BIT AND POWER LOADING TECHNICAL FIELD

The present invention relates generally to the field of a multicarrier optical communication system, and, more particularly, to a device and a method that determine a Forward Error Correction (FEC)-aware bit and power loading for a multicarrier optical communication system. BACKGROUND

Conventional bit- and power-loading mechanisms are typically configured within three steps and by using a pre-FEC channel metric, such as the Signal-to-Noise Ratio (SNR), the Error Vector Magnitude (EVM), the pre-FEC Bit Error Rate (BER), etc. For example, a conventional bit- and power-loading mechanism may be configured within the following three steps: Step 1: Channel sounding by the transmission of Orthogonal Frequency-Division Multiplexing (OFDM) or Discrete Multi-Tone (DMT) Modulation symbol with non-loaded pilots (e.g., uniform power over all subcarriers and also uniform modulation size overall subcarriers). Step 2: Bit- and power-loading configuration identification via the SNR or a related metric, such as the BER or the EVM, is usually performed iteratively, for example, by increasing loading of one subcarrier at each time and until a target rate is reached. Step 3: Feeding back the obtained configuration to a transmitter (Tx) of the system via an error- free channel. FIG.6 schematically illustrates a conventional multicarrier optical communication system 600 for configuring a bit- and power-loading scheme at the receiver side. The conventional multicarrier optical communication system 600 comprises a transmitter 601 and a receiver 602. Moreover, the channel SNR, or a related pre-FEC metric (e.g., the EVM, the pre-FEC BER), is determined at the receiver 602.

The conventional multicarrier optical communication system 600 performs an iterative loading procedure (e.g., by loading one subcarrier at each time) to find (in each iteration) the next best subcarrier to increase in terms of power and the constellation.

However, the conventional bit- and power-loading procedures include identifying the next best subcarrier based on, for example, the channel SNR, related pre-FEC metrics (i.e., the EVM, the pre-FEC BER), etc. The conventional schemes have the disadvantages that they do not consider the applied FEC.

Furthermore, advanced FEC types are known, including a Multi-Level Coding (MLC) that encodes subsets of bit levels in different ways, and thus maps the pre-FEC capacity to a post- FEC capacity, for example, depending on the applied modulation scheme.

FIG. 7 illustrates a conventional MLC scheme 700. The conventional MLC scheme 700 illustrated in FIG. 7 is one example from many different ways to build an MLC. For example, the MLC may be built such that the Bit levels can be varied, more than 2 multi-levels could be used, etc. The conventional MLC scheme 700 includes a FEC A 701 and a FEC B 702. Moreover, one least Significant Bit (LSB) is strongly encoded by the FEC A 701, and all of the Most Significant Bits (MSBs) are weakly encoded by the FEC B 702. Afterwards, both of the encoded bit streams are transmitted to a bit multiplexing unit, and the post-FEC stream is obtained.

However, in such MLC system, using a pre-FEC metric for identifying the next best subcarrier to increase loading, does not reflect the actual post-FEC performance of the MLC system, since the MLC performs differently on different modulation schemes.

FIG. 8 illustrates the performance of a conventional MLC system 800 after applying a constant pre-FEC. As it can be derived from FIG. 8, the target of a constant pre-FEC BER over all subcarriers does not lead to a minimum BER after the FEC correction, however, it may lead only to suboptimal system performance. Overall, the conventional devices and methods have the disadvantage that they do not optimize bit- and power loading using post-FEC BER statistics. Moreover, optimizing bit and power loading using post-FEC BER statistics is impractical, due to the long time that is needed to get those statistics. However, for the multicarrier optical systems that apply MLC, the post-FEC metrics are necessary for an optimal bit-and power loading strategy. It is thus generally desirable to improve devices and methods for supporting FEC-bit and power loading for a multicarrier optical communication system. SUMMARY

