Transmitting or receiving a signal

Abstract

Methods (100, 200), a transmitter (300) and a receiver (400) are provided. In an example aspect, a method (100), performed by a transmitter, of transmitting a signal, is provided. The method (100) comprises forming a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmitting the signal.

Description

[0001] TRANSMITTING OR RECEIVING A SIGNAL

[0002] Technical Field

[0003] Examples of the present disclosure relate to a method, performed by a transmitter, of transmitting a signal, a transmitter for transmitting a signal, a method, performed by a receiver, of receiving a signal, and to a receiver for receiving a signal.

[0004] Background

[0005] Previous work [1 , 2, 3, 4] has investigated how much information can be carried by a single photon. Motivations for this work include the prospect of saving energy in scenarios where the available power may be limited, such as in satellite applications. In [5], it has been shown that a single photon can be used to convey more than 10 bits of information, by associating each single photon with one of the pixels comprised within a grid of detectors (that is, by associating each photon with one of 9072 spatial modes).

[0006] Although this technique works well over short distances, it is not particularly suitable for communication scenarios over longer distances. In pulse position modulation

[0007] (PPM), as described in [6], more than one bit can be transmitted per pulse over a single spatial mode. This technique allows for message transmission that uses much lower total energy, through utilisation of the fact that the “no pulse” state carries no energy. However, this technique does not allow for error correction in the received messages (for example, error correction for lost pulses and / or for new pulses appearing in a block in a received message).

[0008] Summary

[0009] An object of the present invention is to provide a reliable communication scheme for photon-starved scenarios.

[0010] A first aspect of the present disclosure provides a method, performed by a transmitter, of transmitting a signal. The method comprises forming a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmitting the signal.

[0011] Another aspect of the present disclosure provides a method, performed by a receiver, of receiving a signal. The method comprises receiving a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence

[0012] A further aspect of the present disclosure provides a transmitter for transmitting a signal. The transmitter comprises a processor and a memory, the memory containing instructions executable by the processor such that the transmitter is operable to form a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmit the signal.

[0013] A still further aspect of the present disclosure provides a transmitter for transmitting a signal. The transmitter is configured to form a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmit the signal. A still further aspect of the present disclosure provides a receiver for receiving a signal. The receiver comprises a processor and a memory, the memory containing instructions executable by the processor such that the receiver is operable to receive a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

[0014] A still further aspect of the present disclosure provides a receiver, for receiving a signal. The receiver is configured to receive a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

[0015] Brief Description of the Drawings

[0016] For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

[0017] Figure 1 is a flow chart of a method 100, performed by a transmitter, of transmitting a signal;

[0018] Figure 2 is a flow chart of a method 200, performed by a receiver, of receiving a signal;

[0019] Figure 3 is a schematic of an example of a transmitter 300 for transmitting a signal; and

[0020] Figure 4 is a schematic of an example of a receiver 400 for receiving a signal. Detailed

[0021] The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and / or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and / or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

[0022] Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and / or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

[0023] One object of the present invention is to address the lack of error-correction in PPM, and to devise a code that can correct both for lost pulses and for new pulses appearing in a block comprised within a signal, and that is suitable for applications that require low energy use and / or for photon-starved scenarios. Examples of this disclosure are provided using light pulses and photons, though this is merely an example and other signal or energy carrying types may be used instead, such as for example electromagnetic (e.g. radio) or electrical signals.

[0024] Certain embodiments of the present disclosure relate to a communication scheme with error correction for photon-starved scenarios. Certain embodiments of the present disclosure relate to communication schemes inspired by PPM [6] and permutation coding [7] that also exploit a polarisation degree of freedom, allowing for the correction of two common types of errors in a signal, the loss of pulses, and the introduction of unwanted pulses (for example, from reflections). In this disclosure, where a signal is transmitted using photons, a “pulse” may be a single photon or multiple photons.

[0025] Certain embodiments of the present disclosure relate to methods of forming a signal in which a single photon or a pulse in the signal is associated with a variable integer number of time bins. It is noted that signals formed according to these methods may require more time to be transmitted compared to signals formed according to traditional block coded communication protocols.

[0026] Certain embodiments of the present disclosure enable information to be transmitted more reliably compared to PPM techniques [6] (as a result of the error correction adopted for the asymmetric channels described herein), whilst still enabling communication in photon-starved scenarios, or applications where low energy use is important (such as satellite or other free-space telecommunication applications).

[0027] A method of forming and transmitting a signal, that enables errors to be identified and / or corrected in received signal corresponding to the formed signal, is now described. It will be appreciated that certain properties of the formed signal are known to both a transmitter transmitting the formed signal, and a receiver receiving a signal corresponding to the formed signal. This awareness may enable the receiver to identify and / or correct errors in the received signal.

[0028] Figure 1 is a flow chart of a method 100, performed by a transmitter (or computing device), of transmitting a signal.

[0029] The method 100 comprises, at step 102, forming a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

[0030] Step 104 of the method 100 comprises transmitting the signal. As noted above, certain properties of the signal formed at step 102 are known to both the transmitter, and a receiver receiving a signal corresponding to the formed (or transmitted) signal. This awareness may enable the receiver to identify and / or correct errors in the received signal. For example, the receiver may be aware of one or more of the following properties of the transmitted signal: the temporal length of the block, that each subblock within the block has a temporal length that differs from a temporal length of each of the other subblocks within the block, the number of subblocks comprised within the block, the relationship between subblocks that each received symbol (and / or each received energised state) represents.

[0031] In some embodiments, step 102 may comprise forming a block of n subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks. It will be appreciated that this block can be arranged in n! ways, as each subblock length occurs exactly once in the block. The number of bits conveyed in such a block is upper bounded by Iog2(n I) bits, where the ordering of the subblocks represents the information conveyed by the block.

