Method for transmitting a message from a sending unit to a receiving unit

Abstract

The invention relates to a method (100) for transmitting a message (30) comprising k information bits from a transmitting unit (10) to a receiving unit (20), wherein the transmission takes place via a random-access channel and the method comprises the following steps: - Dividing (110) the message into a first message part (32) comprising k1 information bits and a second message part (34) comprising k2 information bits; - Transmission (120) of the first part of the message (32) from the transmitting unit (10) to the receiving unit (20); - Determining (130) transmission parameters for the transmission of the second message part (34), wherein determining the transmission parameters includes reading transmission parameters from a database, and wherein the first message part (32) contains information for reading the transmission parameters from the database; and - Transmission (140) of the second message part (34) from the transmitting unit (10) to the receiving unit (20) using the determined transmission parameters.

Description

[0001] The present invention relates to a method for transmitting a message comprising k information bits from a transmitting unit to a receiving unit, particularly in a satellite communications system, via a random access channel (also referred to as a random access channel or RACH). The present invention also relates to a method for receiving a message from a transmitting unit via a random access channel. Likewise, the present invention relates to a transmitting unit and a receiving unit designed to carry out the aforementioned methods.

[0002] DE 10 2017 209 025 B3 describes a method for transmitting data using a coded slotted ALOHA method. This method uses a single parity check code for encoding. DE 10 2016 214 067 A1 describes a method for transmitting data between a receiver and multiple transmitters. This method provides a transmission channel that connects a receiver to multiple transmitters. DE 10 2015 217 954 A1 describes another method for transmitting data between multiple transmitters and a receiver. In this method, the data is preferably transmitted using a CRDSA method.

[0003] Methods for transmitting messages play a particularly important role in various fields of communications engineering. In particular, small data networks are gaining increasing importance as a new approach for next-generation wireless communications. This paradigm, characterized by the exchange of short information packets, finds its natural application in massive machine-type communications (mMTC) and the Internet of Things (IoT), and has been adopted by commercial solutions in terrestrial and satellite-based communications. In this context, data is generated by a large number of end devices that transmit sporadically and unpredictably, rendering conventional orthogonal channel access methods inefficient.Accordingly, access-free schemes based on variations and enhancements of the basic ALOHA [1] or slotted ALOHA [1] random access (RA) protocols can be used as a means to improve efficiency. Uncoordinated access eliminates the need for a central node to dictate how resources should be distributed among active users. This has a twofold advantage: first, the overhead caused by the resource allocation information sent from the central node to the sender is completely avoided. Second, resources that may be underutilized due to the sporadic nature of the data can be efficiently utilized by other nodes that effectively have data to send.

[0004] In most modern random access solutions, users proactively transmit multiple copies of the same physical layer packet with information about where the packet copies are sent within the data unit or depending on the data, e.g., by using the data as a pseudo-random seed that is fed into a known pseudo-random algorithm that determines where the packet copies are sent.

[0005] Random access techniques are very useful when access to a wireless medium cannot be controlled by user scheduling or by traditional DAMA (Demand Assigned Multiple Access) techniques. This applies, for example, in the following cases: - there are many users, but their activity factor is low (i.e. the percentage of time during which they need to communicate and then access the wireless medium is very low); in this case, even if the statistically aggregated traffic generated by all users can be absorbed by the channel, it may not be possible to set a schedule in advance and schedule all users regularly, as this could imply an excessively long schedule; - the nodes must be simple (e.g. sensor networks) and cannot afford the complexity of a DAMA protocol, they may only need to send a message if they have one; - the propagation time between the nodes and the gateway is very high (e.g., the round-trip time for geostationary satellites is approximately 500 ms). In this case, using a DAMA scheme, the node would have to wait too long before sending its message; - the number of nodes active at any given time varies, and the total population is very large. A shared schedule may require a large overhead (proportional to the total user population, not to the small group of active users) and still be highly suboptimal, since many of the users may have nothing to transmit during their allocated resource.

[0006] In these and other cases, random access techniques can be used particularly effectively.

[0007] The problem with random access schemes is that user transmissions are, by definition, uncorrelated, resulting in low channel efficiency. In recent years, several efforts have been proposed to improve efficiency. These efforts essentially rely on the possibility of transmitters transmitting repetitions of the same burst, possibly with intelligent coding across the repetitions, and on using successive interference cancellation (SIC) at the receiver to resolve burst collisions. Thanks to these ideas, the following improvements in maximum throughput have been achieved, starting from a maximum throughput of 0.36 pk / slot (packets per time slot) for classic slotted ALOHA [1] and diversity slotted ALOHA (diversity slotted ALOHA, see [2]): - Contention Resolution Diversity Slotted ALOHA (CRDSA) with 2 replicas or copies [3]: 0.55 pk / slot (packets per time slot), - CRDSA with more than 2 replicas (CRDSA++) [4]: ​​0.7 pk / slot, - Irregular Repetition Slotted ALOHA (IRSA) [5]: 0.9 pk / slot, - Coded Slotted ALOHA [6]: arbitrarily close to 1 pk / slot.

