阅读说明：本技术 从多个天线传送符号 (Transmitting symbols from multiple antennas ) 是由 M·洛佩兹 L·威尔逊 于 2019-03-11 设计创作，主要内容包括：提供了用于从多个天线传送符号的方法和设备。在一个示例中,一种方法包括从每个天线同时传送乘以矩阵的选定列的相应元素的符号。该矩阵的行数至少是天线的数量,该矩阵的列数至少是9,并且该矩阵包括最大过量的实哈达玛矩阵或者是最大过量的实哈达玛矩阵的子矩阵。(Methods and apparatus are provided for transmitting symbols from multiple antennas. In one example, a method includes simultaneously transmitting from each antenna symbols multiplied by respective elements of a selected column of a matrix. The number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.)

1. A method of transmitting symbols from multiple antennas, the method comprising:

simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected column of the matrix;

wherein the number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

2. The method of claim 1, further comprising:

transmitting at least one further symbol, comprising for each further symbol, simultaneously transmitting from each antenna the further symbol multiplied by a respective element of a column of a matrix associated with the further symbol.

3. The method of claim 2, wherein the selected column and each column associated with each additional symbol comprise different columns of the matrix.

4. A method of transmitting symbols from multiple antennas, the method comprising:

simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected row of the matrix;

wherein the number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

5. The method of claim 4, further comprising:

transmitting at least one further symbol, including for each further symbol, simultaneously transmitting from each antenna the further symbol multiplied by a respective element of a row of a matrix associated with the further symbol.

6. The method of claim 5, wherein the selected row and each row associated with each additional symbol comprise different rows of the matrix.

7. A method as claimed in any one of claims 2, 3, 5 and 6, wherein the symbol and the at least one further symbol comprise at least 9 OFDM symbols.

8. A method as claimed in any preceding claim, wherein the number of antennas is at least 9.

9. The method of any of the preceding claims, wherein the matrix comprises an 8x8, 12x12, or 16x16 matrix.

10. The method of any one of the preceding claims, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

11. the method of any of claims 1 to 9, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

12. the method of any of claims 1 to 9, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

13. the method of any of claims 1 to 9, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

14. a method as claimed in any preceding claim, wherein the symbols comprise OFDM symbols.

15. The method of any of the preceding claims, wherein the symbol comprises a Long Training Field (LTF) symbol.

16. A method of constructing a maximum excess real hadamard matrix, the method comprising:

selecting a real Hadamard matrix;

determining a first excess of the matrix;

negating rows and columns of the matrix;

determining a second excess of the matrix;

reversing the negating step if the second excess is less than the first excess; and

the previous four steps are repeated until the matrix comprises a maximum excess of real hadamard matrices.

17. The method of claim 16, wherein the matrix comprises an 8x8, 12x12, or 16x16 matrix.

18. The method of any one of claims 1 to 15, wherein the matrix is constructed according to the method of claim 14 or 15.

19. A computer program comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any one of claims 1 to 18.

20. A carrier containing the computer program of claim 19, wherein the carrier comprises one of the electronic signals.

21. A computer program product comprising a non-transitory computer readable medium on which the computer program of claim 19 is stored.

22. An apparatus for transmitting symbols from multiple antennas, the apparatus comprising a processor and a memory, the memory containing instructions executable by the processor such that the apparatus is operable to:

simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected column of the matrix;

wherein the number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

23. The apparatus of claim 22, wherein the memory contains instructions executable by the processor such that the apparatus is operable to:

transmitting at least one further symbol, comprising for each further symbol, simultaneously transmitting from each antenna the further symbol multiplied by a respective element of a column of a matrix associated with the further symbol.

24. The apparatus of claim 23, wherein the selected column and each column associated with each additional symbol comprise different columns of the matrix.

25. An apparatus for transmitting symbols from multiple antennas, the apparatus comprising a processor and a memory, the memory containing instructions executable by the processor such that the apparatus is operable to:

simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected row of the matrix;

wherein the number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

26. The apparatus of claim 25, wherein the memory contains instructions executable by the processor such that the apparatus is operable to:

transmitting at least one further symbol, including for each further symbol, simultaneously transmitting from each antenna the further symbol multiplied by a respective element of a row of a matrix associated with the further symbol.