[0032] It will be appreciated that, in some cases, this upper bound may not necessarily translate to a whole number of bits. For example, considering 3 subblocks, these 3 subblocks may be ordered in 6 ways, and Iog2(6) is less than 3 bits but greater than 2 bits (-2.58). In this case, the amount of wasted bits (that is, partial amounts of bits that are theoretically available, but in practice cannot be used to encode information) may be reduced by transmitting two blocks, where each block respectively comprises 3 subblocks. It will be appreciated that these two blocks can be arranged in 6*6=36 ways, enabling 5 bits to be encoded by these blocks, as 2A5=32. In this case, as Iog2(36) - 5.17, less bits are wasted than in comparison to the case where only one block is transmitted.

[0033] In some embodiments, each subblock comprises an integer number of symbol periods, wherein the symbol in each subblock is in one of the symbol periods of the subblock.

[0034] For example, in some embodiments, a subblock (or block) may be defined as an integral number of time bins, where each time bin corresponds to a symbol period. Alternatively, a subblock (or block) may be defined as an integral number of any other encoding resources (where each encoding resource corresponds to a symbol period) that are orthogonal and that can be perfectly discriminated. In some embodiments, the at least two energized states comprise at least two photon polarisation states, or at least two light pulse polarisation states. In some embodiments, the at least two energized states comprise at least two light pulses that each have a number of degrees of freedom (for example, in the form of polarisation).

[0035] In some embodiments, the at least two photon polarisation states comprise a photon with a horizontal linear polarization (H) and a photon with vertical linear polarisation (V). These photon polarisation states are orthogonal, enabling them to be discriminated by a receiver. It will be appreciated that any orthogonal basis for the at least two photon polarisation states may be utilised. For example, in some embodiments, the at least two photon polarisation states comprise a photon with diagonal polarization (D) and a photon with anti-diagonal polarisation (A).

[0036] In some embodiments, each subblock comprises a symbol period that contains one of at least two energized states, such as one of at least two photon polarisation states, or one of at least two light pulse polarisation states. In this disclosure, “horizontal” and “vertical” polarizations are merely example labels given to the two orthogonal polarization states, and are not intended to convey any particular orientation of the polarization states relative to the transmitter, receiver or any other entity. In other scenarios, where for example electrical signals may be used, the energized states may be for example voltages of opposite polarity relative to a reference voltage (e.g. ground), voltages of different levels, phases of a sinusoidal signal, etc.

[0037] For example, for each of the subblocks comprised within a block, a first time bin of each subblock may contain a photon with one of two orthogonal polarisations (for example, a horizontal (H) or vertical (V) linear polarisation). In other embodiments, the first time bin of each subblock may contain a single polarised light pulse with one of two or more orthogonal polarisations.

[0038] In some embodiments, where the symbol corresponds to a first energized state, the symbol indicates that the temporal length of the subblock is greater than the temporal length of the next subblock in the sequence. In some embodiments, where the symbol corresponds to a second energized state, the symbol indicates that the temporal length of the subblock is smaller than the temporal length of the next subblock in the sequence. For example, for any given subblock i (where i = 1 , 2, ... , n-1 , n) comprised within a block of n subblocks, the subblock i may be tagged with a "H" state if the next subblock within the block, subblock i+1 , is longer than the subblock i, and may be tagged with a "V" state if the next subblock within the block, subblock i+1 , is shorter than the subblock i. The last subblock i=n in the block may be tagged with an "H" if the first subblock i=1 is longer than the subblock i=n, and may be tagged with a "V" if the first subblock i=1 is shorter than the subblock i=n.

[0039] That is, utilising at least two distinguishable energised states in this manner may enable a symbol to provide an indication of whether the length of a particular subblock is shorter, or longer, in comparison to another subblock in the sequence. This indication may then be used by a receiver to detect and / or correct errors in a received signal corresponding to a signal formed according to step 102.

[0040] For example, if a block comprising the time ordered sequence of 3 subblocks of lengths 2, 1 and 4 respectively, is transmitted, the states VO H V000 may be transmitted in the signal, where the first V is the photon with a vertical linear polarisation comprised in the subblock of length 2, where the H is the photon with a horizontal linear polarisation comprised in the subblock of length 1 , and where the second V is the photon with a vertical linear polarisation comprised in the subblock of length 4. The first V state indicates that the first subblock (of length 2) is longer than the next subblock in the sequence (the second subblock, of length 1). The first H state indicates that the second subblock (of length 1) is shorter than the next subblock in the sequence (the third subblock, of length 4). The second V state indicates that the third subblock (of length 4) is longer than the first subblock in the sequence (of length 2). That is, in this example, the polarisation of the energised state in each subblock is dependent on the length of the following subblock in the sequence. The 0 states indicate the time bins of the subblocks that contain no energised states. The VO H V000 string indicates how the symbols are ordered in the time bins, starting with the first time bin and ending with the last time bin of the block. The spaces in the sequence are provided for illustrative purposes and do not necessarily represent time delays, pauses or anything else in the sequence (though the presence of such is not excluded).

[0041] In some embodiments, the data is a codeword that is represented by the order of the subblocks in the time ordered sequence. For example, considering 3 subblocks of lengths 1 , 2 and 4 respectively, it will be appreciated that these subblocks can be ordered in 3!=6 ways in a block, 124, 142, 214, 241 , 412 and 421. Each of these 3! orderings then correspond to a codeword that is conveyed by the block. In this example, Iog2(6) ~ 2.58 bits of information can be transmitted per block.

[0042] In some embodiments, each subblock has a temporal length selected from a set of allowed subblock temporal lengths, and a sum of the allowed subblock temporal lengths is equal to a predetermined temporal length of the block.

[0043] In some embodiments, a receiver receiving a signal corresponding to a signal formed according to step 102 is aware of the set of allowed subblock temporal lengths. This awareness may enable the receiver to detect an error in the received signal. For example, when the receiver is aware of the set of allowed subblock lengths, if a signal comprising a subblock with a disallowed length (that is, a length that is not comprised in the set of allowed subblock lengths) is received, the receiver may detect an error in the received signal.