[0008] All these works are based on the following assumptions: - Collisions are always detected at the receiver and result in a complete loss of information about the colliding segments sent via the random access frame; - The signal-to-noise ratio (SNR) is high enough to guarantee perfect decoding, i.e. if a burst is received in a clean timeslot, it will always be successfully decoded; - the channel estimation is ideal (this hypothesis, together with the previous one, guarantees an ideal SIC).

[0009] Under these assumptions, no system can exceed an efficiency of 1 as long as there is only one receiving node [7].

[0010] The channel model and corresponding decoding condition (i.e., the model according to which a packet is considered correctly decoded) assumed in modern slotted random access (also known as the collision channel) is inadequate when considering asynchronous protocols and may be inaccurate in some situations even for slot-synchronous systems. For example, when packets are received with different power levels caused by different propagation paths or different fading, the collision channel is not accurate even with slot synchronization.

[0011] Under these circumstances, the decoding condition must take physical layer aspects into account more precisely. When this is the case, advanced techniques such as combining become particularly interesting. For example, a packet can be considered correctly decoded if it exceeds a certain signal-to-interference-plus-noise ratio that closely approximates the performance of a practical modulation and coding scheme. This condition can be easily extended to incorporate advanced combining techniques prior to decoding by considering combined observation of identical packet copies transmitted by the same users, thereby significantly improving performance.

[0012] Current solutions enabling combination techniques are mainly based on physical layer techniques. For example, in asynchronous or slot-synchronous modern random access schemes (ECRA [8] and IRSA [5], respectively), after the correct reception of a packet copy, the candidate positions of possible copies are evaluated using correlation techniques. In particular, after a message is detected, its twin(s) are extracted from the received signal by correlating the entire message with all candidate positions of possible messages. Disadvantages of the state of the art

[0013] The main drawback of the state-of-the-art lies in the process required to detect and combine the transmissions of multiple copies belonging to the same users. Detection is fundamental to enable the use of any combination method, as well as the maximum ratio combination method. In the latter case, the symbols belonging to the different copies must be weighted according to the signal-to-interference-to-noise ratio (also known as signal-to-interreference-plus-noise ratio, or SINR), so the receiver must be aware of all transmissions that interfere with the transmission under investigation. The second aspect relates to the grouping of copies belonging to the same user, which is important for effectively creating the combined observation by selecting symbols belonging to the different copies of the same user to be sent as input to the decoder.

[0014] The use of physical-layer techniques for detecting and grouping users, e.g., based on correlations, is prone to errors due to the presence of noise and interference. Missing packets leads to an inaccurate estimate of the total interference across the replicas and thus to an overweighting of symbols, which in turn distorts the expected reliability of symbols where interference is not detected. Failure to cluster copies belonging to the same user correctly has even more drastic consequences. Random assignment to copies belonging to different users is very likely to result in decoding errors. Solution to the task

[0015] To achieve the above-mentioned object, the present invention proposes a method for transmitting a message comprising k information bits from a transmitting unit to a receiving unit, wherein the transmission takes place via a random access channel and the method comprises the following steps: - dividing the message into a first message part comprising k1 information bits and a second message part comprising k2 information bits; - Transmission of the first part of the message from the transmitting unit to the receiving unit; - Determining transmission parameters for the transmission of the second message part, wherein determining the transmission parameters comprises reading transmission parameters from a database, and wherein the first message part contains information for reading the transmission parameters from the database; and - Transmission of the second part of the message from the transmitting unit to the receiving unit using the determined transmission parameters.

[0016] The method according to the invention allows information for selecting the transmission parameters with which the second message part is transmitted to be encoded in the k1 information bits of the first message part. This allows the receiving unit to reconstruct the transmission parameters by reading the first message part, which facilitates the decoding of messages received from different transmitting units.

[0017] The first message part can be transmitted directly or in coded form. For example, the first message part can be transmitted using a compressed sensing method (also referred to as compressive sensing or compressed acquisition). The first message part contains information required to read the transmission parameters from the database. In particular, the position of the transmission parameters within the database can be coded in the first message part or in the k1 information bits of the first message part.

[0018] The sum of the information bits of the first message part and the second message part (k1 + k2) can preferably be k. Alternatively, a third message part can also be provided, in which case the sum of the information bits of the first message part and the second message part can deviate from k.

[0019] The database can contain multiple tuples or lists storing different combinations of transfer parameters.

[0020] Each tuple or list can preferably contain two or three numerical values.

[0021] According to some embodiments of the present invention, the database may comprise a total of M tuples or M lists, where M = 2 k 1.

[0022] Furthermore, the method according to the invention can provide that each tuple or each list - has a first numerical value b, which is represented by an integer value representing the number of copies of the second message part generated by the transmitting unit and transmitted to the receiving unit; and - a second numerical value c represented by an integer value describing a seed value used by a method for generating pseudo-random numbers to generate b time slots; and optionally - a third numerical value a, which is represented by a real number having a value in the range [0, 1} and describing a delay normalized to a packet duration, which is used by a transmitting unit to start a local virtual frame.

[0023] In particular, when using an asynchronous random access method, it can be provided that each tuple or list has a first numerical value b, a second numerical value c, and a third numerical value c. In contrast, when using a synchronous random access method, it can preferably be provided that each tuple or list has only a first numerical value b and a second numerical value c.

