the decoded user data using the strategy by means of which the coded packets are generated from the overall codeword and also the code used for coding the packets, Remove the interference caused by the d coded packets in packets of other users using a Successive Interference Cancellation method. A method according to claim 1 or 2, characterized in that a rate-less code, an LDPC code, a turbo code or a convolution code is used to generate the total code word. Method according to one of claims 1 to 3, characterized in that the header of the coded packets is coded separately and with a lower code rate. Method according to one of claims 1 to 4, characterized in that each of the d coded packets contains the entire payload. Method according to one of Claims 1 to 4, characterized in that at least one of the d-coded packets contains the entire user data and the remaining coded packets do not contain all or no user data.

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