[0044] In some embodiments, the set of allowed subblock temporal lengths may comprise a sequence of distinct elements where pairwise sums of these distinct elements are each themselves distinct. For example, the sequence may be the sequence A010672 listed in the On-Line Encyclopedia of Integer Sequences [8],

[0045] For example, in some embodiments, a block of n subblocks may be formed, where the lengths of these subblocks exhaust the first n integers of the A010672 sequence (1 , 2, 4, 7, 12, 20, 29, 38, 52...). As noted above, this sequence has the property that the pairwise sums of distinct elements from the sequence are each themselves distinct.

[0046] Utilising a set of allowed subblock temporal lengths comprising a sequence with such properties, to form a signal for transmission according to step 102, may enable errors to be identified and / or corrected in a received signal corresponding to the formed signal, as described in greater detail below.

[0047] In some embodiments, each subblock comprises only one symbol that represents an energized state. Similarly to PPM, transmitting a signal that comprises subblocks that themselves each comprise only one symbol that represents an energized state, reduces the total amount of energy required to transmit the signal (and the information that signal represents).

[0048] Furthermore, where the at least two energized states comprise at least two photon polarisation states, more than 1 bit of information may be conveyed per photon for the signal when the signal comprises more than n>3 subblocks (that is, when Iog2(nl) / n > 1). However, it will be appreciated that this reduction in the total amount of energy required to transmit the signal is at the expense of a lower information rate (measured in bits per second).

[0049] In some embodiments, for each subblock, the first symbol period of the subblock contains the energized state. In some embodiments, for each subblock, at least one of the remaining symbol periods of the subblock does not contain an energized state.

[0050] Defining which symbol period of each subblock contains the energized state may enable a receiver to determine the start and end point of each subblock (and therefore the length of each subblock) in a received signal corresponding to a signal formed according to step 102. For example, if the receiver receives a signal comprising the states V0 H V000, and the receiver is aware that, for each subblock, the first symbol period of the subblock contains the energized state, the receiver is able to determine that the signal is comprised of a subblock of length 2 (V0, as the receiver determines that the first received energised state in the sequence indicates the beginning of the first subblock), followed by a subblock of length 1 (H, as the receiver determines that the second received energised state in the sequence indicates the beginning of the second subblock), followed by a subblock of length 4 (V000, as the receiver determines that the third received energised state in the sequence indicates the beginning of the third subblock).

[0051] In some embodiments, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from a temporal length of any of the subblocks of the number of subblocks.

[0052] In some embodiments, the receiver is aware that the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from a temporal length of any of the subblocks of the number of subblocks. This awareness may enable the receiver to detect an error in a received signal corresponding to the formed signal, as described in greater detail below. For example, this awareness may enable a receiver to determine that the received signal comprises a subblock with a disallowed block length, that has resulted from the combination of two subblocks that were present in the formed signal.

[0053] In some embodiments, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from the sum of the temporal lengths of any other pair of subblocks.

[0054] In some embodiments, the receiver is aware that the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from the sum of the temporal lengths of any other pair of subblocks. This awareness may enable the receiver to detect and / or correct an error in a received signal corresponding to the formed signal, as described in greater detail below. For example, this awareness may enable a receiver to determine that the received signal comprises a subblock with a disallowed block length (that has resulted from the combination of two subblocks in the that were present in the formed signal), and may enable the receiver to determine which two subblocks that were present in the formed signal have combined to form the disallowed subblock.

[0055] In some embodiments, the symbol indicates the temporal length of the subblock in relation to the next subblock in the sequence. This indicated relation may then be utilised by a receiver to detect and / or correct an error in a received signal corresponding to a signal formed according to step 102, where the error relates to adjacent subblocks in the formed signal.

[0056] Figure 2 is a flow chart of a method 200, performed by a receiver (or computing device), of receiving a signal. The receiver receives a signal corresponding to a transmitted signal, wherein the transmitted signal has been formed in accordance with step 102 described above. The receiver may identify and / or correct errors present in this received signal, as described in greater detail below.

[0057] The method 200 comprises, at step 202, receiving a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

[0058] That is, the receiver receives a signal that corresponds to a signal that has been formed in accordance with step 102 described above. However, it will be appreciated that the received signal may contain an error, and therefore may not be equivalent to the signal transmitted at step 104 described above.

[0059] However, the receiver is aware of certain properties of the transmitted signal (that has been formed in accordance with step 102 described above). For example, the receiver may be aware of one or more of the following properties of the transmitted signal: the temporal length of the block, that each subblock within the block has a temporal length that differs from a temporal length of each of the other subblocks within the block, the number of subblocks comprised within the block, the relationship between subblocks that each received symbol (and / or each received energised state) represents.

[0060] This awareness may enable the receiver to identify and / or correct errors in the received signal, without requiring any additional information from the transmitter that transmitted the signal. For example, an awareness of the relationship between subblocks in the transmitted signal that each received symbol (and / or each received energised state) represents may enable the receiver to correct errors in the received signal.

[0061] In some embodiments, the method 200 further comprises determining the transmitted data based on the order of the subblocks in the time ordered sequence of the received signal. In some embodiments, the data is a codeword that is represented by the order of the subblocks in the time ordered sequence of the transmitted signal, as described with reference to Figure 1. In some embodiments, the method 200 further comprises determining the codeword from the order of the subblocks in the time ordered sequence of the received signal.

[0062] In some embodiments, in which the received signal is equivalent to the signal transmitted at step 104 of the method 100, the receiver does not have to perform error detection or correction, and may determine the codeword based on the order of the subblocks in the time ordered sequence comprised within the received signal. For example, if the receiver receives a signal comprising the states V0 H V000, the receiver may determine that the block comprises the time ordered sequence of 3 subblocks of lengths 2, 1 and 4 respectively. The receiver may then determine that the codeword that the block represents is 214.