[0024] In the method according to the invention, it is provided that the determination of the transmission parameters additionally comprises the following step: - Mapping the k1 information bits to an integer value i; where the integer value i determines the position of the transmission parameters within the database.

[0025] This allows the position of the transmission parameters within the database to be encoded particularly efficiently in the k1 information bits of the first message part. The receiving unit can then reconstruct the transmission parameters after receiving the first message part, thereby ensuring improved reconstruction of the second message part.

[0026] In the method according to the invention, the transmission of the first message part from the transmitting unit to the receiving unit comprises the following steps: - Converting the integer value i into a vector x, where the vector x has exactly one value that is different from zero and that is located at the i-th position of the vector x; - Calculation and transmission of A⋅xuT, where A denotes the sensor matrix and xuT the transposed vector for each user u.

[0027] This allows the receiving unit, which also owns the sensor matrix, to calculate the vector xuT reconstruct and thereby determine the position of the transfer parameter within the database.

[0028] Furthermore, the method according to the invention can provide that the transmission of the second message part from the transmitting unit to the receiving unit comprises the following steps: - encoding the second message part comprising k2 information bits with a coding rate R, wherein n2 characters are generated by the encoding; - generating b copies of the encoded second message part; and - Transmitting the b copies within b time slots, the position of which is determined by performing a pseudo-random number generation process using the seed value c a total of b times.

[0029] The second message part can preferably be encoded using a forward error correction code (also referred to as a forward error correction code or FEC code). The receiving unit can then subsequently reconstruct the second message part using a combining method and the transmission parameters.

[0030] Furthermore, it can be provided that the transmission of the second message part from the transmitting unit to the receiving unit additionally comprises the following step: - Adding a delay a in the transmission of the b copies to determine the start of a local virtual time frame for a user u.

[0031] This additional method step can be used in particular when an asynchronous random access method is used for communication between the transmitting unit and the receiving unit.

[0032] In addition, to achieve the above-mentioned object, a method for receiving a message from a transmitting unit via a random access channel is proposed, wherein the method is implemented on a receiving unit and the method comprises the following steps: - receiving a first message part from a transmitting unit; - determining transmission parameters with which a second message part was transmitted by the transmitting unit, wherein determining the transmission parameters comprises reading transmission parameters from a database, and wherein the first message part contains information for reading the transmission parameters from the database; - receiving the second message part from the transmitting unit using a combining method and using the determined transmission parameters; and - Reconstructing the message based on the first message part and the second message part.

[0033] The reconstruction of the message can be done by simply concatenating the k1 information bits of the first message part and the k2 information bits of the second message part.

[0034] In the reception method according to the invention, it can preferably be provided that the combining method is based on a selection combining method (also referred to as a read-out combining method or in English as selection combining), a constructive combining method, an equal gain combining method or a maximum ratio combining method.

[0035] In the receiving method according to the invention, it can be provided that the database has a total of M tuples or M lists, where M = 2 k 1 and k1 denotes the number of information bits of the first message part.

[0036] It can preferably be provided that each tuple or each list - has a first numerical value b, which is represented by an integer value representing the number of copies of the second message part generated by the transmitting unit and transmitted to the receiving unit; and - a second numerical value c represented by an integer value describing a seed value used by a method for generating pseudo-random numbers to generate b time slots; and optionally - a third numerical value a, which is represented by a real number having a value in the range [0, 1} and describing a delay normalized to a packet duration, which is used by a transmitting unit to start a local virtual frame.

[0037] As already explained above in connection with the transmission method according to the invention, each tuple or list can have a first numerical value b and a second numerical value c if a synchronous random access method is used, while each tuple or list can additionally have a third numerical value a if an asynchronous random access method is used.

[0038] Furthermore, in the receiving method according to the invention, it can be provided that receiving the first message part from the transmitting unit comprises the following steps: - Estimating a vector xuT, where xuT denotes the transpose of a vector sent by the sending unit for a user u, which has only one value that is different from zero and this non-zero value is located at the i-th position of the vector xuT where i is an integer value; - Determine the integer value i; and - Mapping the integer value i to k1 information bits of the first message part.

[0039] The mapping can be done in a simple way by converting the integer value i into a binary number with k1 digits.

[0040] In addition, to solve the problem described above, a transmitting unit comprising a storage unit, a communication unit and a computing unit is proposed, wherein the transmitting unit is designed to: - dividing a message into a first message part comprising k1 information bits and a second message part comprising k2 information bits; - to transmit the first part of the message from the transmitting unit to the receiving unit; - to determine transmission parameters for the transmission of the second message part, and to read the transmission parameters from a database, wherein the first message part contains information for reading the transmission parameters from the database; and - to transmit the second part of the message from the transmitting unit to the receiving unit using the determined transmission parameters.

[0041] The transmitting unit according to the invention can preferably be provided to carry out the steps described above in connection with the transmission method.