27. The apparatus of claim 26, wherein the selected row and each row associated with each additional symbol comprise a different row of the matrix.

28. The method of any one of claims 23, 24, 26 and 27, wherein the symbol and the at least one further symbol comprise at least 9 OFDM.

29. The apparatus of any of claims 22 to 28, wherein the number of antennas is at least 9.

30. The apparatus of any of claims 22 to 29, wherein the matrix comprises an 8x8, 12x12, or 16x16 matrix.

31. The apparatus of any of claims 22 to 30, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

32. the apparatus of any of claims 22 to 30, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

33. the apparatus of any of claims 22 to 2306, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

34. the apparatus of any of claims 22 to 30, wherein the matrix comprises a sub-matrix of a matrix M or M, wherein M comprises or is equivalent to:

。

35. the apparatus of any of claims 22 to 34, wherein the symbols comprise OFDM symbols.

36. The apparatus of any of claims 22-35, wherein the symbols comprise Long Training Field (LTF) symbols.

37. An apparatus for transmitting symbols from multiple antennas, the apparatus configured to:

simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected column of the matrix;

wherein the number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

38. An apparatus for transmitting symbols from multiple antennas, the apparatus configured to:

simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected row of the matrix;

wherein the number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Technical Field

Examples of the present disclosure relate to transmitting symbols, e.g., Long Training Fields (LTFs), from multiple antennas.

Background

Advanced antenna systems may be used to significantly enhance the performance of a wireless communication system in both the Uplink (UL) and Downlink (DL) directions. For example, advanced antennas may provide the possibility to use the spatial domain of the channel to improve the reliability and/or throughput of the transmission, e.g. by transmitting using multiple spatial streams, also referred to as space time streams.

For example, the 802.11-16 standard specifies a set of matrices, often referred to as P-matrices, where rows (and columns) define a set of orthogonal vectors that are used as orthogonal cover codes for channel and pilot estimation when more than one space-time stream is utilized (e.g., non-multiple-input multiple-output MIMO operation). The rows or columns of these P-matrices may be applied to the Long Training Field (LTF) and to the pilots embedded in the data symbols at the time of transmission. The P matrix may be, for example, a Hadamard matrix (Hadamard matrix).

The real Hadamard matrix is a n × n dimensional square matrix H whose entries (entries) consist of +1 and-1 (i.e., only real values), such that H · H^TnI ═ nI. Here, superscript (. degree.)^TDenotes a matrix transpose, and I is an identity matrix. The order of the real hadamard matrix is known to be a multiple of n 1, 2 or 4 (i.e., n 4,8,12,16, …). It can be verified that the hadamard property is preserved by the following operation matrix:

1) negation of a row or column.

2) Permutation (i.e., swapping) of any two rows or columns.

Furthermore, any hadamard matrix can be transformed by means of these operations into a matrix whose first row and first column consist of only + 1. Hadamard matrices in this form are said to be normalized (normalized). The two matrices a and B are said to be equivalent if the hadamard matrix a can be transformed into the hadamard matrix B by successively applying the operations 1 and 2 identified above. Otherwise, the matrix is said to be non-equivalent. Any hadamard matrix is equivalent to the normalized hadamard matrix.

EHT (very high throughput) has been proposed as an enhancement to the IEEE 802.11 standard. In particular, EHT should provide support for up to 16 space-time streams. Currently, the IEEE 802.11-16 standard and its revision 802.11ax support up to 8 space-time streams. Thus, for example, there may be a need for a matrix of order 9 ≦ n ≦ 16 (e.g., P matrix) to provide orthogonal cover codes for Long Training Fields (LTFs) for up to 16 space-time streams.

Constructing a real valued P matrix for 8 or fewer space-time streams is simple and can be done by inspection or by exhaustive computer search. However, as the dimension of the P matrix increases, an exhaustive computer search becomes impractical.

Disclosure of Invention

One aspect of the present disclosure provides a method of transmitting symbols from multiple antennas. The method includes simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected column of the matrix. The number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises a maximum excess (maximum excess) of real hadamard matrices or a sub-matrix of the maximum excess of real hadamard matrices.