[0063] In other embodiments, in which an error is detected in the received signal, the receiver may determine the codeword based on the order of the subblocks in the time ordered sequence, and one or more further properties of the received signal (for example, the lengths of one or more received subblocks and / or one or more received energised states), as described in greater detail below.

[0064] In some embodiments, each subblock of the transmitted signal has a temporal length selected from a set of allowed subblock temporal lengths, and a sum of the allowed subblock temporal lengths is equal to a predetermined temporal length of the block.

[0065] As noted above, in some embodiments, the receiver is aware of the set of allowed subblock temporal lengths. This awareness may enable the receiver to detect an error in the received signal.

[0066] In some embodiments, each subblock comprises only one symbol that corresponds to an energized state.

[0067] In some embodiments, the at least two energized states comprise: at least two photon polarisation states, or at least two light pulse polarisation states.

[0068] In some embodiments, each subblock comprises an integer number of symbol periods, and wherein the symbol in each subblock is in one of the symbol periods of the subblock.

[0069] In some embodiments, for each subblock, the first symbol period of the subblock contains the energized state. In some embodiments, for each subblock, at least one of the remaining symbol periods of the subblock does not contain an energized state.

[0070] As noted above, defining which symbol period of each subblock contains the energized state may enable the receiver to determine the start and end point of each subblock (and therefore the length of each subblock) in the received signal. In some embodiments, for the transmitted signal, the symbol indicates the temporal length of the subblock in relation to the next subblock in the sequence.

[0071] In some embodiments, for the transmitted signal, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from a temporal length of any of the subblocks of the number of subblocks. In other words, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from a temporal length of any of the subblocks in the block.

[0072] In some embodiments, the receiver is aware that the sum of the temporal lengths of any pair of subblocks in the block is equal to a temporal length that differs from a temporal length of any of the subblocks in the block. This awareness may enable the receiver to detect an error in a received signal corresponding to the transmitted signal, as described in greater detail below.

[0073] In some embodiments, for the transmitted signal, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from the sum of the temporal lengths of any other pair of subblocks.

[0074] In some embodiments, the receiver is aware that the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from the sum of the temporal lengths of any other pair of subblocks. This awareness may enable the receiver to detect and / or correct an error in a received signal corresponding to the transmitted signal, as described in greater detail below.

[0075] In some embodiments, where the symbol corresponds to a first energized state, the symbol indicates that the temporal length of the subblock is greater than the temporal length of the next subblock in the sequence of the transmitted signal. In some embodiments, where the symbol corresponds to a second energized state, the symbol indicates that the temporal length of the subblock is smaller than the temporal length of the next subblock in the sequence of the transmitted signal.

[0076] As noted above, utilising at least two distinguishable energised states may enable the symbol to provide an indication of whether the length of a particular subblock is shorter, or longer, in comparison to another subblock in the sequence in the transmitted signal. This indication may then be used by a receiver to detect and / or correct errors in the received signal.

[0077] In some embodiments, the method 200 further comprises determining whether the received signal meets one or more of the following criteria: the received signal comprises a block having a temporal length that is not equal to the predetermined temporal length, the received signal comprises two or more subblocks that have the same temporal length, the received signal comprises a number of subblocks that differs from the number, and / or the received signal comprises a subblock with a temporal length that is not an allowed subblock temporal length.

[0078] In some embodiments, the method 200 further comprises, in response to determining that the received signal meets one or more of the criteria, detecting an error in the received signal.

[0079] For example, if it is determined that the received signal comprises a block having a temporal length that is not equal to the predetermined temporal length, the receiver may detect an error in the received signal (as a block with a particular length was expected in the received signal).

[0080] For example, if it is determined that the received signal comprises two or more subblocks that have the same temporal length, the receiver may detect an addition error in the received signal (e.g. the received signal included more photons or pulses than were expected).

[0081] For example, if it is determined that the received signal comprises a number of subblocks that differs from the number of subblocks that were expected, the receiver may detect an error in the received signal (as a signal comprising the number of subblocks was expected by the receiver).

[0082] In some embodiments, the receiver may detect an addition error if it is determined that the number of subblocks in the received signal is greater than the number of subblocks that were expected. In some embodiments, the receiver may detect a loss error if it is determined that the number of subblocks in the received signal is fewer than the number of subblocks (e.g. the received signal included fewer photons or pulses than were expected). For example, if it is determined that the received signal comprises a subblock with a temporal length that is not an allowed subblock temporal length, the receiver may detect an error in the received signal (as a signal comprising subblocks of only allowed subblock lengths was expected by the receiver).

[0083] In some embodiments, the method 200 further comprises, in response to detecting an error in the received signal, requesting re-transmission of the transmitted signal. For example, where the receiver has detected an error in the received signal, but is unable to perform error correction, the receiver may request that the transmitted signal be retransmitted, e.g. by sending a request to the transmitter or computing device.

[0084] In some embodiments, the method 200 comprises determining that the received signal comprises a number of subblocks that is fewer than the number that were expected. In some embodiments, in response to determining that the received signal comprises a number of subblocks that is fewer than the number that were expected, the receiver may detect a loss error in the received signal. For example, the receiver may determine that one of the energised states (for example, a photon) comprised in the transmitted signal has been lost.

[0085] For example, for a transmitted signal comprising the configuration of subblock lengths (1 ,2,4,7,12) (where each of these subblocks are of an allowed subblock length), after a photon loss, a signal with any of the following configurations of subblock lengths may be received: (3,4,7,12), (1 ,6,7,12), (1 ,2,11 ,12), (1 ,2,4,19).

[0086] It will be appreciated that the first photon may also be lost, but this can be corrected as the start time of each block is assumed to be known by the receiver.