[0042] Finally, the present invention proposes a receiving unit comprising a storage unit, a communication unit and a computing unit, wherein the receiving unit is designed to - to receive a first message part from a transmitting unit; - to determine transmission parameters with which a second message part was transmitted by the transmitting unit and to read the transmission parameters from a database, wherein the first message part contains information for reading the transmission parameters from the database; - to receive the second message part from the transmitting unit using a combining method and using the determined transmission parameters; and - to reconstruct the message based on the first message part and the second message part.

[0043] The receiving unit can preferably be provided to carry out the steps described above in connection with the receiving method. Preferred features of the invention

[0044] In the present invention, a compressed sensing (also referred to as compressive sensing) method can preferably be used to ensure the detection and grouping of users in random access methods, both slot-synchronous (also referred to as slot-synchronous) and asynchronous, and to enable the use of combination techniques. According to some embodiments of the concept, the following can be provided: - A message to be transmitted can be divided into two parts. The first part, which can preferably be relatively small (e.g., 8, 10, or 12 bits long), contains not only the information bits but also serves to select a tuple or list whose dimension and information content depend on the chosen random access method. Practical examples are shown in the following sections. - The first message part can then preferably be transmitted using compressive sensing. In particular, the first message part can be used to map to an integer value within an available range. This integer value can then represent a selected column of a predefined sensing matrix (also called a sensor matrix) transmitted over the communication channel. - In this case, all active users can perform the same operation, so multiple columns of the sensor matrix are selected (ideally one column for each active user). The receiving unit can observe a weighted sum of the columns of the sensor matrix selected by the users' transmitting units, along with noise, e.g., white Gaussian noise. - The second message part can then be encoded preferably with a forward error correction code at a selected coding rate and then copied b times. Depending on the selected random access method, the number of copies (or replicas) can be fixed and the same for all users, or it can be drawn from an optimized probability mass function (also known as a probability mass function) and thus vary from user to user. - It should be noted that the position and number of copies can be part of the information selected with the first message part and are therefore part of the corresponding tuple. - On the receiver side, the first task may then be to determine and estimate which columns of the sensor matrix were active. This can be done using common noisy compressive sensing decoding algorithms (also called noisy compressive sensing decoding methods), such as matching pursuit or approximate message passing. Once the receiver unit has determined which columns are active, it can determine two basic components of the transmission scheme: 1. The first part of each active message 2. The tuple content or list content, which provides additional meta-information about the second phase of the transmission. For example, the receiving unit knows how many users are active, when the transmissions took place, and how many replicas were sent by each user. All of this information can be used, for example, to enable efficient combining procedures. Further preferred details of the invention

[0045] The method proposed by the present invention can be applied in a system in which short packets with k information bits, generated by multiple competing nodes, are to be transmitted in an uncoordinated system based on random access methods. The steps on the transmitter side can be configured as follows: 1. The message to be transmitted can be divided into two parts, a first message part with k1 information bits and a second message part with k2 information bits, where preferably k = k1 + k2. 2. With the first k1 information bits, 2 k1 = M possible messages are encoded. Preferably, the information message can then be mapped to an integer value i in the set S = {1, ..., M}. Subsequently, a (sparse) vector x = [x1,x2, ... x i-1 ,xi ,x i+1 ,...,x M ], in which exactly one value is non-zero and which is, for example, binary. The position of the entry that is non-zero is specified by the integer value i, so that x = [0,0, ...,0,1,0, ..., 0]. 3. The integer value i can also be used to map tuples or lists of dimension M into a lookup table (LUT). The tuples in the lookup table are used to select side information useful for transmitting the second k2 bits of information in the message, and can be used on the receiver side to support decoding and enable combinational methods. Examples include: a. For asynchronous random access protocols, such as ECRA, the tuples or lists can consist of three elements: 1. a: a real number in the range [0, 1). Represents the delay, normalized to a packet duration, selected by a user or transmitting unit for the start of a local virtual frame (assigned to the user or transmitting unit, respectively). A virtual frame is a set of local time slots whose start and end are local to the user or transmitting unit. 2. b: an integer value that specifies the number of packet copies (= replicas) transmitted by a user or a sending unit. 3. c: an integer value representing a seed used by a pseudorandom algorithm known to all transmitting units and all intended receiving units to generate time slots in the range [1, max_slots]. max_slots is a known integer value that can be specified in advance once the duration of the second phase of the scheme, as well as the payload size and coding rate of the replicas sent in the second phase, are selected. b. For slot-synchronous random access protocols, such as IRSA, the tuple can consist of two elements: 1. b: an integer value that specifies the number of packet copies (= replicas) transmitted by a user or a sending unit. 2. c: an integer value representing a seed used by a pseudorandom algorithm known to all transmitting units and all intended receiving units to generate time slots in the range [1, max_slots]. max_slots is a known integer value that can be specified in advance once the duration of the second phase of the scheme, as well as the payload size and coding rate of the replicas sent in the second phase, are selected. 4. The matrix A of dimensions NxM, where N « M, can be created. The matrix A is also referred to as the sensing matrix or sensor matrix. This matrix can preferably be known to all transmitting units as well as to the receiving unit(s). Typically, A is constructed as a random matrix, with entries drawn from a (standard) Gaussian random variable. Other types of constructions are also possible and do not affect the present invention. 5. Each user or each transmitting unit can then A⋅xuT=[0⋮010⋮0], with xuT as created in step 2, to the receiving unit. If xuT is binary, any user u can change column i u of the sensor matrix A for transmission. 6. The second message part, which consists of k2 information bits, can first be encoded with a coding rate R [bits / symbol], generating n2 symbols. Depending on the chosen random access method, the message can then a. b times, where b was previously selected in step 3. b. in the b slots, whose position can be determined by executing the pseudorandom algorithm, are transmitted b times with the seed c, which was also previously selected in step 3. c. if the method is an asynchronous method, a delay of a may be added to the transmission to take into account the start of the user's local virtual frame.