In view of the above-mentioned problems and disadvantages, the embodiments of the invention aim to improve the conventional devices and methods. An objective is thereby to provide a device and a method, which optimize bit and power loading for an optical communication system according to the performance of such system. For example, a FEC-related optimization metric for bit- and power loading is desired. Such bit- and power loading would have direct implications on the performance of the FEC, the achievable coding rate, etc. The objective is achieved by the embodiments provided in the enclosed independent claims. Advantageous implementations of these embodiments are further defined in the dependent claims. A first aspect of the invention provides a device for supporting Forward Error Correction (FEC) bit and power loading in a multicarrier optical communication system using Multi-Level Coding (MLC) wherein the MLC is based on two or more constellations of different size, and wherein the device is configured to determine a FEC property of the optical communication system, determine a target Spectral Efficiency (SE), determine one or more rate increments based on the FEC property and the two or more constellations, wherein each rate increment is associated with a change from a given constellation to the next larger constellation, and determine one or more power increments, for each of a plurality of subcarriers, based on the FEC property and the two or more constellations, wherein each power increment is associated with a change from a given constellation to the next larger constellation allocated to that subcarrier. The device may be, or may incorporated in, a transmitter device and/or a receiver device of the optical communication system. The multicarrier optical communication system may use MLC which en-/de-codes two or more bit levels with parallel en-/de-coders of different strength, moreover, the MLC may be based on two or more constellations of different size. In some embodiments, the bit-loading multitone system may assign sets of subcarriers with different modulations, which may vary in their number of bit levels per symbol. Moreover, since fixed bit levels have been assigned to the parallel en-/de-coders of the MLC, they may encode varying modulations with varying strength. Furthermore, for an optimal bit-and power loading strategy, a post-FEC metric may be necessary. In some embodiments, the device may obtain a FEC-related optimization metric, for example, for bit- and power loading. The embodiments of the invention are not limited to a specific type of FEC. A typical FEC example is MLC. In some embodiments, the device may, for example, (only) assume a nonlinear mapping of pre-FEC to post-FEC capacity in a multicarrier scheme with multiple modulations. Moreover, the device may determine one or more rate increments based on the determined FEC property and the two or more constellations. Furthermore, each rate increment can be associated with a change from a given constellation to the next larger constellation. For example, there may be two constellations including a first constellation being the Quadrature Phase Shift Keying (QPSK) having a smaller size than a second constellation being 16-Quadrature amplitude modulation (16-QAM). Moreover, one rate increment may be determined, which may be associated with a change from the QPSK to the 16-QAM. Furthermore, one power increment may be determined, which may be associated with a change from the QPSK to the 16-QAM, without limiting the present discourse to a specific FEC type, a specific constellations, etc. For instance, the device may determine a metric that combines the FEC- related rate increment and the FEC- and SNR-related power increment , which describe each the effective

change of any increase in constellation size k to k+1 for any subcarrier i. Moreover, both of the rate increment and the power increment may be calculated a priori, once the MLC has been selected. In some embodiments, the device of the first aspect may have the advantage of increasing the performance gain, e.g., by lower transmit power. Moreover, it may achieve a uniform FEC performance across all subcarriers, and may further reduce the chip complexity by reducing the interleaving, which is needed for the FEC to reach its performance. In some embodiments, the post FEC error statistics may not be required, which are also impractical to obtain in real systems. The device may comprise a circuitry. Moreover, the circuitry may comprise hardware and software. The hardware may comprise analog or digital circuitry, or both analog and digital circuitry. In some embodiments, the circuitry comprises one or more processors and a non- volatile memory connected to the one or more processors. The non-volatile memory may carry executable program code which, when executed by the one or more processors, causes the device to perform the operations or methods described herein. In an implementation form of the first aspect, the device is further configured to calculate a bit allocation and/or a power allocation for the plurality of subcarriers based on the determined rate increment and the determined power increment. In particular, the multicarrier bit- and power loading may be optimized towards post-FEC performance (instead of pre-FEC performance in the conventional devices), which is the system performance. In some embodiments, bit- and power loading is following the same complexity, for example, once the FEC-related metrics (i.e., the and the ) are identified.