[0087] In some embodiments, in response to determining that the received signal comprises a number of subblocks that is fewer than the expected number, the method 200 may comprise determining whether a subblock in the received signal has a temporal length that is not an allowed subblock temporal length. In some embodiments, receiver may determine that one of the energised states (for example, a photon) comprised in the transmitted signal has been lost in the subblock in the received signal with the disallowed block length. As illustrated above, when a photon originally present in a signal formed for transmission is lost, a subblock with a length that is not in the list of allowed subblock lengths will be present in the received signal. For example, for a received signal comprising any of the configurations: (3,4,7,12), (1 ,6,7,12), (1 ,2,11 ,12), (1 ,2,4,19), in each of these configurations, there is exactly one subblock with a disallowed length. For example, for each of these configurations, the receiver may identify that the photon has been lost in the disallowed subblocks of length 3, 6, 11 , and 19 respectively.

[0088] In some embodiments, in response to determining that a subblock in the received signal has a temporal length that is not an allowed subblock temporal length, the method 200 may comprise correcting the received signal by identifying two corrected subblocks based on the temporal length of the determined subblock and the symbol in the determined subblock, wherein the temporal length of each of the corrected subblocks is an allowed subblock temporal length.

[0089] For example, the receiver may determine that the subblock in the received signal with the disallowed block length is composed of two subblocks (of allowed subblock lengths) that were present in the transmitted signal. The lengths of these two subblocks that were present in the transmitted signal will add to result in the disallowed block length, and can be determined based on the length of the disallowed subblock, and the set of allowed subblock lengths (which is known to the receiver). The receiver may then utilise the symbol comprised within the disallowed subblock to determine the ordering of these two allowed subblocks in the transmitted signal. In some embodiments, the receiver may utilise the symbol comprised within the disallowed subblock, and an awareness of the relationship between subblocks in the transmitted signal that a received symbol represents, to determine the ordering of these two allowed subblocks in the transmitted signal.

[0090] For example, the receiver may be aware that when a received symbol corresponds to a first energized state, the received symbol indicates that the temporal length of the subblock is greater than the temporal length of the next subblock in the sequence of the transmitted signal, and that when the received symbol corresponds to a second energized state, the received symbol indicates that the temporal length of the subblock is smaller than the temporal length of the next subblock in the sequence of the transmitted signal. In this example, if the symbol comprised within the disallowed subblock corresponds to the first energized state, the receiver may utilise this awareness to determine that the subblock with the disallowed length is composed of two subblocks, where the first subblock length is greater than the second subblock length. If the symbol comprised within the disallowed subblock corresponds to the second energized state, the receiver may utilise this awareness to determine that the subblock with the disallowed length is composed of two subblocks, where the first subblock length is shorter than the second subblock length.

[0091] For example, if the subblock with a disallowed length is initialised with a "H", it can be determined that this subblock with a disallowed length is composed of two subblocks, where the first subblock length is smaller than the second subblock length. For example, if the configuration of the received block is (1 ,2,11 ,12), and the subblock of length 11 is initialised with a "H”, it can be determined that this subblock was composed of the subblocks (4,7) in the transmitted signal, and not the subblocks (7,4). That is, the polarisation of the energised states in the received signal, and an awareness of the relationship between subblocks in the transmitted signal that the polarisation of an energised state represents, may be used to correct errors in the received signal.

[0092] In some embodiments, the method 200 further comprises determining the transmitted data based on the order of the corrected subblocks and the other subblocks in the time ordered sequence of the received signal.

[0093] For example, the receiver may determine, based on a received block (1 ,2,11 ,12), and corrected subblocks (4,7), that the transmitted block was (1 , 2, 4, 7, 12). The receiver may then determine the codeword that was encoded in the transmitted block.

[0094] In some embodiments, the method 200 comprises determining that the received signal comprises a number of subblocks that is greater than the expected number. In some embodiments, in response determining that the received signal comprises a number of subblocks that is greater than the expected number, the receiver may detect an addition error in the received signal. For example, the receiver may determine that the received signal comprises an additional energised state (for example, a photon) that was not comprised in the transmitted signal.

[0095] For example, for a transmitted signal comprising the configuration of subblock lengths (1 ,2,4,7,12), if an additional photon appears inside the (7) subblock, the receiver may receive signal with any of the following configurations of subblock lengths: (1 ,2,4,1 ,6,12), (1 ,2,4,2,5,12), (1 ,2,4,3,4,12), (1 ,2,4,4,3,12), (1 ,2,4,5,2,12), (1 ,2,4,6,1 ,12). For any of these received signals, the receiver would detect more blocks than expected, and detect an error in the received signal.

[0096] In some embodiments, in response to determining that the received signal comprises a number of subblocks that is greater than the number, the method 200 comprises determining whether a subblock in the received signal has a temporal length that is not an allowed subblock temporal length. In some embodiments, the receiver may determine that the presence of an additional energised state (for example, a photon) in the received signal has resulted in the presence of a subblock with a disallowed block length in the received signal. The disallowed subblock in the configuration of the received subblocks then indicates the position of the error to the receiver.

[0097] In some embodiments, in response to determining that a subblock in the received signal has a temporal length that is not an allowed subblock temporal length, the method 200 comprises identifying a further subblock in the received signal, wherein the further subblock is adjacent to the determined subblock and has a temporal length that is equal to that of another subblock in the received signal. The receiver may determine that the further block has resulted from the presence of the additional energised state in the received signal.

[0098] For example, where the receiver receives signal with any of the configurations of subblock lengths (1 ,2,4,1 ,6,12), (1 ,2,4,2,5,12), (1 ,2,4,3,4,12), (1 ,2,4,4,3,12), (1 ,2,4,5,2,12), (1 ,2,4,6,1 ,12), for each respective configuration, next to the subblock with a disallowed length is a subblock whose length already appears once in the received configuration. As noted above, the receiver is aware that each subblock within the block of the transmitted signal has a temporal length that differs from a temporal length of each of the other subblocks within the block. As such, the presence of a subblock whose length already appears once in the received configuration in the received signal indicates an error to the receiver.