[0046] The following steps can then be carried out on the recipient side: 1. The receiving unit can be synchronized with the two phases alternating, ie the compressing sensing phase and the random-access phase. 2. After the compression acquisition phase has ended, the receiving unit can observe the following: y=A∑u=1UxuT+n

[0047] Where A is the known sensor matrix, U is the total number of users transmitting to each other in the current competition, and n is the sampled additive white Gaussian noise of the channel. The vector xuT was described above in step 2 of the transmitter operations.

[0048] For simplicity, we can assume that x is binary. The task of the receiving unit is to deduce, by observing y, which columns of the sensor matrix A have been transmitted. This problem is called noisy compressed sensing. Various standard methods can be used to solve the problem, such as l1 minimization, iterative methods such as Orthogonal Matching Pursuit (OMP) and its further developments, and so-called hard-thresholding methods.

[0049] To solve this problem, the size of y and thus also A must be chosen appropriately. One approach is to determine the number of required measurements appropriately (ie, the size y or equivalently the number of rows in A), provided that the maximum sparsity level, which is ∑u=1UxuT carried (which in the binary case is simply U - the number of competing users).

[0050] So, if U is known, y can be dimensioned such that the desired reconstruction probability is achieved, so that, for example, the probability of a successful reconstruction of xuT 0.99, where N is the number of measurements. 3. Furthermore, it is assumed that the receiving unit xuT could be perfectly reconstructed. Now a fundamental quantity can be determined, which represents two pieces of information: a. If xuT is known, the receiving unit knows the index i u of the non-zero element in xuT. This can first be mapped back to the first k1 bits of user u by simply converting the integer value into a k1 bit string. b. In addition, the receiving unit can derive, for example, a tuple [a,b,c] (for ECRA) or [b,c] (for IRSA) selected by the user using the known look-up table.

[0051] It is important to note that in this case, if the compressed sensing phase is successful, the receiving unit can derive both the first bits k1 and the tuple as well as the tuple [a,b,c] (for ECRA) or [b,c] (for IRSA) for all users. 4. After step 3 has been successfully completed, it can be assumed that all U users have been identified, their k1 bits have been retrieved, and each tuple (or list) [a, b, c] (for ECRA) or [b, c] (for IRSA) has been retrieved. The receiving unit can now proceed as follows: a. Estimation of the time, frequency, and phase deviations perceived by each transmitted copy. Before proceeding to the next step, such impairments can be compensated for in the received signals. b. Then, the observations from the individual copies belonging to the same users can be combined. For example, selection combining, equal gain combining, and maximum ratio combining can be used to construct the observations. Specifically, the following steps can be performed: i. If the selection combination is chosen, the receiving unit can for each i-th symbol siu,d of the user u and for copy d the corresponding ratio of signal to interference and noise γiu,d calculate by relying on the knowledge of each transmission in the random access phase (all tuples are known), by selecting the maximum for each symbol of each copy, ie: d⌣l=arg maxdγiu,d

[0052] Then the selection-combining observation for user u is formed by for each symbol i in the packet, in particular r⌣iu=riu,d⌣l=hiu,d⌣lsiu,d⌣l+niu,d⌣l for each i, where, hiu,d⌣l the perceived channel amplitude over the symbol i of the user u for copy On the other hand, n i the noise and the aggregate interference of symbol i of user u for copy Assuming that this can be well approximated by a Gaussian random variable, the following can be assumed: niu,d⌣l∼CN(0,σ2+σi,u,d⌣l2¯), where σ 2 the noise variance and σi,u,d⌣l2¯ for the total interference level over the symbol i of the user u for copy Without loss of generality, it can also be assumed that E{|siu,d⌣l|2}=1 and the following can be determined: γiu,d⌣l=|hiu,d⌣l|2σ2+σi,u,d⌣l2¯

[0053] In this way, after the selection combination, the observation ř of the entire package is created. ii. If equal-gain combining is applied instead, a coherent, equal-weighted summation of the individual replica symbols is performed. Coherent summation can be performed because the receiver has previously estimated and compensated for the time, frequency, and phase deviation for all copies, including those of user u (see step 4a above).

[0054] In particular, riu˜=∑k=1driu,k=∑k=1dhiu,ksiu,k+niu,k for each i.

[0055] In this way, after the equal-gain combination, the observation r̃ of the entire packet can be created. iii. If maximum ratio combination is applied instead, a coherent weighted sum of the individual replica symbols can be formed. The coherent sum can be performed because the receiver unit has previously estimated and compensated for the time, frequency, and phase offset for all copies, including those of user u (see step 4a above). In addition, different weights can be assigned to the i-th symbol of each copy, depending on the estimated noise and interference level.