Another aspect of the present disclosure provides a method of transmitting symbols from multiple antennas. The method includes simultaneously transmitting from each antenna the symbols multiplied by the corresponding elements of the selected row of the matrix. The number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Further aspects of the disclosure provide methods of constructing a maximum excess real hadamard matrix. The method comprises the following steps: selecting a real Hadamard matrix; determining a first excess of the matrix; negating rows and columns of the matrix; determining a second excess of the matrix; if the second excess is less than the first excess, reversing the negating step; and the previous four steps are repeated until the matrix comprises the maximum excess real hadamard matrix.

Yet another aspect of the present disclosure provides an apparatus for transmitting symbols from multiple antennas. The apparatus includes a processor and a memory. The memory contains instructions executable by the processor such that the apparatus is operable to simultaneously transmit from each antenna symbols multiplied by respective elements of a selected column of the matrix. The number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Another aspect of the disclosure provides an apparatus for transmitting symbols from multiple antennas. The apparatus includes a processor and a memory. The memory contains instructions executable by the processor such that the apparatus is operable to simultaneously transmit from each antenna symbols multiplied by respective elements of a selected row of the matrix. The number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Additional aspects of the present disclosure provide apparatus for transmitting symbols from multiple antennas. The apparatus is configured to simultaneously transmit from each antenna the symbols multiplied by the corresponding elements of the selected column of the matrix. The number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Further aspects of the disclosure provide apparatus for transmitting symbols from multiple antennas. The apparatus is configured to simultaneously transmit from each antenna the symbols multiplied by the corresponding elements of the selected row of the matrix. The number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Drawings

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 accompanying drawings in which:

fig. 1 is an example of a normalized hadamard matrix of order n-12;

fig. 2 is an example of a normalized hadamard matrix of order n-16;

fig. 3 is an example of a normalized hadamard matrix of order n-16;

fig. 4 is an example of a normalized hadamard matrix of order n-16;

fig. 5 is an example of a normalized hadamard matrix of order n-16;

fig. 6 is an example of a normalized hadamard matrix of order n-16;

fig. 7 is a flow chart of an example of a method of transmitting symbols from multiple antennas;

fig. 8 is a flow chart of an example of a method of transmitting symbols from multiple antennas;

fig. 9 is an example of a maximum excess real hadamard matrix of order n-12;

fig. 10 is an example of a maximum excess real hadamard matrix of order n-16;

fig. 11 is an example of a maximum excess real hadamard matrix of order n-16;

fig. 12 is an example of a maximum excess real hadamard matrix of order n-16;

FIG. 13 is a flow diagram of an example of a method of constructing a maximum excess real Hadamard matrix;

fig. 14 shows an example of a device for transmitting symbols from multiple antennas; and

fig. 15 shows an example of a device for transmitting symbols from multiple antennas.

Detailed Description

For purposes of explanation and not limitation, specific details are set forth below, such as particular embodiments or examples. It will be appreciated by one skilled in the art that other examples may be employed in addition to these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not to 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 dedicated 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 appropriate radio communication circuitry. Moreover, the techniques can additionally be considered to be embodied entirely within any form of computer-readable memory (such as solid-state memory, magnetic or optical disk) containing an appropriate set of computer instructions that would cause a processing circuit to carry out the techniques described herein, where appropriate.

Hardware implementations may include or encompass, but are not limited to, Digital Signal Processor (DSP) hardware, reduced instruction set processors, hardware (e.g., digital or analog) circuits including, but not limited to, application specific integrated circuit(s) (ASICs) and/or field programmable gate array(s) (FPGAs), and state machines capable of performing such functions, where appropriate.

At least some embodiments of the present disclosure relate to the design of matrices that reduce computational complexity and/or memory requirements at the transmitter and receiver (e.g., P matrices, and/or matrices whose rows or columns are applicable to symbols such as LTF symbols). For this reason, the following definitions will be useful. The excess σ (H) of the real hadamard matrix H is defined as the sum of all its terms. For example, the attribute indicates an excess (or deficiency) of items including +1 relative to items including-1. The maximum possible excess of the real hadamard matrix of order 12 is 36. Likewise, the maximum possible excess of real hadamard matrices of order n-16 is 64. The real hadamard matrix with the largest possible excess for a given order is referred to herein as the maximum excess hadamard matrix. Despite the row and/or column permutations, there is only one normalized hadamard matrix 100 of order n-12, which is shown in fig. 1. There are also exactly 5 non-equivalent normalized hadamard matrices 200, 300, 400, 500 and 600 of order n-16, shown in fig. 2 to 6, respectively. However, these matrices 100-600 are not maximum excess hadamard matrices.