[0099] In some embodiments, the method 200 comprises correcting the received signal by identifying a combined subblock based on the temporal lengths of the determined subblock and the further subblock, wherein the corrected subblock has a temporal length that is an allowed subblock temporal length, and the temporal length of the corrected subblock is a sum of the temporal lengths of the determined subblock and the further subblock.

[0100] That is, an addition error can be corrected by identifying the disallowed subblock, and the subblock adjacent to it with a length that has already appeared once in the received configuration, and merging these two subblocks. For example, for the received block

[0101] (1.2.4.1.6.12), the receiver may form a corrected subblock (7), that is composed of the second subblock of length (1) (that has already appeared in the block), and the subblock of length (6) (the subblock with the disallowed length).

[0102] In some embodiments, the situation is symmetric. For example, for the received block

[0103] (1.2.4.3.4.12), it is not immediately clear which subblock to merge with the subblock with the disallowed length (that is, it is not clear with subblock of length 4 should be merged with the disallowed subblock of length 3). However, in this scenario, the polarisations of the energised states in the received signal, and an awareness of the relationship between subblocks in the transmitted signal that the polarisation of an energised state represents, can be used to correct this error, in a similar manner as described above.

[0104] In some embodiments, the method 200 further comprises determining the transmitted data based on the order of the combined subblock and the other subblocks in the time ordered sequence of the received signal.

[0105] For example, the receiver may determine, based on a received block (1 ,2,4,1 ,6,12), and the combined subblock (7), that the transmitted block was (1 , 2, 4, 7, 12). The receiver may then determine the codeword that was encoded in the transmitted block.

[0106] In some embodiments, the method 200 may comprise detecting and / or correcting a plurality of errors in the received signal.

[0107] A scenario is considered in which multiple non-adjacent photons are lost in a received signal. In this scenario, the block (1 ,2, 4, 7) is transmitted, encoded as HH0H000V000000 in the transmitted signal. Following the transmission, the receiver receives the signal H00H0000000000, and therefore the block (3,11). The receiver can determine that it is likely that two loss errors occurred (although 3 loss errors and one photon error may have also occurred, the probability of this is considerably lower).

[0108] The receiver may then correct the received subblock of length 3, in a similar manner as described above. As subblock of length 3 is initialised with a "H”, it can be determined that this subblock was composed of the subblocks (1 ,2) in the transmitted signal, and not the subblocks (2,1). The receiver may then correct the received subblock of length 11 , in a similar manner as described above. As subblock of length 11 is initialised with a "H”, it can be determined that this subblock was composed of the subblocks (4,7) in the transmitted signal, and not the subblocks (7,4).

[0109] It will be appreciated that the receiver cannot correct 2 or more adjacent loss errors, and certain other multi-errors, in the received signal, when each energised state comprises a single photon. However, the receiver can detect most errors in the received signal, as the receiver is aware that a fixed number of distinct subblock lengths are present in the transmitted signal. With low probability, there may be errors stemming from a combination of addition and deletion, that could not be detected by the receiver, when each energised state comprises a single photon.

[0110] With reference to Figures 1 and 2, it will be appreciated that, when a general polarized pulse is considered (and not a single photon), more than 2 polarization states (i.e., “H” and “V”) can be distinguished.

[0111] For these cases, the error correction of the received may be extended such that 2 adjacent loss errors in the signal can be corrected.

[0112] For example, for a transmitted signal comprising the configuration of subblock lengths

[0113] (1.2.4.7.12), after 2 adjacent pulse losses, a received signal with the following configuration may be received (1 ,13,12). To perform error correction in this case, the polarisation information requires 6 distinguishable symbols (a,b,c,d,e,f), to account for each of the 3!=6 orderings a=(2,4,7), b=(2,7,4), c=(4,2,7), d=(4,7,2), e=(7,2,4), f=(7,4,2), which enables the correction of two adjacent loss errors. In this example, the sequence

[0114] (1.2.4.7.12) would be encoded as “aaOaOOOdOOOOOOeOOOOOOOOOOO” in the transmitted signal. Figure 3 is a schematic of an example of a transmitter 300 for transmitting a signal. The transmitter 300 comprises processing circuitry 302 (e.g. one or more processors) and a memory 304 in communication with the processing circuitry 302. The memory 304 contains instructions executable by the processing circuitry 302. The transmitter 300 also comprises an interface 306 in communication with the processing circuitry 302. Although the interface 306, processing circuitry 302 and memory 304 are shown connected in series, these may alternatively be interconnected in any other way, for example via a bus.

[0115] In one embodiment, the memory 304 contains instructions executable by the processing circuitry 302 such that the transmitter 300 is operable to form a signal for transmission, wherein the signal comprises a block of a predetermined temporal length that comprises a time ordered sequence of a predetermined number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that represents corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence, and transmit the signal. In some examples, the transmitter 300 is operable to carry out the method 100 described above with reference to Figure 1.

[0116] In some embodiments, a computer program 308 is provided. The computer program 308 comprises instructions which, when executed on at least one processor 302, cause the at least one processor 302 to carry out the method 100 described above with reference to Figure 1 .

[0117] In some embodiments, a carrier 310 containing the computer program 308 described above is provided. The carrier 310 may comprise one of: an electronic signal, optical signal, radio signal or computer readable storage medium.

[0118] In some embodiments, a computer program product 312 comprising non transitory computer readable media having stored thereon the computer program 308 described above is provided.

[0119] Figure 4 is a schematic of an example of a receiver 400 for receiving a signal. The receiver 400 comprises processing circuitry 402 (e.g. one or more processors) and a memory 404 in communication with the processing circuitry 402. The memory 404 contains instructions executable by the processing circuitry 402. The receiver 400 also comprises an interface 406 in communication with the processing circuitry 402. Although the interface 406, processing circuitry 402 and memory 404 are shown connected in series, these may alternatively be interconnected in any other way, for example via a bus.