[0056] The noise can be estimated using data-driven (preamble- and pilot-based estimation) or non-data-driven methods. Thanks to the knowledge of all tuples belonging to all users and an estimate of the received power for each replica, an aggregated interference estimate is available instead.

[0057] In particular, rlu¯=∑k=1d(αk)∗riu,k=∑k=1d(αk)∗hiu,ksiu,k+(αk)∗niu,k for each i, where αk=hiu,kσ2+σi,u,k2¯

[0058] According to the maximum ratio combination and assuming that E{|siu,d⌣l|2}=1 the following can be defined: γiu¯=∑k=1dγiu,k=∑k=1d|hiu,k|2σ2+σi,u,k2¯

[0059] In this way, after the maximum ratio combination, the observation r̅ of the entire package is created. 5. Now, the combined observation of the k2 information bits of the second message part can be created for user u. Depending on the combination method (selection combination, equal gain combination, or maximum ratio combination), the observation ř, r̃, or r̅ is available. The observation now serves as input for the channel decoder, which, if successful, can reliably retrieve the k2 information bits. 6. The complete message information bits of user u can now be reassembled by selecting the k1 information bits that were successfully retrieved after step 3 and the k2 information bits that were successfully decoded after step 5. 7. If the k2 information bits of user u have been successfully decoded, the signal contribution of all its replicas is removed from the received signal using successive interference cancellation (SIC). In addition, the aggregated interference level across the positions where the replicas were sent is updated accordingly, i.e., reduced by the power of the received replicas.

[0060] Steps 4 and 5 can be repeated for all users identified after step 3, and each time the decoding of the second k2 bits is successful, steps 6 and 7 can be performed. If decoding is unsuccessful after step 5, the decoder can proceed to subsequent users, continuing with the previously undecoded users. Once all users have been successfully decoded in the first round of SIC, the receiving unit can proceed to reprocess the users not decoded in the previous round. The iterations are repeated until no user remains or no further user can be successfully decoded.

[0061] The present invention will be explained in more detail below with reference to the figures. Fig. 1 an embodiment of the transmission method according to the invention, Fig. 2 an embodiment of the receiving method according to the invention, Fig. 3 an embodiment of the transmitting unit according to the invention and the receiving unit according to the invention, Fig. 4 shows an embodiment of a message transmitted according to the invention comprising a first message part and a second message part, Fig. 5 the calculated probability of a successful message reconstruction using the method according to the invention, Fig. 6 the calculated packet loss rate using the method according to the invention, and Fig. 7 shows the calculated spectral efficiency for an asynchronous random access scheme.

[0062] In the Fig. 1 shows an exemplary embodiment of the method 100 according to the invention for transmitting a message from a transmitting unit to a receiving unit. In the method according to the invention, in a first method step 110, the message to be transmitted is divided into a first message part and a second message part. The first message part has k1 information bits, while the second message part has k2 information bits. In this case, it can be provided that k1 is selected to be relatively small compared to k2. For example, it can be provided that the first message part has a total of 8 bits, 10 bits, or 12 bits. In a second method step 120, the first message part is transmitted from the transmitting unit to the receiving unit. In addition to the information of the message to be transmitted, the first message part also contains information about how the second message part is to be transmitted.The bits of the first message part thus encode information about the transmission parameters used for transmitting the second message part. In a third method step 130, the transmission parameters for transmitting the second message part are determined, with the transmission parameters being read from a database. The k1 information bits of the first message part contain information about where the transmission parameters are stored within the database.

[0063] For example, the k1 information bits can encode an integer value that represents the position or index of the transmission parameters within the database. In a fourth method step 140, the second message part is transmitted from the transmitting unit to the receiving unit, using the previously determined transmission parameters for this purpose.

[0064] In the Fig. 2 shows an embodiment of the method 200 according to the invention for receiving a message from a transmitting unit by a receiving unit. In the method 200 according to the invention, a first message part is received from a transmitting unit in a first method step 210. The first message part contains, on the one hand, part of the information of the message to be transmitted. On the other hand, the first message part encodes information about the transmission parameters with which a second message part is transmitted. In a second method step 220, the transmission parameters are determined by reading them from a database, wherein the first message part contains information for reading the transmission parameters from the database. In particular, the first message part can encode a position or an index within the database at which the transmission parameters are stored.In a third method step 230, the second message part is received and decoded by the transmitting unit. The determined transmission parameters serve as secondary information, enabling simplified decoding. In a fourth method step 240, the transmitted message is reconstructed based on the first message part and the second message part. For example, the message can be created by concatenating the k1 information bits of the first message part and the k2 information bits of the second message part.

[0065] In the Fig. Figure 3 shows an exemplary embodiment of the transmitting unit 10 and the receiving unit 20 according to the invention. The transmitting unit 10 has a memory unit 12, a communication unit 14, and a computing unit 16. The transmitting unit 10 is designed to carry out the method steps described above in connection with the transmission method according to the invention. The receiving unit 20 also has a memory unit 22, a communication unit 24, and a computing unit 26. The receiving unit 20 is designed to carry out the method steps described above in connection with the receiving method.