Embodiments of the present disclosure propose the use of matrices that may be adapted to provide an orthogonal cover code capable of supporting at least 9 space-time streams. In some examples, the proposed matrix is a real hadamard matrix with the largest possible excess.

The matrix may, for example, provide an orthogonal cover code that enables support for at least 9 space-time streams (e.g., up to 16 space-time streams). Since the orthogonal cover codes are defined according to the hadamard matrix with the largest possible excess, they consist of only ± 1 and have the largest possible number of + 1. In some examples, these two properties may allow for the greatest possible reduction in computational complexity and/or memory usage at the transmitter when compared to using other matrices and cover codes (including real hadamard matrices without the greatest excess).

Fig. 7 is a flow diagram of an example of a method 700 of transmitting symbols from multiple antennas. In some examples, the symbols may include or include Long Training Field (LTF) symbols or one or more pilot symbols, and/or may include OFDM symbols. The method includes simultaneously transmitting symbols from each antenna multiplied by a corresponding element of a selected column of the matrix at step 702. The number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Thus, for example, a symbol may be transmitted and multiplied by an element from a selected column of the matrix, the element corresponding to the antenna from which the symbol is transmitted. The elements may be different for each antenna, although in some examples, the values of the elements may be the same (e.g., selected from ± 1).

In some examples, the number of space-time streams to transmit or being transmitted is made smaller than the order (size, number of rows/columns) of the hadamard matrix. For example, the hadamard matrix may be a 16x16 matrix, and 15 space-time streams may be transmitted. In some examples, the matrix used to provide orthogonal cover codes for the 15 space-time streams may be a 15x16 matrix. In some examples, the number of space-time streams is equal to the number of antennas.

In some examples, more than one (e.g., at least the number of space-time streams) symbol is transmitted. In some examples, the number of times the transmission of the symbol repeats over time (including the first transmission) is equal to the number of columns of the matrix (e.g., 16 columns for a 16x16 hadamard matrix). In some examples, method 700 may include transmitting at least one further symbol, including, for each further symbol, simultaneously transmitting from each antenna the further symbol multiplied by a respective element of a column of a matrix associated with the further symbol. That is, for example, as part of a training sequence, in a first time period, a symbol is transmitted and an element from a first column of the matrix is used; and for subsequent time periods, the symbol is again transmitted and elements from a different column of the matrix are used. In some examples, the symbols may be transmitted one or more times in further subsequent time periods each using a training sequence of a different column of the matrix. Thus, for example, the selected column and each column associated with each further symbol comprise different columns of the matrix.

Fig. 8 shows an alternative example of a method 800 of transmitting symbols from multiple antennas. Method 800 differs from method 700 in that the symbols are multiplied by selected columns of the matrix rather than corresponding elements of the rows. Thus, the method 800 includes simultaneously transmitting from each antenna, at step 802, symbols multiplied by corresponding elements of a selected row of the matrix. The number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices. Any of the alternatives described above with respect to method 700 of fig. 7 are also applicable to method 800 of fig. 8, except that rows are referenced instead of columns and columns are referenced instead of rows where appropriate.

It should be noted that actual implementations of the method 700 or 800 may or may not specifically use rows or columns of the matrix. Instead, for example, a calculation or operation may be performed that effectively results in a symbol transmission as if it had been multiplied by a value that would come from a matrix that includes the maximum excess of real hadamard matrices or a sub-matrix that is the maximum excess of real hadamard matrices, even if other operations, vectors and/or matrices are used instead.