[0120] In one embodiment, the memory 404 contains instructions executable by the processing circuitry 402 such that the receiver 400 is operable to receiving a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block of a predetermined temporal length that comprises a time ordered sequence of a predetermined number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that represents corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence. In some examples, the receiver 400 is operable to carry out the method 200 described above with reference to Figure 2.

[0121] In some embodiments, a computer program 408 is provided. The computer program 408 comprises instructions which, when executed on at least one processor 402, cause the at least one processor 402 to carry out the method 200 described above with reference to Figure 2.

[0122] In some embodiments, a carrier 410 containing the computer program 408 described above is provided. The carrier 410 may comprise one of: an electronic signal, optical signal, radio signal or computer readable storage medium.

[0123] In some embodiments, a computer program product 412 comprising non transitory computer readable media having stored thereon the computer program 408 described above is provided.

[0124] It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim or embodiment, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, “first”, “second” etc are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein may be carried out in any order unless expressly otherwise stated. Any reference signs in the statements shall not be construed so as to limit their scope.

[0125] REFERENCES

[0126] [1] J. Pierce, “Optical channels: Practical limits with photon counting,” IEEE Transactions on Communications, vol. 26, no. 12, pp. 1819-1821, 1978.

[0127] [2] R. McEliece, “Practical codes for photon communication,” IEEE Transactions on Information Theory, vol. 27, no. 4, pp. 393-398, 1981.

[0128] [3] S. Butman, J. Katz, and J. Lesh, “Bandwidth limitations on noiseless optical channel capacity,” IEEE Transactions on Communications, vol. 30, no. 5, pp. 1262-1264, 1982.

[0129] [4] J. Lesh, “Capacity limit of the noiseless, energy-efficient optical ppm channel,” IEEE Transactions on Communications, vol. 31 , no. 4, pp. 546-548,

[0130] 1983.

[0131] [5] T. B. H. Tentrup, T. Hummel, T. A. W. Wolterink, R. llppu, A. P. Mosk, and P. W. H. Pinkse, “Transmitting more than 10 bit with a single photon,” Optics Express, vol. 25, no. 3, p. 2826, 2017.

[0132] [6] Wikipedia contributors, “Pulse-position modulation — Wikipedia, the free encyclopedia.” https: / / en.wikipedia.org / wiki / Pulse-position_modulation, 2022. [Online; Accessed 2022 - 08 - 25],

[0133] [7] I. F. Blake, G. Cohen, and M. Deza, “Coding with permutations,” Information and Control, vol. 43, no. 1, pp. 1-19, 1979.

[0134] [8] Hoey, Dan, Sequence a010672 in the Online Encyclopedia of Integer Sequences, published electronically at https: / / oeis.org, 2016.

Claims

CLAIMS1. A method (100), performed by a transmitter (300), of transmitting a signal, the method (100) comprising: forming a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmitting the signal.

2. The method (100) of claim 1 , wherein each subblock has a temporal length selected from a set of allowed subblock temporal lengths, and a sum of the allowed subblock temporal lengths is equal to a predetermined temporal length of the block.

3. The method (100) of claim 2, wherein the set of allowed subblock temporal lengths comprises a sequence of distinct elements where pairwise sums of these distinct elements are each themselves distinct.

4. The method (100) of any of claims 1 -3, wherein each subblock comprises only one symbol that corresponds to an energized state.

5. The method (100) of any of claims 1-4, wherein each subblock comprises an integer number of symbol periods, and wherein the symbol in each subblock is in one of the symbol periods of the subblock.

6. The method (100) of claim 5, wherein, for each subblock, the first symbol period of the subblock contains the energized state.

7. The method (100) of claim 5 or 6, wherein, for each subblock, at least one of the remaining symbol periods of the subblock does not contain an energized state8. The method (100) of any of claims 1-7, wherein the data is a codeword that is represented by the order of the subblocks in the time ordered sequence.

9. The method (100) of any of claims 1-8, wherein: the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from a temporal length of any of the subblocks of the number of subblocks; and / or the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from the sum of the temporal lengths of any other pair of subblocks.

10. The method (100) of any of claims 1-9, wherein the symbol indicates the temporal length of the subblock in relation to the next subblock in the sequence.

11. The method (100) of claim 10, wherein, when the symbol corresponds to a first energized state, the symbol indicates that the temporal length of the subblock is greater than the temporal length of the next subblock in the sequence, and wherein, when the symbol corresponds to a second energized state, the symbol indicates that the temporal length of the subblock is smaller than the temporal length of the next subblock in the sequence.

12. The method (100) of any of claims 1-11 , wherein the at least two energized states comprise: at least two photon polarisation states, or at least two light pulse polarisation states.

13. A method (200), performed by a receiver (400), of receiving a signal, the method (200) comprising: receiving a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

14. The method (200) of claim 13, comprising determining the transmitted data based on the order of the subblocks in the time ordered sequence of the received signal.

15. The method (200) of claim 13 or 14, wherein each subblock of the transmitted signal has a temporal length selected from a set of allowed subblock temporal lengths, and a sum of the allowed subblock temporal lengths is equal to a predetermined temporal length of the block.

16. The method (200) of claim 15, wherein the set of allowed subblock temporal lengths comprises a sequence of distinct elements where pairwise sums of these distinct elements are each themselves distinct.

17. The method (200) of any of claims 13-16, wherein each subblock comprises only one symbol that corresponds to an energized state.

18. The method (200) of any of claims 13-17, wherein each subblock comprises an integer number of symbol periods, and wherein the symbol in each subblock is in one of the symbol periods of the subblock.

19. The method (200) of claim 18, wherein, for each subblock, the first symbol period of the subblock contains the energized state.