[0066] In the Fig. 4 shows an embodiment of a message 30 transmitted according to the present invention. The message 30 has a total of k information bits. The message 30 has a first message part 32 and a second message part 34. The first message part 32 has k1 information bits, while the second message part 34 has k2 information bits. In particular, k = k1 + k2 (provided the message does not have a further message part). The first message part 32 can preferably have fewer bits than the second message part 34. For example, the first message part can have 8 bits, 10 bits, or 12 bits. The first message part 32 contains information about the message 30 to be transmitted. At the same time, the k1 information bits of the first message part 32 contain information that enables the transmission parameters to be read from a database.In particular, the k1 information bits can encode a position or index within the database at which the transmission parameters for transmitting the second message part 34 are stored. For example, the k1 information bits can encode an integer value indicating a position or index within a database at which the transmission parameters are stored.

[0067] In the Fig. Figure 5 shows the calculated probability of a successful message reconstruction using an embodiment of the method according to the invention. First, the potential performance of the first phase of the disclosure, i.e., the compressive-sensing phase, is considered. When applying a compressive-sensing detection or decoding method, such as the Orthogonal Matching Pursuit (OMP) method, the Fig. 5 shown performance can be achieved. Fig. 5 shows the probability of a successful recovery of the noise-free compressive-sensing phase. In the Fig. 5 shows the performance of OMP for a situation where k1 = 10, so that 2 k1 = M = 1024 and N = 512 (without noise). Up to 25 users can be reliably detected with an expected probability of successful recovery of > 0.99. If a more moderate recovery probability of > 0.9 is accepted, even up to 45 users can be reliably detected in this phase.

[0068] It should be noted that a noise-free scenario was assumed here. Performance will degrade if noise is also taken into account. The effect is a reduction in the sparsity level that can be supported for a desired successful recovery probability, taking into account the number of messages and the number of measurements (number of occupied channel usages).

[0069] To ensure the required probability of success for an increasing number of concurrent users sharing the channel, the system must be robust both in terms of the number of messages allowed 2 k1= M and the number of measurements N. In other words, for the same probability of successful detection, both M and N must be increased accordingly if a larger number of users are to be supported.

[0070] Another way to improve performance is to compare it with newer and more powerful methods that go beyond OMP. Generalized OMP (G-OMP), Subspace Pursuit (SP), and many others have been proposed recently. All of these methods may be better suited to handling the required number of users. It is important to note that the present invention is independent of the detection algorithm chosen for the compressed sensing phase, and any of the above-mentioned solutions can be used within the scope of the present invention.

[0071] Next, the performance of the second phase, the random access phase, will be investigated. It is assumed that an asynchronous random access algorithm is used. In particular, ALOHA (ECRA) with extended contention resolution is used. By assuming perfect detection in the first phase, it can be shown that the Fig. 6 and Fig. 7 can be achieved for the packet loss rate and for the spectral efficiency.

[0072] The results assume perfect detection for the first phase, k2 = 100 information bits, a coding rate (including modulation) of R = 1.5 bits per symbol, and an SNR = 6 dB at the receiver input for all transmitted messages. Furthermore, it is assumed that the received power is the same for all users and that each transmission is repeated twice, i.e., the number of copies per message is 2. Furthermore, it is assumed that a packet is successfully decoded if the average mutual information transmitted over each symbol exceeds the chosen rate. A maximum-ratio combination is applied with perfect knowledge of the weights assumed on a symbol-by-symbol basis, and it is also assumed that interference cancellation is ideal.

[0073] As from the Fig. As can be seen in Figure 6, with the selected parameters a packet loss rate of less than 10 -3can be achieved with up to approximately 45 users, while for 55 users this can be achieved with 10 -2 or for 62 users 10 -1 The spectral efficiency can reach a maximum of 1.7 b / s / Hz while supporting approximately 60 concurrent users.