In a further implementation form of the first aspect, the device is further configured to determine the subcarrier associated with the largest ratio of rate increment over power increment as the cheapest subcarrier. In particular, an iterative loading (e.g., one subcarrier at a time) may be performed including finding in each iteration the next best subcarrier to be increased, for example, by finding the subcarrier i which maximizes according to argmax Furthermore, the next best subcarrier may be selected not only by the SNR, but also by the applied rate through power allocation. In a further implementation form of the first aspect, the device is further configured to allocate the constellation having the smallest size to the plurality of subcarriers, determine the cheapest subcarrier from the plurality of subcarriers, after the allocation of the constellation having the smallest size, allocate the next larger constellation than the constellation having the smallest size to the cheapest subcarrier, determine whether an overall target rate, and therewith an overall SE, is achieved for the plurality of subcarriers, and calculate, when it is determined that the overall target rate is achieved, the bit allocation and/or power allocation based on the allocated constellations. In a further implementation form of the first aspect, the device is further configured to determine the overall target rate based on the SE. In a further implementation form of the first aspect, the device is further configured to determine, when it is determined that the overall target rate is not achieved, a next cheapest subcarrier, allocate the next larger constellation than currently allocated to the next cheapest subcarrier, determine whether the overall target rate is achieved for the plurality of subcarriers, and calculate, when it is determined that the overall target rate is achieved, the bit allocation and/or power allocation based on the allocated constellations. In a further implementation form of the first aspect, the device is further configured to iteratively perform the steps of allocating, determining, and calculating until the overall target rate is achieved. In a further implementation form of the first aspect, the device is further configured to perform the FEC. In a further implementation form of the first aspect, the device is, or is included in, a receiver of the optical communication system. In a further implementation form of the first aspect, the MLC is based on a concatenated MLC comprising a Low-Density-Parity-Check-Code (LDPC) inner code and a staircase outer code. In a further implementation form of the first aspect, the device is further configured to inner- code at least one Least Significant Bit (LSB) based on the LDPC inner code; and/or inner- uncode at least one upper bit-level, if existent For instance, in some embodiments, a QPSK modulation may be used which transmits 1 bit per symbol, and thus has no upper bit levels (MSBs), moreover, the device may only inner code the LDPC (i.e., the existent bit level). The present disclosure is not limited to a specific coding of different bit levels, a specific used MLC, etc. For example, in some embodiments, the device may use two or more LDPCs with different strengths to inner-code different bit levels in a different procedure. In some embodiments, the device may use different numbers and different types of inner codes/outer codes. In some embodiments, different number of stages may be used, for example, two inner code stages, one outer code stage, etc., may be used. In a further implementation form of the first aspect, the multicarrier optical communication system is based on at least one of the following:

- an Intensity-Modulated Direct-Detection (IM-DD) Discrete Multi-Tone (DMT); - a Coherent Optical Orthogonal Frequency Division Multiplexing, CO-OFDM. This is beneficial, since the device and/or its functions may be applicable to any frequency domain-modulating subcarrier multicarrier scheme, including but not limited to the IM-DMT and the CO-OFDM. A second aspect of the invention provides a method for supporting Forward Error Correction (FEC) bit and power loading in a multicarrier optical communication system using Multi-Level Coding (MLC) wherein the MLC is based on two or more constellations of different size, and wherein the method comprises determining a FEC property of the optical communication system, determining a target Spectral Efficiency (SE), determining one or more rate increments based on the FEC property and the two or more constellations, wherein each rate increment is associated with a change from a given constellation to the next larger constellation, and determining one or more power increments, for each of a plurality of subcarriers, based on the FEC property and the two or more constellations, wherein each power increment is associated with a change from a given constellation to the next larger constellation allocated to that subcarrier. In an implementation form of the second aspect, the method further comprises calculating a bit allocation and/or a power allocation for the plurality of subcarriers based on the determined rate increment and the determined power increment. In a further implementation form of the second aspect, the method further comprises allocating the constellation having the smallest size to the plurality of subcarriers, determining the cheapest subcarrier from the plurality of subcarriers, after the allocation of the constellation having the smallest size, allocating the next larger constellation than the constellation having the smallest size to the cheapest subcarrier, determining whether an overall target rate, and therewith an overall SE, is achieved for the plurality of subcarriers, and calculating, when it is determined that the overall target rate is achieved, the bit allocation and/or power allocation based on the allocated constellations. In a further implementation form of the second aspect, the method further comprises determining the subcarrier associated with the largest ratio of rate increment over power increment as the cheapest subcarrier. In a further implementation form of the second aspect, the method further comprises determining the overall target rate based on the SE. In a further implementation form of the second aspect, the method further comprises determining, when it is determined that the overall target rate is not achieved, a next cheapest subcarrier, allocating the next larger constellation than currently allocated to the next cheapest subcarrier, determining whether the overall target rate is achieved for the plurality of subcarriers, and calculating, when it is determined that the overall target rate is achieved, the bit allocation and/or power allocation based on the allocated constellations. In a further implementation form of the second aspect, the method further comprises iteratively performing the steps of allocating, determining, and calculating until the overall target rate is achieved. In a further implementation form of the second aspect, the method further comprises performing the FEC. In a further implementation form of the second aspect, the method is performed by a device that is, or is included in, a receiver of the optical communication system. In a further implementation form of the second aspect, the MLC is based on a concatenated MLC comprising a Low-Density-Parity-Check-Code (LDPC) inner code and a staircase outer code. In a further implementation form of the second aspect, the method further comprises inner- coding the Least Significant Bit (LSB) based on the LDPC inner code; and/or inner-uncoding at least one upper bit-level. In a further implementation form of the second aspect, the multicarrier optical communication system is based on at least one of the following:

- an Intensity-Modulated Direct-Detection (IM-DD) Discrete Multi-Tone (DMT); - a Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM). A third aspect of the invention provides a computer program which, when executed by a computer, causes the method of the second aspect or an implementation form of the second aspect to be performed. A fourth aspect of the invention provides a non-transitory computer-readable recording medium that stores therein a computer program which, when executed by a computer, causes the method of the second aspect or an implementation form of the second aspect to be performed. It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof. BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which FIG.1 is a schematic view of a device for supporting FEC bit- and power loading in a multicarrier optical communication system, according to an embodiment of the invention; FIG.2 is a schematic view of a system including the device for supporting FEC bit- and power loading, when using a concatenated MLC, according to an embodiment of the invention; FIG.3a and 3b illustrate BER and achievable rate curves for the concatenated MLC, respectively; FIG.4 illustrates a comparison of the performance of the device of the invention and a conventional device; FIG.5 is a flowchart of a method for supporting FEC bit- and power loading in a multicarrier optical communication system, according to an embodiment of the invention; FIG.6 schematically illustrates a conventional multicarrier optical communication system for configuring a bit- and power-loading scheme at the receiver side; FIG.7 schematically illustrates a conventional MLC scheme; FIG.8 schematically illustrates performance of a conventional MLC system after applying a constant pre-FEC. DETAILED DESCRIPTION OF EMBODIMENTS FIG. 1 is a schematic view of a device 100 for supporting FEC bit- and power loading in a multicarrier optical communication system 1, according to an embodiment of the invention. The device 100 supports FEC bit- and power loading in the multicarrier optical communication system 1 using Multi-Level Coding (MLC), wherein the MLC is based on two or more constellations C ₁ , C ₂ of different size. For example, the constellations C ₁ is smaller than the constellations C ₂ , and C ₂ is the next larger constellation from C ₁ . The device 100 is configured to determine a FEC property 101 of the optical communication system 1. The device 100 is further configured to determine a target Spectral Efficiency (SE) 102. The device 100 is further configured to determine one or more rate increments 103, 104 based on the FEC property 101 and the two or more constellations C ₁ , C ₂ , wherein each rate increment 103, 104 is associated with a change from a given constellation C ₁ to the next larger constellation C ₂ . The device 100 is further configured to determine one or more power increments 105, 106, for each of a plurality of subcarriers 110, based on the FEC property 101 and the two or more constellations C ₁ , C ₂ , wherein each power increment 105, 106 is associated with a change from a given C ₁ constellation to the next larger constellation C ₂ allocated to that subcarrier. The device 100 may be, or may incorporated, in a transmitter device and/or a receiver device of the optical communication system 1. The device may comprise a circuitry (not shown in FIG. 1). Moreover, the circuitry may comprise hardware and software. The hardware may comprise analog or digital circuitry, or both analog and digital circuitry. In some embodiments, the circuitry comprises one or more processors and a non-volatile memory connected to the one or more processors. The non-volatile memory may carry executable program code which, when executed by the one or more processors, causes the device to perform the operations or methods described herein. FIG.2 is a schematic view of the optical communication system 1 including the device 100 for supporting FEC bit- and power loading (e.g. as of FIG. 1), when using a concatenated MLC, according to an embodiment of the present invention. At first, the MLC may be used, e.g., by the optical communication system 1 and/or by the device 100. For example, the device 100 may be a receiver device in the optical communication system 1. The concatenated MLC comprising the LDPC inner code 201 and the staircase outer code 202. The inner FEC (i.e., LDPC inner code) codes only the least significant bit (LSB), i.e., the lowest bit-level is inner-coded and the upper bit-levels are inner-uncoded. Moreover, the MLC is based on a plurality of constellations C _k of increasing size C ₁ < C ₂ < C ₃ < C ₄ . Each constellation accommodates a number of information bits, i.e., Rk: QPSK, 16 QAM, 64 QAM and 256 QAM. For instance, a given subcarrier i requires the transmit power P _k,i to operate a given constellation of C _k , where k is the size of the given constellation. Furthermore, the device 100 determines the FEC property 101. For example, the given total FEC overhead may be defined as the OH _total . In addition, the net target data rate is the output FEC rate according to the input FEC rate is R _inner , and the outer FEC rate is