20. The method (200) of claim 18 or 19, wherein, for each subblock, at least one of the remaining symbol periods of the subblock does not contain an energized state.21 . The method (200) of any of claims 13-20, wherein: for the transmitted signal, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from a temporal length of any of the subblocks of the number of subblocks; and / or for the transmitted signal, the sum of the temporal lengths of any pair of subblocks of the number of subblocks is equal to a temporal length that differs from the sum of the temporal lengths of any other pair of subblocks.

22. The method (200) of any of claims 13-21 , wherein, for the transmitted signal, the symbol indicates the temporal length of the subblock in relation to the next subblock in the sequence.

23. The method (200) of claim 22, wherein, when the symbol corresponds to a first energized state, the symbol indicates that the temporal length of the subblock is greater than the temporal length of the next subblock in the sequence of the transmitted signal, and wherein, when the symbol corresponds to a second energized state, the symbol indicates that the temporal length of the subblock is smaller than the temporal length of the next subblock in the sequence of the transmitted signal.

24. The method (200) of any of claims 13-23, wherein the at least two energized states comprise: at least two photon polarisation states, or at least two light pulse polarisation states.

25. The method (200) of any of claims 13-24, wherein the data is a codeword that is represented by the order of the subblocks in the time ordered sequence of the transmitted signal.

26. The method (200) of claim 25, further comprising determining the codeword from the order of the subblocks in the time ordered sequence of the received signal.

27. The method (200) of any of claims 15-26, the method (200) further comprising: determining whether the received signal meets one or more of the following criteria: the received signal comprises a block having a temporal length that is not equal to the predetermined temporal length, the received signal comprises two or more subblocks that have the same temporal length, the received signal comprises a number of subblocks that differs from the number, and / or the received signal comprises a subblock with a temporal length that is not an allowed subblock temporal length; and in response to determining that the received signal meets one or more of the criteria, detecting an error in the received signal.

28. The method (200) of claim 27, the method (200) further comprising: in response to detecting an error in the received signal, requesting re-transmission of the transmitted signal.

29. The method (200) of any of claims 13 to 28, the method (200) further comprising: determining that the received signal comprises a number of subblocks that is fewer than the number; in response to determining that the received signal comprises a number of subblocks that is fewer than the number, determining whether a subblock in the received signal has a temporal length that is not an allowed subblock temporal length; and in response to determining that a subblock in the received signal has a temporal length that is not an allowed subblock temporal length, correcting the received signal by identifying two corrected subblocks based on the temporal length of the determined subblock and the symbol in the determined subblock, wherein the temporal length of each of the corrected subblocks is an allowed subblock temporal length.

30. The method (200) of claim 29, the method (200) further comprising determining the transmitted data based on the order of the corrected subblocks and the other subblocks in the time ordered sequence of the received signal.31 . The method (200) of any of claims 13 to 30, the method (200) further comprising: determining that the received signal comprises a number of subblocks that is greater than the number; in response to determining that the received signal comprises a number of subblocks that is greater than the number, determining whether a subblock in the received signal has a temporal length that is not an allowed subblock temporal length; in response to determining that a subblock in the received signal has a temporal length that is not an allowed subblock temporal length, identifying a further subblock in the received signal, wherein the further subblock is adjacent to the determined subblock and has a temporal length that is equal to that of another subblock in the received signal; correcting the received signal by identifying a combined subblock based on the temporal lengths of the determined subblock and the further subblock, wherein the corrected subblock has a temporal length that is an allowed subblock temporal length, and the temporal length of the corrected subblock is a sum of the temporal lengths of the determined subblock and the further subblock.

32. The method (200) of claim 31 , the method (200) further comprising determining the transmitted data based on the order of the combined subblock and the other subblocks in the time ordered sequence of the received signal.

33. A computer program (308, 408) comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out a method (100, 200) according to any of claims 1 to 32.

34. A carrier (310, 410) containing a computer program (308, 408) according to claim 33, wherein the carrier (310, 410) comprises one of an electronic signal, optical signal, radio signal or computer readable storage medium.

35. A computer program product (312, 412) comprising non transitory computer readable media having stored thereon a computer program (308, 408) according to claim 33.

36. A transmitter (300) for transmitting a signal, the transmitter (300) comprising a processor and a memory (304), the memory (304) containing instructions executable by the processor such that the transmitter (300) is operable to: form a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmit the signal.

37. The transmitter (300) of claim 36, wherein the memory (304) contains instructions executable by the processor such that the transmitter (300) is operable to perform the method (100) of any of claims 2 to 12.

38. A transmitter (300), for transmitting a signal, wherein the transmitter (300) is configured to: form a signal for transmission, wherein the signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents data to be transmitted, wherein each subblock comprises at least one symbol that corresponds to one of at leasttwo energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence; and transmit the signal.

39. The transmitter (300) of claim 38, wherein the transmitter (300) is further configured to perform the method (100) of any of claims 2 to 12.

40. A receiver (400) for receiving a signal, the receiver (400) comprising a processor and a memory (404), the memory (404) containing instructions executable by the processor such that the receiver (400) is operable to: receive a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

41. The receiver (400) of claim 40, wherein the memory (404) contains instructions executable by the processor such that the receiver (400) is operable to perform the method (200) of any of claims 14 to 32.

42. A receiver (400), for receiving a signal, wherein the receiver (400) is configured to: receive a signal corresponding to a transmitted signal, wherein the transmitted signal comprises a block that comprises a time ordered sequence of a number of subblocks, each subblock having a temporal length that differs from a temporal length of each of the other subblocks, wherein the order of the subblocks in the time ordered sequence represents transmitted data, wherein each subblock comprises one symbol that corresponds to one of at least two energized states, and wherein the symbol indicates the temporal length of the subblock in relation to another subblock in the sequence.

43. The receiver (400) of claim 42, wherein the receiver (400) is further configured to perform the method (200) of any of claims 14 to 32.

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