In some examples, the number of antennas is at least the number of space-time streams, e.g., at least 9. The matrix may comprise a 12x12 or 16x16 matrix. For example, the matrix includes a sub-matrix of matrix M or M, where M includes or is equivalent to the matrix 900 shown in fig. 9, which is an example of a maximum excess real hadamard matrix of order n-12. For example, embodiments using up to 12 spatial streams and/or transmitting symbols in up to 12 different time periods may use matrix 900 or an equivalent. Similarly, the hadamard matrices may include the matrices 1000-1200 shown in fig. 10-12, respectively, with fig. 10-12 showing examples of maximum excess real hadamard matrices of order n-16. These may be used, for example, in embodiments having up to 16 spatial streams and/or transmitting symbols in up to 16 different time periods.

The matrix 1000 in FIG. 10 may be defined, for example, asWhereinRepresents a Kronecker product (Kronecker product), and

in some examples, where the number of space-time streams is m-9, 10, 11, it is proposed to use sub-matrices of the hadamard matrix to provide orthogonal vectors (e.g., at different time periods) to be applied to the symbols. The matrix to be used may belong to, for example, the m × 12 dimensions and may thus be a sub-matrix of the maximum excess hadamard matrix of order 12. In the case where the number of space-time streams is m 13, 14, 15, an m × 16 dimensional sub-matrix of the maximum excess hadamard matrix of order 16 may be used.

Fig. 13 is a flow diagram of an example of a method 1300 of constructing a maximum excess real hadamard matrix. The method 1300 includes, at step 1302, selecting a real hadamard matrix. This may be, for example, a normalized hadamard matrix such as those shown in fig. 1 to 7, or any other hadamard matrix. The method 1300 further includes, at step 1304, determining a first excess of the matrix and, at step 1306, negating rows and columns of the matrix. The rows and columns may be selected sequentially, randomly, or in any other manner. In some examples, the rows and columns are selected such that after multiple iterations of step 1306, each row/column combination is negated at least once. Thus, for example, for a 16x16 matrix, step 1306 may be repeated at least 256 times (unless the maximum excess matrix is obtained before all combinations are negated).

Next, step 1308 of the method includes determining a second excess of the matrix, which may be different from the first excess determined in step 1304 as a result of the negation in step 1306. In step 1310, the method 1300 includes reversing the negating step if the second excess is less than the first excess (or alternatively, less than or equal to the first excess). Thus, after step 1304-. Finally, step 1312 of the method 1300 includes repeating the previous four steps (i.e., steps 1304-1310) until the matrix includes the maximum excess real Hadamard matrix. In some examples of method 700 or 800, a matrix is constructed according to method 1300.

Diagram 1400 shows an example of a device 1400 for transmitting symbols from multiple antennas. In some examples, device 1400 may be configured to perform method 700 described above with reference to fig. 7, or any of the other examples described herein.

The device 1400 includes a processor 1402 and a memory 1404 in communication with the processor 1402. The memory 1404 contains instructions executable by the processor 1402. In one embodiment, memory 1404 contains instructions executable by processor 1402, such that apparatus 1400 is operable to simultaneously transmit from each antenna symbols multiplied by respective elements of a selected column of a matrix. The number of rows of the matrix is at least the number of antennas, the number of columns of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

Diagram 1500 shows an example of an apparatus 1500 for transmitting symbols from multiple antennas. In some examples, device 1500 may be configured to perform method 800 described above with reference to fig. 8, or any of the other examples described herein.

The device 1500 includes a processor 1502 and a memory 1504 in communication with the processor 1502. The memory 1504 contains instructions executable by the processor 1502. In one embodiment, the memory 1504 contains instructions executable by the processor 1502 such that the apparatus 1500 is operable to simultaneously transmit from each antenna symbols multiplied by respective elements of a selected row of the matrix. The number of columns of the matrix is at least the number of antennas, the number of rows of the matrix is at least 9, and the matrix comprises the maximum excess of real hadamard matrices or sub-matrices of the maximum excess of real hadamard matrices.

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 claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several of the units set forth in the following statements. Where the terms "first", "second", etc. are used, they are merely to be construed as labels to facilitate identification of particular features. In particular, unless explicitly stated otherwise, they are not to be construed as describing a first or second of a plurality of such features (i.e., first or second of such features occurring in time or space). The steps in the methods disclosed herein may be performed in any order, unless explicitly stated otherwise. Any reference signs in the description shall not be construed as limiting their scope.