R _outer . Besides, the device 100 further determines the SE 102. For example, by using two polarizations, and for an exemplary baud rate of S=80 GBaud and an exemplary target net data rate R _total =800 Gbit/s, the spectral efficiency 102 (e.g., in bits/s/Hz/polarization) is determined according to SE = Rtotal/(S*2) = 800/(80*2) = 5. In this example, the device 100 needs to load on average 5 bits on each time-frequency-polarization slot, i.e., QAM symbol. Now that the MLC, the FEC property 101, and the target SE 102, has been chosen, the device 100 further determines the rate increments 103, 104 and the power increments 105, 106. Moreover, the device 100 may determine the one or more rate increments 103, 104, for example, based on defining an information rate increment dR _k+1 = R _k+1 - R _k , which may result from using constellation of C _k+1 (i.e., the next larger constellation) instead of the (current smaller) constellation C _k . Moreover, the device 100 determines the one or more power increments 105, 106, for example, based on defining a power increment dP _k+1,i = P _k+1,i -P _k,i , which may result from using constellation C _k+1 (i.e., the next larger constellation) instead of the (current smaller) constellation C _k on the subcarrier i. The one or more rate increments (dR) 103, 104 may be determined as follows: For a target SE 102, an output FEC rate Rtotal. and an outer code rate Router, the device 100 calculate the inner code rate R _inner when only one bit-level is inner coded, according to the following equations:

Furthermore, with the knowledge of the inner code rate, the device 100 calculates the number of information bits mapped to each QAM constellation size and the rate increments 103, 104, for example, based on the presented qualitative parameters in table I.

Table I: the rate increments and the power increments. wherein ^ _^^^^^ , ^ _^^^^^ , and ^ _^^^^^ can be defined, ^ _^^^^^ = total net rate, ^ _^^^^^ =rate of outer code, and wherein: The device 100 further determines the one or more power increments (dP) 105, 106, as follow: Each constellation size (i.e., QPSK, 16QAM, 64 QAM, 256 QAM) has a required minimum SNR to operate at the required BER (^ _^^^ ) as it is shown using qualitative parameters in table II:

Table II: The required SNR for operation of different constellations The device 100 may determine the values of these SNRs by using the inner code rate R _inner and the required BER (e _^^^ ), for example, from the achievable rate and the BER curves which are illustrated in FIG.3a and FIG.3b. FIG.3a and FIG.3b illustrate diagrams of BER 300a and achievable rate curves 300b for the concatenated MLC, respectively. In FIG.3a, the graph indicated with the reference number 301a corresponds to uncoded BER versus SNR for 16-QAM, bit-level 2, the graph indicated with the reference number 302a corresponds to uncoded BER versus SNR for 256-QAM, bit-level 2, the graph indicated with the reference number 303a corresponds to uncoded BER versus SNR for 256-QAM, bit-level 3, the graph indicated with the reference number 304a corresponds to uncoded BER versus SNR for 256-QAM, bit-level 4, the graph indicated with the reference number 305a corresponds to uncoded BER versus SNR for 64-QAM, bit-level 2, the graph indicated with the reference number 306a corresponds to uncoded BER versus SNR for 64-QAM, bit-level 2, 3, the graph indicated with the reference number 307a corresponds to uncoded BER versus SNR for 64- QAM, bit-level 3, and the graph indicated with the reference number 308a corresponds to uncoded BER versus SNR for 256-QAM, bit-level 2. In FIG.3b, the graph indicated with the reference number 301b corresponds to the achievable binary FEC rate versus SNR for the QPSK, the graph indicated with the reference number 302b corresponds to the achievable binary FEC rate versus SNR for the 16-QAM, the graph indicated with the reference number 303b corresponds to the achievable binary FEC rate versus SNR for the 64-QAM, and the graph indicated with the reference number 304b corresponds to the achievable binary FEC rate versus SNR for the 256 QAM. For instance, for an exemplary required BER of ^ _^^^ of 1∙10-3, the MLC example, the FEC yields the following numbers as listed in Table III:

Table III: the exemplarily values for different parameters After that, the device 100 determined the rate increments (d^ ) 103, 104 and the power increment (d ^), for the FEC 101, a FEC-aware strategy can be applied for bit- and power loading configuration. Without limiting the present disclosure, the device 100 may perform a (joint) bit and power loading by calculating a bit allocation (^ _^ ) and power allocation (^ _^ ) by performing the following operations: 1) The device 100 determines the FEC 101, the SE 102, the rate increments 103, 104 (d^ _^ ⁾ , and the power increments 105, 106 (d^ _^,^ ,).

2) The device 100 allocates the smallest constellation to all subcarriers ^ _^ = 0 , ^ = 0,… , ^.

3) The device 100 finds the next cheapest subcarrier according to:

4) The device 100 increases the allocation on the cheapest subcarrier ^ _^ ® ^ _^ + 1.

5) The device 100 determines if the target rate is achieved, for example, it may determine if, å , moreover, if the tareget rate is achieved, the device 100 goes to next step (step 6), otherwise, it goes back to perform step 2.

6) The device 100 calculates the bit allocation according to , and the power

allocation according to ^ _^ = ^ _^^,^ . FIG.4 illustrates a comparison of the performance of the device 100 of the present invention and a conventional device using a conventional waterfilling method. In the diagram 400 illustrated in FIG. 4, the residual BER to be corrected by outer code is illustrated in Y axis of the diagram 400 versus the SNR offset (dB). As it can be derived, the performance of the device 100 (indicated with 401) outperforms the performance of the conventional device using waterfilling (indicated with 402) with a power saving of 0.21 dB on a laboratory measurement. FIG. 5 shows a method 500 according to an embodiment of the invention for supporting Forward Error Correction (FEC) bit and power loading in a multicarrier optical communication system using Multi-Level Coding (MLC) wherein the MLC is based on two or more constellations of different size C1, C2. The method 500 may be carried out by the device 100, as it described above. The method 500 comprises a step 501 of determining a FEC property 101 of the optical communication system 1. The method 500 further comprises a step 502 of determining a target Spectral Efficiency (SE) 102. The method 500 further comprises a step 503 of determining one or more rate increments 103, 104 based on the FEC property 101 and the two or more constellations C1, C2, wherein each rate increment 103, 104 is associated with a change from a given constellation C1 to the next larger constellation C ₂ . The method 500 further comprises a step 504 of determining one or more power increments 105, 106, for each of a plurality of subcarriers 110, based on the FEC property 101 and the two or more constellations C1, C2, wherein each power increment 105, 106 is associated with a change from a given C1 constellation to the next larger constellation C2 allocated to that subcarrier. The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article“a” or“an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.