[0074] To account for the packet error rate and spectral efficiency of the overall system, the results from Phase 1 and Phase 2 would have to be combined and a complete simulation would be performed. However, the presented results clearly demonstrate that by properly tuning the two phases, the system can achieve remarkable performance, far exceeding that of systems used in practice. BIBLIOGRAPHY [1] N. Abramson, “The ALOHA system - another alternative for computer communications,” in Proc. of 1970 Fall Joint Computer Conf., vol. 37, pp. 281-285, AFIPS Press, 1970 [2] G. Choudhury und S. Rappaport, „Diversity ALOHA - a random access scheme for satellite communications“, IEEE Trans. Commun, vol. 31, pp. 450-457, Mar. 1983 [3] E. Casini, R. De Gaudenzi, and O. del Rio Herrero, „Contention Resolution Diversity Slotted ALOHA (CRDSA): an enhanced random access scheme for satellite access packet networks“, IEEE Trans. Commun., vol. 6, num. 4, pp 1408-1419, April 2007 [4] De Gaudenzi, R.; del Rio Herrero, O.; „Advances in Random Access protocols for satellite networks“, Satellite and Space Communications, 2009. IWSSC 2009. Internationaler Workshop über, S.331-336, 9-11 September 2009 [5] Liva, G.; „Graph-Based Analysis and Optimization of Contention Resolution Diversity Slotted ALOHA,“ Communications, IEEE Transactions on Communications, vol. 59, no.2, pp.477-487, February 2011 [6] E. Paolini, G. Liva, and M. Chiani, „High Throughput Random Access via Codes on Graphs: Coded Slotted ALOHA,“, IEEE ICC 2011 [7] Paolini, E.; Liva, G.; Chiani, M.; “Graph-Based Random Access for the Collision Channel without Feedback: Capacity Bound,” Global Telecommunications Conference (GLOBECOM 2011), 2011 IEEE, pp.1-5, December 5-9, 2011 [8] F. Clazzer, C. Kissling and M. Marchese; “Enhancing Contention Resolution ALOHA Using Combining Techniques,” IEEE Transactions on Communications, vol. 66, number 6, pp 2576-2587, June 2018 LIST OF REFERENCE SYMBOLS 10 transmitter unit 12 Storage unit of the transmitting unit 14 Communication unit of the transmitting unit 16 Computing unit of the transmitting unit 20 receiving unit 22 Storage unit of the receiving unit 24 Communication unit of the receiving unit 26 Computing unit of the receiving unit 30 Message 32 first news part 34 second news part 100 transfer procedures 110 first step of the transfer procedure 120 second step of the transfer procedure 130 third step of the transfer procedure 140 fourth step of the transfer procedure 200 reception procedures 210 first step of the reception procedure 220 second step of the reception procedure 230 third procedural step of the reception procedure 240 fourth procedural step of the reception procedure

Claims

[1] Method (100) for transmitting a message (30) comprising k information bits from a transmitting unit (10) to a receiving unit (20), in particular in a satellite communication system, wherein the transmission takes place via a random access channel and the method comprises the following steps: - dividing (110) the message (30) into a first message part (32) comprising k1 information bits and a second message part (34) comprising k2 information bits; - transmission (120) of the first message part from the transmitting unit (10) to the receiving unit (20); - determining (130) transmission parameters for the transmission of the second message part (34), wherein determining the transmission parameters comprises reading transmission parameters from a database, and wherein the first message part (32) contains information for reading the transmission parameters from the database; and - transmission (140) of the second message part (34) from the transmitting unit (10) to the receiving unit (20) using the determined transmission parameters, wherein the determination of the transmission parameters additionally comprises the following step: - mapping the k1 information bits to an integer value i; wherein the integer value i determines the position of the transmission parameters within the database; and wherein the transmission of the first message part (32) from the transmitting unit (10) to the receiving unit (20) comprises the following steps: - Converting the integer value i into a sparse vector x, where the vector x has exactly one value that is different from zero and that is located at the i-th position of the vector x; - Calculation and transmission of A⋅xuT where A denotes the sensor matrix and xuT the transposed sparse vector for each user u. [2] Method (100) according to claim 1, characterized by that the database has a total of M tuples or M lists, where M = 2 k1 amounts. [3] Method (100) according to claim 2, characterized by that each tuple or list - has a first numerical value b, which is represented by an integer value representing the number of copies of the second message part (34) generated by the transmitting unit (10) and transmitted to the receiving unit (20); and - a second numerical value c represented by an integer value describing a seed value used by a method for generating pseudo-random numbers to generate b time slots; and optionally - a third numerical value a, which is represented by a real number having a value in the range [0, 1} and describing a delay normalized to a packet duration, which is used by a transmitting unit (10) to start a local virtual frame. [4] Method (100) according to one of claims 1 to 3, characterized by that the transmission of the second message part (34) from the transmitting unit (10) to the receiving unit (20) comprises the following steps: - encoding the second message part (34) comprising k2 information bits at a coding rate R, wherein n2 characters are generated by the encoding; - generating b copies of the coded second message part (34); and - Transmitting the b copies within b time slots, the position of which is determined by performing a pseudo-random number generation process using the seed value c a total of b times. [5] Method (100) according to claim 4, characterized by that the transmission of the second message part (34) from the transmitting unit (10) to the receiving unit (20) additionally comprises the following step: - Adding a delay a in the transmission of the b copies to determine the start of a local virtual time frame for a user u. [6] Transmitting unit (10), in particular for a satellite communication system, comprising a storage unit (12), a communication unit (14) and a computing unit (16), wherein the transmitting unit (10) is designed to: - dividing a message (30) into a first message part (32) comprising k1 information bits and a second message part (34) comprising k2 information bits; - to transmit the first message part (32) from the transmitting unit (10) to the receiving unit (20); - to determine transmission parameters for the transmission of the second message part (34) and to read the transmission parameters from a database, wherein the first message part (32) has information for reading the transmission parameters from the database; - to transmit the second message part (34) from the transmitting unit (10) to the receiving unit (20) using the determined transmission parameters, - map k1 information bits to an integer value i; where the integer value i determines the position of the transmission parameters within the database; - convert the integer value i into a sparse vector x, where the vector x has exactly one value that is different from zero and that is located at the i-th position of the vector x; and - A⋅xut A to calculate and transmit, where A denotes the sensor matrix and xuT the transposed sparse vector for each user u.

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